High-resolution P and S tomography of the Carpathian-Pannonian upper mantle and the signature of continental mantle downwelling

Posted by pkm at Mar 17, 2015 10:24 AM |
G. W. Stuart (1), G. A. Houseman (1), Y. Ren (1), C. Ionescu (2), E. Hegedus (3), S. Radovanović (4) & Y. Shen (5) - (1) School of Earth and Environment, University of Leeds, UK; (2) National Institute of Earth Physics, Bucharest, Romania; (3) Eötvös Loránd Geophysical Institute, Budapest, Hungary; (4) Seismological Survey of Serbia, Belgrade, Serbia; (5) Graduate School of Oceanography, University of Rhode Island

We describe a new high-resolution 3D model of the P and S-wave velocity variation in the lithosphere and upper mantle beneath the Carpathian-Pannonian region.   We compare velocity variations of the S-wave model with those of the P-wave model we previously derived and interpret these observations in light of the geodynamical processes that have shaped the lithosphere in this region. The tomographic images were constructed by finite-frequency inversion of P and S relative travel-times measured from teleseismic event data recorded on 54 temporary stations deployed in the South Carpathian Project (2009–2011), 56 temporary stations deployed in the Carpathian Basins Project (2005–2007), and 131 national network broadband stations. Our Vp and Vs models show evidence of lithospheric downwelling in the upper mantle.  Beneath the Eastern Alps, a sub-vertical slab of fast material persists down to ~ 400 km and this fast anomaly extends further to the east and down into the mantle transition zone (MTZ) under the Pannonian Basin. Beneath the SE Carpathians a fast signature extends also to nearly 400 km.  These anomalously fast regions are apparently between 230 and 260 K cooler than ambient mantle at ~200 km depth.  In the MTZ, we image a broad fast anomaly of circular form beneath the entire Pannonian Basin and this fast material appears to have spread outward from beneath the center of the basin. Fast anomalies occur also beneath the Bohemian Massif, parts of the Dinarides and the western edge of the Moesian platform. The signature of mantle downwelling beneath the Vrancea Zone contrasts strongly with that beneath the eastern Alps: at depths between 150 km and 380 km the Vrancea anomaly has high Vp (~ + 2%) and high Vs (~ +3% to +4%); but between 50 km and 150 km depth the amplitude of the Vp anomaly is halved (~ +1%) and the Vs anomaly is actually negative (~ -1.5% to -1%).  We explore possible explanations for this unique seismic signature and conclude that the most likely explanation is that upward migrating fluids released by dehydration reactions at depths of ~150-200 km cause the reduction in velocities above, and moreover are consistent with the high level of intermediate-depth seismicity and Neogene volcanism in that area.  The intermediate-depth seismicity of the Vrancea region and the large volume of high wave-speed material directly beneath it appear to be a consequence of gravitational instability of the mantle lithosphere that is the most recent tectonic event to affect this region.

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