Anthony Lowry

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The Laramide Orogeny, a late Cretaceous mountain building event in the western United States, 1,2,3 has been postulated to result from flat­slab style subduction of the Farallon plate . Possible consequences of flat­slab style subduction on elevation, tectonism and volcanism associated with expected changes in temperature and hydration state of the mantle in the western U.S. has x1 been of considerable interest. However, the impact of flat­slab subduction on modern mantle temperature, water and tectonic stability remains poorly­understood. Here we show that thermal perturbation by Farallon flat slab subduction is still evident today in Moho temperatures of the Rocky Mountain and Great Plains regions of the U.S. Measurements of temperature in the 4 uppermost mantle from tomographic P­wave velocity imagesare compared to models of surface heat flow measurements assuming steady­state lithospheric thermal transfer and reasonable rock thermal properties. Misfit between modeled and measured Moho temperature indicates a violation of the modeling assumptions, likely dominated by transient perturbations on timescales less than the ~100 Myr required for a conductive thermal pulse to traverse the lithosphere. Misfit is dominated by anomalously low measurements in regions deformed by thick­skin tectonism during the Laramide, suggesting the temperature discrepancy may reflect thermal perturbation by Farallon flat­slab subduction. Shallow subducted slab would serve both to chill the base of the lithosphere and insulate it from deeper convective heating. The measurements indicate perturbation of mantle lithospheric temperature to distances of more than 1800 km from the modern plate boundary, where Laramide­style tectonism was weakly expressed. These results may have significant implications for history, evolution and stability of 5 continental lithosphere within EarthScope’s USArray footprint.

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