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Paleozoic strata of the Southern Rocky Mountains and Colorado Plateau overlie a structurally complex basement of metamorphic, igneous, and metasedimentary rocks. The basement complex was completed in Proterozoic time by major continental-scale wrench faulting that produced a northwesterly trending swarm of faults (the Olympic-Wichita Lineament of Baars, 1976) and a northeasterly set (the Colorado Lineament of Warner, 1978). The apparently conjugate sets of fractures intersect in the heart of the Paradox Basin of the eastern Colorado Plateau. Precambrian sense of displacement was dextral along the Olympic-Wichita trend, at least on the Colorado Plateau (Baars and Stevenson, 1982), and sinistral along the Colorado Lineament (Warner, 1978). Thus, sigma one was directed in a north-south orientation (Fig. 1). Age of the northwesterly faults is bracketed to 1.72 Ga to 1.46 Ga (Baars and Ellingson, 1984); younger granitic intrusives (-1.46 Ga) invade older metamorphics, metasediments, and faults without discretion in the San Juan Mountains. Warner (1978) dated the northeast set at about 1.7 Ga.

Metasedimentary rocks (quartzites and metapelites) of the Uncompahgre Formation are juxtaposed with the metamorphic basement (-1.78 Ga) along the fault blocks in the San Juan Mountains of southwest Colorado. The shape and location of the sedimentary basin for these Proterozoic rocks is enigmatic, but stratigraphic similarities and speculative dating would relate the metasediments with the Mazatzal Group of central Arizona and the Ortega Group of north-central New Mexico (C. W. Harris, personal communication, 1987). Ritzma (1987) argued that the age of the Uinta Mountain Group of northern Utah is

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