Transportable arrays in Wyoming, Colorado, and New Mexico

Introduction. Ps and Sp images of sub-moho negative arrivals beneath the intermountain region of western U.S. are made using dense (2-40 km station spacing) arrays shown to the left that recorded for 1-2 years: the CREST, RISTRA, SIEDCAR, LODORE, LARAMIE, BILLINGS array data combined with Transportable Array data. The nominal Sp interpretational zoo includes an LAB arrival thought to manifest the juxtaposition of melt against the base of the thermal lithosphere and a MLD arrival that manifest a low velocity step/notch in the middle of the chemical lithosphere. Our images find a zoo of sub-moho Sp negative features namely sharp offsets, doublets, and dim-outs that sometimes correlate as expected with the upper mantle velocity structure, but sometimes do not. The primary Sp negative arrival is at 70-95 km depth and shallows 20-30 km beneath the Colorado Rocky Mountains (CRM), the Rio Grande Rift (RGR), and at the Basin and Range/Colorado Plateau boundary (BR/CP). A second deeper Sp negative is found at 150-160 km depth in SE New Mexico. The deeper Sp negatives may manifest a deep solidus crossing due to the upwelling of hydrous diapirs from a 410 low-velocity layer imaged by Ps waves beneath the CRM. Interpretation of these Sp features, in tandem with Ps images and surface and body wave tomograms, are helping to constrain the origin (lithospheric potential energy release or upwelling transition zone diapirs) of the positive buoyancy flux required to drive the young (<9 Ma) uplift of this area.