Dating the detachment fault system of the Ruby Mountains, Nevada: Significance for the kinematics of low-angle normal faults

Abstract

[1] The mechanics of low-angle normal faulting and metamorphic core complexes continue to be a subject of debate. We investigate the conditions, timing, and kinematics of slip in the late, upper-crustal stages of core complex evolution of the Ruby Mountains detachment fault at the well-exposed Secret Pass locality with an X-ray diffraction (XRD) and Ar-Ar study of clay gouge samples from three separate faults, two from the low-angle detachment system and one from a high-angle normal fault that soles into the main detachment fault system. XRD analysis and modeling of XRD analysis show that authigenic illite-rich illite/smectite (I/S) in gouge at Secret Pass is distinguishable from clay phases in hanging wall rocks because the I/S in the gouges contains only one-water layer as opposed to the more common two-water I/S phases found in both the hanging wall and footwall. Ar-Ar ages for the monomineralic one-water I/S found in the hanging wall high-angle fault, the main detachment, and a low-angle normal fault structurally above the main detachment are 11.6 ± 0.1 Ma, 12.3 ± 0.1 Ma, and <13.8 ± 0.2 Ma, respectively. The not-quite-flat Ar-Ar spectra indicate the gouge illites grew over some interval of time and not in discrete events. The nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11–13 Ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips <45° and possibly as low as 22°.