Mesozoic Tectonic Evolution of the Tiefort Mountains, NE Mojave Desert, CA

Detailed geologic mapping, petrologic and structural analyses, and U-Pb dating in the Tiefort Mountains, northern Mojave Desert, California, provide constraints on Paleozoic paleogeography and Mesozoic intra-arc tectonics. Geologic mapping in the Tiefort Mountains was used to identify metasedimentary, metaplutonic, and undeformed plutonic rocks. The metasedimentary rocks consist primarily of quartzose schists and gneisses that are correlated with the late-Precambrian Cordilleran miogeoclinal-cratonal sequence of the Johnnie Formation, Stirling Quartzite, and Wood Canyon Formation. Recognition of Precambrian augen gneiss and metasedimentary rocks with miogeoclinal protoliths provides new data on the location of the cratonal-miogeoclinal and miogeoclinal/eugeoclinal paleogeographic boundaries in the Mojave Desert. The metasedimentary units are intruded by Jurassic granite and diorite orthogneisses. Late Jurassic mafic and felsic dikes, and Cretaceous granites and pegmatites intrude the schists, gneisses and cross-cut structural trends. Deformed rocks record amphibolite facies metamorphism and contractional deformation in the Tiefort Mountains. Mylonitic foliations and stretching and mineral lineations that developed at amphibolite facies conditions characterize the structural fabric in the region and are related to Middle to Late Jurassic southeast-vergent ductile shear zones. This early fabric is deformed into broad folds about northwest- and north-trending axes at South Tiefort and Tiefort Mountains, respectively. Pervasively annealed microstructures indicate that metamorphism outlasted deformation throughout the Tiefort Mountains. U-Pb dating of representative deformed and undeformed rock units constrains the timing of plutonism and deformation in the region. Deformation at South Tiefort Mountain is constrained by approximately 170 Ma deformed granite orthogneiss and undeformed cross-cutting 148 Ma dikes that are correlated with the Independence dike swarm of Chen and Moore (1979). At Tiefort Mountain, the age and nature of deformation are less well constrained. U-Pb analyses from deformed diorite and synkinematic biotite granite gneiss yield ages of 162 ± 8 Ma and approximately 105 Ma, respectively. The late Early Cretaceous age of the biotite gneiss indicates that a Cretaceous deformational event may have affected rocks at Tiefort Mountain. Dikes that cross-cut all fabrics at Tiefort Mountain are interpreted to be part of a regional 82-83 Ma intrusive event (Walker et al., 1990b; Miller et al., 1991). The timing of deformation and southeast-verging kinematic indicators within the South Tiefort shear zone indicates that this feature is similar in age and kinematics to ductile thrust faults in the Cronese Hills 20 km SE of the study area (Walker et al., 1990a). Recognition of Middle Jurassic structures in the Tiefort Mountains suggest the presence of a continuous belt of contractile deformation from the Central Mojave Desert and the Cronese Hills through the northern Mojave Desert to the East Sierran thrust system north of the Garlock Fault.