STRUCTURAL AND THERMOCHRONOLOGIC EVIDENCE FOR A CA. 1.6 GA CONTRACTIONAL EVENT IN SOUTHERN WYOMING

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.