Collaborative Research: Lithospheric Structure andEvolution of the Rocky Mountain Transect of the Western U.S.: An Integrated Geological and Geophysical Investigation

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9614208 Johnson This collaborative study is aimed at understanding the evolution of the lithosphere of the Rocky Mountain transect in the North American southwest. This transect is located at the juncture of two globally-unique tectonic regimes, each of fundamental importance for understanding continental tectonics and together representing an outstanding field laboratory for studies of continental lithosphere. First, a 1000 km wide juvenile Proterozoic orogenic belt records rapid accretion of continental materials from mantle sources and their assembly to southern Laurentia along NE-trending boundaries between 1.8 and 1.6 Ga. Second, the present high elevation of the SW in general and the Rocky Mountains in particular is the manifestation of Laramide, Tertiary, and still ongoing modification of Proterozoic Lithosphere that has resulted in a wide NW-trending mantle boundary zone between old, cold, high velocity mantle in the continental interior and hotter, lower velocity mantle in the western U.S. By examining the present-day lithospheric structure across ancient boundaries, the P.I.s hope to better understand the interplay between "active" and "passive" processes during lithospheric evolution. Their principal hypothesis is that lithospheric structures produced during assembly of the southwestern U.S. have profoundly influenced physical and chemical modification of the continental lithosphere during all subsequent magmatic and tectonic events, including those related to the ongoing reorganization of small-scale asthenospheric convection and replacement of lithosphere by asthenosphere. To test this hypothesis, they propose an integrated set of multi-scale experiments using state of the art geophysical, geological and geochemical techniques that, together, are designed to decipher lithosphere structure and evolution, and processes of lithosphere formation and deformation. ***