SUBVOLANIC NORMAL FAULTING AND HYPABYSSAL DIKE INTRUSION IN THE LITTLE CHUCKWALLA MOUNTAINS

Geologic mapping near Graham Pass in the Little Chuckwalla Mountains has revealed complex patterns of high-angle normal faulting and dike intrusion interpreted to have occurred in a subvolcanic setting. Local stratigraphy comprises Proterozoic and Mesozoic crystalline rocks overlain nonconformably by over 1 kilometer of Late Oligocene to Early Miocene sedimentary and volcanic rocks.

The Tertiary sequence, exposed over an area of 25 km², is a complex homocline that strikes N30°E and dips southeast between 10° and 45°. A set of normal faults strikes northwest and breaks the lower part of the homocline into several steep-sided grabens. The main graben is 1 kilometer wide and 450 meters deep, based on stratigraphic separation across bounding faults. Hypabyssal dacite and rhyolite dikes intruded the faults bounding the main graben. These dikes, part of a swarm that also includes andesite dikes, are up to 50 meters thick, and can be traced along strike for over 3 kilometers. The volcanic and sedimentary rocks within the main graben were subjected to intense hydrothermal alteration. The dikes fed eruptions of lava flows and pyroclastic deposits now exposed in the stratigraphically higher parts of the Tertiary sequence. Graben formation and dike intrusion were coeval, and record mid-Tertiary extension along a northeast-striking axis.

The Tertiary sequence and the older structures described above occupy the hanging wall of a steep, northeast-striking normal fault (Graham Pass fault) that roughly parallels Graham Pass Road. The southeast-dipping Graham Pass fault crosscuts the dike swarm, and therefore post-dates graben formation and dike intrusion. Striations on Graham Pass fault surfaces indicate dip-slip movement. Minimum dip separation is 500 meters.