The most recent (682-792 C.E.) volcanic eruption in the Jombolok lava field, East Sayan, Central Asia triggered exodus of Mongolian pre-Chinggis Khaan tribes (778-786 C.E.)

International audienceThis study presents new data on one of the most recent (historical) volcanic eruptions in Central Asia. The Jombolok lava field located in the East Sayan Mountains (Southern Siberia) was formed during Late Pleistocene and Holocene times. At least four phases of volcanic activity have been identified and evidences associated with the last phase have been found in the upper reaches of the Khi-Gol valley and in the Oka-Jombolok basin. The volcanic activity is represented by young basaltic lava located among older lavas. Live and dead trees have been sampled in the young lava field. Nine fragments of wood have been found embedded in lavas of the latest eruption. Dendrochronological analysis, radiocarbon dating and the analysis of historical chronicles have shown that the latest eruption occurred during the period 682-792 A.D. The volcanic activity possibly triggered the migration of Mongolian tribes out of the locality known in historical chronicles as Ergune-Kun towards the Onon River, which, 400 years later, became the place of birth and rise of Chinggis Khaan

Helium isotopes provide no evidence for deep mantle involvement in widespread Cenozoic volcanism across Central Asia

Small-volume alkali basaltic volcanism has occurred intermittently for the past + 30 My across a vast area of thick continental crust from southern Siberia, through Mongolia to northeast China. With a lack of evidence for Basin-and-Range-type crustal extension or rifting, models to explain the widely dispersed, yet long-lived, volcanism tend to favour involvement of one or more mantle plume(s). We examine the range of 3He/4He isotope values in olivine phenocrysts from basalts, and their entrained mantle xenoliths, from Hamar Daban in southern Siberia, and Hangai in central Mongolia, in order to examine whether upwelling lower mantle appears to be present beneath central Asia and thus test the validity of the plume model for this region. Our results show that the maximum 3He/4He value for the Siberian basalts is 8.12 ± 0.2Ra, and the maximum value for Mongolian basalts is 9.5 ± 0.5Ra. These values suggest that there is no significant contribution from a high 3He/4He primordial component that would strongly argue a lower mantle source. Overlap with commonly reported values for MORB leads us to propose that the source of the magmatism derives from the shallow asthenosphere. Alternative models to a deeply sourced mantle plume that may be able to explain the magmatism include: a shallow thermal anomaly confined to the upper mantle but either fed laterally or caused by thermal blanketing of the large Asian landmass; replacement or delamination of the lowermost lithosphere in response to tectonic stresses; or large-scale mantle disturbance or overturn caused by a protracted history of subduction beneath central Asia that ended regionally with the Jurassic closure of the Mongol-Okhotsk Ocean, but continues further afield with the present Indo-Asia collision

Late Mesozoic topographic evolution of western Transbaikalia: evidence for rapid geodynamic changes from the Mongol-Okhotsk collision to widespread rifting

International audienceThe Mesozoic geodynamic evolution of Transbaikalia has been largely controlled by the scissors-like closure of the Mongol –Okhotsk Ocean that separated Siberia from Mongolia–North China continents. Following the oceanic closure, the tectonic evolution of that region was characterized by collisional uplift and subsequent extension that gave rise to the formation of metamorphic core complexes. This complex tectonic setting prevailed simultaneously between 150 Ma and 110 Ma both in Transbaikalia, North Mongolia, and within the North China Craton. Published paleobotanical and paleontological data show that the oldest Mesozoic basins had formed in western Transbaikalia before the estimated age of extension onset. However no precise geochronological age is available for the onset of extension in Transbaikalia. The Tugnuy Basin, as probably the oldest Mesozoic basin in western Transbaikalia, is a key object to date the onset of extension and following changes in tectonic setting. In this study, U–Pb (LA-ICP-MS) dating of detrital zircons from three key Jurassic sediment formations of the Tugnuy Basin are used to identify the potential source areas of the sediments, understand the changes in sediment routing and provide insights on the topographic evolution of western Transbaikalia. Our results show several significant changes in tectonic regime after the closure of the Mongol–Okhotsk Ocean. A wide uplifted plateau formed during the closure of the Mongol-Okhotsk Ocean, determining the Early Jurassic drainage system reaching the Angara-Vitim batholith to the north and shedding sediments to the continental margin to the South. The following collisional event at the end of the Early Jurassic led to the uplift of the collision zone, which partially inverted the drainage system toward the North. A strike-slip displacement induced by the oblique collision initiated some of the early Transbaikalian depressions, such as the Tugnuy Basin at about 168 Ma. A phase of basin inversion, marked by folding and erosion of the Upper Jurassic sediments, could correspond to the short-term collision event that took place during the latest Jurassic–earliest Cretaceous in the eastern Central Asian Orogenic Belt. The following inversion in tectonic regime from compression to extension is consistent with the mid-lower-crustal extension that led to the formation of the numerous metamorphic core complexes throughout northeastern continental Asia during the Early Cretaceous

Jurassic sediments in the Irkut basin and southwestern Transbaikalia: correlations based on paleobotanical and geochronological data

International audienceThe Jurassic growth of mountain ranges along the southern edge of the Siberian platform occurred in an active tectonic setting related to the closure of the Mongol-Okhotsk Ocean. The oceanic subduction and subsequent continent collision events induced compressive deformations at the platform boundary. Understanding the paleogeography related to the Mesozoic closure of the Mongol-Okhotsk Ocean requires dating and correlation of the Jurassic Prisayan Formation in the Irkut basin and Tugnuyskaya Formation in southwestern Transbaikalia. This work presents structural and paleobotanic results within both formations. 40Ar/39Ar dating of underlying volcanics from the upper member of the Ichetuyskaya Formation is used to refine the age of the sediment series and provide probable correlation. The results show that the Tugnuyskaya Formation initiated at the end of the Middle Jurassic-beginning of the Late Jurassic and was not coeval with the Prisayan Formation, whose upper fine-grained members were deposited in the early Middle Jurassic. 40Ar/39Ar dating of volcanics from the upper member of the Ichetuyskaya Formation yielded a Middle Jurassic age of 167.7 ± 1.2 Ma (Bajocian to Bathonian). The paleogeographic data analysis based on facies and mineralogical composition of sediments and on a study of source areas from Sm-Nd data and the U-Pb ages of detrital zircons from the deposits in the southern Irkut basin indicates that the deposition of the Prisayan Formation was followed by the intensification of relief building along the southern edge of the Siberian Platform. Our geochronological data show that active tectonic deformations in southwestern Transbaikalia evidenced in the volcanoclastic Ichetuyskaya Formation in the Tugnuy basin also occurred during the Middle Jurassic. The uppermost sediments of the Tugnuy basin were deposited at the end of the Middle Jurassic-Late Jurassic in a quiet tectonic setting with low relief and lacustrine-boggy depositional environments

Age of the Jombolok lava field (East Sayan): evidence from dendrochronology and radiocarbon dating

International audienceDendrochronology and radiocarbon dating, with reference to remote sensing, digital elevation modeling, geological, and geomorphological data, provide new age constraints for the Jombolok lava field in the East Sayan Mountains (Siberia). The Jombolok lava field originated in the latest Late Pleistocene and underwent at least four phases of volcanic activity recorded in lava flows. Two earliest phases followed shortly one after another more than 13 kyr ago. The third phase corresponding to eruptions of Kropotkin volcano can be timed only relatively. The fourth phase has been dated by dendrochronology and AMS 14C of well-preserved wood buried under the youngest lava which occurs among older lavas near the Jombolok River mouth. The age of this activity is bracketed between the death of trees caused by eruptions 1268-928 years ago and the beginning of new tree growth on the surface of the most recent lavas 900 years ago