Mantle resistance against Gibraltar slab dragging as a key cause of the Messinian Salinity Crisis

The Messinian Salinity Crisis (5.97–5.33 Ma) was caused by the closure of the Atlantic‐Mediterranean gateways that cut through the Gibraltar orogenic system. The geodynamic drivers underlying gateway closure and re‐opening are still debated. Here, we interrogate the gateway successions to find the imprints of surface deformation, infer the timing and nature of associated geodynamic drivers, and test such inferences against numerical simulations of slab dynamics. We find that since the latest Miocene, a tectonic framework was established in the gateway region dominated simultaneously by (a) relative plate convergence, (b) slab tearing under the eastern Betic Cordillera and (c) mantle resistance against north‐northeastward dragging of the Gibraltar slab by the African plate's absolute motion. We propose that mantle‐resisted slab dragging and slab tearing operated in concert closing the gateways that caused the Messinian Salinity Crisis, whereas sinking of heavy oceanic lithosphere located between buoyant continental plates re‐opened the Strait of Gibraltar at 5.33 Ma.

break-off and dynamic topographic rebound (Duggen et al., 2003;Garcia-Castellanos & Villasenor, 2011), rejuvenation of plate convergence and associated crustal deformation affecting the Atlantic-Mediterranean connectivity (Jolivet et al., 2006), or isostatic responses to salt loads (Govers, Meijer, & Krijgsman, 2009). Rollback and slab steepening may have ended the Messinian Salinity Crisis by the opening, or deepening, of the Strait of Gibraltar (Garcia-Castellanos & Villasenor, 2011;Govers, 2009). Linking these models to recent field evidence from the gateway basins (Capella et al., 2017;Flecker et al., 2015) may help identifying which of these processes, or others, opened and closed the gateways at different times.
Here, we address available field evidence of crustal deformation in the upper Miocene-Pliocene sedimentary archive of the Gibraltar orogenic system. We compare the geological reconstruction of the tectonic framework of the past with that of the present-day as reflected in GPS motions and active structures. Then, we test it against predictions obtained from numerical simulation of the Gibraltar subduction (Spakman, Chertova, Berg, & Hinsbergen, 2018) to propose a combination of geodynamic processes that explains the closure and re-opening of gateways at different locations in the Gibraltar arc causing the Messinian Salinity Crisis and its resolution.
In late Oligocene time, the slab reached sufficient length to initiate rollback (Chertova, Spakman, Geenen, et al., 2014a); slab rollback opened the oceanic Algerian Basin (Figure 1a) at high rates (up to ~90 mm/year) in middle to late Miocene time, until the slab arrived in the Gibraltar region in the late Tortonian (~8 Ma) (Do Couto et al., 2016;van Hinsbergen et al., 2014). During this process, a large fragment of crust in the upper plate separated from Eurasia and formed the Alborán domain, presently a highly extended fold-thrust belt that formed during earlier stages of subduction (Booth-Rea et al., 2007;Lonergan & White, 1997). Throughout the Miocene, the Alborán domain was thrust over the African and Iberian margins (Platt et al., 2003) forming the Rif Belt in Morocco and the Betic Cordillera in Spain (Vergés & Fernàndez, 2012) (Figure 1b).
The palaeogeographic re-organization brought about by the over-thrusting of the Alborán domain fundamentally changed the role of the Mediterranean in oceanic circulation that switched from being an oceanic corridor to a landlocked basin controlled by regional climate and connectivity at the sill (Capella et al., 2019).
Importantly, the docking of the Alborán domain around 8 Ma, coincided with cessation, or strong deceleration, of the Gibraltar slab rollback with no unequivocal evidence for continuing subduction up to present-day (Gutscher et al., 2012). Only small-magnitude, distributed shortening and locally also extension and strike-slip deformation occurred under the slow (<5 mm/year), NW-SE relative plate convergence between Africa-Iberia. This regime is proposed to dominate the tectonics of the region since the late Miocene (Jolivet et al., 2006;Pedrera et al., 2011). The docking of the Alborán domain along the Gibraltar arc set the stage for the Messinian Salinity Crisis of which the final onset (5.97 Ma), however, was initiated as much as 2 Myr later.

| G EODYNAMIC CONTROL ON THE E VOLUTI ON OF THE ATL ANTI C-MEDITERR ANE AN G ATE WAYS INFERRED FROM THE L ATE MI O CENE S ED IMENTARY RECORD
Mantle tomography and other seismological investigations provide a clear 3D image of the present-day geometry of the Gibraltar slab showing its lateral continuity with the surface plates, as well as where it is detached and may have delaminated the continental lithospheric mantle (Bezada et al., 2013;Chertova, Spakman, Geenen, et al., 2014a;Gutscher et al., 2002;Heit et al., 2017;Mancilla et al., 2015;Piromallo & Morelli, 2003;Spakman & Wortel, 2004;Villaseñor et al., 2015). The dynamic evolution towards this present-day snapshot requires geological observations of which first-order regional trends can be tested against predictions from numerical modelling of subduction evolution in the overall convergent plate setting (Chertova, Spakman, Geenen, et al., 2014a;Spakman et al., 2018). Following, we summarize the geological observations on the tectonic evolution of the Betic, Gibraltar and Rifian gateways with emphasis on the key interval between 8 and 5 Ma. In the central Betics, a NE-SW to NW-SE extension was recorded from late Miocene onwards, contemporaneously with vertical uplift, while the record was less affected by shortening (Galindo-Zaldívar et al., 2003;Reicherter & Peters, 2005;Rodríguez-Fernández & Sanz de Galdeano, 2006). In the western Betics, NW-SE directed shortening is recorded in intramontane basins (Jiménez-Bonilla, Expósito, Balanyá, Díaz-Azpiroz, & Barcos, 2015) and along the coast of the Gulf of Cadiz. In contrast with the western Betics, the N-S to NW-SE shortening of the eastern Betics was coeval with considerable uplift (Janowski et al., 2017)  Pliocene (Meijninger & Vissers, 2006. The NW-SE to NNW-SSE shortening remains active today in the west (Ruiz-Constán et al., 2009) as well as in the east where it leads to distributed deformation superposed on SW-NE aligned crustal motions that are accommodated by the left-lateral transpressive Eastern Betic Shear Zone (Borque et al., 2019). The late Miocene to Present shortening regime is aligned with the relative convergence direction between Africa and Iberia (e.g. Jolivet et al., 2006), but contributions to shortening from subduction dynamics, particularly after slab rollback has come to a near halt, cannot be excluded.  At present, to the east of ~4°W (Figure 2), the slab below the central-eastern Betics is detached from Iberian lithosphere (Mancilla et al., 2015) and the detached portion of the slab curves into a near E-W orientation below the central-eastern

| Gibraltar
In the area of the Strait of Gibraltar, an accretionary prism consisting of deep-marine clastic deposits known as the Flysch units

| Rif
In the external Rif, Miocene basin evolution was controlled by thin-skinned thrusting associated with the westward drift of the Alborán domain which locally created W-to SW-ward transport kinematics (Platt et al., 2003), and SW-ward migration of the foredeep (Capella et al., , 2017Chalouan et al., 2008)

| TOWARDS A UNIF YING SCENARI O FOR G EODYNAMI C FORCING ON THE G ATE WAYS
These available geological observations constrain the particular tectonic framework that was established at ca. drivers of crustal deformation since the late Miocene have not significantly changed (Pedrera et al., 2011).  shows that after rollback strongly decelerated in the late Miocene, the subducted slab has been dragged laterally through the mantle by the ~NNE-directed absolute motion of the African plate, i.e. almost parallel to the Gibraltar trench ( Figure 2). The mantle surrounding the Gibraltar slab resists this lateral slab transport, which leads to a relative motion between the slab and the advancing African lithosphere causing a SSW-directed indentation of the Moroccan Rif (Figure 2a) or, when viewed in the mantle frame, collision of the African plate with the slab (Figure 2b). This occurs at the transition from slab to African continental lithosphere, which is situated below the internal Rif.

3-D numerical subduction modelling
Numerical predictions of SKS-splitting resulting from upper mantle anisotropy developed by accumulating mantle shear during 35 Myr of subduction evolution (Chertova, Spakman, & Faccenda, 2017) show that observed SKS-splitting (e.g. Diaz et al., 2015Diaz et al., , 2010 can be explained by a combination of local slab-induced mantle flow (anisotropy signature inherited from rollback until ~8 Ma) and long-term externally excited cm/year-scale mantle flow, which overprints effects of slab dragging (~6 mm/year). Hence, observations of SKS-splitting are consistent with the modelled subduction evolution (Chertova, Spakman, Geenen, et al., 2014a;Chertova, Spakman, & Steinberger, 2018;Spakman et al., 2018) including the impact of external mantle flow . Furthermore, the numerical-model prediction of mantle-resisted slab dragging is robust with respect to variations in slab-mantle rheology (Chertova, Spakman, Berg, & Hinsbergen, 2014b;Spakman et al., 2018). Predictions derived for crustal behaviour concern first-order features of regional style of deformation. This assumes that the first-order regional crustal deformation is driven by the regional interaction of the lithospheric-mantle of the continental plates, continental margins, and of the Gibraltar slab, as modelled. In concert with the relative NW-SE relative plate convergence, mantle-resisted slab dragging predicts a regional style of surface deformation that explains the SW-directed deformation and motion of the Rif, nearly orogen-parallel extension of the central-eastern Betics above a progressively tearing slab, and the occurrence of, and sense of motion on the Trans-Alborán and eastern-Betics shear zones, as can still be inferred at present from GPS motions (Figure 2)  .

| Slab-dragging and slab tearing causing the Messinian Salinity Crisis
The indentation of the Rif by mantle-resisted slab dragging together with central-eastern Betics slab detachment (Figure 3) can be combined into a geodynamic model that explains gateway closure and re-opening. The first essential step preconditioning the Gibraltar region for the isolation of the Mediterranean was the early to late Miocene westward slab roll-back during which the Alborán domain was thrust on top of the African continental margin to the south and Iberian margin to the north, with the formation of the Flysch prism in the middle (Figure 1b).
The latter closed the deep, oceanic corridor that had formed between Iberia and Northwest Africa during the Jurassic break-up of Pangea (Vissers, Hinsbergen, Meijer, & Piccardo, 2013). Flexure related to the emplacement of the Alborán domain onto the continental margins led to foreland basin subsidence in the Iberian and African margins which formed the Betic and Rifian Corridors (Figure 1b). Although these still allowed Atlantic-Mediterranean water exchange, it was this tectonic event that led to the first restriction of the Mediterranean Sea, documented in sedimentological, geochemical and faunal records (Capella et al., 2019;Flecker et al., 2015).
After the arrest at ~8 Ma of rapid west-ward rollback, NNEdirected slab dragging became the remaining slab motion . The indentation of the Moroccan continental margin, generated by the mantle resistance against NNE slab dragging, initiated the event of regional thick-skinned tectonics west of the Nekor fault that closed the Rifian Corridor in the early Messinian, at around 7 Ma (Capella et al., , 2017. This SSW indentation is likely also responsible for the thickened crust contiguous to the slab edge (Gil et al., 2014) as well as the on average NNE-SSW contraction observed in the Rif (Capella et al., 2017;Pedrera et al., 2011) rather than NW-SE orientated Africa-Eurasia convergence, whose direction is compatible only with the minor shortening and folding recorded in the eastern Rif and the western Betics (Bernini et al., 2000;Jiménez-Bonilla et al., 2015) (Figure 1b).
We note that the Rifian gateway closure occurred directly to the south of the slab where it is still continuous with Moroccan lithosphere today, thus requiring a different closure mechanism than the uplift proposed to take place following slab tearing and delamination of lithospheric mantle (Duggen et al., 2003). The latter mechanism fails to explain the NE-SW verging thick-skinned tectonics, crustal thickening, and continuous shortening of the western Rif that started in the late Miocene and is still active today.
Between 7-8 Ma, all the Betic gateways were uplifting (Garcia-Castellanos & Villasenor, 2011;van der Schee et al., 2018). The process underlying this uplift is related to the gradual slab tearing moving towards the west (Mancilla et al., 2015), which also appears in the numerical models of slab evolution . Slab tearing under the south Iberian margin decouples the NE-directed Iberian absolute plate motion (Figure 2b) from the westernmost segment of the slab, which is still attached under the western Betics.
The NNE directed slab dragging occurs at lower pace than the NEdirected Iberian motion relative to the mantle (Figure 2b,d), which therefore causes a differential motion leading to the SW-NE extension in the eastern-central Betic region . At

| Opening of the Strait of Gibraltar
Field evidence suggests that by 7 Ma, the Betic and Rifian gateways were shut and uplifted van der Schee et al., 2018). After 7 Ma, an embryonic Strait of Gibraltar took over the connectivity in an area that was either always submerged or barely emergent on top of the Flysch units . Morphological evidence of ENE-to E-orientated faults at the Camarinal Sill (Luján et al., 2011) suggests that strike-slip structures may have controlled connections at a time of little or no vertical motion.
The infilling of the western Alborán Basin indicates a marked westward depocentre shift between the upper Tortonian and Pliocene units (Do Couto et al., 2016). The timing of this event, the direction of depocentre-shift combined with the location of the slab-hinge below the western Alborán Basin, suggests that subsidence was driven by slab pull (Govers, 2009). The evolution of the Strait of Gibraltar before and during the Messinian Salinity Crisis  may be a consequence of progressive slab tearing from the eastern to central Betics. Westward lateral migration of the slab tear point under the Betics increases the gravitational load on the slab that is still attached, shifting depocentres (van der Meulen, Meulenkamp, & Wortel, 1998;Wortel & Spakman, 2000). This tear is currently situated within ~100 km of the Strait of Gibraltar (Figure 2) and has therefore been suggested as a potential driver of the re-opening of the Strait at 5.33 Ma (Garcia-Castellanos & Villasenor, 2011). Between 8-7 Ma, however, slab tearing was likely to have little effect on vertical motions of the Strait of Gibraltar because it was confined to the eastern Betics, i.e. 300-400 km to the ENE, and had not propagated much westward yet.
An alternative possibility is that slow vertical sinking of a slab that stopped significant rollback by ~8 Ma acted as the sole driver of the subsidence that gradually opened the Strait of Gibraltar before and after the Messinian Salinity Crisis. The slab dipping steeply below the Strait of Gibraltar comprises cold oceanic lithosphere of Jurassic age (Gutscher et al., 2012) and it is situated between the buoyant continental lithosphere to which it is connected to the north (western Betics) and south (western Rif). It is therefore possible to infer a F I G U R E 3 (a) ArcGlobe (ESRI™) topography of the Gibraltar region with projection of the paleogeography of the Mediterranean-Atlantic gateways after . The three insets illustrate the timing and mechanisms of vertical motion that led to the temporary isolation (~5.33-7 Ma) of the Mediterranean, and refer to the corresponding geodynamic forces. The east Alborán Arc hypothesis suggests that an emergent, volcanic archipelago could have temporarily connected southeastern Iberia with northern Africa (Booth-Rea, Ranero, & Grevemeyer, 2018). (b) Cartoon interpretation of RGB-slab morphology across the upper mantle modified from . Vectors illustrate the absolute plate motions for Africa and Iberia as well as slower central-eastern Betics motion (as in Figure 2b

| CON CLUS ION
We tested the imprints of the tectonic evolution of the late Miocene Atlantic-Mediterranean gateway basins against plausible geodynamic drivers. Structural constraints from the gateway successions show a tectonic framework not dissimilar to that of the present-day, consist- of Norway through its Centres of Excellence funding scheme, project number 223272. We thank Laurent Jolivet and the anonymous reviewers whose comments helped us to improve the manuscript.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analysed in this study.