PALEO-SUBDUCTION AND MODERN BASALT EXTRACTION STRUCTURES IN THE SOUTHERN ROCKY MOUNTAINS: MULTI-BAND IMAGES FROM THE CD-ROM EXPERIMENT

The CD-ROM seismic experiments targeted two Paleoproterozoic suture zones in the western U.S. in a north-south study corridor that extends from central New Mexico to central Wyoming. Seismic reflection, refraction, and teleseismic measurements were made across the Cheyenne Belt in southern Wyoming, and across the Jemez Lineament in northern New Mexico. The Cheyenne Belt is a profound geologic boundary separating the Archean Wyoming craton from island arcs accreted to the proto-continent in the Paleoproterozoic. The Jemez Lineament is a linear trend of modern volcanics extending SW from southern Colorado to Arizona, and also coincides with the southern edge of the suture between Yavapai and Mazatzal Paleoproterozoic island arc terranes. Karlstrom and Humphreys (1998) have speculated that the ancient accretion boundaries influence Cenozoic tectonism in the western U.S., noting the correlation of NE-SW low velocity upper mantle tomography anomalies with geochemical boundaries and mapped suture zones in the Southern Rocky Mountains.

At the Cheyenne Belt, the combined reflection-refraction-teleseismic datasets show crust and upper mantle subduction structures that are inferred to have formed during continental accretion and stabilization of the southwestern U.S. Of particular note are a north dipping high velocity slab with a fragment of subducted crust imaged in both the P and S tomography and prestack depth migrated receiver function images.

At the Jemez Lineament the reflection data image a bivergent orogen marking the Yavapai-Mazatzal suture in the crust. Refraction velocities in the upper mantle under the suture zone suggest that the upper mantle contains 1% partial melt. In the same upper mantle region P and S tomography show large magnitude low velocity anomalies that correspond to a series of moderately bright but complicated upper mantle events in the pre-stack depth migrated receiver function images. We have modeled this complex series of events in the upper mantle as the source zone of the recently erupted basaltic magmas found along the Jemez Lineament portion of the CD-ROM profile. We speculate that the paleo-suture zone left from continental accretion acts as a crustal conduit for basaltic magmas to pass through the crust, form sills, and erupt.