+ Visit NASA.gov + Contact NASA   + Home Collection > NASA  Author > Dueker, Kenneth G.  Author > Humphreys, Eugene D.  NASA Center > Goddard Space Flight Center  Publication Year > 1991-2000 > 1994  Subject > G-I > Geophysics  Availability Options > Order From Other Sources  Item/Media Type > Journal Article  Note: Start a new navigation search by selecting a link above     Previous Record | Next Record + Back to Results   + Printer Friendly Title: Physical state of the western U.S. upper mantle Author(s): Humphreys, Eugene D.; Dueker, Kenneth G. Abstract: Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100-200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only resonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within approximately 250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of years, the imaged upper mantle structure is young. The following upper mantle processes are hypothesized to account for these findings and inferences. Away from the plate margin, small-scale upper mantle convection driven by partial melt-induced buoyancy of hot upper mantle leads to the production and segregation of melt and the creation of compositional variations. The heterogeneous upper mantle P wave structure of the elevated continental interior is largely a consequence of partial melt variations that are modulated by the compositional variations, and throughout this region we infer high temperatures and low densities. Near the plate margin, relative plate motions force upper mantle flow, although upper mantle flow driven by the positive buoyancy of melt and the negative bouyancy lithosphere is important locally. NASA Center: Goddard Space Flight Center Publication Date: May 10, 1994 Document Source: Other Sources No Digital Version Available: Go to Tips On Ordering Document ID: 19950030730 Accession ID: 95A62329 Publication Information: Journal of Geophysical Research, 99, B5, p. 9635-9650, ISSN 0148-0227, Number of Pages = 16 Contract-Grant-Task Number: NAG5-755 Keywords: EARTH CRUST; EARTH MANTLE; EARTH PLANETARY STRUCTURE; GEODYNAMICS; GEOPHYSICS; P WAVES; PLATES (TECTONICS); SEISMOLOGY; SUBDUCTION (GEOLOGY); TECTONICS; TOPOGRAPHY; UNITED STATES; BUOYANCY; CHEMICAL COMPOSITION; GEOMAGNETISM; HEAT TRANSFER; MAPS; MELTING; SLABS; TABLES (DATA); VARIATIONS; VELOCITY; Notes: Journal of Geophysical Research vol. 99, no. B5 p. 9635-9650 May 10, 1994 Accessibility: Unclassified; Copyright; Unlimited; Publicly available; Updated/Added to NTRS: 2004-11-03 + Back to Top + Sponsored by the NASA Scientific and Technical    Information Program + 2004 Vision for Space Exploration + Freedom of Information Act + NASA Web Privacy Policy and Important Notices + NASA Disclaimers, Copyright Notice,    and Terms and Conditions of Use NASA Official: Calvin Mackey Page Curator: NASA Center for AeroSpace Information (help@sti.nasa.gov) Last Updated: July 5, 2007