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<br />4. REGIONAL TECTONICS <br /> <br />The project area is located in north-central Colorado (Figure 1) in the <br />Southern Rocky Mountain physiographic/tectonic province. The <br />Southern Rocky Mountains aze bounded on the west by the Colorado <br />Plateau, on the northwest and west by the Wyoming Basin and on the <br />east by the Colorado Piedmont and Great Plains. A sliver of the Basin <br />and Range physiographic/tectonic province represented by the Rio <br />Grande rift extends into south-central Colorado and perhaps further to <br />the north as discussed below. <br />For many yeazs, the present tectonic and physiographic features of <br />Colorado were considered a result of an intense period of deformation <br />(Lazamide orogeny) beginning in late Cretaceous time (70+ mybp) and <br />lasting sporadically through perhaps mid-Tertiary time (40+ mybp). <br />From mid-Tertiary time to the present, most of Colorado was assumed <br />to be tectonically dormant, if not entirely inactive. This assumption <br />was reinforced by an apparent lack of earthquake activity and evidence <br />for late Cenozoic tectonism. A distinct period of post-Laramide defor- <br />mation, however, is now recognized beginning in Miocene time (28 <br />mybp) and continuing into Quaternary time (1.9 mybp). This post- <br />Laramide deformation is responsible for the present structural/physio- <br />graphic framework of Colorado. Development of the Rio Grande rift <br />system beginning in Miocene time is perhaps the single most impor- <br />tant tectonic event in the late Cenozoic structural evolution of central <br />Colorado. <br />4.1 RIO GRANDE RIFT <br />The Rio Grande rift, afault-bounded structural trough, extends 575 <br />miles (960 km) from southern New Mexico and west Texas into north- <br />central Colorado. The rift is a major, late tectonic feature characterized <br />by a central graben (down-dropped fault block) containing thick <br />alluvial fill. The margins of the rift are extensively block-faulted <br />producing steep, bounding mountain fronts. The rift is well-defined <br />through central New Mexico into south-central Colorado. A <br />topographic barrier separates the San Luis Valley graben from the <br />Upper Arkansas Valley graben in the Poncha Pass area. The Upper <br />9 <br />