Three-dimensional imaging of the subducting Indian continental lithosphere beneath the southern and central Tibetan Plateau using body-wave finite frequency tomography

Abstract:

Combining the new teleseismic body waves recorded by TIBET-31N passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan Plateau to examine the roles of the upper mantle in the formation of the Tibetan Plateau. Continuous high velocities are observed beneath the Himalayas and Lhasa Terrane with a moderate northward inclining angle. We interpret this high velocity anomaly as the subducting Indian continental lithosphere, which shows a slight east-west variation of northern extent, reaching ~31°N along 85°E and ~30°N along 91°E to 350-km depth. Strong low P- and S-wave velocity anomalies that extend from lower crust to about 200 km depth beneath the Cona rift, Yadong-Gulu rift, Tangra Yum Co rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. And these upper mantle low velocities also show east-west variations of depth extent. The one beneath Tangra Yum Co extends shallower to about 180 km, and the ones west of Yadong-Gulu rift and east of Cona rift have extended more than 300 km. The low velocity west of Yadong-Gulu rift extends further north and appears to connect with the massive upper mantle low velocity beneath central Tibet.

We propose a three-dimensional dynamic model for Indian continental lithosphere that is consistent with these important observations. The most importrant features of this 3-D dynamic model are: the northward moving Indian continental lithosphere is currently subducting beneath the southern Tibetan Plateau with a moderate angle of 40 degrees, reaching to Bangong-Nujiang Suture at depth of 350 km; the subducting Indian continental lithosphere is fragmented beneath the surface rifts with the largest gap beneath the Yadong-Gulu rift; and the fragmentation induced local upwelling asthenosphere beneath the Yadong-Gulu rift flow northward mixing with the hot asthenosphere in central Tibetan Plateau. Establishing the 3-D images of the subducting Indian continental lithosphere is an essential step toward the full understanding of the Indian-Eurasia continental collision and the construction of the Tibetan Plateau.