HR: 1340h
AN: GP43C-1498    [Abstracts]
TI: A refined statistical approach to the paleomagnetic strike test: applications to the Wyoming salient, Sevier fold-thrust belt
AU: Yonkee, A
EM: ayonkee@weber.edu
AF: Weber State University, Department of Geosciences 2507 University Circle, Ogden, UT 84408, United States
AU: * Weil, A B
EM: aweil@brynmawr.edu
AF: Bryn Mawr College, Department of Geology 101 North Merion Ave., Bryn MAwr, PA 19010, United States
AB: Paleomagnetic studies are increasingly being done on curved orogens from around the world. These studies interpret kinematics of curvature in different ways, ranging from mostly primary to largely secondary. Discrepancies in interpretations, sometimes for the same orogen, partly reflect limited sampling, different assumptions, and incomplete statistical analyses. We present a refined statistical approach to strike-tests, in which large-scale structural trends are correlated with restored paleomagnetic trends, and apply this approach to new paleomagnetic and structural data for 164 sites from the Triassic Ankareh Formation, sampled around the Wyoming salient of the Sevier fold-thrust belt. 92 sites preserve a primary Triassic magnetization, and 32 sites preserve a secondary Early Cretaceous component. A weighted least-squares method is used to calculate best fit slopes (which can vary from 0 for primary curvature to 1 for all secondary curvature), confidence intervals, and goodness of fit. Weighting factors are calculated from combined uncertainties in site mean paleomagnetic vectors (typically ~±5° at 1σ), local rotations recorded by variations in mesoscopic structure orientations (~±10°), and structural trend (~±3°). Analysis of Triassic data for the entire salient indicates that 77% (±9% at 2Σ) of present-day curvature is secondary; analyses of individual thrust sheets reveal similar patterns. Residuals have approximately normal distributions for most sites; sites from overturned fold limbs and transfer zones have larger residuals and are not included. Slopes calculated using only uncertainties in paleomagnetic vectors have narrower confidence intervals, but poor goodness of fit, indicating that local rotations contribute to uncertainty. Strike tests are also used to analyze LPS directions estimated from mesoscopic structures and 3-D strain analysis of deformed reduction spots. Best-fit slopes for the entire salient and individual thrust sheets indicate that LPS directions are statistically orthogonal to structural trends. Combined with paleomagnetic data, this indicates that LPS directions had a 23% (±10%) component of primary curvature and can not be directly used to quantify secondary rotation. Confidence intervals were also evaluated using bootstrap statistics and Monte Carlo simulations, which gave similar results. Our approach of systematic paleomagnetic and structural sampling, combined with appropriate weighting to determine confidence intervals and goodness of fit, closely constrains acceptable curvature models for the Wyoming salient.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1594 Instruments and techniques
DE: 8108 Continental tectonics: compressional
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting