Late Cretaceous-Paleocene Extensional Collapse and Disaggregation of the Southernmost Sierra Nevada Batholith

Abstract

Geobarometric studies have documented that most of the metasedimentary wall rocks and plutons presently exposed in the southernmost Sierra Nevada batholith south of the Lake Isabella area were metamorphosed and emplaced at crustal levels significantly deeper (∼15 to 30 km) than the batholithic rocks exposed to the north (depths of ∼3 to 15 km). Field and geophysical studies have suggested that much of the southernmost part of the batholith is underlain along low-angle faults by the Rand Schist. The schist is composed mostly of metagraywacke that has been metamorphosed at relatively high pressures and moderate temperatures. NNW-trending compositional, age, and isotopic boundaries in the plutonic rocks of the central Sierra Nevada appear to be deflected westward in the southernmost part of the batholith. Based on these observations, in conjunction with the implicit assumption that the Sierra Nevada batholith formerly continued unbroken south of the Garlock fault, previous studies have inferred that the batholith was tectonically disrupted following its emplacement during the Cretaceous. Hypotheses to account for this disruption include intraplate oroctinal bending, W-vergent overthrusting, and gravitational collapse of overthickened crust. In this paper, new geologic data from the eastern Tehachapi Mountains, located adjacent to and north of the Garlock fault in the southernmost Sierra Nevada, are integrated with data from previous geologic studies in the region into a new view of the Late Cretaceous-Paleocene tectonic evolution of the region. The thesis of this paper is that part of the southernmost Sierra Nevada batholith was unroofed by extensional faulting in Late Cretaceous-Paleocene time. Unroofing occurred along a regional system of low-angle detachment faults. Remnants of the upper-plate rocks today are scattered across the southern Sierra Nevada region, from the Rand Mountains west to the San Emigdio Mountains, and across the San Andreas fault to the northern Salinian block. Batholithic rocks in the upper plates of the Blackburn Canyon fault of the eastern Tehachapi Mountains, low-angle faults in the Rand Mountains and southeastern Sierra Nevada, and the Pastoria fault of the western Tehachapi Mountains are inferred to have been removed from a position structurally above rocks exposed in the southeastern Sierra Nevada and transported to their present locations along low-angle detachment faults. Some of the granitic and metamorphic rocks in the northern part of the Salinian block are suggested to have originated from a position structurally above deep-level rocks of the southwestern Sierra Nevada. The Paleocene-lower Eocene Goler Formation of the El Paso Mountains and the post-Late Cretaceous to pre-lower Miocene Witnet Formation in the southernmost Sierra Nevada are hypothesized to have been deposited in supradetachment basins that formed adjacent to some of the detachment faults. Regional age constraints for this inferred tectonic unroofing and disaggregation of the southern Sierra Nevada batholith suggest that it occurred between ∼90 to 85 Ma and ∼55 to 50 Ma. Upper-plate rocks of the detachment system appear to have been rotated clockwise by as much as 90° based on differences in the orientation of foliation and contacts between inferred correlative hanging-wall and footwall rocks. Transport of the upper-plate rocks is proposed to have occurred in two stages. First, the upper crust in the southern Sierra Nevada extended in a south to southeast direction, and second, the allochthonous rocks were carried westward at the latitude of the Mojave Desert by a mechanism that may include W-vergent faulting and/or oroclinal bending. The Late Cretaceous NNW extension of the upper crust in the southernmost Sierra Nevada postulated in this study is similar to Late Cretaceous, generally NW-directed, crustal extension that has been recognized to the northeast in the Funeral, Panamint, and Inyo mountains by others. Extensional collapse of the upper crust in the southern Sierra Nevada batholith may be closely linked to the emplacement of Rand Schist beneath the batholith during Late Cretaceous time, as has been suggested in previous studies.