TESTING THE PRESENCE OF FUNDAMENTAL CRUSTAL BOUNDARIES IN THE WESTERN UNITED STATES

Once proposed, inferred crustal boundaries often become entrenched in the geological literature. We re-examine the position of two crustal boundaries in the eastern Great Basin: a) the Cheyenne Belt separating the Archean Wyoming Province from Paleoproterozoic terranes to the south, and b) a boundary separating the Mojave Province from the Yavapai Province. One difficulty in the inference of such boundaries is the sparse and scattered nature of basement exposures in the region of the Colorado Plateau and the Basin and Range Province.

For the Cheyenne Belt, new and re-interpretated existing data strongly indicate that west of ∼109°W longitude [Utah-Colorado border] the Cheyenne Belt has been drawn in the wrong position. Instead of being beneath the Uinta axis, it lies north of the Uinta Mountains and the Farmington Canyon Complex of the northern Wasatch Mountains, and south of the Albion/Raft River/Grouse Creek Ranges, the eastern Snake River Plain, and exposures of Archean rocks in central Wyoming. Caution should be taken when inferring the position of crustal boundaries on the basis of structures like the Uinta axis.

The boundary separating the Mojave block from the Yavapai Province appears to be correct in the east-central Great Basin, although this conclusion is not well supported on the basis of crustal model-age approaches. When examined in detail, data are often ambiguous or contradictory. Model ages can be used to infer possible boundaries that are later tested with simple statistical approaches applied to radiogenic isotope ratios that are sensitive to crustal contamination and crustal age. A statistical test enhances discrimination of boundaries and indicates the level of confidence in the discrimination. However, this approach is critically dependent upon the initial conceptualization of the locations of the boundaries. In the end, all that is revealed with confidence is whether two data sets come from different populations, which may be a necessary but insufficient criterion for discrimination.