Michael West

Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

[1] We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8180 Tectonophysics: Evolution of the Earth: Tomography; KEYWORDS: convection, Rio Grande rift, Colorado Plateau

Crustal structure of northern and southern Tibet from surface wave dispersion analysis

[1] Group and phase velocities of fundamental mode Rayleigh waves, in the period range of 10 to 70 s, are obtained for southern and northern Tibet. Significant variations in crustal velocity structure are found. The group velocity minimum for Tibet occurs at � 33 s and the minimum is � 0.12 km/s lower for southern Tibet than for northern Tibet. At periods greater than 50 s, however, group velocities are up to 0.2 km/s faster in southern Tibet. The group and phase velocities are inverted for layered S wave models. The dispersion observations in southern Tibet can only be fit with a low-velocity layer in the middle crust. In contrast, the velocity models for northern Tibet do not require any lowvelocity zone in the crust. The S wave velocity of the lower crust of southern Tibet is � 0.2 km/s faster than the lower crust of northern Tibet. In southern Tibet the sub-Moho velocity increases with a positive gradient that is similar to a shield, while there is no velocity gradient beneath northern Tibet. The high-velocity lower crust of southern Tibet is consistent with the underthrusting of Indian continental lithosphere. The most plausible explanation of the mid-crustal low velocity zone is the presence of crustal melt resulting from H2O-saturated melting of the interplate shear zone between the underthrusting Indian crust and overflowing Asian crust. The lack of a pronounced crustal low-velocity zone in northern Tibet is an indication of a relatively dry crust. The low S wave velocity in the lower crust of northern Tibet is interpreted to be due to a combination of compositional differences, high temperatures, presumably caused by a high mantle heat flux, and possibly small amounts of partial melt. Combined with all available observations in Tibet, the new surface wave results are consistent with a hot and weak upper mantle beneath northern Tibet. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7218 Seismology: Lithosphere and upper mantle; 7255 Seismology: Surface waves and free oscillations; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: Tibet, crustal velocity structure, surface wave, Rayleigh waves, continental collision

Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau using receiver functions

Received 20 October 2004; accepted 2 March 2005; published 28 May 2005. [1] The seismic structure of the crust and upper mantle of the southwestern United States is examined using receiver functions calculated from teleseismic arrivals recorded in the Colorado Plateau–Rio Grande Rift–Great Plains Seismic Transect (LA RISTRA) experiment. We apply receiver function estimation and filtering methods developed by Wilson and Aster (2005) to produce receiver functions with decreased sensitivity to noise and deconvolutional instability. Crustal thickness and Vp/Vs ratios are estimated using both direct and reverberated P-to-S receiver function modes. We apply regularized receiver function migration methods to produce a multiple-suppressed image of the velocity discontinuity structure of the subsurface. Our results show that crustal thickness averages 44.1 ± 2.3 km beneath the Great Plains (GP) and 45.6 ± 1.1 km beneath the Colorado Plateau (CP). Crustal thinning beneath the Rio Grande Rift (RGR) is broadly symmetric about the rift axis, with the thinnest crust (35 km) located directly beneath the rift axis, suggesting a pure shear stretched lithosphere beneath the RGR. We also observe a prominent northwest dipping discontinuity, ranging from 65 to 85 km deep beneath the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are consistent with preserved ancient lithospheric structures such as relict suture zones associated with Proterozoic subduction. We observe an upper mantle discontinuity at 220–300 km depth that may correlate with similar discontinuities observed beneath eastern North America. We also observe relatively flat discontinuities at 410 and 660 km depth, indicating there is not a large-scale thermal anomaly beneath the RGR at these depths.

Crust and upper mantle shear wave structure of the southwest United States: Implications for rifting and support for high elevation

[1] Surface wave phase velocities from 29 earthquakes are used to map the shear velocity structure to � 350 km depth across the 950-km-long Rio Grande Rift Seismic Transect Experiment (LA RISTRA) seismic array in the southwest United States. Events from a range of back azimuths minimize the effects of multipathing. The resulting velocity model reveals a transition in lithospheric thickness from 200 km in the Great Plains to 45–55 km beneath the Rio Grande Rift, thickening beneath the Colorado Plateau to 120–150 km. The upper mantle low-velocity signature of the rift is roughly twice the width of its surface morphology. An asthenospheric low-velocity channel underlies the region west of the Great Plains and extends to 300 km depth. This channel is likely the result of warm mantle infill behind the sinking Farallon plate. Buoyant forces within this channel are sufficient to support much of the high elevation of the rift and plateau. No evidence for a deep mantle source is found beneath the rift, implying that present rifting is not driven by deep mantle upwelling. Velocities from 55 to 90 km beneath the rift axis are 10% slower than beneath the Great Plains, consistent with small amounts of partial melt. Low velocities extend to 200–300 km depth on either side of the rift but not directly beneath it, forming an inverted-U shape. This feature may reflect mantle that has cooled through vertical advection in a subadiabatic environment. Upwelling may be reinforced by small-scale convection caused by variations in lithospheric thickness and shallow mantle temperatures. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7255 Seismology: Surface waves and free oscillations; 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; KEYWORDS: surface waves, continental rifting, upper mantle structure

Lithospheric structure of the Rio Grande rift

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust–mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rifts surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.

Magma storage beneath Axial volcano on the Juan de Fuca mid-ocean ridge

Axial volcano, which is located near the intersection of the Juan de Fuca ridge and the Cobb–Eickelberg seamount chain beneath the northeast Pacific Ocean, is a locus of volcanic activity thought to be associated with the Cobb hotspot. The volcano rises 700 metres above the ridge, has substantial rift zones extending about 50 kilometres to the north and south, and has erupted as recently as 1998 (ref. 2). Here we present seismological data that constrain the three-dimensional velocity structure beneath the volcano. We image a large low-velocity zone in the crust, consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust, and estimate the total magma volume in the system to be between 5 and 21 km3. This volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption (0.1–0.2 km3). We therefore infer that such volcanic events remove only a small portion of the reservoir that they tap, which must accordingly be long-lived compared to the eruption cycle. On the basis of magma flux estimates, we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years.

A simple approach to the joint inversion of seismic body and surface waves applied to the southwest U.S.

[1] Body and surface wave tomography have complementary strengths when applied to regional-scale studies of the upper mantle. We present a straight-forward technique for their joint inversion which hinges on treating surface waves as horizontally-propagating rays with deep sensitivity kernels. This formulation allows surface wave phase or group measurements to be integrated directly into existing body wave tomography inversions with modest effort. We apply the joint inversion to a synthetic case and to data from the RISTRA project in the southwest U.S. The data variance reductions demonstrate that the joint inversion produces a better fit to the combined dataset, not merely a compromise. For large arrays, this method offers an improvement over augmenting body wave tomography with a one-dimensional model. The joint inversion combines the absolute velocity of a surface wave model with the high resolution afforded by body waves—both qualities that are required to understand regional-scale mantle phenomena. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7255 Seismology: Surface waves and free oscillations; 7294 Seismology: Instruments and techniques; 8109 Tectonophysics: Continental tectonics— extensional (0905); 8180 Tectonophysics: Tomography. Citation: West, M., W. Gao, and S. Grand (2004), A simple approach to the joint inversion of seismic body and surface waves applied to the southwest U.S., Geophys. Res. Lett., 31, L15615,

Shallow-crustal magma chamber beneath the axial high of the Coaxial segment of Juan de Fuca Ridge at the source site of the 1993 eruption

Seismic imaging reveals a shallow-crustal magma chamber beneath the source site of the 1993 eruption on Coaxial segment, Juan de Fuca Ridge. The magma chamber is at least 6 km 3 in volume and contains at least 0.6 km 3 of melt, enough to supply at least several eruptions of a size equal to the one in 1993. No mid-crustal connection of this magma chamber with the magmatic plumbing of nearby Axial volcano (the current expression of the Cobb-Eickelberg hotspot) is evident, confirming previous geochemical and geological studies that argued against mixing between the two. The lack of connectivity implies that magma transport through the uppermost mantle and lower crust are very highly focused into narrow (<5-10 km) conduits.