TERTIARY ROCK UPLIFT SCENARIOS FOR THE COLORADO PLATEAU, BASED ON A SUITE OF GEOLOGIC AND GEOPHYSICAL DATA

This study integrates available data relevant to the tectonic and physiographic evolution of the Colorado Plateau to test a geodynamic model for the Tertiary history of this enigmatic region. The centerpiece of this model is the idea (based on xenolith data) that the Colorado Plateau is underlain by compositionally distinct mantle lithosphere that is depleted in basaltic components relative to the surrounding mantle (e.g., beneath the Basin and Range). We explore the implications of this idea for the Tertiary evolution of the Colorado Plateau in numerical models with constraints derived from Tertiary magmatic patterns, heatflow, xenolith data, seismic velocity structures, and rock erosion and exhumation.

The starting point of these models is the post-Laramide (partial or full) removal of the Farallon slab and associated changes in the physical state of North America lithosphere, including the voluminous middle Tertiary “ignimbrite flare-up.” This work explores the coupling between the timing of surface uplift of the plateau (achieved without significant upper crustal shortening) and physical changes after Farallon slab removal and Tertiary magmatism surrounding the Colorado Plateau. We model the thermal consequences of Farallon slab removal from beneath North America as a conductive relaxation process, where the base of a variably depleted (variable normative density) North America is brought into contact with hot asthenosphere and the system is allowed to then evolve toward conductive steady-state. The results from these first-order models show that heating of a variably depleted chemical boundary layer will lead to variable vertical motions, with more rock uplift in regions of greater depletion (higher Mg #) and thicker chemical boundary layer. This work helps to quantify how much rock uplift can be attributed thermal relaxation of a variably depleted lithosphere as a function of degree of depletion, lateral scale of chemical heterogeneity, and thermal parameters. This estimate of rock uplift is combined with estimates of rock uplift driven by spatially variable erosional unloading of the Colorado Plateau to obtain viable regional rock uplift scenarios.