A THREE-SIDED OROGEN: A NEW TECTONIC MODEL FOR ANCESTRAL ROCKY MOUNTAIN UPLIFT AND BASIN DEVELOPMENT (Invited Presentation)

Many decades of research have resulted in a detailed understanding of the structural and sedimentologic evolution of individual Ancestral Rocky Mountain (ARM) uplifts and associated basins, active during Pennsylvanian-Permian time. However, the overall tectonic mechanisms for uplift and subsidence remain debated. The various proposed tectonic drivers include intra-foreland stress resulting from progressive closure of the Ouachita-Marathon belt; wrenching associated with this closure; left-lateral slip along major fault systems; the activation of pre-existing continental weaknesses; mafic underplating; and flat-slab subduction along the southwest (Sonoran) Laurentian margin. However, none of these models fully accounts for the geometry and kinematics of ARM uplifts and the lack of any associated magmatic activity. The orientation of most ARM uplifts is nearly orthogonal to the Ouachita-Marathon belt, and although some strike slip motion has been documented, most uplift bounding faults experienced SW-NE displacement, 90° offset from that expected if collision along the Ouachita-Marathon belt was the principle source of stress. Flat-slab subduction proposed along the Sonoran margin of Laurentia addresses this issue; however, the expected continent-ward sweep and return of magmatism into the Rockies has never been identified.

To explain the geometry and kinematics of ARM uplifts and the absence of magmatism within the ARM region, we propose that ARM tectonism was driven by sinistral transpression of the Caborca Block along the Sonoran margin from southern California to its current location in northern Mexico. In this scenario, convergent tectonism in the Ouachita-Marathon belt and western Nevada served only as tectonic backstops, but neither was the driving force for SW-NE directed contraction. Recent U-Pb zircon ages for plutonic rocks and volcanic clasts in Caborca suggests that subduction initiated during the Permian, marking the likely end of transpression. This timing is consistent with the observed diminishment in ARM deformation at that time, and also with recent late-Paleozoic global plate models that suggest a shift to convergent motion between the North American and Panthalassan Plates coincident with early ARM tectonism.