CENOZOIC EVOLUTION OF THE 3-D STRAIN FIELD IN THE RIVER MOUNTAINS OF SOUTHERN NEVADA: IMPLICATIONS FOR THE IMPORTANCE OF ORTHORHOMBIC FAULTS IN RAPIDLY EXTENDING TERRAINS

Orthorhombic faulting patterns exposed on the upper plate of a low-angle detachment fault (e.g. River Mountains) are important indicators of 3-D strain during extension. Comprised mostly of 13.5-12 Ma volcanic rocks, the structures record the transition from an early, detachment-style extensional episode (13-11 Ma) to high-angle normal faulting (post-5.5 Ma). This study characterizes the evolving 3-D strain field as a function of discrete fault geometries that have been attributed to either an early or late-stage of post-13.5 Ma extension. New 1:12,000 scale geologic mapping reveals five fault sets striking NW (2 sets), N (2 sets) and ENE based on cross-cutting relationships. Inconsistent structural relationships among fault sets and their bimodal distribution of slickenline rakes are consistent with slip transfer on orthorhombic faults. This fault pattern differs from conjugate fault theory in that a secondary fault set will cut the first set in addition to being cut by the primary set indicating coeval faulting. Preliminary results suggest an early stage of orthorhombic faulting and slip transfer among ENE-striking transfer and NW-striking normal faults prior to deposition of the 9-5 Ma Muddy Creek Formation (MCF). A younger extensional event deforms the MCF and contains a less complicated relationship between faults (i.e. mostly a single fault set with a few orthorhombic faults). These conflicting mechanical and structural styles of extension may reflect (1) an inheritance of older structures in the sub-stratum or (2) a transition into a more uniform and lower strain environment. A stereonet plot of 29 pre-MCF faults indicate three clustered odd-axis poles of principal shortening with a trend and plunge of 59°, 44°; 141°, 41°; and 241°, 15° respectively. This may suggest extension prior to MCF deposition alternated between a SW-NE and NW-SE orientation. A stereonet plot of 27 MCF faults reveals two poles of maximum shortening with a trend and plunge of 41°, 6° and 90°, 49° respectively. These MCF odd-axis poles are similarly expressed in the pre-MCF plot but are rotated 20-40° counterclockwise in the younger strata and may reflect structural variations in the sub-stratum. Horizontal odd-axis orientations (i.e. 241°, 15° in the pre-MCF set and 41°, 6° in the MCF set) require oblique slip on faults as expressed by measured slip rakes in outcrop.