PALEOMAGNETISM OF UPPER PENNSYLVANIAN TO LOWER PERMIAN SANGRE DE CRISTO FORMATION STRATA: EVALUATION OF LATE CRETACEOUS TO EARLY TERTIARY DEFORMATION IN THE SANGRE DE CRISTO MOUNTAINS, SOUTHERN COLORADO

The eastern flank of the Sangre de Cristo Mountains in south-central Colorado exposes thick (>1.0 km) sections of upper Carboniferous through Lower Permian fine-grained, hematite-cemented detrital strata, deposited in response to the Ancestral Rocky Mountain orogeny. We have sampled these rocks in the La Veta Pass area and farther south to quantify aspects of Laramide contractional deformation in forming the Sangre de Cristo Mountains and also, in well-exposed, continuous sections, to evaluate details of the late Paleozoic apparent polar wander path for North America. Most rocks respond well to progressive thermal and chemical demagnetization, with no post-late Paleozoic remanence superimposed on an interpreted primary magnetization. In thermal demagnetization, the principal magnetization component is smoothly and progressively unblocked between about 590 and 685^oC, with no hint of changes in magnetic mineralogy and/or viscous behavior during unblocking. We have collected over 220 sites (one site as one bed, with 7-10 samples per bed). Demagnetization data from over 150 sites yield high-precision means (with all sites with a₉₅ values less than 8^o). In the La Veta Pass area, bedding ranges from about horizontal to overturned, and sections of different orientations yield slightly different results (e.g., Decl.=155.7^o, Inc.=-10.0^o, a₉₅=2.6^o, k=201.7, N=16 sites, overturned; Decl.=153.5, Incl.=17.1, a₉₅=11.4, k=24.6, N=8, 11^o dip to southwest; and Decl.=160.3, Incl.=-0.06, a₉₅=2.9, k=320.2, N=9, 8^o to west). The ensemble of results implies, using an Early Permian reference direction of 144.6/-6.9, a modest clockwise rotation of the rocks. About 15 km south, the Indian Creek area exposes Sangre de Cristo strata in an open, upright anticline. The mean direction from 10 sites is Decl.=170.9, Incl.=-9.8, a₉₅=5.1, k=102, implying some 20+ ^o of clockwise rotation during and since deformation of these rocks.