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Speaker: Anne Sheehan, University of Colorado, Boulder

Abstract: Foreland mountain belts consisting of basement-involved arches are major features of many contractional orogens. These thick-skinned fold and thrust belts occur between cratonic interiors and more intensely shortened compressional orogenic belts. They occur most prominently during low-angle subduction (e.g., the Rocky Mountains of North America and Sierras Pampeanas of South America) and continental collision (e.g., the Tien Shan in Asia).  The dissimilarity between thick-skinned, arch-dominated and more thin-skinned fold and thrust belts as well as their placement far from active tectonic boundaries prompts the following question: do these arches form due to the lithospheric rheology inherent to the zone between mobile belts and cratons, or are they driven by deeper processes such as low-angle subduction?  

Previous geologic and geophysical studies have shown that the upper crustal geometries of fault networks bounding these arches are broadly similar, hence, they may arise from similar lithospheric conditions. However the manifestation of arch shortening at depth and the rheology of the lower crust and upper mantle has been uncertain due to the absence of detailed geophysical imaging, leaving the question of conditions driving deformational style unanswered. This situation is changing rapidly, as foreland arches are the target of deep seismological investigations including in the western United States as part of EarthScope. In this presentation Dr. Sheehan will present recent results from a major passive and active source EarthScope Flexible Array seismological experiment in the Bighorn Mountains in Wyoming.