Abstract

Homer Mountain and surrounding regions are within, or adjacent to, the western part of a broad region of low-angle normal faults exposed within the lower Colorado River trough. During middle Miocene time, upper-plate rocks in the Homer, Sacramento, Dead, and Newberry Mountains moved eastward or northeastward, relative to the lower plate, above single or multiple low-angle normal faults. Deposition of coarse clastic sedimentary rocks occurred during extensional faulting and was accompanied by, and closely followed by, eruption of basaltic volcanics. Upper-plate fault blocks of Miocene volcanic and sedimentary rocks and older, underlying crystalline rocks are tilted gently to steeply to the west or southwest. Low-angle normal faults have complex, sinuous traces due to the irregular form of fault surfaces. Antiformal warping and uplift of the lower plate about a north-south trending axis, due to tectonic denudation and isostatic rebound, divided the regionally east-thickening extensional allochthon into separate synformal and wedge-shaped components. The Homer Mountain area lies within, or adjacent to, the synformal component.

Lower-plate rocks are intruded by numerous, middle Miocene, east-west-trending dikes, which are in turn intruded by subhorizontal dikes. Both sets of dikes are cut and displaced by low-angle normal faults except on the west flank of the Newberry Mountains where an east-west–trending dike cuts the basal low-angle normal fault. K-Ar geochronologic and field data establish the approximate synchroneity of dike emplacement and low-angle faulting. The state of stress in the lower plate, as inferred from dike orientations, differed radically from the state of stress in the upper plate, as inferred from fault geometry. Low-angle faulting and east-northeast-west-southwest distension of upper-plate rocks reflect regional reduction of compression in the east-northeast-west-southwest direction and associated large-scale east-northeast-west-southwest crustal extension. In contrast, concave-upward flexure of the lower plate, in response to tectonic denudation and resultant isostatic uplift, is inferred to have produced local subhorizontal compression at shallow crustal levels in the lower plate that overwhelmed the regional extensional stress and prevented emplacement of dikes oriented perpendicular to the direction of regional extension. North-south crustal extension during dike emplacement appears to have resulted from minor divergence of lower-plate crustal blocks as they were displaced in a west-southwest direction away from the Colorado Plateau.

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