COMPLEX CRUSTAL ASSEMBLY OF SOUTHWESTERN NORTH AMERICA INVOLVING NORTHWEST AND NORTHEAST-STRIKING FABRICS: THE BLACK CANYON OF THE GUNNISON, SOUTHERN COLORADO

The southwest Colorado region, including the Black Canyon of the Gunnison, Uncompahgre Plateau, Dubois and Cochetopa successions is dominated by NW-striking Paleoproterozic foliations and hence is an important region for understanding the geometry of crustal assembly of southwest North America. The NW-striking fabrics (1.72-1.70Ga) are regionally important because they are related to arc assembly of the Yavapai province and may represent the orientation of assembly boundaries other than the dominant NE-striking orogenic fabric (1.70-1.68Ga) that is often attributed to the southward growth of island arc terranes. The NW-striking (F2-folds) of the Black Canyon metamorphic rocks were reoriented into the ~7km wide, NE-striking Black Canyon shear zone (F3-folds) during dextral strike-slip and are an expression of complex kinematics during a change from assembly of outboard arcs to collision with Laurentia during the progressive Yavapai-Mazatzal orogeny. The Black Canyon shear zone was reactivated during dextral southeast-side down shear, when the Vernal Mesa monzogranite and a NW-striking set of en echelon pegmatite tension gashes were emplaced. A new U/Pb date of 1.44Ga for the Vernal Mesa monzogranite provides a direct date for reactivation of the shear zone. Based on our results from the Black Canyon of the Gunnison and a regional fabric correlation we propose the following six stage tectonic model for southwest Colorado: 1) The island-arc stage (ca. 1.79-1.73Ga): The deposition of the Dubois and Cochetopa successions as submarine bimodal volcanism in arcs or related settings perhaps on >1.8Ga basement fragments and the emplacement of calc-alkaline granites (1.759-1.757Ga) 2) Early assembly stage (1.73-1.70Ga): The creation of isoclinal F1-folds followed by NE-SW shortening to create NW-trending F2-folds. 3) The formation of various NE-trending F3-folds and S3-foliations, such as the Black Canyon shear zone, during a change from NE-SW shortening to NW-SE shortening (1.70-1.68Ga). 4) Stabilization of arc crust, erosion and deposition of clean quartzite across the region or in syn-tectonic basins (1.70-1.68Ga). 5) The Mazatzal orogeny ~1.65Ga dominated by NW-SE shortening. 6) Intracontinental dextral transpressional tectonism including the emplacement of the Vernal Mesa monzogranite (1.44Ga).