First-order outstanding questions regarding foreland deformation center on geodynamic processes that form diverse orientations of basement arches, mechanisms of basement deformation and its transfer to the sedimentary cover, and relationships to plate margin stresses. This project will integrate structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic studies of Laramide structures in Wyoming to test various tectonic models of foreland deformation. Testable models include, temporal changes in stress directions (possibly associated with flat-slab subduction), spatial variations in stress and fault orientations (potentially associated with basement anisotropy), and varying components of shear along differently oriented structures. Results of this project will further provide data on structural traps, fracture systems, and fault patterns within the Wyoming foreland, which will improve understanding of hydrocarbon reservoirs and potential sites for carbon dioxide sequestration. Finally, this collaborative investigation will be merged with ongoing paleomagnetic and structural studies of other orogens to compare processes of mountain building in different settings, and will generate large data sets for statistical comparisons of various strain and paleostress methods used in kinematic and mechanical analysis.
The focus of this investigation will be on key structures that display a range of trends and deformation styles in the Wyoming foreland, and on two stratigraphic levels (Triassic redbeds and Jurassic limestone) that are well exposed, carry primary remanent magnetizations, and contain multiple strain markers. Characteristics of minor fault systems will be measured and analyzed to estimate paleostress patterns, strain orientations will be estimated from AMS fabrics and calcite twin analysis, and oriented cores will be drilled for paleomagnetic analysis to quantify timing and magnitudes of vertical-axis rotations. Characteristics of fracture patterns will be quantified for selected well-exposed surfaces to test relationships to fold and fault mechanics. By integrating multiple data sets, limitations of each set will be reduced. Critically, paleomagnetic data will be combined with deformation fabrics (minor faults, AMS, calcite twins, and fractures) and cross sections/3-D restorations to quantify both vertical-axis rotations and spatial-temporal changes in paleostress/strain. This project will provide an expansive data base, building on previous studies, in order to critically test kinematic models and mechanical processes of Laramide foreland deformation.
Scientific Merits. This project has improved our understanding of structural evolution of foreland mountain belts, which deform continental crust far from tectonic plate boundaries (e.g., the continental portions of the Rocky Mountain and Andean belts). Weak, but measurable deformation fabrics, including minor faults and anisotropy of magnetic susceptibility (AMS), were quantified for more than 400 sites located across the Laramide foreland of Wyoming. Paleomagnetic declinations, which record rock movement with respect to the spin axis of the Earth, were also measured to evaluate potential vertical-axis rotations and fully restore paleo-stress fields associated with ancient subduction. This extensive data set has revealed overall SW-NE directed paleo-stress directions, which were at high angles to overall NW-trending basement-cored uplifts across much of the Laramide foreland (e.g., the Wind River Mountains), interpreted to partly reflect basal shear during flat slab subduction beneath thick lithosphere. This pattern is different from E-W paleo-stress directions previously found in the adjacent Sevier fold-thrust belt that are interpreted to reflect stresses transmitted through a growing thrust wedge and hinterland tied to the ancient Cordilleran plate boundary. In detail, trends of basement uplifts and paleo-stress directions display spatial and temporal variations across the foreland related to basement heterogeneities and evolving fault systems. A model for Laramide foreland deformation combining increased stress from flat slab subduction, spatial and temporal variations in stress related to basement weaknesses, and localized wrench shear along parts of variably oriented basement uplifts best explains structural and paleomagnetic data. This work has shown the utility of AMS to determine orientations of subtle deformation fabrics that record paleo-stress fields in weakly deformed rocks, which can be applied to other regions. By integrating multiple techniques to estimate stress-strain directions and full restoration that incorporates vertical-axis rotations, robust constraints have been placed on paleo-deformation patterns and relations to crustal heterogeneities and plate dynamics. Extensive data on fracture networks that contribute to rock permeability have also been collected, which may improve future hydrocarbon exploration and production within foreland structures. Broader Impacts. The project supported education of fifteen undergraduate students at Weber State University, Bryn Mawr College and affiliated Haverford and Swarthmore Colleges, and Carleton College. Ten of these students were female, and most students have gone onto graduate programs. Results were disseminated in four papers published in scientific journals with another four manuscripts in preparation, ten presentations and published abstracts at national and international professional meetings, and a field trip for the Geological Society of America that included participation of international geoscientists. This project promoted collaboration between multiple colleges and universities and merged with ongoing paleomagnetic and structural studies of other mountain systems.
