Sevier–Laramide deformation of the continental interior from calcite twinning analysis, west-central North America

Abstract

Paleozoic–Mesozoic carbonates that cover cratonic western North America contain a regional layer-parallel shortening (LPS) fabric that is preserved by mechanically twinned calcite. Shortening directions are generally parallel to the Sevier thrust-transport direction (E–W) in carbonates of the Idaho–Wyoming portion of the thrust belt and within carbonates as far as 2000 km into the plate interior. The inferred calcite twinning differential stress magnitudes generally decrease across the thrust belt, and decrease exponentially away from the orogenic front into the craton. Synorogenic calcite cements and veins preserve a distinct twinning deformation history: in the thrust belt, twinning strains commonly record local, out-of-transport piggyback strain events with high differential stresses (<150 MPa), whereas in Laramide uplifts and adjacent basins as far east as the Black Hills, twinned vein calcite preserves a sub-horizontal, N–S-shortening strain, with differential stress magnitudes that decrease to the east. Deformation of the plate interior during the Sevier orogeny was dominated by E–W contraction at the plate margin, which changed into dominantly oblique contraction (∼N–S shortening) along western North America during the younger, basement-involved Laramide event.