PRE- AND POST 9.5 MA SHEAR ZONE ACTIVITY AT THE MORMON POINT TURTLEBACK, DEATH VALLEY, CALIFORNIA

The Mormon Point turtleback, on the western front of the Black Mountains, consists of a footwall of ductiley deformed rocks, a brittle shear zone, and a hanging wall of faulted and fractured upper crustal rocks. The footwall rocks consist of a mid-Proterozoic basement gneiss and a late-Proterozoic marble generally assigned to the Noonday Dolomite. Both units contain mylonitic fabrics with a top-to-the-NW sense of shear, consistent with the regional direction of extension. The footwall is intruded by 9.53 +/- .04 Ma. silicic dikes (U-Pb zircon: Miller & Friedman, 2003). These dikes cut most of the footwall but are deflected to the NW near the upper 30-50 meters of exposure. Their angle of deflection is approximately 46 degrees to indicate a shear strain of slightly more than 1. Additionally the amount of NW-directed deflection for each dike amounts to less than 100m of post intrusion displacement.

The deflection of the dikes indicates either two distinct shear episodes, before and after intrusion, or one protracted event. We favor two distinct periods because the 10.44 +/- .22 Ma Smith Mountain Granite (Miller et al., this volume) also cuts most of the shear zone. Therefore the vast majority of ductile extensional strain on the Mormon Point turtleback occurred prior to 10.44 +/- .22 Ma and only limited ductile strain occurred after 9. 53+/- .04 Ma. This evidence supports those models that call for most crustal extension in the region to have occurred earlier than 11 Ma.