DEAD OR JUST RESTING? GEODETIC VERSUS GEOLOGIC SLIP RATES: PAHRUMP SEGMENT OF THE STATELINE FAULT SYSTEM, CALIFORNIA-NEVADA, USA

The Stateline fault system (SFS) is a northwest striking zone of distributed dextral faulting which defines the active eastern margin of the eastern California shear zone (ECSZ). The SFS extends along the California-Nevada border from interstate 15 and Ivanpah to northern Amargosa valley, approximately 20km from Yucca Mountain. Pahrump (2 km to the east) and Las Vegas (~40km to the east) are the nearest cities. The minimum long term geologic slip rate on the SFS is 2.3 ± 0.35 mm/yr based on a 30 ± 3 km offset of 13 Ma volcanics. This slip rate is 20% of the geodetic dextral displacement rate observed across the western Basin and Range but is two to three times the 0.7-1.2 mm/yr geodetic rate measured across the SFS. This suggests that the fault has either slowed down permanently and may be deactivating or that it is simply in a low slip rate transient period and will speed up again in the future. These two hypotheses have very diffferent implications for fault slip processes and regional strain patterns in the ECSZ. To understand the slip history and along-strike geometry of the SLF we are mapping the fault zone in detail with a current emphasis on the northern Pahrump segment in Stewart Valley (2 km west of Pahrump). Using Lidar data we have mapped offset Quaternary deposits and geomorphic features along the eastern margin of Stewart Valley, including, uplifted playa and alluvial fan deposits, offset alluvial fan surfaces, dextrally offset drainages, sag ponds, and push-up ridges. Limited age data and a geomorphic chronosequence suggest that the minimum Holocene slip rate for the SFS in southern Stewart Valley is 2.5 ± 1 mm/yr based on a 23m offset of a small drainage that cuts into an uplifted surface estimated to be early Holocene in age. If correct this estimate is consistent with the slip rate transient hypothesis for the SFS, which suggests that over longer time scales (1ka to 10ka), the geodetic slip rate could increase to match the long-term geologic slip rate.