On the Accumulation of Lithospheric Material in the Pannonian Basin Mantle Transition Zone

Subduction zones are the most efficient consuming boundaries in plate tectonics. But where and how deep does the lithospheric material go afterwards? This question is of great importance as it feeds studies and debates in mantle dynamics. Our present understanding of the fate of descending slabs is essentially based on tomography studies of active subduction zones, such as the Circum-Pacific. These images show three main scenarios (e.g. Bijwaard et al., 1998): direct penetration into the lower mantle, flattening and stagnation in the mantle transition zone (MTZ), and flattening followed by penetration. Here we look at an active and geodynamically complex zone, the Pannonian Basin and its surroundings: oceanic subductions at surface are now completed in the region, but accumulated slab material has been imaged in the MTZ by tomography (Wortel and Spakman, 2000; Piromallo and Morelli, 2003). Our approach is to compute receiver functions from a new, high-resolution deployment as well as permanent station data in order to characterise the structure of the MTZ. We discover that while the top of the MTZ is at its expected depth (410 km); its bottom is clearly depressed below 660 km and shows important undulations. We argue that these are instabilities arising from the uneven accumulation of cold material from successive subduction episodes, and that the observed depressions might develop into viscous drops initiating penetration into the lower mantle.