Thick-structured Proterozoic lithosphere of the Rocky Mountain region

A new tomographic image of the western United States shows three northeast-trending, low-velocity, upper mantle anomalies in the Rocky Mountain-Colorado Plateau region: the Yellowstone, Saint George, and Jemez, lineaments. Each is characterized by small compressional wave-speed anomalies (±2% perturbations) that extend to 200-250 km depth. A fundamental question is whether they represent ongoing asthenospheric convention or old lithospheric compositional bodies. This puzzle is compounded by the observation that each is aligned with both young volcanic fields and Proterozoic crustal grain and/or sutures. We suggest that the low-velocity bodies are lithospheric anomalies and that they were derived from melting of hydrated olivine-poor lithologies (oceanic slabs, their associated sediments, and batholith residue) that were tectonically emplaced during Proterozoic suturing events. Such lithologies would be hydrated by the water in oceanic slabs and sediments trapped during the suturing processes. Our suggestion is consistent with the geochemical fingerprint of most of the young volcanics that indicate melting of an old and chemically diverse lithosphere that often contains a subduction zone trace-element signature. In addition, the sharp and dipping lateral velocity gradients bounding the low-velocity bodies, in particular where extensional deformation is small, suggest these bodies are not upwelling asthenosphere. This suggestion that the low-velocity bodies follow Proterozoic lithospheric sutures is supported by new teleseismic data from the Continental Dynamics-Rocky Mountains project that reveal a surprisingly thick continental lithospheric. In our transect across the Proterozoic Jemez suture, we find the coincidence of young lithospheric volcanism, a low-velocity mantle anomaly, and deep lithospheric layering (to 170 km). In our Wyoming-Colorado transects across the Archean-Proterozoic Cheyenne suture, we find the coincidence of deep mantle layering and a north-dipping, high-velocity slab, which extends (to 200 km depth) from the base of an imbricated Moho, directly under the Cheyenne suture. We suggest this slab was trapped against the edge of the thick, Archean-age Wyoming lithosphere after the subduction polarity flipped from south-to north-directed after and/or during accretion of the first Proterozoic arc along Wyoming's southern margin 1.78-1.75 Ga. Such a tectonic model for the evolution of the Cheyenne belt is consistent with observations along many other Archean-Proterozoic sutures worldwide. Overall, our results demonstrate that Proterozoic crustal sutures in the Rocky Mountain region extend throughout a thick chemical lithosphere and that young lithospheric melting has been focused along old suture zones. The coincidence of old deep structure and young tectonism supports the hypothesis that the lithospheric structure created during Proterozoic assembly provides a first-order control on the complex history of exhumation, deformation, sedimentation, and magmatism of this fascinating, tectonically active region.