Extent and variability of Mesozoic‐Cenozoic multi‐stage salt diapirs in the Southern Permian Basin, Southern North Sea

Salt structures can be used as an archive for tectonic and depositional processes as all salt structures respond distinctively. Few salt tectonic studies have investigated the evolution of multi‐stage salt structures, nevertheless, no previous study had systematically identified, mapped, nor classified, the evolution of multi‐stage salt structures in a regional study. Decades of hydrocarbon exploration and the heavily dense 3D seismic data available, make the Southern North Sea one of the best natural laboratories to investigate the evolution of salt structures. The Southern North Sea salt basin is a Late Permian Zechstein salt mega‐basin containing a myriad of salt structures. The complex tectonic evolution of the Southern North Sea created diverse Mesozoic structural sub‐basins with different tectonostratigraphic evolutions. We defined a nomenclature, linked to the mega‐stratigraphic sequences, for the classification of salt structures. We used a Two‐Way‐Travel‐Time 3D seismic reflection dataset and time‐thickness variations around salt structures to systematically analyse the evolution of salt structures across the diverse structural sub‐basins of the Southern North Sea. Multi‐stage salt diapirs were triggered halokinetically in the Early Triassic and are linked to regional palaeo‐depocentres controlled by the sub‐Zechstein structural configuration. Multi‐stage salt diapirs in the different sub‐basins evolved through three different regional phases and up to five distinctive local stages. The most complex salt diapirs developed in the Central Graben, Sole Pit High and Silver Pit Basin, where multi‐stage salt diapirs showed 4–5 local stages of salt diapirism. The multi‐stage evolution of salt structures should be thoroughly investigated to reduce risks and uncertainties in the energy sector and net zero projects, such as in carbon capturing and storage projects, and energy storage in man‐made salt caverns.


| INTRODUCTION
Salt structures can be used as an archive for tectonic and depositional processes.Individual salt structures not only respond differently to the regional tectonic deformation processes but also respond differently to the subregional and local deformation processes.Consequently, salt structures record the local tectonic and depositional drivers of sub-basin evolution within a regional tectonostratigraphic framework.
The North Sea is divided into two regional salt basins, that is, the Northern and Southern Permian Basin, which extent from the east coast of the United Kingdom into continental Europe (Figure 1).The Southern Permian Basin (SPB), an east-west trending Late Palaeozoic intracontinental extensional basin, is characterized by >1000m-thick Late Permian Zechstein layered halite and evaporites sequence (Doornenbal & Stevenson, 2010;Maystrenko et al., 2012Maystrenko et al., , 2013;;Soto et al., 2017;Ziegler, 1990a).The Southern North Sea (SNS) basin lies within the SPB and comprises the UK central-offshore sector, the Netherlands offshore sector and parts of the offshore sector of Germany (Figure 1a).
Inherited Palaeozoic-structural elements strongly influenced the sub-division of the SNS mega basin into five structurally distinct Mesozoic sub-basins (Alberts & Underhill, 1991;Geluk, 1999Geluk, , 2007;;Peryt et al., 2010;Pharaoh et al., 2010;Van Hoorn, 1987).Moreover, the SNS salt basin contains a plethora of salt structural styles and is the perfect natural laboratory to study multi-stage salt diapirism thanks to the availability of a regional 3D seismic reflection dataset which permits a regional comparative salt tectonic analysis of the complex multi-stage salt structures of the SNS salt basin.
The variation of the Zechstein salt depositional thickness across the different structural sub-basins controlled the Mesozoic-Cenozoic tectonostratigraphic deformation and triggered salt diapirism across the SNS salt basin creating a myriad of salt structures (Warsitzka et al., 2019).The diverse salt structures in the SNS demonstrate variable and complex multi-phase salt tectonic histories which responded differently to the regional tectonic processes and responded differently within the diverse structural subbasins (Stewart & Coward, 1995;Warsitzka et al., 2019).
The understanding of the complex evolution of salt structures has a positive academic, societal and economic impact.For example, these understandings can help us to further define our understandings of salt tectonics, help the energy sector for the re-evaluation of hydrocarbon prospects, help to the identification of CO2 storage repositories and for nuclear and water waste management and disposal.Hitherto, regional to sub-regional salt tectonic studies have focused only on the understanding of a particular salt structural zone of hand-picked salt diapirs in salt basins.For example, regional stratigraphic studies in salt basins have been carried out by Rowan and Weimer (1998) using a high volume of 2D seismic cross-section profiles in the US Gulf of Mexico.Nevertheless, the authors did not consider in detail the complete evolution of salt diapirs and only focused on the Pliocene-Pleistocene stratigraphic succession controlled by the bathymetric relief caused by salt diapirism.Salt tectonic studies that emphasized the relevance of utilizing the surrounding halokinetic sequences and the seismic stratigraphic sequences such as Rowan and Weimer (1998) and Stewart (2020), are essential studies that helped in the conceptualization of the methodology for this project.Previous 3D seismic studies in the Dutch part of the SNS, by ten Veen et al. (2012), have investigated the relationship between the palaeo-depositional salt thickness, the pre-Zechstein salt basement faulting and the salt overburden faulting.Additionally, using a 3D seismic dataset, Harding and Huuse (2015) identified and elucidated the complex evolution of salt diapirs using timethickness variation information across mapped horizons in the Dutch sector of the SNS.Harding and Huuse (2015) demonstrated the importance of a robust study between the relationship of the pre-salt basement structures, salt structures and the supra-salt characteristics utilizing seismic attribute analyses and thickness maps.
Since salt structures record the tectonic and depositional history of a basin, we needed a systematic regional identification, characterization and classification of multi-stage salt diapirs.Due to the availability of a regional mega-survey 3D dataset and revisiting previous salt tectonic studies of the SNS, we developed a robust regional methodology, nomenclature and classification

Highlights
• We introduced a regional systematic identification, evaluation, and correlation of salt structures.• We developed a classification scheme & nomenclature to map multi-stage salt structures utilizing a 3D seismic dataset.• Correlated the kinematic evolution of salt structures to the regional tectonic evolution of the Southern North Sea.• A total of 155 salt structures were identified in the five different structural sub-basins of the Southern North Sea.• Multi-stage salt diapirs evolved through 2-3 regional phases and 3-5 different local stages of salt diapirism.
of multi-stage salt diapirs by mapping the tectonic megastratigraphic sequences across the different structural sub-basins of the SNS salt basin.We utilized the megastratigraphic sequences as the framework to correlate and compare all multi-stage salt structures across the entire SNS, and the different kinematic and halokinetic processes that further control salt mobilization and salt diapirism.Our study also maps the variability of multi-stage salt structures and their regional and local controls by analysing regional to sub-regional time thickness variations across the diverse SNS sub-basins, from the base of the Late Permian Zechstein Supergroup to the seabed.
F I G U R E 1 Regional tectonic map of northeastern Europe and the North Sea and regional seismic cross section (next page).(a) Regional tectonic map of the North Sea with Permian salt basins showing the relationship of the Southern and Northern Permian Basin Zechstein salt distribution.Southern North Sea basin and regional tectonic elements adapted from Doornenbal and Stevenson (2010); Evans et al. (2003); Glennie (1990aGlennie ( , 1990b)); Stewart (2007); Ziegler (1990a).Distribution of more than 1000 oil and gas fields to date in the North Sea after Doornenbal and Stevenson (2010); Evans et al. (2003); Underhill (2009).Next page-(b) Regional seismic cross-section of the Southern North Sea, from WSW to ENE, extending from the United Kingdom to Danish continental waters.CBH, Cleaver Bank High; CG, Central Graben, D-HFZ, Dowsing-Hewett Fault Zone; ECG, East Central Graben; SPH, Sole Pit High; VG, Viking Graben; WCG, West Central Graben.

EAGE
GAITAN and ADAM

| GEOLOGY OF THE SOUTHERN NORTH SEA
The SPB is a salt basin which developed on mid-Palaeozoic crustal units with a NW-SE structural fabric inherited from the Caledonian orogeny (Figure 1a) (McKie, 2017;Stewart & Coward, 1995).The regional geology of the SNS basin is the result of regional tectonics, sea-level fluctuations and basin to sub-basin deformation due to salt tectonics (Cameron et al., 1992;Glennie, 1997;Warsitzka et al., 2019;Ziegler, 1990aZiegler, , 1990b)).
The SNS basin rests on Caledonian crust which comprises the northern foreland section of the Variscan foreland thrust belt.The SPB is bounded to the south by the Rheic suture and the Variscan front.Caledonian structural trends in the basement were partly reactivated as strike-slip fault systems during the Variscan collision and during the Alpine orogeny (Figure 1a).
The eastern part of the SNS salt basin is represented by structural lows, for example, the Central Graben and the Horn Graben, whereas to the west, inversion uplifted the Sole Pit High and Inde Shelf (Figure 1b).Generally, the Mesozoic-Cenozoic sedimentary sequences of the SNS basin can be divided into the foreland, the pre-rift, a main syn-rift phase, post-rift and syn-and post-tectonic phase (Figure 2).
At the end of the Variscan orogeny, the tectonic plate reorganization of the SNS developed extensional faults from the Hardegsen tectonic phase to the Middle and Late Cimmerian tectonic phases in the Jurassic (Figure 2) (Cameron et al., 1992).Restricted playa and sabkhalike environments, in a large intra-continental foreland basin, fostered the cyclic deposition of the Late Permian Zechstein Supergroup (Glennie & Underhill, 1998).Sealevel fluctuations controlled the cyclic deposition of the Zechstein salt, including Zechstein carbonates along the basin margins (Peryt et al., 2010).Early post-salt sedimentation during the Late Permian into Early Triassic times influenced the salt-overburden tectonic evolution of the different SNS sub-basins (Geluk, 1999).Middle to Late Triassic rifting and thermal relaxation deformation triggered rafting and salt halokinetic processes in the SNS (Figure 2) (Stewart et al., 1996;Stewart & Coward, 1995).The Middle and Late Cimmerian phases of rifting during the Jurassic, attributed to the Pangea continental breakup (Woodcock & Strachan, 2000), allowed the transition to full marine conditions and deposition of marine sediments from the Jurassic to the Cenozoic (Figure 2) (Cameron et al., 1992).Localized uplift and erosion during the Early Cretaceous promoted the development of the Late Cimmerian Unconformity, or as used in here, the Base Cretaceous Unconformity (BCU) (Figure 2) (Vejbaek et al., 2010), which is more prominent in basin-marginal areas (Cameron et al., 1992;Grant, Underhill, Hernández-Casado, Barker, et al., 2019;Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019).Sub-Hercynian to Savian pulses of inversion during the Alpine orogeny and localized uplift, were possible due to the reactivation of pre-existing NNW-SSE Palaeozoic-inherited structural elements, for example, the Dowsing-Hewett Fault Zone and the Central Graben system (Figure 1a) (Pharaoh et al., 2010).Much of the Palaeocene-Neogene, if deposited, was locally eroded to the west of the SNS, for example, in the Sole Pit High, due to the Pyranean and Savian pulses of inversion in the Cenozoic (Figures 1b and 2) (Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019).

Sea sub-basins
The complex Mesozoic-Cenozoic tectonic evolution of the SNS, and the inherited Palaeozoic basement structures, promoted the development of five major structural sub-basins and structural highs with different subsidence, inversion and halokinetic histories (Geluk, 1999;Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019;Stewart, 2007;Ziegler, 1990b).Today's structural configuration of the diverse SNS sub-basins includes the Broad Fourteens Basin, Central Graben and partly the Cleaver Bank High, as structural lows, whereas the Sole Pit High, partly the Cleaver Bank High and the Silver Pit Basin are structural highs (Figures 1b and 3) (Doornenbal & Stevenson, 2010).

| Broad Fourteens Basin
The Broad Fourteens Basin (BFB) is a NW-SE trending Mesozoic sub-basin, inverted during the Alpine collision (Figure 2).The BFB is in the southern limit of the SNS and lies offshore of the Netherlands (Figure 3).The BFB is bounded by the Cleaver Bank High to the north, the Winterton High (WH) to the west and the Texel Ijselmeer High (TIH) to the east.

GAITAN and ADAM
During the Pangea breakup in the Early Triassic, the BFB was a tectonically quiet sub-basin (Cameron et al., 1992).The Hardegsen and Early Cimmerian pulses of rifting in the Triassic continued into the Jurassic promoting Early Jurassic sedimentation and salt tectonic processes (Figure 2) (Ziegler, 1990b).North Sea doming during the Middle to Late Cimmerian tectonic phases in the Jurassic caused regional extension and inversion deformation (Figure 2) (de Lugt et al., 2003;Pharaoh et al., 2010).The BFB was inverted during the sub-Hercynian and Laramide tectonic phases in the Late Cretaceous (Figure 2).Inversion during the Late Cretaceous was followed by a regional subsidence phase and overall basin tilt towards the east (Figure 1a,b) (Nalpas et al., 1995).The Cleaver Bank High (CBH) is in the offshore Dutch sector of the SNS and is located at the centre of the study area (Figures 1a,b and 3).To the west, the CBH is bounded by the Silver Pit Basin and the Sole Pit High.To the north, the CBH is bounded by the E-W trending Mid North Sea High (MNSH).To the east, the Central Graben system outlines the extent of the CBH, and to the south, the BFB and TIH (Figure 3).
Salt structures in the Dutch CBH, are atop NW-SE compressive structures reactivated as normal faults during rifting phases in the Devonian (Quirk, 1993).The CBH was tectonically quiet during Triassic times until the Mid-Jurassic (van Gent et al., 2010).The Late Jurassic in the CBH was marked by thermal relaxation during the Late Cimmerian tectonic phase which promoted the regional subsidence of the different Mesozoic SNS sub-basins (Figure 2).A total of three inversion events are documented in the Dutch CBH during the Mesozoic-Cenozoic (Quirk, 1993).
The CBH remained a structural high which encouraged erosion and sediment distribution from the CBH into the adjacent structural sub-basins, that is, the Broad Fourteens Basin and Central Graben (Figure 3) (Duin et al., 2006).Contrariwise, the inversion of the adjacent Broad Fourteens Basin and Central Graben, permitted the accumulation of hundreds of metres of sediments into the CBH during the sub-Hercynian to Savian inversion tectonic phases (Figures 1b and 2) (Stewart & Coward, 1995).

| Central Graben
The Central Graben system extends from the Viking Graben, offshore Norway, to offshore the Netherlands (Figures 1a and 3).The Mesozoic extension and strike-slip deformation attributed to the Pangea breakup (Figure 2), promoted the development of the Central Graben system, the Viking Graben to the north (Figure 1a) and the Horn Graben to the west (Figure 1b).
The SNS Central Graben (CG) is in the Dutch sector of the study area (Figures 1 and 3).The CG is a regional NW-SE to N-S trending set of Palaeozoic-Mesozoic structural grabens extending from the Northern Permian Basin to the Southern Permian Basin (Doornenbal & Stevenson, 2010;Geluk, 2005;Sears et al., 1993).
The CG evolved through pulses of E-W Cimmerian extension during the Triassic and Jurassic (Rank-Friend & Elders, 2004).Inversion of the Central Graben rift flanks during the Sub-Hercynian to Savian inversion tectonic phases (Figure 2), promoted the erosion of hundreds of metres of Cretaceous sediments from the uplifted structural highs (Stewart & Coward, 1995).

| Silver Pit Basin
The Silver Pit Basin (SiPB) is a NE-SW trending, structural sub-basin located mostly in the UK section of the study area (Figure 3).It is bounded to the north by the North Dogger Shelf and the Mid North Sea High (MNSH).To the south, the SiPB is delimited by the Cleaver Bank High and the Sole Pit High.
During the Mesozoic, the SiPB was a tectonically quiet sub-basin unaffected by the major Palaeozoicinherited structural elements and acted as a palaeodepocentres for sediments eroded from the adjacent CBH (ten Veen et al., 2012).Salt halokinetic phases (Figure 2), and salt-overburden deformation during the Jurassic and Early Cretaceous, promoted the widespread development of localized salt-cover faulting (Figure 1b) (Doornenbal & Stevenson, 2010).Mesozoic-Cenozoic gravity sliding deformation triggered salt mobilization processes and developed local NE-SW trending sedimentary basins in the Silver Pit Basin (Figure 3) (Stewart, 2007).

| Sole Pit High
The Sole Pit High (SPH) is a NW-SE trending, inverted structural high located in the UK section of the study area (Figures 1b and 3).It is bounded to the west by the, Palaeozoic-inherited, Dowsing Fault Zone and Hewett Fault Zone, to the west by the East Midland Shelf and to the east by the CBH and SiPB (Figure 3).
The tectonostratigraphic evolution of the SPH was highly controlled by the E-W Cimmerian extension during the Late Triassic and Jurassic (Figure 2).Extension was accommodated by transtensional structural features which were inverted during the Late Cretaceous-Cenozoic (Figure 1b) (Cameron et al., 1992;Pharaoh et al., 2010;Woodcock & Strachan, 2000).Sub-Hercynian to Pyrenian basin-wide inversion of the SNS resulted in a total uplift of about 1500 m (Figure 1b) (Pharaoh et al., 2010;Van Hoorn, 1987;Ziegler, 1990a).Consequently, several hundreds of sediments were eroded from the top of salt structures in the SPH (Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019).

North Sea
Regional tectonics, that is, extension and contractions, trigger salt tectonics and promote the development of salt structures (Figure 4a).In the SNS, Triassic rifting triggered salt diapirism.Pulses of contraction and inversion in the GAITAN and ADAM SNS reactivated, the already established, salt structures rendering a plethora of complex salt structures across the distinct SNS structural sub-basins (Cameron et al., 1992;Glennie & Underhill, 1998;Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019;Ziegler, 1990b).
Tectonic deformation in the SNS is generally represented in two structural deformation styles, basement deformation, also known as thick-skinned deformation, (Figure 4a, i), and salt-overburden deformation, known as thin-skinned deformation (Figure 4a, ii-iii) (Stewart et al., 1996;ten Veen et al., 2012).In basement-controlled systems (Figure 4a, i), the salt layer controls the coupling deformation between the sub-salt structural faults and the salt-overburden deformation.Basement-controlled deformation is observed in the Broad Fourteens Basin (Nalpas et al., 1995), Central Graben and Sole Pit High (Stewart & Coward, 1995;ten Veen et al., 2012).Further studies in the SNS include Koyi and Petersen (1993)  Contraction in the SNS is mostly inherited from the Variscan orogeny and Alpine collision (Figure 2).Saltoverburden deformation is essentially observed in all structural sub-basins (Doornenbal & Stevenson, 2010).Contraction in the salt-overburden buckles the saltoverburden which can lead to salt thrusting and salt extrusion of the salt layer into younger strata and onto the surface (Figure 4a, iii) (Dooley et al., 2007;Hudec & Jackson, 2007).Stewart and Coward (1995) analysed the processes triggering gravity sliding deformation which developed NW-SE trending salt-core buckle folds in the Silver Pit Basin and Sole Pit High.The authors also concluded that salt mobilization in the SNS was triggered during the Mid-Triassic.Moreover, Remmelts (1995) documented that salt mobilization in the SNS continued through the Late Cimmerian in response to regional extension in the Jurassic: also discussed in (Pharaoh et al., 2010).
Extension in the salt-overburden (Figure 4a, ii) narrows the salt-overlying sedimentary layer creating a set of distinctive grabens, normal faults and listric normal faults.Extensional rifting and gravity-driven extension are the most common tectonic processes and the primary salt trigger mechanisms for salt mobilization (Jackson & Vendeville, 1992).
Halokinesis is a type of salt tectonic process in which salt flows with little or no tectonic influence (Jackson & F I G U R E 4 Trigger mechanisms for halokinesis and kinematic stages of salt diapirism.(a) Tectonic and halokinetic salt diapirism initiators.(b) Salt diapir stages "reactive", "active" and "passive".Adapted from Jackson and Talbot (1994); Jackson andVendeville (1992, 1994); Jackson et al. (1994); Vendeville et al. (1994); Vendeville andJackson (1992a, 1992b).Hudec, 2017).The main trigger mechanisms for halokinesis are regional tectonics and differential loading (Hudec & Jackson, 2007;Trusheim, 1960;Vendeville, 2002;Waltham, 1997;Warsitzka et al., 2013).Differential loading is one of the main, non-tectonic processes which triggers salt mobilization and is the primary mechanism triggering halokinesis (Figure 4a, iv).During halokinesis, salt mobilization is triggered due to a density mismatch between the sedimentary load and the overall density of the salt layer caused by the redistribution of sediments that occur during sedimentation, erosional processes and the difference in topography (Hudec & Jackson, 2007;Jackson & Hudec, 2017;Peel, 2014;Vendeville, 2002).Consequently, salt flows from areas where the sedimentary load is high to areas of lower gravitational load.

| Salt diapirism stages
The different stages of salt diapirism are studied based on the age relationships between the time of the overburden deposition and the time of salt flow (Jackson & Hudec, 2017).The complete evolution of multi-stage salt structures is analysed by the stratal deposition characteristics of the adjacent sedimentary sequences and the relationship of the salt-overburden interface.Prior to salt mobilization or structural deformation, the salt structure is inactive and pre-deformation or prefaulting sedimentation uniformly overlies the salt layer.Inactive and pre-deformed stratigraphic sequences are tantamount to pre-kinematic and pre-halokinetic stratigraphic sequences.During salt mobilization or structural deformation, the deposited sedimentary sequences are syn-tectonic or syn-halokinetic stratigraphic sequences.After structural deformation or salt mobilization, the stratigraphic sequences, that is, post-kinematic or posthalokinetic, deposit and uniformly cover the salt structure.The most common kinematic evolution of a salt diapir is a salt structure that has been triggered by extension (reactive) (Figure 4b, i), breaking through its overburden (active) (Figure 4b, ii) and growing near the subsurface (passive) (Figure 4b, iii) (Jackson et al., 1994;Jackson & Vendeville, 1994).
Rifting and gravity-driven extension are the main tectonic mechanisms for "reactive" salt diapirism (Figure 4a, ii-iii and 4b, i)."Reactive" diapirs form as a response to extensional forces (Hudec & Jackson, 2011;Karam & Mitra, 2016;Moragas et al., 2017;Stewart, 2006;Vendeville & Jackson, 1992a, 1992b).Extension weakens and unloads the salt-overburden via the development of a set of grabens and half-grabens (Figure 4a, ii).Normal faulting and graben formation allows salt to rise at the centre of the graben-system because the central graben is the smallest of all, and it is usually filled with water or air unevenly loading the salt layer (Hudec & Jackson, 2007).Extension does not only promote "reactive" diapirism, it also creates asymmetrical salt structures, also known as salt rollers (Figure 4a, i) (Bally, 1981;Lin et al., 1992).In a reactive scenario (Figure 4b, i), pre-kinematic layers are deposited on the undeformed original salt layer.Pre-kinematic layers have a constant thickness and parallel seismic stratigraphic reflectivities.Syn-tectonic sediments thicken towards growth faults.The main characteristics of synextensional sedimentation are seismic stratigraphic divergence onto the footwall of active listric-normal faults and seismic stratigraphic onlap away from normal faults (Figure 4b, i).Post-kinematic layers are deposited after most of the deformation has ceased and display equant and tabular seismic characteristics.
"Active" rise is when a salt structure grows without piercing its overburden driven by contraction and/or pure halokinesis.Contrarily, "active" diapirism, is when salt structures pierce their overburden developing discordant contacts between the salt interface and the overburden sediments (Figure 4b, ii) (Jackson & Hudec, 2017, p. 79)."Active" salt diapirs also develop tectonically or by pure halokinesis (Jackson et al., 1994).During "active" diapirism, syn-deformation sedimentation thickens away from the salt structure and thins towards the crest of the salt diapir (Figure 4b, ii).
Contraction is also a main trigger mechanism for salt mobilization and salt deformation (Figure 4a, iii).Contractional forces in salt basins can buckle the salt layer, develop salt-related thrusting, squeeze pre-existing salt structures, or reactivate salt diapirism (Jackson & Hudec, 2017, pp. 304).Contraction or inversion on preexisting salt structures varies greatly and highly depends on the established salt structural profile (Vendeville & Nilsen, 1995).If the salt diapir is at or near the surface, for example, during "passive" diapirism (Figure 4b, iii), contraction can squeeze salt diapirs allowing salt to flow freely at the surface.

| DATASET
We used the regional SNS Mega-Survey 3D full stack seismic reflection dataset, provided by Petroleum Geo-Services (PGS), for the basin-scale, systematic analysis and correlation of multi-stage salt structures across all SNS structural sub-basins (Figure 5).The UK sector of the SNS Mega-Survey is now owned by the Oil and Gas Authority (OGA).The Dutch and German sector remain under PGS ownership.
The SNS Mega-Survey consists of 44 3D time-migrated seismic volumes.Survey merging of the SNS Mega-Survey was performed for the post-stack time migrated volumes using post-stack techniques.The SNS Mega-Survey extends from offshore Durham and the East Midland Shelf UK to onshore Netherlands (Figure 5).The horizontal inline and crossline spacing of the seismic dataset is 50 m and the vertical sample rate is 4 ms.
The overall quality of the 3D seismic dataset in the Mesozoic-Cenozoic succession is average to good.The vertical resolution of near-vertical and/or vertical salt structures is poor, and the lateral resolution improves as we move away from near-vertical into the seismic stratigraphic sequences.Imaging problems are present around salt diapirs, as it is common in a 3D-seismic dataset due to the high P-wave velocities of salt and evaporites.Some of the most common salt-related imaging problems in the 3D seismic dataset are residual time and phase misties present along the edges of the merged surveys, velocity pull-ups beneath salt structures, seismic noise, seismic multiples near the steep flanks of salt diapirs, seismic noise problems at the carapace of the salt structures due to salt-related faulting, and vessel-acquisition trace problems in the first 300-400 ms (TWTT) from the seabed.

| Mapped horizons
Seismic picks of six seismic horizons were provided by PGS in the original seismic dataset in the seismic interpretation Kingdom software by S&P Global (formerly IHS Markit).The coverage of the six PGS seismic picks in the SNS Mega-Survey varies and is summarized in Table 1.PGS seismic picks have an inline and crossline separation of c. 100-200 m, and in some areas, for example, in the Broad Fourteens Basin, Central Graben and Sole Pit High, the seismic pick inline and crossline separation is even greater, for example, >1000 m.All PGS seismic horizon picks were reinterpreted and gridded using the seismic interpretation Kingdom software.The seabed and Top Palaeogene horizons were also mapped across the whole SNS Mega-survey for a total of eight survey-wide horizons used for this study (Table 1 and Figure 6).
The mapped horizons represent partly formation tops and partly regional unconformities.The regional unconformities represent mega stratigraphic sequence boundaries (Table 1 and Figure 6).The regional horizons mapped for this project are, in chronological order: the base salt, top Zechstein salt, top Early Triassic, top Late Triassic (where present), top Jurassic (where present)/Base Cretaceous Unconformity (BCU), top Cretaceous (Base Cenozoic), top Palaeogene (Base Neogene) and the Seabed (Figure 6).

| METHODOLOGY
The methodology for this study consists of: • Regional to sub-regional mapping of the megastratigraphic sequences and correlation across the different structural sub-basins • Characterization of the geometrical relationships between salt structures and the mega-stratigraphic sequences using regional seismic cross-sections • Generation of time-thickness maps, that is, isochron maps, for regional and local thickness variation and depocentre migration analysis • Characterization of salt-withdrawal palaeo-depocentres using isochron maps to discriminate active from inactive salt structures • Implementation of a unique nomenclature We utilized regionally mappable seismic horizons, unconformities and tectono-stratigraphic sequences as a basin-scale correlative framework and a detailed salt kinematic characterization of individual salt structures reflecting more local salt tectonic and halokinetic processes.Hitherto, this is the only mega-regional salt tectonics study which maps and classifies multi-stage salt diapirs on a mega-regional basin scale.Using our methodology, different types of salt diapirs can be identified and correlated across the different structural sub-basins.

| Regional phase
This study utilized the well-established tectonostratigraphic nomenclature of the North Sea from Ziegler (1990b); Ziegler and Hoorn (1989), and defines five different regional tectonic phases, these are, the prekinematic phase, detached, rift, inversion/contraction and post-tectonic phase (Table 2).The regional phases connect the tectono-stratigraphic evolution of the SNS salt basin to the regional seismic stratigraphic sequences.Additionally, the regional phases describe the basin-scale tectonic processes and correlate the tectono-stratigraphic sequences throughout the different structural sub-basins.Ultimately, the regional phases allow an integration and comparison of all salt structures in the regional salt basin, irrespective of the structural sub-basin in which they are found.
5.1.2| Pre-kinematic phase (pk)   The pre-kinematic (pk) phase (Table 2) is characterized by regional seismic stratigraphic sequences without evidence of tectonic deformation.The pre-kinematic phase is characterized by a quiescent tectonic phase, but it can also display syn-halokinetic deformation due to pure halokinesis or differential loading.Key seismic characteristics of the pre-kinematic phase include, tabular, isopachous and layered geometries; these geometries include synhalokinetic thickness variations across the structural subbasin.No visible active faulting is documented during the pre-kinematic phase in our study. 5.1.3| Detached phase (d)   The detached phase (d) (Table 2) characterizes the regional seismic mega-stratigraphic sequences affected only by gravity-driven processes that control the deformation of the salt-overburden stratigraphic sequences, for example, salt tectonic processes where salt deformation occurs in the supra-salt stratigraphic sequences and where it is not intrinsically related to sub-salt basement deformation.
The key seismic characteristics of the detached phase include syn-kinematic and/or syn-halokinetic seismic stratigraphic sequences and documented gravity-induced deformation of the salt-overburden.Divergent and pinchout seismic characteristics are common near salt structures.Furthermore, the salt horizon acts as a regional detachment where deformation is facilitated and transported downdip. 5.1.4| Rift phase (r)   The rift phase (r) (Table 2) is characterized by regional rifting and crustal extensional processes.It is represented by syn-extensional or syn-rift seismic stratigraphic sequences.Regional extension involves basement structural deformation, nevertheless, basement-related deformation may or may not control salt tectonics.Key seismic characteristics of the rift phase include synkinematic stratigraphic reflectors and may include syn-extension, divergent seismic stratigraphic reflectivities.Depending on different factors controlled by rift extension, salt flow and syn-kinematic sedimentation, salt structures triggered during a rift phase can be pre-diapiric, that is, salt anticlines, or they can be salt diapirs.
5.1.5| Inversion/contraction phase (i)   The inversion/contraction phase (i) (Table 2) represents syn-inversion or syn-contraction seismic stratigraphic sequences and documented regional orogenic-mountain Lithological-Unconformity-horizon Top Bacton: This level corresponds to a regional unconformity in the UK.
In the Netherlands, it represents the lithostratigraphic transition between the Buntsandstein Fm to the Röt Fm. 247 Ma (Cameron et al., 1992;Geluk, 2005) Semi building processes.The inversion/contraction phase can include basement-involved inversion/contraction or basement-detached inversion/contraction at supra-salt levels.The key seismic stratigraphic reflectivities include pinch-out geometries, gently seismic stratigraphic onlap, harpoon structures and uplifted seismic stratigraphic geometries adjacent to near-vertical or vertical salt structures.The regional stratigraphic sequences directly respond to contraction by uplifting, buckling and inverting faults.
5.1.6| Post-tectonic (pt)   The post-tectonic phase (pt) (Table 2) represents a regional quiescent tectonic phase.The seismic stratigraphic sequences deposited during a post-tectonic phase characterize dormant stages of salt structures or halokinetic processes triggered by differential loading after a regional tectonic phase of deformation.Key regional seismic-stratigraphic characteristics of the post-tectonic phase include tabular and uniform stratigraphic geometries, basin-fill geometries and gentle onlap geometries.
Regional thickness variations might be present at a regional scale due to regional crustal doming and thermal relaxation processes.The post-tectonic regional phase can be reactivated by regional tectonics at a later stage.

| Regional tectonostratigraphic correlation
The regional tectonostratigraphic correlation incorporates first-order geometrical relationships between the seismic mega-stratigraphic sequences and the salt layer.The regional phases are expanded and connected to the firstorder set of interpretations that can be studied within the seismic stratigraphic boundaries (e.g., diapiric sequence, F I G U R E 6 Simplified seismic stratigraphy.Seismic W-E cross-section from the Sole Pit High in pseudo-relief.Pseudo-relief is a predefined function in the Kingdom Software that calculates the RMS amplitudes, and subsequently, applies an inverse Hilbert transform function which helps to the visual examination of seismic data.Seismic data courtesy of Oil and Gas Authority and PGS.Mapped horizons are either regional formation tops or stratigraphic unconformities which can be mapped across the entire supra-regional seismic dataset.
The mapped horizons also define the upper and lower limits for the generation of seismic isochron maps used for this project.BCU, Base Cretaceous Unconformity; VPU, velocity pull-up.
concordance, phase; Table 2).The regional correlation and the first-order geometrical observations are the foundation to characterize and compare the different neardiapir processes further controlling salt mobilization and salt diapirism across the different structural sub-basins.
The integration between the regional correlation and the sub-basin seismic stratigraphic observations, permits a correlation of multi-stage salt structures across the study area.

| Diapiric sequence
The first level of observations linked to the tectonic megastratigraphic sequences is whether the stratigraphic sequences recorded a pre-diapiric or diapiric salt structures.This is because any regional phase can trigger salt mobilization or salt diapirism.However, during a pre-kinematic phase, the diapiric sequence can also be a layered diapiric sequence where no salt mobilization nor deformation is observed.A layered sequence is the precursor of the prediapiric and diapiric sequences, which are characterized by salt anticlines or salt diapirs.Post-diapiric sequences are characteristic of the post-tectonic phase where salt structures become dormant and are covered by postkinematic sedimentation (Table 2).
The seismic stratigraphic sequences of a layered sequence are tabular and undeformed; the layered sequence is assumed to be characteristic of any prior tectonic or salt deformation process (Figure 7a).Pre-diapiric sequences are characterized by uniform and continuous seismic stratigraphic sequences concordantly overlying evolving salt anticlines or salt pillows (Figure 7b).Thickness variations may be present.A diapiric sequence is characterized by discontinuous seismic stratigraphic sequences, where salt diapirism processes have pierced, or virtually pierced (Figure 7c), the diapiric sequence.Thickness variations are common.A post-diapiric sequence represents continuous and tabular seismic stratigraphic sequences that cover dormant salt structures (Figure 7d).There can also be thickness variations around halokinetic salt structures.

| Concordance
The concordance discriminates between discordant and concordant salt structures and their overlying stratigraphic sequences (Table 2); for example, it helps discerning between a salt pillow and a salt stock.The diapiric sequence and the concordance are interrelated because the concordance relationship of the seismic stratigraphic sequences surrounding salt structures is linked to the diapiric or pre-diapiric characteristics of individual salt structures.The seismic stratigraphic sequences are concordant when the diapiric sequence is pre-diapiric, layered or post-diapiric and discordant when the diapiric sequence is diapiric.

| Phase
The phase is characterized by the piercing relationship of salt structures and the encasing seismic stratigraphic sequences (Figure 8).The phase discriminates salt diapirs that pierced through the complete stratigraphic succession and the ones that become arrested within a particular seismic stratigraphic sequence (Table 2 and Figure 8a,b).Conversely, if the seismic mega-stratigraphic sequence shows concordant geometries, the phase is non-piercing (Figure 8c).

| Local stage
The local stage represents the structural, kinematic and depositional characteristics of individual salt structures within a particular sub-basin.The local stages allow a systematic identification and comparison of the near and adjacent salt structures within the same structural sub-basin which might have evolved through different non-tectonic processes, for example, sedimentary loading processes, sediment redistribution, erosion and pure halokinesis (Table 2).The local stages are directly linked and correlated with the regional phases.

| Driving mechanisms
The driving mechanisms of salt structures control the different salt-growth characteristics of salt structures within the regional phases.We utilize widely accepted nomenclature from Jackson et al. (1994); Vendeville and Jackson (1992a); Vendeville and Jackson (1992b), and defined the driving mechanisms of a salt structure within the encasing stratigraphic sequences.These are; tectonically active, when salt structures evolve due to regional tectonics, halokinetic, when salt structures evolve due to non-tectonic process such as sedimentary loading, and inactive (Table 2).
A tectonically active salt structure is characterized by pre-diapiric and diapiric sequences and present in tectonically influenced regional phases, for example, detached, rift and inversion/contraction phases.This is because any regional tectonic phase can trigger salt mobilization and salt diapirism.A halokinetic driving mechanism is also characterized by surrounding pre-diapiric and diapiric sequences in all regional phases.This project defines the driving mechanism as inactive in the pre-kinematic and post-regional phases when salt structures are not yet triggered or when they become immobile after a tectonically active or halokinetic stage.

| Kinematic stage
The kinematic stage represents the structural characteristics of salt structures.We utilize already established salt kinematic nomenclature from Vendeville andJackson (1992a, 1992b) (structure; Table 2).For example, Salt anticlines and salt pillows (pA) are characterized by pre-diapiric sequences in all regional phases and will display concordant geometries.Salt anticlines/pillows are controlled by regional tectonics and/or pure halokinesis.Passive diapirism (dP) is present in all regional phases within diapiric sequences.Passive diapirs are mainly halokinetic; however, passive diapirs are also triggered by extensional tectonics.Tectonically active mechanisms control active diapirism (dA) and reactive diapirism (dR) in the detached, rift and inversion/contraction phase.To differentiate between extension-triggered salt structures evolving during a piercing or arrested phase in the rift phase, this project further differentiated extension diapirism (dE) from reactive diapirism (dR) (Jackson & Vendeville, 1990, 1992, 1994;Vendeville & Jackson, 1992a, 1992b).Inversion/contraction controls contractional salt structures, for example, during a tectonically active and non-piercing phase, contraction controls contractional anticlines (pC).During an arrested phase, contraction drives salt thrusting (dT) and/ or contractional diapirism (dC).Dormant diapirs (dD) occur only in the post-tectonic phase.They are characterized by regional quiescence and regional phases of inactivity where salt diapirs are inactive.

| Regional tectono-stratigraphic framework
The regional phases of the five structural sub-basins in the SNS were derived from observed seismic stratigraphic sequences (Figure 6 and Table 1) and regional tectonic phases identified by previous published studies discussed previously.
Congruent with the structural phases discussed in Warsitzka et al. (2019) The location and geometry of the SNS structural subbasins are evident from the top Zechstein horizon structure map (Figure 9a) and regional seismic cross-sections (Figure 10).Furthermore, the Zechstein horizon is F I G U R E 1 0 Interpreted regional seismic cross-section profiles of the Southern North Sea.Seismic data courtesy of Oil and Gas Authority and PGS.
| 2095 EAGE GAITAN and ADAM structurally higher in the Sole Pit High and partly higher in the Silver Pit Basin and is the deepest in the Broad Fourteens Basin and in the Central Graben (Figures 9a  and 10).The Cleaver Bank High sits in between the Sole Pit High and the Central Graben and it is structurally lower towards the Central Graben (Figures 9a and 10a,b).Basement-related salt tectonic deformation is observed in the Broad Fourteens Basin, Central Graben, Sole Pit High and Hantum Fault Zone.Salt overburden fault and fracturing is ubiquitous around salt structures in the SNS salt basin (Figure 10).

| Salt-sediment interactions
There are a total of 155 salt structures in the five different structural sub-basins of the SNS (Figure 9); 55 are discordant salt structures and 100 are concordant.In the Broad Fourteens Basin, c. 9% are discordant salt structures and 7% are concordant.In the Central Graben, C. 42% are discordant salt structures and 8% concordant.In the Cleaver Bank High, 18% are 10 discordant salt structures and 50% concordant.In the Sole Pit High, 18% are 10 discordant salt structures and 8% are concordant.Finally, in the Silver Pit Basin, 13% are discordant salt structures and 27% are concordant (Figure 9).

Base Zechstein salt interval
We used the Zechstein time-thickness variation to distinguish salt structures from relatively undeformed or welded salt (Figure 11).The graph in Figure 11b shows a multimodal distribution in which most of the data are between 0 and 0.5 s TWTT and represents areas in the SNS of thin or relatively undeformed salt.In the same graph, the

EAGE
GAITAN and ADAM distribution pattern changes from 0.5 s TWTT to an exponential decrease in count.We utilized this exponential decrease change from 0.5 s TWTT as a cut-off to identify salt structures.Additionally, we considered a second cut-off (>1.25 s TWTT) where the histogram count flattens near zero to discriminate the tallest salt structures (Figure 11b,c).Using the velocities taken from Geldart and Sheriff (2004) for the average sedimentary sequences (sandstone) encasing salt structures with ϕ = 20%, 0.5 s TWTT represents salt structures taller (thicker) than 940 m.Values greater than 1.25 s TWTT represent salt structures taller (thicker) than 2350 m (Figure 11b,c).Focus is given to the tallest/thickest salt structures, that is, salt structures taller than 2350 m as they must represent the most complex multi-stage salt diapirs in the study area.
A total of 163 salt structures were identified in the SNS using a filter of >0.5 s TWTT (>940 m), 109 concordant and 54 discordant (Table 3a).Out of those 163, 155 salt structures lie within the five different structural sub-basins studied in this project; 8% are in the Broad Fourteens Basin, 20% in the Central Graben, 39% in the Cleaver Bank High, 11% in the Sole Pit High and 22% in the Silver Pit Basin (Table 3b).Out of the 155 salt structures, 65% are concordant and 35% discordant (Table 3a).Out of the 55 discordant salt structures, 27% of salt structures are taller/thicker than 2350 m (>1.25 s).We focused on 22 salt structures of the 155, six concordant and 16 discordant salt structures (Table 3a) based on their height, proximity to pre-salt structural features and distribution across the SNS salt basin.The detailed number of concordant/discordant salt structures in the individual sub-basins is observed in Table 3b.salt processes.Regional palaeo-depocentres display Early time-thicknesses greater than 0.27 s (Figure 12b).Three main regional palaeo-depocentres were identified during the Early Triassic (Figure 12c), that is, a ca.50 km in diameter in the northern part of the BFB, an NW-SE trending, ca.250-km-long regional palaeo-depocentre in the SPH extending from the northern part of the study area to the BFB, and lastly a ca.100-km-long, NNE-SSW trending palaeo-depocentre in the CG.The regional palaeo-depocentres in the CG were controlled by the Palaeozoic-inherited Central Graben system.
Most evolving salt anticlines and salt diapirs and the documented sub-and supra-Zechstein salt active faulting are found within the defined Early Triassic regional palaeo-depocentres (Figure 12b,c).

| Salt mobilization and diapir stages
We identified 119 evolving salt structures in the SNS during the Early Triassic (Figure 12c).Out of the 119 evolving salt structures, 64% are evolving concordant structures and 36% are discordant.ca.25% of the evolving salt structures do not lie within the Early Triassic regional palaeo-depocentres, identified because of local thickness variations in isochron maps (Figure 12b,c).

| Regional and local stages of salt deformation
We selected a total of 11 evolving salt structures in the Early Triassic (Figure 12c) to compare the salt structural variability across all structural sub-basins.Seven out of the 11 filtered salt structures are taller than 2350 m; five are in the CG, one in the CBH and one in the SPH.Only the salt structures in the CG and CBH lie within the regional palaeo-depocentres (Figure 12c).Two of the three evolving concordant structures are within the regional palaeo-depocentre in the SiPB and one in the northern part of the CBH (Figure 12c).Using the regional and local nomenclature (Table 2) a total of two different regional phases are identified in the SNS, that is, the pre-kinematic and the rift regional stage (Figure 12c).The pre-kinematic phase is the dominant regional phase among the different structural sub-basins, for example, in the CBH, SPH and SiPB.In the CBH, SPH and SiPB, six active salt structures were classified to be evolving in a pre-kinematic phase due to halokinesis, that is, two discordant and four concordant (Figure 12).The second dominant regional phase is the rift phase in the CG.We classified five discordant salt structures in the Early Triassic in the CG to be triggered in a rift phase because of the halokinetic evidence and documented active faulting at sub-Zechstein salt levels previously mentioned from salt tectonic studies in the Dutch-SNS.
Salt anticlines are widespread in all structural subbasins (Figure 12c).During the Early Triassic, there is only a variation in regional phases, but there is no variation of local stages (Figure 12c).Local salt-withdrawal palaeo-depocentres are ubiquitous adjacent to salt structures in map view and seismic cross-sectional profiles (Figures 12b,13 and 14).We classified the studied salt structures in the pre-kinematic and rift regional phase as salt anticlines/pillows during the Early Triassic (pA; Figure 12c).Salt-withdrawal palaeodepocentres in the CG are now inverted salt minibasins, also known as turtle structures, in between salt diapirs (pA; Figure 13).In the SPH, local stratigraphic thickening of the Early Triassic stratigraphic sequence is observed at the flanks of E-W trending, tall salt diapirs and near the apex of NNW-SSE trending salt anticlines (pA; Figure 14), and in the northeastern flank of asymmetrical-triggered salt anticlines in the SPH (pA; Figure 15).Halokinesis triggered kilometre-long salt anticlines in the SiPB (Figure 12c).Salt anticlines in the SiPB remained as salt anticlines for their entire kinematic evolution and developed in the CBH during a pre-kinematic stage (Figure 12c).North of the CBH, salt anticlines triggered in the Early Triassic, remained as salt anticlines during their entire kinematic evolution, whereas salt anticlines in the southeastern part of the CBH evolved to discordant salt structures at a later stage (Figure 12c).

| Mid-Late Triassic-rift/ detached phase
The dominant regional phase during the Mid-Late Triassic is the rift phase, represented by active normal faulting, and salt mobilization and salt diapirism in the Central Graben (CG) and in the Sole Pit High (SPH).A detached phase is also present in the Silver Pit Basin (SiPB) influenced by marginal sliding detachment from the North Dogger Shelf and Mid North Sea High (Figure 16a).

| Structural trends
During the Mid-Late Triassic, the NNW-SSE to NNE-SSW trending active faulting in the CG was controlled by the sub-Zechstein salt structural configuration and salt mobilization.Similarly, in the SPH, Mid-Late Triassic normal faults were controlled by the sub-salt NW-SE structural configuration.E-W normal faults in the north of the SPH formed by salt mobilization and the gravity-sliding deformation the North Dogger Shelf and North Sea High.

| Regional depocentres
Thickness of the top Triassic-top Early Triassic isochron highlight regional palaeo-depocentres and localized salt-withdrawal palaeo-depocentres developed during the Mid-Late Triassic (Figure 16b).Thickness variations from the Early Triassic to the Mid-Late Triassic indicate changes in salt mobilization and salt diapirism mode.The regional palaeo-depocentres show Mid-Late Triassic time-thicknesses greater than 0.45 s (Figure 16b).
Regional palaeo-depocentres dwindled during the Mid-Late Triassic compared to the regional palaeo-depocentres observed in the Early Triassic (Figures 12b and 16b).GAITAN and ADAM Three palaeo-depocentres were identified during the Mid-Triassic (Figure 16b,c), that is, two ca.100-km-long, NW-SE trending, regional palaeo-depocentres in the SPH and a ca.100-km-long, N-S trending, regional palaeo-depocentre in the CG.During the Mid-Late Triassic, the regional palaeo-depocentres were only controlled by the Palaeozoic-inherited structural elements of the Central Graben system and the Dowsing-Hewett Fault Zone in the SPH (Figure 16b,c).Fewer concordant and discordant salt structures, compared to the Early Triassic, lie within the Mid-Late Triassic regional palaeo-depocentres (Figure 16b,c).

| Salt mobilization and diapir stages
We identified 96 evolving salt structures in the SNS during the Mid-Late Triassic (Figure 16c).Out of the 96 evolving salt structures, 49% are concordant structures and 51% discordant.Circa 50% of the evolving salt structures lie within the Mid-Late Triassic regional palaeo-depocentres (Figure 16c).

| Regional and local stages of salt deformation
We selected a total of 11 evolving salt structures in the Mid-Late Triassic to compare the salt structural variability across all sub-basins.Seven out of the 11 salt structures are taller than 2350 m; five are in the CG, one in the SPH and one in the SiPB (Figure 16c).Only four of the five salt structures in the CG lie within the regional palaeodepocentre, and only one salt structure in the palaeodepocentre of the SPH.The remaining four evolving salt structures lie within the Mesozoic gravity sliding deformation zone (Figure 16c).
A total of two different regional phases are identified during the Mid-Late Triassic, that is, the detached and the rift regional phase (Figure 16c).The rift phase is the dominant regional phase observed in the CG and SPH.We classified salt structures during the Mid-Late Triassic in the CG and SPH as salt structures evolving through a rift phase because of the continuous sub-Zechstein salt deformation and supra-salt Mesozoic faulting promoted by salt mobilization and salt diapirism (Figure 16).We also classified two concordant and three discordant salt structure to evolve in a detached phase in the SiPB due to gravitysliding deformation from marginal areas (Figure 16c).
In the Mid-Late Triassic, local stages vary across the CG, SPH and SiPB (Figure 16).Local palaeo-depocentres adjacent to salt structures are present in map view and seismic cross-section profiles in the CG, SPH and SiPB (Figures 13,  14 and 16b).We classified salt structures in the CG as passive diapirs (dP; Figure 16c) because local salt-withdrawal palaeo-depocentres shifted closer to the salt structures in the northern part of the study area of the CG from the Early Triassic to the Mid-Late Triassic (Figure 16b).Moreover, where local palaeo-depocentres continued from Early Triassic times (Figure 16b), we classified the adjacent salt structures during the Mid-Late Triassic in the CG as salt anticlines/pillows (pA; Figure 16c), evidenced by inverted salt-withdrawal mini-basins in cross-section profiles during the rift phase (pA; Figure 13).
Localized palaeo-depocentres are identified along flanks of NNW-SSE trending salt anticlines in the SPH (pA; Figure 14) and along flanks of NW-SE trending salt F I G U R E 1 5 Interpreted SW-NE seismic cross-section profile of a multi-stage salt wall in the Sole Pit High-Inde Shelf.The evolution of this NNE-SSW trending salt wall is complex and obscured by extensional-contractional collapse and diapirism during the detached and inversion/contraction regional phase.VPU, velocity pull-ups.Location of cross section in Figure 9. Seismic data courtesy of Oil and Gas Authority and PGS.
anticlines the gravity-sliding deformation the SPH (pA; Figure 15).E-trending salt anticlines in the northern part of the SPH in the gravity-sliding deformation area, were triggered by the southward gravity-deformation transport from the marginal area of the North Dogger Shelf to the north (Figure 16c).Furthermore, NW-SE trending salt anticlines remained as salt anticlines in the SiPB.Local palaeo-depocentres are evident across, NW-SE trending, active salt diapirs in the SiPB (Figure 16b,c).

| Jurassic-rift/detached phase
The dominant regional phase during the Jurassic continued to be the rift phase represented by Early Jurassic normal faulting and evolving salt diapirism in the Central Graben (CG) and Sole Pit High (SPH) (Figure 17).A detached phase was still present in the Silver Pit Basin (SiPB); however, Jurassic sediments are mostly absent in the SiPB (Figure 17). 5.9.1 | During the Jurassic, the SSW to NE-trending active faults in the CG were controlled by salt diapirism processes and by the sub-salt extension of the Central Graben System (Figure 17a,b).In the SPH, the NW-SE trending Jurassic faults were controlled by the sub-salt and Mid-Late Triassic NW-SE structural configuration.E-W normal faults in the north of the SPH formed by salt diapirism, gravity-sliding deformation from the basin margins and extensional tectonics during pulses of Cimmerian rifting (Figure 17). 5.9.2 | Regional depocentres Thickness variations of the top Jurassic-top Triassic isochron highlight regional palaeo-depocentres and localized There is a small increase in the regional palaeodepocentre areal extent in the SPH from the Mid-Late Triassic to the Jurassic and there is now palaeo-depocentres observed in the BFB (Figures 16b,c  and 17b,c).Four main regional palaeo-depocentres formed during the Jurassic (Figure 17b,c), that is, two NW-SE trending palaeo-depocentres in the BFB, a ca.100-km-long, N-S trending palaeo-depocentre in the CG and a ca.150-km-long, NW-SE trending palaeodepocentre in the SPH and part of the SiPB (Figure 17c).During the Jurassic, the BFB, CG and SPH regional palaeo-depocentres were triggered by Jurassic rifting, in the CG, NNE-SSW to NE-SW salt diapirism and Jurassic faulting also controlled the development of the Jurassic regional palaeo-depocentre (Figure 17c).In the SPH, the regional palaeo-depocentre was controlled by the NW-SE trending Dowsing-Hewett Fault Zone, E-W to NW-SE salt diapirism and the Mesozoic gravity-sliding deformation from the North Dogger Shelf and Mid North Sea High (Figure 17c).

| Salt mobilization and diapir stages
We identified 39 evolving salt structures in the SNS during the Jurassic; 26% are concordant salt structures and 74% discordant.Circa 2/3 of the active salt structures lie within the Jurassic regional palaeo-depocentre (Figure 17c).

| Regional and local stages of salt deformation
We selected a total of nine evolving salt structures in the Jurassic to compare the salt structural variability across all sub-basins (Figure 17c).Seven out of the 11 salt structures are taller than 2350 m; five are in the CG, one in the SPH and one in the SiPB (Figure 17c).Five selected discordant salt structures in the CG, two discordant and one concordant in the SPH are within the regional palaeo-depocentres.The only remaining evolving discordant salt structure in the SiPB lies outside the palaeo-depocentre but within the Mesozoic gravity-sliding deformation zone (Figure 17c).
Two regional phases are identified in the Jurassic, that is, the detached and the rift regional phase (Figure 17c).The rift phase is the dominant phase and is observed in the CG and SPH (Figure 17c).We classified six evolving salt structures during the Jurassic in the CG and SPH as salt structures evolving through a rift phase because of the continuous sub-Zechstein salt extension deformation and Mesozoic faulting in the supra-salt cover, salt mobilization and salt diapirism (Figure 17a,c).Three discordant salt structures evolved in the detached phase in the SiPB due to the gravity-sliding deformation from the marginal areas.
There is a variation in local stages in the CG, SPH and SiPB in the Jurassic (Figure 17c).Local palaeo-depocentres adjacent to salt structures in map view and seismic crosssection profiles are observed in the CG, SPH and SiPB (Figures 13,14 and 17c).Passive diapirs (dP) from the Mid-Late Triassic continued evolving as passive diapirs in the CG.Mid-Late Triassic salt anticlines (pA) transitioned to passive diapirs in the CG during the rift phase (pA-dP; Figure 17c) evidenced from local palaeo-depocentres shifting closer to NE-SW trending salt diapirs in the main rift basin of the CG (Figure 17b,c).In the SPH, Jurassic palaeo-depocentres were controlled by the sub-salt structural deformation of the Dowsing-Hewett Fault Zone promoting the salt rise of tall salt anticlines in the SPH in the rift phase.The SiPB was influenced by the subsiding SPH during the Jurassic and the southwestern gravity-sliding deformation from the marginal areas triggering active faulting and active diapirism (dA; Figure 17c).

| Cretaceous-Inversion phase
The dominant regional stage during the Cretaceous is the inversion phase, represented by inverted faults and salt diapirism in the Broad Fourteens Basin (BFB), Central Graben (CG), part of the Cleaver Bank High (CBH) and Sole Pit High (SPH) (Figure 18).Gravity-sliding deformation continued to control supra-salt deformation and salt mobilization in the CBH and Silver Pit Basin (SiPB).

| Structural trends
During the Early Cretaceous, the NNE-SSW trending normal faults in the CG were controlled by salt diapirism and the sub-salt structural configuration.The NNE-SSW trending reverse faults in the CG were controlled by regional inversion along Triassic-Jurassic fault structural trends.In the SPH, Early Cretaceous normal faults were controlled by the sub-salt Dowsing-Hewett Fault Zone and NW-SE fault structural trend during the Triassic-Jurassic (Figure 18a).The NW-SE trending inverted faults in the SPH were controlled by regional inversion along the Dowsing-Hewett Fault Zone (Figure 18a).NNW-SSE normal faults in the SiPB were controlled by salt mobilization and salt diapirism, and by the gravity-sliding deformation from the marginal areas to the north. 5.10.2 | Regional depocentres Thickness of the Cretaceous-Base Cretaceous Unconformity highlight and localized palaeo-depocentres developed during the Cretaceous (Figure 18b).Thickness variations from the Jurassic to the Cretaceous indicate regional inversion and regional palaeo-depocentre formation.The regional palaeo-depocentres represent Cretaceous time-thicknesses >0.65 s (Figure 18b,c).
Regional palaeo-depocentres show a greater areal extent than the palaeo-depocentres observed in the Triassic-Jurassic (Figures 12b, 16b and 17b).Four regional palaeo-depocentres were identified in the Cretaceous (Figure 18b,c), two palaeo-depocentres trending NW-SE are adjacent to the BFB, one adjacent to CG, in Hantum Fault Zone, trending and last one in the part of the CBH (Figure 18b,c).

| Salt mobilization and diapir stages
We identified 129 evolving salt structures in the SNS during the Cretaceous (Figure 18c); 61% are concordant salt structures and 39% are discordant.Circa 80% of the overall evolving salt structures lie within the Cretaceous regional palaeo-depocentres.

| Regional and local stages of salt deformation
We selected a total of 10 evolving salt structures in the Cretaceous (Figure 18c) to compare the salt structural variability across all sub-basins.Seven out of the 10 salt structures are taller than 2350 m; four are in the CG, one in the CBH, one in the SPH and one in the SiPB (Figure 18c).Only two evolving concordant salt structures taller than 2350 m lie within regional palaeo-depocentres: One in the CBH and one in the SiPB (Figure 18c).Three evolving salt structures are taller than 940 m (TWTT), two concordant and one discordant are within the regional palaeo-depocentre in the CBH.The remaining five are within inverted blocks in the CG and SPH (Figure 18b,c).
The Cretaceous is characterized by two regional phases, that is, a pre-kinematic phase in the Early Cretaceous and an inversion phase in the Late Cretaceous (Figure 18c).The pre-kinematic phase is only observed in the CBH, whereas the inversion phase is ubiquitously found across the study area.We classified evolving salt structures in the Cretaceous as contractional salt structures, for example, contractional anticlines (pC) and contractional diapirs (dC), because of the widespread documented inverted faults across the study area and thin syn-kinematic sedimentation towards the carapace of salt structures (Figure 18a,c).
There is a variation of local stages in the Cretaceous across all sub-basins (Figure 18c).Most of the evolving salt structures in the Cretaceous are linked to the regional palaeo-depocentres where localized palaeo-depocentres are evident in map view and seismic cross-sections (Figures 13,15 and 18b).We classified evolving salt diapirs in the BFB, CG and SPH as contractional diapirs (dC) in an inversion phase because these diapirs are now arrested within the Cretaceous stratigraphic sequences and small thickness variations around the evolving contractional diapirs.During the Early Cretaceous, some salt diapirs also displayed a passive diapir (dP) local stage before inversion tectonics.Moreover, we classified two contractional anticlines (pC) in the CBH evidenced by localized thickness variations (Figure 18b).

| Palaeogene-Inversion phase
In the Palaeogene, the inversion regional phase is the dominant phase represented by widespread inverted faults in the Broad Fourteens Basin (BFB), Central Graben (CG) and in the southwestern part of the Cleaver Bank High (CBH).Additionally, the inversion phase is also represented by the regional uplift of the Sole Pit High (SPH) and part of the Silver Pit Basin (SiPB) (Figure 19).

| Structural trends
During the Palaeogene, the NW-SE to N-S trending inverted faults in the BFB, CG and CBH, were controlled by the regional inversion along older Mesozoic structural trends during the Alpine orogeny (Figure 19a).Normal faulting in the CBH, SPH and SiPB, was controlled by salt mobilization and gravity-sliding deformation from adjacent structural margins and towards the regional structural low to the east of the SNS (Figure 19a).
A main regional palaeo-depocentre is observed extending from the SiPB to the Hantum Fault Zone offshore the Dutch-SNS.Gravity-sliding from marginal structural highs, controlled salt mobilization, salt diapirism and the development of NW-SE to NW-SW trending local palaeodepocentres in the SiPB (Figure 19a,b).

| Salt mobilization and diapir stages
We identified 128 evolving salt structures in the SNS during the Palaeogene; 59% are concordant salt structures and 41% are discordant (Figure 19c).Circa 80% of evolving salt structures in the Palaeogene are within the main regional palaeo-depocentre (Figure 19b,c).

Regional and stages of deformation
We selected a of salt structures in the Palaeogene (Figure 19c) compare the salt structural variability across all sub-Nine out of the 14 salt structures are taller than 2350 m; six are in the CG, one in the CBH, one in the SPH and one in the SiPB (Figure 19c).Of the 13 evolving salt structures, 10 lie within the main regional palaeo-depocentre.From the remaining three, two lie within the gravity-sliding deformation zone and one in the Cretaceous-inverted block in the northern part of the CG (Figure 18b,c).
There is no variation of regional phases in the Palaeogene.The inversion phase is observed across all sub-basins (Figure 19c).We classified all 13 salt structures in the Palaeogene as contractional salt structures because of prominent faulting across study area and the contraction tectonics from the (Figure 19).There is a small of local stages across all sub-basins in the Palaeogene (Figure 19c).Local palaeo-depocentres are widespread across all subbasins observed from map view and cross-section profiles (Figures 15 and 19b).We classified 10 evolving salt structures in the BFB, CG and SPH, as contractional diapirs (dC) because of the arrested characteristics of salt diapirs and thickness variations observed from map view and seismic cross sections (Figures 13,15 and 19b).Intra-Palaeogene seismic stratigraphic onlap atop contractional diapirs are evidenced in seismic cross-sections in Figures 13 and 15 in the CG and SPH respectively.Additionally, we classified three contractional anticlines (pC), two in the CBH and one in the SiPB because of their thin Palaeogene sediments at the carapace of the salt anticlines and thickness variations around the salt anticlines evidencing rising salt anticlines in an inversion phase (Figure 19b,c).
5.12 | Qty-Neogene-Inversion/ post-tectonic phase 5.12.1 | Regional depocentres Thickness variations of the Seabed-top Palaeogene isochron highlight regional palaeo-depocentres and localized palaeo-depocentres developed during the Qty-Neogene (Figure 20a).The regional palaeo-depocentres represent Qty-Neogene time-thicknesses >0.60 s (Figure 20).A main regional palaeo-depocentre remained in the eastern part of the study area, that is, in the Central Graben (CG), in part of the Cleaver Bank High (CBH) and in the Silver Pit Basin (SiPB) (Figure 20a).The main palaeo-also the NW-trending Fourteens (BFB).During the Qtythe palaeo-depocentre was controlled by the regional eastward basin-tilt and the subsiding BFB (Figure 20). 5.12.2 | Salt mobilization and diapir stages We identified 98 evolving salt structures in the SNS during the Qty-Neogene (Figure 20b).About 60% concordant salt structures and 40% discordant.Circa 75% of the evolving Qty-Neogene salt structures lie within the main regionaldepocentre (Figure 20b).

| Regional and local stages of salt deformation
We selected 13 salt structures in the Qty-Neogene to compare the salt structural variability across all sub-basins.Ten salt structures are taller than 2350 m; seven are in the CG, one in the CBH, one in the SPH and one in the SiPB (Figure 20b).Only 10 of the 13 salt structures are within the main regional palaeo-depocentre in the BFB, CG and CBH.The remaining three active salt structures are in the CBH, SPH and SiPB (Figure 20b).
The Qty-Neogene is characterized by two regional phases, that is, the inversion and the post-tectonic phase (Figure 20b).The post-tectonic phase is clearly the dominant regional phase observed in the CG, CBH and SiPB as all multi-stage salt structures become dormant.We classified salt structures during the Qty-Neogene in the CG, CBH and SiPB, as contractional salt structures during the early phases of the Qty-Neogene because of local thickness variations atop evolving salt structures and localized active faults (Figure 20b).All multi-stage salt structures become dormant during the late phase of the Qty-Neogene.
There is a minor variation of local stages across the sub-basins in the Qty-Neogene because some multi-stage salt diapirs transitioned from a contractional diapir (dC) to a dormant diapir (dD), for example, in the CG, CBH and in the SiPB (Figure 20b).For reference, we display two salt anticlines (pA) in the post-tectonic phase evidenced by the tabular and stratified intra-seismic characteristics of the Qty-Neogene mega-stratigraphic sequence onlapping onto the crest of the salt anticlines.

| Summary
This study analysed a total of 22 multi-stage salt structures in the SNS salt basin.About 20% of the 22 multi-stage salt structures evolved in 1-2 regional phases, for example, three in the Cleaver Bank High and two in the Sole Pit High (Table 4a).Approximately 80% of multi-stage salt structures evolved in three regional phases, that is, eight in the Central Graben, three in the Cleaver Bank High, one in the Sole Pit High and five in the Silver Pit Basin (Table 4a).Furthermore, of the 22 studied multi-stage salt structures, 23% evolved in 1-2 local stages, for example, three in the Cleaver Bank High and two in the Sole Pit High (Table 4b).Twenty-seven percent evolved in three local stages, for example, three in the Cleaver Bank High and three in the Silver Pit Basin (Table 4b).Twentyseven percent evolved in four local stages, for example, three in the Central Graben, one in the Sole Pit High and two in the Silver Pit Basin.The last 23% of multi-stage salt evolved five local in (Table 4b).

|
Our methodology permitted the identification and sysof multi-stage salt structures in the complex SNS salt basin and will be applicable to salt basins in general.A similar approach was conducted by Warsitzka et al. (2019) which arrived to similar conclusions.However, our regional approach allows further interpretation of the diverse regional and local control factors driving halokinesis and salt diapirism within the different structural sub-basins and within specific sub-basins.
Our study indicates that there is a high variability of multi-stage salt structures in the SNS salt basin as previously implied in Davison et al. (2000); Doornenbal and Stevenson (2010); Harding and Huuse (2015); Stewart (2007); Warsitzka et al. (2019).Moreover, multi-stage salt structure styles and patterns can vary within the same structural sub-basin as local and nontectonic processes controlled the evolution of salt structures.The evolution of multi-stage salt structures was recorded within the regional seismic stratigraphic sequences and the salt diapir-proximal syn-kinematic stratigraphic sequences encasing salt anticlines and diapirs (Davison et al., 2000;Giles & Rowan, 2012;Harding & Huuse, 2015;Pichel & Jackson, 2020;Warsitzka et al., 2019).Isochron maps are an essential tool for the identification, correlation and classification of multistage salt diapirs in salt basins.
Interestingly, our study suggests that most multi-stage salt structures in the SNS were triggered as salt anticlines and not as reactive diapirs as we would expect during a rifting or regional extension regime (Jackson & Hudec, 2017;Jackson & Vendeville, 1994;Karam & Mitra, 2016;Moragas et al., 2017;Stewart, 2006;Vendeville, 2002).Nevertheless, our results agree with what Koyi and Petersen (1993) documented in the Danish Basin and in agreement with Harding and Huuse (2015); Jackson and Vendeville (1994), which suggested that extension also creates buoyancy instabilities which promotes the formation of salt anticlines.

| Basement regional controls
Our methodology located the most complex multi-stage salt diapirs in the Central Graben (CG), in the Sole Pit High (SPH) and in the Silver Pit Basin (SiPB).These three structural sub-basins show a complex regional tectonic history with the most complex salt diapirs which evolved in three regional phases and up to five different local stages.
The main reason for the higher occurrence of complex multi-stage salt diapirism processes in the CG, SPH and SiPB is a combination of the following; (1) a basementinherited rift-graben morphology of the Southern Permian Basin which led to a syn-depositional thickness variations of the Permian Zechstein Supergroup (Peryt et al., 2010), for example, the Central Graben System and the Dowsing-Hewett Fault Zone (Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019;Stewart, 2007;Stewart & Coward, 1995;Warsitzka et al., 2019) (Figure 1), ( 2) gravity-sliding deformation in a detached phase which triggered salt anticlines and controlled the kinematic evolution of multi-stage salt structures in the SPH and SiPB during the Mesozoic and Cenozoic (Stewart, 2007), (3) a palaeo-depositional salt thickness of 500-1000 m in the Central Graben to +1000 m in the SPH-SiPB which determined the style of deformation of the Mesozoic-Cenozoic stratigraphic sequences and salt diapirism (Maystrenko et al., 2012(Maystrenko et al., , 2013;;ten Veen et al., 2012), ( 4

| Chrono-halokinetic stack grouping
We summarize our results using our chrono-halokinetic stacks which we clustered into four different groups to visually identify and compare the regional and local variability of multi-stage salt diapirism (Figure 21).The chrono-halokinetic stacks show the number of interpreted regional phases and local stages.The different groups are formed based on the number of local stages, for example, we grouped multi-stage salt structures which showed 1-2 local stages and 1-2 regional phases into Group 1. Second, multi-stage salt structures which displayed three local stages and three regional phases were grouped into Group 2. Multi-stage salt structures that showed four local stages and three regional phases were grouped into Group 3, and multi-stage salt structures that displayed five local stages and three regional phases were grouped into Group 4 (Figure 21).
F U E 2 Classification salt in the Southern North Summary structures using the chrono-halokinetic stacks.
| 2111 EAGE GAITAN and ADAM 6.2.1 | Group 1 Group 1 contains three salt anticlines which evolved in only one contractional phase and one local stage of halokinesis in the Cleaver Bank High (CBH) during the Late Cretaceous-Cenozoic (CB11; Figure 21).One of the three salt anticlines is characterized by a post-tectonic phase during the Cenozoic (CB12; Figure 21).However, due to the absence of thickness variations, we infer that most salt anticlines will evidence a post-tectonic Group contains one salt anticline and one multi-stage salt diapir in the Sole Pit High (SPH).The salt anticline was triggered halokinetically during a pre-kinematic phase in the Early Triassic and continued evolving as a salt anticline during the rift phase until the Jurassic (SP11).In contrast, the multi-stage salt diapir in the SPH evolved in the detached phase during the Mid-Late Triassic-Jurassic (SP12).The chrono-halokinetic stacks show that the dominant regional phase of Group 1 is the detached phase, which displays a 1-2 regional phase to local stage ratio.The pre-kinematic, rift, inversion/contraction and the post-tectonic phase all show a 1-1 regional phase to local stage ratio.
Group 1 displays three different local stages of salt mobilization and diapirism, for example, salt anticlines in the pre-kinematic, rift, inversion/contraction and posttectonic phase (CB11-12, SP11) and a reactive and active diapir in the detached phase (SP12).The regional controlling factors for deformation in Group 1 were; (1) regional variations of the palaeo-depositional salt thickness near the basin marginal zones in the SPH where palaeosalt thicknesses vary from 500-1000 m in the SPH to +1000 m of original salt thickness in the SiPB as reported by Maystrenko et al. (2013) and where Nalpas et al. (1995) suggested that the style of deformation of the megastratigraphic sequences and salt diapirism is linked to the presence and thickness of the salt layer (Figure 20), (2) a rifting phase which controlled the deposition of +1000 m of Mesozoic sediments into the rift basin of the SPH which promoted salt rise and salt diapiric growth during the Mid-Late Triassic-Jurassic, (3) a detached phase due to gravity-sliding deformation from marginal areas triggered by regional tectonics during the Mid-Late Triassic which triggered reactive diapirism at the periphery of the gravitysliding deformation areal extent.We believe that reactive diapirism in the northern part of the SPH at the periphery of the gravity-sliding deformation zone occurred because of salt-overburden transport to the marginal areas of the gravity deformation zone.A today's example of reactive diapirism linked to salt-overburden transportation is the Paradox salt-overburden in the Colorado River canyon, USA, studied in Schultz-Ela and Walsh ( 2002), (4) a contraction phase beginning from the Late Cretaceous and continued until the Palaeogene which triggered and controlled the development of salt anticlines in the CBH.A source of uncertainty in our regional methodology arises from the absence and erosion of Mesozoic sediments in the CBH which may hinder any possible interpretation of regional phases and local stages of salt mobilization and diapirism (Figures 12,16 and 17).A thick-original salt and the development of a regional palaeo-depocentre in the CBH during the inversion of the contiguous BFB and CG hindered salt diapirism and the piercing of Cenozoic sediments in the CBH.

| Group 2
Group 2 contains six multi-stage salt structures which evolved in three different regional phases and three local stages of salt mobilization and salt diapirism in the Cleaver Bank High (CBH) and in the Silver Pit Basin (SiPB) (Group 2; Figure 21).Multi-stage salt structures in Group 2 display a consistent regional-to-local stage ratio, for example, one regional phase per one local stage (1-1 ratio).
Our chrono-halokinetic stacks show that most multistage salt structures in Group 2 were triggered in a prekinematic phase in the Early Triassic-Jurassic and one multi-stage salt diapir triggered in a detached phase in the Mid-Late Triassic.Three multi-stage salt structures are salt anticlines which evolved as a salt anticline in a detached phase in the SiPB (Si21) and salt anticlines evolved as contractional anticlines in the CBH (CB21).Salt anticlines in the SiPB further evolved during a contraction phase triggering contractional anticlines whereas in the CBH salt anticlines became dormant during the post-tectonic phase in the Qty-Neogene.Multi-stage salt diapirs in Group 2 all evolved in a contraction phase which controlled contractional diapirism during the Late Cretaceous-Cenozoic (CB22, Si22).Additionally, all contractional diapirs became dormant during the post-tectonic phase in the Qty-Neogene.
The regional controlling factors for deformation and salt diapirism in the SiPB in Group 2 were; (1) the basement-detached, gravity-sliding deformation during the Mid-Late Triassic-Early Cretaceous from the North Dogger Shelf and Mid North Sea High which triggered gravity downdip salt overburden contraction and extension developing salt anticlines and buckle folding, as observed in examples of passive margins (Davison, 2007;Hudec et al., 2009;Quirk et al., 2012;Quirk & Pilcher, 2012;Stewart & Coward, 1995;Warsitzka et al., 2013).We believe that salt anticlines never pierced their overburden because of the thick salt-overburden and thick original salt layer, which favoured high-amplitude and more rounded salt anticlines, as documented in Hudec and Jackson (2011), (2) a palaeo-depositional original salt thickness of +1000 m which, as suggested by Maystrenko et al. (2013) and ten Veen et al. ( 2012), controlled the development of high-amplitude NW-SE trending salt anticlines in the SiPB, (3) contractional regional tectonics which controlled the further kinematic evolution of contractional anticlines and contractional diapirs in the CBH and SiPB.Similarly, the controlling factor for deformation and salt mobilization in the CBH is conregional tectonics which promoted the further squeezing of salt anticlines.

| Group 3
Group 3 contains six multi-stage salt diapirs which evolved in three different regional phases and four different local stages of salt diapirism (Group 3; Figure 21).All multi-stage salt diapirs were triggered as salt anticlines in a pre-kinematic (SP3), rift (CG3) and detached (Si3) regional phase.Our chrono-halokinetic stacks show that the dominant regional phases are the rift and inversion/contraction phase, where there is a 1-2 ratio of regional phase per local stage of salt diapirism.
In the SPH, salt anticlines triggered during a prekinematic phase continued evolving as salt anticlines in a detached phase during the Mid-Late Triassic-Jurassic.In the CG, salt anticlines switched to a passive diapir growth mode in the rift phase during the Mid-Late Triassic-Jurassic.One of the main characteristics of multi-stage salt diapirs in the SPH and CG are that salt diapirs transitioned from a passive diapir local stage of salt diapirism to a contractional diapir during the inversion/contraction phase.Based on the absence of faulting and the seismic character of the Cretaceous sequence preserved in cross-section, we assumed that the passive diapir phase continued until the Cretaceous before the inversion phase in the Late Cretaceous.There is no universal example of how multi-stage salt diapirs react to inversion tectonics, however, we demonstrated that salt diapirs in the CG responded to inversion by arching the salt diapir's roof developing outer-arc extensional faulting and gentle seismic-stratigraphic onlap towards the crest of the arched diapir's roof (dC; Figure 13).This agrees with models by Hudec and Jackson (2007) of contractional diapirism.Passive diapirs evolving into contractional diapirs are common and have been documented in the North German Zechstein basin, for example, in Baldschuhn et al. (2001).Moreover, salt anticlines continued evolving as salt anticlines in the SiPB in the early phases of the contraction phase, but sustained contraction triggered salt diapirism during the later phases of contraction during the Palaeogene.Multi-stage salt diapirs in the CG and SiPB also displayed a post-tectonic phase where multi-stage salt diapirs became dormant.
The regional controlling factors for deformation and salt diapirism in Group 3 were as follows: (1) the early development of regional and localized palaeo-depocentres in the Early Triassic in the CG and SPH which triggered salt anticlines, (2) the regional Palaeozoic-inherited structural configuration at sub-salt levels which controlled the continuous development of regional palaeo-depocentres in the CG in the rift phase from the Mid-Late Triassic to the early phases of inversion during the Cretaceous .The rift phase also controlled the basementinvolved extension of the Dowsing-Hewett Fault Zone in the western margin of the SPH (Arthur, 1993;Grant, Underhill, Hernández-Casado, Jamieson, et al., 2019;Stewart & Coward, 1995), (3) the gravity-sliding deformation zone from the North Dogger Shelf and Mid North Sea High (Stewart, 2007) influenced the evolution of salt anticlines during the detached phase.Furthermore, shortening during the inversion/contraction phase of previously established salt anticlines in the SPH, promoted the salt rise and salt collapse of kilometre-long salt walls in the SPH-Inde Shelf (SP3) during the Cretaceous.This is evidenced by the inward-tilted collapsed unconformity in the diapir-proximal stratigraphic sequences encasing the multi-stage salt diapir and in agreement with previously published collapsed structures in the North Sea by Stewart and Coward (1995) and Stewart (2006).6.2.4 | Group 4   Group 4 contains five multi-stage salt diapirs which evolved in three different regional phases and five different local stages of salt diapirism in the CG (Group 4; Figure 21).Group 5 represents the most complex and tallest multi-stage salt diapirs in the SNS.All multi-stage salt diapirs in Group 4 in the CG were triggered as salt anticlines in the rift phase followed by a long-lasting stage of passive diapirism during the Triassic-Early Cretaceous.Multi-stage salt diapirs evolved to contractional salt diapirs in the inversion phase during the Late Cretaceous-Cenozoic, becoming dormant in the post-tectonic phase during the Qty-Neogene.
Our chrono-halokinetic stacks showed that the rift and inversion/contraction phases are the dominant regional phases in the CG, for example, multi-stage salt diapirs showed a 1-2 regional phase to local stage ratio in the rift phase during the Triassic-Jurassic and in the inversion/ contraction phase during the Late Cretaceous-Cenozoic.The main regional controlling factors for deformation in Group 4 were (1) the early development of regional and NE-SW trending, localized palaeo-depocentres in the CG which triggered salt mobilization and salt anticlines during the Late Permian-Early Triassic, (2) rift-extension from the Mid-Late Triassic to the Early Cretaceous in the rift phase permitted the accumulation of +1000 m of Mesozoic sediments in the main rift graben system.Consequently, saltwithdrawal palaeo-depocentres and the spatial shifting of localized palaeo-depocentres comparing isochron maps at different stratigraphic levels, recorded the depletion of the where palaeo-depocentres were located and the change in salt diapiric growth from salt anticlines to passive diapirs in the CG.Once the depletion of the salt layer occurs as salt-withdrawal palaeo-depocentres sink, the palaeo-depocentres invert to form an "inverted salt minibasin" also known as "turtle structure" evidenced by the Early Triassic-Jurassic mega-stratigraphic sequences (pA; Figure 13).More than 2 km of Triassic-Jurassic sediments are preserved within the turtle-structures.These geometries are common and widely documented in salt basins (Seni & Jackson, 1984;Stewart, 2007;Trusheim, 1960;Vendeville, 2002), (3) regional contraction in the CG inverted normal faulting and rejuvenated previously established salt diapirs further controlling salt diapirism squeezing the stem of salt diapirs and uplifting the diapirproximal syn-kinematic and syn-halokinetic sequences encasing multi-stage salt diapirs.All multi-stage salt diapirs in the CG are now dormant salt structures, however, based on what has been discussed in Knox et al. (2010); Vejbaek et al. (2010) in the Southern Permian Basin, halokinesis could have been active until the Quaternary period, for example, in the Northern Polish Platform.
Our study demonstrated that the majority and the most complex multi-stage salt structures in the SNS were controlled by basement-involved tectonics and basin marginal gravity-sliding deformation from the North Dogger Shelf and Mid North Sea High.Salt anticlines are prevalent in the different structural subbasins triggered in a pre-kinematic and detached phase in the Sole Pit High and Silver Pit Basin, and in a rift phase in the deep basin centre of the Central Graben (Figure 21).We interpret that the thick salt layer prior to extension controlled the style of deformation during the rift phase.This interpretation agrees with studies in the Central Graben by Jackson and Vendeville (1994) and Stewart (2007).However, Jackson and Vendeville (1995) and Vendeville and Jackson (1992b) presented a different interpretation in the West Central Shelf where extension mainly triggered reactive diapirism.
Due to the regional character of this work, the interpretation of multi-stage salt structures is limited by using regional palaeo-depocentres and thickness variations adjacent to salt structures.However, the unique nomenclature developed here can be applied to local studies in complex salt basins.Finally, we also acknowledge that our study might be limited by the number of regional mapped horizons and the geological time interval between mapped horizons which determines the resolution of our interpretations.

| CONCLUSIONS
This study introduces a new methodology for the identification, mapping and classification of multi-stage salt structures in a regional salt basin.We developed a multistage salt diapirism classification scheme utilizing the regional tectono-stratigraphic sequences and salt-sediment relationships mapped from a 3D seismic reflection dataset.Additionally, we compared how multi-stage salt diapirism evolved from the Early Triassic to the Quaternary across the different structural sub-basins of the SNS.Finally, we summarized our interpretations into chrono-halokinetic stacks, which group salt structures based on the number of regional phases identified.
Multi-stage salt structures are prevalent in the diverse structural sub-basins of the SNS.The most complex multistage salt structures evolved in three different regional phases in sub-basins controlled by basement-involved salt tectonics; the most complex multi-stage salt diapirs were triggered as salt anticlines/pillows, rather than reactive or passive diapirism.Of the 155 salt structures highlighted in this study in the different sub-basins, about 65% of salt structures are salt anticlines.
The most complex multi-stage salt structures evolved in four, and up to five, local stages of salt diapirism, for example, in the Central Graben, Sole Pit High and Silver Pit Basin.The most complex multi-stage salt diapirs are linked to the rifting and inversion of sub-salt structures, for example, in the Central Graben and Sole Pit High.The most complex salt structures are km-tall salt stocks with squeezed stems, encased by near-vertical seismic stratigraphic sequences and uplifted carapaces; away from these multi-stage salt diapirs, inverted salt mini-basin geometries and salt-welding of the source layer is common.The most complex salt structures in the Central Graben developed due to irregular salt deposition and the early commencement of salt mobilization just after salt deposition.Furthermore, a thicker salt layer hindered the development of km-tall multi-stage salt diapirs, favouring thin-skinned deformation and the creation of tens-ofkilometres-wide salt anticlines in the Sole Pit High and Cleaver Bank High.
Multi-stage salt structures are of economic and societal interest, and relevant to the hydrocarbon industry and energy storage projects.For example, for the last two decades, man-made salt caverns have been utilized to store hydrogen gas.Depleted reservoirs near salt structures are also ideal due to the seal and trap characteristics of salt structures, for example, salt stocks and salt canopies.Additionally, due to the high thermal conductivity of salt, salt structures transmit heat to shallower depths in the subsurface which can be exploited by the geothermal industry.Therefore, it is paramount to understand the evolution of multi-stage salt diapirs to mitigate risks and

| FURTHER WORK
This study the foundation for subsequent regional salt tectonic studies.Additionally, this study highlighted the Central Graben, Sole Pit High and Silver Pit Basin structural sub-basins as the three most convoluted subbasins containing a plethora of multi-stage salt structures.Further studies will elucidate the link between the kinematic evolution of multi-stage salt structures and the syn-halokinetic sedimentary sequences encasing the multi-stage salt diapirs using a 3D seismic dataset, a variety of sub-regional to local isochron maps and geometrical seismic attribute maps to highlight the structural features around multi-stage salt structures.Finally, we encourage other salt tectonic researchers to apply the methodology and test the robustness of this classification in different salt basins worldwide.We predict that minor modifications or additions will be needed to include salt structures non-existent in the Southern North Sea, for example, salt canopies.

F
Regional salt tectonics map of the Southern North Sea.Study area is delimited by black polygon representing the 3D PGS MegaSurvey seismic dataset.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HaFZ, Hantum Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High; TIH, Texel Ijselmeer High; WH, Winterton High.Gravity-sliding deformation zone after Stewart (2007).EAGE GAITAN and ADAM 2.1.2| Cleaver Bank High and ten Veen et al. (2012) which studied the relationship between basement faults, the Zechstein salt structures and the salt-overburden faults in the SNS, also discussed in Geluk et al. (2007), Geluk (2007) and Peryt et al. (2010).de Jager (2003) studied and linked the NW-SE spatial relationship between the Palaeozoic-inherited faults and the orientation of salt structures in the Dutch SNS.ten Veen et al. (2012) concluded that salt structural trends in the SNS Central Graben and Sole Pit High are linked to N-S and NW-SE sub-Zechstein salt faults, respectively, mimicking the general trend of the Palaeozoic-inherited structural elements.

F
I G U R E 5 Location of dataset limits in the Southern North Sea and northwestern Europe.

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I G U R E 7 Cartoon examples of the diapiric sequences and the relationship with the salt's interface.F I G U R E 8 Cartoon examples of the different phases of evolution of salt structures.
, the pre-kinematic phase (pk) encompasses the Early Triassic in the Cleaver Bank High, Sole Pit High and Silver Pit Basin where the Early Triassic megastratigraphic sequence is not controlled by tectonics nor gravity-sliding deformation.The detached stage (d) comprises the Mid-Late Triassic and Jurassic (where present) mega-stratigraphic sequences in the Cleaver Bank High and Silver Pit Basin where the stratigraphic sequences are influenced by the gravity-sliding deformation from the Mid North Sea High and North Dogger Shelf.The rift phase (r) encompasses the Early Triassic, Mid-Late Triassic, Jurassic and the lower part of the Cretaceous mega-stratigraphic sequences in the Broad Fourteens Basin and Central Graben.The inversion/contraction phase (i) represents the upper part of the Cretaceous, Palaeogene and part of the Qty-Neogene mega-stratigraphic sequences and is widely expressed in the Broad Fourteens Basin, Central Graben, Cleaver Bank High and Sole Pit High.The post-tectonic (pt) phase comprises the Qty-Neogene mega-stratigraphic sequences across the whole SNS.

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I G U R E 9 Regional Top Zechstein structural map of the Southern North Sea.(a) Top Zechstein salt horizon (TWTT).(b) Salt structural concordance map.Zechstein salt TWTT structural map shown with the different Southern North Sea structural sub-basins in (a) and salt structure polygons drawn from (a) in (b).Extensive diapirism is observed in the Central Graben, Broad Fourteens Basin and Sole Pit High.Structural lows are the Broad Fourteens Basin, Central Graben and the Cleaver Bank High, whereas the Sole Pit High is observed as a structural high.Underlying pre-salt faults after Doornenbal and Stevenson (2010).Location of Figures 13-15 is shown in (a).BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.

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I G U R E 1 1 (a) Area of study showing sub-Zechstein salt structural features and general Mesozoic faults.Discordant and concordant salt structures from 0.5 to 1.25 s (TWTT) and >1.25 s (TWTT) are also shown.Discordant salt structures >1.25 s (TWTT) are shown in a darker shade of pink representing the tallest salt structures.(b) Regional TWTT Top Zechstein salt-Base Zechstein salt grid values histogram showing a multi-modal distribution which represents the merge of several surveys.(c) Zoomed-in view of (b) representing only salt structures >1.25 s (TWTT).The zoomed-in section displays an exponentially decreasing distribution.The x-axis of the plotted histograms represents the height and/or thickness (TWTT) from the top of the Zechstein salt to the base of the Zechstein salt horizon.The y-axis displays the count of values for each time interval (TWTT).

5. 7 |
Early Triassic-pre-kinematic/ rift phase The dominant regional phases during the Early Triassic are the pre-kinematic and the rift phase, for example, in the Broad Fourteens Basin (BFB), Central Graben (CG), Cleaver Bank High (CBH) and the Sole Pit High (SPH), represented by active Early Triassic faulting in the CG, influenced by early pulses of basement-involved extension, and salt mobilization in the BFB, CBH and SPH (Figure12).

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A B L E 3 (a) Number of salt structures in the Southern North Sea and structural sub-basins with >0.5-1.25 s TWTT and >1.25 s filters.(b) Number of salt structures in the individual structural sub-basins shown and differentiated between concordant and discordant.Triassic, the active Palaeozoicinherited structural elements controlled the development of NNW-SSE trending salt structures in the CG and NW-SE trending evolving salt structures in the SPH (Figure 12a).The inherited Palaeozoic-structural and salt structural trend in the CG controlled the development of NW-SE to NNE-SSW trending, Early Triassic faults (CG; Figure 12a), whereas in the SPH, the inherited Palaeozoic-structural and salt structural trend controlled the formation of NW-SE trending Mesozoic faults (SPH; Figure 12a).
5.7.2 | Regional depocentresThickness variations of the top Early Triassic-top Zechstein salt isochron highlight regional palaeodepocentres and more localized salt-withdrawal basins.More localized thickness changes indicate synkinematic deposition related to salt mobilization and F I G U R E 1 2 Early Triassic seismic stratigraphic level.Observations in variations of structural configuration and erosional to nondepositional characteristics can be distinguished between the diverse Southern North Sea sub-basins.(a) TWTT structure map of the Top Early Triassic with Early Triassic active faults.Gaps on TWTT structure and isochron maps are areas where salt structures have pierced the stratigraphic level.Active faults are modified after Doornenbal and Stevenson (2010); Stewart (2007).(b) Top Early Triassic-Top Zechstein salt isochron (TWTT) map.(c) Synthesis map of the tectonostratigraphic evolution of the Early Triassic in relation to salt diapirism and palaeo-depocentre development.Southern North Sea sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.

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I G U R E 1 3 Interpreted NNW-SSE seismic cross-sectional profile of two highly squeezed multi-stage salt walls in the Central Graben.Salt walls are separated by an inverted salt-withdrawal depocentre or turtle structure.Thick Mesozoic sediments, ca. 2 s, are observed during the rift and inverted regional phase of the Central Graben in the northwestern part of the cross-section.VPU, velocity pull ups.Location of cross section in Figure9.Seismic data courtesy of Oil and Gas Authority and PGS.F I G U R E 1 4 Interpreted SW-NE seismic cross-section profile of salt structures in the Sole Pit High. Figure of a salt anticline adjacent to a semi-pinched-off salt wall.Thickening of the Early Triassic sediments is observed at the apex of the salt anticline.From the Early Triassic to the Jurassic in the rift phase, sediments are concordant to the salt interface.ca.1.2 s (TWTT) of Triassic-Jurassic sediments are preserved in the rift/detached phase within the two adjacent salt structures.Note the transpressional flower structural feature SW of the cross-section.VPU , velocity pull-ups.Location of cross section in Figure 9. Seismic data courtesy of Oil and Gas Authority and PGS.

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I G U R E 1 6 Mid-Late Triassic seismic stratigraphic level.Observations in variations of structural configuration and erosional to non-depositional characteristics can be distinguished between the diverse Southern North Sea structural sub-basins.Note the absence of sediments in the Cleaver Bank High, Sole Pit High and TIH.(a) TWTT structure map of the Top Triassic with Mid-Late Triassic active faults.Gaps on TWTT structure and isochron maps are areas where salt structures have pierced the stratigraphic level.Active faults are modified after Doornenbal and Stevenson (2010); Stewart (2007).(b) Top Triassic-Top Early Triassic isochron (TWTT) map.(c) Synthesis map of the tectonostratigraphic evolution of the Mid-Late Triassic in relation to salt diapirism and palaeo-depocentre development.Southern North Sea structural sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.

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Jurassic seismic stratigraphic level.Observations in variations of structural configuration and erosional to non-depositional characteristics can be distinguished between the diverse Southern North Sea structural sub-basins.Note the absence of sediments in the Cleaver Bank High, Silver Pit Basin, Sole Pit High and TIH.(a) TWTT structure map of the Top Jurassic with Jurassic active faults.Gaps on TWTT structure and isochron maps are areas where salt structures have pierced the stratigraphic level.Active faults are modified after Doornenbal and Stevenson (2010); Stewart (2007).(b) Top Jurassic-Top Triassic isochron (TWTT) map.(c) Synthesis map of the tectonostratigraphic evolution of the Jurassic in relation to salt diapirism and palaeo-depocentre development.Southern North Sea structural sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.salt-palaeo-depocentres developed during (Figure Thickness variations from Mid-Late Triassic to the Jurassic indicate changes in salt mobilization and salt diapirism mode.The regional palaeo-depocentres represent Jurassic time-thicknesses >0.70 s (Figure 17b,c).

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Cretaceous seismic stratigraphic level.Observations in variations of structural configuration and erosional to nondepositional characteristics can be distinguished between the diverse Southern North Sea structural sub-basins.Note the absence of sediments in the Sole Pit High.(a) TWTT structure map of the Top Cretaceous with Cretaceous active faults.Gaps on TWTT structure and isochron maps are areas where salt structures have pierced the stratigraphic level.Active faults are modified after Doornenbal and Stevenson (2010); Stewart (2007).(b) Top Cretaceous-Base Cretaceous Unconformity isochron (TWTT) map.(c) Synthesis map of the tectonostratigraphic evolution of the Cretaceous in relation to salt diapirism and palaeo-depocentre development.Southern North Sea structural sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.

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Palaeogene seismic stratigraphic level.Observations in variations of structural configuration and erosional to nondepositional characteristics can be distinguished between the diverse Southern North Sea structural sub-basins.(a) TWTT structure map of the Top Palaeogene with Cenozoic active faults.Active faults are modified after Doornenbal and Stevenson (2010); Stewart (2007).(b) Top Palaeogene-Top Cretaceous isochron (TWTT) map.(c) Synthesis map of the tectonostratigraphic evolution of the Palaeogene in relation to salt diapirism and palaeo-depocentre development.Southern North Sea structural sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.

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I G U R E 2 0 Qty-Neogene seismic stratigraphic levels.TWTT structure map of Seabed not shown.Observations in variations of structural configuration and erosional to non-depositional characteristics can be distinguished between the diverse Southern North Sea structural sub-basins.(a) Qty-Neogene-Top Neogene isochron (TWTT) map.(b) Synthesis map of the tectonostratigraphic evolution of the Qty-Neogene in relation to salt diapirism and palaeo-depocentre development.Southern North Sea structural sub-basins and structural features are labelled.BFB, Broad Fourteens Basin; CBH, Cleaver Bank High; CG, Central Graben; DFZ, Dowsing Fault Zone; HFZ, Hewett Fault Zone; SiPB, Silver Pit Basin; SPH, Sole Pit High.
Summary of salt structures identified in the Southern North Sea.(a) Regional phases in the SNS.(b) Local stages in structural sub-basins.

seismic picks (coverage) Mapped horizon Horizon type Isochron Description & approximate age General seismo-stratigraphy
Information of mapped horizons for this study with descriptors.
(Cameron et al., 1992)iassicLithological horizon Top Triassic: This level corresponds to the mudstone/sandstone boundary of the Penarth Gp and the Lias Gp (UK).Equivalent to the transition between the Sleen Fm to the Aalburg Fm in the Netherlands.201Ma(Cameron et al., 1992)