Yu. S. Biske

Tectono-Stratigraphic framework and Palaeozoic evolution of the Chinese South Tianshan

A stratigraphic and structural study was carried out in the central part of the Chinese South Tianshan (STS) within a 50–100 km-wide transect centered on the Dushanzi-Kuqa road (83°–85° E). Our data elucidate the tectonic structure and evolution of the Palaeozoic sedimentary basin, document overthrust structures in the late Carboniferous-early Permian orogenic belt and suggest correlations between the western and eastern segments of the STS in Kyrgyzstan and China.We recognise a series of lithotectonic units in the study area that have different stratigraphic characteristics and were formed within (a) continental margin and slope of the Kazakhstan continent, (b) Turkestan (South Tianshan) ocean, (c) intra-oceanic carbonate sea-mounts, which at least partly evolved on top of an extinct island arc, (d) a back-arc oceanic-crust basin, (e) external deeper marine and internal shallow-marine areas of the Tarim shelf and (f) Tarim craton. The overall structure of the basin was similar within Kyrgyzstan and China. The main distinction of the western areas is a lack of ophiolites on the southern flank of the belt, a poorly expressed arc in the axial part, and a more complicated facial setting of the central area, where carbonate banks were separated by deeper marine depressions with cherty deposits. The eastern sector is defined by a continental arc that evolved on the northern margin of the Tarim craton in the Silurian and became separated from the continent in the latest Silurian-early Devonian. There is also a middle Palaeozoic metamorphic belt on the southern flank of the STS.A pre-Carboniferous unconformity, previously assumed throughout the study area, is only confirmed within the continental massifs of Kazakhstan and Tarim. As in the western areas, the unconformity does not exist within the STS. Continuous sedimentation in the STS occurred from the Early Devonian to the early Bashkirian in marginal parts of the belt and up to Gzhelian age in the axial part. Convergence began in the Bashkirian (320–315 Ma) and caused subduction of oceanic crust in the northern and southern areas of the STS to the north and south, respectively. A back arc basin in the south was closed in the Moscovian, and since that time top-to-the-south thrusting and overthrusting prevailed throughout the STS.The time of onset of collision of Kazakhstan with Tarim was not younger in age than Kasimovian, based on the age of initiation of a turbidite foreland basin on the northern margin of Tarim. Thrust deformation during the Late Pennsylvanian to early Permian was synchronous within Kyrgyzstan and China; it occurred in a collisional setting and was accompanied by accumulation of turbidites and olistostromes. Broad termination of thrusting, followed by folding and uplift of the area in the middle Asselian indicates the beginning of a rigid collisional phase. Emplacement of early Permian stitching granite plutons in the STS and adjacent areas of Kazakhstan and Tarim completes the formation of the collisional orogen within Kyrgyzstan and northwestern China.

Geodynamics of late Paleozoic magmatism in the Tien Shan and its framework

The Devonian-Permian history of magmatic activity in the Tien Shan and its framework has been considered using new isotopic datings. It has been shown that the intensity of magmatism and composition of igneous rocks are controlled by interaction of the local thermal upper mantle state (plumes) and dynamics of the lithosphere on a broader regional scale (plate motion). The Kazakhstan paleocontinent, which partly included the present-day Tien Shan and Kyzylkum, was formed in the Late Ordovician-Early Silurian as a result of amalgamation of ancient continental masses and island arcs. In the Early Devonian, heating of the mantle resulted in the within-plate basaltic volcanism in the southern framework of the Kazakhstan paleocontinent (Turkestan paleoocean) and development of suprasubduction magmatism over an extensive area at its margin. In the Middle-Late Devonian, the margins of the Turkestan paleoocean were passive; the area of within-plate oceanic magmatism shifted eastward, and the active margin was retained at the junction with the Balkhash-Junggar paleoocean. A new period of active magmatism was induced by an overall shortening of the region under the settings of plate convergence. The process started in the Early Carboniferous at the Junggar-Balkhash margin of the Kazakhstan paleocontinent and the southern (Paleotethian) margin of the Karakum-Tajik paleocontinent. In the Late Carboniferous, magmatism developed along the northern boundary of the Turkestan paleoocean, which was closing between them. The disappearance of deepwater oceanic basins by the end of the Carboniferous was accompanied by collisional granitic magmatism, which inherited the paleolocations of subduction zones.Postcollision magmatism fell in the Early Permian with a peak at 280 Ma ago. In contrast to Late Carboniferous granitic rocks, the localization of Early Permian granitoids is more independent of collision sutures. The magmatism of this time comprises: (1) continuation of the suprasubduction process (I-granites, etc.) with transition to the bimodal type in the Tien Shan segment of the Kazakhstan paleocontinent that formed; (2) superposition of A-granites on the outer Hercynides and foredeep at the margin of the Tarim paleocontinent (Kokshaal-Halyktau) and emplacement of various granitoids (I, S, and A types, up to alkali syenite) in the linear Kyzylkum-Alay Orogen; and (3) within-plate basalts and alkaline intrusions in the Tarim paleocontinent. Synchronism of the maximum manifestation and atypical combination of igneous rock associations with spreading of magmatism over the foreland can be readily explained by the effect of the Tarim plume on the lithosphere. Having reached maximum intensity by the Early Permian, this plume could have imparted a more distinct thermal expression to collision. The localization of granitoids in the upper crust was controlled by postcollision regional strike-slip faults and antiforms at the last stage of Paleozoic convergence.

Structures of the late palaeozoic thrust belt in the Chinese South Tian Shan

Aiming to resolve contradictions in tectonic models and to establish a correlation between Chinese and Kyrgyz sectors of the South Tian Shan we carried out stratigraphic and structural studies in Chinese part of the belt along the Bayinbuluk—Kuqa transect. New data indicate that Chinese South Tian Shan is dominated by top-to-the-south structures, which were formed during the latest Carboniferous and Early Permian. Major allochthons of the Devonian carbonates, thrusted on the Gzhelian and Asselian turbidites, are revealed in the northern part of the belt. Imbricated thrust packages and recumbent folds in deeper marine Devonian and Carboniferous rocks are common in the South. Postkinematic granites yield U-Pb ages of 285–275 Ma, which indicate that thrust deformation ceased by the middle of the Early Permian. The same direction of motion and similar age of deformations in Kyrgyz and Chinese sectors of the South Tian Shan prove, that top-to-the-south structures were formed during the same structural episode, which corresponds to the main collisional stage within entire belt.

Paleoseismodeformations of hard rocks in the Karelian isthmus

Within the limits of the Fennoscandian (Baltic) crystalline shield, late and postglacial shattering of hard rocks due to strong ancient seismic effects has been studied at several sites [1–5]. The approach of these studies implied the search for evidence of a seis� mic origin of dislocations and, in some cases, determi� nation of the dislocation age. These studies have also been carried out by the authors in southern Karelia and northern Leningrad oblast. The present work is devoted to elaboration of criteria and distinguishing of several coeval events, and to parameterization of these events in terms of seismology standards. This commu� nication presents the first, basic part of such a study. The source parameters were defined on the basis of models and calculations from the initial values of dis� placements for each of the distinguished events. The remarkable shattering of hard rocks in the out� crops near Krasnyi Sokol settlement, northern Kare� lian Isthmus (Leningrad oblast, near the town of Kamennogorsk), had been noticed as early as the 19th century by the famous geologist A.A. Inostrantsev. Since then, this site, which is unique in the expressive� ness of fresh ruptured structures, had not been investi� gated. In 2010–2012, it was studied in detail by the authors and was named “Inostrantsev’s cave” [6]. The site is located within the limits of the NE wing of the large Vuoksa fault zone [7] bearing recent signs of acti� vation. Owing to intensive longitudinal and transverse fracturing of the hard rock massif, the inner structure and shattering conditions were able to be studied down to 5–7 m depth. Here, a complex of seismotectonic dislocations with displacements and deformations of various character is developed in the hard rocks; in the framing, seismogravitational structures are found. Importantly, both these types are closely coupled and correlate with each other. The present work will con� sider only the dislocation in the hard rock massif proper. A significant feature is the clear predominance of signs of horizontal displacements in the two mutu� ally perpendicular directions at the nearly horizontal basis of the shattered part of the massif. The Vuoksa fault zone, which is marked by the river system of the same name, is represented by a system of

Continuous Sections of the Devonian and Carboniferous Carbonates and Timing of Collision in the Chinese South Tianshan

New stratigraphic data indicate that carbonate sedimentation in the axial part of the Chinese South Tianshan (STS) occurred continuously from the Early and Middle Devonian to the latest Carboniferous. This reflects steady subsidence in quiet tectonic regime and excludes major collisional events in the STS during this time. Pre-Carboniferous structural events, previously identified in the marginal parts of the Kazakhstan and Tarim continental massifs, did not extend to the STS basin. Continental collision in the Chinese STS occurred during the latest Carboniferous and early Permian, as in adjacent areas to the west in Kyrgyzstan.