Rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the Funeral Mountains, Death Valley, California

Citations Over Time

Abstract

The Funeral Mountains in eastern California preserve a record of Early Cretaceous (?) metamorphism followed by ductile deformation, uplift, and low‐angle normal (detachment) faulting. 40 Ar/ 39 Ar age spectra indicate that cooling and uplift of the lower plate began in Cretaceous time. Uplift was accommodated by normal‐sense movement along a wide northwest 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, σ and δ porphyroclasts, grain shape fabrics, and folds indicate that the upper surfaces moved toward 299° ±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 detachment fault, which formed subparallel to lower‐plate mylonitic fabrics. Fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 Ma. Reconstruction along the movement vector places the Grapevine Mountains over the Funeral Mountains, having been displaced at least 40 km. Isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest. The maximum temperature and pressure was determined on a sample from Monarch Canyon using thermobarometry, 700°C at a depth of 32 km. At Chloride Cliff, 5 km southeast of Monarch Canyon, 4 samples yielded 575°–600°C at depths of 19–27 km. At Indian Pass, 17 km southeast of Monarch Canyon, a temperature of 490°C was determined. In the southern Funeral Mountains, about 50 km southeast of Monarch Canyon, conodont color alteration indexes indicate temperatures of 325°–425°C. These data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism. Thermochronologic data (K‐Ar on muscovite, biotite, and hornblende, 40 Ar/ 39 Ar on hornblende, and fission track on apatite, titanite, and zircon) indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined 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 documented. The findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt.

Related Papers

• → Synchronous Development of Mylonite and Pseudotachylyte in Ductile Shear Zone: An Example from the Chitradurga Eastern Margin Shear Zone, Karnataka(2008)4 cited
• Discovery of sinistral-normal ductile shear zone in Proterozoic Bendong granodiorite pluton(2003)
• Detachment Fault in Dinggye Area in the Middle Part of the Himalayan Orogen(2003)
• Research on microstructures of the mylonite in Xilinhe region of Jilin province and its geological significance(2003)
• DETACHMENT FAULT IN DINGGYE AREA IN THE MIDDLE PART OF THE HIMALAYAN OROGEN(2003)