Quaternary faulting history along the Deep Springs fault, California

Abstract

New geologic mapping, structural studies, geochronology, and diffusion erosion modeling along the Deep Springs fault, California, shed light on its Quaternary faulting history. The Deep Springs fault, a 26km-long, predominantly north-northeaststriking, west-northwestdipping normal fault bounding the eastern side of Deep Springs Valley, cuts Jurassic batholithic rocks nonconformably overlain by middle Miocene to Pleistocene stream gravels, coarse-grained sand, tuffaceous sand, unwelded to partially welded tuff, and Bishop ash, as well as Quaternary coarse- to finegrained alluvial fan deposits. The 40 Ar/ 39 Ar geochronology yields ages of 3.09 0.08 Ma for the unwelded tuff and 753 4k a for the Bishop ash. Holocene debris flows, a landslide, and recent alluvium bury the youngest fault scarp. The Deep Springs fault is characterized by multiple fault planes and fault scarps that become progressively younger toward the basin. The dip of the fault plane varies from 20 to 87 and fault-plane and striation measurements indicate an average orientation of N23E, 47NW, and N77E, 49SW, respectively. The offset of Bishop ash and underlying tuff across the Deep Springs fault indicates horizontal extension and vertical slip rates of 0.7 and 0.9 mm/yr, respectively, since the eruption of the Bishop Tuff, and 0.2 and 0.2 mm/yr, respectively, since the eruption of the unwelded tuff. If the vertical slip rate since the eruption of the Bishop Tuff has remained constant through time, then slip along the Deep Springs fault began ca. 1.7 Ma. Younger fault scarps to the west of the bedrock fault cut Quaternary deposits; scarp offset ranges from 0.8 to 17.5 m and scarp slope angle ranges from 8 to 37. Topographic profiling of the smallest, least eroded fault scarps, with an average surface offset of 2.7 m, indicates that these scarps developed as the result of a single earthquake and ruptured an 20km-long segment of the fault. Radiocarbon analyses on detrital charcoal, located in the footwall block of one of these scarps, yield an age of 1.960 0.055 ka. Diffusion erosion modeling of these fault scarps yields an elapsed time of 1.7 0.5 k.y. since these fault scarps formed. Making reasonable assumptions about the depth of this earthquake and shear modulus, we estimate a moment magnitude, MW 7.0, for this earthquake. The Deep Springs fault is one of several displacement-transfer normal faults that define a zone of distributed deformation between subparallel right-lateral strike-slip faults east of the Sierra Nevada that make up the northern part of the eastern California shear zone. The young age and recent earthquake activity along the Deep Springs fault are consistent with a model proposed for the kinematic evolution of this part of the eastern California shear zone.