LATE CRETACEOUS MAGMATIC-TECTONIC EVOLUTION OF THE CENTRAL MOJAVE DESERT

Laramide orogenesis in the Mojave Desert region is hypothesized to have initiated as one or more seamounts subducted beneath the Late Cretaceous continental margin. Based on published and recently acquired geochronology, Late Cretaceous plutonism is continuous in this region from 83 – 74 Ma; therefore, magmatic rocks represent a critical record to test models of the tectonic transition from steep slab to shallow slab subduction during the Laramide orogeny. Major element, trace element and isotope geochemistry of Cretaceous plutons from Joshua Tree National Park and the Cadiz Valley batholith in the central Mojave Desert are consistent with an origin from a subduction environment and lack significant deflections through time, as would be predicted in models that call for the convective loss of the lower crust and asthenospheric ingress. Pluton trace element chemistry implies a thick crustal source for all Late Cretaceous Mojave Desert plutons, particularly for the Cadiz Valley batholith, which intruded from 76.5 – 74.5 Ma. Structural evidence also indicates mid-crustal compression in the western-most Joshua Tree National Park region (Friesenhahn, 2018). All data from plutons in the central Mojave Desert are consistent with eastward-migrating arc magmatism continuing until ~74 Ma, synchronous with or followed shortly thereafter by extensional unroofing (Wells and Hoisch, 2008). These results are inconsistent with a ~85 Ma docking of a Shatsky Rise conjugate initiating a very shallowly subducting slab. Results are consistent with overall shallowing of subduction (Jones et al., 2011), the shutoff of magmatism in the region, and a ~70 – 65 Ma dramatic eastward migration of magmatism (Chapman et al., 2018) driven by the passage of a seamount beneath the Mojave Desert. Additional future work on the Teutonia Batholith will elucidate the state of magmatism in the Mojave during to 88 – 82 Ma period, synchronous with the Sierra Crest magmatic event to the north in the Sierra Nevada batholith.