GEOLOGIC MAP OF THE FAIRVIEW VALLEY AREA, SAN BERNARDINO COUNTY, CALIFORNIA

Previous geologic mapping in the Fairview Valley area of west central Mojave Desert has been mostly small scale or reconnaissance level. For this work Fairview Valley area was mapped in detail at a scale of 1:12,000. Google Earth and other air photos and GPS were also used to verify locations. Bedrock includes Late Proterozoic and Paleozoic carbonate dominated metasedimentary rocks, Permo-Triassic, Jurassic and Cretaceous intrusives, Jurassic Fairview Valley Formation and Lower Sidewinder volcanic rocks. Several alluvial units are also present. Geologic structure is complex, the result of multiple Mesozoic folding, metamorphic, intrusive, compression and extension deformational events. Cenozoic activity includes debris flows, landslides and active faulting continuing into modern time.

One of the most prominent features in the Fairview Valley area is the Helendale Fault Zone (HFZ), identified on the California State Alquist Priolo fault map as active (Hart 1977). The HFZ is considered to be a right lateral strike slip fault, postulated to have up to several kilometers of movement, and is one of several major northwest trending lateral faults. The HFZ has been traced for about 90 km. and has evidence of recurrent surface rupture in Holocene time (200-10,000 yrs). Alluvial and basement rocks in Fairview Valley all indicate about the same amount of displacement, and have been displaced together by right lateral faulting in a zone of faulting >2km wide that contains many faults, active during Pleistocene thru recent time.

Regional and local structural grain trends northwest parallel to the long axis of the valley. Aksoy (1993) interpreted the valley to be a graben, gravity data suggesting right stepping en echelon strands formed a pullapart basin up to 3 kilometers wide. His cross sections suggest Fairview Valley is fault bounded on the west and east. Recent detailed mapping indicates the HFZ is a long lived zone of structural weakness in which many faults are present within the valley and adjacent bedrock, some of which predate and or have been reactivated by the modern Helendale Fault. Accumulated evidence in Fairview Valley indicates the modern HFZ forms a zone >2.5 km wide in which numerous faults and fractures are present, some of which have been episodically active during Pleistocene thru recent time.