PALEOPROTEROZOIC KINEMATICS AND GEOCHRONOLOGY OF A CRYSTALLINE BASEMENT COMPLEX IN THE NOPAH RANGE, TECOPA PASS, CALIFORNIA

Paleoproterozoic thermal and deformational events that occurred in the central 1.8 Ga Mojave province during the assembly of southwestern Laurentia are recorded within basement units of the Nopah Range (Tecopa Pass, Calif.) located in the southeast portion of the Death Valley extensional corridor. The southern end of the Nopah Range exposes a crystalline basement complex comprised of a megacrystic granite and two distinct migmatite units overlain by unmetamorphosed Neoproterozoic continental margin sequences. Consistent with other portions of the Mojave terrane, the complex is strongly deformed and contains a pervasive upper amphibolite-facies metamorphic signature. A single deformation fabric (D1) is characterized in the southern Nopahs by a widespread NW-striking, E- and W-dipping foliation that is axial planar to sheath folds of migmatitic leucosome; a NW-SE, gently to moderately plunging stretching mineral lineation is present on foliation surfaces. D1 kinematics are dominated by an oblique shear sense with dextral and reverse components and interpreted to be southeast vergent. In situ EMPA monazite U-Th-total Pb geochronometric results on both the migmatitic and granitic units yield intracrystalline domain ages of 1762 ± 6 Ma and 1750 ± 6 Ma, respectively, suggesting probable initial crystallization at that time as a result of midcrustal anatexis during the waning stages of northward accretion of Mojave to the Archean Wyoming craton. Monazite overgrowths and prominent crystal replacement domains from both units yield younger ages of 1732 ± 4 Ma (migmatite) and 1727 ± 5 Ma (granite). Although it is difficult to assign a date and mechanism for the D1 event, collisional orogenesis between the Mojave and younger juvenile Yavapai province to the southeast is thought to have occurred between ca. 1740 and 1720 Ma. Thus, the ca. 1730 Ma date likely corresponds to widespread Yavapai orogenesis recorded in the central Mojavian, which involved transpression-related crustal thickening. Laurentide crustal growth can be regarded as a continuum of convergence between individual terranes; however the recognition of discrete pulses of tectonothermal events that record the accretion of tectonic blocks can lead to a more comprehensive understanding of the processes and rate of crustal growth.