THERMO-KINEMATIC MODELING OF DETACHMENT FAULTING: FUNERAL MOUNTAINS, CALIFORNIA

Low angle normal faults (detachments) play a fundamental role in lithospheric strain and pose potential seismic hazard, but the mechanics of their formation and slip remain enigmatic. Numerical simulations and fault rock studies often invoke strain softening or weak fault-zone materials to explain low-angle frictional slip, but many continental detachment faults initiated and evolved in strong rocks at peak crustal strength. Thus a universal physical mechanism for detachment fault initiation is lacking.

To explore this problem, we have generated thermo-kinematic models of footwall exhumation below the Boundary Canyon detachment in Death Valley, California. The models simulate advection and conduction of heat in a 2D grid using velocities generated from geologic reconstructions (made using Move structural modeling software; thermal model using FetKin code). Particles placed along the detachment fault are tracked through temperature-time space, and model thermo-chronometric ages are calculated. Model ages are compared with muscovite ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He thermo-chronometric data from the footwall (Wells et al., in prep). Fault geometry and exhumation pathways are adjusted until model and actual closure ages converge. The iterations produce incremental fault geometries and surrounding thermal states, which are constrained by metamorphic phase relations and pre- to syn-extensional Cenozoic basins.

Mechanical analysis of the initial fault suggests that rotation of the maximum principal stress away from vertical is required to breach a failure envelope of modest static friction (0.4). Continuum numerical models have shown that stress field rotation is possible through shear tractions or flexural forces applied to the base of the brittle crust via lower crustal flow (Yin, 1989) or remnant crustal root isostasy (Spencer and Chase, 1989), respectively.