2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 8
Presentation Time: 3:20 PM


DUEBENDORFER, Ernest, Department of Geology, Northern Arizona Univ, Flagstaff, AZ 86011, CHAMBERLAIN, Kevin, Geology & Geophysics Department, Univ of Wyoming, Laramie, WY 82071-3006, HEIZLER, Matthew T., New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining & Technology, Socorro, NM 87801 and HARPER, Kathleen, Department of Geology and Geophysics, Univ of Wyoming, P.O. Box 3006, Laramie, WY 82701, Ernie.D@nau.edu

Structural and thermochronologic studies in the Sierra Madre, southern Wyoming, indicate that parts of the 1.78-1.75 Ga Cheyenne belt, the Archean-Proterozoic boundary, were reactivated at ca 1.6 Ga. New shear zones, most notably the brittle Battle Lake thrust-tear fault system (BLFS), also formed at this time. We suggest that the BLFS displaced parts of the Cheyenne belt at least 20 km to the north-northwest and that it corresponds to the surface expression of a strong south-dipping reflector imaged by regional seismic studies (CD-ROM).

40Ar/ 39Ar and U-Pb thermochronology defines broad age patterns in the Sierra Madre that are delineated by discrete deformation zones. North of the amphibolite-grade, 1.78 Ga, Divide Peak mylonite zone (DPMZ, part of the Cheyenne belt), 40Ar/ 39Ar hornblende and biotite and U-Pb apatite dates suggest cooling below 300°C by 2.0-1.9 Ga. South of the DPMZ but north of the BLFS, hornblendes yield apparent ages of ca. 1.80-1.78 Ga that may record cooling associated with Cheyenne-belt tectonism. South of the BLFS, a hornblende sample has a well-defined weighted mean age of 1618±2 Ma. A synkinematic muscovite from the BLFS yielded two 40Ar/ 39Ar ages of 1597±1 Ma and 1592±3 Ma. We interpret the young ages to record a deformational event in the southern Sierra Madre between 1.62 and 1.59 Ga. Similar ages for deformation have been documented from the Medicine Bow Mts. to the east (Strickland et al., this meeting).

The proximity of samples (in some cases less than 1 km across discrete structures) with different 40Ar/ 39Ar hornblende ages argues against either partial resetting during protracted cooling or partial resetting during ca. 1.4 Ga magmatic and thermal events. If this “event” is shown to be regional in extent, it may represent a previously unrecognized period of contractional deformation in southwestern Laurentia. The time interval between 1.62 and 1.59 Ga is not widely represented in the geologic record worldwide. Of the proposed conjugate margins of western Laurentia, only Australia records significant tectonism at this time. A more thorough understanding of the timing and extent of this deformation may have implications for Rodinia reconstructions.