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The Sangre de Cristo Range, from Blanca Peak northward to the Arkansas River in Colorado, is composed mostly of Precambrian crystalline rocks and upper Paleozoic clastic sedimentary rocks. These rocks were folded and faulted by Laramide compressional forces from the Late Cretaceous to Eocene. Laramide structures are large arcuate thrust plates that intersect and overlap one another to form a northwest-trending belt that extends across the range from Huerfano Park to Valley View Hot Springs. All of the thrust plates within the range are bounded by west-dipping faults, some of which extend into the basement of Precambrian crystalline rocks. Along the east side of the range, the Alvarado fault is interpreted tentatively as an east-dipping thrust, bringing Precambrian crystal ine rocks west over Paleozoic rocks. Thrust plates were folded internally before and during thrusting; some plates of Paleozoic rock contain folds that tighten and decrease in amplitude toward the leading edge of the plate. Stacked plates consisting of Precambrian and Paleozoic strata have been folded concordantly after thrusting. Thrust faults are mainly high to medium-angle reverse faults along the leading edge of thrust plates, but they flatten to about 30° at depth. Total shortening within the range is at least 8 km (5 mi) at the latitude of Westcliffe and at least 14 km (9 mi) farther south near the latitude of the Great Sand Dunes.
During the Neogene, the Sangre de Cristo Range was uplifted, and the adjoining San Luis and Wet Mountain Valleys were downdropped by extensional rift faulting. Rifting followed late Oligocene intrusion of stocks, sills, and dikes of mafic to felsic igneous rock into the Precambrian and Paleozoic rocks of the range. The horst of the Sangre de Cristo Range probably began to rise in the late Oligocene, rose rapidly in the early Miocene, and rose rapidly again in the late Miocene and Quaternary. Flows of mafic lava were erupted from faults along the southwest side of the Wet Mountain Valley and in the San Luis Valley. Zones of Laramide thrusts along the east and west sides of the range were reactivated to form the Sangre de Cristo and Alvarado normal faults, respectively. The floor of the Neogene sedimentary and volcanic fill of the San Luis Valley has been downdropped 2,000-7,000 m (6,600-23,000 ft) below the top of the range, and the floor of the Wet Mountains Valley has been downdropped about 2,000 m (6,600 ft) below the range. Rifting is still in progress in the San Luis Valley, west of the range, but may have ceased in the Wet Mountain Valley.
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