Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

被引:22
作者
Beghein, Caroline [1 ]
Snoke, J. Arthur [2 ]
Fouch, Matthew J. [3 ]
机构
[1] Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90095 USA
[2] Virginia Polytech Inst & State Univ, Dept Geosci, Blacksburg, VA 24061 USA
[3] Arizona State Univ, Sch Earth & Space Explorat, Bateman Phys Sci Ctr F Wing, Tempe, AZ 85287 USA
关键词
Rayleigh-waves; anisotropy; lithosphere; asthenosphere; crust; USArray; Great Basin; WESTERN UNITED-STATES; MODE SURFACE-WAVES; SEISMIC ANISOTROPY; MANTLE FLOW; COLORADO PLATEAU; RANGE PROVINCE; TOMOGRAPHY; BENEATH; INVERSION; DEFORMATION;
D O I
10.1016/j.epsl.2009.11.036
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semicircular pattern of fast seismic directions. Away from central Nevada the fast directions are similar to N-S in the eastern Great Basin, NW-SE in the Walker Lane region, and they transition from E-W to N-S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28-102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an similar to E-W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere-asthenosphere boundary at depths of similar to 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain similar to 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component. (C) 2009 Elsevier B.V. All rights reserved.
引用
收藏
页码:467 / 478
页数:12
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