Several models have been proposed to describe the tectonic evolution of SE Tibet and its marginal areas. Hence, high-resolution crustal velocity models are essential to address this controversy. With waveform data from 73 broadband stations in southwest China and northern Vietnam, we invert for a 3-D shear wave velocity model of the crust and uppermost mantle from ambient noise tomography. Our model reveals that the midlower crustal low-velocity zone in the Xiaojiang Fault Zone extends farther southward across the Red River Fault to Vietnam and is approximately bounded by the Xiaojiang and Dien Bien Phu faults to the east. We suggest that the observed low-velocity zone represents a mechanically weak zone in the mid-lower crust, which may serve as a channel for efficient southward material transport in SE Tibet. With our results and previous evidence, we propose a combined model that integrates rigid block extrusion and crustal channel flow to describe the large-scale material transport in SE Tibet. We further propose a two-phase material transport model in SE Tibet after the India-Eurasia plate collision: (1) rigid block extrusion between the right-lateral Sagaing Fault and left-lateral Red River Fault during the early Oligocene-early Miocene and (2) a combined model of rigid block extrusion and material channel flow in the mid-lower crust from the late Miocene to the present. The southward crustal material transport is likely to be diverted along two major channels around the more rigid crust beneath the inner zone of the Emeishan large igneous province.

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Abundant earthquakes clustered within a particular zone often reflect an active geological feature, such as clustering seismicity along a fault zone and a huge number of volcanic-earthquakes around the erupting conduit. Herein we perform a double-difference tomographic inversion and relocate the seismicity at the long-resting tatun volcano group (tVG) in northern taiwan. A dramatic improvement of the earthquake location model surprisingly show that, from 2014 to 2017, two clustered seismic zones are identified in the TVG. One major group of events (>1000) persistently clustered within a ~500 m diameter vertical conduit with a ~2 km height. The clustering seismicity conduit is just located nearby Dayoukeng, one of the strongest fumaroles in the tVG, and is connected to a fracture zone characterized by low Vp/Vs in the shallow crust. the other group of events is clustered within a spherelike zone beneath Mt. Chihsin around the depths between 0.5 km and 2 km. Both seismic zones are probably triggered by the significantly volcanic gases and fluids ascending from the deep magma reservoir. combined with a variety of results from literature, the seismicity conduit near the strong fumarole is the evidence for an active volcano and also identifies a likely pathway for ascending magma if the TVG erupts again in the future. But possibility of developing different magma pathways at other clustered seismic zones such as beneath Mt. chihsin may not be totally excluded. The evaluation of whether or not a volcano will erupt often relies on the classification of it as active, dormant, or extinct; however, the exact definition of an active volcano may still be debatable. Two criteria are often employed to classify an active volcano: (1) the volcano erupted within the past ~10,000 years or (2) the identification of magma chambers beneath the volcano 1. However, those definitions are not set in stone because some eruptions have surprisingly occurred in dormant or extinct volcanoes, which are long-resting volcanoes without evidence of any magma chambers beneath them. Thus, some other criteria may be involved in the evaluation of the possibility of eruptions in long-resting volcanoes, in particular. The Tatun volcano group (hereinafter "TVG") in the northern tip of Taiwan is a typical long-resting volcano because there was no eruption record in human history. From the volcanic hazard point of view, it is important to know whether or not the TVG is active because it is located near the Taipei metropolis, with more than 6 million residents living in both Taipei City and New Taipei City in northern Taiwan (Fig. 1). The distance between Mt. Chihsin (the highest peak of 1120 m in the TVG) and the Taipei 101 skyscraper building (a landmark in downtown of Taipei City 2) is less than 15 km. In other words, the TVG is a typical "City on Volcano" case if it is active. Apart from the Taipei metropolis, it is worth highlighting the two nuclear power plants located around the northern boundary of the TVG. Thus, even a small volcanic eruption at th...

It is conventionally believed that magma generation beneath the volcanic arc is triggered by the infiltration of fluids or melts derived from the subducted slab. However, recently geochemical analyses argue the arc magma may be formed by mélange diapirs that are physically mixed by sediment, altered oceanic crust, fluids, and mantle above the subducted slab. Further numerical modeling predicts that the mantle wedge diapirs have significant seismic velocity anomalies, even though these have not been observed yet. Here we show that unambiguously later P-waves scattered from some obstacles in the mantle wedge are well recorded at a dense seismic array (Formosa Array) in northern Taiwan. It is the first detection of seismic scattering obstacles in the mantle wedge. Although the exact shape and size of the scattered obstacles are not well constrained by the arrival-times of the later P-waves, the first order approximation of several spheres with radius of ~ 1 km provides a plausible interpretation. Since these obstacles were located just beneath the magma reservoirs around depths between 60 and 95 km, we conclude they may be mantle wedge diapirs that are likely associated with magma generation beneath active volcanoes.