VELOCITY STRUCTURE OF THE ROCKY MOUNTAINS BASED ON DEEP PROBE '95 AND CD-ROM '99 SEISMIC REFRACTION/WIDE-ANGLE REFLECTION DATA

The Deep Probe '95 and CD-ROM '99 refraction wide/angle reflection experiments were designed to illuminate the lithospheric structure of the Rocky Mountains. Both projects focused on Precambrian features and their effects on Phanerozoic deformation. The Deep Probe profile crossed such geologic features as the Yavapai-Mazatzal Transition zone, the Cheyenne belt, the Great Falls Tectonic zone, the Medicine Hat block, the Vulcan structure, and several Archean blocks within the Hearne and Slave Provinces. The CD-ROM profile crossed such geologic features as the Jemez lineament, the Colorado mineral belt, and the Cheyenne belt. The CD-ROM profile ended at the Wyoming shot point, which was re-occupied from Deep Probe. South of the Wyoming shot point, these profiles diverge and are separated by the Rio Grande rift.

Major results from the Deep Probe velocity model were the observation of the Cheyenne belt suture zone at depth where the crust thickens from 40-50 km and then subsequent thinning of the crust to the north to about 40 km indicating a relict rift. In northern Wyoming, the crust then thickens dramatically where a high-velocity lower-crustal (HVLC) layer is present with a thickness of about 20 km and continues to the north over 100 km. Final results from the CD-ROM velocity model show a small step in the Moho in northern New Mexico, which could indicate the location of the Jemez lineament. The northern end of the model shows similar thickening and thinning of the crust in association with the Cheyenne belt and a relict rift, which were seen in the Deep Probe model. The southern end of the model shows a HVLC layer, which ranges in thickness from 5-10 km and is most evident under the Great Plains. The HVLC layer identified in the Deep Probe model is not the same as the layer identified in the CD-ROM model. The Deep Probe HVLC layer is interpreted to have formed in the Archean or late Proterozoic through a series of crustal building events. In contrast, the interpretation of the CD-ROM layer is that it formed in the Proterozoic (1.7-1.6 Ga) and was subsequently modified in association with widespread magmatism (1.4 Ga). Although the process for creating such HVLC layers are probably very similar, the age, extent and crustal structure of the HVLC layers from these profiles are distinctly different.