The dominant orogenic fabric in Proterozoic rocks of the southwestern U.S. includes a series of NE-striking shear zones that are commonly interpreted as suture zones across which blocks of juvenile crust were assembled to the southern margin of Laurentia. New structural and geochronological data from southwestern Colorado suggest that fabrics related to assembly of tectonostratigraphic terranes in this area strike northwest. The NW-striking foliations represent deformation at ca. 10-20 km paleodepths (ca. 1.77-1.71 Ga), and are parallel to magnetic anomalies and to gradients in mantle velocity structure. The agreement between these data sets suggests that the NW-striking structures are important at lithospheric scale, extend to >100 km depth, and may record assembly of southwestern Colorado across NW-striking tectonic boundaries. Geochronologic data indicate that northwest (central Colorado)-and northeast (Cheyenne belt)-striking boundaries developed simultaneously during accretion of southwestern Laurentia between ca. 1.78-1.73 Ga. We propose that the Yavapai province at ca. 1.75 Ga may have involved a complex arcuate subduction system, with multiple arcs, analogous to that of the modern Banda Sea, in the Indonesia region.

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[1] Seismic reflection data show $40°S-dipping reflectors that extend from the Cheyenne belt (CB-Archean-Proterozoic suture) to 20 km depths, and profound differences in crustal reflectivity across the suture zone, which is itself imaged as an interwedging boundary. Archean crust has a Moho reflection (at 40 km) and abundant upper crustal reflectivity; Proterozoic crust has no Moho reflection and is distinctly less reflective. Reflective zones south of the Cheyenne belt include 40°S-and N-dipping reflections that extend from the Farwell Mountain zone to 20 km depths; these are interpreted to be a complex cryptic suture zone between the 1.78 -1.76 Ga Green Mountain block and 1.75 -1.72 Ga Rawah block.