The lithosphere is a boundary layer above the convecting upper mantle, influencing convection in a manner that has not yet been well determined or modeled. Also, the lithosphere, especially on continents, records certain aspects of the dynamic mantle over most of Earth's history. Lithospheric processes such as volcanism and epiorogenic uplift can strongly affect the Earth's weather, and possibly, over the longer term, climate.

The Project.

"Lithospheric Processes" is oriented toward a better understanding of fundamental deformational and magmatic processes affecting the Earth's lithosphere. Processes include mechanisms by which lithosphere is thinned -- or maintained without thinning -- during extension, the possibility of lateral transfer of lower crust to maintain a uniform crustal thickness during extension, mechanisms by which melts are generated and rise to the surface in different tectonic environments, and lateral variations of seismic properties in the crust and underlying mantle. This project incorporates a variety of geological, geochemical, and geophysical techniques, some standard, others innovative, to investigate the origin and evolution of Earth's lithosphere. Included in this project are studies oriented toward such processes as fluid transport that have application to environmental concerns, and problems of social importance such as volcanic and earthquake hazards. We include studies of the asthenosphere, which underlies the lithosphere, because understanding the lithosphere typically involves comparison with the Earth's asthenosphere, and because many processes affecting the lithosphere typically originate in the asthenosphere. The emphasis under this project is on studies that have global application to understanding processes by which lithosphere is created and modified and its relationship to the underlying asthenosphere.

The present work builds on and will broaden the expertise available at Los Alamos related to fundamental geophysical and geochemical studies of the Earth. In addition, researchers at Los Alamos have been instrumental in developing advanced laboratory and numerical techniques for investigating fundamental processes of the lithosphere. Such techniques include computer codes to model the dynamics of volcanic eruptions, and high-precision mass spectrometric techniques for isotopic analysis of trace constitutent of rocks. The overall goal of these studies is to understand the dynamic processes by which Earth's lithosphere is formed and modified by convective processes within the mantle, and ultimately to allow geological observations made at the Earth's surface to constrain results of numerical modelling of convection in the Earth's mantle and core. These goals are being pursued by promoting collaboration between Laboratory scientists and university faculty and students.

Approach and progress to date. This project uses a variety of geochemical and geophysical approaches to understanding the lithsphere and underlying asthenosphere. Included are: (1) seismic surface and body waves, generated by naturally occurring earthquake activity, used to construct better regional and global velocity models for the crust and upper mantle and to understand crustal structure and fault mechanisms in the western US; (2) the geochemistry, including U and Th isotopic compositions, of volcanic rocks from selected tectonic environments to better understand melt-generation and eruption processes in a variety of tectonic settings; and (3) new numerical codes, developed and applied to modeling and predicting volcanic phenomena. In addition, this work combines petrological, geochemical, and geochronological studies of young volcanic rocks in Ethiopia and Kenya to understand volcanism in the African rift. These studies are important for understanding continental rifting, but also have application to the paleoanthropology of the area, allowing us to extend our understanding of hominid evolution back in time to nearly 5 million years ago.

Achievements include:

(1) Determining that the crust-mantle boundary beneath the southern Great Basin is flat at 30 km depth. This result is not to be expected a priori, since the large amount of extension that the Basin and Range has undergone since about 30 million years ago should give rise to a highly thinned crust. In addition, different parts of the Basin and Range province have undergone different amounts of crustal thinning, leading to crust that is variably thick. When combined with seismic velocities, a flat boundary may suggest that crustal thickness is maintained by underplating of melts derived from the upper mantle, and possibly by (lateral) ductile flow of lower crust.

(2) Determining that ocean-island basalts originate by simple batch melting of a small amounts (<6.5%) of garnet-bearing source at a uniform depth. In contrast with some earlier experimental data, Th is partitioned more strongly into melts than U. In contrast, mid-ocean ridge basalts result from mixing of melts generated from a range of depths. These results are significant in confirming that primary melting processes, rather than secondary processes such as volatile transfer, control the trace element and isotopic compositions of these rocks.

(3) We have computed epicenters for 204 earthquakes from the central Rio Grande rift, New Mexico, as well as focal mechanism solutions for 30 of these events. First motions for some of these events do not fit standard source models, possibly the result of groundwater circulation in a geothermal reservoir. These anomalous earthquakes suggest that studies of microseismicity might be useful as an exploration tool for geothermal reservoirs in other areas.

Collaborators:

 

D. J. DePaolo, Department of Geology and Geophysics, University of California, Berkeley

 

M. C. Fehler, EES-4, Los Alamos

 

T. Hearn, Physics Department, New Mexico State University

 

G. Heiken, EES-1, Los Alamos

 

Leigh House, EES-4, Los Alamos

 

T. Lay, Institute of Tectonics, University of California, Santa Cruz

 

M. T. Murrell, CST-8, Los Alamos

 

J. J. Papike, Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico

 

A. Sanford, Geoscience Department, New Mexico Institute of Mining and Technology

 

S. R. Taylor, EES-3, Los Alamos

 

T. White, Department of Anthropology, University of California, Berkeley

 

K. H. Wohletz, EES-1, Los Alamos

 

G. WoldeGabriel, EES-1, Los Alamos

Publications:

 

 

Zhang, Y.-S., Tanimoto, T., and Stolper, E., 1994, S-wave velocity, basalt chemistry and bathymetry along the Mid-Atlantic Ridge, Phys. Earth Planet. Inter., 84, 79-94.

 

Hearn, T. M., Rosca, A.C., and Fehler, M. C., 1994, Pn tomography beneath the southern Great Basin, Geophys. Res. Lett. (in press).

 

White, T. D., Suwa, G., Hart, W. K., Walters, R. C., WoldeGabriel, G., de Heinzelin, J., Clark, J. D., Asfaw, B., and Vrba, E., 1993, New discoveries of Australopithecus at Maka in Ethiopia, Nature, 366, 261-265.

 

Clark, J. D., de Heinzelin, J., Schick, K. D., Hart, W. K., White, T. D., WoldeGabriel, G., Walter, R. C., Suwa, G., Asfaw, B., Vrba, E., and H.-Selassie, Y., 1994, African Homo erectus: Old radiometric ages and young Oldowan assemblages in the Middle Awash Valley, Ethiopia, Science, 264, 1907-1910.

 

Sims, K. W. W., DePaolo, D. J., Murrell, M. T., Baldridge, W. S., Goldstein, S. J., and Clague, D. A., 1994, Mechanisms of magma generation beneath Hawaii and mid-ocean ridges: U-Th and Sm-Nd isotopic evidence, Science, v. 267, p. 508-512.

For more information, contact Scott Baldridge (505-667-4338 or sbaldridge@lanl.gov).