KINEMATIC INVERSION OF PLATE MOTION AND THRUST PLATE ROTATIONS WITHIN THE LA VETA PASS THRUST, COLORADO

The La Veta Pass thrust (LVPT) is a ~30km segment of the Sangre de Cristo frontal thrust system of southern Colorado. The fault forms a continuous structure that begins in the south along the eastern flank of Cuchara Peak and trends due north about 15 km where it has a trend of 320°. Kinematic analysis of small-fault populations within hanging wall strata reveal ~20° discrepancy between the hangingwall vergence from north to south. In the north, the mean shortening direction trends 046°, which yields a typical Laramide-age vergence of ~ 052°. To the south, small-fault populations yield a shortening direction of 050°, which is consistent with a vergence direction of 075° suggesting ~20° of clockwise rotation of this part of the thrust sheet. The thrust lies entirely within exposed sections of the Upper Carboniferous to Lower Permian detrital strata deposited during the Ancestral Rocky Mountain orogeny. Previously reported demagnetization data and results of on-going work on over 245 sites collected within Sangre de Cristo Formation strata exposed along the thrust yield declinations that range between 142-162° at or near La Veta Pass to 161-181° at the Indian Creek locality, which is located along the north-south trending segment of the LVPT. When compared to an Early Permian reference declination of about 144°, the paleomagnetic analysis demonstrates a clockwise rotation of the thrust related rocks that increases from north to south, which is consistent with the ~20° clockwise rotation demonstrated by the small-fault population data. Thrust sheet rotation variations are generally attributed to either a displacement gradient model or the existence of a lateral ramp beneath the thrust sheet. In the former, rotations should be maximized and opposite in sense at the thrust tips with minimal rotation near the center of the thrust. In the latter, rotations are maximized in zones of oblique motion and serve to accommodate a horizontal slip gradients between offset footwall ramps. Our analysis supports an interpretation that the obliquity of slip along the north-south trending segment of LVPT results in a slip-model similar to a lateral ramp. The paleomagnetic data and the inversion of plate motion from small-fault populations demonstrate a level of kinematic consistency that would not have been observed had we applied a simple stress/strain inversion to the kinematic data.