Spatial and temporal evolution of rift systems: some implications at the scale of a plate boundary

Unravelling the mechanisms of continental lithosphere extension that control the formation and evolution of rift systems leading to continental breakup and creation of plate boundaries remains one of the main challenges in tectonics. Combined studies conducted in present-day rifted margins and their onshore fossil analogues reveal contrasted crustal architectures and the complex spatial and temporal development of rift systems.

The Bay of Biscay and Pyrenees are part of a Late Jurassic to Mid Cretaceous rift system located at the transition between the Iberian and European plates. The transition from preserved oceanic and rift domains to the west to their complete inversion in the east provides access to seismically imaged, drilled and exposed parts of a rift system. The originality of this natural laboratory enabled the development and application of an innovative offshore-onshore approach. We combine observations from seismic reflection data, gravity inversion results and field mapping to characterize, identify and map former rift domains from offshore to onshore, i.e. from a present-day margin to remnants of former margins preserved in the Pyrenean orogen.

This new mapping approach highlights the complex architecture of the paleo Iberian- European plate boundary that is characterized by the existence of strongly segmented rift systems spatially disconnected by weakly thinned continental ribbons (Figure 1). Rift system segmentation seems to be partially inherited from the pre-rift structuration and controls the formation of the rift systems as well as lateral variations of the architecture.

Based on the restoration, subsidence and deformation history of the rift systems, we illustrate the evolution of strain partitioning between them and present the implications at the scale of the Iberian-European plate boundary. Our results show that the deformation history is more complex and polyphase than previously assumed. We suggest that the onset of the Late Jurassic to Early Cretaceous rifting was partitioned between a set of distinct left-lateral transtensional rift systems. A plate kinematic reorganization at Aptian-Albian time resulted in the onset of seafloor spreading in the Western Bay of Biscay and extreme crustal and lithosphere thinning in intra-continental rift basins to the east.

Eventually, this work may provide insights on the partitioning of the deformation at transform to transtensional plate boundaries and may represent an analogue to unravel the initiation of segmented or strongly oblique shear margins observed worldwide. In spite of its transient nature, the Iberian-European plate boundary may provide new insights on (1) processes preceding lithosphere break-up, and (2) the complex partitioning of extensional deformation in propagating rift systems observed at nascent plate boundaries.