D. Dorjnamjaa

Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan connection

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process. The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the faults left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south.

A structural transect across the Mongolian Western Altai: Active transpressional mountain building in central Asia

We present results from the first detailed geological transect across the Mongolian Western Altai using modern methods of structural geology and fault kinematic analysis. Our purpose was to document the structures responsible for Cenozoic uplift of the range in order to better understand processes of intracontinental mountain building. Historical right-lateral strike-slip and oblique-slip earthquakes have previously been documented from the Western Altai, and many mountain fronts are marked by active fault scarps indicating current tectonic activity and uplift. The dominant structures in the range are long (>200 km) NNW trending right-lateral strike-slip faults. Our transect can be divided into three separate domains that contain active, right-lateral strike-slip master faults and thrust faults with opposing vergence. The current deformation regime is thus transpressional. Each domain has an asymmetric flower structure cross-sectional geometry, and the transect as a whole is interpreted as three separate large flower structures. The mechanism of uplift along the transect appears to be horizontal and vertical growth of flower structures rooted into the dominant right-lateral strike-slip faults. The major Bulgan Fault forms the southern structural boundary to the range and is a 3.5-km-wide brittle-ductile zone that has accommodated reverse and left-lateral strike-slip displacements. It appears to be linked to the North Gobi Fault Zone to the east and Irtysh Fault zone to the west and thus may be over 900 km in length. Two major ductile left-lateral extensional shear zones were identified in the interior of the range that appear to be preserved structures related to a regional Paleozoic or Mesozoic extensional event. Basement rocks along the transect are dominantly metavolcanic, metasedimentary, or intrusive units probably representing a Paleozoic accretionary prism and arc complex. The extent to which Cenozoic uplift has been accommodated by reactivation of older structures and inversion of older basins is unknown and will require further study. As previously suggested by others, Cenozoic uplift of the Altai is interpreted to be due to NE-SW directed compressional stress resulting from the Indo-Eurasian collision 2500 km to the south.

Quaternary alluvial fans in the Gobi of southern Mongolia: evidence for neotectonics and climate change

Alluvial fans in southern Monglia occur along a group of narrow discontinuous mountain ranges which formed as transpressional uplifts along a series of strike-slip faults. They provide information on the nature of neotectonic activity in the eastern Gobi Altai range and on palaeoclimate change. Alluvial fan formation was dominated by various geomorphological processes largely controlled by climatic changes related to an increase in aridity throughout late Quaternary times. Their sedimentology shows that initially they experienced humid conditions, when the sedimentary environments were dominated by perennial streams, followed by a period of increasing aridity, during which coarse fanglomerates were deposited in alluvial fans by ephemerial streams and active-layer structures were produced by permafrost within the alluvial fan sediments. With climatic amelioration during early Holocene times, the permafrost degraded and fan incision and entrenchment dominated. Sedimentation was then confined to the upper reaches of the fans, adjacent to steep mountain slopes, and within the entrenched channels. The alluvial fans have been neotectonically deformed, faulted and their surface warped by small thrust faults that propagate from the mountain fronts into their forelands. Localised uplift rates are in the order of 0.1 to 1 m Ka -1 .

Geometry and style of partitioned deformation within a late Cenozoic transpressional zone in the eastern Gobi Altai Mountains, Mongolia

Abstract The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with peaks averaging 2000–3000 m in elevation. The region is seismically active and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengths and at stepover zones. This report summarizes structural field investigations made in the easternmost Gobi Altai to document the structural geometry and style of late Cenozoic transpressional deformation in the region in order to better understand processes of intracontinental mountain building and the distant intracontinental strain response to the Indo-Eurasian collision. The Artsa Bogd range marks the northeastern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-lateral oblique-slip faults. The northern side of the range is also bounded by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments. The southern margin of Artsa Bogd appears tectonically inactive but contains S-vergent thrust faults and left-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion of a Mesozoic basin. The isolated, high and narrow Tsost Uul range south of Artsa Bogd occupies a restraining bend position along the left-lateral Tsost Uul strike-slip fault system. Major faults within the range define a half-flower structure cross-sectional geometry. To the south of the Tsost Uul range, the Gobi Bulag left-lateral strike-slip fault system is marked by small push-up ridges and one major restraining bend mountain where the fault steps to the right near its western end. Throughout the region, Late Cretaceous-Tertiary basalts and Tertiary and Quaternary sediments are deformed by the major fault systems indicating late Cenozoic fault activity. These fault systems and the ranges formed along them occur at fairly regular intervals (approximately 20 km) between the North Gobi Altai fault system and the Gobi Tien Shan fault system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of discrete linear belts of Cenozoic E-W left-lateral transpressional deformation south of the Hangay Dome. The regular spacing of the fault systems may suggest more uniform distributed left-lateral flow at depth. Eastward-directed lower crustal and lithospheric mantle flow is suggested by existing seismic anisotropy data for the eastern Gobi Altai and is believed to be the driving force for the upper crustal deformation.

Timing of formation of forebergs in the northeastern Gobi Altai, Mongolia: implications for estimating mountain uplift rates and earthquake recurrence intervals

Bedrock and Quaternary alluvial fans in the forelands of the northern Gobi Altai have been faulted and warped by foreland-propagating thrust faults to form elongate hills known as forebergs. Forebergs near Bogd town, north of Arsta Bogd, and at Ih Hetsüü, north of Baga Bogd were mapped and their formation constrained using luminescence dating of lacustrine and fanglomerates sediments to give localized uplift rates of c. 0.05 m ka−1, and between c. 1.6 and 4.1 m ka−1, respectively. Comparisons between vertical surface rupture during the 1957 Gobi Altai earthquake with the age and amount of uplift of lacustrine sediments provide a tentative estimate of between 1288 ± 382 to 497 ± 120 years for the average recurrence interval of great earthquakes in the Baga Bogd region. These localized uplifts rates support the view that the Gobi Altai Mountains probably took between 50 and 25 million years to form and were, therefore, coeval with the uplift of the Himalayas and the Tibetan Plateau.

The landscape evolution of Nemegt Uul: a late Cenozoic transpressional uplift in the Gobi Altai, southern Mongolia

Abstract The geomorphology and structural geology of Nemegt Uul, Southern Mongolia, is examined as an example of a mountain range that has formed within a restraining bend along a major intracontinental strike-slip fault system, the Gobi-Tien Shan fault system. Structural and geomorphological analysis demonstrates that the mountain belt is young and has been differentially tilted and eroded. A geomorphological model is developed showing that uplift and erosion have resulted in the formation of deeply incised mountains, alluvial fans, badlands, desert pavements and dunes.

Association of macro- and microfossils in the Vendian (Ediacaran) postglacial successions in Western Mongolia

The Vendian (Ediacaran) beds of the Zavkhan Basin, in the upper part of the Tsagaanolom Formation (<632 ± 14 Ma), yielded a new “Zavkhan” association of algae, microfossils, and problematic organisms, which is established in the series of alternating chert-carbonate shale with phosphorite interbeds. This association is distinct in the predomination of large (250 μm and over) sphaeromorphic microfossils of the genera Tasmanites, Archaeooides, and Leiosphaeridia, whereas acanthomorph acritarchs are represented by rarely found Cavaspina sp. and Tanarium sp. Multicellular algae included fragments of encrusting or foliate thalli with pseudoparenchymatous structure of polygonal cells characteristic of Rhodophyta algae (Thallophycoides sp.), and cordlike thalli of Vendotaenid algae Tyrasotaenia podolica. These layers of siltstone contain imprints of the problematic Vendian macrofossil Beltanelliformis brunsae. In their stratigraphic position, chemostratigraphic data, and fossil assemblage, the “Zavkhan” association can be assigned to the Upper Vendian.

Precambrian and Cambrian Regional Stratigraphy of Mongolia

In this paper, we demonstrate the Mongolian Precambrian and Cambrian stratigraphic classification of sedimentary, sedimentary–volcanogenic, volcanogenic, and metamorphic rock sediments spread over the territory of Mongolia according to the new stratigraphic classification approved by the International Stratigraphic Commission. The Mongolian republic is situated in the central part of the Asiatic continent. Geological studies in different parts of Mongolia began in the middle of the twentieth century, but regular and extensive examination started in the 1970s and 1980s along with stratigraphic analyses of the oldest deposits within the sedimentary basins. The Joint Russian–Mongolian Scientific Research Geological and Palaeontological Expeditions have played a leading role in the study of the region. These studies commenced more than 40 years ago and are still in progress to some extent. Precambrian rock units, exposed mainly in northern Mongolian, have been divided into three regional metacomplexes (Baidrag, Buteel, and Bumbuger) and four sedimentary formations (Muren, Khug, Darkhad or Zavkhan, and Khubsugul or Tsagaanolom). Isotopic age determinations on rocks and minerals from Archean granulite–gneiss blocks demonstrate an evolution from ~3.1 to 2.6 Ga. The Palaeoproterozoic sedimentary complex is well exposed in northern and central Mongolia and is distinguished by marbles, various calciphyres, amphibolites, and iron quartzites. The age of the complex is 1600–2050 Ma. Detailed studies of the middle Neoproterozoic and early Cambrian faunal (comeosiliceous polyactinal sponges, ichnofossils, soft-bodied fauna, medusa, chiolites, anabarites, archaeocyathids, trilobites) and floral (stromatolites, microphytolite assemblages, cyanobacterial mats, microfossils, calcareous algae) fossils provide the opportunity to construct the first regional stratigraphic subdivision of different facies sediments. Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only a few of them contain unequivocal macroscopic fossils. Precambrian glacial marine deposits (Ediacaran Maikhanuul Formation) were discovered in the Zavkhan region of western Mongolia in the 1990s; they discordantly overlie the volcanic rocks of the Zavkhan Formation (732–777 Ma) and are conformably overlain by limestones of the Tsagaanolom Formation. Recent studies suggest that the discoidal macroscopic remains from the intertillite beds of the Maikhanuul Formation, as well the assemblage of microfossils from the Tsagaanolom Formation, are comparable to those from the Doushantuo Formation of South China (600–550 Ma).

A new look at the Precambrian and Cambrian event chronostratigraphic correlation of Mongolia

The Mongolian Precambrian and Cambrian event chronostratigraphic classification of sedimentary, sedimentary—volcanogenic, volcanogenic, and metamorphic rock sediments spread over the territory of Mongolia according to the new chronostratigraphic classification approved by the International Stratigraphic Commission is discussed. These ancient rock units are readily divided into two independent complexes. The lower complex (1000–3500 m) is represented by the Archean and Early Proterozoic crystalline basement and the upper complex is the latest Precambrian–Cambrian sediments (220–7500 m). A detailed study of abundant Neoproterozoic and Early Cambrian faunal (ichnofossils, hexactinellid sponges, archaeocyaths, trilobites, etc.) and floral (stromatolites, microphytolites, cyanobacterial mats, microfossils, etc.) fossils provides the first regional chronostratigraphic subdivision of different facies sediments. Every stage is characterized by distinctive geohistorical and biological events.

Prasinophyte green algae Tasmanites and problematic fossils in the Upper Vendian Biota of the Zavkhan Basin, western Mongolia

A new species of spheromorphic microfossils of Tasmanites with a characteristically thick cover is established in the Zavkhan association of algae, microfossils, and problematic Upper Vendian organisms from the upper part of the siliciclastic–carbonate section of the Tsagaanolom Formation (632 ± 14 Ma) of the Zavkhan Basin in western Mongolia. Representatives of this genus are widespread in the Phanerozoic beds and their accumulations are recorded in the Domanik facies and near hydrocarbon deposits.