Séminaire ISTerre

---

Waveform tomography of eastern Asia: Deep Subduction beneath NE China & Lithospheric foundering and underthrusting beneath Tibet

vendredi 26 février 2021 - 15h00

Fenglin Niu - Department of Earth Science, Rice University, US
--- 
We have conducted full waveform inversions with a large waveform dataset to obtain high-resolution 3D P- and S-wave velocity models beneath eastern Asia. Our models revealed sharper, more intense high-velocity slabs in the upper mantle under the southern Kuril, Japan, and Ryukyu arcs, than previous studies have found. Beneath northeast China, the subducting Pacific plate appears to possess a gap where a cylindrical slow velocity anomaly emerges and extends to a shallow depth right beneath the Changbaishan volcano, which is the largest active magmatic center in China. We speculate the slow anomaly represents hot and buoyant asthenosphere, which is brought down by the subducting Pacific slab and is able to rise through a gap in the deep stagnant slab. The upwelling is feeding the Changbaishan volcanic complex through decompression melting. Further more, our P-wave tomography model suggests that the stagnant Pacific slab actually consists of at least two separated segments. While the current subducting Pacific slab clearly shows a westward penetration with increasing depth, there are two other isolated high velocity anomalies that appear to be detached from a continuous slab to the surface beneath the Korean Peninsula and the Bohai and Yellow Sea. These high velocity segments suggest that slab detachment might have occurred during Oligocene, which may have triggered the observed lateral shift in magmatism in northeast China and initiated the opening of the Japan Sea in the early Miocene. Beneath South-Central Tibet, we observed a T-shaped high wave speed structure and interpreted it as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the northern Tibetan lithosphere. Surface uplift of northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle. On the other hand, the southern Tibet also possesses a high velocity keel with S-wave speed comparable to that beneath the North American craton, suggesting that plateau is likely to evolve into a craton due to continued under-accretion of the Indian continent.

---

Equipe organisatrice : Ondes et structures

Séminaire uniquement en visio

Informations de visio :

https://univ-grenoble-alpes-fr.zoom.us/j/97295763658?pwd=Z0Y4OEJtNXFOMnRwcFlXNWl1eFpSQT09
Meeting ID : 972 9576 3658
Passcode : 783219