Aral I. Okay

Tethyan sutures of northern Turkey

Abstract The two main Tethyan sutures of Turkey, the İzmir-Ankara-Erzincan and the Intra-Pontide sutures, are reviewed through several well-studied transects crossing the suture regions. Both sutures have formed during the Early Tertiary continental collisions following northward subduction of Tethyan oceanic lithosphere. The İzmir-Ankara-Erzincan suture is represented along most of its c. 2000 km length by Paleocene and younger thrust, which emplace the upper crustal rocks of the northern continent over that of the southern continent with an intervening tectonic layer of Cretaceous subduction-accretion complexes. These thrusts constitutes a profound stratigraphic, structural, magmatic and metamorphic break, of at least Carboniferous to Palaeocene age and form the main boundary between Laurasia and Gondwana in the Turkish transect. Voluminous subduction-accretion complexes of Triassic and Cretaceous ages occur respectively to the north and south of the suture giving the antithetic subduction polarities during these two periods. This, and evidence for a major accretionary orogeny of Late Triassic age north of the İzmir-Ankara-Erzincan suture suggest that two separate oceanic lithospheres, of Carboniferous to Triassic (Palaeo-Tethys) and of Triassic to Cretaceous ages (Neo-Tethys) respectively have been consumed along the suture. The final continental collision along the İzmir-Ankara-Erzincan suture was slightly diachronous and occurred in the earliest Palaeocene to the west and in the Late Palaeocene to the east. The c. 800 km long Intra-Pontide suture is younger in age and have formed during the Early Eocene and younger continental collisions linked to the opening of the Western Black Sea Basin as an oceanic back-arc basin. At present the North Anatolian Fault, which came into existence in the Late Miocene, follows the course of the older Intra-Pontide suture.

Kinematic history of the opening of the Black Sea and its effect on the surrounding regions

The Black Sea consists of two oceanic basins separated by the mid-Black Sea ridge. The east-west-oriented west Black Sea basin opened as a back-arc rift in the Cretaceous by tearing a Hercynian continental sliver, the Istanbul zone, from the present-day Odessa shelf. The Istanbul zone, which was initially contiguous with the Moesian platform in the west, moved south during the Late Cretaceous-Paleocene with respect to the Odessa shelf along two transform faults: the dextral west Black Sea and the sinistral west Crimean faults. It collided in the early Eocene with a Cimmeride zone in the south, thereby ending the extension in the western Black Sea and deactivating both the west Black Sea and the west Crimean faults as strike-slip faults. The east Black Sea basin opened as a result of the counterclockwise rotation of an east Black Sea block around a rotation pole located north of the Crimea. This block was bounded by the west Crimean fault, the southern margin of the eastern Black Sea, and the southern frontal thrusts of the Greater Caucasus. The rotation of the east Black Sea block was contemporaneous with the rifting of the west Black Sea basin but lasted until the Miocene, resulting in continuous compression along the Greater Caucasus.

Tectonics of an ultrahigh-pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China

Ultrahigh-pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino-Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino-Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm-Nd, Rb-Sr, and Ar-Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial eNd values indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan-type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100-km-thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh-pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.

Evidence for intracontinental thrust-related exhumation of the ultra-high-pressure rocks in China

Coesite- and diamond-bearing ultra-high-pressure metamorphic rocks produced during the Triassic continental collision between the Sino-Korean and Yangtze plates are offset by about 530 km along the sinistral Tanlu fault in eastern China. The Tanlu fault terminates abruptly south of the ultra-high-pressure metamorphic terrain in Dabie Shan and does not extend into the foreland fold belt along the Yangtze River. This suggests that in this region the Mesozoic major sinistral displacement along the Tanlu fault was transferred to southward- directed thrusting, which led to the exhumation of the ultra-high-pressure metamorphic rocks formed earlier. Postcollisional Himalayan-type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in China.

An active, deep marine strike‐slip basin along the North Anatolian fault in Turkey

The Tekirdag depression within the Marmara Sea in the Mediterranean region is an active, rhomb-shaped strike-slip basin along the North Anatolian fault with a basin floor at a water depth of −1150 m. New multichannel seismic reflection data and on-land geological studies indicate that the basin is forming along a releasing bend of the strike-slip fault and is filled with syntransform sediments of Pliocene-Quaternary age. The basin is bounded on one side by the North Anatolian fault and on the other side by a subparallel normal fault, which forms the steep submarine slope. In cross section the basin is strongly asymmetric with the thickness of the syntransform strata increasing from a few tens of meters on the submarine slope to over 2.5 km adjacent to the North Anatolian fault. Seismic sections also show that the slope-forming normal fault connects at depth to the North Anatolian fault, implying that the basin is completely detached from its substratum. The whole structure can be envisaged as a huge, rather flat, negative flower structure. The releasing bend of the North Anatolian fault, responsible for the formation of the basin, is flanked by a constraining bend. Along the constraining bend, the syntransform strata are being underthrust, implying a recent change in the direction of the regional displacement vector. This thrusting is responsible for the uplift of the submarine slope to a height of 924 m, possibly by a mechanism of elastic rebound. Regional geology suggests that most of the syntransform strata are lacustrine with only the topmost few hundred meters consisting of deep marine clays. The anomalous present depth of the Tekirdag depression is due to reduced Quaternary sedimentation coupled with high rates of displacement along the North Anatolian fault, which amounts to 20 mm/yr in the Marmara Sea region.

Coeval plutonism and metamorphism in a latest Oligocene metamorphic core complex in northwest Turkey

A metamorphic core complex of latest Oligocene age crops out in the Kazdag˘ mountain range in northwest Turkey. The footwall of the core complex consists of gneiss, amphibolite and marble metamorphosed at 5 ± 1 kbar and 640° ± 50 °C. The average muscovite and biotite Rb/Sr ages from the gneisses are 19 Ma and 22 Ma, respectively, and imply high temperature metamorphism during latest Oligocene times. The hangingwall is made up of an unmetamorphosed Lower Tertiary oceanic accretionary melange with Upper Cretaceous eclogite lenses. The hangingwall and footwall are separated by an extensional ductile shear zone, two kilometres thick. Mylonites and underlying high-grade metamorphic rocks show a N-trending mineral lineation with the structural fabrics indicat- ing down-dip, top-to-the-north shear sense. The shear zone, the accretionary melange and the high- grade metamorphic rocks are cut by an undeformed granitoid with a 21 Ma Rb/Sr biotite age, analytically indistinguishable from the Rb/Sr biotite ages in the surrounding footwall gneisses. The estimated pressure of the metamorphism, and that of the granitoid emplacement, indicate that the high-grade metamorphic rocks were rapidly exhumed at ~ 24 Ma from a depth of ~ 14 km to ~ 7 km by activity along the shear zone. The subsequent exhumation of the metamorphic rocks to the surface occurred during Pliocene-Quaternary times in a transpressive ridge between two overstepping fault segments of the North Anatolian Fault zone. The high-grade metamorphic rocks of the Kazdag ˘ range are surrounded by voluminous calc-alkaline volcanic and plutonic rocks of Late Oligocene-Early Miocene age, which formed above the northward-dipping Hellenic subduction zone. The magmatic arc setting of the core complex and stratigraphic evidence for subdued topography in northwest Turkey prior to the onset of extension suggest that the latest Oligocene regional extension was primarily related to the roll-back of the subduction zone rather than to the gravitational collapse.

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Abstract Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Obduction, subduction and collision as reflected in the Upper Cretaceous-Lower Eocene sedimentary record of western Turkey

Late Cretaceous–Early Eocene Tethyan evolution of western Turkey is characterized by ophiolite obduction, high-pressure/low-temperature metamorphism, subduction, arc magmatism and continent–continent collision. The imprints of these events in the Upper Cretaceous–Lower Eocene sedimentary record of western Anatolia are studied in thirty-eight well-described stratigraphic sections. During the Late Cretaceous period, western Turkey consisted of two continents, the Pontides in the north and the Anatolide-Taurides in the south. These continental masses were separated by the Izmir-Ankara Neo-Tethyan ocean. During the convergence the Pontides formed the upper plate, the Anatolide-Taurides the lower plate. The arc magmatism in the Pontides along the Black Sea coast is biostratigraphically tightly constrained in time between the late Turonian and latest Campanian. Ophiolite obduction over the passive margin of the Anatolide-Tauride Block started in the Santonian soon after the inception of subduction in the Turonian. As a result, large areas of the Anatolide-Tauride Block subsided and became a region of pelagic carbonate sedimentation during the Campanian. The leading margin of the Anatolide-Tauride Block was buried deeply and was deformed and metamorphosed to blueschist facies during Campanian times. The Campanian arc volcanic rocks in the Pontides are conformably overlain by shaley limestone of Maastrichtian–Palaeocene age. However, Maastrichtian sedimentary sequences north of the Tethyan suture are of fore-arc type suggesting that although arc magmatism ceased by the end of the Campanian age, continent–continent collision was delayed until Palaeocene time, when there was a change from marine to continental sedimentation in the fore-arc basins. The interval between the end of the arc magmatism and continent–continent collision may have been related to a northward jump of the subduction zone at the end of Campanian time, or to continued obduction during the Maastrichtian.

Apatite fission-track data for the Miocene Arabia-Eurasia collision

The collision between the Eurasian and Arabian plates along the 2400-km-long BitlisZagros thrust zone isolated the Mediterranean from the Indian Ocean and has been linked to extension of the Aegean, rifting of the Red Sea, and the formation of the North and East Anatolian fault systems. However, the timing of the collision is poorly constrained, and estimates range from Late Cretaceous to late Miocene. Here, we report the fi rst apatite fi ssiontrack (AFT) ages from the Bitlis-Zagros thrust zone. The AFT samples are distributed over the 450 km length of the Bitlis thrust zone in southeast Turkey and include metamorphic rocks and Eocene sandstones. Despite the disparate lithology and large distance, the AFT ages point consistently to exhumation between 18 and 13 Ma. The AFT ages, along with a critical appraisal of regional stratigraphy, indicate that the last oceanic lithosphere between the Arabian and Eurasian plates was consumed by the early Miocene (ca. 20 Ma). The results imply that Aegean extension predated the Arabia-Eurasia collision.

Post-collision magmatism and tectonics in northwest Anatolia

A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavşanli and Sakarya zones. The 40Ar/39Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of ∼3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures <10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (<0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.