Upper mantle deformation in the western US determined from seismic anisotropy; 1: Regional overview
- Univ. of Nevada, Reno, NV (United States). Seismological Lab.
- Carnegie Institution, Washington, DC (United States)
The examination of seismic anisotropy via shear-wave splitting measurements is an exciting new tool for the study of tectonic deformation. Simple measurement of polarization ([phi]) and time separation ([delta]t) between two split (birefringent) shear waves lead to inference about fast direction, and degree of anisotropy or length of anisotropic path. For the case of teleseismic phases, these can in turn be related to the deformation of olivine and hence to tectonic movements. The authors are using such measurements to study the strain within the upper mantle beneath the western US, from the coast of California to the Great Plains. The wide variety of tectonic regimes, including stable regimes, subduction, transform faulting, and extension, allows one to examine the correlation between the deformation of the upper mantle and the surface tectonics. Comparing their own and other studies, [phi] is often consistent within fairly wide geographic regions (hundreds of kms), but can change over short distances (tens of kms), that are not always correlated with obvious surface features. Measurements of fast directions of anisotropy vary from E-W in California and west-central Nevada, to NE-SW in Oregon, northwestern Nevada, the southern Great Basin and the northern Rio Grande Rift, to NW-SE in the north-central Basin and Ranges and NNE/SSW along the western border of the Colorado Plateau. These variations in [phi] and [delta]t are interpreted as changes in the direction of olivine alignment in the mantle and in depth of penetration of the aligned material and/or in the degree of alignment. Average [delta]t of about 1.0 s suggests the path must extend into the mantle or else be highly anisotropic, while the variation over short distances suggests an anisotropic source close to the surface; therefore suggest that it originates in the upper mantle. The coherence across wide geographic regions suggests a coherent pattern of deformation on scales of hundreds of kms.
- OSTI ID:
- 5023627
- Report Number(s):
- CONF-9305259-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
EARTH MANTLE
SEISMIC SURVEYS
DATA ANALYSIS
USA
ANISOTROPY
ATTENUATION
DEFORMATION
OLIVINE
RESPONSE FUNCTIONS
SEISMIC S WAVES
TECTONICS
WAVE PROPAGATION
DEVELOPED COUNTRIES
FUNCTIONS
GEOPHYSICAL SURVEYS
MINERALS
NORTH AMERICA
SEISMIC WAVES
SILICATE MINERALS
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580000* - Geosciences