Cocorp deep seismic reflection profiling in the northern Sierra Nevada, California

Abstract

A COCORP seismic reflection profile across the northern Sierra Nevada in California shows several east‐dipping zones of discontinuous reflections. Correlation with surface geology suggests that these zones probably originate from faults of the Foothills fault system. In particular, the Melones fault, which coincides with the “Mother Lode” of the central and southern Sierra foothills, appears to be marked by prominent reflections in the midcrust. Migration of the COCORP data suggests that these faults are approximately planar, have moderately steep east dips (35°–47°), and penetrate at least to midcrustal depths (>20 km). At present it is unclear whether these faults are primary Nevadan thrusts, “late” Nevadan backthrusts (retrocharriage), or younger Cretaceous or Cenozoic faults, also known to occur in the region. Other more problematic features imaged on the profile include a prominent west‐dipping zone of reflections in the midcrust beneath the Eastern belt, and subhorizontal reflections at 22‐ to 26‐km depth beneath the Tahoe graben. The former might represent a west‐dipping thrust analogous to the Taylorsville thrust cropping out to the north of the survey route. The latter might represent the base of the Sierra Nevada batholith, the westward extension of any one of several thrust systems cropping out in Nevada, a low‐angle extensional detachment, or Moho.