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Geodynamics of the Gulf of California from surface wave tomography

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Author: Zhang, X. · Paulssen, H.
Type:article
Date:2012
Source:Physics of the Earth and Planetary Interiors, 192-193, 59-67
Identifier: 461836
Keywords: Geosciences · Gulf of California · Radial anisotropy · Surface waves · Tomography · Continental rifting · Crustal extension · Depth range · Gulf of California · High velocity · Horizontal shear · Low velocities · Low-velocity anomaly · Mantle structure · Microplates · Olivine crystals · Plate boundaries · Radial anisotropy · Radially anisotropic · Rayleigh · Seismic station · Shear velocity models · Shear-wave velocity · Slab windows · Surface-wave tomography · Tomographic · Tomographic inversion · Upper mantle · Vertical alignment · Vertical flows · Vertical shear · Anisotropy · Olivine · Shear waves · Surface waves · Tectonics · Tomography · Geodynamics · crustal structure · geodynamics · mantle structure · microplate · North American plate · Pacific plate · plate boundary · Rayleigh wave · rifting · S-wave · seismic anisotropy · seismic tomography · slab · subduction · surface wave · upper mantle · wave velocity · Gulf of California · Pacific Ocean · Vesicular stomatitis virus · Energy Efficiency · Energy / Geological Survey Netherlands · Earth & Environment · SGE - Sustainable Geo Energy · EELS - Earth, Environmental and Life Sciences

Abstract

The Gulf of California, which forms part of the Pacific-North American plate boundary, is an ideal place to investigate upper mantle dynamics in a continental rifting area. With 19 seismic stations located around the gulf, the NARS-Baja experiment (2002-2008) was designed to image its crustal and mantle structure. Here we present results of a tomographic inversion of Love and Rayleigh interstation phase velocity measurements for a radially anisotropic shear velocity model of the Gulf of California. This study confirms the overall low shear-wave velocities in the upper 200km of the mantle found in other Rayleigh wave studies, and the presence of a positive shear-wave velocity anomaly at depths of roughly 80-160km beneath the central gulf (Zhang et al., 2009). In addition, we find that the horizontal shear velocity (VSH) is generally higher than the vertical shear velocity (VSV). For the northern gulf, however, there is strong indication for VSV>VSH in the 40-60km depth range. This region also has anomalously low shear-wave velocities down to 100km depth. Combining these observations, we suggest that the low velocity anomaly and the negative radial anisotropy (VSH<VSV) beneath the northern gulf are related to vertical flow of low velocity material in an area of a slab window, either by the vertical alignment of olivine crystals or by a fabric of vertically oriented sheets of melt. The high-velocity anomaly beneath the central gulf is interpreted as a remnant slab fragment which inhibits vertical flow from the deeper mantle. Our tomographic results indicate that the formation of the Gulf of California cannot be explained by simple models of crustal extension or dynamic upwelling. Instead, its structure and geodynamics are caused by the cessation of subduction by stalled microplates in the central and southern gulf and the presence of a slab window in the north. © 2012 Elsevier B.V.