Alexander Mikolaichuk

Paleomagnetism of Paleogene basalts from the Tien Shan, Kyrgyzstan: rigid Eurasia and dipole geomagnetic field

Several conflicting explanations, invoking persistent non-dipole fields or tectonic motion on various scales, were suggested to account for anomalously low paleomagnetic inclinations in Cenozoic rocks of Central Asia. In order to better understand this problem we undertook a paleomagnetic study of Paleogene basalts from a part of the Tien Shan close to the China^Kyrgyzstan border. Stepwise thermal demagnetization showed that only a single-component remanence of reversed polarity is present in these rocks above 200^260‡C. The overall mean direction of this remanence (D = 194.6‡, I =354.0‡, K95 = 3.8‡, n = 18 sites) agrees well with the Eurasian reference directions for the Paleogene which is in sharp contrast with most earlier published Cenozoic inclinations from Central Asia which are 20^30‡ shallower than the expected values. Good agreement of the reference inclinations and those from basalts rules out models which invoke non-dipole fields and/or large-scale tectonic motions for explanation of the Cenozoic inclination anomaly. Instead, a mechanism related to natural remanent magnetization acquisition in redbeds is strongly indicated. The above conclusion also implies that the differences between the reference data and measured inclinations in Cretaceous redbeds from Central Asia are also of non-tectonic origin and a revision of our views on evolution of this region is required. fl 2002 Elsevier Science B.V. All rights reserved.

Cenozoic deformation and exhumation history of the Central Kyrgyz Tien Shan

New low-temperature thermochronological data from 80 samples in eastern Kyrgyzstan are combined with previously published data from 61 samples to constrain exhumation in a number of mountain ranges in the Central Kyrgyz Tien Shan. All sampled ranges are found to have a broadly consistent Cenozoic exhumation history, characterized by initially low cooling rates (<1°C/Myr) followed by a series of increases in exhumation that occurred diachronously across the region in the late Cenozoic that are interpreted to record the onset of deformation in different mountain ranges. Combined with geological estimates for the onset of proximal deformation, our data suggest that the Central Kyrgyz Tien Shan started deforming in the late Oligocene-early Miocene, leading to the development of several, widely spaced mountain ranges separated by large intermontane basins. Subsequently, more ranges have been constructed in response to significant shortening increases across the Central Kyrgyz Tien Shan, notably in the late Miocene. The order of range construction is interpreted to reflect variations in the susceptibility of inherited structures to reactivation. Reactivated structures are also shown to have significance along strike variations in fault vergence and displacement, which have influenced the development and growth of individual mountain ranges. Moreover, the timing of deformation allows the former extent of many intermontane basins that have since been partitioned to be inferred; this can be linked to the highly time-transgressive onset of late Cenozoic coarse clastic sedimentation.

Crustal-scale structure of South Tien Shan: implications for subduction polarity and Cenozoic reactivation

Abstract Based on new structural and petrological investigations, we present two crustal-scale cross-sections of the Kyrgyz South Tien Shan, and correlations of main faults and units between Kyrgyzstan and China. The overall structure corresponds to a doubly-vergent mountain belt. The Kyrgyz and Chinese areas exhibit identical structural and metamorphic histories. To the west, the Atbashi Range comprises high-pressure oceanic and continental units stacked by north-verging thrusts above a low metamorphic accretionary prism. High-pressure (HP) gneisses are bound to their south by a south-dipping detachment exhibiting mantle relicts. The high-pressure oceanic and continental units underwent similar pressure–temperature (P–T) paths with peak conditions of around 500 °C–20 kbar, followed by rapid exhumation. The overall south-dipping structure and kinematics indicate a south-dipping subduction of the Central Tien Shan Ocean at 320–310 Ma, ending with the docking of the Tarim block to the Kazakh continent. To the east, the Pobeda Massif shows a narrow push-up structure. A major north-vergent thrust exhumes deep-crustal-level granulites, constituting the highest summits, which were thrust towards the north onto low-grade Devonian–Carboniferous schists. The southern part of South Tien Shan is made up of a south-verging thrust stack that formed later during ongoing convergence, reactivated throughout post-30 Ma phases.

Talas–Fergana Fault Cenozoic timing of deformation and its relation to Pamir indentation

Abstract Regional strike-slip faults are widely distributed in continental interiors and play a major role in the distribution of far-field deformation due to continental collisions. Constraining the deformation history of the Talas–Fergana Fault (TFF), one of the largest of such faults in the Himalayan deformed interior, is vital to comprehend the hinterland kinematics of the India–Asia collision. New apatite fission track results from the NW Tien Shan define a rapid exhumation event at c. 25 Ma. This event is correlated with a synchronous pulse in the South Tien Shan, implying that both ranges experienced a simultaneous onset of rapid exhumation. We suggest that strike-slip motion along the TFF commenced at c. 25 Ma, facilitating counter-clockwise rotation of the Fergana Basin and enabling exhumation of the linked horsetail splays. Pamir indentation, located south of the Western Tien Shan, is postulated to be underway by c. 20 Ma. Recently published results suggest synchronous strike-slip deformation in the western Tarim Basin and eastern flank of the Pamir. Based on our results and published data, we are able to connect Tarim and Pamir deformation to the onset of TFF slip. We suggest that this pre-existing regional structure was responsible for transferring Pamir-induced shortening to the NW Tien Shan. Supplementary material: Supplementary material is available at http://www.geolsoc.org.uk/SUP18845

Exhumation history of the western Kyrgyz Tien Shan: Implications for intramontane basin formation

The dextral Talas-Fergana Fault separates the western from the central Tien Shan. Recent work has shed light on the Cenozoic evolution of the eastern and central Tien Shan; much less attention has been paid to the western Tien Shan. In this contribution we present new thermochronological ages for the Fergana and Alai ranges that, combined with the available data set, constrain the Cenozoic exhumation history of the western Tien Shan. Following a tectonically quiet early Cenozoic period, we suggest an onset of exhumation at ~25 Ma. This early onset was followed by a period of slower exhumation and in some areas minor reheating. A final, strong late Miocene rapid cooling event is well represented in the western Tien Shan as in other sectors of the range. The early onset of uplift of the western Tien Shan dissected the previously continuous westernmost Parathethyan Sea, progressively isolating basins (e.g., Fergana, Tarim, and Alai basins) in the central Asian hinterland. Moreover, the coeval timing of late Miocene uplift along the length of entire Tien Shan implies that neither the Pamir nor Tarim can be the sole driver for exhumation of the entire range.

The stratigraphic, sedimentological and structural evolution of the southern margin of the Kazakhstan continent in the Tien Shan Range during the Devonian to Permian

Abstract The passive margin carbonate platform in the Middle Tien Shan rests on Givetian–Frasnian red siliciclastic strata. It evolved from an attached carbonate platform in the Famennian and early Tournaisian to an isolated carbonate platform in the late Tournaisian to early Bashkirian. The open-ocean side of the platform was reef-rimmed, whereas the continental side was both reef- and shoal-rimmed. Platform interiors exhibit low-energy facies during the Famennian to early Visean and high-energy facies during the late Visean to Bashkirian. Eustatic sea-level rises in the middle Tournaisian, early Visean and near the Visean and Serpukhovian boundary caused major reorganizations in platform architecture. Deformation in the middle Bashkirian reflects the onset of a convergent margin. Flexural loading by an orogenic thrust wedge controlled basin subsidence along the southern edge of the Middle Tien Shan in the Late Pennsylvanian to Asselian. Cessation of deposition in the Asselian followed by folding and granitoid plutonism reflects the onset of a rigid collision. Devonian to Permian carbonates represent outcrop analogues of coeval oil- and gas-rich carbonate platforms in the North Caspian basin and can be used for comparative and predictive sedimentological studies. Palaeozoic carbonate reservoir facies may host subsurface Cenozoic oil fields in the Fergana Basin.

Lateral change of sources for the Cretaceous-Paleogene magmatism of the Tian Shan

The Southern and Northern-Central Tian Shan are sharply different in the character of the evolution of Cretaceous-Paleogene magmatism. The Southern Tian Shan comprises a picrobasalt-trachybasalt-basanite-phonotephrite-phonolite volcanic series, which was formed over a considerable time interval from 122 to 46 Ma, whereas the Northern-Central Tian Shan hosts a foidite-basanite-trachybasalt-basaltbasaltic andesite volcanic association, which erupted within a rather narrow time interval between 61 and 53 Ma. The entire volcanic series of the former region was derived from a shallow garnet-free mantle source. The volcanic assemblage of the latter region included basanites and foidites derived from a deep garnet-bearing mantle source, whereas trachybasalt, basalt, and basaltic andesite melts were generated in the lower crust. It is supposed that the change of sources and different evolutionary trends of Cretaceous-Paleogene magmatism in the Southern and Northern-Central Tian Shan were caused by the activation of the heterogeneous lithosphere beneath the converging shores of the Late Paleozoic Turkestan paleoocean.