Mantle Transition Zone Discontinuities beneath Tibetan Plateau and Adjacent Regions

Abstract:

The 410 and 660 km discontinuities (d410 and d660) bordering the mantle transition zone (MTZ) beneath Tibetan Plateau and Himalayas are imaged to understand mantle deformation and dynamics associated with the indentation of the northern Indian Plate into Eurasia. A total of about 200,000 high-quality P-to-S receiver functions recorded by 804 broadband seismic stations are stacked over 420 continuous overlapping circular bins with a radius of 1 degree. After moveout correction based on the 1-D IASP91 velocity model, the resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. The Himalayas possesses an apparent depression of d410 of about 2 km and an 8 km depressed d660, leading to an MTZ that is about 6 km thicker than normal. Another prominent feature in the study area is up to 7 km apparent thickening of the MTZ beneath Tibetan Plateau. This thickening is associated with an 8 km apparent depression of the d410 and 15 km depression of the d660. All these observations, when combined with the recent tomographic results, indicate that the apparent MTZ thickening beneath Himalaya and Tibetan Plateau is mainly caused by the high velocity anomaly related to the stagnated Indian slab in the MTZ.