Subducting slabs stagnant in the mantle transition zone

Abstract

The P wave velocity structure beneath the Western Pacific is found from the International Seismological Center first arrival data. Special attention was paid to the deep structure beneath the Wadati‐Benioff zone. We discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks. This way of discretization minimizes a problem with tomographic studies of regional scale: difficulty in making corrections for the effects outside the region of interest. Our solution is iterative with the alternate step of the relocation of earthquakes, using the whole mantle model of Inoue et al. (1990) as a starting model. A first‐order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model. The essential result depends little on whether the reference spherical model is smooth or discontinuous near 400‐ and 670‐km depths. We examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere. The resolution is depth dependent but in general good enough to see the slab configuration beneath the Southern Kurile‐Japan‐Izu‐Bonin arcs and the Java arc. It is relatively poor beneath the Northern Kurile and Mariana arcs. The seismic image of subducting slab beneath the Southern Kurile to Bonin arcs bends to subhorizontal near the leading edge of the Wadati‐Benioff zone and extends continentward over a distance of more than 1000 km. The subhorizontal portion of the slab connects a high‐velocity blob to the bottom that reaches a depth of at least 800 km across the 670‐km discontinuity under the Japan arc. Although the image of the Java slab directly penetrates the 670‐km discontinuity, it then bends to a shallow dip with a considerable spread, reaching a depth of about 1200 km. These results suggest that descending slabs of lithosphere in the Western Pacific tend to be stagnant in the transition zone under a subtle control of the 670‐km discontinuity. Although stagnant slab materials eventually descend into the lower mantle, they no longer maintain their original configuration below the 670‐km discontinuity.

Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number

“…The approximate Ra and P values would suggest that we are today either in the transitional regime or just in the whole mantle convection regime and very unlikely to be in the layered regime. Seismological evidence seems to strongly support this, with observations of subducting slabs descending from the upper to the lower mantle (Grand et al, 1997;van der Hilst et al, 1997;Creager and Jordan, 1984) and also observations of stagnant slabs, which might reflect some resistance at this boundary (Fukao et al, 1992) …”

Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia

“…This largescale high velocity structure may have a complex origin. The eastern part of these anomalies has been well documented in tomographic studies (e.g., van der Hilst et al, 1991;Fukao et al, 1992) and has been interpreted as "slab deflection" at the 660 km discontinuity as a result of slab roll back ocean ward (van der Hilst and Seno, 1993). The strong anomalies in the transition zone east of ∼120 • E can probably be explained as the subducted ocean lithosphere of the western Pacific and Philippine Sea plates (Fig.…”

“…A number of P-wave tomographic studies have concentrated on complex morphology of the subducting oceanic lithospheric slabs beneath the western Pacific, the Philippine Sea, and Indonesia (e.g., van der Hilst et al, 1991;Fukao et al, 1992;Widiyantoro and van der Hilst, 1997). Using data from temporary seismic arrays, receiver function studies have focused on the crust and shallow mantle beneath central Tibet (e.g., Kind et al, 1996;Kosarev et al, 1999;Kind et al, 2002).…”

“…The major large-scale features produced by the collision and post-collisional convergence of the Indian and Eurasian plates include the Tibetan Plateau, where the continental crust has approximately doubled in thickness (Molnar and Tapponnier, 1975;Tapponnier et al, 2001;Rapine et al, 2003). Offshore, oceanic lithosphere is predominantly of early Cenozoic age, and there is ongoing subduction of the Pacific, Philippine Sea, and Indo Australian plates beneath Eurasia (van der Hilst et al, 1991;Fukao et al, 1992;Widiyantoro and van der Hilst, 1996) with eastward slab roll back since Oligocene times (van der Hilst and Seno, 1993;Northrup et al, 1995). Between these active plate boundary zones the continental lithosphere was subjected to extension during early Cenozoic time (Tapponnier and Molnar, 1977; Fig.…”

Upper mantle anisotropy beneath Japan from shear wave splitting

“…(b) The locations of all F-net stations mentioned by name in the text. Hilst et al, 1991;Fukao et al, 1992) and seismicity (e.g., Engdahl et al, 1998;Gudmundsson and Sambridge, 1998).…”