The Ups and Downs of the Ancestral Rocky Mountains

Soreghan, G.S.¹, M. Charles Gilbert¹, G. Randy Keller¹ (1) University of Oklahoma, Norman, OK

The Late Paleozoic Ancestral Rocky Mountains (ARM) constitute amagmatic, largely basement-cored uplifts and linked basins in western equatorial Pangaea, far from any plate boundary. The Uncompahgre uplift (western ARM, Colorado) exhibits at least 6 km of structural relief that formed during ARM orogenesis. The conglomeratic, Pennsylvanian-Permian Cutler Formation (proximal Paradox basin) wedges toward the uplift. New field data indicate the youngest Cutler strata onlap basement and bury ~1 km of relief carved in the uplift, preserved in the form of modern Unaweep Canyon, which harbors a basal unit of late Paleozoic age. The Wichita uplift (eastern ARM, Oklahoma) was worn down to a near seal-level peneplain in the latest Pennsylvanian after ~7 km of uplift. A distinct and late-stage uplift occurred in earliest Permian, yielding relief of ~1 km. Burial and preservation of this topography requires active subsidence of at least 1 km in the early-middle Permian. Preservation in both these widely separated sites of significant paleorelief beneath Permian strata, together with regional evidence for broad disappearance of the coarse clastic mantles around ARM uplifts by mid-Permian time, suggests the uplifts and their immediate surroundings succumbed to active (tectonic) subsidence soon after reaching their apogee. The regional character of this signature, and uniform magnitude, imply an epeirogenic control. Geophysical data show that both of these features involve large-scale reverse faulting that brought large mafic massifs from mid-crustal depths to the surface. Thus, one model for epeirogenic subsidence is lithospheric flexure due to the loads represented by masses.