EXAMPLE OF A SUPRADETACHMENT BASIN WITHIN A PULL-APART TECTONIC SETTING - MORMON-POINT, DEATH-VALLEY, CALIFORNIA

The geological features now exposed at Mormon Point, Death Valley, rev eal processes of extension that continue to be active, but are conceal ed beneath the east side of Death Valley. Late Cenozoic sedimentary ro cks at Mormon Point crop out in the hangingwall of the Mormon Point lo w-angle normal fault zone, a fault zone that formed within a releasing bend of the oblique-slip (right-normal slip) fault zone along the eas t side of Death Valley. The late Cenozoic sedimentary rocks were part of the valley when the low-angle fault zone was active, but during lat e Quaternary time they became part of the Black Mountains block and we re uplifted. Rocks and structures exposed at Mormon Point are an examp le of the types of features developed in a releasing bend along the ma rgins of a major pull-apart structure, and in this example they are ve ry similar to features associated with regional detachment faults. The oldest sedimentary rocks in the hangingwall of the Mormon Point low-a ngle fault zone dip steeply to moderately east or north-east and were faulted and rotated in an extensional kinematic environment different from that recorded by rocks and structures associated with younger roc ks in the hangingwall. Some of the younger parts of the late Cenozoic sedimentary rocks were deposited, faulted and rotated during movement on the Mormon Point low-angle normal fault. Progressively, strata are less faulted and less rotated. The Mormon Point low-angle normal fault has an irregular fault surface whose segments define intersections th at plunge 18 degrees-30 degrees, N10 degrees-40 degrees W, with a maxi mum of 22 degrees, N22 degrees W that we interpret to be the general d irection of slip. Thus, even though Death Valley trends north, movemen t on the faults responsible for its formation was at least locally nor th-northwest. Gouge and disrupted conglomerates along the faults are i nterpreted to have formed either as adjustments to accommodate space p roblems at the corners of blocks or along faults that bounded blocks d uring their displacement and rotation. The younger units of the late C enozoic sedimentary rock sequence and the geomorphic surfaces develope d on them are rarely faulted, not rotated, and overlap the Mormon Poin t low-angle faults. Active faults cut Holocene alluvium north of the l ate Cenozoic rocks and form the present boundary between Mormon Point and the Black Mountains. The distribution of active faults defines a r eleasing bend that mimics the older releasing bend formed by the Mormo n Point low-angle fault zone. Rocks and structures similar to those ex posed above the Mormon Point low-angle fault zone are probably forming today beneath the east side of Death Valley north-west of Mormon Poin t.