Tracing the Trans-European Suture Zone into the Mantle Transition Zone
Abstract
The depth to which lithospheric roots of cratons influence the surrounding mantle has important consequences for our understanding of the thermal structure of the mantle and its geodynamics. For example, it has been proposed that the cratonic keel of the Kaapvaal craton extends even into the mantle transition zone (MTZ). Here, we use more than 19,000 P-receiver functions from 479 stations in central and eastern Europe to map the MTZ discontinuities beneath the western boundary of the East European Craton (EEC) and adjacent Phanerozoic Europe, across the Trans-European Suture Zone (TESZ), the most fundamental lithospheric boundary in Europe. The main data source in our study is the PASSEQ experiment, which achieved the densest coverage across the TESZ yet with about 200 temporary stations installed from 2006 to 2008 in Germany, the Czech Republic, Poland and Lithuania. These recordings are supplemented by national and regional networks and broad-band data from older temporary deployments. Receiver functions use P-to-S converted teleseismic waves, which yield relative travel times to the 410 km and 660 km discontinuities. They are especially useful in mapping variations in MTZ thickness. We observe significantly shorter travel times, by as much as 2 s compared to standard Earth models, for conversions from both MTZ discontinuities within the craton. This is an effect of the cratonic root and can be explained by about 5% higher velocities in the upper mantle, compared to IASP91, beneath the craton. By contrast, the differential travel time across the MTZ is normal to slightly raised. This implies that any insulating effect of the continental root does not reach the MTZ, comparable to recent observations for the North American Craton. In contrast to earlier observations in Siberia, we do not find any trace of a discontinuity at 520 km depth. If this discontinuity is caused by the transition of wadsleyite to ringwoodite, this means that the MTZ is rather dry beneath the EEC. Within most of covered Phanerozoic Europe, the MTZ differential travel time is remarkably uniform and in agreement with IASP91. No widespread thermal effects of the various episodes of Caledonian and Variscan subduction that took place during the amalgamation of the continent remain. Only more recent tectonic events can be traced. For example, we find evidence of cold material, i.e. the subducted lithosphere of the Alpine Tethys, pooled on top of the 660 km discontinuity, at the contact between Alpine and Carpathian arcs, the subvertical high-velocity anomaly of the Eastern Alpine slab, and heating by 100 K to 250 K at 410 km depth beneath the broader Eifel area. The most interesting result is a previously unobserved increase in MTZ differential travel time by 1.5 s in a linear region of about 350 km width along the TESZ. Taking into account results of recent S-tomographies, raised water content in the MTZ cannot be the main cause for the observed travel time increase. Accordingly, we explain the increase, equivalent to a 15 km thicker MTZ, by a temperature decrease of about 80 K. It could either be due to a remnant of (mainly Variscan) subduction along the present TESZ which got stuck in the MTZ, or an indication of downwelling due to small-scale, edge-driven convection caused by the contrast in lithospheric thickness across the TESZ.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI21C..05K
- Keywords:
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- 7203 SEISMOLOGY / Body waves;
- 7208 SEISMOLOGY / Mantle;
- 8103 TECTONOPHYSICS / Continental cratons;
- 9335 GEOGRAPHIC LOCATION / Europe