New Mexico Geological Society Annual Spring Meeting — Abstracts

The Orocopia Schist, is an assemblage of meta-sedimentary and basaltic rocks that were subducted and underplated beneath the southwest U.S. during the Laramide Orogeny. The introduction of low-density crustal material into the deep crust may have significantly modified the composition and rheology of the lithosphere. However, the distribution of the Orocopia Schist in the subsurface is largely unknown. Flat-slab subduction, provides key insights into the tectonic evolution of the southern U.S. Cordillera. This study investigates the distribution of Orocopia Schist by examining the isotopic composition of mid-Cenozoic igneous rocks that assimilated crustal material during intrusion and emplacement. Igneous rocks that assimilated Orocopia Schist have heavy oxygen isotopes (high δ18O) and juvenile Lu-Hf radiogenic isotopes (εHf), whereas igneous rocks that did not assimilate Orocopia Schist have light oxygen isotopes and more evolved radiogenic isotopes. We collected and analyzed multiple samples from southeast California to southeast Arizona. The initial results suggest that the Orocopia Schist is present in the subsurface at least as far east as Phoenix, Arizona. By integrating our results with existing geologic and geophysical constraints, findings will help clarify the role of subducted sediment in the formation and evolution of continental crust in southwestern North America.