Hangay Dome Research Articles

Morphotectonic analysis of strike-slip faults in the Sayan-Tuva Upland (North Mongolia and South Siberia): Age and displacement rates

The Sayan-Tuva Upland is the northernmost positive relief of Central Asia, associated with the India-Asia collision. It formed on the boundary of the Siberian Craton in late Miocene-early Pliocene time and is characterized by uplift and shear displacement of lithospheric blocks. In this study, we answer the question of how deformation is distributed within the Sayan-Tuva Upland and what are the rates and age of horizontal displacements. Using morphotectonic and palaeoseismological analyses, we have calculated the displacement rates and onset of activation of three faults: Erzin-Agardag, Sayan-Tuva and Kaakhem, which are fragments of large strike-slip fault systems that cut across the entire Sayan-Tuva Upland. For the Erzin-Agardag Fault, the rate of left-lateral displacement is 0.7–1.4 mm/yr and the age of strike-slip reactivation is estimated to be 2.1–1.1 Ma. For the Sayan-Tuva and Kaakhem faults, the minimum horizontal displacement rates are 0.9 ± 0.1 and 0.6 ± 0.1 mm/yr, and the maximum ages of the onset of strike-slip displacements are 2.4 Ma and 5 Ma, respectively. We propose that 1.1–2.1 Ma ago there was a kinematic change that activated left-lateral strike-slip faults, leading to the formation of the modern kinematic model, characterized by eastward movement of lithospheric blocks along inherited fault systems between the Hangay Dome and the Siberian Craton. Analysis of slip distribution and trenching across the Erzin-Agardag Fault allowed estimating the mean recurrence interval of the ∼ M7.8 earthquakes between 9.4 and 4.7 ka. The Erzin-Agardag Fault has produced multiple displacements with an amplitude of ∼ 6.6 m and follows a characteristic slip model.

Upper mantle structure beneath the Mongolian region from multimode surface waves: Implications for the western margin of Amurian plate

Multimode phase speeds of surface waves are used to build a new radially anisotropic S wave model in the eastern Eurasian and Mongolian regions. Our dataset includes seismic waveforms of over 1655 teleseismic events (Mw≥5.8) from 2009 to 2021, recorded at permanent and temporary stations in and around Mongolia. The multimode dispersion curves of Love and Rayleigh waves were extracted using the nonlinear waveform fitting method for individual seismograms. Then, we retrieved phase speed maps for each mode and frequency, incorporating finite-frequency effects. Finally, localized multimode dispersion curves extracted from the phase speed maps were inverted for local 1-D SV and SH wave profiles, which are combined into a radially anisotropic 3-D shear wave model. Our new model exhibits significant lateral variations of S wave speeds at 70–100 km depth beneath Mongolia, i.e., slow anomalies in the tectonically active western Mongolia in contrast to fast anomalies in stable eastern Mongolia. In the radial anisotropy model, SH waves are faster than SV waves in most areas of the Mongolian lithosphere above 100 km depth, except for the northeast of the Altay Mountains. The Hangay Dome region is characterized by significantly slower velocities that may relate to its uplifting. A large-scale low velocity beneath the northeast of the Hangay Dome with a slower SV wave speed than SH may indicate the existence of partially molten layers. This study also reveals distinct lateral variations of S wave speeds across the boundary between the Amurian and Eurasian plates, characterized by the fast anomaly in eastern Mongolia, corresponding to the lithosphere in the western Amurian plate.

Crustal Structures From Receiver Functions and Gravity Modeling in Central Mongolia

Abstract3D forward gravity modeling combined with receiver function (RF) analysis characterizes the crustal structures of the southern part of the Mongolian collage. The seismic signals of the 48 stations of the MOBAL2003 and the IRIS‐PASSCAL experiments were analyzed to get the RFs. This analysis revealed a significant difference between the crustal structures of the Hangay dome and the tectonic zones in the south. In addition, seismic stations south of the Hangay dome display significant signals related to the occurrence of a low‐velocity zone at lower crustal level confirmed by the gravity anomalies. Finally, these seismic analysis inputs, the boundaries, the lithologies, and the density values from rock samples of the different tectonic zones constitute the starting points from the 3D forward gravity modeling. The resulting crustal density model indicates: (a) the likely absence of a Precambrian basement block beneath the Hangay dome, (b) an alternation of two low‐velocity/low‐density zones (LVLDZs) with high‐density zones in the Baydrag microcontinent interpreted as fragments of early Tonian plutons, (c) the occurrence of an LVLDZ at the lower crustal level beneath the Lake zone, the Mongol‐Altai Accretionary Wedge, and the Trans‐Altai Zone. Therefore, the combination of the seismic RF with gravity analysis and modeling reveals new crustal structures of the Mongolian collage and enhances the occurrence and the extent of an LVLDZ at lower crustal level. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolian collage.

Lithospheric deformation in Mongolia revealed by anisotropic Rayleigh wave tomography: Implications for the mechanism of intracontinental orogeny

The Mongolian Plateau is situated in the eastern part of the Central Asian Orogenic Belt that was formed during the Paleozoic era. In the Cenozoic era, the plateau has been reactivated, leading to significant topographic uplift, volcanism, and seismic activity. Several destructive earthquakes, ranging from magnitudes 6 to 8, have occurred in the 20th century. But the mechanism of these neotectonics remains unclear. To better understand these processes, we determined S wave velocity and azimuthal anisotropy in the crust and upper mantle under Mongolia using Rayleigh wave dispersion curves. Our findings indicate that the lithosphere is dominated by NW-SE fast velocity directions (FVDs). However, we observed different patterns in two specific regions. The lithospheric anisotropy under the Hangay Dome is weak, suggesting that it is accompanied by significant mantle upwelling under the dome. On the other hand, the FVDs in the Gobi Desert exhibit a circular pattern that wraps around the Ordos Block, which may have been caused by past and present deformation imposed on the stable block. These results offer new insights into the lithospheric deformation and dynamics of the Mongolian Plateau.

Hydrous Melting Driven Upwelling From the Mantle Transition Zone in the Mongolia Plateau Revealed by Receiver Function Analysis

AbstractThe detailed images of the mantle transition zone (MTZ) give insights into the nature of mantle upwelling from the deep Earth. We use receiver function to investigate the MTZ structure beneath the Mongolia Plateau using earthquakes recorded by 223 stations deployed in the Central Asian Orogenic Belt. The topographies of the 410 km (D410) and 660 km (D660) discontinuities are both characterized by slight depressions in most parts of the Mongolia Plateau. An obvious low‐velocity layer (LVL) atop the MTZ with an average thickness of ∼40 km is observed beneath this study area, indicating the hydrous partial molten layer atop the D410 and a hydrous MTZ. The D410 depth of ∼410 km and the LVL up to ∼60 km thick beneath the Dariganga volcano imply that there is a chemically distinct hydrous upwelling related to the subducted Pacific slab rather than a classic thermal‐dominated upwelling. The ∼10 km thinned MTZs are found beneath the western Hangay Dome and the Middle Gobi volcano. A narrow NS‐trending MTZ thickening zone extending from the Hovsgol rift to the central Hangay Dome is mainly caused by a ∼10 km depression of D660. We speculate that the mantle upwelling beneath the Hangay Dome may be related to the stagnant slab descending through the MTZ, rather than being rooted from the deep lower mantle. Hydrous melting‐driven upwellings from the MTZ beneath the Mongolia Plateau play a key role in intraplate volcanism.

Seismotectonics of Mongolia and Baikal Rift Zone Controlled by Lithospheric Structures

AbstractActive tectonics are developing in Mongolia and the Baikal rift zone, where large crustal earthquakes (Mw ≥ 6.0) occur frequently. To clarify the mechanism of these earthquakes, we determine a high‐resolution 3‐D P‐wave velocity model of the crust and upper mantle beneath Mongolia and the Baikal rift zone. We find an annular high‐velocity zone in the lithosphere; whereas low‐velocity anomalies exist both inside and around the high‐velocity ring as well as beneath the rift zone. Strong interactions between the high‐velocity ring and the surrounding terranes may have induced many moderate‐to‐large earthquakes (Mw 5–7). By contrast, mega‐earthquakes (Mw ≥ 8.0) occurred right above the high‐velocity ring. We propose that the high‐velocity ring is strong as a whole; it sustains the stress arising from the India‐Asia collision and causes the mega‐earthquakes. Meanwhile, the ring could protect the Hangay Dome from the regional compression, leading to weak seismicity under the dome.

Rayleigh wave tomography of central and southern Mongolia

The Mongolian Plateau located between the Siberian Craton and the North China Craton developed after multiple tectonic evolutions. It is proposed to be related to the far-field effect of the Indo-Asian collision in the Cenozoic. In this study, we inverted for high-resolution S wave velocity structures of Mongolia using Rayleigh wave tomography to constrain the plateau evolution. In particular, we obtained crustal and upper mantle seismic structures under Gobi Desert by combining Mongolian and Chinese datasets. The study reveals strong low-velocity anomalies in the lower crust immediately beneath the Hangay Dome, while the low-velocity zones are imaged beneath the dome margins in the upper mantle. The results imply that the Hangay Dome was formed by mantle upwelling into the crust from major faults following lithospheric delamination. Under the South Gobi Desert, we imaged several inclined high-velocity patches in the upper mantle, indicating the subducted slabs formed in the Paleozoic. The ancient structures in the upper mantle led the Indo-Asian collision to propagate further to southern Mongolia.

Lateral variations in crustal Lg attenuation in and around the Hangay Dome, Mongolia

Lateral variations in crustal Lg attenuation in and around the Hangay Dome, Mongolia

Metasomatism of the off-cratonic lithospheric mantle beneath Hangay Dome, Mongolia: Constraints from trace-element modelling of lherzolite xenoliths

Interactions of hydrous fluid and melt with dry mantle rocks are known to result in metasomatic alteration of the lithospheric mantle. Here we investigate such interactions that occurred beneath the Hangay Dome, Mongolia, in 22 mantle xenoliths, which were recovered from Cenozoic basalts at the Tsagan, Zala, Horgo, and Shavaryn-Tsaram localities near the village of Tariat. The xenoliths are medium- to coarse-grained spinel lherzolites that exhibit variable degrees of reaction with silicate melt and fluid (indicated by their Fe- and LREE-enrichment and the presence of secondary clinopyroxene, amphibole, phlogopite, apatite, and sulfide). According to their normalized REE patterns and microstructures, the spinel lherzolites were divided into three groups. Group 1 lherzolites contain LREE-depleted clinopyroxene and whole-rock compositions, exhibit a greater number of preserved deformation textures, and are the least affected by metasomatism. These lherzolites are interpreted to represent the sub-continental lithosphere before the rejuvenation processes that occurred during the Cenozoic. Group 3 lherzolites are characterized by partial annealing of pre-existing textures, and LREE-enrichment in clinopyroxenes and whole-rock compositions compared to Group 1 lherzolites. Group 3 lherzolites are interpreted to be the result of the interaction of depleted lithospheric mantle with a basaltic melt during the Cenozoic. The lack of correlation between the intensity of metasomatism, the degree of annealing, and the calculated temperatures of the lherzolites suggests that the Cenozoic basaltic melt percolation postdates the static recrystallization. Group 2 lherzolites exhibit characteristics from both Groups 1 and 3.Numerical modelling of the interaction between depleted lherzolites and basaltic melts evidences that: (i) a single initial liquid may fractionate to produce a range of element patterns: the observed spectrum of REE compositions of the Tariat lherzolites cannot have resulted from simple mixing of basaltic melt with a depleted mantle rocks; instead, it can be explained by chromatographic fractionation during reactive porous melt flow; (ii) highly fractionated element patterns are derived from conventional initial melt compositions and do not imply the existence of exotic melts; and (iii) strong element fractionation can be produced along short distances even at the thin section and mineral scale; this opposes the view that long percolation distances are required to produce significant chromatographic effects: the clinopyroxene core-rim disequilibrium demonstrates that REE variations in clinopyroxene rims were acquired in response to interactions with a more evolved REE-rich melt.

High‐Resolution Crustal and Uppermost Mantle Structure Beneath Central Mongolia From Rayleigh Waves and Receiver Functions

AbstractIn this study, I present a high‐resolution 3‐D Vs model of the crust and uppermost mantle beneath central Mongolia, based on seismic data recorded at the Central Mongolia Seismic Experiment array and some other networks. The model is constructed by a Bayesian Monte Carlo inversion that jointly interprets Rayleigh wave phase speeds and receiver functions. The Rayleigh wave dispersion measurements are derived from both ambient noise and teleseismic earthquake data. To improve ambient noise data coverage, a recently developed three‐station interferometry (Zhang et al., 2020, https://doi.org/10.1093/gji/ggaa046) technique is applied to produce high‐quality three‐station interferograms that augment the ambient noise data base. There are several prominent geological and tectonic features captured by the model, including (1) the Khövsgöl rift system is imaged as a high‐speed anomaly in the upper crust, whose southern edge follows the Bulnay fault. (2) The estimated crustal thickness map identifies relatively thick crust beneath the Khövsgöl rift system, the Hangay Dome, and the Gobi‐Altai Mountains. (3) A crustal high‐velocity anomaly is imaged beneath the Taryatu‐Chulutu volcanic field, which is interpreted as a solidified intrusive magma body. (4) Dispersed low‐velocity anomalies in the mantle identify locations with possible partial melting. The melting zones could be related to small‐scale asthenospheric upwelling and convective removal of lithosphere, which produces uplift of the Hangay Dome.

Lithospheric structures beneath the western Mongolian Plateau: Insight from S wave receiver function

• S wave receiver functions reveals lithospheric structures in western Mongolia. • Two velocity discontinuities (i.e., LAB and MLD) are notable in the upper mantle . • Lower part of the lithosphere beneath the Hangay Dome has been delaminated. The Mongolian Plateau is located in the western segment of the Central Asian Orogenic Belt. In the Cenozoic, the western plateau was affected by the India-Asian collision; the Hangay Dome of a low-relief plateau has been formed. However, the mechanism of the uplift of the plateau and particularly the Hangay Dome is debated. In this study, we image the lithosphere-asthenosphere boundary (LAB) with S wave receiver functions. The LAB is much shallower beneath the western Mongolian Plateau (~80–90 km) than beneath the North China Craton (~150 km) and Siberian Craton (~180 km). We also found another possible deep LAB at 200–250 km depths beneath the Hangay Dome, indicating a lithospheric delamination beneath western Mongolia. Thus we conclude that the consequential asthenospheric upwelling caused the tectonic uplift of the Hangay Dome.

Determining 1D velocity model from local earthquake data in the South Hangay region, central Mongolia

One dimensional (1D) velocity models are still widely used for computing earthquake locations at seismological centers. The location accuracy of an earthquake strongly depends on the velocity model used to compute the location. In the past, the local velocity model developed for the Hangay region was lacking precision due to insufficient data. Within the framework of the “Intracontinental Deformation and Surface Uplift- Geodynamic Evolution of the Hangay Dome, Mongolia, Central Asia” project [15], 72 seismic Broadband stations network were deployed in the Hangay Dome. This gives us an opportunity to estimate the crustal velocity structure of the South Hangay region using recorded local earthquake data. For this purpose, available velocity models for the South Hangay region have been re-evaluated. By simultaneous invertion P- and S-wave arrival times using VELEST algorithm, we estimated minimum 1D velocity models, station corrections, hypocentre locations, and origin times for the south Hangay region. Consequently, 1D crustal velocity model is proposed for the South Hangay region. This new model is expected to improve the accuracy of the routine hypocenter determination and as initial reference models for seismic tomography study.

Upper mantle and mantle transition zone thermal and water content anomalies beneath NE Asia: Constraints from receiver function imaging of the 410 and 660 km discontinuities

The 410 and 660 km discontinuities (d410 and d660) bordering the mantle transition zone (MTZ) beneath NE Asia, including NE China, Eastern Mongolia, and southern Siberia, are imaged in successive circular bins with a radius of 1 degree by stacking a total of 274,413 P-to-s radial receiver functions recorded by 799 broadband seismic stations. After moveout correction based on the 1-D IASP91 Earth model, the resulting apparent depths of the discontinuities exhibit significant and spatially systematic variations. Three approximately N-S elongated narrow zones with significantly thickened MTZ are observed, which may be associated with the thermal effect and dehydration of subducted slabs. The major volcanoes in NE China are underlain by a d660 that is apparently depressed by ∼19 km, which can be interpreted by the presence of an anomalously high water concentration in the lower MTZ released from the stagnated slabs. Low wavespeed anomalies above the d410 west of the Datong volcanic fields are underlain by an MTZ with normal thickness, and are attributable by dehydration of the leading portion of the stagnant Pacific slab in the MTZ that is revealed in an N-S oriented narrow zone east of this area. The lateral shift of the upper mantle low wavespeed zone and the area with thickened MTZ may suggest a westward drift of the upper mantle relative to the subducted slab. An abnormally thin MTZ is observed beneath the Hangay Dome in central Mongolia, suggesting the possible existence of thermal upwelling from the lower mantle through the MTZ.

Tomographic Pn Velocity and Anisotropy Structure in Mongolia and the Adjacent Regions

AbstractWe present new high‐resolution Pn velocity and anisotropy models beneath Mongolia and the adjacent regions by inverting 169,406 Pn arrivals. The data are selected from 786 permanent stations and 106 portable seismographs recently deployed in Mongolia and China's Inner Mongolia. The availability of new data acquired has allowed us to explore the uppermost mantle structures in great detail in this region, and the 2° × 2° model resolution is capable of distinguishing small‐scale geological features. Pn average velocity is 8.18 km/s, substantially faster than the global average. Distinct contrast in the velocity of the uppermost mantle is observed between the central part and two ends of the Baikal Rift, with the presence of higher velocities under the Central Baikal Rift, indicating strong variation of lithospheric thinning across the entire Baikal Rift. Low velocity beneath the Hangay Dome implies partial melting of mantle reflected by asthenospheric upwelling. Azimuthal anisotropy in the upper and lower mantle lid is measured by grouping the travel time data into two phases by epicenter distance. The obvious depth dependence of Pn anisotropy beneath the Hentey Mountains suggests different origins of fabrics. The lower anisotropy may origin from asthenospheric flow, while the shallower fabric may exhibit the preserved lattice preferred orientation anisotropy in the uppermost mantle. Meanwhile, depth‐dependent anisotropic structures and significantly low velocity are found beneath Tien Shan, most possibly suggesting that the lithospheric mantle thinning is experienced by either delamination or local asthenospheric upwelling.

Microstructures, Water Contents, and Seismic Properties of the Mantle Lithosphere Beneath the Northern Limit of the Hangay Dome, Mongolia

AbstractTo constrain the deformation, thermal evolution, and seismic properties of the mantle lithosphere beneath the Hangay Dome, we have analyzed the microstructures, crystal preferred orientations (CPO), and hydrogen concentrations of olivine and pyroxenes of 50 mantle xenoliths carried up by Cenozoic basalts from Zala, Haer, and Shavaryn‐Tsaram from Tariat, Mongolia. Most xenoliths are medium‐ to coarse‐grained spinel‐lherzolites, but four contain garnet + spinel. Coarse granular, highly annealed microstructures predominate. The microstructures are associated with well‐developed CPO, typical of deformation under high temperature, moderate pressure, and dry conditions. The hydrogen concentrations in olivine, orthopyroxene, and clinopyroxene are low and range around 5, 75, and 147 ppm H2O wt, respectively. Together, microstructures and CPO indicate that ductile deformation was followed by static recrystallization, which has annealed the microstructures but preserved the CPO and, hence, the anisotropy of physical properties. Lack of correlation between annealing and equilibrium temperatures suggest that the annealing is due to a long quiescence episode since the last deformation episode. Here, there is not evidence that the formation of Hangay Dome is associated with recent deformation in the lithospheric mantle. Calculated seismic properties show moderate seismic anisotropy, with fast propagation of P waves and polarization of S waves parallel to the flow direction and low birefringence for S waves propagating obliquely to the flow plane. The results are consistent with weak P wave anomalies but not with the strong low S wave velocity anomalies predicted by some tomographic models or with the high conductivity inferred from magnetotelluric data for the lithospheric mantle beneath the Hangay Dome.

Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

Paleotopography and erosion rates in the central Hangay Dome, Mongolia: Landscape evolution since the mid-Miocene

Standing over 2km above the surrounding topography and flanked by orogen-scale strike-slip faults, the Hangay Dome in central Mongolia is characterized by long wavelength high topography, basaltic eruptions spanning 30million years, and an abundance of flat-topped summit plateaus. However, despite decades of research, the origin and timing of the intraplate Hangay Dome uplift continues to be debated. Using Landsat imagery, GIS, and cosmogenic beryllium-10, we employ geomorphic investigations of (1) paleotopography preserved beneath basalt flows of known age, (2) erosion rates at various temporal scales, and (3) the relative contribution of glacial activity to total erosion to provide insight into the nature of landscape evolution in the Egiin Davaa region of the central Hangay Dome since the middle Miocene. Reconstruction of paleo-valleys cut into Paleozoic basement rock that exhibit a degree of local relief (>600m) similar to the modern landscape, sluggish mean erosion rates (<45mMyr−1), and dominance of glacial erosion suggest that there has been no dramatic change in tectonic forcing of the study area since ∼13Ma, and that high amplitude climate oscillations beginning in the Pliocene have led to an environment influenced primarily by the activity of glaciers. These results provide support for uplift onset during the Oligocene or early Miocene, quantify landscape evolution since the middle Miocene, and underscore the importance of considering geomorphic archives found on Earth’s surface when building models of intra-continental epeirogeny.

Links between climate, erosion, uplift, and topography during intracontinental mountain building of the Hangay Dome, Mongolia

The Hangay mountain range, a dome in central Mongolia, provides a window into understanding how climate influences the erosion and resulting geomorphic and sedimentary signatures of continental topography. Specifically, asymmetric erosion of the Hangay, associated with a distinct orographic precipitation gradient, offers a natural experiment for exploring uplift, erosion, and the isostatic response to erosional unloading. The flat‐topped Hangay peaks preserve low‐relief remnant surfaces that provide markers of rock uplift. This makes it possible to map the deformation of a former planar surface during doming and hence to estimate the total extent of erosion by the difference from present day topography. Erosion into the Hangay surface has been significant but incomplete; the morphology of the range indicates a nonequilibrium landscape that may have persisted for millions to tens of millions of years, implying a long response time in this semiarid climate. The extent of erosion across the range correlates with mean annual precipitation. Variability in present‐day peak heights across the north‐south climatic and erosional gradient provides empirical support for the generally accepted theory that climate‐driven erosion will increase the height of mountain peaks by generating greater surface uplift through isostasy. Correction for this isostatic response makes it possible to reconstruct primary surface uplift of the Hangay. Results highlight the importance of considering the interplay between climate, erosion, and uplift in shaping intracontinental topography and thus when interpreting the geomorphic, sedimentary, and geodynamic signatures associated with such topography.

Tectonic setting and structural evolution of the Late Cenozoic Gobi Altai orogen

Abstract The Gobi Altai is an intraplate, intracontinental transpressional orogen in southern Mongolia that formed in the Late Cenozoic as a distant response to the Indo-Eurasia collision. The modern range formed within crust constructed by successive terrane accretion and ocean suturing events and widespread granite plutonism throughout the Palaeozoic. Modern reactivation of the Gobi Altai crust and the kinematics of Quaternary faults are fundamentally controlled by Palaeozoic basement structural trends, the location of rigid Precambrian blocks, orientation of SH max and possible thermal weakening of the lower crust as a result of an extensive history of Mesozoic–Cenozoic basaltic volcanism in the region, and the presence of thermally elevated asthenosphere under the Hangay Dome to the north. Modern mountain building processes in the Gobi Altai typically involve reactivation of NW–SE-striking basement structures in thrust mode and development of linking east–west left-lateral strike-slip faults that crosscut basement structures within an overall left-lateral transpressional regime. Restraining bends, other transpressional ridges and thrust basement blocks are the main range type, but are discontinuously distributed and separated by internally drained basins filling with modern alluvial deposits. Unlike a contractional thrust belt, there is no orogenic foreland or hinterland, and thrusts are both NE and SW directed with no evidence for a basal décollement. Normal faults related to widespread Cretaceous rifting in the region appear to be unfavourably oriented for Late Cenozoic reactivation despite widespread topographic inversion of Cretaceous basin sequences. Because the Gobi Altai is an actively developing youthful mountain range in an arid region with low erosion rates, it provides an excellent opportunity to study the way in which a continental interior reactivates as a result a distant continental collision. In addition, it offers important insights into how other more advanced intracontinental transpressional orogens may have developed during earlier stages of their evolution.

Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2–0.3 m s−1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.