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To clarify the deep structure of the Taiwan Orogen formed by the collision between the Eurasian and the Philippine Sea plates, we determine a robust 3-D shear-wave velocity (Vs) model of the crust and upper mantle beneath the Taiwan Island and its surrounding regions by applying a novel joint inversion method to local and teleseismic P and S wave travel times, Rayleigh-wave phase velocities and S-wave receiver functions. A notable new feature revealed by our model is the morphologic variation of the subducting Eurasian slab along the orogen belt. Beneath South Taiwan, the subducting Eurasian slab is imaged as a continuous high-Vs zone down to ∼400 km depth with an obvious dip angle change at ∼70 km depth. Beneath Central Taiwan, the Eurasian slab is imaged as intermittent high-Vs anomalies in the upper mantle, whereas the Eurasian slab exhibits as a sheet-like high-Vs zone with a low dip angle down to only ∼100 km depth beneath North Taiwan. These results suggest that parts of the subducting Eurasian slab have been detached beneath the Philippine Sea east off the Taiwan Island, which may result in the remnant slab rebounding to underplate the upper plate and so promote mountain building on the Taiwan Island. • A 3-D Vs model of the Taiwan Orogen is derived from 4 kinds of seismic data. • Shape variations of the Eurasian slab are revealed along the orogen belt. • Parts of the Eurasian slab are detached beneath the Taiwan Orogen. [ABSTRACT FROM AUTHOR]

Increasing evidence shows that the low-velocity layer (LVL) atop the mantle transition zone (MTZ) may play a critical role in Earth's evolution. The existence and formation of the LVL in continental interiors remain ambiguous. This study investigates the P-wave velocity structure atop the MTZ by matching P- and sP-wave triplicated waveforms simultaneously in the western Central Asian Orogenic Belt far away from continental margins. Two groups of nearly orthogonal triplications are allocated using records of four earthquakes at stations from the China National Seismic Network and the Incorporated Research Institutions for Seismology. Our observations resolve an LVL with 21–23 km thickness and a P-wave velocity drop of approximately −5.7% to −5.8% atop the MTZ. The two nearly orthogonal directions show little variation in the velocity structure of the LVL. It is inferred that the LVL results from partial melting induced by water and/or other volatiles released from the subducted Paleo-Asian oceanic slab and, hence, has existed since the Paleozoic, with possible variations in melt fraction. The long-lived partially molten LVL may play an important role in the intraplate magmatism and mantle dynamics in the Central Asian Orogenic Belt. Our results provide strong seismic evidence for the LVL existing in the continental interior, indicating that the LVL may be pervasive in Eurasia because of vast subductions of ancient plates; this provides new insights into the material circulation in the ancient oceanic subduction zones. • P-wave velocity at depth 410 km by modelling P and sP triplicated waveforms. • A low-velocity layer atop mantle transition zone in Central Asian Orogenic Belt. • Pervasive upper mantle melting in Eurasia induced by vast ancient subductions. [ABSTRACT FROM AUTHOR]

• 24904 RFs from 1182 events were collected by 307 stations to study MTZ. • Effect of velocity of upper mantle was studied via IASP91 and a 3D velocity model. • MTZ thickening was seen under the Western Alps due to uplift of discontinuities. • Localized arc-shaped thinning of MTZ was located along the alpine orogenic belt. • Temperature rise by mantle upwelling promoted topographic uplift of orogenic belt. To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift. [ABSTRACT FROM AUTHOR]

Abstract: A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30Myr), and small terranes that formed and were exhumed on timescales of <10Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth''s crust and mantle. [Copyright &y& Elsevier]