Extensional fault geometry and evolution within rifted margin hyper-extended continental crust leading to mantle exhumation and allochthon formation

Abstract. Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the top-most mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface which develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling hinge model of Buck (1988) and compare modelling results with the seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on seismic reflection data can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking and the development of new high-angle short-cut fault segments within the hanging-wall. This results in the transfer and isostatic rotation of triangular pieces of hanging-wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few km in lateral extent and thickness. While earthquake seismology favours a planar fault geometry with in the brittle seismogenic crust, seismic reflection imaging suggests a more listric geometry. Our modelling results show that a sequence of extensional listric or planar faults with identical parameters (i.e. location, heave, surface dip, Te) produce very similar sea-bed bathymetric relief. Listric and planar fault geometries do however produce distinct Moho and allochthon shapes. We propose that the initial fault geometry, prior to flexural isostatic rotation, is planar in the seismogenic crust becoming listric at depth as the brittle plastic transition is approached. Extensional faulting and thinning of hyper-extended continental crust may eventually lead to mantle exhumation. Where extensional faulting is in-sequence, this results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast out-of- sequence faulting results in a transition to exhumed mantle with bathymetric relief.