Seismic evidence for ductile necking of the mid‐lower crust beneath the Columbrets Basin (Western Mediterranean)

The Columbrets Basin is the largest Mesozoic rift basin of the Valencia Trough in the Western Mediterranean. The analysis of a seismic‐reflection survey makes it possible to reconstruct the tectonic fabric underlying the sedimentary basin, including the structure of the top of the lower crust and the Moho. It is proposed that the ductile deformation of the mid‐lower crust was the main mechanism controlling the basin geometry, with the radial flow of mid‐lower crust coeval with the reactivation of two large‐offset SW‐dipping normal faults, inherited from the precursor Permian–Triassic rifting. Mid‐lower crustal necking occurred below the major depocenters, immediately before hyperextension. Our results provide new insight into the formation of circular‐shaped basins and the evolution of depth‐dependent extensional processes during rifting.


| INTRODUC TI ON
Rift segmentation can be associated with pre-existing crustal heterogeneities. Likewise, extensional stresses concentrate along oblique inherited discontinuities, promoting en-echelon fault patterns (Jammes et al., 2009). The trend of those faults and their associated sedimentary depocentres lie between the strike of preexisting crustal anisotropies and the vector orthogonal to the extension direction. In such scenarios, brittle faulting is often confined to the upper crust being coeval with plastic flow in the mid-lower crust (Brun et al., 2018;Clerc et al., 2018;Huismans & Beaumont, 2011).
Thus, the upper crust could stretch and thin less than the entire crust, commonly referred to as a depth-dependent stretching mode (Davis & Kusznir, 2004;Huismans & Beaumont, 2011). One issue that may arise related to this process is the extensional discrepancy, which refers to the amount of crustal thinning not accounted for by the displacement of upper crustal faults. However, the structure and, consequently, the detailed behaviour of the mid-lower crust during rifting remain enigmatic, mainly due to the resolution and limited coverage of the available seismic surveys. Moreover, most examples describe successful hyperextended rifted margins such as the Gabon margin (Clerc et al., 2018) or the South China Sea (Zhao et al., 2021), where crustal extension culminated with mantle exhumation and oceanic spreading. Failed rift systems, alternatively, provide key observations to understand the structure and behaviour of the mid-lower crust structure prior to oceanization.  Figure S1) makes it possible to visualize the structure of the sedimentary basin at the lithosphere scale, as well as the seismic signature of the Moho and the mid-lower crust. This paper presents a 3D reconstruction of the Columbrets Basin, together with sequential restoration of key seismic profiles, showing the lithosphere evolution from the Middle Jurassic to present-day. The main objectives are to discuss (i) the deformational mechanisms that led to crustal thinning, (ii) the behaviour of the mid-lower crust during oblique rifting and (iii) the localization of extension along inherited crustal-scale weak zones.

| The Columbrets Basin
The Columbrets Basin is located at the linking zone between the Iberian Chain, the Betic Cordillera and the Catalan Coastal Chain. It is surrounded

Statement of significance
The interpretation of a high-resolution 2D seismicreflection survey across the Columbrets Basin allows reconstructing of the structure of the Moho and the top of the lower crust underneath and formulating a new kinematic model of evolution. Results identify a basinbounding large-offset extensional fault that was coeval to the radial flow of the mid-lower crust. This mode of midlower crustal thinning provides new insight to understand the formation of circular-shaped basins and the evolution of depth-dependent extensional processes during rifting.  Roca et al., 1999). (c) Location of the seismic lines and well data used in this study. Well data (Lanaja, 1987) infill consists of a 15-km-thick Mesozoic to Cenozoic succession, deposited above a thinned continental crust (Etheve et al., 2018;Roma et al., 2018;Viñas-Gaza, 2016). The basin underwent two Mesozoic rifting events related to the opening of the Western Tethys and the Iberian intraplate rift, and a Cenozoic tectonic inversion event related to the convergence between Africa and Eurasia (Salas et al., 2001). Thus, the original basin was partially inverted during the formation of the Iberian Chain to the west (Guimerà, 2018) and the Betic Cordillera to the southwest (Etheve et al., 2016;Fontboté et al., 1990).
The basin experienced mild inversion during late Eocene-Aquitanian times marked by a regional-scale erosional unconformity.
Tectonic inversion was followed by Early to Middle Miocene rapid subsidence, related to the onset of extension in the Valencia Trough, which is represented by 2-6 km of Lower Miocene platform carbonates and conglomerates, followed by Middle Miocene to recent clastic sediments (Ayala et al., 2015), separated by the Messinian unconformity (Roca et al., 1999). Such an extensional phase was accompanied by two Late Oligocene to Serravalian and Tortonian to present magmatic episodes (Martí et al., 1992) (Figures 1a and 2b).

F I G U R E 2
Depth-converted seismic interpretation of seismic profiles along-strike (a and b) and across (c-h) the Columbrets Fault. Notice that the fault throw decreases towards the east as the Mesozoic sedimentary package thins. Seismic profiles from the SGV01 survey were interpreted in time and converted to depth along with the interpreted horizons using interval velocities obtained from an available Expanded Spread Profile in the basin (ESP-7 in Torné et al., 1992;Pascal et al., 1992).  (Figure 3a). Above, the upper crust is rotated by 20° and displays sigmoidal reflectors, which are interpreted as drag folds associated with shearing above the mid-lower crust (Figure 4).
Similar ductile thinning of the mid-lower crust has been documented in other seismic reflection and gravity modelling studies in the area (Dañobeitia et al., 1992;Torné et al., 1992).

| Tectonic evolution of the Columbrets Basin
The interpreted seismic profile SGV01-204 (Figure 1d) has been sequentially restored in six major steps ( Figure 5)

| D ISCUSS I ON AND CON CLUS I ON S
The published models that deal with the structure and evolution of the Columbrets Basin can be grouped into three main categories.
The first one considers a large NW-dipping extensional detachment  Figure 1c for location. The restoration workflow follows the methodology by Ramos et al. (2020), taking into account the sedimentary decompaction of each stratigraphic interval and compaction curves for average lithologies. Percentage lithologies and average density values were calculated using nearby well data (Lanaja, 1987). Furthermore, the occurrence of parallel Permo-Triassic structures onshore and Early-Mid Jurassic volcanism in the Serra d'Espadà, in the Central Iberian rift system (Salas et al., 2001;Figure 1a), suggests that the WNW-ESE trend of these large-offset faults is likely inherited from pre-existing basement-involved faults. These pre-existing faults represent the northern boundary of the ENE-WSW oblique fault system, which nucleated diapiric structures to the south but did not generate major thickness changes within the Columbrets Basin or the mid-lower crust (Figure 3c).
Brittle deformation in the upper crust appears to be accompanied by the ductile necking of the mid-lower crust ( Figure 5). This mode of deformation is probably related to the ductile rheological behaviour of a felsic composition of the mid-lower crust during extension (Weinberg et al., 2007). Necking near the base of the crust was probably favoured by high temperatures produced by the rising asthenosphere (Huismans & Beaumont, 2011), which would signifi- Ductile flow within the mid-lower crust may have been the primary control on the formation of the circular-shaped basin. Further geophysical data could help to refine the geodynamical mechanisms behind the formation of these types of basins.
In summary, it is suggested that strain localization along a preexisting high-angle fault evolved from a reactivated steeply-dipping fault to a crustal-scale shear zone. The increased viscosity related to a high thermal gradient promoted ductile flow within the mid-lower crust, leading to crustal necking immediately before hyperextension.
Our results provide an exceptional example of depth-dependent stretching, where 80% of thinning is accommodated by ductile deformation in the mid-lower crust and the remaining 20% by extensional faulting in the upper crust. This mode of mid-lower crustal thinning may provide new insight to understand the formation of circular-shaped basins and the evolution of depth-dependent extensional processes during rifting.
On the other hand, most of the published kinematic reconstructions of the West Mediterranean region show a gap in this sector of the Tethys realm, so the linkage between the Tethys and the Iberian rift system remains largely unconstrained (Angrand et al., 2020;Angrand & Mouthereau, 2021;Nirrengarten et al., 2018;Pedrera et al., 2020;Tavani et al., 2018). The findings of this work can help to pinpoint the kinematic plate reconstructions in eastern Iberia during the Mesozoic. Some of these reconstructions emphasize the role of the Permian-Triassic diffuse rift system in the nucleation and development of extensional basins during the Mesozoic (e.g., Angrand & Mouthereau, 2021), which are in line with our results.

ACK N O WLE D G E M ENTS
This work is part of the projects REViSE-Betics-PID2020-119651RBI00 (Spanish Science Ministry) and FIPS, PY20-01387 (Junta de Andalucía), also funded by the 'Ayudas Extraordinarias Menciones Excelencia Severo Ochoa' (IGME-CSIC). We thank Stefano Tavani, F. Mouthereau and an anonymous reviewer for their constructive comments and suggestions on the manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in Base de Datoby the Spanish Ministry of Ecological Transition at https://datos.gob.es/es/catal ogo/ea001 0987-base-de-datos -dehidro carburos.