Lhasa Block Research Articles

Geochemistry of Miocene trachytes in Bugasi, Lhasa block, Tibetan Plateau: Mixing products between mantle- and crust-derived melts?

It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of an enriched mantle region or lower crustal materials. The Miocene Bugasi volcanic rocks (BVR) in the western part of the Lhasa block are composed mainly of trachyandesites and trachytes, both of which belong to the shoshonite series. The trachytes show somewhat transitional compositions between the mantle-derived trachyandesites of the BVR and the crust-derived potassic rocks of Konglongxiang, most evident in their Sr, Ba, and Ni concentrations, Nb/Ta, Rb/Sr, Th/Nb, Zr/Nb, and Ba/Rb trace element ratios, and Sr and Nd isotopic compositions. These features, coupled with the relatively high Cr and Ni concentrations and Mg#, suggest that the trachytes are the product of mixing between mantle-derived and lower crust-derived melts.

Crustal–lithospheric structure and continental extrusion of Tibet

Abstract: Crustal shortening and thickening to c . 70–85 km in the Tibetan Plateau occurred both before and mainly after the c . 50 Ma India–Asia collision. Potassic–ultrapotassic shoshonitic and adakitic lavas erupted across the Qiangtang ( c . 50–29 Ma) and Lhasa blocks ( c . 30–10 Ma) indicate a hot mantle, thick crust and eclogitic root during that period. The progressive northward underthrusting of cold, Indian mantle lithosphere since collision shut off the source in the Lhasa block at c . 10 Ma. Late Miocene–Pleistocene shoshonitic volcanic rocks in northern Tibet require hot mantle. We review the major tectonic processes proposed for Tibet including ‘rigid-block', continuum and crustal flow as well as the geological history of the major strike-slip faults. We examine controversies concerning the cumulative geological offsets and the discrepancies between geological, Quaternary and geodetic slip rates. Low present-day slip rates measured from global positioning system and InSAR along the Karakoram and Altyn Tagh Faults in addition to slow long-term geological rates can only account for limited eastward extrusion of Tibet since Mid-Miocene time. We conclude that despite being prominent geomorphological features sometimes with wide mylonite zones, the faults cut earlier formed metamorphic and igneous rocks and show limited offsets. Concentrated strain at the surface is dissipated deeper into wide ductile shear zones.

Lithologic Boundaries in Permian Post‐glacial Sediments of the Gondwana‐affinity Regions of China: Typical Sections, Age Range and Correlation

Abstract:In Southwest China, Gondwana‐affinity Permo‐Carboniferous deposits are known to occur in the Northern Himalayas, the Lhasa Block and the South Qiangtang Block, the Baoshan Block and the Tengchong Bock. The three‐fold clastic successions, namely diamictite–pebbly mudstone–dark mudstone and shale are commonly interpreted as representing deposits of glacial–deglacial–post‐glacial periods in marine environments. Deposits, for example, the Dingjiazhai Formation in the Baoshan Block, the Kongshuhe Formation in the Tengchong Block, the Yongzhu Group and the Poindo Group in the Lhasa Block, are all succeeded by carbonate deposits. This marks a significant change from post‐glacial clastic environment to a carbonate environment. Available paleontological data show that the change from post‐glacial clastic environment to carbonate environment took place in the Baoshan, Tengchong and Lhasa Block at the beginning of the Artinskian. The carbonate environment in the Baoshan Block was spoiled by the eruption of the Woniusi Basalts, and in the Xainza area of the Lhasa Block was shortly replaced by clastic environment until the Kungurian. In the northern Himalayas limestones began to occur in the Late Permian in a fluctuating manner.

Effects of fault movement and material properties on deformation and stress fields of Tibetan Plateau

We compare the factors which affect the movement of Tibetan Plateau by building three types of finite element models: an elastic materials (M-EC), a continuous model composed by non-linear materials (M-PC), and an elastic model with discontinuous fault movements (M-ET). Both in M-ET and M-EC, the materials in Qiangtang and Lhasa block are elastic, and in M-ET, discontinuous movement of faults is considered for evaluating the effects of strike-slip faults. In model M-PC Druker-Prager plastic materials are used in Qiangtang and Lhasa block. Comparisons of the numerical simulation and the GPS observations show following characteristics: (1) Under present tectonic environment, short-term deformation of Tibetan Plateau can be simulated well by elastic models; (2) Discontinuous fault activities increase the lateral extrusion of the eastern part of Tibetan Plateau, reduce the stress field level in Qiangtang, Tarim and Qaidam blocks and strengthen the E-W extensional force in the east and the west parts of Qiangtang block; (3) Properties of plastic materials reduce the total stress field and the E-W extensional force, thus, the normal fault earthquakes in southern Tibet is mainly owed to the effect of active fault movement. Based on the numerical simulations we speculate that faults movement may play a more important role on the kinematic pattern of Tibetan Plateau than bulk properties.

3-D lithospheric structure beneath southern Tibet from Rayleigh-wave tomography with a 2-D seismic array

SUMMARY A 3-D shear wave velocity model has been established for the lithosphere of southern Tibet through Rayleigh-wave tomography. The teleseismic data are acquired from a 2-D seismic array selected out of the second phase of the Hi-CLIMB project. We first inverted for a 2-D phase velocity model using two-plane-wave tomography approach, and then inverted for the 3-D shear wave velocity model with the 2-D phase velocity model and the map of crustal thickness derived from receiver functions. The results reveal a pervasive low velocity anomaly in the middle crust of the Lhasa block. We interpret this low velocity anomaly as the presence of wholesale mid-crustal channel flow. Prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north–south trending rifts: the Tangra Yum Co rift and the Pumqu Xianza rift. We propose a possible scenario for the formation of these rifts: the entire lithosphere is involved in the early-stage rifting, while the late-stage rifting is restricted in the upper crust. In the non-rift regions, a slab-like high velocity anomaly is traced beneath both the Himalayas and Lhasa block. Combining all the observations, we suggest that the central part of southern Tibet is currently underlain by the broken Indian lower crust and mantle lithosphere.

Geomagnetic polarity transitions recorded in the Miocene lavas of the Wuyu basin, Tibet

Geomagnetic polarity transitions were the most important phenomena in the study of the geomagnetic field evolution and the earth’s deep interior dynamics. The Miocene postcollisional ultrapotassic-potassic rocks are widely distributed in the Lhasa Block of southern Tibet. Paleomagnetic studies on the Wuyu Formation indicated that these volcanic rocks recorded the C5Bn.2n-C5Bn.1r and C5ACr-C5ACn geomagnetic polarity transitions. The virtual geomagnetic poles (VGPs) of these two polarity transitions were located in the Atlantic and the central Pacific, respectively. Due to data scarcity, it remains unclear whether VGP paths of Miocene geomagnetic polarity transitions were longitudinal preference and/or 90° away from the sampling sites.

Rifting and strike-slip shear in central Tibet and the geometry, age and kinematics of upper crustal extension in Tibet

Abstract The youngest deformation structures on the Tibet Plateau are about NNE-trending grabens. We first combine remote-sensing structural and geomorphological studies with structural field observations and literature seismological data to study the Muga Purou rift that stretches at c . 86°E across central Tibet and highlight a complex deformation field. ENE-striking faults are dominated by sinistral strike–slip motion; NNE-striking faults have normal kinematics and outline a right-stepping en-echelon array of grabens, also suggesting sinistral strike–slip; along NW-striking fault sets, the arrangement of grabens may indicate a dextral strike–slip component. Thus, in central Tibet, rifts comprise mostly grabens connected to strike–slip fault zones or are arranged en-echelon to accommodate sinistral wrenching; overall strain geometry is constrictional, in which NNE–SSW and subvertical shortening is balanced by WNW–ESE extension. The overwhelmingly shallow earthquakes only locally outline active faults; clusters seem to trace linkage or propagation zones of know structures. The earthquake pattern, the neotectonic mapping, and the local fault–slip analyses emphasize a distributed, heterogeneous pattern of deformation within a developing regional structure and indicate that strain concentration is weak in the uppermost crust of central Tibet. Thus, the geometry of neotectonic deformation is different from that in southern Tibet. Next, we use structural and palaeomagnetic data along the Zagaya section of southern central Tibet to outline significant block rotation and sinistral strike–slip SE of the Muga Purou rift. Our analysis supports earlier interpretations of reactivation of the Bangong–Nujiang suture as a neotectonic strike–slip belt. Then, we review the existing and provide new geochronology on the onset of neotectonic deformation in Tibet and suggest that the currently active neotectonic deformation started c . 5 Ma ago. It was preceded by c . north–south shortening and c . east–west lengthening within a regime that comprises strike–slip and low-angle normal faults; these were active at c . 18–7 Ma. The c . east-striking, sinistral Damxung shear zone and the c . NE-trending Nyainqentanghla sinistral-normal detachment allow speculations about the nature of this deformation: the ductile, low-angle detachments may be part of or connect to a mid-crustal décollement layer in which the strike–slip zones root; they may be unrelated to crustal extension. Finally, we propose a kinematic model that traces neotectonic particle flow across Tibet and speculate on the origin of structural differences in southern and central Tibet. Particles accelerate and move eastwards from western Tibet. Flow lines first diverge as the plateau is widening. At c . 92°E, the flow lines start to converge and particles accelerate; this area is characterized by the appearance of the major though-going strike–slip faults of eastern-central Tibet. The flow lines turn southeastward and converge most between the Assam–Namche Barwa and Gongha syntaxes; here the particles reach their highest velocity. The flow lines diverge south of the cord between the syntaxes. This neotectonic kinematic pattern correlates well with the decade-long velocity field derived from GPS-geodesy. The difference between the structural geometries of the rifts in central and southern Tibet may be an effect of the basal shear associated with the subduction of the Indian plate. The boundary between the nearly pure extensional province of the southern Tibet and the strike–slip and normal faulting one of central Tibet runs obliquely across the Lhasa block. Published P-wave tomographic imaging showed that the distance over which Indian lithosphere has thrust under Tibet decreases from west to east; this suggests that the distinct spatial variation in the mantle structure along the collision zone is responsible for the surface distribution of rift structures in Tibet. Supplementary material: Containing supporting data is available at http://www.geolsoc.org.uk/SUP18446 .

Preservation of a large-scale bedrock peneplain suggests long-term landscape stability in southern Tibet

Extensive areas with low relief constitute the dominant geomorphic element of the central and southern Tibetan Plateau and were previously interpreted to result from the progressive filling of internally drained basins with sediments derived from adjacent mountain ranges that have been largely eroded. Here we document a completely different low-relief landscape in the Lhasa block of southern Tibet (~31°N, ~90°E), where a large-scale peneplain carved into Cretaceous granitoids is well preserved north of lake Nam. Around the town of Bangoin this bedrock peneplain occurs at a constant elevation of ~5,300 m in a region not affected by active faulting. The main planation surface has been incised by streams resulting in a local relief of up to 700 m. Progressive fluvial incision generated additional flat bedrock surfaces of limited extent that occur at lower elevations than the peneplain. Local hillslope gradients in the peneplain region increase with decreasing elevation, which is in marked contrast to the morphology of nearby mountain ranges that are uplifted in the footwall blocks of active normal faults. The subsiding hanging walls of these normal faults provide the necessary accomodation space for the sediment produced by erosion of the peneplain region. These relations demonstrate that the bedrock peneplain is the most stable geomorphic element present in southern Tibet.

Late Guadalupian (Middle Permian) Fusuline Fauna from the Xiala Formation in Xainza County, Central Tibet: Implication of the Rifting Time of the Lhasa Block

A fusuline fauna consisting of 9 species of 4 genera from the Xiala Formation of the Mujiucuo section, Xainza County, Tibet, China is described. The fusuline fauna is dominated byNankinellaandChusenellaand indicates a Midian (Late Guadalupian) age. The earliest record of fusuline fauna during the Midian in the Lhasa Block suggests that the block rifted later than the Qiangtang Block to the north and the Baoshan and Tengchong blocks to the east, all of which yield much earlier fusuline faunas of Yakhtashian (Artinskian) age, but had drifted away from Gondwana to a relatively warm temperate zone in the Late Guadalupian (Middle Permian).

Metamorphism, anatexis, zircon ages and tectonic evolution of the Gongshan block in the northern Indochina continent—An eastern extension of the Lhasa Block

The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~ 22 Ma. Leucosome and S-type granite of 22–53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages. This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore, the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~ 53–22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.

Position of the Lhasa terrane prior to India-Asia collision derived from palaeomagnetic inclinations of 53 Ma old dykes of the Linzhou Basin: constraints on the age of collision and post-collisional shortening within the Tibetan Plateau

The Lhasa terrane comprises the southern Eurasian margin since at least mid-Cretaceous. Therefore paleomagnetic data from the Lhasa terrane are crucial to reconstruct the pre-collisional southern margin of Eurasia. Previous paleomagnetic data from the Cretaceous Takena Formation provide a rough constraint on the pre-collisional paleolatitudinal position of the southern Eurasian margin. However, Paleogene paleomagnetic data from the Lhasa terrane, which can provide more direct constraints, are very limited. In addition, reliability of the existing data is ambiguous because of a possible later overprint. In our new study 17 sites were drilled from mafic dykes intruded in the T1 unit of the Linzizong Formation of the Linzhou Basin near Lhasa for paleomagnetic studies (Figure 1). The dykes were Ar 40 /Ar 39 dated to c. 53 Ma by Yue and Ding (2006). From ten sites a higher coercivity component demagnetized between 20 and 100 mT could be isolated. Detailed rockmagnetic analyses reveal Ti-rich titanomagnetite as the remanence carrier, which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin. This is supported by a positive fold test. The occurrence of normal and reverse polarity directions within single sites, the subvolcanic character of the dykes, and the within-site scatter, as well as the scatter between site mean directions, indicate that paleosecular variation is recorded within sites. Tilt correction was performed using the bedding of overlying fluvial- lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement. Bedding-corrected directions give an overall mean direction of D/I = 12.3°/27.2° (α95 = 10.6°, k = 21.7, N = 10) corresponding to a paleolatitude of 13.2° ± 5.8°N at a reference location on the suture zone (20°N/90°E). Comparison with previous Cretaceous data mainly from the Takena Formation yields a stable position of the Lhasa terrane during Cretaceous and Early Eocene. This indicates that no large north-south crustal shortening occurred along the southern Eurasian margin between Late Cretaceous and Early Eocene. The difference between expected paleolatitudes determined from the APWP of Eurasia and observed paleolatitudes reveals relative northward movement of the Lhasa block of ca. 1847±763 km since Early Eocene. This is attributed to indentation of India into Asia and implies a considerable amount of north-south crustal shortening. Two models of “Greater India” were used to determine the extended northern margin of India: Ali and Aitchison (2005) proposed a Greater India based on bathymetric features in the Eastern Indian Ocean and Patzelt and others (1996) determined the paleolatitude of northern India using paleomagnetic investigations on Paleocene sediments of the Zongpu Formation at Gamba and Duela. Intersection of the pre-collisional southern Eurasian margin and the northern margin of India yields a collision age of the Indian and Eurasian continents of c. 5349 Ma. An initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data (e.g. Patriat and Achache 1984, Klootwijk and others, 1992, Guillot and others, 2003). A significantly later collision age at the Eocene/Oligocene boundary as proposed by Aitchison and others (2007) is not in agreement with our new and previous paleomagnetic data from the Lhasa Block.

New paleomagnetic results of Paleocene volcanic rocks from the Lhasa block: Tectonic implications for the collision of India and Asia

A combined geochronological and paleomagnetic investigation has been performed on Paleocene volcanic sequences in the Lhasa block near the localities of Mendui (30.1°N/90.9°E). A total of 15 sites have been sampled from rhyolitic tuffs. Stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I = 359.0°/26.1° with α 95 = 9.2° and N = 14 sites, corresponding to a paleopole at 73.6°N, 274.3°E with A 95 = 7.3°. Positive fold tests suggest a primary origin for the characteristic remanence. In order to provide a more accurate pole, we propose to combine site-mean directions from this study and Achache et al.'s (1984). The combined average palaeomagnetic direction from early Paleocene volcanic rocks is D = 356.6°, I = 25.9°, κ = 21.7, α 95 = 6.8° after tilt correction, N = 22 sites, corresponding to a pole at 73.2°N, 282.4°E with A 95 = 5.4°. The paleomagnetic results yield a paleolatitude of 13.6 ± 5.4°N for the southern margin of Eurasia at ∼ 55 Ma. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, significant crustal shortening of 1400 ± 600 km and 2000 ± 550 km respectively may have occurred between the southern margin of Eurasia and the stable India, and within Eurasia since the collision of India and Eurasia.

New constraints to the onset of the India–Asia collision: Paleomagnetic reconnaissance on the Linzizong Group in the Lhasa Block, China

To better understand the onset of the India–Asia collision, a paleomagnetic study has been conducted on Paleogene volcanic rocks and sediments of the Linzizong Group (∼ 64–44 Ma) from the Linzhou and Namling basins of the Lhasa Block at the southern margin of the Asian continent. Following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully defined from the Dianzhong (∼ 64–60 Ma), Nianbo and Pana (∼ 60–44 Ma) formations. These ChRMs are of dual polarity, and pass a fold test with some of them in volcanic rocks carried by oxidation-induced hematite; we thus interpret them as primary remanences. Paleomagnetic results from the bulk of the collection imply that the southern leading edge of Asia, i.e. the Lhasa Block, remained essentially stationary in low latitudes at ∼ 10°N without paleomagnetically-detectable tectonic rotation and latitudinal variation during the formation of the Linzizong Group. Consequently, a large-scale post-collisional northward convergence in the Tibetan plateau and central Asia is very likely to have occurred after formation of the Linzizong Group, and the definitive paleoposition of the Lhasa Block during the period embracing the interval between ∼ 64 and 44 Ma provides further constraints on the latitudinal position of the initial contact between India and Asia. This evidence rules out the model for a younger (∼ 35 Ma) India–Asia collision and implies that the initial India–Asia collision occurred between ∼ 65 and 50 Ma. Given that Greater India comprised the present Indian continent together with a ∼ 950 km extension beyond its northern margin prior to the rifting of Gondwana in the Middle Jurassic, we conclude that the initial India–Asia collision most probably occurred between ∼ 55 and 60 Ma.

New paleomagnetic results from the Lhasa block: Revised estimation of latitudinal shortening across Tibet and implications for dating the India–Asia collision

Knowing how and when the Tibetan Plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths. Previous paleomagnetic work suggested that intracontinental shortening from deformation related to the India–Asia collision played a major role in building the Tibetan Plateau. However, these studies were derived from red bed facies, which are often afflicted by inclination shallowing, leading to overestimates of continental shortening. We report new paleomagnetic data from the Lhasa block, the southern continental margin of Asia before the India–Asia collision. A high temperature, characteristic remanent magnetization (ChRM) was isolated from 43 sites of Late Cretaceous red beds, 32 sites of Late Cretaceous lava flows, and nine sites of Eocene tuffs. All the ChRM directions are significantly different from the present-day geomagnetic field, and the mean directions of red beds, lava flows and tuffs pass the fold test. The distribution patterns of ChRM directions derived from volcanic rocks are consistent with a secular variation model, suggesting that secular variation was likely averaged out. In contrast, the red bed ChRM directions exhibit an east–west elongated distribution pattern, indicative of inclination flattening. The mean ChRM inclination of the red beds, 23.5°±2.5°, is consistent with previous results, but significantly shallower than that of 41.9°±4.4° carried by the coeval lava flows. An elongation―inclination analysis of the red bed ChRM directions yielded an unflattened mean inclination of 39.7°, with 95% confidence limits between 37.7° and 41.9°. The paleolatitudes of the Late Cretaceous volcanic rocks and inclination-corrected sediments suggest about 10° less shortening occurred between Lhasa and Siberia than previous estimates, with total shortening lying between 3.3°±2.6° and 7.3°±5.4° since the Late Cretaceous. The mean ChRM inclination derived from the Eocene tuffs, 51.8°±5.2°, is indistinguishable from the expected reference direction. Our new results imply that India–Asia convergence was accommodated mainly through consumption of the India plate and yield a much younger age (ca. 43Ma) for the initial India–Asia collision than previously estimated by paleomagnetic data.

Quantifying crustal flow in Tibet with magnetotelluric data

Several classes of geodynamic models have been proposed to explain the evolution of the Himalayan orogen and the Tibetan plateau. Models invoking flow in a weak lower crustal layer have been successful in explaining a number of geological and geophysical observations. Geophysical observations indicate that the weak layer at mid- to lower crustal depths in Tibet may be the result of partial melting. Laboratory studies indicate an order reduction in strength for melt fractions of 5–10%. By relating these laboratory studies to magnetotelluric observations, which are sensitive to the presence of partial melts, it is possible to estimate the flow parameters associated with the channel flow model. Calculations show that in the Central Tibetan Plateau, if a layer consists of partially molten felsic rocks, then conductances in the range 7000–27,000 S will produce flow velocities of the order 1 cm/a. Beneath the southern part of the Lhasa block and the Qiangtang terrane magnetotelluric studies indicate conductances of up to 20,000 S. These conductances suggest effective viscosities of 2.5 × 10 18 to 3 × 10 20 Pa s, corresponding to flow velocities between 0.02 and 4.5 cm/a. Together with higher pressure gradients near the margins of the plateau these flow parameters clearly support the validity of the channel flow model in these parts of Tibet. Lower crustal conductances (3000 S) are reported beneath the northern Lhasa block and deformation by channel flow in this region is unlikely. Assuming a felsic rock composition, the minimum effective viscosity of the crust beneath the northern Lhasa block is estimated as 3 × 10 20 Pa s. The corresponding flow velocities do not exceed 0.04 cm/a unless higher temperatures than suggested by seismic observations are assumed.

Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite ( P = 14–18 kbar, T ≈ 800 °C) yield U–Pb ages of 24.0 ± 0.3 Ma and 18.8 ± 0.3 Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490 ± 3 Ma and 24.2 ± 0.4 Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0 ± 0.4 Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼ 24 Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17 Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼ 0.6 to ∼ 2.0 Ga with peak at 0.8–1.2 Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼ 600 Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼ 24 Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene.

Crustal structure variation along 30°N in the eastern Tibetan Plateau and its tectonic implications

We determined crustal structure along the latitude 30°N through the eastern Tibetan Plateau using a teleseismic receiver function analysis. The data came mostly from seismic stations deployed in eastern Tibet and western Sichuan region from 2004 to 2006. Crustal thickness and Vp/Vs ratio at each station were estimated by the H– k stacking method. On the profile, the mean crustal thickness and Vp/Vs ratio were found to be 62.3 km and 1.74 in the Lhasa block, 71.2 km and 1.79 near the Bangong–Nujiang suture, 66.3 km and 1.80 in the Qiangtang block, 59.8 km and 1.81 in the Songpan–Garze block, and 42.9 km and 1.76 in the Yangtze block, respectively. The estimated crustal thicknesses are consistent with predictions based on the topography and the Airy isostasy, except near the Bangong–Nujiang suture and in the Qiangtang block where the crust is 5–10 km thicker than predicted, indicating that the crust may be denser, possibly due to mafic underplating. We also inverted receiver functions for crustal velocity structure along the profile, which reveals a low S-wave velocity zone in the lower crust beneath the eastern Tibetan Plateau, although the extent of the low-velocity zone varies considerably. The low-velocity zone, together with previous results, suggests limited partial melting and localized crustal flow in the lower crust of the eastern Tibetan Plateau.

Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet.

Tectonic setting of the Nepal Himalaya and its potential for hydrocarbon exploration

Nepal lies at the collision zone between the Indian subcontinent and the Tibetan Plateau of the Eurasian continent. It is made up of enormous tectonic stacking of sedimentary and metamorphic rocks with granite intrusions that resulted from the collision and under-plating of the Indian Craton with the Lhasa block of Tibet. The five major tectonic zones separated from each other by thrust contacts from south to north are the Terai, Siwalik or Sub Himalaya, Lesser Himalaya, Higher Himalaya and Tibetan Tethys. On the northern margin of the Indian subcontinent, foreland sedimentary basins began to develop immediately after the terminal collision between the northward drifting Indian Plate and relatively passive Eurasian Plate in Late Eocene time. The southern part of Nepal known as the Terai and Siwalik foothill, lies in the northern margin of the Ganga Basin and Purnea Basin that extend from India. Such basins with thick accumulation of sediments are considered as the potential area for petroleum exploration.
 Regional scale seismic reflection, gravity and magnetic data combined with surface mapping and basin analysis have established the subsurface framework of southern Nepal. Geological settings potential for hydrocarbon prospects recognized in Nepal include structural traps related to normal faulting involving pre-Siwalik formation and thrusting involving Siwaliks, structural traps associated with frontal blind thrusts, anticlines and thrust-faults, basement controlled structures and stratigraphic pinchouts.
 Drilling data consists of only one well drilled in the eastern part of Nepal. Oil and gas seeps have been observed in Dailekh area emanating through deep faults. Geochemical analyses of these seep samples indicate that these oil and gas have geologic origin from mature source rocks. Various outcrop samples from different parts of the country have been found rich in organic carbon. Source-rock maturity basin modeling constructed for various sections indicates that the level of thermal maturity is within oil and gas generating window. The Potwar Basin to the west in Pakistan and Assam Basin to the east in India having similar geologic setting to that of Nepal are producing oil and gas for a long time. In the Indo-Gangetic Plain across the border on Indian side, many deep wells have recorded the presence of gas and high content of organic carbon. Assessment of the available data acquired so far indicate that there is a fairly good possibility of discovering petroleum resource in Nepal.

古生物地理から見た東アジアの古生代テクトニクス－南部北上古陸，ハンカ地塊，ラサ地塊の古地理－

古生物地理から見た東アジアの古生代テクトニクス－南部北上古陸，ハンカ地塊，ラサ地塊の古地理－