Upper mantle tomographic V_p and V_s images of the Rocky mountains in Wyoming, Colorado and New Mexico: Evidence for a thick heterogeneous chemical lithosphere

Upper mantle tomographic body wave images from the CD-ROM deployment reveal two major lithospheric anomalies across two primary structural boundaries in the southern Rocky Mountains: a ~200 km deep high velocity north-dipping “Cheyenne slab” beneath the Archean-Proterozoic Cheyenne belt, and a 100 km deep low velocity “Jemez body” beneath the Proterozoic-Proterozoic Jemez suture. The Cheyenne slab is most likely a slab fragment accreted against the Archean Wyoming during the Proterozoic arc collision processes. This interpretation suggests that the ancient slab’s thermal signature has been diffused away and non-thermal explanations for the high velocity slab are required. Tomographic modeling of possible chemical and anisotropic velocity variations associated with the slab shows that our isotropic velocity images can be explained via non-thermal models. In addition, the de-correlation of the P- and S-velocity images and the CD-ROM shear-wave splitting modeling are consistent with a dipping slab. The Jemez body plausibly results from the combination of low-solidus materials in the suture lithosphere and the late Cenozoic regional heating of the lithosphere. The 100 km deep lithospheric layering and the uniform shear-wave splitting measurements support our contention that the Jemez body is a lithospheric anomaly. A third low velocity structure extends beneath the middle Rio Grande Rift to 300 km depth. This anomaly may manifest a thermal upwelling that could increase heat flow into the lithosphere. Our results suggest that lithospheric heterogeneities related to fossil accretionary processes have been preserved in the Precambrian sutures, and are preferentially affecting the subsequent tectonism in this region.