THE POINT OF ROCKS DETACHMENT: A MAJOR TRANSPRESSIONAL FAULT ALONG THE LAS VEGAS VALLEY SHEAR ZONE, SOUTHEASTERN NEVADA

The Las Vegas Valley shear zone (LVVsz) is a lateral fault along which about 50 km of right-lateral movement has taken place between ca.15-10 Ma as suggested by folded Miocene strata within a positive flower structure exposed north of the Specter Range. The shear zone extends northwestward about 100 km from Las Vegas to Mercury, Nevada where it bends westward beneath the southern Specter Range. Geologic mapping southwest of Mercury shows that the Point of Rocks detachment that crops out in the northernmost Spring Mountains is a major fault zone that dips gently north toward the Specter Range although the fault is locally folded about west-trending hinges. Rare kinematic indicators record south-directed transport. The detachment is marked by tens of meters of generally pebbly cataclastic breccia derived mainly from carbonate strata of the Late Cambrian Nopah Formation, which forms most of the exposed hanging wall. Beneath the breccia about 1.4 km of strata are missing between Nopah beds and fault slices of much lower Early Cambrian Wood Canyon Formation. The footwall comprises two parts: 1) Neoproterozoic and Cambrian strata folded about north-trending hinges and 2) an adjacent basin. The Nopah Formation and underlying fault rocks have been pushed across the folded strata of the footwall and into the adjacent lake where they are recorded by slab-like masses of breccia intercalated with bodies of carbonate clast conglomerate. The Point-of-Rocks detachment is a contractional feature that is compatible with transpression expected along the left-step restraining bend in the LVVsz.