TECTONIC DEVELOPMENT OF THE COLORADO PLATEAU AS INFERRED FROM CRUSTAL AND UPPER MANTLE STRUCTURE

Does lithospheric structure reflect patterns imparted by recent deformation or does it exhibit preserved features that have persisted since accretion? Modern advances in seismic processing techniques combined with increased sampling have lead to dramatic improvements in our ability to image crustal and upper mantle structure and address this question. Combining data from several temporary seismic deployments across a large region of the western Unites States, we have created maps and cross-sections of crustal structure traversing from the Basin and Range to the Great Plains utilizing seismic techniques that isolate P-to-s converted phases produced by discontinuities in seismic wave-speeds (such as the Moho). Comparing crustal thicknesses to surface elevations indicates very little correlation, suggesting a contribution from the mantle is needed to explain some of the high elevations.

Focused investigations into of the crust within the Colorado Plateau from dense seismic arrays produce detailed images of the crust within localized areas and offer insight into how smaller regions fit into the large-scale framework of western US tectonics. The signal from the Moho within the undeformed Colorado Plateau appears much more blurred than in the extended Basin and Range indicating that the boundary between the crust and mantle may be somewhat diffuse. Abrupt lateral changes in both Moho and inter-crustal structure can be found within the plateau that appear to correspond to the sutures between the Yavapai and Mazatzal terranes that comprise part of the North American continent. This seismic observation along with the locations of positive isostatic anomalies can be explained by the presence of a high-density layer that is confined to the 10-20 km depth range of the upper crust in the Four Corners region of the plateau. The extent of this structure appears to be controlled by the ancient terrane boundaries and may be an inherited feature. If such a body exists, as suggested by the gravity and seismic data, it could locally provide strength for the Colorado Plateau and help it resist deformation. Furthermore, differences in Moho structure collocated with terrane boundaries suggest that the Proterozoic lithosphere associated with these terranes has been preserved and thus cast doubt on models of plateau uplift that require complete lithospheric removal.