PLIO-PLEISTOCENE DEFORMATION AT ALVORD MOUNTAIN, CENTRAL MOJAVE DESERT, CA

Neogene to recent displacement between the Pacific and North American plates is accommodated partly by strike-slip faults in the Mojave tectonic block, east of the main San Andreas fault. However, estimates of slip along individual faults within the Mojave block are variable and, in some cases, are poorly constrained. Slip estimates depend critically on understanding Tertiary to Recent deformation between major fault-bounded blocks. Alvord Mtn. sits within a crustal block that is bounded to the north and south by major EW-striking high-angle faults, and previous regional studies have suggested that Alvord Mtn. has been rotated clockwise about a vertical axis from 20 to as much as 50 degrees. Alvord Mtn. was thus studied to better understand how deformation within the block is related to displacement on the major block-bounding faults. At Alvord Mountain, early Miocene to Pleistocene volcanic and sedimentary strata rest nonconformably on Paleozoic-Mesozoic igneous and metamorphic rocks, and are folded into a broad NNW-plunging anticline. The anticline is cut by predominantly NE-striking faults, but a few earlier faults that cut out Tertiary strata are folded and offset by later faults. Along the major mapped faults, numerous small faults with a few meters to a few 10's of meters of offset are observed. These smaller faults also strike predominantly to the northeast. Offset strata and measured fault striae indicate predominantly normal left-oblique slip across the faults. Minimum combined displacement on mapped faults is at least one km. An upper bound on slip across the mapped faults is difficult to estimate but it cannot be greater than slip on the Afton fault (approximately 5 km), which forms the southeastern boundary of Alvord Mountain. The timing of faulting is poorly constrained but many faults cut Plio-Pleistocene gravels. These results imply that: (1) Alvord Mtn. cannot be treated purely as a rotating rigid crustal block; (2) structures that are compatible with both transpression (fold) and transtension (faults with a normal component of displacement) may be superimposed between major block-bounding faults; (3) detailed study of structures between block-bounding faults reveals a complex strain history that is important to incorporate into regional shear zone models.