Crustal and uppermost mantle structure along the Deep Probe seismic profile

Abstract

The Rocky Mountain region has undergone a complex tectonic history that includes Proterozoic accretion to form the North American craton, late Paleozoic deformation, Cretaceous to early Tertiary shortening, and Oligocene to Recent extension. Understanding the effects of these events on lithospheric structure was the primary goal of the Deep Probe seismic experiment. This is a lithospheric-scale study of the Rocky Mountain region that attempted to image crust and upper mantle structures up to 500 km depth to provide insights on the effect of various tectonic events on today's continental structure. To accomplish this goal, instruments were deployed along a 2400-km-long transect from New Mexico to Canada to record explosions 10 times more powerful than those employed in conventional crustal studies. The Deep Probe results provide new constraints on the location and geometry of the Archean–Proterozoic boundary near the Colorado–Wyoming border, as well as new information on crustal thickness, and uppermost mantle velocities along the profile. Geophysical modeling of the profile used well log and geologic data to evaluate the composition and structure of the uppermost crust. Seismic refraction and reflection, gravity, and receiver function studies were employed to constrain properties of the lower crust and upper mantle structure. The final model shows that seismic velocities along the Deep Probe profile range from 3.5 km/s in the basins to over 8.2 km/s in the upper mantle. At the southern end of the profile, the model indicates a crustal thickness of about 35 km beneath the Basin and Range province. The crust gradually thickens to about 40 to 45 km going north along the profile into the Colorado Plateau. An area of 50 km-thick crust under northwestern Colorado may reflect Proterozoic tectonism related to the suture zone between the Archean and Proterozoic terranes. Northwestward thinning of the crust to about 40 km under southern Wyoming is interpreted as evidence for a relict (2.0 Ga) passive continental margin. The crust in the Archean Wyoming province thickens to over 50 km going north, and then thins again under southern Canada. This thickening is due to a lowermost crustal layer that is about 20 km thick and is confined to the Archean Wyoming province. This lower crustal layer has velocities ranging from 7.05 to 7.3 km/s, which corresponds to a mafic composition. Thus, this layer is interpreted as mafic material that was probably underplated during the Archean. The uppermost mantle of the Archean Wyoming province has lower velocities (∼8.1 km/s) on average than typical cratonal areas, which is consistent with it being located in and adjacent to the North American Cordillera, which has undergone significant recent tectonism.