THE CLARK MOUNTAIN FAULT COMPLEX: EVIDENCE FOR JURASSIC EXTENSION IN THE EASTERN MOJAVE

The Clark Mountain fault complex is comprised of two subparallel faults striking north-south along the eastern flank of Clark Mountain and Mohawk Ridge. The westernmost of the two faults dips gently to the west (10-30 degrees). Brecciation is absent in both footwall and hanging wall rocks. Along Mohawk Ridge, hanging wall rocks are comprised of Cambrian carbonates while the footwall is Proterozoic gneiss. To the north, footwall lithology is more variable consisting of both Cambrian and Proterozoic units. Footwall rocks are strongly folded, the folds consistently vergent to the northeast. The geometry and appearance of this fault suggest it is most likely a thrust fault, the Keaney/Mollusk Mine (KMM) thrust, proposed by Birchfiel and Davis (1971). North of the Colosseum Mine, a mid-Cretaceous (100 ma?) granitic pluton cuts the thrust, while south of I-15 Jurassic (167 ma) Delfonte volcanics lie in the footwall of the thrust. This constrains thrusting to late Jurassic-early Cretaceous. The second major fault lies to the east in the footwall of the KMM thrust. This fault is poorly exposed, but generally dips steeply (50-80 degrees) to the west. The footwall is comprised of Proterozoic basement while the hanging wall consists of brecciated lower Cambrian Zabriskie and Cararra Formations. This fault is characterized by dip-slip, west side down, normal movement. We term this fault the Clark Mountain normal fault (CM). Locally, the KMM thrust overrides the CM fault suggesting normal faulting predates thrusting. This is consistent with observations by Burchfiel and Davis (1988) that the South fault, south of I-15, and also a normal fault, predates KMM thrusting. We believe the South fault represents the southern extension of the CM fault. Since Delfonte volcanics are cut by the South fault, our model proposes mid-Jurassic extension (the CM fault and perhaps the Ivanpah faults) followed by later northeast-directed late Jurassic-early Cretaceous thrusting (KMM thrust).