Coexisting ductile and brittle deformation shape magma-poor margins

Abstract:

Rifted margins are the places where continental break-up, mantle alteration and new oceanic crust formation occur. Many conjugate margins portray a striking structural asymmetry, with the wide side characterised by large-offset, low-angle detachment faults, whose formation mechanisms and role in the lithosphere's asymmetric extension remains much debated. Here we combine observations and a novel numerical modeling technique to show that the configuration of magma-poor margins is explained by the across-fault lateral coexistence of ductile shearing at low-angle, leading to detachment formation in the hangingwall of active faults, and brittle faulting providing water for mantle serpentinisation in their footwall. The detachment is a composite structure formed by the propagation at depth of high-angle sequentially active faults, as low-angle, narrow ductile shear zones, in a restricted lower crustal channel above the Moho, which back-rotated to sub-horizontal once inactive. Contrary to well-established ideas suggesting the detachment caused conjugate asymmetry and formed only after complete crustal embrittlement, we demonstrate that the detachment is the culmination of an asymmetry formation process that started earlier in the rifting history and requires some ductile deformation. Thus small across-fault variations between brittle and ductile deformation result in large-scale detachment formation and shape overall margin asymmetry and mantle serpentinisation patterns.