827 International Geology Review, Vol. 45, 2003, p. 827–840. Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/03/687/827-14 $25.00 Development of an Oblique Subduction Zone— Tectonic Evolution of the Tethys Suture Zone in Southeast Turkey ALI ELMAS 1 stanbul University, Engineering Faculty, Department of Geology, 34850 Avcilar, stanbul, Turkey AND YÜCEL YILMAZ Kadir Has University, Fatih, stanbul, Turkey Abstract The Southeast Anatolian orogen, part of the Mediterranean-Himalayan orogenic belt, may be divided into three approximately E-W–trending zones. From south to north, they are the Arabian platform, an imbricated zone, and a nappe zone. The eastern part of the Arabian platform includes a lower Paleozoic to lower Miocene sedimentary sequence and ophiolitic rocks that were obducted on the platform during Late Cretaceous time. Also, in the eastern part of the nappe zone consisting of lower and upper nappe packages, three major Paleocene–Oligocene deformational phases occurred, related to late–early Paleocene–early Eocene, late Lutetian, and late Oligocene nappe emplacements. These amalgamated nappes collided with the Arabian plate and were welded onto it at the latest stage of the orogenic evolution during late early Miocene time. The imbricated zone consists of units that were compressed and sandwiched between the nappes and the Arabian platform. In the nappe and imbricated zones, various island-arc volcanics and associated units developed related to the northward subduction of Neotethys during the Late Cretaceous to the late Oligocene. The ages of volcanic units decrease from west to east. This reflects oblique subduction of the ocean floor; its remnant presently underlies the northerly nappes. Introduction THE SOUTHEAST ANATOLIAN BELT constitutes the western part of the Bitlis-Zagros orogen (Fig. 1A), and is a good example of a collision-related moun- tain range ( engör and Kidd, 1979; engör and Yil- maz, 1981). It may be divided into three major tectonic zones (Figs. 1B and 2): from south to north, the Arabian platform, an imbricated zone, and a nappe zone (Yilmaz, 1993). The Arabian platform represents a thick autoch- thonous sedimentary succession of early Paleozoic to Miocene age (Sungurlu, 1974; Perinçek, 1990; Yilmaz, 1993). The nappe zone and imbricated zone consist essentially of metamorphic and ophiolitic associations (Yilmaz et al., 1993). Within these zones, various Upper Cretaceous to Eocene volca- nosedimentary units are present; they include the Yüksekova Group, the Helete volcanics, and the Maden Group (Table 1). The Yüksekova Group and the Helete volcanics are products of island-arc volcanism developed on an ophiolitic basement ( engör and Yilmaz, 1981; Hempton and Savci, 1982; Tarhan, 1986; Yigitbas, 1989). The former formed during Late Cretaceous to early Eocene time, and the latter developed during middle to late Eocene time (Yilmaz et al., 1993); in contrast, the Maden Group was deposited in a back-arc basin behind (i.e., north of) the Helete volcanic chain dur- ing middle Eocene time (Yigitbas, 1989; Yigitbas and Yilmaz, 1996a). However, field observations and geochemical data obtained in the eastern part of the Southeast Anatolian orogenic belt indicate the presence of young island-arc volcanic activity, from late Lutetian to late Oligocene time (Gövelek volca- nics; Elmas, 1992, 1994; Table 1). The existence of this volcanic unit necessitates re-evaluation of the evolution of the Southeast Anatolian orogenic belt. In this paper, field observations derived from dif- ferent tectonic units of the region are presented first, and then the Cretaceous to Oligocene island-arc vol- canic associations are compared. Finally, in light of 1 Corresponding author; email: alielmas@istanbul.edu.tr