HR: 17:20h
AN: GP34A-05    [Abstracts]
TI: Describing the 3-D kinematic history of the Wyoming-Idaho fold thrust belt: an integrated approach
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
AU: Yonkee, A
EM: ayonkee@weber.edu
AF: Weber State University, Department of Geosciences 2507 University Circle, Ogden, UT 84408, United States
AU: Sussman, A
EM: spring@lanl.gov
AF: Los Alamos National Lab, Los Alamos National Lab, Los Alamos, NM 87545, United States
AB: The kinematic history of fold-thrust belts is recorded in their 3D displacement field (a combination of translation, internal strain and rotation). In an effort to describe the complete deformation field of the Wyoming-Idaho fold- thrust belt we have gathered an extensive amount of structural and paleomagnetic data from throughout the belt between Salt Lake City, Utah and Jackson, Wyoming. Paleomagnetic and anisotropy of magnetic susceptibility (AMS) data are reported here from 160 sites collected from the Triassic Ankareh Formation. The paleomagnetic data indicate that the Ankareh Formation provides a good record of the Earth's ancient magnetic field. At least three components of magnetization are revealed: a viscous magnetization acquired in a recent field, and two characteristic remanence directions that linearly decay towards the origin of demagnetization diagrams and are either post to syn-folding with a Tertiary-like inclination for Wyoming, or pre-folding with an inclination that agrees well with the expected local inclination value for the Middle to Late Triassic for Wyoming. Declinations of in situ remanent magnetization show considerable regional variation (up to 90 degrees), suggesting that significant relative rotations have occurred around the salient since the time of magnetization acquisition. The AMS data reveal several different AMS ellipsoid shapes, ranging from a primary fabric to a tectonic fabric that is geometrically correlated to the trend of regional fold axes. About 23% of the sites retain a primary sedimentary fabric. About two-thirds of the sites have an oblate fabric with a well defined magnetic lineation that is consistently parallel to regional fold axis trend. The general parallelism of the oblate fabric with bedding indicates a composite fabric between a primary sedimentary fabric and an early tectonic layer parallel shortening fabric in which the magnetic lineation represents either an intersection lineation or a stretching lineation. About 13% of the sites have a more developed tectonic fabric represented by a prolate AMS ellipsoid with its long axis parallel to the trend of regional fold axes. As a whole, the paleomagnetic declinations and magnetic susceptibility fabric orientations from the Wyoming-Idaho fold-thrust belt exhibit a fan pattern that is in close agreement with the plan- view arcuate shape of the belt. Where possible, strain data were collected in conjunction with magnetic data. These data are geometrically correlated with AMS fabric orientations, and a strike test reveals correlation between strain orientations and paleomagnetic vector trends. Integrated, these datasets indicate that the observed final curvature of structural trends in the Wyoming-Idaho fold-thrust belt reflects about 1/3 primary curvature and 2/3 secondary tectonic curvature. This pattern suggests a significant amount of vertical-axis rotation has taken place within individual thrust sheets subsequent to early shortening of the orogenic wedge, which must be taken into account in any robust 3-D kinematic model of thrust-belt formation.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8000 STRUCTURAL GEOLOGY
DE: 8005 Folds and folding
DE: 8100 TECTONOPHYSICS
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly