Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38 km thick crust beneath the Shillong Plateau which increases to ∼37–38 km beneath the Brahmaputra valley and ∼46–48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8 km along with a low velocity zone within ∼8–13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18 km and low velocity zone within ∼18–36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30⁰ toward northwest.     

Crustal structure in an onshore–offshore transitional zone near Hong Kong,northern South China Sea

To study the crustal structure beneath the onshore–offshore transitional zone, a wide-angle onshore–offshore seismic experiment was carried out in northern South China Sea near Hong Kong, using large volume airgun sources at sea and seismic stations on land. The crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the Dangan Islands based on the characteristics of Pg and Multiple Pg, and the crustal structure beneath the sedimentary layer is relatively simple. The Moho depth is about 25–28 km along the profile and the P-wave velocity increases gradually with depth. The velocities in the upper crust range from 5.5 to 6.4 km/s, while that in the lower crust is 6.4–6.9 km/s. It also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75–90 km distance, which suggests that the Littoral Fault Zone (LFZ) exists beneath the onshore–offshore transitional zone. The magnetism anomalies, bouguer gravity anomalies and active seismic zone along the coastline imply the LFZ is a main tectonic fault in the onshore–offshore area. Combined with two previously published profiles in the continental South China (L–G profile) and in the northern margin of South China Sea (OBS1993) respectively, we constructed a land-sea super cross-section about 1000 km long. The results show the onshore–offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was imaged along L–G profile. However, the high velocity layer (HVL) in the lower crust was detected along OBS1993. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the LFZ could be the boundary fault between them.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Crustal thickness and velocity structure beneath Singapore’s seismic network

We estimated the crustal thickness and velocity structure beneath the five stations comprising the Republic of Singapore’s seismic network. Our data set was composed of 697 teleseismic receiver functions and 7 months of broad-band data that was cross-correlated to produce inter-station Green’s functions. Surface wave group velocities were extracted from the Green’s functions to obtain dispersion data for a path from central Sumatra to Singapore in order to provide a complimentary data set to the receiver functions. Crustal thickness was estimated via an H − k stacking technique, and high-resolution 1D P-wave velocity profiles were generated beneath each station by jointly inverting receiver function stacks and the group velocity data using a linearised time-domain inversion scheme. Crustal thickness beneath four stations was found to be between 28.0 km and 32.0 km, while one station in the northeast of Singapore indicates 24.0 km thick crust. This implies a significant crustal thinning beneath Singapore over the lateral extent of 50.0 km. Inversion results exhibit several crustal features that are observable in the derived models at all five stations, indicating that they exist across Singapore as a whole. There appears to be an upper-crustal high-velocity zone beneath Singapore, underlain by a velocity inversion. Station NTU shows slower near-surface velocities than the other stations, consistent with its situation above the sedimentary Jurong formation. These results expand the available global velocity data set, as well as being useful for assessing the seismic hazard in Singapore.     

Crustal and lithospheric thinning beneath the seismogenic Kachchh rift zone,Gujarat (India): Its implications toward the generation of the 2001 Bhuj earthquake sequence

A high-resolution passive seismic experiment in the Kachchh rift zone of the western India has produced an excellent dataset of several thousands teleseismic events. From this network, 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions (for the 30–310° back-azimuth ranges), which show a positive phase at 4.5–6.1 s delay time and a strong negative phase at 8.0–11.0 s. These phases have been modeled by a velocity increase at Moho (i.e. 34–43 km) and a velocity decrease at 62–92 km depth. The estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3–7 km in crustal thickness and 6–14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un-rifted parts of the Kachchh rift zone. On an average, the Kachchh region is characterized by a thin lithosphere of 75.9 ± 5.9 km. The marked velocity decrease associated with the lithosphere–asthenoshere boundary (LAB), observed over an area of 120 km × 80 km, and the isotropic study of xenoliths from Kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of CO₂ rich lherzolite beneath the Kachchh seismic zone that might have caused by rifting episode (at 88 Ma) and the associated Deccan thermal-plume interaction (at 65 Ma) episodes. Thus, the coincidence of the area of the major aftershock activity and the Moho as well as asthenospheric upwarping beneath the central Kachchh rift zone suggests that these pockets of CO₂-rich lherzolite partial melts could perhaps provide a high input of volatiles containing CO₂ into the lower crust, which might contribute significantly in the seismo-genesis of continued aftershock activity in the region. It is also inferred that large stresses in the denser and stronger lower crust (at 14–34 km depths) induced by ongoing Banni upliftment, crustal intrusive, marked lateral variation in crustal thickness and related sub-crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the Kachchh seismic zone.     

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.     

Seismic signature of the mantle transition zone beneath eastern Tibet and Sichuan Basin

The strong interaction between the eastward flow escaping from Tibet and the rigid Sichuan Basin resulted in the rise of the Longmenshan. However, the detailed dynamics in the mantle remains controversial. In this study, the structure of the mantle transition zone (MTZ) beneath eastern Tibet and Sichuan Basin is investigated using 5080 receiver functions from 51 broadband stations. The depth of the 410 km discontinuity is close to the global average, except for the Longmenshan where the 410 and 660-km discontinuities are found to be depressed by up to 10–25 km and 5–10 km, respectively. The observed simultaneous depressions of the 410 and 660-km discontinuities distributed along the LMS, together with proofs from tomography and regional tectonics, suggest that asthenospheric flow sinks into the MTZ, resulting in a high velocity zone, as well as variation in the MTZ thickness. The depressions are not from the traditional Clapeyron slopes or temperature variation. Also, the depression of the 410 km discontinuity and the dehydration of wadsleyite are syngenetic, both of which originate from the dry mantle flow traveling across the old 410 km interface.     

Crustal structure in the Tengchong volcanic area and position of the magma chambers

In this paper, we extract 1500 P receiver functions in the Tengchong volcanic area from 211 teleseismic events recorded at nine digital seismic stations. A common conversion point stacking technique is used to improve the signal-to-noise ratio and to get the time delays of the Ps, PpPs, PsPs + PpSs phases within grids of 10 km × 10 km. Finally, the crustal thickness and Poisson’s ratio are calculated. The results show that the crustal thickness ranges from 28 to 40 km and the Poisson’s ratio ranges from 0.28 to 0.36. There exist two mantle-uplifting sites each with a horizontal scale of about 30 km × 30 km, one in Mazhan–Tengchong–Maanshan and the other in Wuhe–Longjiang–Tuantian. The high Poisson’s ratio is consistently located within these two sites. Recorded shocks with Ms > 2.0 reveal that most of the shocks are distributed around the two sites and few are located at the centers. The shocks, the geothermal distribution, and the crustal structure suggest that the magma is still active, and the two mantle-uplifting sites detected may be the positions of two magma chambers in the crust.     

Alkaline series related to Early-Middle Miocene intra-continental rifting in a collision zone: An example from Polatlı, Central Anatolia,Turkey

A large volcanic area (∼7600 km²), the Galatean Volcanic Province (GVP), developed in northwest Central Anatolia during the Miocene along the Neo-Tethys Ocean suture zone possibly by post-collisional processes. The GVP mainly comprises 20–14 My old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier (Late Cretaceous) subduction-related events. 100 km south of the GVP, near Polatlı, Ankara, basaltic rocks that cover large areas are intercalated with the Miocene deposits of the Beypazarı basin, an intra-continental subsidence zone at the southwest of the GVP. Field observations, geochemistry and K–Ar age dating of the Polatlı volcanites show that they are Early (19.9 Ma) to mid (14.1 Ma) Miocene in age, covering an area as large as 215 km². Variations in lava thickness and the thickness of the underlying silicified/baked zones suggest that the basaltic lavas erupted from a southern source, possibly from the Eskişehir fault zone, and flowed northwards. Most Polatlı samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent. They do not display any characteristic to suggest a subductional component. Although the GVP and Polatlı lavas formed close in time and space, they were derived from different mantle sources. Considering the positions of these two magmatic regions with regard to the Tethyan suture zone, we propose that the mantle beneath the GVP and near the suture zone memorised the earlier subduction while the mantle beneath Polatlı that is located about 100 km further from the suture zone remained apparently unchanged. After a significant volume of magma was consumed in the GVP, a later (∼10 My) and last activity (Güvem activity) has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear. Considering that the western Anatolian crust is proposed to undergo extension since the Late Oligocene–Early Miocene times, the Early Miocene intra-plate Polatlı activity may have developed within this extensional tectonic regime. Combined with regional data, Polatlı data also provide broad estimations on how long a subductional event continues to modify the mantle after the subduction ceased (at least ∼20 My), how long the subductional signature is preserved during significant magmatism (between 6 and 10 My) and how far the subductional effect disappears laterally on the mantle with respect to the collision zone (<100 km).     

Basement configuration of the Jhagadia–Rajpipla profile in the western part of Deccan syneclise,India from travel-time inversion of seismic refraction and wide-angle reflection data

2-D velocity structure up to the basement is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the SW–NE trending Jhagadia–Rajpipla profile, located on the western part of Deccan syneclise in the Narmada–Tapti region. The study region is mostly covered by alluvium. Inversion of refraction and wide-angle reflection data reveals four layered velocity structure above the basement. The first two layers with P-wave velocities of 1.95–2.3 km s^?1 and 2.7–3.05 km s^?1 represent the Recent and Quaternary sediments respectively. The thickness of these sediments varies from 0.15 km to 3.4 km. The third layer with a P-wave velocity of 4.8–5.1 km s^?1 corresponds to the Deccan volcanics, whose thickness varies from 0.5 km to 1.0 km. Presence of a low velocity zone (LVZ) below the high velocity volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data and the wide-angle reflection from top of the LVZ present immediately after the first arrival refraction from Deccan Trap layer. The thickness of the low velocity Mesozoic sediments varies from 0.3 km to 1.7 km. The basement with a P-wave velocity of 5.9–6.15 km s^?1 lies at a depth of 4.9 km near Jhagadia and shallows to 1.2 km towards northeast near Rajpipla. The results indicate presence of low velocity Mesozoic sediments hidden below the Deccan Trap layer in the western part of the Deccan syneclise.     

3D thermal structure of the continental lithosphere beneath China and adjacent regions

Based on the Crust2.0 model and the topography data of Chinese continent and its adjacent regions, a three-dimensional finite element model is constructed in terms of the spherical coordinate system. In our numerical model, the average annual ground temperature from 195 meteorological stations and temperature of upper mantle derived from the seismic velocities are adopted as the top and bottom boundary conditions, respectively. The observed thermal conductivity and heat production from P wave velocity based on empirical formula are employed in our numerical model as well. The comparison between the calculated and observed surface heat flow proved that our results are reliable. The temperature beneath the Precambrian cratons is lower than that of other areas for 100–300 °C also. The typical temperature rang at the Moho is estimated to be 800–1000 °C beneath the Tibetan plateau and 500–700 °C beneath the Precambrian cratons (such as Indian plate, Sichuan basin, South China, North China and Tarim), respectively. The thermal state in the eastern part of Sino-Korean craton at the depth deeper than 60 km indicates that it was destructed. The thermal structure in center of Tibetan plateau (especially beneath Qiangtang area) supports the proposed flow of lower crustal or upper mantle material to the east. Generally, the distribution of volcanoes in Chinese continent is consistent with the high temperature areas in the crust or upper mantle. There are many obvious thermal transition zones across the orogenic belts. The thermal transition zone between eastern and western parts in the crust of Chinese continent is consistent with the north–south seismic zone.     

A new map of the lava flow field of Nyamulagira (D.R. Congo) from satellite imagery

Nyamulagira (3058 m a.s.l.), a volcano of the Virunga volcanic province in the western branch of the East African Rift, is Africa’s most active volcano with one eruption every 2–4 years. It represents a hazard for the Virunga National Park and its vicinity. Despite such a frequent activity, Nyamulagira remains poorly studied. The only existing volcanological map was produced in the sixties by Thonnard et al. (1965). The occurrence of 19 eruptions since its publication makes it obsolete. In the present study we mapped the Nyamulagira lava flows from 1938 up to the last eruption to date in 2010 using optical (Landsat, ASTER) and radar (ENVISAT-ASAR, ERS, JERS) imagery. The results are integrated into a Geographical Information System (GIS) and coupled with additional data sources. GIS use makes the new database a flexible – and easy-to-update – tool for scientific purposes as well as for risk, environmental and humanitarian management. Here a new lava flow map was produced. Volumes of the successive lava flows and affected areas of the Virunga National Park were estimated.     

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m²) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high V_p (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (V_p = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.     

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120 s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15 km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1 km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2 km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150 km) beneath the Taiwan Strait is about 0.1–0.3 km/s lower than the AK135 model. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves.     

Mineral chemical constraints on the petrogenesis of mafic and intermediate volcanic rocks from the Erciyes and Hasandağ volcanoes,Central Turkey

Hasandağ and Erciyes stratovolcanoes, which produced both calc-alkaline and alkaline eruptive products, are the two important volcanic complexes in Central Anatolia. There are three geochemical evolution stages in the history of the Hasandağ strato volcanic complex: (1) Keçikalesi tholeiitic, (2) Hasandağ calc-alkaline and (3) Hasandağ alkaline. Volcanologic and petrologic characteristics of the Hasandağ and Erciyes calc-alkaline series show that water played an important role on the genesis of these rocks. These rocks are phenocryst-rich with vesicular texture, and contain hydrous mineral phases. The approximate pressure and temperature estimates obtained from the mineral chemistry studies of the Hasandağ strato volcanic complex indicate crystallization temperature of 1100 °C with 2.5–3.4 kbar pressure interval for the first stage of Keçikalesi tholeiitic volcanism, and about 850 °C temperatures with 4.3–9.6 kbar pressure intervals for the second stage of Hasandağ calc-alkaline volcanism.The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline, (2) Koçdağ calc-alkaline and (3) Erciyes calc-alkaline. The temperature of Koçdağ alkaline volcanism is 1097–1181 °C and in a range of 5.1–6.7 kbar pressure, for Koçdağ calc-alkaline volcanism 850–1050 °C temperature to 2.0–6.6 kbar pressure interval, and for Erciyes calc-alkaline volcanism about 950 °C temperature, to 3.2–7.9 kbar pressure intervals were calculated. Polybaric origin of magma chambers for calc-alkaline and alkaline rocks and disequilibrium parameters observed in phenocrysts indicate that the rocks were affected by magma mixing processes in crustal magma chambers. The disequilibrium features of amphibole and plagioclase phenocrysts in these rocks point the latent heat in magma chambers and periodic recharging with mafic magma chambers and also show that magmas reequilibrate before the eruption.     

Volcanic eruptions in the Longgang volcanic field,northeastern China,during the past 15,000 years

The Longgang volcanic field, located in northeastern China, is volcanically active with a number of eruptions during the Quaternary but the chronology of the eruptions is poorly defined. Some tephra layers are well preserved in the annually laminated sediments of maar lakes in the region, and facilitate the construction of a much improved chronological framework for the volcanic history of the area. The results of our investigations reveal that three basaltic explosive eruptions occurred at AD 460, 11460 cal yr BP and 14000 cal yr BP, respectively. The largest explosive basaltic eruption (AD 460) produced a thick black scoria layer in the Longgang volcanic field, including lakes. The tephra distribution and chronological data suggest that this eruption is likely to be from the Jinlongdingzi volcano. Two basaltic flood eruptions occurred at Jinlongdingzi. The earlier basaltic eruption produced a lava flow that spread over a forest and encased standing trees. Two radiocarbon ages obtained from charcoal samples collected from the burned remains of these trees are 1828–1989 cal yr BP and 2164–2359 cal yr BP. In the most recent stage of volcanism, the lava flow extended only ca. 2 km, and flowed into Lake Dalongwan. From the present status of the forest ecosystem, which has not yet reached the fully mature successional stage, we estimate that this lava is very young (ca. a few hundreds years old). Jinlongdingzi is a potentially dangerous volcano. Monitoring and assessment of the potential hazards in the Longgang volcanic field should be carried out in the future.     

Crustal structure of the Gujarat region,India: New constraints from the analysis of teleseismic receiver functions

In this study, receiver function analysis is carried out at 32 broadband stations spread all over the Gujarat region, located in the western part of India to image the sedimentary structure and investigate the crustal composition for the entire region. The powerful Genetic Algorithm technique is applied to the receiver functions to derive S-velocity structure beneath each site. A detail image in terms of basement depths and Moho thickness for the entire Gujarat region is obtained for the first time. Gujarat comprises of three distinct regions: Kachchh, Saurashtra and Mainland. In Kachchh region, depth of the basement varies from around 1.5 km in the eastern part to 6 km in the western part and around 2–3 km in the northern part to 4–5 km in the southern part. In the Saurashtra region, there is not much variation in the depth of the basement and is between 3 km and 4 km. In Gujarat mainland part, the basement depth is 5–8 km in the Cambay basin and western edge of Narmada basin. In other parts of the mainland, it is 3–4 km. The depth of Moho beneath each site is obtained using stacking algorithm approach. The Moho is at shallower depth (26–30 km) in the western part of Kachchh region. In the eastern part and epicentral zone of the 2001 Bhuj earthquake, large variation in the Moho depths is noticed (36–46 km). In the Saurashtra region, the crust is more thick in the northern part. It varies from 36–38 km in the southern part to 42–44 km in the northern part. In the mainland region, the crust is more thick (40–44 km) in the northern and southern part and is shallow in Cambay and Narmada basins (32–36 km). The large variations of Poisson’s ratio across Gujarat region may be interpreted as heterogeneity in crustal composition. High values of σ (∼0.30) at many sites in Kachchh and few sites in Saurashtra and Mainland regions may be related to the existence of high-velocity lower crust with a mafic/ultramafic composition and, locally, to the presence of partial melt. The existing tectono-sedimentary models proposed by various researchers were also examined.     

Seismic structure and rheology of the crust under mainland China

The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65 km beneath the Tibetan Plateau, about 35 km beneath South China, and about 36–38 km beneath North China and Northeastern China. For the basins, the thickness is ~ 55 km beneath Qaidam, ~ 50 km beneath Tarim, ~ 40 km beneath Sichuan and ~ 35 km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures.     

Major element composition of concentrate garnets in Proterozoic kimberlites from the Eastern Dharwar Craton,India: Implications on sub-continental lithospheric mantle

This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr₂O₃ contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.     

Tectonic controls on post-subduction granite genesis and emplacement: The late Caledonian suite of Britain and Ireland

Rates of magma emplacement commonly vary as a function of tectonic setting. The late Caledonian granites of Britain and Ireland are associated with closure of the Iapetus Ocean and were emplaced into a varying regime of transpression and transtension throughout the Silurian and into the early Devonian. Here we evaluate a new approach for examining how magma volumes vary as a function of tectonic setting. Available radiometric ages from the late Caledonian granites are used to calculate probability density functions (age spectra), with each pluton weighted by outcrop area as a proxy for its volume. These spectra confirm an absence of magmatic activity during Iapetus subduction between c. 455 Ma and 425 Ma and a dominance of post-subduction magmas between c. 425 Ma and 380 Ma. We review possible reasons why, despite the widespread outcrop of the late Caledonian granites, magmatism appears absent during Iapetus subduction. These include shallow angle subduction or extensive erosion and tectonic removal of the arc.In contrast to previous work, we find no strong difference in the age or major element chemistry of post-subduction granites across all terranes. We propose a common causal mechanism in which the down-going Iapetus oceanic slab peeled back and detached beneath the suture following final Iapetus closure. The lithospheric mantle was delaminated beneath the suture and for about 100 km back beneath the Avalonian margin. While magma generation is largely a function of gravitationally driven lithosphere delamination, strike slip dominated kinematics in the overlying continental crust is what modulated granitic magma emplacement. Early Devonian (419–404 Ma) transtension permitted large volumes of granite emplacement, whereas the subsequent Acadian (late Early Devonian, 404–394 Ma) transpression reduced and eventually suppressed magma emplacement.