Yücel Yılmaz

Genesis of collision volcanism in Eastern Anatolia, Turkey

Late Cenozoic volcanism in Eastern Anatolia extends in a broad SW - NE trending belt across the Arabia - Eurasia collision zone, from the Arabian foreland basin in the southwest and to the Kars Plateau and Lesser Caucasus in the northeast. Foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic - alkaline composition. Volcanism on the thickened crust north of the Bitlis Thrust Zone varies from the mildly alkaline volcano, Nemrut, and older Mus volcanics in the south, through the transitional calc-alkaline/alkaline volcanoes Bingol and Suphan and the alkaline volcano Tendurek to the calc-alkaline volcano Ararat and older Kars plateau volcanics in the north. Isotope (Sr, Nd) and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the Arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time; and that lavas from the alkaline volcanic area around Mus, and the volcanoes Nemrut and Tendurek north of the Bitlis Thrust Zone were derived from a lithospheric source of similar composition, either from the same, underthrust, Arabian continent or from the Bitlis Massif microcontinent. By contrast, the transitional lavas from Bingol and Suphan and the calc-alkaline lavas from Ararat and Kars were derived from lithosphere carrying a distinct subduction signature inherited from pre-collision subduction events. Positive correlations between 87Sr/86Sr and SiO2 and Rb/Nb and SiO2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone. Trace element covariation diagrams such as Y-Rb indicate the importance of hornblende crystallization at depth (and orthopyroxene at shallow levels) within the calc-alkaline provinces, in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas. Trace element diagrams, based on the covariation of compatible and incompatible elements, point to moderate - low degrees of partial melting with residual clinopyroxene throughout, and residual garnet in the foreland province. Consideration of mineral stabilities, mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths, probably beneath the amphibole and dolomite breakdown curves. Perturbation of the thickened lithosphere by delamination of the thermal boundary layer, perhaps coupled with local stretching associated with pull-apart basins on strike-slip fault systems, is then sufficient to generate melt, the composition of that melt being largely dependent on the enrichment history of the lithosphere in question. In Eastern Anatolia, volcanism appears to have started between about 8 and 6 Ma ago, some 5 Ma after the start of rapid uplift of the East Anatolian Plateau.

When Did the Western Anatolian Grabens Begin to Develop

Abstract To solve a long-lasting controversy on the timing and mechanism of generation of the western Anatolian graben system, new data have been collected from a mapping project in western Anatolia, which reveal that initially north-south trending graben basins were formed under an east-west extensional regime during Early Miocene times. The extensional openings associated with approximately north-south trending oblique slip faults provided access for calc-alkaline, hybrid magmas to reach the surface. A north-south extensional regime began during Late Miocene time. During this period a major breakaway fault was formed. Part of the lower plate was uplifted and cropped out later in the Bozdağ, Horst, and above the upper plate approximately north-south trending cross-grabens were developed. Along these fault systems, alkaline basalt lavas were extruded. The north-south extension was interrupted at the end of Late Miocene or Early Pliocene times, as evidenced by a regional horizontal erosional surface which developed across Neogene rocks, including Upper Miocene-Lower Pliocene strata. This erosion nearly obliterated the previously formed topographic irregularities, including the Bozdağ elevation. Later, the erosional surface was disrupted and the structures which controlled development of the Lower-Upper Miocene rocks were cut by approximately east-west trending normal faults formed by rejuvenated north-south extension. This has led to development of the present-day east-west trending grabens during Plio-Quaternary time.

Two contrasting magmatic associations of NW Anatolia and their tectonic significance

Abstract In northwestern Anatolia two magmatic episodes are distinguished. Initially an intermediate to felsic calc-alkaline association was formed during the Oligocene-Early Miocene. In this period, granitic plutons were intruded into shallow levels in the crust. They are associated with hypabyssal and volcanic rocks. This magmatic event is late/post collisional with respect to the Tethyan collision, which occurred during the Late Cretaceous-Eocene period. The magmatic activity occurred when the region was still suffering a N–S directed compression, which is the result of continuing convergence after the collision. Consequently the magmas passed through an excessively thickened continental crust and, therefore, were contaminated by the crustal materials. The magmatic rocks of this phase are commonly high-K calcalkaline and partly shoshonitic and hybrid. Their compositions reveal crystallization from mantle-derived magmas contaminated by a high amount of crustal components. This magmatic event may thus be regarded as a Tibetan type. The geological signature of the magmas is also similar to the arc-derived magmas. The reason for this is that the metasomatic mantle where the magmas formed was permanently enriched when the subduction and total consumption of the NeoTethyan ocean floor occurred. The second magmatic phase occurred during the Late Miocene-Pliocene. Sporadically developed alkaline basalts were formed during this period. They show geochemical affinities similar to rift-type basalts. This genetical implication is supported by the structural data, which reveal that the E–W trending grabens of the western Anatolia developed in this period under N–S extensional regime.

Geology of the quaternary volcanic centres of the east Anatolia

Abstract Following the collision along the Bitlis–Zagros suture, a north–south convergence between the Arabian Platform and Laurasia has continued uninterrupted until the present. As a result, the continental crust has been shortened, thickened and consequently elevated to form the Turkish–Iranian high plateau. On the high plateau volcanic activity began during the Neogene, intensified during the late Miocene–Pliocene and continued until historical times. Large volcanic centres have been developed during the Quaternary which form significant peaks above the Turkish–Iranian high plateau. Among the Quaternary volcanoes, the major volcanic centres are Ararat, Tendurek, Suphan and Nemrut. Ararat (Agri Dagi) is the largest volcanic center and is a compound stratovolcano, consisting of Greater Ararat and lesser Ararat. The former represents the highest elevation of Anatolia reaching over 5000 m in height. Tendurek is a double-peaked shield volcano, which produced a voluminous amount of basalt lava as extensive pahoehoe, and aa flows. It has an ill-defined semi-caldera. Suphan is an isolated stratovolcano, capped by silicic dome. It represents the second highest topographic elevation in Anatolia, with a height of over 4000 m. A cluster of subsidiary cones and small domes surrounds the volcano. Nemrut is the largest member of a group of volcanoes, which trend north–south. It is a stratovolcano, having a well-defined collapse caldera and a caldera lake. Various volcanic ejecta have been extruded from these volcanic centres over the last 1 to 2 million years. The Quaternary volcanic centres, although temporally and spatially closely associated, display a wide range of lavas from basalt to rhyolite. The volcanoes have diverse compositional trends; Ararat is distinctly subalkaline, Suphan is mildly subalkaline, Nemrut is mildly alkaline and Tendurek is strongly alkaline. The major and trace element compositions together with the isotope ratios indicate that their magmas were generated from a heterogeneous mantle source. Each of the volcanic centres has undergone a partly different magmatic evolution.

Ophiolitic and metamorphic assemblages of southeast Anatolia and their significance in the geological evolution of the orogenic belt

The southeast Anatolian orogen may be divided into three roughly east-west trending structural zones formed as a result of continental collision between the Taurus platform and the Arabian continent. Along the orogenic belt, metamorphic and ophiolitic rocks occur widely. The ophiolites represent remnants of the ocean or oceans which were totally consumed between these converging continental blocks during Late Cretaceous to Miocene period. Metamorphic rocks formed from the oceanic as well as the continental rocks which were incorporated into a nappe stack during the consumption of the oceanic lithosphere and the progressive southward advance of the nappes toward the Arabian continent. The metamorphic units, together with the ophiolite associations, provide stratigraphic and petrologic evidence indicating time, place, and environment of formation of these units; the metamorphic units also provide evidence of nappe transportation stages which are complementary to the data derived from the sedimentary successions in the evaluation of the orogenic evolution of southeast Anatolian orogen.

Geological evolution of the late Mesozoic continental margin of Northwestern Anatolia

Abstract The Armutlu peninsula is a composite tectonic entity made up of sections of the Sakarya continent, the Rhodope-Pontide fragment and an ophiolite. These were assembled following a continental collision between Gondwanaland and Laurasia during the Late Cretaceous. The northern margin of the Sakarya continent underwent progressively increasing deformation prior to and during the advancing collision, due to continued convergence between the two continents. Initially, the leading edge of the continent subsided under the load of an approaching ophiolitic slab. Following this, a north-directed thrusting and folding occurred during the Turonian. Progressive elimination and eventual closure of the ocean preceded the thrusting of northerly situated, collisiondashinduced, nappe packages over the leading edge of the Sakarya continent. The nappe-laden edge of the continental margin then collapsed and steadily subsided under the heavy load of the ophiolitic slab and the northern continental fragment. Consequently, the nappe packages and the ophiolite were collectively metamorphosed during the ConiaciandashSantonian interval. During the subsidence the main body of the Sakarya continent partially detached from its collapsed edge along a fault zone and thus suffered an independent but less severe deformation, which lasted until the uplift of the collapsed edge in the Campanian. From the late Campanian onward, throughout later orogenic stages, the metamorphic and nondashmetamorphic units amalgamated into a single tectonic entity, forming a basement for younger cover rocks.

Pre-Cenozoic tectono-stratigraphic components of the Western Pontides and their geological evolution

The Western Pontides of northern Turkey are a tectonic mosaic formed as a result of progressive welding of continental and oceanic fragments during the Palaeozoic and Mesozoic. In this region, three approximately east–west trending zones can be distinguished: the Pontide Zone representing the Pontides sensu stricto; the Sakarya Zone which is regarded as the southerly continental fragment; and the Armutlu–Ovacik Zone which is viewed as a tectonic mixture of the two zones. The Pontide Zone records development of an ensimatic island arc which are emplaced upon a continental fragment of Laurasian origin prior to the development of the Ordovician sediments. On top of this amalgamated basement, association thick sediments were deposited during the Palaeozoic and Early Mesozoic. During the Dogger, Palaeotethyan ophiolites were obducted onto the Pontides. The thick pile of cover sediments and the ophiolite slab were exhumed along a detachment surface while the underlying rocks were elevated to form a metamorphic core complex during the Early Cretaceous. The metamorphic rocks are exposed along the Ballidag, Sunnice, Almacik and Armutlu mountain ranges. This east–west trending structural high separated two coeval basins, the Ulus Basin and the Boyali Basin, which are located to the north and south respectively. During the Late Cretaceous, collision occurred between the Sakarya continent and the Pontide Zone. The continental convergence affected the region until the late Early Eocene. The present tectonic style of the region was established during this phase. From the Middle Eocene onwards only structural rearrangements have occurred. Copyright

The Çubukludağ graben, south of İzmir: its tectonic significance in the Neogene geological evolution of the western Anatolia

Abstract Field studies on the Neogene successions in south of Izmir reveal that subsequent Neogene continental basins were developed in the region. Initially a vast lake basin was formed during the Early–Middle Miocene period. The lacustrine sediments underwent an approximately N–S shortening deformation to the end of Middle Miocene. A small portion of the basin fill was later trapped within the N–S-trending, fault-bounded graben basin, the Cubukludag graben, opened during the Late Miocene. Oblique-slip normal faults with minor sinistral displacement are formed possibly under N–S extensional regime, and controlled the sediment deposition. Following this the region suffered a phase of denudation which produced a regionwide erosional surface suggesting that the extension interrupted to the end of Late Miocene–Early Pliocene period. After this event the E–W-trending major grabens and horsts of western Anatolia began to form. The graben bounding faults cut across the Upper Miocene–Pliocene lacustrine sediments and fragmented the erosional surface. The Cubukludag graben began to work as a cross graben between the E–W grabens, since that period.

New evidence and solution to the Maden complex controversy of the Southeast Anatolian orogenic belt (Turkey)

The volcanosedimentary units of Late Mesozoic-Tertiary age that outcrop in the Southeast Anatolian orogenic belt are commonly referred to as the Maden complex. There is a long-lasting controversy over its definition, age, stratigraphic and structural position, and the origin, and thus, the orogenic evolution. To solve this problem, large strips across the Southeast Anatolian orogenic belt have been studied extensively, and different rock groups which were regarded previously as the Maden unit have been differentiated. Their major characteristics and differences have been identified. The Maden unit sensu stricto is here redefined as a volcanosedimentary succession of Middle Eocene age representing a short-lived back-arc basin which reached the stage of an embryonic ocean. Presently, the Maden group occurs mainly within the lower nappe stack of the nappe zone of the Southeast Anatolian orogen. It rests stratigraphically on an amalgamated nappe package consisting of the different metamorphic tectonic units and, in turn, is overlain tectonically by the upper nappe units.

Geology of the northern side of the Gulf of Edremit and its tectonic significance for the development of the Aegean grabens

Abstract This paper describes the Neogene evolution of northwestern Anatolia based on geological data collected in the course of a new mapping program. The geological history of the region, as recorded by the Neogene sedimentary and magmatic rocks that overlie the Paleozoic–Triassic basement, began after a lake invasion during the Early Miocene period with the deposition of shale-dominated successions. They were accompanied by calc-alkaline intermediate lavas and pyroclastic rocks ejected through NNE trending fractures and faults. The Lower–Middle Miocene successions were deformed under a compressional regime at the end of the Middle Miocene. The deposition of the overlying Upper Miocene–Lower Pliocene successions was restricted to within NE–SW trending graben basins. The graben bounding faults are oblique with a major strike-slip displacement, formed under approximately the N–S extension. The morphological irregularities formed during the Miocene graben formations were obliterated during a severe erosional phase to the end of the deposition of this lacustrine succession. The present E–W graben system as exemplified from the well-developed Edremit graben, postdates the erosional phase, which has formed during the Plio-Quaternary period.