UPPER MANTLE TOMOGRAPHIC VP AND VS IMAGES OF THE MIDDLE ROCKY MOUNTAINS IN WYOMING, COLORADO AND NEW MEXICO: EVIDENCE FOR A THICK HETEROGENEOUS CHEMICAL LITHOSPHERE

Upper mantle tomographic compressional (Vp) and shear (Vs) wave images from a one-year 50 broadband PASSCAL station Continental Dynamics of the Rocky Mountains (CD-ROM) deployment are presented. The deployment was designed to image lithospheric scale variations across two primary geologic sutures: the Archean-Proterozoic Cheyenne suture and the Proterozoic-Proterozoic Jemez suture. Our most robust findings are: (1) A north dipping high velocity “slab” extends from the Moho to ~200 km depth beneath the Cheyenne suture. The correlation of Vp and Vs images, anisotropic travel-time modeling and inversion suggest that half of the velocity anomaly is derived from dipping seismic anisotropy, and the rest from isotropic high velocity. Shear-wave splitting measurements show dramatic change of seismic anisotropy domain across the Cheyenne belt, and forward modeling favors a north dipping anisotropy beneath Cheyenne belt. We suggest this high velocity slab is a Proterozoic slab fragment trapped in Archean Wyoming craton during the 1.78-1.74 Ga period of arc collisions. Geological evidence favors a south-dipping model for the Cheyenne suture. Thus our observed north-dipping slab suggests a polarity flip of the fossil subduction system. The flip of subduction polarity is also seen in the North Banda sea in Australia; (2) A 100 km wide, 3% low P-wave velocity body is imaged extend from the Moho to 100 km depth beneath the Jemez suture. Given that the Jemez suture is currently being exploited as a late Cenozoic volcanic lineament, we suggest that lower-solidus mantle compositions are producing basaltic magmas with primarily lithospheric geochemical fingerprints; 3) A low velocity anomaly (stronger on the S-wave image) exists beneath the middle segment of the Rio Grande Rift in the San Luis Valley. This anomaly is spatially connected to a “pipe” of low Vs velocities that extends to 300 km depth. This may suggest a deep thermal upwelling resides at least beneath the middle Rio Grande Rift. Given the profound variations in upper mantle Vp and Vs velocity structure, we suggest our observed velocity anomalies result from a thick, variable chemical lithosphere that was created during the accretionary events of the Rocky Mountain regions. Further correlations between the velocity structure and the mantle layering support this conclusion.