Mеsozoic Tectonothermal Evolution of the Zagan Metamorphic Core Complex in Western Transbaikalia: 40Ar/39Ar and FTA Dating

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-trac

Late Permian palaeomagnetic data east and west of the Urals

We studied Upper Permian redbeds from two areas, one between the Urals and the Volga River in the southeastern part of Baltica and the other in north Kazakhstan within the Ural-Mongol belt, which are about 900 km apart; a limited collection of Lower-Middle Triassic volcanics from north Kazakhstan was also studied. A high-temperature component that shows rectilinear decay to the origin was isolated from most samples of all three collections. For the Late Permian of north Kazakhstan, the area-mean direction of this component is D = 224.3°, I =−56.8°, k = 161, Α 95 = 2.7°, N = 18 sites, palaeopole at 53.4°N, 161.3°E; the fold test is positive. The Triassic result ( D = 55.9°, I =+69.1°, k = 208, Α 95 = 4.2°, N = 7 sites, pole at 57.0°N, 134.1°E) is confirmed by a positive reversal test. The corresponding palaeomagnetic poles from north Kazakhstan show good agreement with the APWP for Baltica, thus indicating no substantial motion between the two areas that are separated by the Urals. Our new mean Late Permian direction for SE Baltica ( D = 42.2°, I = 39.2°, k = 94, Α 95 = 3.5°, N = 17 sites; palaeopole at 45.6°N, 170.2°E) is confirmed as near-primary by a positive tilt test and the presence of dual-polarity directions. The corresponding pole also falls on the APWP of Baltica, but is far-sided with respect to the coeval reference poles, as the observed mean inclination is shallower than expected by 13°± 4°. In principle, lower-than-expected inclinations may be attributed to one or more of the following causes: relative tectonic displacements, quadrupole and octupole terms in the geomagnetic field, higher-order harmonics (incl. secular variation) of the same field, random scatter, non-removed overprints, or inclination error during remanence acquisition and/or diagenetic compaction. Our analysis shows that most mechanisms from the above list cannot explain the observed pattern, leaving as the most likely option that it must be accounted for by inclination shallowing. Comparison with selected coeval results from eastern Baltica (all within Russia) shows that all of them are biased in the same way. This implies that they cannot be used for analysis of geomagnetic field characteristics, such as non-dipole contributions, without a more adequate knowledge of the required correction for inclination shallowing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71899/1/j.1365-246X.2008.03727.x.pd

Geomorphic study of seismically active areas using remote sensing data. Case of the Gorny Altai (Siberia) affected by the 2003 Altai earthquake

This paper shows that a multitemporal, multiscale, and multisource remote sensing dataset represents an efficient tool for studying morphotectonics in seismically active areas, with an application in Siberia. The focus is placed on the use of high resolution imagery including Corona, Orbview 3, Ikonos 2 (available on Google Earth), and Landsat images combined with four different digital elevation models (DEMs) built using various satellite data. DEMs are the version 2 SRTM 3 arc-second and version 3 SRTM 1 arc-second released by USGS, the X-SAR DEM, and the version 2 Aster GDEM. In the specific case of the Gorny Altai, the remote sensing dataset composed of DEMs and satellite images provide relevant evidence of the geomorphological consequences of the 2003 Altai earthquake characterized by large landslides, block tilting, and ground-cracks. Ikonos imagery reveals the en-échelon faults compatible with a dextral strike-skip faulting. Archive satellite data allow us detecting new faults generated by the earthquake, but also the pre-existing fault network, with a specific emphasis on the use of Corona archive from the 1960’s. The best global DEMs (SRTM 1 arc-second and Aster GDEM) are well-correlated. Generally, the Aster GDEM presents a lower horizontal accuracy than the SRTM DEM, whereas the vertical accuracy is relatively similar. In the case of the largest landslide induced by the 2003 Altai earthquake (about 1 km2), the comparison of the pre-seismic topographic profile obtained by SRTM and the post-seismic topographic profile obtained by Aster GDEM is of great interest. Following the landslide episode, it allows us defining a zone of depletion and a zone of accumulation. The limit between the hard Palaeozoic rocks (sandstone, etc.) and the loose Quaternary sediments appears clearly as a zone of weakness. Before the 2003 earthquake, a round track was already detected in the Corona images, corresponding either to an old landslide or a precursor stage of the major landslide. More generally, the dextral strike-slip faulting is accompanied by the uplift of the northeastern segment, close to the Chagan Uzun block. In the Kuskunnur-Taltura-Chagan river area, this uplift is revealed by the abnormal elevation of the Kuskunnur river compared to the elevation of the Taltura river. The present geomorphological study is a complement to dendrochronological and radiocarbon dating of earthquake triggered landslides, rockfalls and seismically cut fossil soils

Structural control on Meso-Cenozoic tectonic reactivation and denudation in the Siberian Altai: insights from multi-method thermochronometry

Abstract not availableS. Glorie, J. De Grave, M.M. Buslov, F.I. Zhimulev, M.A. Elburg, P. Van den haut

Meso-Cenozoic building of the northern Central Asian Orogenic Belt: thermotectonic history of the Tuva region

The Tuvinian and West-Sayan mountain ranges form part of the Central Asian Orogenic Belt (CAOB); more specifically, they align along the Altai-Sayan-Hangay zone. Its Precambrian-Paleozoic basement has been subjected to Mesozoic and Cenozoic tectonic reactivation. Two north-south transects across the mountain belts and intervening basins of Tuva were sampled for apatite fission-track (AFT) thermochronology in order to elucidate the thermal history of the Tuvinian basement in relation to the Mesozoic and Cenozoic reactivation of the CAOB. Most AFT ages are Late Cretaceous and range between 55 and 115. Ma. Mean lengths of confined fission tracks are relatively long with most values between 13 and 14. μm. Thermal history modeling shows a rapid Late Jurassic-Cretaceous cooling for the sampled Tuvinian crystalline rocks, related to exhumation of the Paleozoic basement. This exhumation is possibly related to the building and subsequent orogenic collapse of the Mongol-Okhotsk orogen that formed between the Siberian and North China-Mongolian (Sino-Korean or Amurian) continental blocks during the Late Mesozoic. Far-field effects of this orogeny and its collapse, might have affected the Baikal, Altai and Sayan units of the Central Asian Orogenic Belt, including the Tuvinian basement. Also, at the Mesozoic southern Eurasian margin, growth of the Asian continent continued and several collision-accretion events asserted distal tectonic influence into the CAOB. After a Paleogene period of stability, thermal history models for some samples hint at a renewed period of basement cooling during the Neogene. In support of this Neogene event, a single sample from the main West Sayan fault zone contains an apatite population with ~. 2. Ma reset AFT ages. This is interpreted in the framework of ongoing building of the modern Central Asian orogens and associated fault movements and exhumation of the basement, presumably related with the ongoing India-Eurasia convergence. © 2014 Elsevier B.V.Johan De Grave, Elien De Pelsmaeker, Fedor I. Zhimulev, Stijn Glorie, Mikhail M. Buslov, Peter Van den haut

Variable slab-mantle interaction in a nascent Neoproterozoic arc-back-arc system generating boninitic-tholeiitic lavas and magnesian andesites

Subduction zones are major sites of elemental recycling on Earth, and they are pivotal in exploring the mechanisms of crustal growth and differentiation. The late Neoproterozoic mafic volcanic suite from the southeastern Gorny Altai terrane represents magmatic products of the nascent Kuznetsk-Altai intra-oceanic island arc southwest off the Siberian continent. These rocks preserve key information on the subarc mantle source and slab-mantle interaction during the early phase of ocean-ocean subduction, and therefore they might shed light on the geodynamic evolution of subduction zones worldwide. The mafic volcanic suite can be geochemically classified into four rock types, i.e., group I tholeiitic lavas with relatively high TiO2 contents, group II tholeiitic lavas with comparatively lower TiO2, boninitic rocks, and magnesian andesites. Trace-element and Nd isotopic compositions demonstrate that group I tholeiitic lavas possibly formed by decompression melting of a fertile asthenospheric mantle source with subtle involvement of slab-derived components. In contrast, the precursor magma of the boninitic rocks was probably derived from depleted clinopyroxene-poor lherzolite metasomatized by slab-derived aqueous fluid and minute amounts of hydrous siliceous melt from the subducted sediments. The high Ti/Zr and low Zr/Sm ratios of these rocks further suggest that garnet was possibly a residual phase during partial fusion of the subducted sediments. These data and their close spatial association suggest that group I tholeiitic lavas and the boninitic rocks possibly formed in a back-arc-related setting. Group II tholeiitic lavas show geochemical compositions intermediate between those of the boninitic rocks and the group I tholeiitic ones. We infer that group II either represents island-arc tholeiites that were generated during the proto-arc stage, or they were formed by mixing of precursor magmas of group I tholeiitic lavas and the boninitic rocks. Geochemical modeling reveals that the magnesian andesites probably formed more adjacent to the trench, with significant contribution of a fertile asthenospheric mantle source and sediment-derived siliceous melt with amphibole in the residue. Our data suggest that initiation or propagation of a back-arc basin in response to slab rollback was a possible geodynamic cause for the volcanic activities in the southeastern Gorny Altai terrane, where the upwelling asthenosphere and variable components from the subducted slab and overlying mantle wedge all made significant contributions to a potentially nascent arc-back-arc system. The available data for similar rock associations worldwide imply that a similar tectonic scenario may prevail during the early phase of intra-oceanic island arcs and contribute to significant mass transfer from the upwelling asthenospheric mantle to the arc crust

Emplacement and exhumation of the Kuznetsk-Alatau basement (Siberia) : implications for the tectonic evolution of the Central Asian Orogenic Belt and sediment supply to the Kuznetsk, Minusa and West Siberian Basins

New geochronological data [zircon U/Pb, titanite fission-track (TFT) and apatite fission-track (AFT) dating and apatite (U-Th-Sm)/He thermochronology] and thermal history modelling yield constraints on the development of the granitoid basement of the Kuznetsk-Alatau Mountains, southern Siberia. The final stages of magmatism in the Kuznetsk-Alatau palaeo-island-arc are Late Cambrian, and collision of the arc with Siberia occurred in the Early Ordovician. The basement was exhumed by the Early Devonian. Continuous Devonian-Early Triassic sedimentation filled the adjoining Kuznetsk and Minusa basins and buried (and re-heated) the Kuznetsk-Alatau basement. After initial Pangaea break-up and Siberian flood-basalt magmatism, the basement reached TFT and AFT retention-temperatures in the Middle Triassic and Early Cretaceous, respectively, during denudation-induced cooling