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The Weston fault, striking N 10-ISOW, terminates the Mosquito fault 6 miles (10 km) southeast of Leadville and crosses the crest of the Mosquito Range into the South Park basin. The Weston fault is a high- angle normal or reverse fault juxtaposing Precambrian basement on the east against similaz basement and overlying Paleozoic sedimentary rocks on the west. To the northwest, the Weston fault decreases in displacement and eventually dies ou[ in the northern part of the Leadville mining district. 5.2 UPPER ARKANSAS VALLEY GRABEN The Upper Arkansas Valley graben extends from Poncha Pass near Salida to Tennessee Pass north of Leadville. USBR (1980, 1990), as part of studies conducted for Twin Lakes and Sugazloaf dams, divided the Upper Arkansas Valley graben into north and south grabens separated by a horst of Precambrian rock near the town of Granite, about 17 miles (28 km) north of Buena Vista. The northern graben extends from south of Twin Lakes northward to Tennessee Pass and encompasses Tennessee Pazk, a broad alluviated flat northwest of Leadville. The graben is roughly triangulaz in shape, about 15 miles (24 km) long, 8 miles (13 km) wide at its widest point north of Twin Lakes, and tapers markedly towazd Tennessee Pass. The graben along its west side is bounded by the Sawatch fault system and along the east by mapped and inferred faults including those in the Leadville area and master faults (Mosquito, Weston) bounding the west flank of the Park Range uplift. The Sawatch fault separates the axial graben from the Sawatch Range uplift to the west. Maximum thickness of Neogene fill in the graben is estimated to be 3000 to 4000 feet based on gravity data. Maximum fault displacement inferred from thickness of graben fill sediments is 8000 to 9000 feet. Many of the faults mapped in the north graben (Tweto, 1961; Tweto and Case, 1972; Tweto and Reed, 1973) are inferred based on graben geometry, distribution of bedrock and vaziation in sedimentary fill thicknesses determined by drilling and geophysical data. For the most part, many of these faults have no surface expression either because they aze covered and do not displace younger su~cial materials or because they do not, in fact, exist. Studies conducted by USBR in the azea did not disclose any faults cutting mid to late Quaternary deposits. 13