A comparison of the effectiveness of clast based finite strain analysis techniques to AMS in sandstones from the Sevier Thrust Belt, Wyoming

Abstract

Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.