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Arkansas Valley graben, although markedly narrower, is similar geologically to its San Luis Valley counterpart and can be traced northward to Tennessee Pass neaz Leadville. North of Tennessee Pass, the characteristic axial graben is absent, but north-striking, range- bounding, high-angle normal and reverse faults continue along the same structural trend. Grose (1974) states that this faulting may be considered an integral part of the Rio Grande Rift system. Rifling began in mid-Miocene time (25+ mybp) and continued through the Pleistocene (1.9 mybp) and at least locally into the Holocene (<10,000 YbP)• USBR (1980, 1990), in seismotectonic hazard evaluations for Twin Lakes and Sugarloaf dams, divided the Upper Arkansas Valley graben into north and south grabens separated by a horst (uplifted fault block) of crystalline basement rock near the town of Granite about 17 miles (28 km) north of Buena Vista. Cleaz evidence of late Pleistocene to Holocene movement on the Sawatch fault is present in the south graben. Evidence for comparable young movement in the north graben, however, is absent or equivocal. The increasing age of fault- ing and generally decreasing late Quaternary scarp heights from south to north coupled with apparent structural segmentation of the graben near Granite indicate that the north and south grabens are distinct and sepazate structural elements within this part of the Rio Grande rifr. No late Quaternary to Holocene faulting has been specifically identified in the north graben. Tweto (1979) distinguished the "Rio Grande rift" from the "Rio Grande rift system." The use of "Rio Grande rift" is restricted to the more-or-less continuous axial graben. The "Rio Grande rift system," on the other hand, includes not only the axial rift, but also Neogene block faults many miles distant from the main rift structures. These coextensive faults, at least in part, aze believed to represent reactiva- tion of older structures dating back to Precambrian time (>600 mybp). The "rift system" as defined by Tweto (1979) encompasses most of central Colorado and includes a number of faults either in or bounding the Colorado Front Range. Rift system faults, however, are believed to exhibit significantly lower displacement rates than their axial counterparts. 10