Mylonite, cataclasite, and gouge: Reconstruction of mechanical heterogeneity along a low-angle normal fault: Death valley, USA

The spectrum of slip behavior in crustal faults generates various rock types that can inform the mechanics of earthquake genesis. However, a single fault exposure may contain evidence of slip at various depths and temperatures due to progressive fault rock formation and overprinting during exhumation. Here, we unravel the spatiotemporal evolution of mechanical transitions along the Boundary Canyon detachment, a low-angle normal fault northeast of Death Valley, USA. Field, microstructural, and geochemical characterizations of fault rocks are compared to existing laboratory experiments and combined with a thermo-kinematic model of fault evolution. Together, these constrain the depths of mechanical transitions along the fault and reveal the evolution of earthquake nucleation zone thickness. Fault exposures from different initial paleodepths passed through the mechanical transitions during footwall exhumation, resulting in overprinting of mylonite by cataclasite and ubiquitous late formation of foliated, illite-rich gouge within the uppermost crust. We present evidence of coseismic low-angle normal fault slip (e.g., injection veins of cataclasite, laminar and grain-inertial fluidization). Coseismic slip likely nucleated at strength contrasts within the fault zone (i.e., contacts between quartzite breccia and calc-mylonite; quartz ribbons and mylonite matrix; breccia and clay gouge) at ≈ 5–9.5 km depth. Observations including mutually overprinting cataclasite/ultramylonite and exposures of pulverized gouge support that dynamic rupture propagated down-dip through the brittle-ductile transition zone (≈10–11 km depth) and up-dip through velocity-strengthening fault patches (≈0–5 km depth). Rapid fault exhumation increased the geotherm, leading to upward advection of the brittle-ductile transition and shallowing/thinning of the earthquake nucleation zone. This process may explain the rarity of large magnitude earthquakes on low-angle normal faults.