The upper mantle beneath the Gulf of California from surface wave dispersion. Geologica Ultraiectina (299)

Abstract

This thesis is a study on upper mantle shear velocity structure beneath the Gulf of California. Surface wave interstation dispersion data were measured in the Gulf of California area and vicinity to obtain a 3-D shear velocity structure of the upper mantle. This work has particular significance for understanding the tectonic evolution history in this region and the potential processes that control lithospheric rupture and the initiation of seafloor spreading in the Gulf of California. The NARS-Baja project is designed as a passive seismic experiment to explore the mantle beneath the Gulf of California region. This thesis presents the results on the upper mantle structure beneath the Gulf of California from surface wave dispersion analysis. The study contains three aspects: the Rayleigh wave phase velocity structure with azimuthal anisotropy in the Gulf of California region; a shear velocity model of Gulf of California and its vicinity inferred from Rayleigh wave dispersion data; the radially anisotropic shear wave velocity model inferred from Love and Rayleigh wave dispersion . The seismic models of the upper mantle beneath the Gulf of California and surrounding regions show some interesting features. The average shear-velocity structure has a strong low-velocity zone beneath a thin lid. The average phase velocity data are even suggestive of lower velocities than the global average down into the transition zone. The low velocities at asthenospheric depths are indicative of fluids and/or melt. A striking feature of our seismic models is a relatively high shear-velocity anomaly beneath the central-southern gulf extending from a depth of roughly 120 to 160 km. This anomaly is interpreted as a remnant of a subducted Farallon microplate fragment . There is no evidence for such a slab remnant beneath the northern part of the gulf, in agreement with the tectonic interpretation of the region. We therefore interpret this region as having a slab window, similar to the area at the mouth of the gulf. Additional evidence on the upper mantle structure is obtained from anisotropy. An along-rift-axis variation is observed in the azimuthally anisotropic phase velocity maps of the fundamental mode Rayleigh waves, as well as in the radially anisotropic structure . The obtained upper mantle structure sheds light onto the interior of this dynamic plate boundary developing area. Our new tectonic interpretation of evolution the Gulf of California is based upon tectonic reconstructions, volcanic evidence, and our seismic interpretation of a stalled slab remnant. In our model, the development of the slab window beneath the North American lithosphere and pre-existing fracture zones in the subducting plate are essential in the tectonic evolution. The details of the subduction dynamics and the plate boundary relocation can therefore be largely be attributed to pre-existing weak features in the plates. Ridges and fracture zones of the subducted plate can determine tearing and fracturing of the slab, whereas faults and extended back-arc basins in the overriding plate can be essential to the plate boundary relocation.