Geometry and Kinematics of Continental Deformation in Zones of Collision: Examples from Central Europe and Eastern Mediterranean

Consideration of world-wide epicenter distribution has shown that deformation in continental lithosphere is not narrowly confined to well-defined plate boundaries but is present in wide, diffuse plate boundary zones. Early studies on the seismicity of the peri-Mediterranean area resulted in the division of the lithosphere in that region into a number of small plates, or microplates. Later studies in central Asia, which integrated seismicity with Quaternary geology, indicated, however, that a continuum approach may be more realistic to describe continental tectonics. This study concentrates on geometry and timing of continental deformation that resulted from continental collision in Central Europe and Eastern Mediterranean. In Central Europe continental collision occurred along the Alps during the Lutetian/Priabonian boundary, Foreland deformation in the form of rifting at high angles to the orogen (the Upper Rhine Graben) and strike-slip faulting at about 45° to 60° to the orogen followed the collision. Rifting was nearly synchronous with the collision; strike-slip faulting happened about 20 m.y. after the collision. In the Eastern Mediterranean the North Anatolian Transform and the Turkish-Iranian Plateau were the main objects of study. The North Anatolian transform fault is a morphologically distinct and seismically active strike-slip fault which extends for about 1200 km from Karliova to the Gulf of Saros along the Black Sea mountains of N. Anatolia. It takes up the relative motion between the Black Sea and the Anatolian plates, thereby connecting the E. Anatolian convergent zone with the Hellenic Trench through the complex plate-boundary zone of the Aegean. For most of its length, the transform has a typical strike-slip fault zone morphology, characterized by a narrow \u27rift zone\u27, offset, captured and dammed streams, sag ponds and other deformed morphological features. The fault zone is a broad region of extensively crushed country rock cut by a number of parallel and/or anastomosing strike-slip faults. The transform has periods of seismic activity the last of which, from 1939 to the present, is characterized by frequent 6The Turkish-Iranian Plateau (Fig. 5.1) is a high region with an average elevation of about 1.5 km. During the late Miocene the last piece of oceanic lithosphere between the Eurasian and Arabian continents was eliminated at the Bitlis/Zagros suture zone. Continued convergence across the collision site resulted in the shortening of the plateau across strike by thickening and by sideways motion of parts of it. Predominantly calc-alkaline vulcanism is present on the highest portions of the area, despite the absence of a descending slab of lithosphere. Surface geology and vulcanism of the Turkish-Iranian Plateau resemble greatly those of the Tibetan Plateau, and both are underlain by a zone of seismic attenuation. From a comparison of these features and their tectonic setting, we argue that the two plateaux are homologous structures, albeit at different stages of their evolution. Both areas appear to be tectonically alive and actively shortening. Available evidence lends little support to the hypothesis of large-scale underthrusting of continental lithosphere and of plastic-rigid indentation where such high plateaux, located directly in front of the rigid indenter, are considered to be tectonically dead. Their peculiar features are best explained in terms of shortening and thickening the continental crust whereby its lower levels are partially melted to give rise to calc-alkaline surface vulcanism. Minor associated alkaline volcanism may be due to local longitudinal cracking of the crust to provide access to mantle. In conclusion, it appears that although the existing mechanical models of continental collision processes satisfy the first-order properties of collision zones they fail to predict the geological (particularly the temporal) details of these areas. Detailed field-mapping rather than attempting to refine the existing theoretical models seems necessary