TRANSIENT VERSUS LONG-TERM STRAIN ACCUMULATION ON THE BLACKWATER FAULT, EASTERN CALIFORNIA SHEAR ZONE

The geodetic rate of strain accumulation across the Eastern California Shear Zone exceeds the summed long-term slip rate of active faults in the region. This discrepancy indicates either a transient high regional strain rate or that long-term slip rates are underestimated. This problem is particularly acute on the Blackwater fault, a right-lateral strike-slip fault northwest of Barstow, CA, where a recent geodetic study suggests 7±3mm/yr of transient strain accumulation that clearly exceeds the 1-2mm/yr long-term slip rate. However, this long-term slip rate of the Blackwater fault is estimated from offset of Mid-Miocene markers that pre-date the presently active strike-slip fault and an assumed onset of faulting between 6 and 10 Ma. A Pliocene or Quaternary onset of faulting could support a much higher long-term slip rate consistent with geodetic measurements. New geologic mapping and geochronlogic studies of offset Plio-Quaternary lava flows were conducted to test the slip rate and transient strain accumulation proposed for the Blackwater fault. Four distinctive basalt flows in the Black Mountain / Superior Valley region are correlated across the trace of the southern Blackwater fault. The lowest of these flows covered a fault line scarp, indicating faulting prior to emplacement of the flow. The northwest margins of the upper two flows are preserved on a low-relief surface formed by the lower two flows. This margin forms a piercing line offset 1.8±0.1 km in a right-lateral sense across the Blackwater fault. Existing geochronology from the southwest Black Mountains supports a latest Pliocene age of basaltic volcanism here, which is consistent with the degraded flow morphology and suggests a Quaternary slip rate on the southern Blackwater fault below 1 mm/yr. A dacite lava flow at Blackwater Well is offset between 0.2 and 0.5 km across the northern Blackwater fault. Although undated, similar flows in the nearby Lava Mountains are Late Pliocene age, again supporting a low (<1mm/yr) slip rate on the Blackwater fault. Geochronology of the offset lava flows, in progress, will further refine these slip rate measurements. Altogether, the data suggests that the Quaternary slip rate of the Blackwater fault is less than 1 mm/yr and that the geodetically measured strain accumulation here is likely to be a transient phenomenon.