Title: Laramide and Neogene Structure of the Northen Sangre De Cristo Range, South-Central Colorado
Abstract: The Sangre d@ 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 have been folded and faulted by Laramide compressional forces from Late Cretaceous to Eocene time. Laramide structures are large arcuate thrust plates that intersect and overlap one another to form a northwesterly trending belt that extends across the mountains 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 crystalline rocks west over Paleozoic rocks. Thrust plates of Paleozoic rocks, and possibly those of Precambrian rocks, are internally folded; the folds tighten and decrease in amplitude toward the leading edge of the plate. Thrust faults are dominantly high- to medium-angle reverse faults along the leading edge of the thrust plates but flatten at depth. Total shortening within the range is approximately 8 km at the latitude of Westcliffe and about 14 km farther south near the latitude of the Great Sand Dunes. During Neogene time the Sangre de Cristo Range was uplifted and the adjoining San Luis and Wet Mountain valleys were down-dropped 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 sedimentary rocks of the range. The horst of the Sangre de Cristo Range probably began to rise in late Oligocene time, rose rapidly in early Miocene time, and rose again in late Miocene to Quaternary time. 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 thrusting along the west and east margins of the range were reactivated to form the Sangre de Cristo and Alvarado normal faults, respectively, so that the floor of the Neogene sedimentary and volcanic fill of the San Luis Valley is 2,00@7,00O”m below the top of the range and the floor of the Wet Mountain Valley fill is about 2,000 m below the range. As it was uplifted, theentire range may have been tilted gently eastward. Rifting is still in progress in the San Luis Valley, west of the range, but may have ceased in the Wet Mountain Valley.
Publication Year: 1983
Publication Date: 1983-01-01
Language: en
Type: article
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Cited By Count: 11
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