THE ARKANSAS/BLUE RIVER TRANSITION ZONE OF THE RIO GRANDE RIFT AND ITS POSSIBLE IMPLICATIONS FOR THE STRUCTURE OF SOUTH PARK (Invited Presentation)

The Rio Grande Rift (RGR) follows the Upper Arkansas Valley northward to Leadville and its rift-flank uplift on the west (Sawatch Range) tilts to the west. At Leadville, the RGR transitions to the east in the Blue River Valley and continues north to Kremmling. In the area of transition, the Mosquito and Tenmile Ranges tilt to the east- opposite from the Sawatch tilt. They are separated into two blocks by a major, north-south lineament that downdrops the Tenmile Range from the Mosquito Range. The lineament continues northward and becomes the eastern bounding fault of the Gore Range uplift and Blue River graben. The Gore Range tilts to the west.

South Park sits on the east side of the Mosquito Range, outside of the main axis of the RGR. A northwest trending lineament crosses South Park that divides it into two, quite different structural areas. North of the lineament the strata tilt to the east, the igneous rocks are predominantly plutonic, and it appears to have been a site of erosion during the Neogene. South of the lineament the strata are fairly flat-lying, the igneous rocks are predominantly volcanic, and it was a site of Neogene deposition. Because the northern, tilted area in South Park is spatially coincident with the extent of the Mosquito Range they may be genetically related.