Geophysics and Tectonics Seminar winter-2020

Cratons are often described as rigid lithospheric blocks, capable of resisting deformation over extended periods of time, and to first order this narrative appears to be true. And yet, uplift occurred across the Wyoming craton during the Laramide Orogeny, creating the Black Hills in western South Dakota. Despite this evidence for decratonization, seismic tomography reveals high velocities in the upper mantle between the Bighorn Mountains and the Black Hills at depths of up to 250 km, a region we refer to as the Thunder Basin Block (TBB). The anomaly has been interpreted as either a stable piece of lithosphere, which may or may not be original to the craton, or a weakened remnant in the process of delamination. The proximity of the block’s edge to the Black Hills suggests that the block may have played a role in their uplift. Our seismic experiment, nicknamed CIELO (Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny), operated from September 2017-2019 in order to better constrain the geographic limits and physical properties of the TBB. Preliminary teleseismic P delay times within the region, corrected for topography and basin structure, find early arrivals related to high-velocity mantle structure. These early results are consistent with a +2.5% anomaly over the upper 200 km of the mantle that sharply terminates at the western edge of the Black Hills and may mark the limit of the TBB. Ps receiver functions show rapid lateral variations in crustal thickness beneath the Black Hills, with a minimum of 39 km along the western edge, which is co-located with the margin of the high-velocity feature, thickening to a maximum of 58 km along the eastern edge of the mountains. There is also strong evidence for anomalous discontinuities within the mid-crust across the Black Hills. These preliminary results show that there is pronounced and complex structure at the eastern edge of the Thunder Basin Block throughout both the crust and upper mantle, suggesting a connection between the uplift of the Black Hills and the internal, potentially modified structure of the craton.