INTEGRATING AMS AND PALEOMAGNETISM TO FULLY RESTORE SHORTENING DIRECTIONS ACROSS COMPLEX OROGENIC SYSTEMS (Invited Presentation)

Anisotropy of Magnetic Susceptibility (AMS) is an increasingly applied technique for detecting subtle shortening fabrics in weakly deformed sedimentary rocks that may be difficult to detect using other methods. AMS measures the preferred orientations of large numbers of magnetic grains within a small rock volume, based on the directional variability of magnetic susceptibility. AMS, like strain, is a second order tensor represented by an ellipsoid (and by the magnitudes and orientations of its three eigenvectors). Although AMS has been widely used to estimate shortening directions in various tectonic settings, caution is needed to establish contributions from multiple grain mineralogies and fabrics, and the timing of fabric acquisition, including depositional and tectonic phases. Electron microscopy and x-ray diffractions studies show that multiple mechanisms contribute to AMS fabrics in sedimentary rocks, including micro-kinking, rotation, and growth of paramagnetic phyllosilicate grains, pressure solution, and shape preferred orientations of ferromagnetic grains. As with other tectonic structures, effects of subsequent deformation must be properly restored in order to interpret AMS fabrics in their correct reference frame. This is particularly important for layer-parallel shortening (LPS) fabrics that typically develop during early deformation phases in fold-thrust belts and forelands, and undergo subsequent tilting and vertical-axis rotation during differential fault slip.

Example studies from the Sevier fold-thrust and Laramide foreland belts highlight how AMS, paleomagnetic, strain, and electron microscopy analyses can be integrated to robustly constrain three-dimensional deformation histories and test various kinematic and mechanical models, including the roles of preexisting crustal architecture, evolving topography, and plate interactions on orogenic wedge development and evolution of map-view fault and paleo-stress/strain patterns.