Abstract

Hematite-cemented strata of the Neoproterozoic-age (~ 900 to 750 Ma) Uinta Mountain Group (UMG) provide paleomagnetic data that can be used to test the hypothesis that the easternmost Uinta Mountains, northwest Colorado and northeast Utah, experienced clockwise sense rotation in response to Laramide crustal shortening. In the north-central Uinta Mountains, Flaming Gorge area, UMG bedding strikes east-west to northwest-southeast and has characteristic remanent magnetizations (ChRMs) of west declination and shallow inclination (as well as an antipodal magnetization). Examples (in stratigraphic coordinates) include the Carter Creek locality (Decl.= 266.0°, Incl.= −8.0°, α₉₅ = 5.4°, pole of 5.8°S, 160.1°E, N = 5 sites); Dowd Springs folded locality (Decl.= 269.0°, Incl.= −11.8°, α₉₅ = 11.5°, pole of 4.8°S, 156.5°E, N = 6 sites); Dowd Springs unfolded locality (Decl.= 101.8°, Incl.= −6.9°, α₉₅ = 13.4°, pole of 11.3°S, 155.6°E, N = 5 sites); Flaming Gorge dam locality (Decl.= 272.7°, Incl.= −6.3°, α₉₅ = 9.8°, and pole at 0.0°S, 156.6°E, N = 6 sites); Sheep Creek Canyon locality (Decl. = 96.2°, Incl.= −5.6°, α₉₅ = 4.7°, pole of 6.6°S, 158.4°E, N = 23 sites). This magnetization is interpreted as a primary, Neoproterozoic magnetization, acquired when Laurentia was at low, equatorial latitudes, during the early stages of breakup of the Neoproterozoic Rodinia supercontinent. In the eastern Uinta Mountains, structural trends and bedding strikes have a northwest to north-south trend and two sampling localities have ChRM directions with west declinations and shallow, negative to positive inclinations (as well as an antipodal magnetization). In stratigraphic coordinates, the Bull Canyon locality yields a mean of Decl.= 272.0°, Incl.= 9.0°, α₉₅= 5.9° (pole of 4.4°N, 163.3°E, N = 9 sites) and the Talamantes Creek - Sparks locality yields a mean of Decl.= 87.9°, Incl.= −9.9°, α₉₅= 8.2° (pole of 1.9°S, 166.4°E, N = 12sites). Many additional eastern UMG localities carry only a north-directed and steep positive inclination remanence, which is interpreted as a secondary magnetization acquired after the end of Laramide deformation. At other localities, this pervasive remanence is unblocked early during stepwise thermal demagnetization and reveals the ancient west and shallow inclination remanence (or its east-directed antipode) at higher unblocking temperatures. Comparison of ancient ChRMs from the central and eastern Uinta Mountain localities shows no statistically significant vertical axis rotation of UMG rocks between the two areas. Fault kinematic data, collected from selected UMG and Mesozoic strata from the eastern part of the range, are interpreted to show evidence for left-lateral transpression and, at least locally, a horizontal, NW-SE directed maximum shortening direction. The paleomagnetic data are interpreted to indicate that Laramide-age deformation of the eastern Uinta Mountains proceeded without significant vertical axis rotation, and that the map-scale curvature observed today in the eastern Uinta Mountains is likely due to reactivation of a pre-existing, rift-related structural grain, rather than a reflection of considerable left-slip transpression between the Uinta Mountains and adjacent, buttressing parts of the foreland.