H. Gürsoy

Tectonic Setting and Evolution of the Sivas Basin, Central Anatolia, Turkey

The Sivas Basin is one of several Central Anatolian basins. It developed mainly after the closure of the northern branch of Neotethys. Its location between the Kirsehir Massif and the Taurides impl...

Palaeomagnetic study of the Karaman and Karapinar volcanic complexes, central Turkey: neotectonic rotation in the south-central sector of the Anatolian Block

Abstract In the Anatolian sector of the Afro–Eurasian collision zone a palaeotectonic collisional phase (Paleocene to Miocene) responsible for emplacement of the Pontide and Tauride orogens has been replaced by a neotectonic phase of continental deformation (Late Miocene/Early Pliocene to Recent). The latter phase appears to have been accommodated mainly by crustal thickening during Late Miocene and Pliocene times, but was succeeded by complex differential rotations of fault blocks during crustal extrusion in Late Pliocene and Quaternary times. In this study we have investigated palaeomagnetism of Miocene–Recent volcanic rocks comprising the western extension of the Central Anatolian Volcanic Province located in the south-central part of the Anatolian Block with the aim of resolving deformations near to the border with the Tauride orogen. Rock magnetic investigations identify low-Ti magnetite assemblages of primary cooling-related origin. These have predominant multidomain structures but significant fractions of single domains are always present; low-temperature alteration is largely absent. The Karaman Volcanic Complex (Late Pliocene) shows a net rotation of −5.7±6.9° not significantly different from the regional field axis during Recent times. The Karapinar Volcanic Field (Brunhes epoch) identifies a larger net rotation of −23.1±12.0° in a restricted sample. The adjoining Karacadag Volcanic Complex (Late Miocene–Pliocene) and Middle Miocene lavas beneath the Hasandag Complex define net rotations of −8.1±5.9° and −16.4±8.9° respectively. Analysis of palaeomagnetic results from Late Cretaceous–Recent rock units emplaced in Anatolia during the palaeotectonic and neotectonic regimes shows that rates of rotation have accelerated in post-Pliocene times as crustal thickening has given way to tectonic escape. A near-uniform anticlockwise rotation of 25–35° has characterised much of this block during the most recent phase of deformation and appears to have occurred in common with the Eurasian Plate to the north of the North Anatolian Fault Zone. Whilst this rotation appears to extend south eastwards across the Ecemis Fault Zone towards the East Anatolian Fault, the present study shows that smaller differential anticlockwise rotations have characterised the south-central region of the block where it has interacted at its southwestern margin with oroclinal bending focussed on the Isparta angle.

Deformational behaviour of continental lithosphere deduced from block rotations across the North Anatolian fault zone in Turkey

Abstract Theoretical considerations of lithosphere deformation across transform plate boundaries predict an expression in terms of 3istributed deformation. The magnitude of rotation is expected to diminish away from the fault zone in a way which depends on the length of the fault, the amount of displacement, and the ductility of the lithosphere. Palaeomagnetic studies across the North Anatolian transform fault zone, which separates the Eurasian Plate and Anatolian Block in northern Turkey, show that clockwise rotations predicted from the sense of dextral motion are indeed present and have attained finite rotations of up to 270° during the ∼ 5 Ma history of Neotectonic deformation. Such rotations are, however, confined to narrow ( ∼ 10 km wide) zones between system-bounding faults and appear to have resulted from rotation in ball-bearing fashion of equidimensional blocks a few kilometres in size. Outside of this zone only anticlockwise rotations are observed; these are unrelated to deformation across the fault zone and record regional anticlockwise rotation of Turkey which is complementing clockwise rotation of Greece and accompanying Neogene opening of the Aegean Sea. The observed behaviour of continental lithosphere satisfies no plausible value of power law behaviour. We therefore conclude that relative motion across this transform boundary occurs as a discrete zone of intense deformation within a brittle layer comprising the seismogenic upper crust. This is presumed to be detached from a continuum deformation response to shearing in the lower crust and mantle beneath.

A PALAEOMAGNETIC STUDY OF THE SIVAS BASIN, CENTRAL TURKEY : CRUSTAL DEFORMATION DURING LATERAL EXTRUSION OF THE ANATOLIAN BLOCK

Abstract The Sivas Basin is a complex collage of Eocene and younger rocks located within the wedge-shaped eastern margin of the Anatolian Block between the (dextral) North Anatolian Fault Zone and the (sinistral) Eastern Anatolian Fault Zone. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomittant sideways expulsion of the Anatolian Block. Post-collisional deformation since mid-Miocene times has been dominated by NS to NWSE compression expressed by thrusting and strike-slip faulting. Cretaceous and Eocene rocks were magnetically overprinted to variable degrees during the collisional phase although these overprints have since been rotated mostly anticlockwise. Rocks emplaced during the neotectonic history are high-fidelity palaeomagnetic recorders of subsequent block movements. Regional anticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. An absence of perceptible differences between group mean rotations identified from Miocene, Pliocene and Quaternary units shows that most regional rotation has been concentrated within the latest phase of the neotectonic history during Quaternary times at an average rate of ∼ 10°/Ma. Commencement of this rotation postdates initiation of the North Anatolian Fault Zone implying that compression following collision was accomodated initially by crustal thickening during Late Miocene and Pliocene times. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NESW sinistral faults to achieve the crustal shortening resulting from NS compression. These fault orientations and their sense of motion are explained by a Prandtl model involving deformation of a triangular plastic terrane (the Anatolian Block) between two rigid plates (Eurasia and Afro-Arabia). The variations in regional rotation identified by palaeomagnetism show that average contemporary anticlockwise rotation of Anatolia revealed by GPS data (∼ 1.2°/Ma) is achieved by variable, and locally large, block rotations between major thrusts and strike-slip faults.

Palaeomagnetic study of crustal deformation across an intracontinental transform: the North Anatolian Fault Zone in Northern Turkey

Abstract Eocene volcanic rocks spanning the North Anatolian Fault Zone in north central Turkey have a common reversed polarity and appear to record a short term volcanic episode useful for identifying subsequent tectonic rotations. Although regional differences are present, no distributed clockwise rotations caused by dextral motion across the fault zone since mid-Miocene times are found. Instead variable anticlockwise block rotations demonstrate that this fault system does not obey theoretical models for crustal behaviour across continental transforms. Deformation is found to be highly inhomogeneous with a narrow zone of intense clockwise rotation recognised within blocks bounded by strike-slip faults above, and parallel to, the fundamental lineament. Further from the lineament no systematic rotations with respect to the major bounding plates are detected. A zone of c. 30° anticlockwise rotation in the east may be either a consequence of emplacement of the Pontides or an ongoing consequence of continental collision. Slightly larger rotations south of the fault probably record block rotations into Anatolia as this region is being extruded westwards by continuing impingement of Afro-Arabia into the Eurasian Plate.

Palaeomagnetism and magnetic properties of the Cappadocian ignimbrite succession, central Turkey and Neogene tectonics of the Anatolian collage

Abstract The Cappadocian ignimbrite succession of central-southern Anatolia comprises at least nine major and two minor calc-alkaline rhyolitic sheets emplaced at 1–2-Ma intervals between 11.2 and 1.1 Ma. It records the last phase of Neotethyan subduction during final emplacement of the Tauride orogen in southern Turkey. This study reports magnetostratigraphy and describes associated rock magnetic properties. Remanence resides in Ti-poor titanomagnetites. Haematisation is locally produced by post-emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation. Weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane. Directions of kmax with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing. Older ignimbrites (Upper and Lower Goreme, Akdag-Zelve) from the Cardak Centre are all of normal polarity. Later ignimbrites, partly erupted from the Derinkuyu Centre, comprise the Sarimaden (R), Cemilkoy (R), Tahar (R), Kizilkaya (R), Incesu (N) and Valibaba-Sofular (R) ignimbrites. The overall (reversed) group mean is D/I=174/−51° (N=10 units, R=9.84, α95=6.6°, k=55) and all magnetisation directions from the Upper Goreme (9.0 Ma) onwards are rotated anticlockwise with respect to Eurasian and African palaeofields. This sense of rotation characterises most of central Anatolia and averages 9±5° in this sector. The rotation rate from 8 to 1 Ma BP was ∼1.25°/Ma but it appears to have accelerated during the latter part of the Quaternary to about an order higher than rates determined from GPS. Rotation has resulted from extrusion of fault blocks during tectonic escape of the Anatolian collage to the southwest and followed crustal thickening as the Afro-Arabia Plate has continued to impinge differentially into Eurasia. Cenozoic magnetic inclinations are systematically shallower across Anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates. Only part of this difference (∼400 km) can be accommodated by northward movement and crustal thickening in Central Anatolia since mid-Miocene times. Shallow inclinations observed in the Aegean region extend into Turkey; they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly. The pattern of neotectonic declinations across Anatolia shows rotations that are strongly anticlockwise rotated in the east near the Arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage. Rotations also seem to become generally younger towards the south. Crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the Tauride Arc and, by inference, the Cyprus Arc.

Regional Significance of Neotectonic Counterclockwise Rotation in Central Turkey

Counterclockwise rotation is a characteristic feature of the results of most paleomagnetic studies of the Pontides and Anatolides of central Turkey, applicable to regions both north and south of the North Anatolian fault zone. In this paper, we report new data from Eocene volcanics and assess existing data from the calc-alkaline volcanic suites of this age. Although there are regional variations, probably resulting from rotations of individual fault blocks, an average counterclockwise rotation of ∼33° is identified across a region extending from 34° to 38° E Long. A mean Eocene paleolatitude of 27° N is compatible with ongoing northward movement and residual closure of a few degrees across the Pontide orogen during the latter part of its paleotectonic history. It seems probable that this rotated domain extends as far west as the Aegean graben system of western Turkey and as far south as the Taurides. Paleomagnetic evidence from younger volcanics suggests that the bulk of the rotation occurred during Quate...

Differential neotectonic rotations in Anatolia and the Tauride Arc: palaeomagnetic investigation of the Erenlerdaǧ Volcanic Complex and Isparta volcanic district, south–central Turkey

Abstract: In the Anatolian sector of the Alpine–Himalayan collisional belt a palaeotectonic phase of terrane accretion has been succeeded by a neotectonic phase of intracontinental deformation as the Afro-Arabian plate has continued to impinge differentially into the accreted collage. The resulting tectonic escape has, during the last few million years, extruded and rotated blocks in a southwesterly direction. This paper reports a palaeomagnetic investigation of the Erenlerdaǧ Volcanic Complex and Isparta alkaline volcanic district undertaken to extend analysis of crustal rotation into the southwestern part of the Anatolian region, and investigate the interaction with the Isparta angle and the extensional province of western Turkey. The Erenlerdaǧ volcanism comprises three phases of volcanism. The oldest Sille volcanics (11.7–11.4 Ma) yield a mean D=150° I=−52° rotated anticlockwise during early stages of crustal thickening and deformation before a later Miocene episode (c. 10.9–8.9 Ma, D=183°, I=−47°) and a Mio-Pliocene episode (D=179°, I=−51°). The latter two episodes indicate that no significant rotation has resulted during the neotectonic crustal extrusion in this southwestern sector of Anatolia. Further to the west within the Isparta Angle a 4.7–4.0 Ma alkaline episode yields a mean of D=186°, I=−53° rotated slightly clockwise. The pattern of palaeomagnetic declinations across Anatolia shows rotations that are strongly anticlockwise in the east near the Arabian pincer and diminish towards the west to become zero or slightly clockwise at the western extremity of the collage. The timing of rotation also appears to become younger towards the south. Crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the Tauride Arc and, by inference, the Cyprian Arc. Where good age control exists in Cappadocia and the Sivas Basin rotations are found to be concentrated within the last few million years and are up to an order higher than rates deduced from global positioning system. The palaeomagnetic data imply that the neotectonic deformation following collision initially produced crustal thickening resulting in uplift of the Anatolian Plateau and was only subsequently accommodated by major differential block rotation during tectonic escape.

Tectonics of the Sivas Basin: The Neogene Record of the Anatolian Accretion Along the Inner Tauric Suture

The Sivas Basin extends over a major crustal structure underlying the contact zone between the Tauride and Pontide belts. The Kirsehir block, a continental crustal element lying between the main belts, introduces a subordinate suture in front of the Pontides—the Inner Tauride suture. The junction of the two main sutures occurs between Hafikand Imranli. Four structural zones have been considered. The northern basement of the basin, which includes both the Kirsehir continental crust and thrust sheets of ophiolite and pelagic sediments, forms an imbricate stack with an Eocene cover. The Eocene cover shows two distinct sequences: marine neritic and continental basalts overlying the Kirsehir basement, and deltaic and basinal deposits lying to the southeast. Southward tectonic stacking of the entire pile has occurred repeatedly since Oligocene time. The Sivas Basin proper is separated from the Kirsehir basement by the Kizilirmak Basin. This new structural unit consists of nearly undeformed, middle Miocene sands...

Neotectonic deformation in the western sector of tectonic escape in Anatolia: palaeomagnetic study of the Afyon region, central Turkey

Abstract Following final closure of the Neotethyan Ocean during the late Miocene, deformation in central Turkey has led to crustal thickening and uplift to produce the Anatolian Plateau followed by westward extrusion of terranes by strike–slip. Widespread volcanism has accompanied this latter (neotectonic) phase, and palaeomagnetic study of the volcanism shows a coherent record of differential block rotations, indicating that the Anatolian region is not a plate (or ‘platelet’) sensu stricto but is undergoing distributed internal deformation. To evaluate the scale of neotectonic rotations in the transition zone near the western limit of tectonic escape and the border of the extensional domain in central-west Turkey, we have studied the palaeomagnetism at 82 sites in volcanic suites distributed along a ∼140-km lineament with north–south trend and ranging in age from 18 to 8 Ma. Comparable deflection of magnetic remanence from the present field direction is identified along the full length of the lineament. A mean clockwise rotation of 12.3±4.2° is determined for this western sector of the Anatolian strike–slip province. Since similar rotations are observed in the youngest and oldest units, this cumulative rotation occurred after the late Miocene. When interpreted together with results elsewhere in Anatolia, it is inferred that the rotation is later than crustal thickening and uplift of the Anatolian Plateau and entirely a facet of the tectonic escape. Inclinations are mostly ∼10° shallower than the predicted Miocene field and are considered to reflect the presence of a persistent inclination anomaly in the Mediterranean region. Larger rotations departing from the regional trend are also observed within the study region, but are confined to the vicinity of major faults, notably those bounding the Afyon-Aksehir Graben. The pattern of neotectonic declinations across Anatolia identifies strong anticlockwise rotation in the east near the Arabian pincer with progressive reduction in the amount of rotation towards the west; it becomes zero or slightly clockwise at the western extremity of the accreted terrane collage. Rotations also appear to become generally younger towards the south. Crustal deformation has therefore been distributed, and the net effect of terrane extrusion to the west and south has been to expand the curvature of the Tauride Arc. The westward radial expansion of the extruded terranes is inferred to combine with backroll on the Hellenic Arc to produce the contemporary extensional province in western Turkey.