Reconciling Baja-BC and Flat Shatsky Conjugate Subduction beneath Western U.S.

Eugene Humphreys, University of Oregon
Edward Clennett, University of Texas at Austin
Jonny Wu, University of Arizona
Nicholas Van Buer, Cal State, Pomona

Abstract

We reconcile the subduction of Shatsky Rise conjugate beneath southern California at 90 Ma with the presence of the Insular superterrane (Baja-BC) to the west of North America. The Shatsky Rise, which is currently 1100 km west of Japan, formed by plume volcanism at the Pacific-Farallon-Izanagi triple junction. We model formation of its conjugate oceanic plateau on the Farallon plate that was - despite large ridge jumps - at least 600 km long and 300 km wide. This Shatsky conjugate began subducting beneath southern California at ~90 Ma and progressed from southern California to the Colorado Plateau while against the base of North America, and arrived under Wyoming at ~70 Ma. Evidence includes: removal of the entire mantle lithosphere and most of the crust of the southern Sierra Nevada; the emplacement of the Rand schist; the cooling, hydration and erosion of basal Colorado Plateau and Colorado mantle lithosphere; the creation of Rocky Mountain faulting by the forced NE motion of the Colorado Plateau; and the replacement of Wyoming mantle lithosphere below ~150 km with Shatsky conjugate mantle lithosphere, which is seismically imaged beneath Wyoming and NE Montana. The occurrence of Mojave plutonism continuing until ~75 Ma occurred where the Farallon slab drooped off the southern margin of the Shatsky conjugate. The southeast sweep of plutonic extinction and emplacement of Orocopia and similar schists is attributed to the conjugate of the Ojin Rise seamount trend of the Shatsky conjugate that swept across southern California and SW Arizona. The Shatsky conjugate path would not intersect with Insular if the superterrane was either proximal to North America and at a relatively northern position within paleomagnetic uncertainty, or if Insular was longitudinally-separated from North America and ~1000 km offshore of southern California (this is our preferred option). We conclude Shatsky conjugate flat-slab subduction and northward translation of the Insular terrane (Baja-BC) are not mutually exclusive hypotheses.

Session

Cordilleran tectonics