Geologic Structure

The Turkmenian portion of South Caspian Basin is bordered to the east by the Kopet-Dagh foldbelt, whereas to the south it is bordered by the Elburz range in Iran. Rows of sub-parallel folds are present throughout the area (Figure 7-33). Some of the folds have shale diapirs, whereas others have active mud volcanoes. Two structural stages have been identified in the South Caspian region. The first one ranges in age from the Mesozoic to Lower Neogene time and the second one from Upper Neogene through Quaternary (Neo-Alpine Orogeny). The structural trends of the two stages are similar. The Mesozoic to Lower Neogene trends are generally much broader than the younger ones. Due to continental collision and consequent "tectonic escape," the trends of the fold axes change dramatically from onshore to offshore in the Turkmenian portion of the South Caspian area. Due to direct collision, the onshore trends are curvilinear and are concave to the southeast. Owing to generalized NNE-SSW compression, the offshore fold axes trend northwest (Figure 7-33).

Sediments in the South Caspian Basin are underlain by oceanic crust. According to Berberian (1983), the oceanic crust is a Tethys Sea remnant which was not subducted during the Alpine-Himalayan orogenies. Ulmishek and Klemme (1990) suggested that the ocean crust may have formed during the Middle Cretaceous time associated with rifting. The writers do not agree with these views, because the analyses of these authors do not take into account the regional evolution of the Caucasus, the type of volcanism present through geologic time (particularly the Mesozoic), the closing of Paleo-Tethys, and the opening of the Neo-Tethys Ocean when the "Kimmerian Archipelago" was colliding with Eurasia in Late Triassic-Middle Jurassic time. The writers, however, agree with the thorough and consistent analyses of Adamia (1982) and Zonenshain et al. (1986), in which the South Caspian Basin opened as a back-arc-type basin.

^Tehran

51 52 53 54 55 56

2oToet so3 so 100 ---r- Basement faults (fades divisions)

—-1--Pliocene-Quaternary fold (Akhmedov and Kevorkov, 1983)

Figure 7-33. Late Alpine (Late Pliocene) fold axes in the South Caspian Basin.

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