Mongol Okhotsk Fold Belt Research Articles

Geochemistry and provenances of the Jurassic terrigenous rocks of the Upper Amur and Zeya–Dep troughs, eastern Central Asian fold belt

This paper reports the results of complex geochemical and Sm–Nd isotope-geochemical studies of terrigenous rocks of the Upper Amur and Zeya–Dep troughs, as well as U–Pb geochronological studies of detrital zircons. It is established that the studied troughs have orogenic nature, which is of key significance for understanding the geodynamic evolution of East Asia in the Mesozoic. Such interpretation is consistent with structural features of the troughs (migration of basin axis inward the continent with time, stratigraphic rejuvenation in the same direction), which are typical of foreland basins regarded as analogues of foreland (marginal) troughs. Obtained data indicate that orogenic processes responsible for the formation of the Mongol-Okhotsk fold belt began in the Early Jurassic.

Kinematics of recent tectonic motions in the east of the Mongol-Okhotsk Fold Belt

We present the first data of GPS measurements on recent motions in the east of the Mongol-Okhotsk Fold Belt. Processing of GPS data yielded a vector field of the velocities of observation point displacements in the geodynamic network of the Upper Amur region. Comprehensive analysis of geological and geophysical data and the estimated velocities of site displacements in this network have shown kinematic heterogeneity of the present-day Mongol-Okhotsk Fold Belt and an intricate deformation pattern there. The tectonic regime within the belt permits considering it a buffer or transit zone, where tectonic stresses arise as a result of the different kinematics of the surrounding tectonic structures.

The velocity structure of the upper mantle beneath Central Asia

Data on the deep structure of the Earth's crust and upper mantle are necessary for understanding and explaining the tectonic processes occurring in differ� ent regions of the world. The obtained information can be used for constructing geodynamical models that enable both explanation and prediction of further evolution of these processes. Therefore, of special interest are regions where riftogenesis and intensive orogenesis are currently observed. It was shown earlier (1-3) that the upper mantle beneath Central Asia is of an inhomogeneous struc� ture, and in both the lateral and vertical directions, at that. Such an inhomogeneous structure indicates the complex geodynamical regime of this region. The aim of the present communication is to investigate the deep structure of the discussed region in more detail and in a greater depth interval. The investigation of the Earth's crust and upper mantle velocity structure was carried out by the sur� facewave tomography for the case of a spherical sur� face (4). The materials for our study were records of surface waves from distant earthquakes of M > 5.5 (1991-2009), made in the "DRYa" channels of digital broadband seismic stations of the IRIS network. The range of epicentral distances was 1500-16 000 km. The dispersion curves of the group velocity for the main mode of the Rayleigh wave were calculated uti� lizing the spectralandtemporal analysis procedure (5) for more than 3200 seismic paths crossing the study region in different directions. In calculations, wave periods of 10 to 250 s were used, which, according to (6), corresponds to a depth extent of 650-700 km. Use of the representative sampling of dispersion curves has allowed us to calculate maps of distributions of the group velocities of the Rayleigh wave for the periods mentioned and to estimate the resolution of the map� ping results by calculation of the effective smoothing radius (4), which was 250-350 km in the lateral direc� tion. Based on these maps, local dispersion curves in particular points of the study region were calculated, and then these curves were turned into Swave velocity sections. The main tendencies of the mantle structure are illustrated by two 2D Swave velocity sections (Fig. 1). The A-B profile crosses the structures of the Mon� gol-Okhotsk Fold Belt in the territories of NE China and Transbaikalia, the Baikal Rift Zone, and the adja� cent areas of the Siberian Craton. The C-D profile starts in the Sayan Mountains, crosses the Khangai Arch and part of the Gobi desert, ending in China, near the Chinese-Mongolian boundary. The characteristic peculiarity of the discussed velocity sections is the presence of a clearly expressed asthenospheric layer possessing Swave velocities of 4.3-4.4 km/s. Beneath the Baikal Rift Zone, the top of this layer lies at about 120 km depth, while beneath other tectonically active structures it is located in the vicinity of the crustal base. The maximal thickness (150 km) of the asthenosphere is reached beneath the Khangai Arch, while in other parts of the profiles it is about 70 km thick.

Tectonic reconstruction of Uda-Murgal arc and the Late Jurassic and Early Cretaceous convergent margin of Northeast Asia–Northwest Pacific

Abstract. A long tectonic zone composed of Upper Jurassic to Lower Cretaceous volcanic and sedimentary rocks is recognized along the Asian continent margin from the Mongol-Okhotsk fold and thrust belt on the south to the Chukotka Peninsula on the north. This belt represents the Uda-Murgal arc, which was developed along the convergent margin between Northeast Asia and Northwest Meso-Pacific. Several segments are identified in this arc based upon the volcanic and sedimentary rock assemblages, their respective compositions and basement structures. The southern and central parts of the Uda-Murgal arc were a continental margin belt with heterogeneous basement represented by metamorphic rocks of the Siberian craton, the Verkhoyansk terrigenous complex of Siberian passive margin and the Koni-Taigonos Late Paleozoic to Early Mesozoic island arc with accreted oceanic terranes. At the present day latitude of the Pekulney and Chukotka segments there was an ensimatic island arc with relicts of the South Anyui oceanic basin in a backarc basin. Accretionary prisms of the Uda-Murgal arc and accreted terranes contain fragments of Permian, Triassic to Jurassic and Jurassic to Cretaceous (Tithonian–Valanginian) oceanic crust and Jurassic ensimatic island arcs. Paleomagnetic and faunal data show significant displacement of these oceanic complexes and the terranes of the Taigonos Peninsula were originally parts of the Izanagi oceanic plate.

The Minas Azules Deposit—An Example of Orogenic Lode Gold Mineralization in the Sierra de Rinconada, Northern Argentina

The Minas Azules gold deposit, located in the northernmost part of the Sierra de Rinconada, Argentina, is hosted by low-grade metamorphosed turbidites of Middle Ordovician age. Quartz veinhosted, stockwork and disseminated gold mineralization is structurally controlled and intimately associated with a large cylindrical-type anticline that developed during the Upper Ordovician-Lower Silurian Ocloyic orogeny. Several styles of gold-bearing quartz veins were emplaced during folding and the later stages of episodic thrust faulting, but the main gold mineralization episode occurred at a particular stage in the deformation history and can be shown to be related to pressure solution reactions that occurred following cleavage development and prior to the onset of transpressional deformation. Due to post-ore faulting, mineralization is confined entirely to the eastern flank of the Minas Azules anticline. Petrological and geochemical studies reveal that, within the eastern flank, hydrothermal alteration of the wallrock is well-developed and, together with low-grade disseminated mineralization, extends for at least 70 meters from the vein system. Alteration in the Minas Azules deposit includes chloritization, sericitization, (de)silicification, sulfidation, and carbonatization. Lithogeochemical changes related to alteration are characterized by the addition of K2O, CO2, As, Au, S, and concomitant depletion of SiO2, MgO, and Na2O. Based on the style of mineralization, relative timing of ore formation, and characteristic alteration features, the Minas Azules deposit bears many similarities with well-documented orogenic gold deposits in Phanerozoic fold belts elsewhere (e.g., Tasmanides, eastern Australia; Meguma Terrane, Nova Scotia; Tien Shan, Central Asia; Mongol-Okhotsk fold belt, Russian Far East) and highlights the potential of the Sierra Rinconada as a significant producer of orogenic lode-gold and disseminated mineralization in the Andes.

Paleozoic geodynamic evolution of the Mongol-Okhotsk fold belt

Geodynamic reconstructions of the Mongol-Okhotsk belt for different Paleozoic eras have been made on the basis of studies of ophiolite, island-arc, innerplate and other geological complexes. Paleomagnetic and paleoclimatic data for continents and microcontinents have been used for palinspastic analysis. It has been found that the Mongol-Okhotsk fold belt occurred in the Late Proterozoic (Riphean-Vend). The further development of the belt in the Paleozoic is related to the opening and partial closing of the Paleooceanic basin. At present the Mongol-Okhotsk belt is a typical collision nappe-fold structure, complicated by innercontinental rifts. It includes geological complexes of different geodynamic nature (oceanic, transitional, continental).