The Late Cretaceous to Paleogene (from about 90 to 50 million years ago) Sevier and Laramide orogens of western North America exemplify the two end-member types of continental crustal shortening: thin-skin and thick-skin. Thin-skin orogens are characterized by continuous fold-thrust belts in sedimentary rocks above shallowly dipping detachments (similar to the Valley-and-Ridge of the Appalachian Mountains), while thick-skin orogens are characterized by discontinuous basement uplifts located far from active plate margins (e.g., the Wind River and Uinta Mountains of the Rocky Mountains west). There are several outstanding questions about the temporal and spatial interactions between these two types of deformation systems, the most controversial of which is the interaction between intraplate deformation and overall plate dynamics. Because the Sevier and Laramide orogens occurred before the Paleogene, any information about plate dynamics has to be inferred from modeling or indirect geologic observations. The Subandeans and the Sierras Pampeanas of the Argentinian Andes are modern, actively deforming thin- and thick-skin systems that are analogous to the ancient Sevier and Laramide orogens, making them the perfect systems to test various geodynamic models and explore outstanding questions related to deformation styles in contractional tectonic environments.
My project is to use anisotropy of magnetic susceptibility (AMS) and paleomagnetism to analyze a preliminary suite of rock samples collected in the spring of 2013 from the active Sierras Pampeana and the Subandean ranges in Argentina. AMS analysis will be used to evaluate weak tectonic fabrics, and thus provide estimates of shortening directions recorded within the samples. The resulting data will be used to model how stresses are being transmitted through the various Andean contractional systems in order to test various models of crustal and plate boundary interactions. Paleomagnetism analysis will be used to quantify the spatial distribution of vertical-axis rotation in the various systems, allowing further testing of kinematic models for the area. The data from these preliminary samples will be used to construct a preliminary tectonic model that will be the basis of a grant proposal to the National Science Foundation for a multi-year research project led by the Bryn Mawr College Geology Department.