Is large scale subduction made unlikely by the mediterranean deep seismicity?

Abstract

The deep seismicity in the Mediterranean region does not have the pattern that the alleged convergence of Africa and Eurasia should produce. Often, where subduction slabs and Wadati-Benioff zones should be present – showing intermediate and deep hypocenters – only shallow intracrustal seismicity is detected. Most geoscientists admit, without a valid explanation, that in this region subduction occurs largely aseismically. Inspection of South Tyrrhenian, Aegean and South Carpathian deep foci zones makes clear that these isolated narrow plumes (or clusters, filaments) of hypocentres cannot be sites of active subduction but that they are related to uplift of deep mantle material. Their presence under actively rising part of orogens – besides many additional clues coming from a number of different fields –leads to a unified interpretation of the involved phenomena, and to a new interpretation of the orogenic processes and fold belt building. The evidence points to vertical displacements of materials as the main process responsible for deep earthquakes, volcanic phenomena and orogenesis. Several tens of km of overthrusts and underthrusts should not be mistaken for large-scale subduction, and the limit of 50-70 km (the roots of an orogen) should be considered the maximum depth of occurrence of metamorphism. Into these limits of depth, the nonlithostatic overpressures due to the surfaceward mantle flow, the association of fluids, extreme magnitude earthquakes and deviatoric stress can be the causes both of those metamorphosed facies (until now presumed to come from depth up to 200 km) and of a shallower than supposed synthesis of biogenic and abiogenic hydrocarbons.