Miocene Low-Angle Normal Faulting and Dike Emplacement, Homer Mountain and Surrounding Areas, Southeastern California and Southernmost Nevada
Miocene low-angle normal faulting and dike emplacement, Homer Mountain and surrounding areas, southeastern California and southernmost Nevada JON E. SPENCER* U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025 ABSTRACT tions, differed radically from the state of that collectively accommodated as much as 50% stress in the upper plate, as inferred from to 100% extension of upper-plate rocks (Ander- Homer Mountain and surrounding regions fault geometry. Low-angle faulting and east- son, 1971). In many areas, normal faults within are within, or adjacent to, the western part of northeast-west-southwest distension of up- upper-plate rocks merge with, or are truncated a broad region of low-angle normal faults ex- per-plate rocks reflect regional reduction of by, a basal, subhorizontal fault often referred to posed within the lower Colorado River compression in the east-northeast-west- as a "detachment fault" (for example, see Davis trough. During middle Miocene time, upper- southwest direction and associated large- and others, 1980). The term "detachment fault" plate rocks in the Homer, Sacramento, Dead, scale east-northeast-west-southwest crustal is used here to indicate a low-angle normal fault and Newberry Mountains moved eastward or extension. In contrast, concave-upward flex- that formed at a low angle (for example, Wer- northeastward, relative to the lower plate, ure of the lower plate, in response to tectonic nicke and others, 1984; Reynolds and Spsncer, above single or multiple low-angle normal denudation and resultant isostatic uplift, is in- 1985). The interpretation that detachment faults faults. Deposition of coarse clastic sedimen- ferred to have produced local subhorizontal are rooted faults that accommodate crustal ex- tary rocks occurred during extensional fault- compression at shallow crustal levels in the tension (Wernicke, 1981; Howard and John, ing and was accompanied by, and closely lower plate that overwhelmed the regional 1983; Davis and others, 1983; Allmendinger followed by, eruption of basaltic volcanics.
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