Seismic Attenuation and Anisotropy in the Carpathians and Adjacent Basins: Upper Mantle Role in the Last Stages of Tethyan Closure

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EAR-0230336Raymond M. RussoThe Carpathian arc marks the suture of a now-closed major embayment ofTethys, the expansive ocean basin that once stretched along the southernmargin of Eurasia from the western Mediterranean to Indochina. The arcis unique in several respects: Closure of the remnant Tethys here entailssome degree of very recent (Miocene-present) continent-continent collisionand disappearance of, most likely, a part of the Tethyan oceanic lithosphereinto the upper mantle. However, study of Carpathian tectonics may have wideapplication, since the processes now visible there must have taken placeelsewhere in the past along the diachronously closing Tethys margin.The unusual seismicity, volcanism, and surface deformation at the Vranceabend zone of the Carpathian arc (45.5 N lat., 25.5 E long.) has yieldeda debate about their cause(s): one viable tectonic scenario invokessubduction of a piece of Tethyan oceanic lithosphere, a remnant of whichis presumed to be just detaching from continental lithosphere of theEast European and Moesian platforms. A second model holds that oceaniclithosphere subduction ended some time in the late Miocene, and that sincethen a portion of East European or Moesian platform continental lithospherehas been delaminating along a horizontal mid-lithospheric interface anddripping down into the upper mantle. The predicted distribution of highvelocity `slab' and low velocity asthenosphere in the upper mantle beneathVrancea zone differs between these models and can be determined seismicallyby a combination of tomography (ongoing in Romania and independent of thisproject) and measurements of seismic attenuation and shear wave splitting,which delineate the distribution of cold lithosphere and warm asthenosphere,and the flow directions of any upper mantle asthenosphere. The activeseismicity at Vrancea can be exploited to delimit both slab distributionand upper mantle flow directions via measurements of seismic attenuationand shear wave splitting. Targeted tests of mantle responses to Tethysclosure will include establishment of the presence and thickness ofmantle asthenosphere beneath the study region, observations of mantleflow directions, estimation of seismic attenuation and associatedinferences concerning mantle temperature fields, and, ultimately, astrong test of competing hypotheses put forth to explain the unusualseismicity and volcanism of the Carpathian arc. Finally, improvedunderstanding of seismic wave propagation and attenuation in the studyregion will directly enhance our understanding of seismic hazard inthis populous area.--