Basin evolution in a folding lithosphere: Altai–Sayan and Tien Shan belts in Central Asia

Chang J., Qiu N., Li J.

The northwestern edge of the Tarim Basin, which contains the Kalpin Uplift and the north Bachu Uplift, lies between the South Tianshan and Tarim continental block and its tectonic evolution is closely correlated with the Tianshan. The pre-Mesozoic units are widely exposed in the northwestern edge of the Tarim Basin, which allow detailed geological observation and sample collection. In this paper, the apatite (U–Th)/He thermochronology was used to study the tectono-thermal evolution of the northwestern edge of the Tarim Basin. The sandstone samples from the Upper Neoproterozoic, Silurian, Devonian, Carboniferous and Permian units, etc. on the northwestern edge of the Tarim Basin were collected and used to determine apatite (U–Th)/He ages. The apatite He ages in the Kalpin Uplift reveal the tectonic uplift events during the Late Cretaceous (96–70 Ma) and the Miocene (22–20 Ma). These uplift events are linked to collisions of the Lhasa Block and the India plate with the southern Eurasian continental margin, respectively. However, apatite He ages in the north Bachu Uplift show only the Eocene event at ∼40 Ma because the west Kunlun orogenic belt thrust over the southwestern edge of the Tarim Basin during the Eocene. The thermal modeling results reveal four cooling events in the Kalpin Uplift since the Late Neoproterozoic, corresponding to 460–445 Ma, 350–320 Ma, 220–210 Ma and 20–10 Ma but only one episode with 220–210 Ma in the north Bachu Uplift since the Devonian. Based on the thermal modeling results, the north Bachu Uplift did not experience the cooling events at 460–445 Ma, 350–320 Ma and 20–10 Ma shown in the Kalpin Uplift. This paper provides a new insight into the tectonic evolution of the Tianshan and the northwestern Tarim Basin.

Makarov V.I.

Abstract The crustal orogeny which formed the present-day Tien Shan results from the complex interaction between two independent processes. The first is the lateral (horizontal) compression related to the collision between the Indian and Eurasian Plates. The second is the rearrangement and flow of crustal material at different levels of the lithosphere beneath the mountain belt. Two broad morphologic and genetic types of mountain basins (intramontane and intermontane/foreland) are proposed as indicators of specific geodynamic factors controlling the topographic relief of the orogenic belt. The first-type basins, having the upper crustal roots, reflect mainly the N–S crustal compression and correspond to simple (elementary) longitudinal folds with basement involvement. The second type basins developed mostly in response to deep processes in the upper mantle and lower crust.

Glorie S., De Grave J., Delvaux D., Buslov M.M., Zhimulev F.I., Vanhaecke F., Elburg M.A., Van den haute P.

The Irtysh shear zone (ISZ) is an important structure in the framework of the Central Asian Orogenic Belt (CAOB). It represents the site of final collision of Kazakhstan with Siberia during Hercynian times and records up to 1000 km of lateral displacement during subsequent reorganization in the CAOB edifice. We present new zircon U/Pb, apatite fission track and fault kinematic data along the ISZ and consequently derived its tectonic history with emphasis on its formation and reactivation episodes. Carboniferous (∼340–320 Ma) zircon U/Pb ages were obtained for the syn- and post-collisional Kalba–Narym intrusives, dating their emplacement in the framework of the Siberia–Kazakhstan collision. During this period, the ISZ experienced an ‘early brittle’ left-lateral, mainly transtensional stress regime. Late Carboniferous–Early Permian post-collisional intrusives were emplaced and the stress regime changed to a ‘late brittle’ regime, characterized by more compressional conditions, indicating rheological strengthening as a response to cessation of ductile shearing and cooling of the ISZ crust. Apatite fission track data and thermal history modeling reveal Late Cretaceous (∼100–70 Ma) cooling of the ISZ basement rocks as a response to denudation of a bordering Late Mesozoic Altai orogen. After this denudation event, the tectonic activity ceased during the Late Mesozoic–Early Cenozoic. A final step of cooling (from ∼25 Ma), exhibited by some of the thermal history models, may reflect reactivation of the ISZ and initiation of Cenozoic Altai mountain building. The Late Plio-Pleistocene phase of mountain building coincides with a new change in the Palaeostress field, characterized by minor transpressional, right-lateral shear conditions.

Champagnac J., Molnar P., Sue C., Herman F.

[1] By regressing simple, independent variables that describe climate and tectonic processes against measures of topography and relief of 69 mountain ranges worldwide, we quantify the relative importance of these processes in shaping observed landscapes. Climate variables include latitude (as a surrogate for mean annual temperature and insolation, but most importantly for the likelihood of glaciation) and mean annual precipitation. To quantify tectonics we use shortening rates across each range. As a measure of topography, we use mean and maximum elevations and relief calculated over different length scales. We show that the combination of climate (negative correlation) and tectonics (positive correlation) explain substantial fractions (>25%, but

Chang H., Ao H., An Z., Fang X., Song Y., Qiang X.

The left-lateral strike-slip Altyn Tagh Fault (ATF) forming the northern boundary of the Tibetan Plateau accommodates parts of the overall convergence between the colliding Indian and Eurasian plates. Precise dating of the ATF activity is essential for understanding possible mechanisms of Tibetan Plateau deformation and uplift. Here we report a magnetostratigraphic study of the Suerkuli Basin deposits recording depositional changes during the ATF activity. Field investigations reveal a remarkable and widespread change in depositional environment in the Suerkuli Basin, i.e. a transformation from low-energy lacustrine deposits (grayish-green mud-siltstone and brown mud-siltstone) into high-energy alluviul fan deposits (poorly sorted gray pebble and cobble conglomerates). Detailed magnetostratigraphy of the 390-m-thick Daban section, at the southeastern margin of the Suerkuli Basin (38°43.09′N, 90°58.84′E), shows that this change in depositional facies occurred at ∼3.2 Ma, accompanied by a remarkable increase in sediment accumulation rate. We attribute this depositional change to the Piocene tectonic activity of the middle ATF although the contribution of the Pliocene global climate deterioration cannot be excluded.

Wang Y., Deng T., Flynn L., Wang X., Yin A., Xu Y., Parker W., Lochner E., Zhang C., Biasatti D.

The linkage between tectonic forces and climate evolution remains a matter of much debate and speculation. Here we present high-resolution oxygen and carbon isotope data from an ancient lake basin in the central Himalaya. These data, together with sedimentologic evidence, reveal major changes in drainage systems and depositional settings at � 7.2, � 5.5 and � 3.2 Ma. These environmental changes appear to be driven by regional-scale tectonics. The oxygen isotope record also reveals alternating wet and dry climates with periodicities of 24 and 100 kyr that were likely controlled by orbital forcing. Paleo-temperatures, estimated using a fossil-based oxygen isotope ‘‘paleo-thermometer’’, are 21 ± 6 Ca t� 7 Ma, which is � 19 ± 6 C higher than the present-day mean annual temperature in the same area. The much warmer environment inferred here is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates. The estimated temperature difference would require the study area to have been raised by � 2–2.5 km since � 7 Ma to its current elevation of 4100–4500 m above sea level. This result can be interpreted as either indicating the presence of a low-altitude intermountain basin in the hanging wall of the already formed Main Central Thrust or a protracted development of a north-trending rift basin that has experienced changes in drainage system and depositional environment through time.

De Grave J., Glorie S., Buslov M.M., Izmer A., Fournier-Carrie A., Batalev V.Y., Vanhaecke F., Elburg M., Van den haute P.

The Song-Kul Basin sits on a plateau at the Northern and Middle Kyrgyz Tien Shan junction. It is a lacustrine basin, occupied by Lake Song-Kul and predominantly developed on igneous basement. This basement was targeted for a multi-method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present-day morphology. Zircon U/Pb dating by LA-ICP-MS revealed four different magmatic episodes: a Late Cambrian (~ 500 Ma) island arc system, a Late Ordovician (~ 450 Ma) subduction related intrusion, an Early Permian (~ 290 Ma) collisional stage, and a Middle to Late Permian (~ 260 Ma) post-collisional magmatic pulse. Middle to Late Triassic (~ 200–230 Ma) titanite fission-track ages and Late Triassic – Early Jurassic (~ 180–210 Ma) apatite fission-track ages and thermal history modeling indicate the Song-Kul basement was already emplaced in the shallow crust at that time. An exhumed fossil apatite fission-track partial annealing zone is recognized in the bordering Song-Kul mountain ranges. The area experienced only minor post-Early Mesozoic denudation. The igneous basement was slowly brought to apatite (U–Th)/He retention temperatures in the Late Cretaceous–Palaeogene. Miocene to present reactivation of the Tien Shan does not manifestly affect this part of the orogen.

Glorie S., De Grave J., Buslov M.M., Zhimulev F.I., Stockli D.F., Batalev V.Y., Izmer A., Van den haute P., Vanhaecke F., Elburg M.A.

[1] Multimethod chronology was applied on intrusives bordering the Kyrgyz South Tien Shan suture (STSs) to decipher the timing of (1) formation and amalgamation of the suturing units and (2) intracontinental deformation that built the bordering mountain ranges. Zircon U/Pb data indicate similarities between the Tien Shan and Tarim Precambrian crust. Caledonian (∼440–410 Ma) and Hercynian (∼310–280 Ma) zircon U/Pb ages were found at the edge of the STSs, related to subduction and closure of the Turkestan Ocean and the formation of the suture itself. Permian-Triassic (∼280–210 Ma) titanite fission track and zircon (U-Th)/He data record the first signs of exhumation when the STSs evolved into a shear zone and the adjacent Tarim basin started to subside. Low-temperature thermochronological (apatite fission track, zircon and apatite (U-Th)/He) analyses reveal three distinct cooling phases, becoming younger toward the STSs center: (1) Jurassic-Cretaceous cooling ages provide evidence that a Mesozoic South Tien Shan orogen formed as a response to the Cimmerian orogeny; (2) Early Paleogene (∼60–45 Ma) data indicate a renewed pulse of STSs reactivation during the Early Cenozoic; (3) Neogene ages constrain the onset of the modern Tien Shan mountain building to the Late Oligocene (∼30–25 Ma), which intensified during the Miocene (∼10–8 Ma) and Pliocene (∼3–2 Ma). The Cenozoic signals may reflect renewed responses to collisions at the southern Eurasian border (i.e., the Kohistan-Dras and India-Eurasia collisions). This progressive rejuvenation of the STSs demonstrates that deformation has not migrated steadily into the forelands, but was focused on pre-existing basement structures.

Li C., Guo Z., Dupont-Nivet G.

► A structural analysis based on growth strata and interpreted seismic data . ► The shortening magnitude and rates estimation in the northern Chinese Tian Shan. ► Discussing the tectonic propagation types of the three fold-and-thrust belts. To understand the reactivation and intensified uplift of the Tian Shan range in the Cenozoic, the age of development of the associated series of anticlinal belts formed in the southern and northern foreland basins must be constrained. To estimate the shortening magnitude and rates in the northern foreland basin, we provide here regional structural analysis based on identified growth strata dated with existing magnetostratigraphy, together with balanced cross sections from interpreted seismic data. These results indicate that three paralleled rows of anticlinal belts have developed sequentially from south to north accommodating a total shortening of ∼15 km at the location of the structurally restored seismic section provided here. These three belts present different structural deformational styles with the southern (Qingshuihe) anticline as a basement-involved fold, the middle (Huoerguosi) anticline as a fault-bend fold and the northern (Anjihai) anticline as a fault-propagation fold. Growth strata inferred from seismic profiles start stratigraphically far below growth strata observed on the outcrop. The latter coincide with accelerated folding of the anticlinal belts at ∼6 Ma for the southern, ∼2 Ma for the middle ∼1 Ma for the northern. Our results imply that the northern Tian Shan foreland rates of deformation were lower until late Miocene and increased in more recent times to values in line with GPS-derived rates.

Gao J., Klemd R., Qian Q., Zhang X., Li J., Jiang T., Yang Y.

article i nfo A ca. 600 m-long, 0.5-20 m-wide NW-SE trending granite dike crosscuts the high pressure-low temperature (HP-LT) Tianshan metamorphic belt, the foliation of which is parallel to the main ENE regional trend in the Chinese South Tianshan Orogen. It is mainly composed of plagioclase, K-feldspar, quartz, muscovite, biotite and secondary chlorite, while fluorite, zircon and xenotime occur as accessories. The immediate country rock is a quartz-biotite-plagioclase schist, which grades several tens of meters away from the granite dike into a chlorite-mica-albite schist. The latter schist is intimately intercalated with blueschist layers and boudins. The A/CNK value of the granite dike samples varies from 1.15 to 1.27 indicating a strongly peraluminous composition. CaO/Na2O ranges from 0.06 to 0.17 and Al2O3/TiO2 from 240 to 525, similar to the ratios of strongly peraluminous (SP) granites exposed in 'high-pressure' collision zones such as the Himalayas. A zircon U-Pb age of 285 Ma was obtained for the granite dike, thus constraining the upper limit for the age of HP-LT metamorphism. The petrological and geochemical data suggest that the SP leucogranite dike intruded during the exhumation of overthickened crust in the post-collisional setting between the Yili (-Central Tianshan) and Tarim blocks. The dataset presented here in conjunction with previously published data corroborate that the HP-LT metamorphism must have occurred earlier than the Permian in the Tianshan Orogen. Therefore, the collision between the Yili (-Central Tianshan) and Tarim blocks and the final amalgamation of the Southwestern Altaids must have been terminated in Late Paleozoic and not in Triassic times as previously suggested.

Bagdassarov N., Batalev V., Egorova V.

[1] The shortening of Tien Shan and the evolution of its lithosphere have been evaluated from P-T geothermobarometry of xenoliths and from comparison of their electrical conductivity with conductivities obtained from the inversion of magnetotelluric (MT) data. Spinel lherzolite and granulite xenoliths found in basaltic outcrop Ortosuu represent upper mantle and crust beneath southern Tien Shan. The spinel lherzolite xenoliths correspond to the lithospheric mantle close to the crust-mantle boundary. The studied mantle xenoliths record two types of the upper mantle processes: a low-degree partial melting (about 7–10%) and a cryptic metasomatism. The granulite xenoliths are fragments of the lower crust captured from differing depths. The temperature and pressure estimates of the garnet granulite xenoliths indicate that they were derived from near the crust-mantle boundary. P-T equilibration conditions of mafic granulites and spinel lherzolites infer that the paleogeotherm corresponded to the heat flux 80–85 mW m−2 about 70–66 Myr ago. The present-day heat flux in the region is about 55–60 mW m−2. The position of the Moho discontinuity 70–66 Myr ago was at a depth of 30–35 km, and the present depth of the Moho boundary is 55–60 km. The observed seismic P wave velocities above and below the present-day Moho boundary are 7.3 and 7.9 km s−1, respectively. Elastic P velocities measured on xenoliths samples in laboratory and extrapolated to PT conditions of the Moho discontinuity (1.8 GPa and ∼750°C) are 6.8 km s−1 for the mafic granulite and 8.0 km s−1 for the spinel lherzolite. The electrical conductivity of xenoliths has been measured at 0.8–1.0 GPa and in the range between 500°C and 850°C for mafic granulites and at 1.0–1.5 GPa and from 500°C to 1050°C for spinel lherzolites. The contrast in conductivities between mafic granulite and spinel lherzolite samples under P-T conditions corresponding to the geotherm with the heat flux ∼60 mW m−2 agrees well with the contrast of MT electrical conductivity below and above the present-day Moho boundary at a depth of 55–60 km. Thus, the thickening of the Tien Shan lithosphere is about 25 ± 5 km. Before the shortening of Tien Shan about 20–30 Myr ago the lithosphere was rather hot, with a temperature of 500°C at a 15 km depth and of 850°C at the Moho boundary. In comparison to Tien Shan, the lithosphere beneath the neighboring Tarim Basin was rather cold, 350°C at the depth of 15 km and 500°C at the Moho boundary. These temperature differences were the main factors that caused the mechanical weakening of the Tien Shan crust and upper mantle. The modern strain rates of the Tien Shan shortening are 10−14 × 3 to 10−15 s−1. The total strength of the lithosphere beneath Tien Shan at present is 8–4 × 1012 N m−1. In the past the lithosphere strength beneath Tien Shan was ∼1012 N m−1, which was more than 1000 times weaker in comparison with the lithospheric strength of Tarim Basin at the beginning of the continental compression in the region.

Cloetingh S., Burov E.

Duchkov A.D., Rychkova K.M., Lebedev V.I., Kamenskii I.L., Sokolova L.S.

Abstract Concentrations of helium isotopes were measured in gas and water samples from 28 thermal mineral springs in Tuva and adjacent regions of Buryatia and Gorny Altai. It is shown that fluids from 16 springs are rich in mantle helium (4–35%). With regard to the air contamination of the samples, the corrected ratios of helium isotopes (Rcor = 3He/4He) in these springs vary from 5.3 × 10–8 to 422 × 10–8. Using these Rcor values, we estimated the heat flow; these estimates were then applied to calculate the deep-level temperatures and thickness of thermal lithosphere. According to these parameters, the Tuva region is divided into two parts. Eastern Tuva (from ~96° E to the boundary with Buryatia) is characterized by abnormal helium isotope ratios and heat flow indicating the intense heating of the Earth’s crust in eastern Tuva: At a depth of 50 km, a temperature reaches 1000–1200 °C, and the thickness of thermal lithosphere is reduced to 70–50 km. This testifies to a rift process west (probably, up to 96° E) of the Baikal Rift Zone. In western Tuva, the average heat flow is much lower, ~45–50 mW/m2, which is commensurate with that in the Altai–Sayan folded area as a whole. The deep-level temperatures here are twice lower, and the lithosphere thickness increases to 150 km.

De Grave J., Buslov M.M., Van Den Haute P., Metcalf J., Dehandschutter B., McWilliams M.O.

Babichev A.V., Novikov I.S., Polyansky O.P., Korobeynikov S.N.

Abstract The Altai lithospheric structure has been generally understood due to available high-resolution digital models. As a further step in modeling, we have simulated the structure of southeastern Altai as interaction of eight blocks which comprise or surround the Chuya and Kurai basins, proceeding from the basic configuration of blocks and earthquake mechanisms. Should the stresses in the system remain invariable, the western periphery of the Kurai basin will deform to let the Uimen-Sumulta fault join the Chuya (western end of Tolbonur) fault and evolve further as a single shear zone. The best fit model was one with slip along a single border fault in the middle of the area between two rheologically different terranes. This setting corresponds to a fault boundary between the more plastic Gorny Altai and more rigid Teletskoe-Chulyshman domains, which is consistent with current crustal movements from GPS data. In addition to scientific significance, models of this kind have practical applications as they highlight areas of stress buildup prone to release in large earthquakes. The new approach was applied to simulate the stress and strain patterns of central and southeastern Gorny Altai, and the models were tested against available geomorphological and seismotectonic data.

Guo Q., Wen Y., Xu C., Zhao X.

Abstract The Keping fold-and-thrust belt (KFB), situated at the southern front of the Tianshan orogenic belt, represents a typical thin-skinned imbricated structure resulting from the uplift and southward-thrusting orogeny of the Tianshan. The KFB is believed to accommodate a considerable portion of the north–south convergence of crustal shortening and thickening across the Tianshan, making it an ideal region for investigating intracontinental orogenic processes. In this study, we used four ascending tracks and two descending tracks of the Sentinel-1 A/B data collected over six years to construct Interferometric Synthetic Aperture Radar (InSAR) line-of-sight velocity fields. Subsequently, the 3D interseismic velocity of the KFB was determined by integrating Global Positioning System and InSAR observations. Finally, we employed 2D edge dislocation models to quantify the kinematic parameters of the folds in the western KFB constrained by the vertical velocities. Our results indicate that the western KFB exhibits a crustal shortening rate of 6.3–7.8 mm/yr, which accounts for approximately one-third of the north–south shortening rate observed in the Tianshan orogenic belt. Among the fault zones, the West Keping fault at the thrust front exhibits the highest slip rate, with slip rates increasing from east to west. The maximum crustal shortening rate in the western segment reaches 3.6 mm/yr, constituting half of the total shortening within the KFB. Combined with topographical and geomorphological studies, we propose that the intensity of the southwestward-propagating orogeny from the Tianshan toward the KFB may have diminished.

Emanov A.F., Emanov A.A., Novikov I.S., Gladyshev E.A., Fateev A.V., Polyansky P.O., Shevkunova E.V., Ershov R.A., Arapov V.V., Krivov A.A.

Abstract —Years after the Chuya earthquake of 2003, geological structures adjacent to the focal area of the Chuya earthquake are still seismically active. The Aigulak focal area is one of them, but energetically the most pronounced. Detailed studies have been carried out with the network of stations of the Altai seismological testing site, supplemented by temporary stations. The region activated in the form of a local and compact structure measuring 10 × 10 km with focal depths from the first 100 m to 20 km. The focal area is not a subsequent activation along the same fault with the Chuya earthquake, but is located on a subparallel fault in the nodal region with its branching into three faults. The seismic activation of the Aigulak focal area is not an aftershock process after a major earthquake, but is an activated structure with a dynamically changing seismic process. An intensive process has formed since the earthquake in 2012 with ML = 6.1 with a gradual decrease in the number of earthquakes, and in 2019 the Aigulak earthquake with ML = 5.5 occurred with a very strong aftershock process after it. Our results of an area study of earthquake density in the focal zone indicate a change in the regime over time: from chaotic to self-organizing along short faults. We conclude that the focal area has not reached the maximum level of seismic energy release.

Khukhuudei U., Kusky T., Windley B.F., Otgonbayar O., Wang L., Nie J., Wenjiao X., Zhang L., Song X.

Nurbekova R., Shi X., Hazlett R., Misch D., Fustic M., Sachsenhofer R.F.

Extensive nanoindentation testing over a range of deflection depths of up to 4 μm yielded a large dataset, providing a viable framework for the statistical assessment of the mechanical properties, specifically elastic modulus (E) and hardness (H), of compositionally diverse organic-rich mudstone samples. The data from indentations as shallow as 300–400 nm were clustered using the k-means algorithm to identify three mechanical categories in the samples: a soft pseudophase (e.g., organic matter, gypsum, and clay minerals), a stiff pseudophase (e.g., quartz and feldspar), and a transitional composite-like pseudophase bridging the soft and hard minerals. The initially diverse values of E and H for the mechanical pseudophases were observed to converge to a constant value at indentations beyond 2–2.5 μm (varying between different samples), implying the existence of a minimal probing depth for assessing the bulk E and H of heterogeneous mudstone samples. The obtained bulk E and H values (8–21 GPa and 0.3–0.9 GPa, respectively) demonstrated a strong correlation with the mineralogical composition of the indented samples. Despite containing a notable proportion of mechanically stiff components (>45 vol%), the bulk mechanical parameters determined in this study were significantly lower than those reported for major shale formations such as the Barnett and Longmaxi Shale. This discrepancy is primarily due to the presence of organic matter with low thermal maturity (Ro < 0.6%), which constitutes

Buslov M.M., Kulikova A.V., Sklyarov E.V., Travin A.V.

Abstract A model of tectonothermal evolution of the Zagan metamorphic core complex (MCC) based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating is discussed. A relationship with the long-range impact of processes from the collision of the North China (Amurian–North China) block with the Siberian continent in the Mesozoic era is proposed. The Zagan MСС was formed in the Cretaceous period on the southern flank of a high mountain uplift of Western Transbaikalia, composed of late Paleozoic granitoids of the Angara–Vitim batholith. According to 40Ar/39Ar dating of amphiboles and micas from the mylonite zone, the active development time of the Zagan MCC corresponds to the early Cretaceous epoch (131, 114 Ma). The tectonic exposure of the core from about 15 km to the depths of about 10 km occurred at a rate of tectonic erosion of 0.4–0.3 mm/year as a result of post-collisional extension of the Mongol–Okhotsk orogen. Apatite fission-track dating shows that further exhumation and cooling of the rocks to about 3 km occurred in the lower-upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year.MCC- metamorphic core complexes, AFT- apatite fission-track

Buslov M.M., Kulikova A.V., Sklyarov E.V., Travin A.V.

Abstract A model of tectonothermal evolution of the Zagan metamorphic core complex (MCC) based on the new data from 40Ar/39Ar dating of amphibole, mica, and apatite fission-track dating is discussed. A relationship with the long-range impact of processes from the collision of the North China (Amurian–North China) block with the Siberian continent in the Mesozoic era is proposed. The Zagan MСС was formed in the Cretaceous period on the southern flank of a high mountain uplift of Western Transbaikalia, composed of late Paleozoic granitoids of the Angara–Vitim batholith. According to 40Ar/39Ar dating of amphiboles and micas from the mylonite zone, the active development time of the Zagan MCC corresponds to the early Cretaceous epoch (131, 114 Ma). The tectonic exposure of the core from about 15 km to the depths of about 10 km occurred at a rate of tectonic erosion of 0.4–0.3 mm/year as a result of post-collisional extension of the Mongol–Okhotsk orogen. Apatite fission-track dating shows that further exhumation and cooling of the rocks to about 3 km occurred in the lower-upper Cretaceous epoch (112, 87 Ma). The erosional denudation rate was about 0.3 mm/year.MCC- metamorphic core complexes, AFT- apatite fission-track

Díez Fernández R., de Vicente G., Moreno‐Martín D., Fernández C., Arenas R., Rubio Pascual F.J.

AbstractMetamorphic basements are usually considered rigid and isotropic at a large scale. However, basements contain inherited weaknesses that may potentially accommodate superimposed contraction (e.g., fault reactivation), and that favor fold nucleation (e.g., penetrative foliations). If these conditions are met, what could be the factors that impede the development of basement folds or their recognition? Actual basement folding is rarely documented, especially for large dimensions. Here we provide a case example, discussed from the perspective of structural analysis of surface data and sustained by geophysical data. The basement of the Spanish‐Portuguese Central System is defined by an Alpine mega‐fold (Hiendelaencina Antiform) that trends parallel to this mountain range and affects the basement and its sedimentary cover, collectively. The wavelength of this fold matches or even surpasses the thickness of the crust that hosts it (36–41 km). The Moho under this mega‐fold is displaced by an Alpine fault that accounts for incipient intraplate continental subduction. The topography of the mantle may reflect an up‐warping compatible with the mega‐fold observed on the surface. Mega‐folding is observed in the hanging wall of the Berzosa Fault, which emerges as a SE‐dipping, Variscan (Paleozoic), extensional fault reactivated as a basal decollement upon Alpine (Cenozoic) contraction. The mega‐fold was formed after well‐oriented planar anisotropies in the basement (foliation and bedding). The development of this fold was assisted by heterogeneous shearing (coeval thrusting) plus the buttressing effect of pre‐existing, near‐vertical, crustal‐scale faults (Somolinos and Somosierra), which inhibited slip‐upsection through the basal decollement (Berzosa Fault).

Deev E.V., Panin A.V., Solomina O.N., Bricheva S.S., Borodovskiy A.P., Entin A.L., Kurbanov R.N.

张 闯.

Zhang C.

Sandstone-type uranium deposits (STUDs) are the most important global source of uranium. However, it is unclear why STUDs have a non-random distribution in time and space. It is generally thought that STUDs are formed by the circulation of groundwater in sandstone rocks. The groundwater is typically oxidized and sourced from local precipitation, which suggests the regional climate may have a role in the formation of STUDs. The groundwater circulation is mainly affected by basin evolution, which means that regional tectonism may also control the formation of STUDs. In this study, the author examined STUDs in Asia, and compiled previously reported ages for STUDs and compared these with the uplift history of the major ore-hosting regions and the late Mesozoic–Cenozoic climatic evolution of Asia. Apart from a few uranium deposits in the Transural region, most of the STUDs in Asia were formed during the Late Cretaceous to Quaternary, and can be classified into three stages: Late Cretaceous–early Paleogene (80–50 Ma; stage I), Oligocene–mid-Miocene (25–17 Ma; stage II), and late Miocene–present (8–0 Ma; stage III). The formation of STUDs in Asia was closely related to regional uplift caused by India–Eurasia collision, subduction of oceanic plates, and increased humidity during greenhouse climate periods and intensification of the Asian Monsoon.

Zhi D., Zhang J., Wu T., Wu A., Tang Y., Liu Y., Cao J.

The northwestern Junggar Basin in the southwestern Central Asian Orogenic Belt is a typical petroliferous basin. The widely distributed reservoirs in Jurassic–Cretaceous strata indicate that the region records Yanshanian–Himalayan tectonic activity, which affected the accumulation and distribution of petroleum. The mechanism of this effect, however, has not been fully explored. To fill the knowledge gap, we studied the structural geology and geochemistry of the well-exposed Wuerhe bitumen deposit. Our results indicate that deformation and hydrocarbon accumulation in the northwestern Junggar Basin during the Yanshanian–Himalayan geodynamic transformation involved two main stages. During the Yanshanian orogeny, a high-angle extensional fault system formed in Jurassic–Cretaceous strata at intermediate to shallow depths owing to dextral shear deformation in the orogenic belt. This fault system connected at depth with the Permian–Triassic oil–gas system, resulting in oil ascending to form fault-controlled reservoirs (e.g., a veined bitumen deposit). During the Himalayan orogeny, this fault system was deactivated owing to sinistral shear caused by far-field stress related to uplift of the Tibetan Plateau. This and the reservoir densification caused by cementation formed favorable hydrocarbon preservation and accumulation conditions. Therefore, the secondary oil reservoirs that formed during the Yanshanian–Himalayan tectonic transformation and the primary oil reservoirs that formed during Hercynian–Indosinian orogenies form a total and complex petroleum system comprising conventional and unconventional petroleum reservoirs. This might be a common feature of oil–gas accumulation in the Central Asian Orogenic Belt and highlights the potential for petroleum exploration at intermediate–shallow depths.

Sha Y., Shi Z., Zhou P., Lei J., Mao X.

Geological records from the representative monsoon-aridity climate regimes, the Chinese Loess Plateau (CLP) and Tarim Basin (TB), demonstrate that the climate has experienced distinct reorganization since the late Oligocene/early Miocene. Meanwhile, incremental evidence indicates asynchronous growth of the Tibetan Plateau (TP) and Central Asian Orogenic Belt. Therefore, how the growth of these topographies affects the Asian climate regimes need further numerical examination. By successively including the main TP, Pamir Plateau (Pr), Tian Shan Mountains (TS) and Mongolian Plateau (MP) in our numerical experiments, the effects of the stepwise uplift are examined. For the arid interior, the main TP, Pr, TS and MP all cause significant reduction of precipitation and moisture, presenting a persistent aridification process. The most intense reduction is resulted by the main TP in warm season. The summer precipitation over the CLP experienced a persistent intensification with the uplift of the main TP, Pr and TS, indicating a persistent strengthening of the East Asian summer monsoon. After that, the summer precipitation over the CLP is suppressed with the MP uplift, which is consistent with the MP-induced decrease over the arid inland. The annual mean precipitation change over the CLP resembles the change in summer. Therefore, the topography-induced precipitation evolution over the TB and CLP exhibits different processes. Meanwhile, the precipitation over the TB initially portrays a spring-winter dominant pattern as the TP significantly suppressed the precipitation in summer. The summer precipitation is then intensified with the inclusion of the Pr-TS and the precipitation seasonality is transformed to peak in summer. The results indicate the differentiation of Asian climate regimes affected by the main TP, Pr-TS and MP. The results might provide new support for the evolution of mid-latitude deserts and Neogene Red Clay and Quaternary loess-paleosol deposits on the CLP since the late Oligocene.

Nurbekova R., Smirnova N., Goncharev I., Sachsenhofer R.F., Hazlett R., Smirnov G., Yensepbayev T., Mametov S., Fustic M.

The Zaysan Basin is a petroliferous basin in eastern Kazakhstan. Its upper Carboniferous–Permian sedimentary succession outcrops in the Kenderlyk Trough and includes, from base to top, the Kenderlyk, Karaungur, Tarancha, and Maychat formations, which contain 5 to 65 m-thick oil shale deposits—the principal subject of this study. Results from 49 outcrop samples show high total organic carbon (TOC) contents (1.2–21%, mean 7.8%), extract yields (1.2–15 mg HC/g rock, mean 6.8 mg HC/g rock), and ultimate expulsion potential (UEP; up to 23 mmboe/km2), making these oil shale deposits among the best source rocks in the world. Sedimentological, organic geochemical and organic petrographic analyses suggest an overall evolution of the basin from a deep/semi-deep lake during deposition of the Kenderlyk, Karaungur, and Tarancha formations to a deltaic setting during deposition of the Maychat Formation. The organic material in the Kenderlyk, Karaungur, and Tarancha formations is predominately composed of Type I and mixed I/III kerogens with high hydrocarbon generation potential (S1 + S2; up to 172.8 mg HC/g rock), high hydrogen index (HI; up to 838 mg HC/g TOC), and low oxygen index (OI; < 60 mg CO2/g TOC). The sporadic influx of terrigenous organic matter resulted in layers with lower HI and higher OI index, and increased inertinite and vitrinite contents. The Maychat Formation includes hydrogen-poor Type III kerogen with low HI (< 200 mg HC/g TOC) and high OI index (> 60 mg CO2/g TOC) in the Kenderlyk Trough. While a highly oxidizing environment during deposition of the Maychat Formation is postulated for the Kenderlyk Trough, it is likely that oxygen-depleted conditions in the depocenter of the Zaysan Basin favored accumulation of Type I kerogen. The oil-source correlation shows that the produced oils are chemically distinct from the source rock extracts of the Kenderlyk, Karaungur, and Tarancha formations. We propose that these source rocks are “massive”, where the retention of generated oil is too high, causing the hydrocarbons to “bleed” from the source rock's edge only. The expelled oil has probably charged the lower Permian deposits, which have not yet been explored. Regional geological cross-sections and seismic lines allow for selecting sweet spots, characterized by high TOC, yields, and temperatures needed for oil generation and unconventional hydrocarbon resource development. This study provides the play concept and a risk assessment analogue for tectonically and magmatically dynamic settings and basins with multiple organic rich strata. Furthermore, the results and proposed concepts may play a significant role in future petroleum exploration and development activities in the Zaysan Basin. In addition, this highly multidisciplinary study emphasizes the significance of integrating several data sources and weighing contradictory information to get the most reasonable conclusion.

Tveritinova T.Y., Marinin A.V., Deev E.V.

In the Katun Fault Zone, the structural-paragenetic analysis of stress indicators and the method of cataclastic analysis of discontinuous faults have been used to determine the features of the structure of this fault and the conditions of its formation at the Late Alpine stage. It is proved that the Katun Fault Zone is the strike-slip fault that develops at different sites in transpressive or transtensive conditions. The newest grabens along the zone were formed under strike-slip fault displacements and locally manifested conditions of horizontal extension or horizontal extension with shear.

Tveritinova T.Y., Marinin A.V., Deev E.V.

The kinematic characteristics of faults included in the structure of the active Katun Fault in Gorny Altai have been determined by methods of structural–paragenetic and cataclastic analysis of disjunctives (faults). It is proven that the Katun Fault is a strike-slip fault developing in different areas under transpressive or transtensional conditions. The most recent grabens along the Katun fault were formed under conditions of strike-slip displacements and locally manifested horizontal extension or extension with shear.