Catherine M. Snelson

Crustal and uppermost mantle structure along the Deep Probe seismic profile

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 todays 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.

Amplification of Seismic Waves by the Seattle Basin, Washington State

Recordings of the 1999 M w 7.6 Chi-Chi (Taiwan) earthquake, two local earthquakes, and five blasts show seismic-wave amplification over a large sedimentary basin in the U.S. Pacific Northwest. For weak ground motions from the Chi-Chi earthquake, the Seattle basin amplified 0.2- to 0.8-Hz waves by factors of 8 to 16 relative to bedrock sites west of the basin. The amplification and peak frequency change during the Chi-Chi coda: the initial S -wave arrivals (0–30 sec) had maximum amplifications of 12 at 0.5–0.8 Hz, whereas later arrivals (35–65 sec) reached amplifications of 16 at 0.3–0.5 Hz. Analysis of local events in the 1.0- to 10.0-Hz frequency range show fourfold amplifications for 1.0-Hz weak ground motion over the Seattle basin. Amplifications decrease as frequencies increase above 1.0 Hz, with frequencies above 7 Hz showing lower amplitudes over the basin than at bedrock sites. Modeling shows that resonance in low-impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification, and the larger amplification at later arrival times suggests surface waves also play a substantial role. These results emphasize the importance of shallow deposits in determining ground motions over large basins.

Chemical Explosion Experiments to Improve Nuclear Test Monitoring

A series of chemical explosions, called the Source Physics Experiments (SPE), is being conducted under the auspices of the U.S. Department of Energy’s National Nuclear Security Administration (NNSA) to develop a new more physics-based paradigm for nuclear test monitoring. Currently, monitoring relies on semi-empirical models to discriminate explosions from earthquakes and to estimate key parameters such as yield. While these models have been highly successful monitoring established test sites, there is concern that future tests could occur in media and at scale depths of burial outside of our empirical experience. This is highlighted by North Korean tests, which exhibit poor performance of a reliable discriminant, mb:Ms (Selby et al., 2012), possibly due to source emplacement and differences in seismic responses for nascent and established test sites. The goal of SPE is to replace these semi-empirical relationships with numerical techniques grounded in a physical basis and thus applicable to any geologic setting or depth.

Geophysical studies of crustal structure in the Rocky Mountain region A review

The Rocky Mountains have fascinated the geological community for over 100 years, but crustal-scale geophysical studies are relatively rare in this region. However, a knowledge of crustal structure is essential if we are to fully understand the regions tectonic history. Thus, we have compiled and synthesized existing information on crustal structure in order to provide as complete a picture as possible at this time. We have focused on Wyoming, Colorado, and New Mexico where there are enough data to make useful correlations with geologic features. The Rocky Mountain region includes the crest of a broad uplift on which the Southern Rocky Mountains are located. In turn, the Southern Rocky Mountains are bisected by the Rio Grande rift. In the Rio Grande rift, distinct crustal thinning (at least 5 km relative to adjacent areas) can be documented from Albuquerque, New Mexico southward. The area of thinned crust widens southward, as does the physiographic expression of the rift. The thinnest crust documented to date (about 28 km) is found west of El Paso, Texas. In contrast with East Africa, the crustal thinning is gradual from the rift valley to the shoulders, perhaps reflecting the back-arc thermal regime that existed prior to rifting. The thickest crust in the region (about 53 km) appears to be associated with both the Southern Rockies in Colorado and the topographically lower Great Plains in Colorado and New Mexico. This lack of correlation between topography and crustal thickness implies that the mantle is playing a major role in the attainment of isostatic balance in this area. Magmatic modification of the crust during rifting appears to have been minor. However, the modification due to the voluminous mid-Tertiary magmatism in the Datil–Mogollon volcanic field (southwestern New Mexico) and San Juan volcanic field (southwestern Colorado) is substantial. In the Datil–Mogollon field, a batholith that accounts for about one fifth of the crustal thickness has been detected in the upper crust, and a feature of similar dimensions is indicated in the San Juan region. There is evidence of the crust thinning northward from Colorado into Wyoming, which could be a relic of Archean rifting of the southern margin of the Wyoming craton.