Active Tectonics
The world map of plate tectonics
Tectonic settings
Caucasus lies between Black Sea and Caspian Sea, within a broad zone of
deformation, which forms part of Alpine-Himalayan collision belt. The
present-day tectonics of the area are dominated by the motion of Arabian
plate northward relative to the Eurasian plate.
The Geological history of the Caucasus region is complex, and is described
by several investigators. A breath outline of main aspects of the geological
evolution directly relevant to understanding the main features of the
present geological and tectonic structures of the Caucasus region are
presented here according to McCormack (McCormack 1994). From Jurassic to
Paleocene times (roughly 140 to 35 Ma), the present Greater Caucasus area
was back-arc basin, produced by subduction of the Tethys Ocean northwards
under the Russian Platform. In late Oligocene (30 Ma) the Red Sea began to
open, causing the Arabian plate to move northwards with respect to Africa
and closing the Tethys Ocean in this region. Closure occurred about 20 Ma
and subduction transferred to the northern boundary of marginal basin. The
marginal basin was in turn rapidly closed up and continent-continent
collision began 5-3.5 Ma. It si proposed from gravity and seismic reflection
data, that oceanic crust still underlies the west and east Black Sea
depressions, the Southern Caspian Basin, and the eastern part of the Mtkvari
Basin.
Tectonic structures of the
Caucasus region
The initiation of
continent-continent collision caused the folding and thrusting of the
Greater Caucasus upwards and they are now the highest mountains in the
western segment of the Alpine-Himalayan belt. The resulting present day
structural units of the Caucasus region are shown in simplified form in.
The Eurasian and Arabian plates converge at 28 mm/y along 26°N near the
Caucasus. subduction of continental crust is avoided by lateral extrusion of
the Turkish block, Azerbaijanian block and northwestern Iranian block and by
crustal thickening through under thrusting of accreted terrines (Triep et
al. 1995). Philip et al (1989) have proposed the existence of a large
left-lateral strike-slip fault - the Borjomi-Kazbegi fault, which they
suggest is a major active feature in the area.
They propose that the structure acts as a transform fault dividing the Greater Caucasus into two distinct parts: the eastern Caucasus, bounded by thrusts to the north and south, which is seismically active, and the western Caucasus, bounded by thrusts only to the south and dipping down gently to the Russian Platform to the north, which is comparatively seismically quiescent (McCormack, 1994). In another model for this region (Jackson 1992), the oblique component of the convergence predicted between Arabia and Eurasia is Partitioned into shortening in the north, perpendicular to the strike of the Greater Caucasus and into right-lateral strike-slip on ESE-WNW striking faults farther south . A large thrust bounds the southern margin of the high plateau area of eastern Turkey, and there is another, lying just to the north of Lake Sevan, Lesser Caucasus. Between the Greater and Lesser Caucasus Intermountain Depression has been trapped in the collision. Jackson shows by summing the moment of large earthquakes that all of the strike-slip motion can be accounted for by motion in earthquakes, but that only 10-30% of the shortening of Caucasus is produced in this way, the rest presumably occurring as a seismic creep.
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