Rapid large-amplitude vertical motions generated by subduction slab roll-back in back-arc basins (Valencia Trough, Western Mediterranean)

The formation and evolution of back-arc basins is complex controlled by subduction dynamics, lithosphere delamination, magmatism, slab roll-back and extension. In such a complex geodynamic context, it is difficult to decipher the mechanisms which controls sedimentary basin subsidence history and distinguish the contribution of lithosphere tectonics from dynamic topography.

Here we focus on one of the main basins of the Western Mediterranean, the Valencia Trough, which formed in the Cenozoic in relation with the slab roll-back of the Tethyan oceanic lithosphere. More specifically, we investigate the subsidence and geodynamic context related to the formation of a regionally observed unconformity, which separates Mesozoic from latest Palaeogene to Neogene sediments, and here referred to as the Miocene Unconformity.

Using a dense grid of seismic reflection data, well data and 3D flexural backstripping, we show that the Miocene Unconformity subsided by more than 1.5 km from ~17 Ma to the present day at an average rate of 90 m/Myr in the SW Valencia Trough. The absence of Cenozoic extensional faults affecting the basement shown by seismic data indicates that this rapid subsidence is not caused by Cenozoic rifting. This subsidence cannot be explained by flexural loading related to the adjacent thin-skin Betic fold and thrust belt either, which only affects subsidence observed near the deformation front. Subduction dynamic subsidence generated by the positive mass anomaly of the subducting slab in the mantle is another mechanism that can control the subsidence evolution of back-arc basins. However, since the formation of the Miocene unconformity, the subduction has propagated westwards and southwards and has slowed or ceased under the Valencia Trough, which would have resulted in the progressive diminution of subduction dynamic subsidence, generating a relative uplift rather than subsidence.

We propose an alternative mechanism and interpret the 1.5 km subsidence of the Miocene Unconformity as the collapse of a back-arc transient uplift event. Erosion during the uplift, resulting in the formation of the unconformity, is estimated to exceed 4 km. This transient uplift was likely caused by heating of back-arc lithosphere and asthenosphere, combined with mantle dynamic uplift, both caused by segmentation of Tethyan subduction resulting in slab tear. Rapid subsidence subsequently resulted from the removal of mantle flow dynamic support from the Tethyan subduction slab roll-back and thermal equilibration.

Our observations and interpretation of rapid back-arc kilometre-scale uplift and collapse might have global applicability to explain some of the observed vertical motions and the subsidence evolution of other back-arc regions experiencing subduction segmentation and slab tear during subduction slab roll-back.