Alpine mantle transition zone imaged by receiver functions

Assessing the thermal state of the mantle is essential to understand mantle dynamics and provides important clues about present and past tectonic regimes. The tectonic processes related to the Cenozoic subduction of the Alpine Tethys likely affected the regional upper-mantle structure. We use receiver functions to determine the mantle transition zone thickness beneath the greater Alpine region as a proxy for its current thermal state. Compilation of a dataset of nearly 7000 high-quality receiver functions from 98 stations yields a reliable and stable image of the Alpine mantle transition zone. Significant signal from P-to-S conversions at the 410- and 660-km discontinuities is clearly observed. The 410-km discontinuity is found at a depth close to that predicted by the IASP91 global model; its seismic signature is generally strong except south of the Alps. Signal from the 660-km discontinuity is strong, continuous and emerges throughout the whole Alpine region at depths greater than predicted by the global model. The mantle transition zone is not or only slightly thickened (<= 10 km) north of the Alps, but is up to 40 km thicker than normal south of the Alps and in the Western Alps outlining a roughly 300 km-wide and 800 km-long, ENE-WSW directed anomaly. Thickening, assuming conversions are due to olivine phase transformations, suggests that the Alpine mantle transition zone is at least 300 K colder than normal, if the thickness anomaly were caused by equal amounts of shallowing and deepening of the 410- and 660-km discontinuity, respectively. However, the thickness anomaly seems mainly due to depression of the 660-km discontinuity, which suggests that temperature at the bottom of the mantle transition zone could actually be up to 800 K cooler than ambient mantle while temperatures at 410-km depth may be close to normal. Both location and amount of suggested thermal anomaly agree with tectonic reconstruction and large-scale tomography models of the Euro-Mediterranean region. The cold material accumulated at the 660-km discontinuity is interpreted as remnants from the oceanic mantle lithosphere that detached from the Eurasian plate after closure of the Alpine Tethys. (c) 2008 Elsevier B.V. All rights reserved.