NEW SURFICIAL GEOLOGIC MAPPING REVEALS CHANGE FROM PLEISTOCENE DEXTRAL STRIKE SLIP TO HOLOCENE TRANSPRESSION ALONG THE SODA-AVAWATZ FAULT ZONE, EASTERN MOJAVE DESERT, CALIFORNIA

The Soda Mountains, eastern Mojave Desert, expose Precambrian and younger rocks that have been uplifted by Mesozoic and Cenozoic faulting. In the eastern Soda Mountains, adjacent to Soda and Silver Lake Playas, faulting includes several roughly parallel, NW-trending sub-vertical faults that offset Tertiary and Quaternary alluvial sedimentary deposits. This zone of deformation, the Soda-Avawatz fault zone (SAFZ) is considered to be the eastern most expression of the Eastern California Shear Zone (ECSZ). Previous work has used the displacement of upper Miocene(?) alluvial strata to infer dominantly dextral strike slip. However, whether upper Pleistocene and Holocene alluvium have been displaced along these faults, and by how much, remains an open question.

New detailed surficial mapping of the piedmont areas to the north, east and south of the NW-trending eastern spur of the Soda Mountains demonstrates that late Pleistocene (unit Qia2, ~70-120ka) units have been tilted and faulted, whereas younger Pleistocene (Qia1, ~20-40 ka) and Holocene (Qya4, ~9-14 ka) units have been uplifted and incised.

In the north, Qia2 surfaces have been back tilted such that the topographic gradient is opposite to the paleoflow direction, and along the eastern piedmont they have been dextrally offset by up to ~130 m, implying a slip rate of up to ~1.9 mm/yr.

Younger Qia1 and Qya4 units are not offset by faults. Instead, along the eastern piedmont theses surfaces exhibit decreasing vertical separation from NW to SE along the NW-striking eastern Soda Mountains front. Additionally, Qya4 alluvial surfaces in the southern Piedmont are deeply (>5 m) incised, with washes having wine glass-shaped cross sections. Together these observations indicate that Qia1 and younger alluvial surfaces have been recently tilted and incised. One possible model is SE-directed progressive uplift related to transpressional folding along the SAFZ.

These observations indicate a transition from dominantly strike-slip displacement along the SAFZ prior to Qia1 time, followed by transpressional folding. This implies that either dextral slip along this part of the ECSZ has moved to a different structure, such as the Bristol Mountains fault zone to the SE, or that deformation along this portion of the ECSZ has changed from strike slip to transpressional regimes.