Deep seismogenic zone Vrancea as an indicator of geodynamic processes

Abstract

Tectonophysical interpretation of 80 mechanisms of earthquake sources (80—172 km deep) during 1940—2011 has been performed for seismogenic zone Vranchea. Functioning nodal planes of mechanisms have been identified as the planes of uplifts and uplift-shifts that were formed under conditions of sub-horizontal pressure. Structural scheme of deep zone Vranchea has been plotted where functioning nodal planes have been united into thrust-fault, uplifts and uplift-shift zones extending in northeastern, northwestern, north-northwestern and sub-latitudinal directions. It has been shown that orientation of zone Vranchea shift planes is close to orientation of shift planes of flysh Carpathians and large fault zones intersecting it. These results contradict two main concepts of zone Vranchea formation based on mechanisms of tearing off slabs of oceanic lithosphere or delamination of continental lithosphere because they assume the turn of a segment of lithosphere from sub-horizontal to sub-vertical position and a fairly strong deformation of lithosphere including the lower crust on the areas adjacent to the zone. According to DSS and seismotomographic data, the structure of the earth’s crust and upper mantle in the area of eastern oro-wedge of Carpathians is little different from the structure of the crust and mantle in other parts of the Carpathian section of Alpines. Alternative concept has been proposed issued from consideration that the Vranchea zone is not a petrologic body – a part of oceanic or continental plate included into subduction-collision processes and moved to present position but more probably the deformation zone of overall lateral (horizontal) pressing appeared in situ as a result of cramming of a narrow enough indentor of a micro-plate Tissa-Dakya into produced by them oro wedge of the Carpathians. Overall lateral pressing favored the resistance to indentor movement from the side of the northern part of the Mysian microplane, Gobrogea and the West Black Sea microplate and also centriclinal decline of fault zones limiting its movement from the north and south. Such formation of Vranchea zone in situ is confirmed by its disposition in the zone of intersection of large-scale fault zones — Trans-European suture zone, South-Carpathian zone et al.