SLOW APPARENT POLAR WANDER FOR NORTH-AMERICA IN THE LATE TRIASSIC AND LARGE COLORADO PLATEAU ROTATION

Several recent analyses of North American paleomagnetic data suggest fast apparent polar wander (APW) (approximately 0.75-degrees/m.y.) during the Late Triassic and a modest amount (approximately 5-degrees) of Colorado Plateau clockwise rotation. Paleomagnetic poles from the lower (Carnian), middle (Norian), and upper (Hettangian) stratigraphic intervals of the Newark Basin, however, indicate very slow APW over the Late Triassic and provide an alternative interpretation for plateau rotation. The middle Newark pole is supported by positive fold and reversal tests, precluding remagnetization, and agrees well with the pole reported from the Norian Upper Shale Member of the Chinle Formation from east central New Mexico as well as the 214 Ma Manicouagan pole from Quebec. These poles provide a well-defined mean Norian reference pole for cratonic North America at 57.7-degrees-N 91.0-degrees-E A95=3.8-degrees. Paleomagnetic poles from the Chinle Formation on the Colorado Plateau (Owl Rock Member, Church Rock Member, and our new result from the upper Chinle in Utah) are also well grouped, consistent with slow APW over the Norian, but give a mean pole position (57.7-degrees-N 65.6-degrees-E A95=2.5-degrees) that differs significantly from the Norian pole for cratonic North America. The North American Norian poles can be closely reconciled by a 13.5-degrees +/- 3.5-degrees correction for accumulated post-Triassic clockwise rotation of the Colorado Plateau associated with Laramide deformation and Cenozoic opening of the Rio Grande Rift. This estimate of Colorado Plateau rotation is consistent with a systematic discrepancy between plateau and cratonic poles for the Early Triassic, whereas available late Paleozoic and Jurassic poles are judged not to provide definitive constraints on plateau rotation. A revised Triassic and Early Jurassic APW path for North America shows that the virtual standstill in the Norian, the last 15 m.y. of the Triassic, was preceded and followed by intervals of fast (approximately 1-degrees/m.y.) angular plate velocity.