Distribution, origin and transport process of boulders deposited by the 2004 Indian Ocean tsunami at Pakarang Cape, Thailand

The tsunami of 2004 in the Indian Ocean transported thousands of meters-long boulders shoreward at Pakarang Cape, Thailand. We investigated size, position and long axis orientation of 467 boulders at the cape. Most of boulders found at the cape are well rounded, ellipsoid in shape, without sharp broken edges. They were fragments of reef rocks and their sizes were estimated to be < 14m³ (22.7t). The distribution pattern and orientation of long axis of boulders reflect the inundation pattern and behavior of the tsunami waves. It was found that there is no clear evidence indicating monotonous fine/coarse shoreward trends of these boulders along each transect line. On the other hand, the large boulders were deposited repeatedly along the three arcuate lines at the intertidal zone with a spacing of approximately 136m interval. This distribution pattern may suggest that long-lasting oscillatory flows might have repositioned the boulders and separated the big ones from small. No boulders were found on land, indicating that the hydraulic force of the tsunami wave rapidly dissipated on reaching the land due to the higher bottom friction and the presence of a steep slope. We further conducted numerical calculation of tsunami inundation at Pakarang Cape. According to the calculation, the sea receded and the major part of the tidal bench (area with boulders at present) was exposed above the sea surface before the arrival of the first tsunami wave. The first tsunami wave arrived at the cape from west to east at approximately 130min after the tsunami generation, and then inundated inlands. Our calculation shows that tsunami wave was focused around the offshore by a small cove at the reef edge and spread afterwards in a fan-like shape on the tidal bench. The critical wave velocities necessary to move the largest and average-size boulders by sliding can be estimated to be approximately 3.2 and 2.0m/s, respectively. The numerical result indicates that the maximum current velocity of the first tsunami wave was estimated to be from 8 to 15m/s between the reef edge and approximately 500m further offshore. This range is large enough for moving even the largest boulder shoreward. These suggest that the tsunami waves that were directed eastward, struck the reef rocks and coral colonies, originally located on the shallow sea bottom near the reef edge, and detached and transported the boulders shoreward.     

试用地球系统科学观解读2004年印度洋地震海啸

2004年印度洋地震海啸是本世纪初全球发生的最为惨重的自然灾害.这次地震海啸涉及地球的岩石圈、水圈、大气圈和生物圈,甚至还有地外星球和月球的作用,造成能量与物质之间的相互转化与传递,说明地球是一个完整的统一整体.因此,对地震海啸等自然灾害必须采用地球系统科学观进行分析和研究,找出彼此之间的相互关系、形成机制和演化规律,并用信息化、全球化和可持续发展的地球科学观来研究和防御地震海啸.     

Run-up and Inundation Pattern Developed During the Indian Ocean Tsunami of December 26, 2004 Along the Coast of Tamilnadu (India)

The tsunami run-up, inundation and damage pattern observed along the coast of Tamilnadu (India) during the deadliest Indian Ocean tsunami of December 26, 2004 is documented in this paper. The tsunami caused severe damage and claimed many victims in the coastal areas of eleven countries, bordering the Indian Ocean. Along the coast of Indian mainland, the damage was caused by the tsunami only. Largest tsunami run-up and inundation was observed along the coast of Nagapattinam district and was about 10–12 m and 3.0 km, respectively. The measured inundation data were strongly scattered in direct relationship to the morphology of the seashore and the tsunami run-up. Lowest tsunami run-up and inundation was measured along the coast of Thanjavur, Puddukkotai and Ramnathpuram districts of Tamilnadu in the Palk Strait. The presence of shadow of Sri Lanka, the interferences of direct/receded waves with the reflected waves from Sri Lanka and Maldive Islands and variation in the width of continental shelf were the main cause of large variation in tsunami run-up along the coast of Tamilnadu.     

The Indian Ocean tsunami of December 26, 2004: observations in Sri Lanka and Thailand

On December 26, 2004 a great earthquake (M _W 9.3) occurred off the western coast of Sumatra triggering a series of tsunami waves that propagated across the Indian Ocean causing damage and life loss in 12 countries. This paper summarizes the observations of lifeline performance, building damage and its distribution, and the social and economic impact of the tsunami made by the Earthquake Engineering Field Investigation Team (EEFIT) in Thailand and Sri Lanka. EEFIT operates under the umbrella of the UK’s Institution of Structural Engineers. It is observed that good engineering practice can reduce economic losses, but additional measures are required to reduce risk to life.     

Preliminary estimation of the tsunami hazards associated with the Makran subduction zone at the northwestern Indian Ocean

We present a preliminary estimation of tsunami hazard associated with the Makran subduction zone (MSZ) at the northwestern Indian Ocean. Makran is one of the two main tsunamigenic zones in the Indian Ocean, which has produced some tsunamis in the past. Northwestern Indian Ocean remains one of the least studied regions in the world in terms of tsunami hazard assessment. Hence, a scenario-based method is employed to provide an estimation of tsunami hazard in this region for the first time. The numerical modeling of tsunami is verified using historical observations of the 1945 Makran tsunami. Then, a number of tsunamis each resulting from a 1945-type earthquake (M _w 8.1) and spaced evenly along the MSZ are simulated. The results indicate that by moving a 1945-type earthquake along the MSZ, the southern coasts of Iran and Pakistan will experience the largest waves with heights of between 5 and 7 m, depending on the location of the source. The tsunami will reach a height of about 5 m and 2 m in northern coast of Oman and eastern coast of the United Arab Emirates, respectively.     

Sheltered coastal environments as archives of paleo-tsunami deposits: Observations from the 2004 Indian Ocean tsunami

The 2004 earthquake left several traces of coseismic land deformation and tsunami deposits, both on the islands along the plate boundary and distant shores of the Indian Ocean rim countries. Researchers are now exploring these sites to develop a chronology of past events. Where the coastal regions are also inundated by storm surges, there is an additional challenge to discriminate between the deposits formed by these two processes. Paleo-tsunami research relies largely on finding deposits where preservation potential is high and storm surge origin can be excluded. During the past decade of our work along the Andaman and Nicobar Islands and the east coast of India, we have observed that the 2004 tsunami deposits are best preserved in lagoons, inland streams and also on elevated terraces. Chronological evidence for older events obtained from such sites is better correlated with those from Thailand, Sri Lanka and Indonesia, reiterating their usefulness in tsunami geology studies.     

Impacts from the 2004 Indian Ocean Tsunami: analysing the potential protecting role of environmental features

The tsunami that deeply impacted the North Indian Ocean shores on 26 December 2004, called for urgent rehabilitation of coastal infrastructures to restore the livelihood of local populations. A spatial and statistical analysis was performed to identify what geomorphological and biological configurations (mangroves forests, coral and other coastal vegetation) are susceptible to decrease or increase coastal vulnerability to tsunami. The results indicate that the width of flooded land strip was, in vast majority, influenced by the distance to fault lines as well as inclination and length of proximal slope. Areas covered by seagrass beds were less impacted, whereas areas behind coral reefs were more affected. The mangroves forests identified in the study were all located in sheltered areas, thus preventing to address the potential protecting role of mangroves forests.     

Socotra Island,Yemen: field survey of the 2004 Indian Ocean tsunami

The tsunami of 26th December 2004 severely affected Yemen’s Socotra Island with a death at a distance of 4,600 km from the epicenter of the Magnitude 9.0 earthquake. Yemen allowed a detailed assessment of the far-field impact of a tsunami in the main propagation direction. The UNESCO mission surveyed 12 impacted towns on the north and south shores covering from the east to the west tip of Socotra. The international team members were on the ground in Yemen from 11 to 19 October 2006. The team measured tsunami run-up heights and inundation distances based on the location of watermarks on buildings and eyewitness accounts. Maximum run-up heights were typically on the order of 2–6 m. Each measurement was located by means of global positioning systems (GPS) and photographed. Numerous eyewitness interviews were recorded on video. The tsunami impact on Socotra is compared with other locations along the shores of the Indian Ocean.     

Plio-Pleistocene palaeoceanography of the tropical Indian Ocean (ODP Hole 709C): Radiolarian and CaCO3 evidence

Sediments spanning the Olduvai subchron (1.79–1.95 Ma) of ODP Hole 709C in the tropical Indian Ocean were sampled at 5-cm intervals. High-resolution quantitative radiolarian and CaCO₃% data have been used as proxies in interpreting palaeoceanographical conditions, an a comparison made with previously obtained oxygen isotope data. Radiolarian data were subjected to principal components analysis, resulting in four varimax factors. Factor 1 accounted for the greatest variation and comprised well-know warm-water and cold-water taxa, with high +ve and - ve scores respectively, which were used in the construction of a temperature index. Results indicate a tripartite temporal division of Olduvai palaeoceanography. In the early Olduvai, prior to 1.895 Ma, and the late Olduvai, approximating the Plio-Pleistocene boundary at 1.81 Ma, divergence-driven equatorial upwelling and high-productivity coincide with isotope-defined ‘interglacials’, and with generally low CaCO₃%. However, during the mid-Olduvai, upwelling, productivity and sedimentation rates were all reduced. These changes in palaeoceanography are thought to be linked directly to eccentricity modulated variations in the precession of the Earth's orbit. The early and late Olduvai coincide with periods of minimum eccentricity, when precessional reinforcemnet of the southwest monsoon is weakest, coastal upwelling along the Oman margin is reduced, and zonal wind-driven equatorial divergence upwelling is increased. The mid-Olduvai, however, corresponds with an eccentricity maximum, during which monsoonal activity is stronger and equatorial divergence upwelling weak.     

Morpho‐sedimentary characteristics of proximal gravel braided river deposits in a Froude‐scaled physical model

A Froude‐scaled physical model of a proximal gravel‐bed braided river was used to connect the river morphological characteristics, and sedimentary processes and forms, to deposit geometry. High resolution continuous three‐dimensional topographic data were acquired from sequential photogrammetric digital elevation models paired with grain‐size surface maps derived from image analysis of textural properties of the surface. From these data, the full three‐dimensional development of the braided river deposit and grain‐size sorting patterns was compiled over an experimental time period of 41 h during which the model river reworked a large portion of the braided channel. The minimum surface of the deposit is developed progressively over time by erosion, migration and avulsion of channels, and by local scour at channel confluences. The maximum surface of the deposit is formed by amalgamation of braid bar surfaces and has less overall relief than the minimum surface. Confluence scour constitutes about 5% of the area of the minimum surface. Migration of individual confluences is limited to distances of the order of the width and length of the confluence, so that confluences do not form laterally extensive deposits and basal surfaces. Maximum and minimum surfaces have very similar grain‐size distributions, and there is no extensive basal coarse layer. Deposit maximum thickness is strongly associated with large channel confluences which occur as deeper areas along the main channel belt and make up a large proportion of the thickest portions of the deposit.     

沧东凹陷始新统孔店组二段深水细粒沉积岩中的软沉积物变形构造*

湖相深水细粒沉积岩中的软沉积物变形构造主要发育在泥岩和泥晶碳酸盐岩为主的地层中,大部分因为发育规模小(镜下尺度),在岩心观察中很容易被忽略掉。本文以沧东凹陷G108-8井孔店组二段(孔二段)细粒沉积岩为研究对象,通过密集的镜下观察和精细岩心描述,识别出同沉积微断裂、液化岩脉、微褶皱变形、微重荷变形和杂乱变形等多种类型的软沉积物变形构造。研究区深水细粒沉积岩中的软沉积物变形构造总体表现为规模小、垂向无明显重力流砂体伴生、原地成因、沉积纹层发育等特点。从应力调节方式的角度考虑,可以将软沉积物变形构造的形成机制划分为上覆应力卸载、侧向应力挤压和层内应力释放等3种类型,不同的形成机制分别是对不同沉积环境变化的响应。通过湖相深水细粒沉积岩软沉积物变形构造的研究,可以恢复古沉积环境变化、预测重力流方位,对深水细粒沉积岩的油气勘探也具有一定的指导意义。     

Geochemical and Nd–Pb isotopic characteristics of the Tethyan asthenosphere: implications for the origin of the Indian Ocean mantle domain

It is unclear why the Pb, Nd, and Sr isotopic composition of the modern mid-ocean ridge basalts (MORB) from the Indian Ocean is different from that of the North Atlantic and Pacific Oceans. A possible explanation for this is that the Indian MORB-type isotopic signature is a long-lived regional feature of the mantle, as evidently shown by the isotopic composition of the 350 Ma MORB-like Mian-Lue northern ophiolite, which was formed in the same region presently occupied by the Indian Ocean. However, this hypothesis is in conflict with the lack of Indian MORB-type isotopic signature in a number of 150 Ma Tethyan and Indian Ocean crusts. To further constrain the origin of the Indian MORB-type isotopic signature, we analyze the geochemical and Pb, Nd, and Sr isotopic composition of representative mafic rocks from four Tethyan ophiolites ranging in age from 90 to 360 Ma. The Sr isotopic composition of the samples is unreliable due to alteration, but the age-corrected Nd and Pb isotopic ratios and geochemical data indicate that these Tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear Indian MORB-type isotopic signature. We therefore conclude that the bulk of the Indian suboceanic mantle was most probably inherited from the Tethyan asthenosphere. A few regions in both the Tethyan and Indian Oceans, however, are most probably underlain by Pacific and North Atlantic MORB-type mantle (and vice-versa) because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins. The Indian MORB-type isotopic signature of the western Pacific marginal basin crusts could be due to either flow of the Indian Ocean mantle into the western Pacific or to endogenous production of such an isotopic signature from delaminated East-Asian sublithospheric materials during closure of the Tethys Ocean.     

Source of a freshwater influx at the last glacial maximum in the Indian Ocean: An alternative interpretation

Recent analysis of a sediment core in the eastern Arabian Sea revealed a negative pulse of about 1% in the δ¹⁸O value of the planktonic Foraminifera around the last glacial maximum (LGM). This pulse has been attributed to (i) increased runoff into the Bay of Bengal from the east-flowing south Indian rivers due to enhancement of the northeast winter monsoon, and (ii) an increase in Arabian sea-surface temperature caused by the weakening of the southwest monsoon at the LGM. We show that the speculation on which the latter hypothesis is based, is not supported by observational data and cannot fully account for the observed magnitude of the spike. With a view to assessing the validity of the first mechanism, we have modelled the mixed layer of the Bay of Bengal as a well-mixed box. The model calculations show that to account for the pulse requires a change of about 10% in either the annual rate of river input or its isotopic composition. For the northeast monsoon to account for the pulse it would mean that the rainfall should have increased by a factor of five to ten during the LGM. No evidence for such an increase is indicated in the available palaeoclimatic data. We explain the freshwater spike by invoking increased discharge of glacial meltwater from the Tibetan plateau into the Bay of Bengal. We show that the proxy climate data from the Indo-Tibetan region that has become available recently provides substantial evidence for the occurrence of a warming event around the LGM, which supports our mechanism.     

X‐ray tomography of tsunami deposits: Towards a new depositional model of tsunami deposits

X‐ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in Andalusia, Spain (1755 Lisbon tsunami). The technique allows characterization of grain‐size distribution, structures, component analysis and sedimentary fabric of fine‐grained unconsolidated tsunami deposits at resolutions down to particle scale. The results are validated by comparing to data obtained using other techniques such as laser diffraction, anisotropy of magnetic susceptibility and X‐ray microfluorescence on the same deposits. Specific technical details such as sampling, scanning and image processing methods, and further improvements are addressed. The use of X‐ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines, rip‐up clasts, crude laminations, convolutions, floating outsized clasts and contacts between successive units. This analysis of the 1755 tsunami deposits using X‐ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits. Deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets, with laminated mudlines corresponding to the basal frictional region of these carpets. The onset of the tsunami backwash is marked by a micro‐vortex resembling Kelvin–Helmoltz instabilities.     

Soft‐sediment deformation structures in a Pleistocene glaciolacustrine delta and their implications for the recognition of subenvironments in delta deposits

The development of soft‐sediment deformation structures in clastic sediments is now reasonably well‐understood but their development in various deltaic subenvironments is not. A sedimentological analysis of a Pleistocene (ca 13·1 to 15 ¹⁰Be ka) Gilbert‐type glaciolacustine delta with gravity‐induced slides and slumps in the Mosty‐Danowo tunnel valley (north‐western Poland) provides more insight, because the various soft‐sediment deformation structures in these deposits were considered in the context of their specific deltaic subenvironment. The sediments show three main groups of soft‐sediment deformation structures in layers between undeformed sediments. The first group consists of deformed cross‐bedding (inclined, overturned, recumbent, complex and sheath folds), large‐scale folds (recumbent and sheath folds) and pillows forming plastic deformations. The second group comprises pillar structures (isolated and stress), clastic dykes with sand volcanoes and clastic megadykes as examples of water‐escape structures. The third group consists of faults (normal and reverse) and extensional fissures (small fissures and neptunian dykes). Some of the deformations developed shortly after deposition of the deformed sediment, other structures developed later. This development must be ascribed to hydroplastic movement in a quasi‐solid state, and due to fluidization and liquefaction of the rapidly deposited, water‐saturated deltaic sediments. The various types of deformations were triggered by: (i) a high sedimentation rate; (ii) erosion (by wave action or meltwater currents); and (iii) ice‐sheet loading and seasonal changes in the ablation rate. Analysis of these triggers, in combination with the deformational mechanisms, have resulted – on the basis of the spatial distribution of the various types of soft‐sediment deformation structures in the delta under study – in a model for the development of soft‐sediment deformation structures in the topsets, foresets and bottomsets of deltas. This analysis not only increases the understanding of the deformation processes in both modern and ancient deltaic settings but also helps to distinguish between the various subenvironments in ancient deltaic deposits.     

Multi‐scale analysis of a migrating submarine channel system in a tectonically‐confined basin: The Miocene Gorgoglione Flysch Formation,southern Italy

The Miocene Gorgoglione Flysch Formation records the stratigraphic product of protracted sediment transfer and deposition through a long‐lived submarine channel system developed in a narrow and elongate thrust‐top basin of the Southern Apennines (Italy). Channel‐fill deposits are exposed in an outcrop belt approximately 500 m thick and 15 km long, oriented oblique to the palaeoflow, which was roughly south‐eastward. These exceptional exposures of channel‐fill strata allow the stacking architectures and the evolution of the channel system to be analyzed at multiple scales, enabling the effects of syn‐sedimentary thrust tectonics and basin confinement on the depositional system development to be deciphered. Two end‐member types of elementary channel architecture have been identified: high‐aspect‐ratio, weakly‐confined channels, and low‐aspect‐ratio, incisional channels. Their systematic stacking results in a complex pattern of seismic‐scale depositional architectures that determines the stratigraphic framework of the deep‐water system. From the base of the succession, two prominent channel complex sets have been recognized, namely CS1 and CS2, consisting of amalgamated incisional channel elements and weakly‐confined channel elements. These channelized units are overlain by isolated incisional channels, erosional into mud‐prone slope deposits. The juxtaposition of different channel architectures is interpreted to have been governed by regional thrust‐tectonics, in combination with a high subsidence rate that promoted significant aggradation. In this scenario, the alternating ‘in sequence’ and ‘out of sequence’ tectonic pulses of the basin‐bounding thrusts controlled the activation of coarse‐clastic inputs in the basin and the resulting stacking architectures of channelized units. The tectonically‐driven confinement of the depositional system limited the lateral offset in channel stacking, preventing large‐scale avulsions. This study represents an excellent opportunity to analyze the stratigraphic evolution of a submarine channel system in tectonically‐active settings from an outcrop perspective. It should find wide applicability in analogous depositional systems, whose stratigraphic architecture has been influenced by tectonically‐controlled lateral confinement and associated lateral tilting.     

Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation,Karoo Basin,South Africa

Submarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub‐parallel sections, passes from extensionally deformed material (clastic dykes and down‐dip facing low‐angle shear surfaces) down‐dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up‐dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross‐cutting relationships indicate an up‐depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km² in the bounding undeformed stratigraphy indicate a low‐gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large‐scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine‐grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down‐dip movement in a low‐gradient, fine‐grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.     

Estimates of large magnitude Late Cambrian earthquakes from seismogenic soft‐sediment deformation structures: Central Rocky Mountains

A variety of unusual early post‐depositional deformation structures exist in grainstone and flat‐pebble conglomerate beds of Upper Cambrian strata, western Colorado, including slide scarps, thrusted beds, irregular blocks and internally deformed beds. Thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined (15° to 40°) ramp, locally producing hanging wall lenses with fault‐bend geometries. Thrust plane orientations are widely distributed, and in some cases nearly oppositely oriented in close proximity, indicating that they did not form as failures acted upon by gravity forces. Irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces. These features represent parts of beds that detached, moved up onto and some distances across, the laterally adjacent undisturbed bed surfaces. Deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm‐induced deformation, because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks. Finally, internally deformed beds are characterized by large blocks, fitted fabrics of highly irregular fragments and contorted lamination, which represent heterogeneous deformation, such as brecciation and liquefaction. The deformation structures were produced by earthquakes linked to the reactivation of Mesoproterozoic, crustal‐scale shear zones in the central Rockies during the Late Cambrian. Analysis of the deformation structures indicates very large body forces and calculated earthquake‐generated ground motion velocities of ca 1·6 m sec^?1. These correspond to moment magnitudes of ca 7·0 or more and a Mercalli Intensity of X+. These are the only known magnitude estimates of Phanerozoic (other than Quaternary) large‐intensity earthquakes for the Rocky Mountain region, and they are as large as, or larger than, previous estimates of Proterozoic earthquakes along these major shear zones of the central Rockies.     

Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo‐Asian Ocean suture extends further east than expected

Newly discovered eclogites, Early Cambrian carbonates and chloritoid‐bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan‐Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan‐type ophiolitic sequence. The eclogites correspond geochemically to T‐MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well‐equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs_0.24–0.36, alm_0.43–0.56, py_0.05–0.18, sps_0.00–0.18, . The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20?22.5 kbar and 590?610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ～600?630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet‐chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.