Evidence of a Developing Cross Valley from Fault Scarp Morphology in the Eureka Valley, Eastern California, U.S.A

Eureka Valley, CA, NW Death Valley National Park, is one of a series of valleys formed by oblique extension within the Eastern California shear zone and Walker Lane belt. In Eureka Valley, extension is accommodated by the normal-oblique, north-south trending Eureka Valley fault zone (EVFZ) that bounds the valley’s east side at the foot of the Last Chance Range. A Mw 6.1 earthquake on May 17, 1993 is attributed to a northeast-southwest-trending trace of EVFZ located in the west-central portion of the valley. The earthquake produced ground deformation best detected by remote sensing with only minor ground cracks observed after the earthquake. The minor ground rupture is in stark contrast to the prominent fault scarps found along valley-bounding fault on the east side. Here we present measured fault scarp profiles offsetting Quaternary alluvial-fan deposits along traces of the EVFZ at the foot of the Last Chance Range. The displacements and maximum fault scarp slope angles are consistent with Mw 7-7.5 events. In the south, fault scarps offset older ~70 ka alluvial-fan deposits. In the north, fault scarps trend parallel to the 1993 aftershock pattern and offset ~30 ka alluvial-fan deposits with scarp angles suggesting events <2 ka. Based on the age differences of offset fan deposits, we infer that the southern EVFZ has been inactive for >30 ka. In contrast, the northern EVFZ has produced at least 2 earthquakes in the last 30 ka. The parallel trends of the northern scarps and the 1993 event suggest that a cross-valley fault is developing in Eureka Valley. The development of a cross-valley fault is consistent with clay models and field observations of a basin in the late extension phase. Cross-valley faults are known barriers and facilitators of oil migration; however, in the case of Eureka Valley, gravity data suggests that the basin sediments in the north are thicker than the south - suggesting that cross-valley faulting has persisted for a sufficient time to influence depocenter location.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California