Paleoseismic and geomorphic evidence for recent rupture on the Hidalgo fault, Eastern California Shear Zone

The Calico-Hidalgo-Mesquite Lake fault system has the highest slip rate of the Eastern California shear zone (ECSZ), but the southern portion (Hidalgo fault) has not been shown to participate in the temporal clustering of surface-rupturing earthquakes across this system. Does this fault participate in the temporal clusters, or does it produce major earthquakes at a different tempo than the surrounding system? The Hidalgo fault exhibits a geomorphically-youthful trace midway up the west flank of Hidalgo Mountain. Oblique-right slip has produced remote uphill-facing scarps that locally create small, sediment-filled depressions. To investigate the paleoearthquake history of the Hidalgo fault, we hand excavated two trenches which cross two parallel, sediment-filled depressions that overlie the width of the faulted zone. Additionally, we conducted local-scale tectonic geomorphic mapping and built a high-resolution topographic model. The stratigraphy within the small depressions consists of mostly massive-bedded, fine-grained sediments and hillslope/scarp-derived coarse-grained units. We found evidence for multiple earthquakes within the trenches based on dip-slip displacements of stratigraphic units, with the youngest event well-constrained by a truncated unit that is overlain by scarp-derived colluvium and a ~10 cm thick laminated horizon. Along strike from the offsets in the trench, the fault clearly displaces channels and talus lobes. We infer that the most recent event occurred during the late Holocene based on the shallow extent of faulting (upward termination ~20 cm from surface), the friable nature of the unfaulted sediments, and the youthful geomorphic preservation of surface rupture-related landforms. Thus, we suspect the age of the most recent event is consistent with the historical ECSZ cluster episode (present to ~1ka), but certainly not older than the penultumate (~5ka) cluster. Additional earthquakes potentially occurred during previous clusters. This evidence, along with trenching on the Mesquite Lake fault, represents two cases for a mid to late Holocene earthquake in this part of the ECSZ. Pending luminescence ages from the pre- and post-faulting stratigraphy will provide key constraints on the event timing.