PALEOMAGNETISM OF UPPER TRIASSIC (CHUGWATER GROUP) STRATA ON THE NORTHEAST FLANK OF THE GROS VENTRE RANGE, NORTHWEST WYOMING, AND IMPLICATIONS FOR LARAMIDE-STYLE DEFORMATION

The Gros Ventre Range in northwest Wyoming exposes a dominantly gently northeast-to north dipping section of Phanerozoic strata, usually nonconformably overlying Precambrian crystalline rocks locally exposed along the southwest margin of the range due to southwest displacement along the Cache reverse fault system. Locally, northwest-southeast trending folds, some of which are overturned and show a southwest vergence, affect the Phanerozoic section and presumably reflect small-magnitude throw along additional structures in the Precambrian basement. Paleomagnetic data have been gathered in Upper Triassic strata, well exposed in the Gros Ventre River valley, to test the hypothesis that the Phanerozoic strata in the Gros Ventre Range, regardless of degree of internal deformation have not experienced vertical axis rotation as a result of southwest-directed, Laramide-style deformation. Our sampling concentrated on thin beds of hematite-cemented fine sandstones and sandy siltstones above and below the Alcove Limestone. Most materials respond well to progressive thermal and chemical demagnetization, with the general range of laboratory unblocking temperatures between about 610 and 680^oC, indicating that hematite is the principal magnetization carrier. At the site (individual bed) level, magnetizations identified in demagnetization are of low dispersion (alpha-95's less than 10^o). Structurally corrected site mean (and thus locality mean) directions, overall regardless of the orientation of the strata, are of west-northwest to northwest declination (300 to 330^o) and shallow inclination (and antipodes), and are thus discordant, in a counterclockwise sense, with expected field directions of Late Triassic age. We tentatively interpret these data as revealing that parts of the Phanerozoic cover of the Gros Ventre Range became detached from the basement, and rotated in a counterclockwise sense, during early Tertiary Laramide-style deformation of the range and interaction with thin-skinned structures of the overthrust belt immediately to the west. We are testing this hypothesis with additional sampling in the southeast part of the range.