GEOLOGIC EVIDENCE OF EARTHQUAKES ALONG A MIOCENE DETACHMENT

The mechanics of slip on low-angle extensional faults (LANFs) is unfavorable in Andersonian mechanics, creating a long-standing conundrum in tectonics. However, other field evidence implies that LANFs may sometimes generate earthquakes rupturing to the surface. We present preliminary observations from the Miocene Waterman Hills Detachment (Mojave Metamorphic Core Complex), which juxtaposes Miocene volcanic and sedimentary sequences onto amphibolite-facies metasedimentary and basic rocks, intruded by syn-extensional granodiorites. We confirm the syn-exhumation ductile strain history described by previous works and add to it, by documenting for the first time, three subsequent stages of brittle deformation that records earthquake ruptures extending into the near-surface.

Strong L-tectonites began to form at amphibolite facies concurrent with the intrusion of granitoids at ~22 Ma (zircon U-Pb ages by Walker et al., 1995). As the footwall domed and exhumed through greenschist-facies conditions, extension localized, resulting in the development of extreme L-tectonites and ultramylonites. The transition to brittle shear strain at low angle is recorded by gently-dipping chlorite-epidote-cemented cataclasites. Meter-thick lenses of pulverized rock are found in meta-chert and quarto-felspathic mylonites in the uppermost 20-30 m of the footwall. The final stages of brittle faulting record the interaction of volcanic rocks and fluids with the crystalline footwall, through tourmaline-cemented cataclasites and fault mirrors, crosscut by steeply-dipping calcite/siderite-hematite cemented faults cutting both the footwall and hanging wall volcanic rocks, pinning the current juxtaposition during final slip on the detachment.

The discovery of pulverized rocks isolated to the lithologies which are most rigid at low temperature is consistent with stress anomalies caused by propagating earthquake rupture into the shallow seismogenic zone at sub-metamorphic conditions. We will further elucidate the P-T-x path related to the detachment embrittlement to refine the seismogenic history.

References

Walker et al., 1995. Connection between igneous activity and extension in the central Mojave metamorphic core complex, California. J. Geophys. Res. Solid Earth, https://doi.org/10.1029/94JB03132