3.7.3 Tectonic Geomorphology and differential vertical motions

The Greater Caucasus is a typical doubly verging mountain-belt with two external fold-and-thrust belts. In its eastern part the pro-wedge front is located to the South and overrides the Kura Basin, whereas the retro-wedge front is located to the North and overrides the Terek-Caspian Basin (Sobornov, 1994; Sobornov, 1996; Ulminshek, 2001). Tectonic structures, such as fault-related folds, thrusts, imbrications, klippen, relay structures, and late transverse structures, such as the large “anti-Caucasian” strike-slip faults, are typical for shallow level fold-and-thrust belt deformations (Gamkrelidze and Rubinstein, 1974; Gamkrelidze and Gamkrelidze, 1977; Kopp and Shcherba, 1985; Dotduyev, 1986; Gamkrelidze and Beridze, 1991; Sholpo, 1993; Ulminshek, 2001). This is corroborated by studies showing that the general metamorphism and internal deformation are weak; schistosity is weak and is only developed in the central parts of the mountain range. Results from apatite fission track studies on basement rocks in Georgia (Kral and Gurbanov, 1996) confirm the prevailing low grade metamorphism in the central part of the orogen. Folds and thrusts generally trend NW-SE to WNW-ESE in the Eastern Great Caucasus, schistosity is axial surface parallel and fold vergence is related to the transport direction. Syn-sedimentary paleotectonic structures of both extensional and compressional (inversion) origin are overprinted by syn-orogenic tectonics.

The evolution of especially the eastern part of the Greater Caucasus is linked to the evolution of the South Caspian Basin. Although it is widely accepted that this basin was initiated by Mesozoic back-arc extension related to the subduction of the Tethys plate to the south, most of the subsidence presently observed occurred during the Tertiary. An additional set of mechanisms must be invoked to explain the younger, greatly accelerated Pliocene-Quaternary phase of subsidence that occurred in a compressional setting, contemporaneous with the subsidence of Caucasus-related foreland basins and the uplift and erosion of the Caucasus Orogen. This phase of rapid subsidence of the South Caspian Basin coincided with the onset of tectonic loading of the crust, in the South by the Elborz orogenic belt and in the North by the Apsheron Sill in the prolongation of the Greater Caucasus. There remains a controversy regarding the timing of the opening of the South Caspian Basin and regarding the presence one or two different sub-basins (Ershov et al., 1999).

The present-day topography of the Caucasus – East Anatolian area reflects the distribution and arrangement of megatectonic features (Fig. 68). Active and dormant mountain fronts shape the topography along the southern margin of the Greater Caucasus and permit to establish the chronology of major tectonic events. In Azerbaijan, the active thrust front coincides with the Karamarian anticline that involves Quaternary sediments of the Kura Basin. This anticline is related to an active blind thrust, with classic wind and water gaps pointing to its lateral growth. This active southern thrust front extends into the foothills of the Lesser Caucasus in western Azerbaijan. Gentle SSW-directed folding and thrusting produces topographic “bumps” in large alluvial fans, as well as tilting of terraces. So far, only few studies have directly addressed the tectonic geomorphology of the Greater Caucasus such as a study of the Alazani Basin on the southern slope of the Great Caucasus in Georgia (Triep et al., 1995), or relate to larger areas as the Kura Basin or the Caspian Sea. Studies on river deltas are further indicators for an active mountain belt. The occurrence of terrace systems documents continued river incision presumably owing to uplift of the mountain range. On the northern slopes of the Eastern Greater Caucasus this uplift gave rise to the development of important cliffs that are upheld by Quaternary material. Connecting these erosional “events” to the terrace systems along the Caspian Sea opens the prospect of quantifying uplift vs. subsidence.

Uplift of the highest summits in the central and northern parts of the Greater Caucasus is highlighted by the occurrence of marine Plio-Pleistocene sediments at elevations in excess of 2500 m above MSL. In the northern part of the mountain belt deposits of the river Samur are deeply (>400m) incised by its present tributaries. Rivers entrench and cannibalize their own deposits, starting in the Early Tertiary, and thus mirror the active tectonic growth and dynamic evolution of topography. Terraces are found along the Caspian Sea (Brod, 1962; Shirinov, 1973; Shirinov, 1975), with the highest, some 230,000 years old terraces being located at about 300m above MSL. Terraces younger than 1.8 Ma occur up to an elevation of some 475m in the mountain valleys, but many more terraces are present up to altitudes of 2000 m and above.

Neotectonic analyses on the larger Caucasus area include studies on crustal deformation, GPS measurements, earthquakes and plate tectonics (McClusky et al., 2000; Vernant et al., 2004a). These investigate the geodynamics and the large-scale structure of the lithosphere in the larger Caucasus-Caspian-Iran area. Regional compression is N-S directed, with an average deformation of 14mm/y across the eastern part of the Greater Caucasus and uplift rates of the order of 8 mm/y. This is in agreement with studies on paleostresses derived from recent brittle faults that cut the whole mountain range and control the development of prominent morphological features such as deep gorges and mountain crests (Fig. 69).