FAULT GEOMETRY AND KINEMATIC CONTROLS ON FAULT ROCK EVOLUTION IN EXTENSIONAL BASINS

The structural and rheological evolution of fault zones along mature active basin-bounding faults in the upper-crust influence their seismic behavior and potential for fluid permeability, relevant for understanding their earthquake hazards and commercial importance. Studies show that the structure and evolution of fault rocks are largely controlled by thermal and fluid history, and mineralogical alterations of the fault rocks. Yet, there remains a limited understanding of how along-fault variations in fault geometry and fault kinematics might modulate fault rock architecture and rheological evolution. To investigate this problem, we explore an excellently exposed segment of the NNW-trending Northern Black Mountain Fault, Death Valley, California where the fault zone defines a dextral strike-slip (-145° to -153° rake) dominated ~100-m wide left-bend 1.29 km (GG-section), to the north of which the fault defines a rectilinear slickensided principal slip surface (GC-section) and to the south of which it defines a <10-m wide left-bend with dextral oblique-normal slip kinematics (-24° to 56° rake; DC-section). Our field mapping of GG fault rocks reveals distinct hydrothermally-altered cataclasite bands defining a ‘reclined’ fold with a fault-parallel SE-plunging axis. Microstructure analysis of veining, microfracturing, brecciation, and their cross-cutting relationships in the cataclasite bands show a more evolved band of fault rocks at the core of the fold and a relatively less evolved brecciated fault rocks in the outer margins where juvenile thrust micro-faults suggest localized contraction within the fault zone. We interpret a restraining relay zone where transpressive kinematics and associated slip-obliquity along with cycles of earthquake rupture and fluid-rock interaction history control the spatial variation in maturity and exhumation of fault rocks. These observations provide insights into the rheological evolution of oblique-slip fault zones with major implications for fault mechanics and earthquake occurrences.