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.