STRAIN TRANSFER FROM THE NORTHERN RIO GRANDE RIFT TO THE UINTA ARCH ALONG THE CRESTED BUTTE LINEAMENT

The Oligocene transition between the end of Laramide compression and subsequent extension along Rio Grande Rift remains poorly defined. Both events affected the eastern margin of the Colorado Plateau and there are many examples of structural along common structures. Wawrzyniec et al., (2002) proposed an early extension direction directed toward 300° primarily documented in the Cripple Creek Diatreme (32-27 Ma), which is not considered part of the Rift but likely formed during the onset of continental extension. As documented by brittle structures in the diatreme the extension direction for early rifting was associated with 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 along east-west trending Uinta uplift. Moreover, no associated fault through the Southern Rockies was proposed to accommodate such motion during this time frame. Evidence of dextral transfer of strain toward the WNW near Poncha Pass at the northern end of the San Luis Valley is interpreted to have occurred ~20-16 Ma, which supports a WNW transfer of the plateau at this latitude. At the margin of the Uinta Basin, more recent studies have shown that the E-W Douglas Creek Fault zone accommodated two phases of deformation; a left-lateral phase during the Laramide, and a right-lateral phase during early Rio Grande Rifting (Bader, 2009). In the dextral phase, the shortening direction of the fault is concordant with the extension direction associated with rifting. In the on-going study seeks to evaluate the possibility that a connecting structure between these two settings is positioned along the Crested Butte Lineament. Along the lineament a series of intrusions suggest that the lineament was active during both Laramide contraction and later Rio Grande Rifting with most of the intrusions having emplacement ages of ~30Ma. Thermal modeling of the Uinta Basin demonstrates that gilsonite dikes of the eastern Uinta Basin are of similar age. Using terrestrial lidar and regional airphoto coverage, this study is documenting a range of fracture sets that demonstrate asymmetric fracture patterns consistent with strike-slip transfer of strain contemporaneous or later than regional intrusive activity.