2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 43
Presentation Time: 8:00 AM-12:00 PM


CATHER, Steven M., New Mexico Bureau of Geology, New Mexico Tech, 801 Leroy Pl, Socorro, 87801, READ, Adam S., New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, KARL, Krainer, Institute for Geology and Paleontology, University of Innsbruck, Inrain 52, Innsbruck, Austria, LUCAS, Spencer G., New Mexico Museum of Natural History, 1801 Mountain Road N.W, Albuquerque, NM 87104, KELLEY, Shari A., Earth and Environmental Science, New Mexico Tech, 801 Bullock St, Socorro, NM 87801, KUES, Barry S., Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, ALLEN, Bruce D., New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Albuquerque, NM 87106 and TIMMONS, J. Michael, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, steve@gis.nmt.edu

We studied the provenance, paleocurrents, sedimentary facies, and paleontology of Pennsylvanian coarse-grained, syntectonic deposits immediately east of the Picuris–Pecos fault (PPF) to test hypotheses for the timing of the 37-km dextral separation of Proterozoic lithotypes and structures on the PPF. Near the Rio Chiquito on the northern flank of the Truchas uplift, a well-exposed, 394-m thick succession of mostly fluvial sandstone, conglomerate, and talus breccia abuts the PPF, and consists entirely of detritus derived from the metasedimentary Hondo Group. These deposits, which we correlate to the Flechado Formation, display southeasterly paleocurrents and fine markedly to the east. We interpret them as fault-scarp deposits. The unit intertongues upsection with marine shales that contain a latest Atokan–early Desmoinesian brachiopod fauna. A limestone 80 m above the top of the Flechado Formation contains the early Desmoinesian fusulinids Beedeina and Wedekindellina. Immediately west of the PPF at Rio Chiquito, fine-grained, mostly arkosic Pennsylvanian beds overlie granite-gneiss. The nearest potential source terrane for the metasedimentary detritus in the Flechado Formation is in the Picuris Mountains, now dextrally separated from the Rio Chiquito exposures by 20 km. These relationships require ≥20 km of post-Atokan dextral slip on the PPF, which is supported by a ~70–90° clockwise rotation of strike of bedding in the Flechado Formation near the fault.

South of the Truchas uplift, poorly exposed Pennsylvanian conglomeratic strata east of the PPF show a southward transition from quartzarenite (Ortega Quartzite provenance) to arkose (granite-gneiss and metavolcanic provenance) just south of Cave Creek in the Pecos Wilderness. Paleocurrent data indicate that these Pennsylvanian sediments were derived from west of the PPF. The Pennsylvanian quartzarenite–arkose transition lies ~5 km south of the projected southern limit of the metaquartzite terrane in the subsurface east of the PPF. It thus appears that a small sinistral separation of the southern boundary of the metaquartzite terrane existed in the Early Pennsylvanian.

Our results provide definitive evidence that at least 20 km, and probably all, of the 37-km dextral separation on the PPF occurred after the Early Pennsylvanian.