A TRANSCURRENT EMPLACEMENT MODEL FOR THE CRESTED BUTTE LACCOLITH AND ASSOCIATED STRAIN TRANSFER FROM THE RIO GRANDE RIFT TO THE UINTA ARCH, SOUTHERN ROCKY MOUNTAINS, USA

The Oligocene transition between Laramide compression and subsequent extension along the eastern margin of the Colorado Plateau remains poorly defined. Wawrzyniec et al., (2002) proposed an early extension direction of 300° that was primarily derived from the Cripple Creek Diatreme (32-27 Ma). The extension direction was associated with the onset of Rio Grande Rifting and plate motion of the Colorado Plateau toward 307°. A key criticism of this hypothesis is that WNW extension along the eastern margin of the Plateau would necessarily place the northern edge of the plateau into compression against the east-west trending Uinta uplift. Moreover, no strain transfer mechanism through the Southern Rockies was proposed to accommodate such motion during this time frame. Mapping and geophysical investigations have been initiated along structures found between the Elk and West Elk Mountains of central Colorado. Near Crested Butte several petrographically similar laccoliths were emplaced along a trend defined by NW-SE trending valleys. The reported emplacement age for the Crested Butte laccolith is 30 Ma. The geometry of these laccoliths, associated regional lineaments, post emplacement joint fabrics, and fault exposures strongly suggests that the intrusions were emplaced within a tensional bridge between right-stepping, dextral shear zones having a WNW directed extension direction. A similar extension direction is corroborated by the orientation of dikes associated with the nearby Treasure Mountain Dome (12.5 Ma). In a regional context, thermal models of the Piceance basin, which is located south of the Uinta Uplift, indicate that the petroleum system source rocks reached peak thermal maturity at ~30Ma. These dikes can be meters in width, have a mean trend of ~300° and are thought to have been emplaced at or near peak thermal conditions of the basin. Given these timing relationships, WNW directed motion of the plateau could have accommodated rift related extension, generated structures that modified the mode of laccolith emplacement in Central Colorado, and produced modest WNW directed shortening south of the Laramide-age Uinta uplift. If correct, the laccolith related structures may represent the missing kinematic link between foreland extension and the broader Western Cordillera.