Geomorphic analysis of late Quaternary faulting on Hilton Creek, Round Valley and Coyote warp faults, east-central Sierra Nevada, California, USA

Abstract

Geomorphic analysis of fault scarps and faulted surficial deposits allows for interpretation of late-Quaternary faulting on the Hilton Creek and Round Valley faults, two major range-front faults in the Sierra Nevada frontal fault system, as well as three faults on the north flank of the Coyote warp, a structure that separates the Round Valley fault from range-front faults to the south. Repeated movement on the two frontal faults during the last 15,000–25,000 years is evident from fault scarps on Tioga (oxygen isotope stage 2) surfaces. The scarp data indicate that vertical slip on the Round Valley fault may have been generally consistent along strike during this time period, and may have been characterized by 1.6–1.8 m of slip per faulting event. If so, 4 events may have occurred in either the last 7900–15,000 years or the last 13,000–15,000 years, 5–7 events may have occurred in the last 15,000–20,000 years, and 7–9 events may have occurred in the last 20,000–25,000 years, which suggests possible repeat times for faulting of 2000 to 4000 years for the Round Valley fault. Bevels in scarp slopes and displacement of probable Holocene-age surfaces support Holocene movement on the fault. On the Hilton Creek fault, scarps indicate that vertical slip increases northward along strike, reaching a maximum on the north side of McGee N Creek. Based on the size of scarps, individual displacements at McGee N Creek must have been either larger or more frequent than individual displacements on the Round Valley fault. The size of single-event displacements at McGee N Creek is unconstrained. However, if they have been on the order of 3–4 m per event, a size well within limits associated with normal faulting, then 7–9 faulting events could have occurred during the last 20,000–25,000 years, as proposed for the Round Valley fault, leaving open the possibility raised by other workers that the two faults may fail together. Holocene movement on the Hilton Creek fault is supported by morphology of the fault scarps and data from a trench at McGee N Creek that suggest a major surface-faulting event has occurred there within the last 3800 cal yr B.P. Estimating displacement on Tahoe (oxygen isotope stages 4–6) and pre-Tahoe (probably oxygen isotope stage 6) moraines from crest-height relations reveals that slip rates have been higher — perhaps twice as high — on the Hilton Creek fault at McGee N Creek than on the Round Valley fault throughout the late Quaternary. In contrast to the range-front faults, Coyote warp faults in the study area show no signs of Holocene movement. The lack of scarps on Tioga moraines, and the degraded morphology of scarps on adjacent Tahoe and pre-Tahoe moraines indicate that these faults have not ruptured in at least the last 15,000–25,000 years.