EOCENE TO RECENT, NORTHWEST-DIRECTED EXTENSION OF THE NORTH AMERICAN CORDILLERA: A KINEMATIC LINK BETWEEN THE EXTENSION IN THE NORTHERN BASIN AND RANGE PROVINCE AND THE OPENING OF THE RIO GRANDE RIFT

Kinematic data collected along structures associated with the Northern Rio Grande Rift indicate a component of NW-directed extension. The faults studied were active in association with formation of the Eocene “Echo-Park” intermountain basins, during emplacement of the Oligocene Cripple Creek Diatreme, and during late Cenozoic formation of rift related basins. With respect to the North American craton, the interpreted NW-directed extension has been kinematically linked to opening of the rift and NW-directed movement of the Colorado Plateau. Such movement is consistent with overall dextral shear along the western margin of North America since the late Mesozoic. These interpretations are contrary to previously proposed models that invoke a component of sinistral shear along the N-S-trending rift. If the alternative model of dextral shear is valid, and it is assumed that the plateau behaves as a coherent block during extension, then the western margin of the plateau should display compatible kinematics. In northeastern Nevada, WNW-directed regional extension began by late Eocene time and was at least locally synchronous with widespread late Eocene to early Oligocene intermediate composition volcanism. Exhumation of the Ruby Mountains-East Humboldt Range metamorphic core complex likely began at this time and continued episodically to the Miocene. Within the core complex, kinematic indicators include composite fabrics, shear bands, and crystallographic textures indicating WNW-directed normal-sense shear within a ~1-km-thick mylonitic zone. The highest strain rates associated with core complex exhumation probably occurred during the Oligocene to earliest Miocene, a period when the eastern margin was experiencing only localized extension and regional dextral transcurrent deformation. Opening of the rift accelerated during the Miocene reaching a maximum in late Miocene time. Because plate boundary force models for Cordilleran extension predict high strain rates between 12 and 8 Ma, these results suggest that the high NW-directed extension rate may have been partitioned increasingly into the Rio Grande Rift during this time frame. Perhaps plate boundary forces thus played a significant role in directing extension across the southwestern U.S. Cordillera.