Abstract

A fundamental tectonic boundary appears to have existed below the site of the present-day Colorado Plateau to Great Basin Transition Zone since Precambrian times. The Plateau proper has seen little deformation since Middle Proterozoic continental assembly apart from Cenozoic uplift and limited thick-skinned contraction and calc-alkaline plutonism. In contrast, the Great Basin region has been subject to repeated episodes of both contractional and extensional tectonism, and extensional activity continues into the modern day. Evidence exists that the Colorado Plateau at its western margin is being converted to lithosphere with rifted Great Basin properties. Some models for migrating extension call upon progressive gravitational collapse of thicker crust of the plateau margin as it warms, possibly aided by hardening of the previously rifted lithosphere (i.e., Great Basin interior) via crustal thinning and cooling.

However, this rather homogeneous and temporally gradual model of deformation has only partial applicability to evolution of the western Colorado Plateau and eastern Great Basin. On the one hand, the limited degree of block style faulting, high elevation, and high apparent elastic thickness of the Transition Zone resemble properties of the Colorado Plateau. On the other, heat flow, upper mantle velocities, and deep electrical conductivity of the transition are more like those of the active eastern Great Basin. We suggest that non-uniform extension occurs below the Transition Zone, with thinning of the lower crust and mantle lithosphere by a degree greater than that of the upper crust. Transition Zone extension may be distinctly earlier than that of the eastern Great Basin, with exhumation of the latter occuring quite uniformly in the Early to Middle Miocene, but the former perhaps initiating significantly later (Late Miocene-Early Pliocene). Estimates of total extension and lower crustal material movement for the eastern Great Basin in the Middle Miocene range widely depending upon the assumed role of the Sevier Desert detachment, but present-day extension appears unrelated to this feature.

At present, we lack basic geophysical and geological data to resolve even gross contributions of force versus strength, let alone individual force or strength components, in driving deformation of the Transition Zone. Gravitational forces are expected to dominate, while Pacific plate boundary forces are considered negligible, but impact of the Colorado Plateau lithospheric keel upon elevated Great Basin asthenosphere may contribute. Strength indicators for the Transition Zone are conflicting. Uncertainties about the roles of low-angle detachments versus lower crustal flow in Transition Zone rifting reflect a limited knowledge of crust/mantle structure, and thus of processes of extensional consumption of formerly stable lithosphere here. The role of magmatism in the Transition Zone remains cryptic also. Some seismic models suggest a high-velocity “rift pillow” near the Transition Zone, which would imply focused extension and magmatism here, while others show only gradual progression in crustal thickness from west to east. The southern part of the Transition Zone has been highly modified by prior calc-alkaline plutonism which significantly affects upper crustal deformation style, but its influence on whole-lithosphere strength is unclear.