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5. PROJECT TECTONIC SETTING The project area is located in the northern Rio Grande rift within the main axial graben. Dominant structural/physiographic features of the project area include the Park Range uplift, the Sawatch Range uplift, and the intervening upper Arkansas Valley graben. 5.1 PARK RANGE UPLIFT The Pazk Range uplift is a major anticlinal feature extending 200 miles from south of Rawlins, Wyoming to near Salida, Colorado. Major physiographic subdivisions of the uplift include from north to south, the Pazk Range, Gore Range, Tenmile Range, Mosquito Range and the Arkansas Hills. In general, the uplift is bounded on the west by a series of major high- angle normal and reverse faults believed to be related to the Rio Grande rift including the Mosquito, Gore, and Steamboat Springs faults. Maximum vertical displacement on the Mosquito fault near Leadville is estimated to be 8000+ feet. On the east, the uplift is separated from the Williams/Front Range uplift by a series of struc- tural basins including North, Middle and South pazks. Locally, major normal or reverse faults divide the east flank of the Pazk Range uplift from southern Middle Park and South Park. The Mosquito fault, bounding the west side of the Mosquito Range, abuts Precambrian basement and intruded Paleozoic rocks on the east against Paleozoic sediments and Tertiary intrusives on the west. The fault zone is structurally complex with individual fault strands separated by blocks of relatively intact bedrock. Sedimentary beds on the west side of the fault show the effects of drag reaching dips of almost 90 degrees adjacent to the fault zone. The Mosquito fault is interpreted to be a high-angle normal to reverse system with strikes ranging from due north to about N 20°E. The London fault, a splay off the main Mosquito fault, strikes N 30°W and crosses the crest of the Mosquito Range near Mosquito Peak about 7 miles (1l km) northeast of Leadville. 12