ANISOTROPY OF MAGNETIC SUSCEPTIBILITY OF TRIASSIC RED BEDS FROM THE IDAHO-WYOMING SALIENT: EVOLUTION FROM EARLY LAYER PARALLEL SHORTENING TO FINAL OROGENIC CURVATURE

To better understand the importance of vertical-axis rotations in the Idaho-Wyoming Salient, samples were collected from over 100 sites for both paleomagnetic and anisotropy of magnetic susceptibility (AMS) analysis.  Sampling was concentrated in the Triassic Ankareh Formation, with emphasis on fine-grained hematite stained sandstones, silt and mudstones.  The magnetic susceptibility of these samples originates from ferromagnetic hematite and paramagnetic phyllosilicates.  Several different AMS ellipsoid shapes were observed, ranging from a primary fabric to a tectonic fabric that is geometrically correlated to the trend of regional fold axes.  Approximately 10% of the sites retain a primary sedimentary fabric, with K_min closely perpendicular to the bedding plane, and no well defined magnetic lineation. Approximately two-thirds of the sites have an oblate fabric with a well defined magnetic lineation that is found in different orientations (61% parallel, 31% acute, 8% orthogonal) with respect to regional fold axis trend.  The general parallelism of the oblate fabric with bedding likely 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 magnetic fabric from the Idaho-Wyoming Salient exhibits a fan pattern that is in close agreement with the plan-view arcuate shape of the thrust-belt.  

The existence of well defined magnetic lineations from throughout the Idaho-Wyoming Salient provide a complementary dataset to existing paleomagnetic, fracture, and strain data that can be used to construct an integrated kinematic model of orogen evolution.  These data indicate a significant amount of vertical-axis rotation has taken place within individual thrust sheets subsequent to early shortening of the orogenic wedge.  Moreover, the good correlation between observed paleomagnetic rotations and deviations in magnetic lineation orientation indicates that the AMS of weakly deformed red beds can serve as a proxy of early stress directions that can be tracked through time.