SEISMIC EVIDENCE FOR THE PRESERVATION OF PROTEROZOIC TERRANES WITHIN THE COLORADO PLATEAU

Continental deformation during the Laramide orogeny occurred well away from the margins of the North American plate. The Colorado Plateau represents an intact uplifted region in western North America that escaped Laramide deformation and is now surrounded by the deformed Basin and Range, Rocky Mountains, and Rio Grande Rift. Studies have sought to determine if buoyancy forces originating at subcrustal depths are needed to support the elevations of the plateau or if its crust is sufficiently thick to support the plateau topography. Similarly, both the crust and mantle have been investigated in attempts to identify the factors that have allowed the plateau to resist deformation. Because the plateau has not been reworked by recent tectonic events it appears to have retained structures related to continental accretion, whose features provide insight into the factors leading to the differing evolution of the plateau compared to its surrounding regions.

We will present detailed observations of crustal structure of the southeastern portion of the Colorado Plateau from seismic data collected during recent deployments of dense temporary broadband arrays. The crust towards the center of the plateau within this area is near 40 km thick and thins to 35 km at the eastern margin of the plateau. The amplitude of the Moho signal also varies within the plateau appearing brighter along its margin and much weaker and diffuse in the interior of the plateau. Changes in the character of the Moho signal are also accompanied by abrupt differences in mid-crustal structure that we interpret to mark the boundaries of the Proterozoic terranes that comprise the North American continent. The observed differences in structure between terranes extend across the entire thickness of the crust and perhaps into the mantle indicating the preservation of terranes as coherent lithospheric columns. The preservation of boundaries between terranes indicates that it is unlikely that extensive crustal flow could have traversed across the entire plateau without disturbing the structure of each terrane, and argues for support mechanism other than mid-crustal flow.