Co-seismic selective rock pulverization in the footwall of a Miocene low-angle normal fault

Large-magnitude earthquakes capable of rupturing to the surface have rarely occurred along detachment faults or low-angle normal faults (LANFs), leaving their seismic potential and related co-seismic damage poorly constrained. Here we document the presence of pulverized rocks formed during final slip stages of the Miocene Waterman Hills detachment (Mojave extensional belt, California).

The Waterman Hills detachment accommodated ~40-50 km of top-to-NE extension and juxtaposes syn-extensional Miocene volcanic and sedimentary sequences onto syn-extensional granodiorites intruded into amphibolite-facies meta-sedimentary and meta-igneous rocks. Early bivergent doming of the metamorphic core complex initiated at amphibolite-facies conditions and localized into NE-dipping mylonites in the greenschist facies, coevally with intrusion and stretching of the granodioritic pluton at ~7-10 km depths. Progressive strain localization led to greenschist-facies mylonitic and ultramylonitic horizons that were overprinted by brittle faulting. The latter includes: (i) gently-dipping (≤30°) anastomosing foliated cataclasites and breccias cemented by chlorite + quartz + epidote ± calcite ± albite, crosscut by (ii) pseudotachylytes and tourmaline-cemented ultracataclasites and fault surfaces, crosscut by (iii) steeply-dipping (>60°) calcite-hematite-cemented faults and veins. The tourmaline- and calcite-cemented faults crosscut both the footwall and hanging wall rocks, pinning the current juxtaposition during final stages of the detachment slip.

Patchy meter-thick lenses of pulverized siliceous rocks are found in the uppermost ~11 m of the footwall damage zone and show very little evidence of post-pulverization displacement. Pulverization occurred at the latest, shallowest stage of detachment faulting, only in the stiffest, fine-grained siliceous footwall lithologies, consistent with the inference that co-seismic tensile stress perturbation due to propagating seismic ruptures caused rock pulverization. The pulverized rocks recorded repeated events of extensional fracturing and healing.

We interpret these pulverized rocks to have recorded the cumulative effects of multiple M_W 5-6 earthquakes propagating to depths ≤ 2 km, in agreement with experimental constraints on dynamic rock pulverization. Our discovery represents the first documentation of dynamic off-fault damage in the footwall of a LANF and demonstrates that shallow portions of LANFs can locally experience co-seismic stress conditions sufficient to induce pulverization, despite their unfavorable orientation for slip.