Seismic structure of the upper mantle in a central Pacific corridor

Abstract

The seismic structure of the Tonga-Hawaii corridor has been investigated by combining two data sets: Revenaugh and Jordan's reflectivity profile from ScS reverberations, which provides travel times to and impedance contrasts across the major mantle discontinuities, and 1500 new observations of frequency-dependent phase delays for the three-component S, SS, and SSS body waves and the R1 and G1 surface waves, which constrain the velocity structure within this layered framework. The shear waves turning in the upper mantle showed significant splitting of the SH and SV components, indicative of shallow polarization anisotropy. The data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric, radially anisotropic structure. The final model, PA5, is characterized by a highvelocity, anisotropic lid, bounded at 68 km depth by a large (negative) G discontinuity; a lowvelocity, anisotropic layer below G, extending to a small L discontinuity at 166 km; an isotropic, steep-gradient region between 166 km and 415 km; and transition-zone discontinuities at 415, 507, and 651 km. The depth of the radial anisotropy in PA5 is shallower than in most previous studies based on surface waves and higher modes. The average value of radial shear anisotropy in the lid, +3.7%, is consistent with the magnitude expected from the spreading-controlled models of olivine orientation, while anisotropy in the low-velocity zone, which is required by our data set, could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present-day plate motions. On the basis of recent work by Hirth and Kohlstedt, we suggest that the G discontinuity is caused by a rapid increase in the water content of mantle minerals with depth, marking the fossilized lower boundary of the melt separation zone active during crust formation. The high-gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval. The average shear-velocity gradient in the transition zone is lower than in most previous seismic models, in better agreement with the predictions for a pyrolitic composition. [e.g., Forsyth, 1975a; Regan and Anderson, 1984; Farra and Vinnik, 1994], and discontinuity structure [e.g., Revenaugh and Jordan, 1987, 1989; Vidale and Benz, 1992] of the upper mantle. Most recently, global and regional 3-D tomography has begun to provide more detailed maps of the lateral heterogeneity in the Pacific mantle [e.g., Woodhouse and Dziewonski, 1984; Montagner and Tanimoto, 1991; Zhang and Tanimoto, 1993; Su et al., 1994]. While such tomographic studies have yielded useful images of the major geographic features, their data sets and model parameterizations have thus far suffered from inadequate Copyright 1996 by the American Geophysical Union. P•nor number O6IRO 1 gg9. 0148-0227/96/96JB-01882509.00 resolution of verti...

Geophysical evidence from the MELT area for compositional controls on oceanic plates

“…The resulting dry region of upper mantle will have a viscosity significantly greater than the underlying asthenosphere, creating a rheological boundary layer 3 that is compositionally and not thermally controlled. Our observations are broadly consistent with seismic data from the central Pacific that show a discontinuity (G) at 68km depth, corresponding to the same dry/wet transition 27 . This boundary corresponds to the base of the layer of low electrical conductivity and high shear wave velocity observed in the MELT area.…”

Implications of grain size evolution on the seismic structure of the oceanic upper mantle

“…Older regions of the oceanic mantle have likely experienced additional shear 430 associated with small-scale convection and/or larger-scale buoyancy-driven flow, which will 431 tend to re-align the orientation of shear in the asthenosphere relative to the fossil seafloor fabric. 432 The largest discrepancy between the modeled and observed velocity structures is the inability 447 of our model to predict the high velocity lid above 60-70 km (G-discontinuity) for old ages that 448 is observed in reverberation phases (e.g., Gaherty et al, 1996;Tan & Helmberger, 2007). If this 449 high velocity lid is caused by anelastic effects, it would require a sharp change in mantle 450…”

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

“…First, a discontinuity at the base of the lithosphere has been observed in receiver function studies (Collins et al, 2002;Li et al, 2004;Vinnik et al, 2005), with whole mantle ScS reverberations (Courtier et al, 2007;Gaherty et al, 1996;Revenaugh and Jordan, 1991), and with multiple S waveforms (Tan and Helmberger, 2007). No discontinuity is expected from temperature sensitivity alone and there is no indication that the depth of the discontinuity is dependent on the age of the seafloor.…”