USING FIELD EVIDENCE TO DISTINGUISH BETWEEN ROOTED AND ROOTLESS DETACHMENT FAULTS IN THE MORMON MOUNTAINS, SOUTHEASTERN NEVADA

Crustal extension of 25km in the Mormon Mountains, Nevada has been attributed to a low-angle fault, known as the Mormon Peak Detachment (MPD). However, an alternative interpretation of this feature as a series of independent rootless detachments is more consistent with the mapped surface geology. Structural contours show that the MPD is not smoothly domed, but consists instead of several discrete, planar surfaces. Kinematic indicators on these constrain motion as parallel to the present, local MPD dip, often perpendicular to the published W to WSW extension direction. Some mapped large high-angle normal faults are cut by the MPD, while others offset it by as much as 100 m, showing that it is neither the oldest nor the youngest extensional structure in the range. The fault zone is characterized by an asymmetric distribution of deformation, with as much as 20 m of clast-rotated megabreccia above the fault but less than 10- 0 m below. Clastic dikes intrude fault zone material into the upper plate. Veins in the fault zone do not show cross-cutting relations. This field evidence is consistent with a single episode of gravitationally driven upper plate motion and not with repeated seismic cycles on a through-going structure. Finally, a previously unrecognized high-angle normal fault mapped on the west side of the Mormon Mountains projects northward into a similar structure imaged in a seismic reflection profile further north and accounts for differential rotation of Paleozoic strata in the south. We suggest that the evolution of the Mormon Mountains was controlled by this structure which built up an area of high relief that partially collapsed in a series of coherent block slides along rootless detachments. Our studies have found no compelling evidence to support the published interpretation of rooted low-angle normal faults, and therefore that estimates of extension across the Mormon Mountains may be an order of magnitude too large.