Fault Geometry Evolution During Hyper-Extension: Formation of Sub-horizontal Reflectors and Allochthons

The geometry and evolution of extensional faults with large offsets during rifting leading to continental breakup is hotly debated. We examine, using flexural isostatic modelling, extensional fault geometry evolution within the hyperextended domain and the transition to exhumed mantle during magma-poor rifted margin formation. Flexural response modelling is used to predict the isostatic rotation and bending of the active fault plane and also the geometries of earlier faults within footwall and hanging-wall. Faults are assumed to have an initial steep dip of 60 at the surface. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on seismic reflection data, often interpreted as seismically active low angle faults, can be generated by the flexural isostatic rotation of faults with initially high angle geometry; modelling results show that there is no requirement for sub-horizontal active faulting. With increase in fault extension, flexural isostatic rotation results in the decrease in fault dip at the point of footwall emergence (i.e. the rolling hinge effect). The emergence angle  decreases to asymptotic values of ~ 30 , the precise value depending on Te and whether the initial fault geometry is listric or planar. Shallow emergent fault angles result in 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. 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. This indicates that sea-bed relief cannot be used to distinguish listric from planar fault geometry. Listric and planar fault geometries do however produce distinct Moho and allochthon shapes. 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. We illustrate these model predictions with examples from seismic reflection data.