Mode of Continental Thinning and Breakup Along the North Iberian-Armorican Conjugate Margins in the Bay of Biscay

Abstract

Continental breakup involves multiple extension styles; each linked to unique structural and sedimentary processes. High-angle, low-angle, listric and antilistric normal faults develop at different stages and positions during rifting. The Bay of Biscay and its margins, a prototypical example of magma-poor rifted margins during Jurassic-Cretaceous, offer insights into the development and interaction of these faults related to lithospheric rheology. To this end, a margin-to-margin crustal scale cross-section is constructed and sequentially restored from a detailed review of available geological and seismic data and gravity and magnetic modeling. The first rifting phase was controlled by high-angle normal faults affecting the strong upper levels of the crust, pre-thinned during the previous Permian-Triassic rifting. These faults rotated and converged at depth, forming a continent-dipping detachment in the weaker middle crust, enabling crustal breakup and mantle exhumation in the pre-thinned sector, and developing the supradetachment Asturias Basin. Partial melting during Permian-Triassic rifting strengthened the lower crust, enhancing crust-mantle coupling during the second rifting phase. During mantle exhumation, weak serpentinized peridotites facilitated the formation of antilistric faults, coinciding with an abrupt increase in extension rates. Subsequently, the rates of extension began to decrease, culminating in the upwelling of the asthenosphere to form a proto-oceanic ridge before the spreading cessation. Finally, the North-Iberian Margin underwent a moderate inversion during the Alpine contraction. These results shed light on the influence of prior tectonic events, interactions between normal fault systems, and their link to lithospheric rheology during rifting, with implications for other magma-poor rifted margins.