South-central Mongolia Research Articles

Geomorphic constraints on the development of a blind-thrust induced landform, south-central Mongolia: Insights into foreberg growth

Forebergs are landforms characterized by low, elongated ridges or hills rising above the surrounding alluvial fans or floodplains, and are typically formed by folding associated with thrust faulting. Forebergs in the Gobi-Altay range, south-central Mongolia, have developed in the forelands of mountains transpressionally uplifted in restraining bends along the E–W-trending sinistral strike-slip Bogd fault. Along the easternmost part of the Bogd fault, six forebergs have formed in the foreland of the Artz Bogd restraining-bend. We surveyed one of these forebergs to ascertain the fault geometry associated with its formation and understand how it has developed over time. This foreberg is ~6 km long and ~1 km wide and rises about 40 m above the low-lying floodplain. We also constrained its evolution using cosmogenic 10Be denudation rates measured from individual catchments along the foreberg, and morphometric analyses using a high-resolution DEM generated from images captured by an unmanned aerial vehicle. We suggest that the foreberg evolved via fault-propagation folding, developing an asymmetric across-foreberg topographic profile. This asymmetry results in differences in 10Be catchment-wide denudation rates and geomorphic metrics between its northern and southern slopes, causing migration of the drainage divide of the growing foreberg to adjust to the tectonic regime. By analyzing the along-foreberg variation in denudation rates and geomorphic metrics, we infer that the foreberg formed via lateral growth and linking of four discrete segments. Our study illuminates the formation of a unique landform and sheds light on how a structure enlarges both vertically and laterally during the early stage of thrust-related fold developments.

Evidence for southward subduction of the Mongol-Okhotsk oceanic plate: Implications from Mesozoic adakitic lavas from Mongolia

A combination of new 40Ar/39Ar dating results, major- and trace-element data, plus Sr-Nd-Pb-Hf isotope data, are used to investigate the petrogenesis of Triassic high-Si adakite (HSA), Cretaceous low-Si adakite-like (LSA) lavas, and Cretaceous high-K and shoshonitic trachyandesite lavas, from eastern and south-central Mongolia. All samples are light rare-earth element and large-ion lithophile element enriched but depleted in some high-field strength elements (notably Nb, Ta and Ti). Two alternative models are proposed to explain the petrogenesis of the HSA samples. (1) A southward-subducting Mongol-Okhotsk slab underwent partial melting in the Triassic during the closure of the Mongol-Okhotsk Ocean, with the resultant melts assimilating mantle and crustal material. Alternatively (2), a basaltic underplate of thickened (>50 km; >1.5 GPa), eclogitic lower crust foundered into the underlying mantle, and underwent partial melting with minor contamination from mantle material and some shallow-level crustal contamination. The LSA samples are interpreted as melts derived from a lithospheric mantle wedge that was previously metasomatised by slab melts. Similarly, the trachyandesite lavas are interpreted as melts deriving from a subduction-enriched subcontinental lithospheric mantle. The spatial distribution of these samples implies that metasomatism likely occurred due to a southward-subducting Mongol-Okhotsk slab associated with the closure of the Mongol-Okhotsk Ocean. When this interpretation is combined with previous evidence for a northward-subducting Mongol-Okhotsk slab it advocates that the Mongol-Okhotsk Ocean closed with double-sided subduction.

Cambrian–Ordovician magmatism of the Ikh-Mongol Arc System exemplified by the Khantaishir Magmatic Complex (Lake Zone, south–central Mongolia)

The Khantaishir Magmatic Complex (KMC) (south–central Mongolia) exposes a section of a magmatic system consisting of deep crustal, ultramafic cumulates (coarse-grained Amp gabbros and hornblendites; c. 0.35–0.5GPa) to shallower crustal levels dominated by Amp–Bt tonalites (c. 0.1–0.2GPa). The magmatic rocks were emplaced during most of the Cambrian (c. 538–495Ma) and are mostly geochemically primitive (Mg#~50), Na-rich and metaluminous. The (normal-) calc-alkaline signature and characteristic trace-element enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE) relative to high-field strength elements (HFSE) suggest an origin within a magmatic arc. Multiple intrusions of basic magma derived from a subduction-modified depleted mantle developed by fractional crystallization and/or accumulation of (Ol, Cpx) Amp+Bt, later joined by Pl. Magma mixing with, or without, exchange of xenocrysts between compositionally dissimilar melt batches was also important. Over time, partial melting of older, lower crustal metabasic rocks became increasingly significant, again with a strong subduction signature. The lack of zircon inheritance in the magmatic products and rather high zircon εHft values (all >+3, but for most samples>+8) as well as whole-rock Sr–Nd isotopic compositions imply that the arc was not founded on mature continental crust. It was probably located at the margin of the Baydrag microcontinent, dominated by accreted metabasic rocks of an older (early Tonian?) island arc covered by a thin layer of subordinate metasediments containing detrital zircons with Tonian and ill-defined Palaeoproterozoic ages. The KMC represents a small vestige of an extensive Cambrian–Ordovician subduction system (termed here the Ikh-Mongol Arc System), bordering the western margin of a chain of Precambrian microcontinents (Tuva-Mongolia, Zabkhan and Baydrag) that, together with accreted Neoproterozoic marginal basins (the Lake Zone), defines the external part of the Mongolian orocline.

Complicated seismic anisotropy beneath south-central Mongolia and its geodynamic implications

Two years of high-quality broadband seismic data from 69 temporary stations deployed in south-central Mongolia provide an opportunity to study the anisotropy-forming mechanisms in this area. The majority of shear wave splitting observations determined from the analysis of teleseismic SKS phase are characterized by NW–SE trending fast directions with large splitting delay times (greater than 2.0 s at six stations), which is inferred to be generated by active asthenospheric flow. The variation of the fast direction may be associated with deflection of asthenosphere around the deep Siberian cratonic keel at the base of the lithosphere. Several of the NE–SW trending fast directions with relatively small delay times observed in the Gobi Desert are parallel to the strike of the main faults and sutures, which may represent lithospheric deformation. In addition, it is inferred that small-scale hot mantle upwelling is responsible for generating a cluster of null measurements observed on the south of the Hentiy Mountain.

The crustal structure of south central Mongolia using receiver functions

The crustal structure of south central Mongolia using receiver functions

Subsidence and drowning of a carbonate platform in south-central Mongolia (Gobi Altai region) during the late Eifelian to early Givetian: A synthesis of conodont data, magnetic susceptibility, and paleoecology

New and published stratigraphic data are integrated herein to resolve the age and depositional setting of Middle Devonian strata in the Gobi-Altai Terrane of south-central Mongolia (Bayanhongar Province). The Tsagaankhaalga Formation (Emsian?–Eifelian) is composed primarily of tabular sandy carbonates; it is sharply overlain by dark, fine-grained volcanoclastic marine sedimentary rock of the Govialtai Formation (Givetian), which is comparatively unfossiliferous, except for sporadic, fossiliferous carbonate interbeds. The latter unit contains abundant tentaculites as well as a newly recovered collection of conodonts assigned to the lower Givetian varcus Zone (timorensis, rhenana, or ansatus). A positive shift in bulk magnetic susceptibility coincides with the Tsagaankhaalga/Govialtai contact, and elevated values within the Govialtai correspond to tuffaceous and rhyolitic strata.The transition from carbonate facies of the Tsagaankhaalga Formation to the volcanoclastic shales of the Govialtai Formation is interpreted as the result of local tectonic activity. This may be qualitatively described as a transition from a shallow marine carbonate shoal with an epibenthic fauna (Tsagaankhaalga), to a deeper, clastic dominated environment with a nektic fauna (Govialtai). The pulse of tectonism must predate the lower Givetian, and it echoes the “Tsakhir Event”, which is recorded by the transition from Ordovician–Silurian carbonates to Lochkovian conglomerate. These findings provide useful biostratigraphic control and insight into the complex tectonic evolution of the Gobi-Altai Terrane and adjacent blocks on the Eurasian Craton.

华北夏季降水转型环流特征分析

本文利用日降水量资料和NCEP/NCAR再分析大气环流等资料，对华北夏季降水雨型年代际转换及环流特征作综合分析，结果表明：1) 我国东部夏季降水型的转换可划为5个时段，1961~1965年华北、东北都明显多雨；1966~1980年华北多雨，东北正常偏少；1981~2000年东北多雨，华北正常偏少；2001~2010年华北、东北少雨，淮河流域明显偏多；2011~2013年华北、东北多雨，江淮流域偏少。2) 近几年华北、东北夏季雨季与1961~1965年相似，降水量明显偏多，但环流形势与1961~1965年有明显不同。在夏季，海平面气压场上，1961~1965年，蒙古低压中心位于蒙古中南部，低压显著加深；而2011~2013年，蒙古低压中心位于蒙古东部至华北、东北地区，低压明显偏弱。在500 hPa高度场上，1961~1965年，西伯利亚槽、华北槽都有所加深，贝加尔湖脊减弱，“阶梯槽”的形势造成华北、东北降水偏多；而2011~2013年，贝加尔湖脊正常，鄂霍次克海位势高度升高，副高偏北，华北槽受东部阻挡的作用加强，结果造成华北、东北夏季降水偏多。在850 hPa风场上，1961~1965年，东亚有明显偏南风异常，蒙古地区有强大气旋性环流异常，与东亚偏南气流在华北、东北西侧产生风向辐合，造成华北、东北降水异常偏多；而2011~2013年，东亚地区无南风异常，但有明显的东南风异常，风速明显小于1961~1965年偏南风，再加上蒙古地区无气旋性环流辐合带来的动力上升条件，造成华北、东北夏季降水虽然比常年偏多，但少于1961~1965年。所以，虽然近年华北和东北夏季降水同时明显增多，与1961~1965年类似，但环流特征明显不同，突出的是1961~1965年为偏南风异常，即东亚夏季风偏强，而2011~2013年为东南风异常，而东亚夏季风并无明显加强。 In this paper, a comprehensive analysis on decadal shift of precipitation rainfall pattern and cir-culation characteristics during summer in North China is made based on such data as daily preci-pitation, general atmospheric circulation reanalyzed by NCEP/NCAR. The results of the analysis indicate that: 1) The shift of precipitation rainfall pattern during summer in Eastern China can be divided into 5 periods, i.e., 1961-1965 with significantly more rainfall both in North China and Northeast China, 1966-1980 with more rainfall in North China but normally less rainfall in North-east China, 1981-2000 with more rainfall in Northeast China but normally less rainfall in North China, 2001-2010 with less rainfall in North China and Northeast China but significantly more rainfall in Huaihe River Basin, and 2011-2013 with more rainfall in North China and Northeast China but less rainfall in Yangtze-Huaihe River Basin. 2) In recent years, the precipitation in rainy season (summer) in North China and Northeast China is similar to that during 1961-1965, which is significantly high. However, the circulation pattern is significantly different from that during 1961-1965. For the sea-level pressure field in summer, the low-pressure center in Mongolia was located in South Central Mongolia during 1961-1965, with low pressure trough significantly dee-pened; while it was located in Eastern Mongolia and toward North China and Northeast China during 2011-2013, with low pressure trough significantly weakened. At 500 hPa height field, Siberia trough and North China trough were deepened but Baikal Lake Ridge was weakened during 1961-1965. Besides, the “step trough” brought more rainfall to North China and Northeast China. During 2011-2013, however, the Baikal Lake Ridge was normal, the geopotential height of Okhotsk Sea was increased, the subtropical high was located northward, and the blocking effect by the eastern part on the North China trough was strengthened, resulting in more rainfall during summer in North China and Northeast China. At 850 hPa wind field, southerly wind was obviously abnormal in East Asia and strong cyclonic circulation anomaly also occurred in Mongolian region during 1961-1965. Wind direction convergence was caused in the west of North China and Northeast China with the southerly airflow in East Asia, resulting in abnormally more rainfall in North China and Northeast China. During 2011-2013, however, south wind was normal in East Asia but southeast wind was obviously abnormal, with wind speed significantly lower than that of southerly wind during 1961-1965. Moreover, there was no dynamic condition created by cyclonic circulation convergence in Mongolian region. As a result, the rainfall in summer in North China and Northeast China is more than that under normal condition but less than that during 1961- 1965. Therefore, similar to the period of 1961-1965, the precipitation in North China and Northeast China has increased significantly in recent years, but the circulation characteristics are markedly different. A significant difference was that the southerly wind was abnormal during 1961- 1965 (namely the summer monsoon in East Asia was strong); while the southeast wind was abnormal during 2011-2013 (namely the summer monsoon in East Asia was not markedly strong).

Raptor Pit Mortality in Mongolia and a Call to Identify and Modify Death Traps Wherever they Occur

Raptor Pit Mortality in Mongolia and a Call to Identify and Modify Death Traps Wherever they Occur

Pastoralism and Protected Area Management in Mongolia's Gobi Gurvansaikhan National Park

AbstractThe Gobi Gurvansaikhan National Park was established in south central Mongolia in 1993 and is used by over 1100 families with pastoralism as their main means of livelihood. Research conducted in 1998–2000 to analyse grazing management problems identified a number of issues and concerns, including a significant increase in the number of herders and the size of the herd; variations in herd size reflecting differences in wealth; problems with marketing of livestock or livestock products; declining stock movements because of transportation costs and loss of water sources; and significant competition and conflicts for grazing areas. The socio‐economic problems associated with Mongolia's transition to a market system, coupled with the expansion of protected areas, mean that herders have to adapt to both the current economic system and changes in land use. Although some aspects of the development of the park can be seen as a positive influence on maintaining pastoral livelihoods in this area, the national goal of protecting 30 per cent of the country, doubling the area of Mongolia currently under protected area status, could have negative effects on pastoral livelihoods, unless ministry officials, protected area administrators and pastoralists can work effectively to solve resource problems.

Palaeomagnetic study of Vendian and Early Cambrian rocks of South Siberia and Central Mongolia: was the Siberian platform assembled at this time?

A palaeomagnetic study of Vendian and Early Cambrian sediments from the Angara block of the Siberian platform: Shaman (52.08°N, 108.83°E) and Minya (58.0°N, 110.0°E) Formations, and the Tuva-Mongolian block: Tsagan-Olom and Bayan-Gol Formations (46.76°N, 96.37°E) isolated three different components of magnetization through thermal demagnetization. The stable high-temperature characteristic remanence directions show both normal and reverse polarities. The mean palaeopoles computed after these high-temperature components are: 32.0°S/71.1°E (dp/dm=6.9°/13.8°) for the Vendian Shaman Formation (10 sites, 80 samples), 33.7°S/37.2°E (dp/dm=8.6°/14.7°) for the Vendian Minya Formation (12 samples), 22.8°S/28.4°E (dp/dm=10.8°/21.6°) for the Vendian Tsagan-Olom Formation (4 sites, 25 samples) and 21.4°S/167.1°E (dp/dm=9.6°/19.1°) for the Early Cambrian Bayan-Gol Formation (6 sites, 49 samples). From a compilation of Vendian and Early Cambrian palaeopoles from the Anabar, Angara and Aldan blocks of the Siberian platform and Tuva-Mongolia block, we propose a model where these blocks were situated in an equatorial to low south palaeolatitude position, with their present-day southern boundaries facing the north pole. From the analysis of the scatter of these poles, we conclude that the Siberian platform might not have fully amalgamated by this time, and that significant rotations occurred after the Early Cambrian. Our new palaeopoles for the Tuva-Mongolia block, together with previously published ones, show that this block was already adjacent to Siberia by the Vendian and Early Cambrian. We propose that the large counterclockwise rotation of the Tuva-Mongolia block with respect to Angara block could mark the end of the closure of the part of the Palaeo-Asian ocean separating these two blocks, and could account for the occurrence of Vendian-Early Cambrian ophiolites in the region.

Biological assessment of Three Beauties of the Gobi National Conservation Park, Mongolia

Three Beauties of the Gobi National Conservation Park was established in south central Mongolia in 1994. We performed a biological assessment of the parks wildlife and other biological/ecological resources from 1995-to assist in conservation management initiatives. Ground and aerial surveys collected data on 130 vertebrate species, several of which are listed as threatened or endangered globally or in Mongolia. We analyzed group structures and estimated population sizes for goitred gazelle (Gazella subgutturosa), Mongolian gazelle (Procapra gutturosa), ibex (Capra sibirica), and argali (Ovis ammon). We report on 20 species previously not reported in the park, including 17 birds, 2 mammals, and 1 reptile. The park is rich in diversity and abundance of raptors, and we observed 29 species of raptors. Several vertebrates appear to be faring better in the park than in other regions of Mongolia or the world. Under a new protected areas law (1994), the Mongolian Protected Areas Bureau is required to establish management zones within the park. Data from this study should be used to facilitate this process.

SKS splitting beneath continental rift zones

We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift‐orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift‐parallel again, although a small number of measurements are still rift‐orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N‐S trending rift. All three rifts are underlain by low‐velocity upper mantle as determined from teleseismic tomography. Rift‐related mantle flow provides a plausible interpretation for the rift‐orthogonal fast directions. The rift‐parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small‐scale mantle convection with rift‐parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift‐parallel fast directions are caused by oriented magmatic cracks.

Nd-Isotopic and Rare-Earth Element Geochemical Constraints on the Origin of the Wenduermiao Iron Deposit, Inner Mongolia, China

The Wenduermiao deposit, a large, low-grade iron ore deposit, occurs in the upper Proterozoic metavolcanic-sedimentary sequences of the Wenduermiao Group in south-central Inner Mongolia. The Wenduermiao Group consists mainly of metabasaltic rock, greenschist, marble, finely crystalline quartzite, and related metachemical sediments. Most of the iron ore occurs as stratiform and podiform deposits, or as massive lenses, along the transition zone of greenschist, chlorite-biotite-sericite-albite-quartz rocks, or calcite-amphibole-albite-quartz rocks. Two important ore types, chlorite-magnetite and calcite-magnetite, have been recognized and are interpreted as forming from different ore fluids. Primary mineralization in the deposit consists of martite and magnetite, with smaller amounts of specularite, goethite, and limonite. Six greenschist samples taken from the wallrock of the deposit have flat rare-earth element (REE) patterns similar to oceanic tholeiite. Two finely crystalline quartzite samples show negat...

Depositional environments and fades transitions of dinosaur-bearing Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People's Republic of China)

Campanian redbeds near Bayan Mandahu consist of six lithosomes, which in turn comprise twelve lithofacies. The distribution of these lithosomes along a 12 km escarpment resolves into three coeval paleogeographic zones representing a proximal-to-distal depositional gradient. Zone 1, the most proximal, consists of alluvial, lacustrine, and eolian sediments. It is interpreted as an eolian influenced, distal alluvial-fan or braid-plain environment. Farther basinward, zone 2 is dominated by structureless eolian sandstones and caliche profiles with minor occurrences of alluvial, lacustrine, and eolian dune deposits. It is interpreted as a zone of vertically aggrading eolian sand, stabilized by vegetation and a high water table. Zone 3, the most distal, is dominated by eolian dune and structureless eolian deposits with lacustrine and interdune fluvial deposits. It is interpreted as the southern margin of an eolian dune field that was dominated by transverse and barchan dunes. U-shaped, mature caliche profiles can be traced for up to 1.5 km and are abundant throughout zones 2 and 3. They indicate lengthy depositional hiatuses and a swaley paleotopography. Djadokhta Formation paleoenvironments and fossil assemblages in south-central Mongolia compare closely with zones 2 and 3 at Bayan Mandahu, indicating a consistency in climate and depositional processes throughout the Gobi Basin during Campanian times. The depositional environments of modern arid and semiarid basin margins in Central Asia (e.g., Junggar, Tarim) are excellent analogues for the Campanian environments and paleogeography of the Gobi Basin margin.

Baselevel, Grade and Peneplain

Baselevel, Grade and Peneplain