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.     

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.     

Chalcophile element constraints on magma differentiation of Quaternary volcanoes in Tengchong,SW China

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.     

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.     

Crustal lineament control on mineralization in the Anarak area of Central Iran

The Anarak mining area in Iran is identified as one of the best known examples of mineralization located at the intersection of two important crustal-scale lineaments, which are the E_W striking Great Kavir fault and the NW–SE trending Urumieh-Dokhtar Magmatic Belt. The objective of this research is to investigate the role of the lineaments and their subsidiaries at Anarak area in the genesis of mineral deposits. Lineaments are long-lived features which control or affect ancient as well as young crustal sequences. Lineaments are also deep-seated, because they commonly control the emplacement of deep crustal or mantle-derived magmas. Metallogenic areas of up to ~ 1000 km² might be observed in areas where broad structural zones intersect, because they can facilitate the localization of magmatic or hydrothermal centers. The two lineaments that intersect in the Anarak area were both very active during mineralization. Therefore, it is likely that pull-apart basins or other dilatational sites at the intersection point were not preserved for substantial lengths of time, and single large magma conduits would have been divided into smaller conduits for magma penetration and hydrothermal circulation. The presence of these small conduits in addition to a small magma input could produce conditions for the circulation of mainly meteoric waters, in which the magmatism has acted as a heat source for the mineralization system. This phenomenon has led to the generation of a metallogenic area with several similar small deposits within a circular area of radius ~ 30 km centered at the intersection point. The two most famous deposits in this area are Talmessi and Meskani in which mineralization had occurred in two separate stages: first stage — fissure-filling copper sulfide mineralization associated with Eocene magmatism (veins, veinlets, and stockworks). The second is an overprinting stage which occurred after a fairly long interval which involves the formation of Ni–Co arsenides and U oxide minerals.     

3D geological modeling for prediction of subsurface Mo targets in the Luanchuan district,China

Three-dimensional (3D) district-scale geoscience information for the Luanchuan Mo district was integrated for understanding the development of its regional geology and ore-forming processes and for decision-making about potential targets for mineral exploration. The methodology and datasets used were: (1) construction of an initial geological model (25 km × 20 km × 2.5 km) using 1:10,000 scale geological map, nine geological cross-sections and gravity and magnetic data; (2) construction of three large-scale Mo deposits model (5 km × 4 km × 2.5 km) using 1:2000 scale geological and topographic maps, 288 boreholes (total core length of 158,700 m), and 32 1:2000 scale cross-sections; (3) 3D inversion of 1:25,000 scale gravity and magnetic data for identification metallogenic anomaly zones which are associated with Jurassic intrusions; (4) extraction of ore-controlling formation and sequence of the Luanchuan Group using the large-scale 3D models of Mo deposits and results of analysis of lithogeochemical samples from outcrops and borehole cores; (5) identification of ore-forming and ore-controlling faults using the large-scale 3D model of Mo deposits and mineralized Jurassic granite porphyry stocks; (6) boost weights-of-evidence and concentration–volume (C–V) fractal analyses to integrate metallogenic information and to identify and classify potential Mo targets. Four classes of exploration targets were identified using C–V modeling and 3D known orebodies model: the first and second class targets are mainly located in three large magma-skarn type deposit camps, occupying ~ 1.4 km³ with total estimated reserve of ~ 2.3 Mt; the third class targets, which are mainly located in Huangbeiling and Yuku deposit camps comprising concealed magma-skarn type deposits, occupy ~ 2.8 km³ and represent a new target exploration zone in the Luanchuan district; the fourth class targets, which are located in the Huoshenmiao, Majuan, and Daping zones, occupy ~ 15 km³ and represent potential mineral resources with likely similar orebody features as the Yuku deposit.     

High-temperature,high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian,NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.     

Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent

We here present the results of the inverse modeling of crustal S-phases recorded from a 400-km-long seismic profile, with azimuth nearly N30W, from Lianxian, near Hunan Province, to Gangkou Island, near Guangzhou City, Guangdong Province, in the southern margin of South China continent. The finding in this case is that many shot gathers provided by this wide-angle seismic experiment show relatively strong reflected and refracted S-phases, in particular some crustal refractions (Sg waves) and Moho reflections (SmS waves or simply Sm waves). The P-wave velocity structure of the crust and uppermost mantle was already obtained through the interpretation of vertical-component shot gathers. Now, with constraints introduced by the P-wave velocity architecture and after picking up S-wave traveltime data on the seismograms, we have obtained the S-velocity model of the crust by adjusting these traveltimes but keeping the geometry of the crustal reflectors. Our results demonstrate: (1) the average crustal S-velocity is about 3.64 km/s to the northwest of the Wuchuan-Sihui fault, and 3.62 km/s to the southeast of this fault; (2) relatively constant S-velocity of about 3.42 km/s for the upper crust, 3.55 km/s for the middle crust and laterally varying shear velocity around 3.82 km/s for the lower crust; (3) correspondingly, Vp/Vs ratio is 1.73 for the upper crust, 1.71 for the middle crust and 1.74 for the lower crust. Both shear velocities and Vp/Vs ratio correlate well with the major active faults that break the study area, and show significant changes especially in the upper crust. High Poisson’s ratio (1.8) is observed at shallow depth beneath the Minzhong depression to the southeast of the Wuchuan-Sihui fault and the Huiyuan depression in the southern margin of South China continent. In contrast, a very low Vp/Vs ratio (1.68) is observed between 8 and 14 km depth beneath Huiyuan. At deeper depth, a high Vp/Vs ratio (1.76) is observed in the lower crust beneath the Minzhong depression.     

Crustal structure beneath two seismic broadband stations revealed from teleseismic P-wave receiver function analysis in the Virunga volcanic area,Western Rift Valley of Africa

The shear velocity structure beneath the Virunga volcanic area was estimated by using an average solution in the time domain inversion of stacked teleseismic receiver functions provided by two seismic broadband stations KUNENE (KNN) and KIBUMBA (KBB). These two stations are 29 km apart and located at the eastern and western escarpment of the Western Rift Valley of Africa in the Virunga area, respectively. The velocity model was presented as P-wave velocity models. From these models, the crust mantle transition zone beneath the area sampled by KNN and KBB in the Virunga area was determined at depth from about 36 to 39 km and 30 to 41 km, respectively. A low velocity zone was observed below stations KNN and KBB at depths between 20–30 km and 18–28 km, respectively, and with average velocity 5.9 km/s and 6.0 km/s. This low velocity zone may probably related to a magma chamber or a melt-rich sill. The models show also high velocity material (6.8–7.4 km/s) lying beneath stations KNN and KBB at depths 3–20 km and 3–10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. The result obtained in this study was applied to the determination of epicentres during the period prior to the 27 November 2006 Nyamuragira eruption. This eruption was preceded by a swarm of hybrid volcanic earthquakes with clear P-waves onset. Using the receiver function model was found to improve the location of events. The located events correlate well with the location of the eruptive site and data provided by the INSAR observations of surface deformation associated with eruption.     

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).     

Delineation of intra crustal horizon in Eastern Dharwar Craton – An aeromagnetic evidence

A correlative study of two mutually independent geophysical properties like magnetic susceptibility variations and shear wave velocity structure of the crust has been carried out in a part of the Eastern Dharwar Craton of Indian peninsular shield. Analysis of the aeromagnetic anomaly field over an area of 35,000 km² comprising the peninsular gneissic basement complex and a part of Cuddapah Basin has resulted in identification of two distinct magnetic horizons: one at a depth of 2 km and the other at a depth of 12 km. Correlation of these results with the inferences made by the inversion of Rayleigh wave phase velocity and other geophysical studies has confirmed the presence of a crustal layer at a depth of 12 km. This horizon has been inferred to be the depth to the lower boundary of the upper crust in this region.     

Long wavelength gravity anomalies over India: Crustal and lithospheric structures and its flexure

Long wavelength gravity anomalies over India were obtained from terrestrial gravity data through two independent methods: (i) wavelength filtering and (ii) removing crustal effects. The gravity fields due to the lithospheric mantle obtained from two methods were quite comparable. The long wavelength gravity anomalies were interpreted in terms of variations in the depth of the lithosphere–asthenosphere boundary (LAB) and the Moho with appropriate densities, that are constrained from seismic results at certain points. Modeling of the long wavelength gravity anomaly along a N–S profile (77°E) suggest that the thickness of the lithosphere for a density contrast of 0.05 g/cm³ with the asthenosphere is maximum of ∼190 km along the Himalayan front that reduces to ∼155 km under the southern part of the Ganga and the Vindhyan basins increasing to ∼175 km south of the Satpura Mobile belt, reducing to ∼155–140 km under the Eastern Dharwar craton (EDC) and from there consistently decreasing south wards to ∼120 km under the southernmost part of India, known as Southern Granulite Terrain (SGT).The crustal model clearly shows three distinct terrains of different bulk densities, and thicknesses, north of the SMB under the Ganga and the Vindhyan basins, and south of it the Eastern Dharwar Craton (EDC) and the Southern Granulite Terrain (SGT) of bulk densities 2.87, 2.90 and 2.96 g/cm³, respectively. It is confirmed from the exposed rock types as the SGT is composed of high bulk density lower crustal rocks and mafic/ultramafic intrusives while the EDC represent typical granite/gneisses rocks and the basement under the Vindhyan and Ganga basins towards the north are composed of Bundelkhand granite massif of the lower density. The crustal thickness along this profile varies from ∼37–38 km under the EDC, increasing to ∼40–45 km under the SGT and ∼40–42 km under the northern part of the Ganga basin with a bulge up to ∼36 km under its southern part. Reduced lithospheric and crustal thicknesses under the Vindhyan and the Ganga basins are attributed to the lithospheric flexure of the Indian plate due to Himalaya. Crustal bulge due to lithospheric flexure is well reflected in isostatic Moho based on flexural model of average effective elastic thickness of ∼40 km. Lithospheric flexure causes high heat flow that is aided by large crustal scale fault system of mobile belts and their extensions northwards in this section, which may be responsible for lower crustal bulk density in the northern part. A low density and high thermal regime in north India north of the SMB compared to south India, however does not conform to the high S-wave velocity in the northern part and thus it is attributed to changes in composition between the northern and the southern parts indicating a reworked lithosphere. Some of the long wavelength gravity anomalies along the east and the west coasts of India are attributed to the intrusives that caused the breakup of India from Antarctica, and Africa, Madagascar and Seychelles along the east and the west coasts of India, respectively.     

Linking the Tengchong Terrane in SW Yunnan with the Lhasa Terrane in southern Tibet through magmatic correlation

New zircon U–Pb data, along with the data reported in the literature, reveal five phases of magmatic activity in the Tengchong Terrane since the Early Paleozoic with spatial and temporal variations summarized as Cambrian–Ordovician (500–460 Ma) to the east, minor Triassic (245–206 Ma) in the east and west, abundant Early Cretaceous (131–114 Ma) in the east, extensive Late Cretaceous (77–65 Ma) in the central region, and Paleocene–Eocene (65–49 Ma) in the central and western Tengchong Terrane, in which the Cretaceous–Eocene magmatism migrated from east to west. The increased zircon ε_Hf(t) of the Early Cretaceous granitoids from − 12.3 to − 1.4 at ca. 131–122 Ma to − 4.6 to + 7.1 at ca. 122–114 Ma, identified for the first time in this study, and the magmatic flare-up at ca. 53 Ma in the central and western Tengchong Terrane indicate increased contributions from mantle- or juvenile crust-derived components. The spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern Tibet. Such similarities, together with the data of stratigraphy and paleobiogeography, enable us to propose that the Tengchong Terrane in SW Yunnan is most likely linked with the Lhasa Terrane in southern Tibet, both of which experienced similar tectonomagmatic histories since the Early Paleozoic.     

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.     

Two types of ultrapotassic plutonic rocks in the Bohemian Massif — Coeval intrusions at different crustal levels

We present U–Pb zircon age determinations of two Variscan ultrapotassic plutonic rocks from the Moldanubian Zone (Bohemian Massif). Equant, multifaceted zircons without inherited cores from a two-pyroxene–biotite quartz monzonite of the Jihlava Pluton yielded a precise age of 335.12 ± 0.57 Ma, interpreted as dating magma crystallization. The majority of both tabular and prismatic grains from the amphibole–biotite melagranite (“durbachite”) from the T?ebí? Pluton plot along a discordia intersecting the concordia at 334.8 ± 3.2 Ma; prismatic zircon grains commonly contain inherited cores and yield an upper intercept age of 2.2 Ga, indicating early Proterozoic inheritance. We therefore suggest that both types of the ultrapotassic plutonic rocks from the Bohemian Massif crystallized at ca 335 Ma, and the previously published ages higher than ca 340 Ma for “durbachites” were biased by a small amount of unresolved inheritance. The ultrapotassic magma emplacement in the middle crust was related to rapid exhumation of a deep crustal segment, considered as isothermal decompression between high-pressure (～ 340 Ma) and medium-pressure (～ 333 Ma) stages recorded in granulites. Mineral assemblages as well as external and internal zircon morphology suggest that the Jihlava intrusion was deep and dry, whereas the T?ebí? intrusion was shallow and wet. Low ε_Hf values of zircons (? 4.4 to ? 7.5) in both rock types suggest a similar source with a predominant crustal component. However, inherited grains in the T?ebí? melagranite indicate its contamination with crustal material during emplacement, and thus possibly a slower rate of exhumation and/or of magma ascent through the crust.     

Petrogenesis of the Guangtoushan granitoid suite,central China: Implications for Early Mesozoic geodynamic evolution of the Qinling Orogenic Belt

The Qinling Orogenic Belt marks the link between the South China and North China Blocks and is an important region to understand the geological evolution of the Chinese mainland as well as the Asian tectonic collage. However, the tectonic affinity and geodynamic evolution of the South Qinling Tectonic Belt (SQTB), a main unit of the Qinling Orogenic Belt, remains debated. Here we present detailed geological, geochemical and zircon U–Pb–Hf isotopic studies on the Zhangjiaba, Xinyuan, Jiangjiaping, Guangtoushan and Huoshaodian plutons from the Guangtoushan granitoid suite (GGS) in the western segment of the SQTB. Combining geology, geochronology and whole-rock geochemistry, we identify four distinct episodes of magmatism as: (1) ~ 230–228 Ma quartz diorites and granodiorites, (2) ~ 224 Ma fine-grained granodiorites and monzogranites, (3) ~ 218 Ma porphyritic monzogranites and (4) ~ 215 Ma high-Mg# quartz diorites and granodiorites as well as coeval muscovite monzogranites. The ~ 230–228 Ma quartz diorites and granodiorites were generated by magma mixing between a mafic melt from mantle source and a granodioritic melt derived from partial melting of Neoproterozoic rocks in the lower continental crust related to a continental arc regime. The ~ 224 Ma fine-grained granodiorites and monzogranites were formed through partial melting of a transitional source with interlayers of basaltic rocks and greywackes in the deep zones of the continental arc. The ~ 218 Ma porphyritic monzogranites originated from partial melting of metamorphosed greywackes in lower crustal levels, suggesting underthrusting of middle or upper crustal materials into lower crustal depths. The ~ 215 Ma high-Mg# quartz diorites and granodiorites (with Mg# values higher than 60) were derived from an enriched mantle altered by sediment-derived melts. Injection of hot mantle-derived magmas led to the emergence of the ~ 215 Ma S-type granites at the final stage.Integrating our studies with previous data, we propose that the Mianlue oceanic crust was still subducting beneath the SQTB during ~ 248–224 Ma, and final closure of the Mianlue oceanic basin occurred between ~ 223 Ma and ~ 218 Ma. After continental collision between the South China Block and the SQTB, slab break-off occurred, following which the SQTB transformed into post-collisional extension setting.     

Geochemistry,provenance, and metamorphic evolution of Gabal Samra Neoproterozoic metapelites,Sinai, Egypt

Metapelites are exposed at Wadi Ba’ba, east of Abu Zenima city; represent the northwestern extension of the Fieran-Solaf Metamorphic Complex, Sinai Peninsula, Egypt. The metapelites are characterized by qtz + pl (An_24–28) + bt + grt ± crd ± sil mineral assemblage, indicating upper amphibolite facies with peak metamorphic conditions of 700 °C and pressures of 7 kbar, as determined by conventional geothermobarometeric methods. This resulted in incipient migmatization, forms patches of leucosomes and melanosomes. Geochemical investigation indicates that the precursor sediments of the metapelites had been deposited as immature Fe-rich shales from source materials of dominantly intermediate composition. Source area exhibited weak to moderate chemical weathering in a tectonically active continental marginal basin within a continental-arc system. A strong shallow-dipping foliation, characterizing the metapelites, was folded around an open antiform with sub-horizontal south plunging hinge.Phase equilibria calculations in the KFMASH system indicate that the peak metamorphic conditions formed at 730–750 °C and 6.8–7.9 kbar. This was followed by a retrogression formed at 770–785 °C and 3.9–4.5 kbar. Hence, this implies an isothermal decompression and rapid exhumation of the metapelites from depth (25–29 km) in the lower crustal level at peak conditions, continuous to include shallow to middle crustal level (14–17 km), at overprint retrograde conditions. Subsequent isobaric cooling took place at 720–750 °C and 3.6–4.5 kbar. The resulting isothermal decompression followed by isobaric cooling clockwise P–T path of the metapelites is more likely, in which the high-temperatures attained maximum conditions during isothermal decompression were enhanced by heat flux, due to the presence of an active magmatic arc that formed on top of subducting young lithosphere. This is supported by a moderate geothermal gradient of 27–43 °C/km and dating compatibility of the Sinai granitoids and the metamorphic complexes. The P–T path segment records the tectonothermal histories of crustal thickening as a result of the East and West Gondwana collision at the metamorphic peak. This was subsequent by extensional and crustal thinning with syn-metamorphic magmatic intrusions, during P–T path retrogression, which resulted in the final assembly of the Arabian–Nubian Shield during Neoproterozoic.     

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.     

Chemical composition of crustal xenoliths from southwestern Syria: Characterization of the upper part of the lower crust beneath the Arabian plate

The chemical bulk rock composition of 37 xenoliths, brought from depths of 25–30 km to the surface by penetrating Cenozoic alkali basaltic magma, from the Shamah Harrat, southwestern Syria, was determined by XRF spectroscopy. The geochemical character of these xenoliths points to original marls and within-plate igneous rocks. To obtain the mean chemical composition of the corresponding upper portion of the lower crust, the compositions of the 37 xenoliths were averaged and a leucogranitic and upper crustal component was added to account for assimilation by the Cenozoic magmas. This mean is more basic (SiO₂—50.5 wt%) and richer in HFSE, LREE, and LILE compared to compositions of the lower crust given by Taylor and McLennan [1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312pp.] and Rudnick and Gao [2005. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. Treatise on Geochemistry, vol. 3. Elsevier, Amsterdam, pp. 1–64]. Calculations of the seismic compressional-wave velocity from our compositional mean, using the PERPLE_X computer software, yielded values around 6.85 km/s, which are in accordance with reported seismic studies for the corresponding depth levels (6.7–7.1 km/s).     

Deciphering an Archean mantle plume: Abitibi greenstone belt,Canada

The 2724–2722 Ma Stoughton-Roquemaure Group (SRG) of the Abitibi greenstone belt (the Archean Superior Province, Canada) is a ≤ 2 km thick komatiite–basalt succession intermittently exposed for about 50 km along strike. The ultramafic and mafic rocks occur mainly as pillowed, brecciated, and massive flows with well preserved spinifex textures in the komatiites. Volcanological, comparative stratigraphic and geochemical studies of the group along a volcanic marker horizon at the base of the succession allow the assessment of magma emplacement processes and mantle source rocks. Major feeder channels, secondary distributary tubes surrounded by pillowed flows with minor breccias and hyaloclastites display facies architecture of small volume flow fields (1–2 km³). Within the SRG, Al-depleted (ADK; Barberton-type) and Al-undepleted (AUK; Munro-type) komatiitic lavas are intercalated with tholeiitic basalt flows at a m- to 10s of m scale. Basalts and komatiites are inferred to be mantle plume-related; both rock types form two groups with characteristics of ADK and AUK including Al₂O₃/TiO₂ ~ 9–12 for ADK versus 17–22 for AUK, as well as (Gd/Yb)_n with > 1.3 versus ~ 1, respectively. The interdigitation of compositionally different flow units, limited extent of SRG volcanic rocks and facies architecture with the prevalence of small volume flows argue for a relatively small, heterogeneous mantle plume during the incipient stage of the evolution of the Archean Abitibi belt. Assuming that the scale of heterogeneities is comparable to the field expression of compositional changes and stratigraphy, it can be suggested that geochemical plume ‘layering’ is on 10s to 100s of m-scale. The evolution of this Archean mantle plume from inception to demise compares favorably with the Yellowstone hotspot which is assumed to have developed over 17 m.y. and had a diameter of about 300 km.