The Great Unconformity (Powell, 1876) of the SW USA represents a complex differential unroofing of middle crustal rocks over an area of — km prior to its inundation by late Precambrian to Paleozoic marine sediments. In many regions, the hiatus represented by this angular unconformity (>1 billion years) is longer than the preserved rock record above and below it. The key to understanding this episode of regional denudation is to decipher variations in the location, timing, and style of differential uplift in tectonic blocks across the region.

This project uses Ar-Ar thermochronology on K-feldspars from key regions to derive complete cooling histories for the temperature range 150-300°C. Because K-feldspar Ar kinematic parameters can be determined during the age spectrum analysis, the age information can be used to quantitatively track this critical segment of the cooling history. These data will be combined with structural studies of block boundaries and ongoing thermochronologic studies of the higher T (>300°C) and lower T (fission track–100°C) portions of the cooling paths. The K-feldspar data set is essential to fill the gap in our understanding of the history and processes of development of this first order geodynamic feature, and its relationships to regional tectonic events. Structural studies of block boundaries are essential to understand kinematics of deformation during denudation.

The combined data set will test the hypothesis that the unroofing was multistage, different block to block, but ultimately was driven by erosion of an elevated plateau, with resulting denudation of an average of 10 km of crust. We will evaluate the relative importance of: 1) plateau building and uplift that was driven by crustal thickening during 1.4 Ga tectonism and "anorogenic" magmatism; 2) block uplift due to 1.1 Ga tectonism and magmatic underplating inboard of the Grenville orogeny, and 3) denudation 0.8-0.6 Ga that took place during breakup of Rodinia, possibly in response to mantle buoyancy introduced by upwelling asthenosphere. Beyond regional tectonics, we hope to better understand plateau uplift processes and intracratonic deformation. The protracted 10-15 km of unroofing of a huge plateau region over 100’s of m.y. may be a predictable isostatic response to intracratonic magmatic-tectonic events.