THE 85 MA TRAINWRECK: DECONSTRUCTING THE DEBRIS THAT MADE THE CORDILLERAN OROGEN OF TODAY (Invited Presentation)

Prior to ~85 Ma, the southwestern U.S. approximates the cartoon vision of a continental arc environment with an accretionary prism, a forearc basin, an arc, backarc fold-and-thrust belt, and a foreland foredeep basin. But starting ~85 Ma these elements are disrupted in puzzling ways that yet define the modern southwest. By ~85 Ma, the first of the Pelona-Orocopia-Rand (POR) schist bodies was emplaced. At about the same time, the Sierra Nevada experienced its final paroxysm of magmatic activity just as the Sevier belt in southern Nevada stopped moving. Much farther to the northeast, the foreland experienced subsidence from some subsurface cause. As the Sierra shut down, sporadic two-mica granites began to be intruded to the east. By ~75 Ma, the southwestern margin had, from north to south, the cooling Sierran batholith, the locus of known Rand Schist emplacement, a zone of substantial granitoid emplacement, the beginnings of emplacement of the Pelona and Orocopia schists, and then an arc migrating to the east into southern Arizona. East of the early Laramide structures of the Colorado Plateau, subsidence intensified in the foreland as some of the earliest basement-cored uplifts rose in southwestern Montana and igneous activity began along the Colorado Mineral Belt. A short time later, the POR schists were being unroofed by extensional structures as Laramide deformation focused beyond the Colorado Plateau. While it seems highly likely these events share some common origin, the nature of that origin remains unclear.

These relations suggest questions whose answers might reveal the causes of the Laramide and associated events. Why was the Sierra Nevada magmatically dead for roughly 70 million years? If POR schists really extend under the entire Mojave, what is the role of the c. 75 Ma magmatic episode? How did the schists approach the surface, in some cases within a few million years of emplacement? Why did the Keystone Fault stop and “dynamic” subsidence begin at nearly the same time? What triggered the emplacement of the 2-mica granites? If Late K extension in the Mojave is related to POR schist emplacement, why is such extension seen elsewhere? I will briefly explore simple conceptual models to highlight the most troublesome relationships most difficult to force into any simple model.