RISE AND TILT OF METAMORPHIC ROCKS IN THE LOWER PLATE OF A DETACHMENTFAULT IN THE FUNERAL MOUNTAINS, DEATH-VALLEY, CALIFORNIA

The Funeral Mountains in eastern California preserve a record of Early Cretaceous (?) metamorphism followed by ductile deformation, uplift, and low-angle normal (detachment) faulting. Ar-40/Ar-39 age spectra in dicate that cooling and uplift of the lower plate began in Cretaceous time. Uplift was accommodated by normal-sense movement along a wide no rthwest dipping shear zone. Mylonitic fabrics, some of which have been dated as Late Cretaceous, deformed older high-temperature metamorphic textures. Analyses of shear bands, mica fish, sigma and delta porphyr oclasts, grain shape fabrics, and folds indicate that the upper surfac es moved toward 299-degrees +/- 12 (top to the northwest) relative to lower surfaces. Uplift continued until the near present, the youngest phase being accommodated by top-to-the-northwest movement along the de tachment fault, which formed subparallel to lower-plate mylonitic fabr ics. Fission track apatite data indicate that exposure of the lower pl ate to the surface occurred sometime after 6 Ma. Reconstruction along the movement vector places the Grapevine Mountains over the Funeral Mo untains, having been displaced at least 40 km. Isograds and thermobaro metry in pelitic schist from the lower plate indicate increasing press ures and temperatures of equilibration toward the northwest. The maxim um temperature and pressure was determined on a sample from Monarch Ca nyon using thermobarometry, 700-degrees-C at a depth of 32 km. At Chlo ride Cliff, 5 km southeast of Monarch Canyon, 4 samples yielded 575-de grees-600-degrees-C at depths of 19-27 km. At Indian Pass, 17 km south east of Monarch Canyon, a temperature of 490-degrees-C was determined. In the southern Funeral Mountains, about 50 km southeast of Monarch C anyon, conodont color alteration indexes indicate temperatures of 325- 425-degrees-C. These data indicate that the lower plate is presently t ilted strongly to the southeast from the orientation it maintained at the peak of metamorphism. Thermochronologic data (K-Ar on muscovite, b iotite, and hornblende, Ar-40/Ar-39 on hornblende, and fission track o n apatite, titanite, and zircon) indicate that both tilting and the tr ansition from ductile to brittle styles of quartz deformation are conf ined to the interval 21-6 Ma; during the latter part of this interval (11-6 Ma), rapid uplift and movement along the detachment fault are do cumented. The findings support current theories of detachment fault ev olution in which a dipping fault surface undergoes rotation to a subho rizontal orientation while the lower late undergoes a com arable tilt.