VELOCITY STRUCTURE ALONG THE CD-ROM '99 SEISMIC REFRACTION/WIDE-ANGLE REFLECTION EXPERIMENT

The Continental Dynamics Rocky Mountains Project (CD-ROM '99) is a collaborative interdisciplinary study involving ~14 American universities and the University of Karlsruhe, Germany and focuses on Precambrian features and their effects on Phanerozoic deformation. One of the major field efforts in the CD-ROM project took place during August, 1999. The University of Texas at El Paso and the University of Karlsruhe, with the assistance of several other institutions, collected data along a ~950 km long seismic refraction/wide-angle reflection profile. As a result, the line trends SSE-NNW from about Fort Sumner, New Mexico to the Gas Hills, Wyoming. It crossed such features as the Jemez lineament, the Colorado mineral belt, and the Cheyenne belt (a prominent Proterozoic suture). Station spacing was nominally 800 m using ~600 instruments during two deployments. Eleven shot points were fired ranging in size from ~900 to 4500 kg and were nominally spaced at 100 km intervals along the profile. All the data show strong Pg arrivals. PmP (upper mantle reflection) is prominent for most shots as well as is PiP (intra-crustal reflection). Pn (upper mantle refraction) arrivals are apparent beyond 200 km on most record sections. For record sections that cover the SRM-GP (Southern Rocky Mountains and Great Plains) the Pi (intra-crustal refraction) and PiP phases are clearly evident on record sections where there is a high signal to noise ratio whereas on more noisy record sections these phases are more subjective. Initial 2-D modeling indicates crustal thicknesses ranging from ~45 to 50 km in New Mexico and Colorado. In northern Colorado the crust begins to thin from ~50 to ~40 km in Wyoming, near the Cheyenne belt. A high-velocity lower crustal layer of about ~10 km thickness is evident in the SRM-GP portion of the model. One interpretation of this high-velocity lower crustal layer is that it is a fundamental crustal element of the undisturbed Proterozoic terranes. However, there have been many Phanerozoic events where this layer could have been inflated, attenuated, or removed throughout the Rocky Mountains.