ESTABLISHING AMS AS A ROBUST PROXY FOR LAYER-PARALLEL SHORTENING IN WEAKLY DEFORMED CLASTIC ROCKS FROM COMPLEX RETROARC AND FORELAND SYSTEMS

The Anisotropy of Magnetic Susceptibility (AMS) of weakly deformed sedimentary rocks has become an increasingly useful technique for detecting subtle shortening fabrics that may not be detected by other structural geology techniques. AMS provides a measure of the preferred orientations of large numbers of magnetic grains within a small rock volume, based on the directional variability of a sample’s magnetic susceptibility in response to applied magnetic fields of different directions. AMS, like strain, is a second order tensor that can be represented by an ellipsoid and by the magnitudes and orientations of its three eigenvalues and eigenvectors. AMS ellipsoid shapes, which range from oblate, to triaxial, to prolate, measure the combined contributions from dia-, para-, as well as ferro-magnetic mineral grains, which may form at different times and by different mechanisms in a given sample, thus complicating interpretations of measured fabrics. AMS has been widely and successfully used to measure directions of layer-parallel shortening (LPS) in fold-thrust belts and forelands, although caution is needed to establish contributing grain mineralogies and fabrics. LPS typically occurs during the early deformational phases of fold-thrust belts and forelands, and results in subtle preferred orientations of mineral grains by multiple mechanisms, such as grain rotation, pressure solution, and neocrystallization. Example AMS studies of weekly deformed clastic red beds will be presented from the Sevier fold-thrust and the Laramide foreland belts of the U.S. Cordillera, as well as the Frontal and Pre-Cordillera belts and the Sierras Pampeanas of the South American Andes. The observed AMS ellipsoid shapes reflect accumulated fabrics of varying intensity. Primary sedimentary fabrics primarily control AMS foliation, which is subparallel to bedding, whereas AMS lineation is controlled mostly by overprinting tectonic LPS fabrics. In most sampled sites AMS fabrics are oblate to triaxial with the AMS foliation parallel to bedding and AMS lineation parallel to the intersection of LPS fabrics with bedding, and to the regional structural trend. Comparison to independent strain and fault kinematic data, suggest that AMS is a robust recorder of deformation fabrics in many weekly deformed clastic rocks.