TRANSIENT SHORTENING STRAIN ACROSS THE CRESCENT VALLEY NORMAL FAULT BASED ON SPACE-GEODESY AND PALEOSEISMOLOGY, NORTH-CENTRAL NEVADA, BASIN AND RANGE PROVINCE

Coordinated geologic and geodetic measurements across the Crescent Valley fault (CVF), Nevada, reveal a profound mismatch in deformation rates. Since 1996, the two ranges on either side of the CVF have been moving toward each other at ~ 2 mm/yr, indicating shortening. In contrast, new reconnaissance mapping and paleoseismological analyses along the CVF also indicate that this fault is one of the more active normal faults of the Basin and Range province. The 50 km long Cortez Mountains range front is characterized by relief of up to 1.3 km, steep (up to 36°) triangular facets, and young (late Pleistocene to late Holocene) alluvial fans cut by normal fault scarps. Vertical displacement across the CVF is ~3 km; since 15 Ma the average long-term vertical displacement rate is ~0.2 mm/yr. Topographic profiling shows that fault scarps, 2-7 m high, are the result of a single rupture event and cut late Holocene alluvial fans. A trench across a faulted alluvial fan at Fourmile Canyon reveals a vertical displacement of ~4.5 m distributed across two normal faults. 14C on charcoal from a buried offset surface in the hanging wall of the trench and from the base of the overlying colluvial wedge tightly bracket the age of the most recent earthquake to between 2.8 ± 0.1 and 2.7 ± 0.1 ka (calibrated AMS ¹⁴C dates). At present, ~3 ka after the most recent event, between 15% and 75% of the average extensional strain should have re-accumulated across the fault, which should translate into a geodetically measurable velocity. Combined, the geodetic and geologic data imply that the shortening rate of 2 mm/yr is transient, and limited in duration to between 5 years (the time span of geodetic measurements) and ~3 ka, the time at which extensional strain was presumably still accumulating.