The Carpathian arc marks the suture of a now-closed major embayment of Tethys, the expansive ocean basin that once stretched along the southern margin of Eurasia from the western Mediterranean to Indochina. The arc is unique in several respects: Closure of the remnant Tethys here entails some degree of very recent (Miocene-present) continent-continent collision and disappearance of, most likely, a part of the Tethyan oceanic lithosphere into the upper mantle. However, study of Carpathian tectonics may have wide application, since the processes now visible there must have taken place elsewhere in the past along the diachronously closing Tethys margin.
The unusual seismicity, volcanism, and surface deformation at the Vrancea bend zone of the Carpathian arc (45.5 N lat., 25.5 E long.) has yielded a debate about their cause(s): one viable tectonic scenario invokes subduction of a piece of Tethyan oceanic lithosphere, a remnant of which is presumed to be just detaching from continental lithosphere of the East European and Moesian platforms. A second model holds that oceanic lithosphere subduction ended some time in the late Miocene, and that since then a portion of East European or Moesian platform continental lithosphere has been delaminating along a horizontal mid-lithospheric interface and dripping down into the upper mantle. The predicted distribution of high velocity `slab' and low velocity asthenosphere in the upper mantle beneath Vrancea zone differs between these models and can be determined seismically by a combination of tomography (ongoing in Romania and independent of this project) 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 active seismicity at Vrancea can be exploited to delimit both slab distribution and upper mantle flow directions via measurements of seismic attenuation and shear wave splitting. Targeted tests of mantle responses to Tethys closure will include establishment of the presence and thickness of mantle asthenosphere beneath the study region, observations of mantle flow directions, estimation of seismic attenuation and associated inferences concerning mantle temperature fields, and, ultimately, a strong test of competing hypotheses put forth to explain the unusual seismicity and volcanism of the Carpathian arc. Finally, improved understanding of seismic wave propagation and attenuation in the study region will directly enhance our understanding of seismic hazard in this populous area.