Water is essential for the formation of a magmatic arc by lowering the melting temperature of materials in the mantle wedge. As such, it is logical to attribute the absence of a magmatic arc to insufficient water released from the subducting plate, although a number of other factors may cause volcanic arc quiescence as well, such as a slab window or flat slab subduction. In this contribution, we present a possible but testable correlation between the occurrence of a magmatic arc and seamount subduction in light of bathymetric data obtained near trenches. This correlation, if it holds true, in turn means that a magmatic arc is unlikely to occur when the subducting slabs have not been severely fractured and that one of the main reasons for excluding effects such as the slab window or flat slab subduction may be that the plate is not accompanied by seamounts. Therefore, the role that seamount subduction plays in recycling water back into the mantle deserves more attention from the earth sciences community.
The Tibetan Plateau, known as " the roof of the world” and " the third pole of the earth”, is a product of the collision between India and Asia during the last ~50 Ma. The regional tectonics–in particular, growth and expansion of the plateau–has been attributed primarily to deformation within the lithosphere. The role and pattern of the underlying asthenospheric flow, however, remain mostly unaddressed. In light of recent seismic tomographic images and published seismic anisotropic descriptions of the upper mantle, here we propose that an entrained mantle flow is likely to exist under Tibet, induced by the northward advancing Indian plate. The direction of mantle flow is characterized by a gradual rotation from northward in the south to eastward in the north as a result of deflection by the deep root of the Tarim block. The presence of an underlying mantle flow is not only able to account for the west-east oriented fast-axis of seismic anisotropy in northern Tibet, but can also adequately explain the sporadic null splitting in southern Tibet. Specifically, the null splitting results, at least in part, from upwellings of asthenospheric flow through tears of the underthrusting Indian plate that have been revealed by various seismic observations. The mantle flow may in turn promote the block extrusion under Tibet that has been observed in GPS measurements; hot asthenospheric upwellings may also lead to widespread post-collisional magmatism in southern Tibet.
The 660-km discontinuity that separates the Earth's upper and lower mantle has primarily been attributed to phase changes in olivine and other minerals. Resolving the sharpness is essential for predicting the composition of the mantle and for understanding its dynamic effects. In this study, we used S-to-P conversions from the 660-km interface, termed S660P, arriving in the P-wave coda from one earthquake in the Izu–Bonin subduction zone recorded by stations in Alaska. The S660P signals were of high quality, providing us an unprecedented opportunity to resolve the sharpness of the discontinuity. Our study demonstrated, based on the impedance contrast given by the IASP91 model, that the discontinuity has a transitional thickness of ~5 km. In addition, we observed a prominent arrival right after the S660P, which was best explained by S-to-P conversions from a deeper discontinuity at a depth of ~720 km with a transitional thickness of ~20 km, termed S720P. The 720-km discontinuity is most likely the result of a phase transition from majoritic garnet to perovskite in the segregated oceanic crust (mainly the mid-oceanic ridge basalt composition) at the uppermost lower mantle beneath this area. The inferred phase changes are also consistent with predictions from mineral physics experiments.