NEW QUATERNARY GEOLOGY MAP OF FAULTS, NORTHERN MOJAVE DESERT, CALIFORNIA

Dozens of previously unmapped east- and northwest-striking faults displaying Quaternary offsets have been identified by recent US Geological Survey 1:100,000-scale mapping conducted in the northern Mojave Desert. Collectively these structures exhibit a pattern that is more complicated than previous models of discrete domains of like-trending structures. The newly mapped faults differ in location and pattern from previously mapped faults, much as the recent Landers and Hector Mine fault ruptures diverged from previously mapped faults. Many faults strike north to northwest and express dextral offset, and a domain of east-striking faults, previously identified, is verified. However, northwest-striking dextral and east-striking sinistral faults are interleaved in complex patterns that indicate they are mutually active, and are not isolated in separate domains. These new patterns suggest large-scale reorganization that has not been recognized by previous geologic mapping.

Detailed study of several faults at 1:24,000 scale provides local verification of the 1:100,000-scale mapping. Strike-slip faults with low slip rates typically have inconsistent topographic expression, making them difficult to recognize by stereoscopic methods and to trace for long distances in the field. However, we were able to identify many previously unmapped Quaternary faults by using the outcrop patterns of Quaternary and older deposits, coupled with detailed mapping of local fault scarps, faceted hillslopes, and ground water disruptions. Many previously mapped faults cut early to middle Quaternary deposits, but not younger deposits, indicating they have been inactive since the middle Quaternary. In contrast, fault scarps that cut late Quaternary deposits outline faults that strike obliquely to the older faults and appear to connect across the older fault systems. This supports the concept of a larger reorganization of faulting in the late Quaternary by providing evidence for both fault reorientation and crosscutting relations with previously mapped faults at many locations across the Mojave Desert.