MANTLE STRUCTURE BENEATH THE ROCKY MOUNTAINS: CD-ROM PASSIVE EXPERIMENT

The study of the continental mantle lithosphere is starting to provide insight into the role the sub-crustal lithosphere plays in the creation, stabilization and modification of continents. Recent seismic images near suture zones show reflectors in the mantle lithosphere most plausibly explained as “fossilized” subducted slabs. Upper mantle velocity images often show distinct velocity heterogeneity between differing crustal provinces in the upper 100 - 200 km. Mantle xenoliths from different age provinces often show significant compositional variations. The nature and source of these observations are still under debate, do these heterogeneities come from the crust or are they from the mid to lower mantle?

The Continental Dynamics - Rocky Mountains (CD-ROM) experiment seeks to constrain the evolution, stabilization and modification of the continental lithosphere. We present the detailed results of images of the mantle constructed using broadband teleseismic receiver functions beneath the interior western United States, with focus on the southern Rocky Mountains. The targets of this experiment were the Cheyenne suture, an Archean continent – Proterozoic arc terrane boundary, and the Jemez suture/volcanic lineament, which separates the Proterozoic Mazatzal and Yavapai provinces. The main features observed underneath the Cheyenne belt are: 1) a Proterozoic oceanic slab most plausibly tectonically emplaced beneath the rifted Archean margin; 2) Structurally separate, old and thick (>150 km) lithosphere as evidenced by changes in mantle layering across the Cheyenne Suture. Below the Jemez volcanic lineament two strong sub-crustal layers are imaged down to 100 km depth that correlate well with a low velocity zone. This layering is interpreted to map the extent of the lithosphere and most plausibly results from changes in lithosphere’s chemical makeup. The mantle transition zone in this region shows 30 km of topography. This would suggest significant thermal heterogeneities possibly originating from mass transport across the transition zone, supplying the upper mantle with the observed heterogeneities.