Geology topics

K.H. Olsen

Publications and source records attributed to K.H. Olsen.

3 recordsLinked to original sources

USGS2012

The Yellowstone-Snake River Plain seismic profiling experiment: Crustal structure of the Eastern Snake River Plain

Seismic refraction profiles recorded along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 Yellowstone-Snake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q-1) structure of the ESRP. Travel-time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an upper-crustal layer (compressional velocity ≅6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediate-velocity (≅6.5 km/s) layer extends from ≅10 to ≅20 km depth. A thick (≅22 km) lower crust of compressional velocity 6.8 km/s, a total crustal thickness of ≅42 km, and a P n velocity of ≅7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to low-Q values in the volcanic rocks (Q p = 20 to 200) and throughout the crust (Q p = 160 to 300). Based on these characteristics of the crustal structure and volcanic-age progression data, it is suggested that the ESRP has resulted from an intensive period of intrusion of mantle-derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate layer. Crustal subsidence and periodic basaltic volcanism as represented by the ESRP complete the sequence of crustal evolution.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

Journal of Geophysical Research B: Solid Earth↗

USGS1984

Upwarp of anomalous asthenosphere beneath the Rio Grande rift

Continental rifts are possible analogues of mid-ocean ridges, although major plate tectonic features are less clearly observed1. Current thermal models of mid-ocean ridges2-4 consist of solid lithospheric plates overlying the hotter, less viscous asthenosphere, with plate thickness increasing away from the ridge axis. The lithospheric lower boundary lies at or near the melting point isotherm, so that at greater depths higher temperatures account for lower viscosity, lower seismic velocities and possibly partial melting. Upwarp of this boundary at the ridge axis concentrates heat there, thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium. At slow spreading ridges, a major central graben forms owing to the mechanics of magma injection into the crust5. Topography, heat flow, gravity and seismic studies support these models. On the continents, a low-velocity channel has been observed, although it is poorly developed beneath ancient cratons6-9. Plate tectonic models have been applied to continental basins and margins10-12, but further similarities to the oceanic models remain elusive. Topographic uplift is often ascribed to Airy type isostatic compensation caused by crustal thickening, rather than thermal compensation in the asthenosphere. Here we discuss the Rio Grande rift, in southwestern United States. Teleseismic P-wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening. ?? 1984 Nature Publishing Group.

Nature↗