CENOZOIC MAGMATIC AND STRUCTURAL EVOLUTION OF THE CENTRAL MOJAVE DESERT, CALIFORNIA: NEW CONSTRAINTS FROM 40AR/39AR GEOCHRONOLOGY AND THERMOCHRONOLOGY

New detailed mapping, structural studies, and 40Ar/39Ar geochronology-thermochronology focused on the central Mojave metamorphic core complex (CMMCC) and surrounding ranges (Calico Mts., Mud, Gravel, and Kramer Hills, Lead Mt., Boron) builds on the large existing geologic database (work by T. Dibblee, R. Dokka, A. Glazner, D. Walker, J. Bartley, J. Fletcher, and others) to shed new light on the early to middle Cenozoic tectono-magmatic evolution of the central Mojave Desert, California. Major late-Cretaceous crustal thickening, metamorphism, and plutonism (e.g. Fletcher, 2002 GSA Mem. 195) was closely followed by rapid cooling and exhumation of basement rocks from ~70 to 65 Ma in an area extending from Barstow to the Rand Mountains. Much of the amphibolite- to greenschist-facies NE-directed mylonitic shear fabric in the footwall of the CMMCC appears to be related to this older unroofing event, suggesting a polygenetic (Laramide and Miocene) core complex exhumation history. A thermochronological transect across the CMMCC footwall indicates that renewed exhumation by rapid NE-directed slip on the bounding detachment fault occurred between ~21 and 17.5 Ma, with ~15-30 km of Miocene slip, at rates of >= 1cm/yr. Isochronous Miocene temperatures differences across the footwall suggest it has been tilted SW ~30-60°.

Cenozoic basalt to rhyolite volcanic activity north of Barstow began ~22 Ma and appears to be distinctly episodic, with widespread eruptions mainly at 22-21 Ma, 19.5-18.5 Ma, and 17.2-15 Ma, and no clear spatial trends. Sedimentation occurred in broad basins throughout the early to middle Miocene (pre-, syn-, and post-extension), with dramatic lateral facies variations from coarse-grained volcaniclastic and plutoniclastic deposits in subaerial and nearshore environments to fine-grained lacustrine deposits offshore. The traditional subdivision of these sedimentary sequences into coarse-grained, “syn-extensional” Pickhandle Formation, and fine-grained lacustrine, “post-tectonic” Barstow Formation is oversimplified and breaks down in detail.