Valentin S. Burtman

Late Cenozoic slip on the Talas-Ferghana fault, the Tien Shan, central Asia

Although Cenozoic crustal shortening and thickening by thrust faulting have built the present Tien Shan, active right-lateral shear on the northwest-trending Talas-Ferghana fault appears to be the most rapid localized deformation in the belt. Ephemeral stream valleys have been offset right-laterally tens of metres. New and published radiocarbon dates of organic material deposited in depressions blocked by offset ridges place upper bounds on the average Holocene slip rate at 18 localities. Uncertainties allow 14 upper bounds to overlap the range of 8–16 mm/yr, and 95% confidence limits on such bounds at 11 sites are entirely within this range. We infer that the rate of ≈10 mm/yr is not simply an upper bound, but applies to the late Holocene Epoch. Although the bounds on rates permit more rapid slip in the northwest than the southeast, they do not place a useful constraint on variations in slip rate along the fault. Offsets of Paleozoic facies boundaries imply a total right-lateral shear of 180–250 km, but Early Cretaceous sedimentary rock appears to have been offset only 60 ± 10 km. Published paleomagnetic declinations of Cretaceous- Miocene rock demonstrate 20°–30° of counterclockwise rotation of the Ferghana Valley, which lies just west of the Talas-Ferghana fault, with respect to stable parts of Eurasia and 20° ± 11° with respect to the central Tien Shan east of the fault. These declinations are consistent with a maximum northwestward translation of 70–210 km of the Ferghana Valley at the Talas-Ferghana fault and, therefore, with a similar maximum horizontal shortening across the Chatkal Ranges, which lie between the Ferghana Valley and the Kazakh platform. Estimates of crustal thickness beneath the Chatkal Ranges, however, permit only 60–100 km of Cenozoic shortening. If <100 km of slip on the Talas-Ferghana fault accumulated at a constant rate of 10 mm/yr, it would imply an initiation of slip more recently than ca. 10 Ma, long after India collided with the rest of Eurasia.

Cenozoic crustal shortening between the Pamir and Tien Shan and a reconstruction of the Pamir–Tien Shan transition zone for the Cretaceous and Palaeogene

The magnitude of the Late Cenozoic crustal shortening during convergence of the Pamir and Tien Shan was determined using a contemporary pattern consisting of facies zones, palaeomagnetic data (regarding the rotation of tectonic units) and data on the structure of the Tadjik Depression. By Late Cenozoic, Cretaceous and Palaeogene facies zones were cut by the Vakhsh‐Trans-Alay overthrust and Darvaz strike‐slip faults and a significant part of the Cretaceous‐Palaeogene Tadjik Basin was overthrust by the Pamir massif. The sediments of easternmost part of the basin are preserved in the Tarim Depression. The facies zones of the southern slope of the Afghan‐Tadjik Basin were deformed and moved northward. A pattern of facies zones indicates a convergence between the Pamir and Tien Shan over a distance of 300‐400 km. A number of cross-sections through the Tadjik Depression were used to establish the structure before folding. A rotation of tectonic units, indicated by structural data, conforms to the angles of rotation as determined in palaeomagnetic studies. The data suggest 300 km of convergence between the Pamir and Tien Shan. Stratigraphic, lithological, structural and palaeomagnetic data formed the basis for the construction of the palinspastically-restored palaeogeographic and sedimentologic environments for the Tadjik shallow sea basin which was situated between the Pamir and Tien Shan before their convergence in the Late Cenozoic. The maps were constructed for the eight stratigraphic levels of the Cretaceous and Palaeogene. The Tadjik Sea was a bay in the enormous Turan Sea. In the Early Aptian this bay was located in what is now the Afghan‐Tadjik Basin. In Late Cretaceous, the eastern shore of the bay lay 600‐700 km further eastward and in the Eocene, marine environments extended even further eastward but after the Rupelian continental environments occupied all of this region.

Permian paleomagnetism of the Tien Shan fold belt, Central Asia: post-collisional rotations and deformation

Permian volcanic and sedimentary rocks were sampled from eight localities in the western and central parts of the Tien Shan fold belt. High-temperature, sometimes intermediate-temperature components isolated from these rocks at seven localities after stepwise thermal demagnetization are shown either to predate folding or be acquired during deformation; the conglomerate test at some localities is positive. The observed inclinations fit rather well with the Eurasian reference data, whereas the declinations are strongly deflected westward; westerly declinations have already been observed from the other parts of the Tien Shan (from the Turan plate in the west to the northern rim of Tarim and the Urumque area in the east). Our analysis shows that a considerable counterclockwise rotation of the Tien Shan fold belt as a rigid body is geologically improbable. We hypothesize that a sinistral shear zone existed over the fold belt thus accounting for systematically westerly declinations. This zone is about 300 km wide and is traced along the Tien Shan fold belt for 2500 km. A large area of Permian alkali magmatism in the West and Central Tien Shan is interpreted as an extensional domain conjugated with the shear zone. This shear zone can be accounted for by translation of the Kara Kum and Tarim blocks along the Eurasian boundary after their oblique collision in the Late Carboniferous. Two phases of rotation are recognized in the Tien Shan. The earlier rotation took place under shear strain during the D3 stage of deformation in the Artinskian‐Ufimian. The later rotation is connected with transpression (D4 stage of deformation) and could occur from the Late Permian to Early Jurassic.

Paleomagnetism of Cretaceous red beds from Tadzhikistan and Cenozoic deformation due to India-Eurasia collision

Abstract We have carried out structural and paleomagnetic studies in the Tadzhik depression in order to evaluate the main features of the Alpine tectonics of this area. About 340 cores from 43 sites of Lower Cretaceous red beds were sampled from four different localities in the basin and adjacent ranges. A well-defined component of magnetization (A) of normal polarity with high unblocking temperatures up to 650–670°C was isolated from all the sites. Another component of magnetization (B) with unblocking temperatures between 650 and 680°C was isolated from only fifteen sites; this component is bipolar. The fold test is positive for both components. We believe that component A was acquired during the Cretaceous long interval of normal polarity. Comparison with Eurasian reference data shows significant counterclockwise rotation of a locality close to the Pamir wedge ( R = 51 ± 5°) and another counterclockwise rotation from the inner part of the basin ( R = 15 ± 5°). No significant rotations are observed at the two other localities on the periphery of the Tadzhik basin.

Lower and Middle Jurassic paleomagnetic results from the south Lesser Caucasus and the evolution of the Mesozoic Tethys ocean

Abstract We studied Lower Jurassic (Pliensbachian-Toarcian) basalts and tuffs and Middle Jurassic (Bajocian) limestones and marls from 9 sites in the southern part of the Lesser Caucasus (ca. 39.3°N, 45.4°E). A component showing rectilinear decay to the origin was reliably isolated from most volcanics. This characteristic component reveals nearly antipodal directions for both polarities and passes the fold test; the conglomerate test performed on lava boulders from the Aalenian basal conglomerates is also positive. We think that a primary remanence ( D = 20°, I = 38° a 95 = 5°) was isolated from the volcanics. In contrast, Middle Jurassic sediments yielded controversial results, and the only reliably isolated intermediate-temperature component (ITC) does not pass the fold test. The ITC directions show a considerable improvement in data grouping during incremental unfolding, with a maximum at 40% unfolding. We argue, however, that the thus obtained mean direction does not correspond to any paleofield and is most probably an artefact. When compared to reference data for the Eurasian and African plates, the Lower Jurassic mean inclination of 21°N ± 4° agrees well with the latter, thus implying that, in the Early Jurassic, the area studied belonged to Gondwana and was separated by the Tethys ocean from Eurasia, in agreement with paleontological data. We tried to locate the position of the boundary between the Eurasian and Gondwanian realms in the Caucasus region in the Early-Middle Jurassic; unfortunately, the available Jurassic paleomagnetic data from other tectonic units of the region did not reveal any clear pattern.

Eocene paleomagnetism of the Caucasus (southwest Georgia): oroclinal bending in the Arabian syntaxis

Abstract The Caucasus is very important for our understanding of tectonic evolution of the Alpine belt, but only a few reliable paleomagnetic results were reported from this region so far. We studied a collection of more than 300 samples of middle Eocene volcanics and volcano-sedimentary rocks from 10 localities in the Adjaro–Trialet tectonic zone (ATZ) in the western part of the Caucasus. Stepwise thermal demagnetization isolates a characteristic remanent magnetization (ChRM) in 19 sites out of 31 studied. ChRM reversed directions prevail, and a few vectors of normal polarity are antipodal to the reversed ones after tilt correction. The fold test is positive too, and we consider the ChRM primary. Analysis of Tertiary declinations and strikes of Alpine folds in the Adjaro–Trialet zone and the Pontides in Northern Turkey shows a large data scatter; Late Cretaceous data from the same region, however, reveal good correlation between paleomagnetic and structural data. Combining Late Cretaceous and Tertiary data indicates oroclinal bending of the Alpine structures which are locally complicated with different deformation. The overall mean Tertiary inclination is slightly shallower than the reference Eurasian inclination recalculated from one apparent polar wander path (APWP), but agrees with other. This finding is in accord with geological evidence on moderate post-Eocene shortening across the Caucasus. We did not find any indication of long-lived paleomagnetic anomalies, such as to Cenozoic anomalously shallow inclinations further to the east in Central Asia.

Origin of structural arcs of the Carpathian-Balkan region

Burtman, VS., 1986. Origin of structural arcs of the &pat&an-Balkan region. In: L.P. Zonenshain (Editor), Tectonics of the Eurasian Fold Belts. Tectonophysics, 127: 245-260. The Late Cretaceous reconstruction shows that the inner zones of the West, East, and South Carpathians were adjacent to the Dinarides and Hellenides at that time, forming together with them a linear fold system with bilateral vergence. This fold system had a northwest strike. In Paleogene-early Miocene times the general deformation of the given part of the Alpine belt took place. It was caused by rotation of its Asia Minor-Balkan part relative to the more northern part of the belt. This rotation (30°-40” counter-clockwise) was accompanied by a shortening of the considered segment of the Alpine belt by 30%. Hence exfoliation of the Carpathian-Pontide and Dinaric-Tauride branches of the Alpine belt from each other, and their disharmonious plastic deformation, resulted in the formation of the Aegean and Tauride structural arcs and the Carpathian structural loop. During formation of the Carpathian loop, the Gemeric-Tatric tectonic block moved northwards along the Pemrinic oceanic substratum and rotated over 90” counter-clockwise relative to the Eastern Alps.

Upper Cretaceous paleomagnetic data from Shikotan Island, Kuril Arc: Implications for plate kinematics

Abstract Maastrichtian tuffaceous sandstones and siltstones were sampled from four sites in the northern part of Shikotan Island in the Lesser Kuril Islands. A characteristic component (ChRM) isolated from most samples passes the reversal and fold tests and is most probably a primary remanence. The ChRM mean direction ( D = 334° , I = 56° , k = 36 , α 95 = 3.2° ) is 14° ± 3.5° steeper than the Pacific reference direction and 13 ± 4° more gentle than the Eurasian reference direction. We assume that the Lesser Kuril Islands together with their southwestern extension into the Nemuro Peninsula, Hokkaido were originally an island arc situated at about 36°N in the central-west Pacific. Soon afterwards, probably by the end of the Late Cretaceous, the Nemuro-Shikotan island arc became inactive and started moving with the Pacific plate. In the Miocene, about 15 m.y. ago, the Nemuro-Shikotan island arc collided with the Eurasian plate, overrode the subduction zone, and occupied its present-day position on the Pacific side of the Kuril island arc. Although this scenario fits the available paleomagnetic data and kinematics of the Pacific plate itself, it disagrees somewhat with the kinematic models for the North Pacific as a whole. In particular, by the end of the Late Cretaceous, the northern margin of the Pacific plate together with the extinct Nemuro-Shikotan island arc should have been in an area close to the transform or ridge-type Kula-Pacific boundary. The available paleomagnetic data from Northeast Asia, however, indicate that active islands arcs existed in mid-northern latitudes of the modern Pacific Ocean in the Cretaceous.

Palaeomagnetism of Upper Cretaceous Rocks from the Caucasus and its Implications for Tectonics

The Caucasus region was subdivided into two domains according to their Alpine structural patterns. The WNW trends prevail within the northern domain (the Great Caucasus), and fold axes outline a large arc within the southern one (the Lesser Caucasus). Sedimentary rocks of Late Cretaceous age have been sampled in Dagestan near the northern margin of the Great Caucasus, and on both flanks of the Lesser Caucasus arc. Directions of a prefolding, most probably primary, component of natural remanent magnetization have been obtained after treatments in the laboratory. The Upper Cretaceous limestones from Dageston yielded a mean paleopole at 74°N and 151°E, closely matching the Cretaceous-Palaeogene part of the Eurasian polar wander path. Declinations ranging from 353° to 37° correspond to changes of fold axis trends within the Lesser Caucasus thus pointing to tectonic origin of the arc. The inclinations for the northern and southern domains differ by 8°+3° We conclude that the Lesser Caucasus moved about 900+350 kilometers northward with respect to stable Eurasia since Late Cretaceous.

Kazakhstan and Tarim microcontinents on the Devonian paleotectonic reconstructions

По палеомагнитным данным установлено палеоширотное положение Казахстанского и Таримского микроконтинентов в девонском периоде. Определение палеоширот основано на результатах изучения на 19 участках доскладчатой высокотемпературной компоненты намагниченности пород. Вычислено положение центра Казахстанского микроконтинента в раннем-среднем девоне на широте 24.6 ± 5.5°, в позднем девоне – на широте 22.7 ± 4.6°. Центральная часть Таримского микроконтинента в раннем-среднем девоне находилась на широте 6.1 ± 4.2°. Было предложено значительное количество палеотектонических схем Центральной Азии с различным дизайном и детализацией. Среди палеотектонических реконструкций можно выделить три группы. На многих реконструкциях распределение террейнов и океанических островных дуг подобно наблюдаемому ныне в Индонезии. К второй группе относятся реконструкции, на которых террейны образуют дугу, которая соединяла Балтийский и Сибирский палеоконтиненты. На реконструкциях третьей группы Казахстанский и Таримский микроконтиненты имеют изолированные позиции в палеоокеане. Мы рассмотрели положение Казахстанского и Таримского микроконтинентов в 19 ордовикских палеотектонических реконструкциях, опубликованных после 2000 года. В результате, предложен путь согласования палеотектонических реконструкций с палеомагнитными данными.Devonian latitudes of the Kazakhstan and Tarim microcontinents obtained from paleomagnetic data. Definition based on the results of the study pre-folded high-temperature components of magnetization of rocks. The article uses the results of paleomagnetic studies of Devonian sedimentary and magmatic rocks, which formed on the continental crust. In these studies, conducted by various researchers, the high-temperature pre folded primary component of magnetization detected in Devonian rocks on 19 plots. Based on that data the latitude of 24.6 ± 5.5° determined for the Center of Kazakhstan microcontinent in the Early-Middle Devonian and 22.7 ± 4.6° in the Late Devonian. The Early-Middle Devonian latitude 6.1 ± 4.2° determined for the Сenter of the Tarim microcontinent. A significant number of paleotectonic schemes of Asia with different design and detail were proposed. We reviewed the position of the Kazakhstan and Tarim microcontinents in 19 paleo-tectonic reconstructions published after 2000. There are three groups of paleoreconstructions among them. On many reconstructions, the location of continental terranes and island arcs of Kazakhstan and Central Asia in the Early and Middle Paleozoic resembles the modern structure of the Indonesian region. On other reconstructions, these terranes form an arc that connected two paleocontinents in the Paleozoic - the Baltic and the Siberian ones. In the alternative design of reconstructions, the terranes have a relatively isolated position in the Paleoocean. As a result, а way of for co-ordination of matching paleotectonic reconstructions with paleomagnetic data is proposed.