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<br />The south graben extends from north of Buena Vista to Poncha Pass, a <br />length of about 40 miles (67 km). Width of the graben ranges from <br />about 1.5 miles (2.5 km) at the northern end to about 12 miles (20 km) <br />neaz the town of Salida. In the Trout Creek dam site azea, the graben <br />is 7 miles (11.7 km) wide. Topographically, the graben is defined by <br />the Sawatch Range uplift on the west, the Arkansas Hills on the east, <br />and the intervening Arkansas Valley. The Sawatch fault, exhibiting <br />10,000 feet (3050 m) of vertical displacement, sepazates the Sawatch <br />Range uplift from the axial graben (Colman, 1985). USBR (1980, <br />1440) documented recurrent late Pleistocene to Holocene displacement <br />on the Sawatch fault between the base of Mt. Antero and Cottonwood <br />Creek west of Buena Vista. Trenches excavated across late Quaternary <br />fault scarps (Ostenaa and others, 1981) indicate the Sawatch fault <br />produced at least six surface faulting events in the past 100,000 to <br />150,000 yeazs. Surface displacements and rupture lengths indicate <br />each of these surface faulting events produced M~ 6.25 to 7.25 <br />earthquakes. <br />The east side of the south graben near the proposed dam site is defined <br />by the.[ransition from relatively flat Arkansas Valley to the rugged <br />Precambrian terrain of the Arkansas Hills. Scott {1975} shows numer- <br />ous high-angle normal faults, striking north-northwest, displacing <br />Precambrian crystalline rocks in the Arkansas Hills. Regional studies <br />by USBR and site-specific studies at the Trout Creek dam site (Irish, <br />1992) did not disclose evidence for late Quaternary displacement on <br />any of these faults. Accordingly, we conclude the locus of late Quater- <br />nary activity in the south graben is on the Sawatch fault. <br />5.3 TECTONIC HISTORY OF THE TROUT CREEK AREA <br />The tectonic history of the project azea begins with onset of the <br />Lazamide Orogeny in late Cretaceous time. East-west directed <br />compressional forces produced broad anticlinal uplifts along the Front <br />Range, Mosquito/Pazk Range and Sawatch Range structural axes. <br />Uplift was accomplished both by folding and displacement along <br />bounding faults, some of which may have had Precambrian ancestry. <br />Preexisting faults favorably oriented to the compressional stress field <br />were reactivated and new faults formed to accommodate regional <br />14 <br />