Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates

Kawakatsu, Hitoshi; Kumar, Prakash; Takei, Yasuko; Shinohara, Masanao; Kanazawa, Toshihiko; Araki, Eiichiro; Suyehiro, Kiyoshi

doi:10.1126/science.1169499

Cited by 509 publications

(646 citation statements)

References 23 publications

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“…This mode of segregation was predicted with two-phase flow theory (1) and subsequently discovered in experiments (2,3). It has been proposed that meltenriched bands, if present in the mantle of Earth, would permit rapid extraction of melt (4), produce significant anisotropy in seismic wave propagation (5), and provide a mechanism for the seismic discontinuity that is, in some places, associated with the lithosphere-asthenosphere boundary (6). The emergence (7) and low angle (8) of melt-enriched bands under simple-shear deformation can be reproduced using two-phase flow theory with a non-Newtonian, isotropic viscosity.…”

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confidence: 82%

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Kohlstedt

Katz

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the center of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory.viscous anisotropy | melt segregation | partial melts | basalt | olivine S hear deformation of partially molten rocks gives rise to melt segregation into sheets (bands in cross-section) that emerge at a low angle to the shear plane. This mode of segregation was predicted with two-phase flow theory (1) and subsequently discovered in experiments (2, 3). It has been proposed that meltenriched bands, if present in the mantle of Earth, would permit rapid extraction of melt (4), produce significant anisotropy in seismic wave propagation (5), and provide a mechanism for the seismic discontinuity that is, in some places, associated with the lithosphere-asthenosphere boundary (6). The emergence (7) and low angle (8) of melt-enriched bands under simple-shear deformation can be reproduced using two-phase flow theory with a non-Newtonian, isotropic viscosity. This theory describes the flow of a low-viscosity liquid (melt) through a permeable and viscously deformable solid matrix (grains) (9). However, an unrealistically strong stress dependence of viscosity was required to match the low angle of bands observed in experiments (8).This disagreement between models and experiments found a possible resolution by the incorporation of anisotropic viscosity arising from coherent alignment of melt pockets between grains [i.e., melt-preferred orientation (MPO)] in response to a deviatoric stress (10-13).Crucially, with the inclusion of viscous anisotropy, two-phase flow theory also predicts a simultaneous but distinct mode of melt segregation driven by large-scale gradients in shear stress. This mode is termed base-state melt segregation (13-15). Base-state melt segregation is not predicted if viscosity is isotropic; thus, its occurrence in experiments represents a test of the hypothesis that M...

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confidence: 82%

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Kohlstedt

Katz

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…Recent laboratory (Hilairet et al, 2007) and seismological (Kawakatsu and Watada, 2007) studies suggested the existence of such layers (water-rich layer) on the upper surfaces of the subducting slab. Thin low-viscosity layers rich in melt have also been suggested on the lower boundary (lithosphere-asthenosphere boundary) (Kawakatsu et al, 2009). Such thin low-viscosity layers on the subducting slab surface may enable such a rapid adjustment.…”

Section: A Simple Two-dimensional Modelmentioning

confidence: 99%

Accelerated pacific plate subduction following interplate thrust earthquakes at the Japan trench

Heki

Mitsui

2013

Earth and Planetary Science Letters

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Interplate thrust earthquakes are usually followed by afterslips, and they let the fore-arc move slowly trenchward. However, we do not know if the subducting oceanic plate is accelerated landward after such earthquakes. The westward velocity of Global Positioning System (GPS) stations in NE Japan show gradient decreasing from east to west reflecting the E-W contractional strain built up by the inter-plate coupling. Here we show that such coupling significantly enhanced (~1.5 times) after the 2003 Tokachi-Oki earthquake (M w 8.0), Hokkaido, in the segments adjacent to the ruptured fault. The coupling seems to be further enhanced (~3 times) after the 2011 Tohoku-Oki earthquake (M w 9.0). It is unlikely that interplate friction suddenly increased over such a large region, and relatively strong pre-2003 coupling there would not allow such enhancements even if full coupling is attained. Hence they are attributable to the temporary acceleration of the Pacific Plate subduction. We propose a simple 2-dimensional model in which down-dip acceleration of the slab let the force balance rapidly recover promoted by a thin low-viscosity layer on the slab surface. The accelerated subduction would account for temporary activations of regional interplate seismicity after megathrust earthquakes.

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“…This exercise can test whether there is a gradual increase in lithospheric thickness from C→D (Figure 11), as required by the proposed SCS spreading model. It has been demonstrated that the thickness of generated oceanic crust correlates with its formation age [93][94][95][96][97]. Therefore, one can calculate the formation age of oceanic crust using the estimated lithospheric thickness, which in turn can be used to estimate the initiation and termination ages of SCS spreading.…”

Section: Spreading History Of the South China Seamentioning

confidence: 99%

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

et al. 2011

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The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS: pre-spreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puzzling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better understanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogenesis, but also is heavily dependent on systematic sampling of seafloor rocks.

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Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates

Cited by 509 publications

References 23 publications

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Experimental test of the viscous anisotropy hypothesis for partially molten rocks

Accelerated pacific plate subduction following interplate thrust earthquakes at the Japan trench

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

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