PRELIMINARY (U-TH)/HE THERMOCHRONOLOGICAL CONSTRAINTS ON TIMING AND MAGNITUDE OF TERTIARY DENUDATION OF THE COLORADO PLATEAU, UTAH

Geologists have long puzzled over the relatively undeformed nature and evolution of the Colorado Plateau. A number of models have attempted to predict amounts and timing of uplift and erosion that are poorly constrained by geological data. The presence of Late Cretaceous marine rocks on the Colorado Plateau indicates that the modern elevation of ~2 km was attained after the Cretaceous; otherwise the timing of relief development is poorly constrained because few Middle to Late Cenozoic deposits are preserved on or near the plateau. A reversal of drainage toward the south and southwest, similar to modern drainage patterns, occurred during Oligocene to Miocene time. This drainage reversal provides information about the development of relative elevation difference between the Plateau and Basin and Range province to the south, but does not necessarily coincide with the time that high elevation was reached. We have attempted to constrain the absolute timing of development of Laramide monoclines (Kaibab, Circle Cliffs, Monument Uplifts, and San Rafael Swell) in southern Utah and their subsequent Tertiary denudation history through apatite (U-Th)/He thermochronometry. Preliminary results of 40 analyzed samples from the four uplifts clearly indicate that the (U-Th)/He system does not record Laramide exhumation, but rather the effects of post-Oligocene erosional denudation of the Colorado Plateau. Apatite (U-Th)/He ages range from 44.5 to 13.4 Ma (Monument uplift), 44.5 to 11.5 Ma (San Rafael Swell), 35.0 to 20.4 Ma (East Kaibab), and 32.5 to 17.3 Ma (Circle Cliffs). These thermochronological transects were collected at a distance from the major canyons and thus should not record cooling related to river incision. Apatite (U-Th)/He data from all four uplifts generally correlate with elevation, although current topographic relief outside of major river drainages or volcanic centers allows access to only small vertical exposure window, not exceeding ~400m. Apparent age correlation with elevation indicates that most samples resided in exhumed HePRZs and underwent slow erosional denudation (1-4 m/m.y.) prior to late Miocene exhumation. These data also indicate that erosional exhumation started <10 Ma and affected the entire Colorado Plateau in southern Utah.