Local Re-Cratonization of the Wyoming Province and the Uplift of the Black Hills

Abstract:

The Archean Wyoming Craton is a primary building block of the core of the North American continent. It resisted tectonic deformation for over a billion years, but during the Laramide orogeny significant crustal shortening was accommodated by basement-involved thrusting. Xenoliths suggest that ~50 km of cratonic mantle were removed from the Wyoming Province by basal erosion during this time. This orogenic event resulted in the formation of several mountain ranges in Wyoming which are part of the Rocky Mountains. The eastern limit of the Rocky Mountains in Wyoming is typically defined by the Big Horn and Laramie ranges. The Black Hills represent a topographic anomaly having a similar trend to the Big Horn and the Laramie Mts, but lying ~200 km NE of the Rocky Mountain Front (RMF); while the intervening region is relatively undeformed. Subduction of the conjugate to the Shatsky Rise is the leading hypothesis to explain the flattening of the Farallon slab that lead to the Laramide orogeny. The leading edge of the Shatsky conjugate would have been the conjugate to the Tamu Massif, the largest known single volcano on the planet. The reconstructed path of the Tamu conjugate plausibly places it beneath the region between the RMF and the Black Hills, where a high seismic velocity anomaly is observed by body wave tomography to depths exceeding 200 km. We propose that subcretion of the highly depleted mantle lithosphere of the Tamu cojugate re-cratonized a region of the Wyoming Province forming a distinct rigid block. We further suggest that the new block successfully resisted tectonic deformation and transferred the stresses of the Laramide orogeny northeastward to generate the basement-cored uplift of the Black Hills.