LITHOSPHERIC VELOCITY STRUCTURE ACROSS THE CHEYENNE BELT NEAR LARAMIE, WYOMING

The Cheyenne Belt of southeastern Wyoming, a major study area for the Continental Dynamics Rocky Mountains project (CD-ROM), is a 1.78 ga, south dipping suture of Proterozoic islands arcs accreted onto the Archean Wyoming craton. Our goal is to gain a better understanding of the deep structure of this suture. A fundamental question is whether the collision that created the suture was thick or thin skinned. In August of 2000, 31 telemetered Passcal broadband seismometers were deployed for 8 months across the Cheyenne Belt near Laramie, Wyoming. We measured P and S wave travel time residuals to determine a velocity structure of the crust and subcrustal lithosphere beneath this array. Earlier studies of xenolith suites in this area suggest the suture dips to the south to a depth of 200 km. Also, the xenolith studies indicate a strong contrast in lower crust and upper mantle structure across the suture. Morozova et al. (2002) show strong contrasts in reflectivity between the Archean and Proterozoic crust. Our preliminary results show large velocity variations along the transect. The upper crust of the northern (Archean) portion contains very slow velocities, while the southern (Proterozoic) portion contains fast upper crust velocities. This suggests structural contrasts across the suture. The P and S tomographic wave images will give us complimentary information that will allow us to constrain rock types.