Southern Block Research Articles

Anomalous Concentration of Uranium in Groundwater in Parts of Salem District, Central Tamil Nadu, India

Anomalous concentration of Uranium in groundwater has been identified around Virakkal, Veerapanpalayam, Pakkanadu, Erithottam, Kammampatti, Kumarappalyam, Idappadiand Moolakkadu areas in parts of Salem district, Tamil Nadu. In this study, an area of 970 sq. km from Mettur in the north to Kumarapalayam in the south parallel to the Koratti shear zone has been examined for uranium content in groundwater. Systematic sampling of groundwater (n=78) from bore well, open well and hand pump have been collected. The samples were analysed for uranium concentration using LED Fluorimeter. Analysis resulted higher concentration of uranium along the southern extension of Koratti Shear Zone where the emplacement of younger granites, syenite and carbonatites has been reported. These lithounits might have played a vital role in the contribution of uranium to the groundwater. Based on the result, two areas have been identified such as North Block and South Block for detailed investigation.58 systematic samples from North block and 52 samples from South block have been collected and analysed. Uranium values assayed in these areas are many times higher than the prescribed limit of World Health organisation and Atomic Energy Regulatory Board of 30ppb and 60ppb respectively. In northern block, among 58 samples 18 samples were assayed higher than 30ppb in which the samples from Veerappanpalayam assayed upto 545.81 ppb. Apart from Veerappanpalayam, many areas such as Mosakumarapalayam (273.90), Arasiramani (250.49ppb), Vellarivalli (131.32ppb), Kodarapalayam (106.52ppb), Savuthanur (174.79ppb), Veerapanpalayam-2 (373.43ppb) and Kotamppalayam (100ppb) were assayed more than 100ppb. Similarly, in the southern block, 52 samples were assayed in which 15 samples were higher than the prescribed limit in which Kuppampatti analyzed 402.29ppb, Virakkal and Thannikuttampatti assayed more than 100ppb. Among the total samples (n=188), 28% of samples were falling under non-potable category. The Present study has spatially brought to light significant areas where groundwater enriched by uranium concentration. It is also provided indirect evidences for uranium exploration in the study area.

Ore-forming mechanism and its relationship with deformational and metamorphic episodes at Haimur gold mine, Nubian Shield, Egypt

AbstractThe Haimur area represents the central part of the Wadi Allaqi region in the southern block of the Egyptian Eastern Desert near Nasser Lake and the Nile valley. It is made up of ophiolitic assemblage comprising serpentinite and talc carbonate, listwenite, metagabbro/amphibolite and metabasalt and island arc assemblages. The orogenic gold deposits in the Haimur area occur in the form of smoky/white sulphide-bearing quartz / quartz-carbonate veins or lenses cutting through the listwenite zone-related rocks. The NE-trending auriferous veins were formed due to an extensional to transtensional shearing related to NW Najd shear tectonics. Gold was observed in association with late-pyrite and chalcopyrite paragenetically formed in the gold phase (second phase), while the first phase is the sulphide phase including early-pyrite and arsenopyrite. Arsenopyrite was formed during early metamorphic recrystallization at a high-temperature range between 405 and 512 °C. However, gold was formed at lower temperature through retrograde metamorphism. Three types of fluid inclusions have been recognized: aqueous (type-I), mixed aqueous–carbonic (type-II) and hydrocarbonic (type-III). The P–T conditions of trapping were obtained from the isochore lines of intersection between T = 300–320 °C and pressure range of 60–180 MPa. The Haimur gold deposit was supposed to be derived from metamorphic fluids created by dehydration and decarbonation of ophiolitic mélange assemblages and volcano-sedimentary rocks, in which the gold was transported as Au(HS)2– complexes and precipitated in reduced environment. The strong linkage between the deformation and metamorphism triggered gold deposition in structurally favourable sites related to the Najd fault system.

Geochronology, whole-rock geochemistry and Sr-Nd isotopes of the Bhanupratappur mafic dyke swarm: Evidence for a common Paleoproterozoic LIP event at 2.37–2.36 Ga in the Bastar and Dharwar cratons

Mafic dykes and dyke swarms in continental settings provide information on the evolution of the subcontinental mantle and can be key elements in the reconstruction of paleo-geographic settings of now separated crustal terranes. This study focuses on the petrogenesis and geochronology of mafic dykes of the WNW (~125°) trending Bhanupratappur swarm in the central Bastar Craton, central India. Dykes of the Bhanupratappur swarm yield an average U-Pb (ID-TIMS) baddeleyite age of 2360 ± 4 Ma, which is interpreted as their emplacement age. The compositions of the dykes range from tholeiitic basalt to basaltic-andesite. Their rare earth element and multi-element patterns indicate the involvement of a crustal component in their petrogenesis. The whole rock initial 87Sr/86Sr2360 Ma ranges from 0.70097 to 0.70506 with most being more radiogenic than the contemporaneous undifferentiated mantle reservoir (i.e. 87Sr/86Sr2360 Ma = 0.70173). The initial εNd 2360 Ma (+0.85 to −2.7) are chondritic to sub-chondritic. The Sr-Nd Isotope composition and major- and trace element chemistry suggest an enriched-heterogeneous mantle source. The closely matching ages and chemistry of the Bhanupratappur swarm (2360 Ma) and the Karimnagar-Bangalore swarms (2363–2369 Ma) of the Dharwar Craton indicate affinities to a common Large Igneous Province, which further implies that the Bastar and Dharwar cratons were already juxtaposed at 2.37–2.36 Ga. The dykes of the Bhanupratappur (WNW-trending) and Bangalore (E-W trending) swarms converge towards the east indicating a plume center in the east. If the Karimnagar swarm was also linked (and was converging) to the same plume, the present-day mismatch in the orientations of the Karimnagar dykes (NE- to ENE-trending) with the Bangalore and Bhanupratappur dykes may indicate a ~55° counterclockwise rotation of the northern block of the Eastern Dharwar Craton with respect to the southern block after 2.37–2.36 Ga.

A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens, China: Insights from geochronology and structural geology

The Qinling-Qilian connection zone (QQCZ) is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens, China. Here we present U–Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block (NCB) and detailed regional structural data. Three episodes of fold deformation (D1, D2 and D3) are distinguished in the QQCZ, with the former two occurred during the early Paleozoic. The D1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D2 and D3 deformations. The D2 deformation characterized by tight folds, associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE–SSW-directed compression, which may be induced by the collision between the NCB and the southern blocks. The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands. The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance, which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework. In addition, the remarkable age peak at ~435 ​Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens, which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician–Early Silurian. Based on structural, detrital zircon and metamorphic data, we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.

Seismic Hazard Assessment of Dhanbad City, India, by deterministic approach

Dhanbad is the largest producer of coal in the state of Jharkhand, India, and hence, it is one of the most important cities in the country. Considering the present-day importance and future growth of the city, assessment of seismic hazard is extremely needed, especially for the mining authorities and the city planners. The prime objective of this study is to develop seismic hazard map of Dhanbad City for the bedrock level following the deterministic approach. In view of the absence of the region-specific Ground Motion Prediction Equations, the concept of logic tree comprising four different attenuation models is adopted. It is observed that the Peak Ground Acceleration in the range of 0.10–0.26 g may be expected in the city. Specifically, southern blocks of the city such as Baghmara, Jharia and Baliapur are found to be most vulnerable to the seismic hazard. Besides, developed seismic hazard map is divided into four zones and smooth acceleration response spectrums are proposed for the zones as per the expected seismicity in different parts of the city.

PROSPECTS FOR HYDROCARBONS DEPOSITS SEARCHING IN CARBONIFEROUS SEDIMENTS OF THE AXIAL AND SOUTHERN SLOPE AREAS OF DNIEPER-DONETS DEPRESSION

The main task of the research was to analyze of the hydrocarbons carboniferous objects of the axial and southern slope zones of the DDD with the aim of predicting further geological prospecting works. Based on the results of the research, promising objects have been identified that determine further areas of prospecting and exploration in the perspective deposits of the Lower and Upper Carboniferous in the study area of the region.It was revealed that among the Paleozoic productive and promising complexes in the axial and southern slope zones of the DDD, on number of identified and prepared objects and their resource base, a priority complex is Carboniferous. The primary objects in that complex are Borzakovska, Brusivska, Uspenovska, Osterska and Rozdolevska structures. We are associate the priority direction of prospecting works in combined traps with anticlinal and non-anticlinal complicated structures in the Lower Carboniferous deposits with those structures. Author is proposing to carry out priority detailed seismic exploration work on the Gersenvanovskaya and Topoleva identified structures to prepare them for prospecting drilling.It has been established that the second place (after the Lower Carboniferous complex) in terms of priority and prospectively is Medium Carboniferous deposits, in which within the study area on primary importance on the searching for hydrocarbon deposits are prepared Dovgopolivska, Svyatogirska, Stepnovska, Eastern-Shebelinska and other structures. In addition, Krasnoglazovsky and Western-Ukrainian promising objects are recommended for preparation by seismic exploration for drilling.It has been proven that in the Upper Carboniferous - Lower Permian deposits, where more than half of the proven gas reserves are located. The further prospects should be associated only with the zones and blocks near salt bunch. Most perspective for hydrocarbon searching in sediments C3+P1 is Yuzhnorozpashnovsky and Severnozhkovsky objects. Western-Ukrainian object, Verkhnelannovsky northern and southern blocks and Onezshska structure are required seismic survey preparation for drilling.Due to reason, that priority structures for further exploration are located, in most, in the axial zone of the region, the main concentration of prospecting works should be carry out within that zone, not taking into account the existing depths of objects. Within the southern slope zone at shallower depths, Brusivskaya, Ostersky and Uspensky structures are prepared for drilling. The implementation of prospecting and geophysical work on the recommended objects of the studied territory of the DDD will be allow to identify new hydrocarbon deposits and replenish the fund of promising objects, that prepared for deep drilling.

Contemporary deformation and strain analysis in South Korea based on long-term (2000–2018) GNSS measurements

South Korea forms the Korean Peninsula and is considered to be tectonically more stable than the other regions of Eastern Asia. Global Navigation Satellite System (GNSS) sites installed in the country contribute to geoscientific studies on contemporary deformation, strain pattern, and current kinematics of tectonic blocks. In the current study, we used long-term GNSS measurements (from 1 January 2000 to 31 December 2018) to study block-wise geodetic deformation and strain pattern of the country, including the co-seismic effect of the 2011 Tohoku-Oki (TO) Earthquake. Measured velocities of GNSS sites are modelled via exponential smoothing method to analyse the GNSS time-series and obtain the secular tectonic crustal deformation. The analysis of strain pattern and vertical displacements indicates that extensional deformation followed compressional deformation from the northern Korean blocks to the southern Korean blocks. Finally, we examined the angular velocity and pole of rotation of the South Korean blocks using pre-seismic and post-seismic displacements of the GNSS sites. The results indicate that the angular velocity of the northern blocks was lower, while the southern block velocity is higher after the TO earthquake. We believe that the integrated approaches applied in the study would provide better insights into the active deformation of South Korea.

EVALUACIÓN DE LA AMENAZA POR TSUNAMI EN POBLACIONES DEL SUR, CENTRO Y NORTE DEL LITORAL PACÍFICO COLOMBIANO

La geomorfología marina en el Pacífico colombiano, se caracteriza por presentar una zona de convergencia de las placas de Nazca y Sudamérica, generando un proceso de subducción Nazca por debajo de la Sudamericana, la alta actividad sísmica en la zona de subducción Colombo-Ecuatoriana. Se han presentado sismos precursores de tsunami, como lo son, los eventos de 1906 y 1979, que afectaron poblaciones costeras del sur del Pacífico colombiano. En su parte norte se evidencia una alta actividad sísmica que podría generar eventos fuertes precursores de tsunami afectando a las poblaciones costeras. Se evaluó la amenaza por tsunami a lo largo del Litoral Pacífico colombiano, a partir del cálculo de la altura del tsunami en las poblaciones de Tumaco, Curay, Juanchaco, Buenaventura, Ciudad Mutis y Juradó. Se utilizó el método determinista, para lo cual se simularon escenarios de tsunami, empleando dos fuentes sísmicas una para el bloque sur y otra para el bloque norte. Se utilizaron dominios de cálculo, construidos con datos batimétricos y altimétricos de alta resolución espacial. El modelo para la simulación de los escenarios de tsunami fue el modelo TUNAMI N2. Como resultado, se obtuvo el tiempo de llegada de la primera ola de tsunami y la inundación máxima en cada población costera. Se concluye que el área más vulnerable y afectada en caso de un tsunami en el Pacífico colombiano es Tumaco.

Geochemical and petrographic variations in pseudotachylyte along the Outer Hebrides Fault Zone, Scotland

The islands of the Outer Hebrides, Scotland host a c. 200 km long fault zone that was active between 1100 and 250 Ma, during which time a belt of fault rock several kilometres wide was created. The fault zone hosts crush breccias, phyllonite, mylonite and pseudotachylyte. The latter unit is often found in major fault zones and within meteorite impact structures. Although pseudotachylyte is widely believed to be a product of frictional melting within a fault zone, there is still controversy over the specific mechanisms of formation. Pseudotachylyte was collected from the southern block of the Outer Hebrides Fault Zone. A geochemical mixing model was applied to reconstruct the geochemistry of the pseudotachylyte matrix using the locally available lithologies. The results show that the pseudotachylyte could have been derived from the local country rock, although some mixing was necessary to provide accurate reconstructions, suggesting a degree of mixing between melts derived from different protoliths. Petrographic observations identified additional characteristics of melt transportation, including transported clasts, aligned fragments, melt segregation and injection of the melt into tensile fractures. The latter probably served as the transport mechanism, drawing melt into the low-pressure tensile structures that developed adjacent to the melt-forming zones within the fault plane. Supplementary Material : Sample site co-ordinates and electron microprobe settings are available at: https://doi.org/10.6084/m9.figshare.c.4567931

Spatio-temporal evolution of the deformation around the Rio-Patras fault (Greece) observed by synthetic aperture radar interferometry from 1993 to 2017

ABSTRACTWe analysed the ground deformation across two blocks defined by the Rio-Patras fault from 1993 to 2017 using multi-temporal Synthetic Aperture Radar Interferometry (InSAR) techniques. Our main objective was to contribute to the assessment of seismic hazard near the large city of Patras. Multiple data-sets were used, each one covering different temporal periods. Descending and ascending acquisitions, providing different viewing geometries contribute to fully determine the ground displacement in 3D. The data-sets used are from the European Space Agency’s (ESA) European Remote Sensing (ERS), Environmental Satellite (ENVISAT) and SENTINEL-1 as well as German Aerospace Center (DLR) ’s TERRASAR-X missions. Considering ESA’s missions covering both acquisition geometries and long periods, the southern block, showing lack of a sufficient number of scatterers does not allow the displacement characterization. In contrary, the northern block is characterized by a high number of scatterers having values of maximum likehood ranging from −3.5 to −4.3 mm year−1 for ascending geometry and from −1.6 to −2.7 mm year−1 for the descending one. The fact that both geometries show negative values of displacements are consistent with downlift movement and at the same time the quantitative differentiation probably indicates an horizontal component as well.

Contrastive analysis of gravity and magnetic anomalies between North China Craton and Indian Shield

The evolution correlation between the North China Craton (NCC) and the Indian Shield (IND) is very important for plate reconstruction. Recently, more and more research results show that the NCC and IND had been pieced together in the Paleoproterozoic. In order to analyse the correlation between the NCC and IND, we use the latest gravity and magnetic data to study deep geophysical characteristics of the NCC and IND respectively, and we also use a two‐dimensional discrete wavelet transform method to discuss the division scheme of the basement tectonic unit in the Paleoproterozoic at the point of gravity and magnetic field. On the basis of gravity and magnetic anomalies, the NCC is divided into the Eastern Block, Western Block, and the Trans‐North China Orogen, which are further divided into the secondary tectonic zones and blocks. The IND is divided into the Southern Block, the Northern Block, and the Central Indian Tectonic Zone, and they also are further divided into secondary tectonic zones and ancient cratons. On the basis of that, according to the four Paleoproterozoic plate assemblage schemes proposed by predecessors, we compared the geophysical characteristics and the correlation through the regional gravity and magnetic anomalies between the two continents. The results show that the faults and tectonic boundaries in the Trans‐North China Orogen of the NCC correspond well to those in the Central Indian Tectonic Zone of IND. Moreover, the gravitational and magnetic anomalies in the Eastern Block of the NCC are quite similar to those in the Southern Block of IND, which shows that the NCC and IND might have been pieced together during the Paleoproterozoic. The geophysical result that verifies the correlation between the NCC and IND based on gravity and magnetic fields provides the evidence for Paleoproterozoic plate reconstruction and constrains the assembling patterns between the NCC and the IND.

Crust and shallow mantle structure of south India by inverting interpolated receiver function with surface wave dispersion

The 3-D shear wave velocity structure beneath south India crust and shallow mantle up to depth 130 km, have been determined from the joint inversion of surface wave dispersion and interpolated receiver function (RF). The P wave RF interpolation helps to equalize the lateral sampling of the RF and the surface wave dispersion, which mainly simplifies the RFs. One of the major difficulties due to back azimuthal variations of receiver functions is suppressed by this scheme of spatial interpolation. We combine the interpolated receiver functions with Rayleigh wave dispersions to estimate the shear wave velocity structure beneath the region. This study reveals the significant variations in crustal thickness and nature of intra crustal layering of southern India shield. The crustal thickness of south India varies from 40 km in Deccan Volcanic Province (DVP), 38 km in East Dharwar Craton (EDC), 44 km in West Dharwar Craton, northern block (WDC-N), 50 km in West Dharwar Craton, southern block (WDC-S), 48 km in Southern Granulite Terrain (SGT) and 38 km in Kerala Khondalite Belt (KKB). The average upper crustal thickness is ∼5 km and that of middle crust is ∼15 km. The lower crust shows significant variations in terms of thickness (varies from ∼8 to 32 km) as well as the high velocity mafic layer. The shear wave velocity variations in the model correlate strongly with the different geological variations along different tectonic blocks of southern India. The DVP, surprisingly, does not have any distinct signature of volcanism in the underlying crust and shallow mantle. The shallow mantle part of entire south India has similar characteristics except for EDC where the velocity is higher beyond 85 km depth.

Female Work Participation in Nadia District, West Bengal: Spatio-Temporal Analysis

The work participation of female is a significant indicator for advancement of society and their economic liberty. But in our country and state, the female workforce participation rate is still low and Nadia district is no exception. In this paper an attempted has been made to assessing the concentration and distribution of female worker, male female work participation disparity, category wise male female main and marginal worker and relationship between literacy rate and female work participation rate. Concentration of female worker is analyzed with the help of location quotient and work participation disparity is assessed by using the shoper's index method modified by Kundu and Rao. Relationship between literacy rate and female work participation rate is examined with the application of regression analysis. From the result analysis it is revelled that, female work participation rate is highly concentrated in southern part of the district while the rate is quite low in the northern part. A wide gap is found between male female work participation rates. Male work participation rate is too high than female work participation rate in every C.D. block. Disparity between MWPR and FWPR is relatively high in northern C.D. blocks than southern block and female work participation rate is strongly negatively co-related with female literacy rate. So, strengthening of women economic power and development of society women participation in workforce is very important.

Emplacement ages of Paleoproterozoic mafic dyke swarms in eastern Dharwar craton, India: Implications for paleoreconstructions and support for a ∼30° change in dyke trends from south to north

Large igneous provinces (LIPs) and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents. The Dharwar craton of southern Peninsular India represents a substantial portion of Archean crust and has been considered to be a principal constituent of Superia, Sclavia, Nuna/Columbia and Rodinia supercontinents. The craton is intruded by numerous regional-scale mafic dyke swarms of which only a few have robustly constrained emplacement ages. Through this study, the LIP record of the Dharwar craton has been improved by U-Pb geochronology of 18 dykes, which together comprise seven generations of Paleoproterozoic dyke swarms with emplacement ages within the 2.37–1.79 Ga age interval. From oldest to youngest, the new ages (integrated with U-Pb ages previously reported for the Hampi swarm) define the following eight swarms with their currently recommended names:NE–SW to ESE–WNW trending ca. 2.37 Ga Bangalore-Karimnagar swarm.N–S to NNE–SSW trending ca. 2.25 Ga Ippaguda-Dhiburahalli swarm.N–S to NNW–SSE trending ca. 2.22 Ga Kandlamadugu swarm.NW–SE to WNW–ESE trending ca. 2.21 Ga Anantapur-Kunigal swarm.NW–SE to WNW–ESE trending ca. 2.18 Ga Mahbubnagar-Dandeli swarm.N–S, NW–SE, and ENE–WSW trending ca. 2.08 Ga Devarabanda swarm.E–W trending 1.88–1.89 Ga Hampi swarm.NW–SE ca. 1.79 Ga Pebbair swarm.Comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features, such as regional Dharwar greenstone belts and the late Archean (ca. 2.5 Ga) Closepet Granite batholith, have led to the hypothesis that the northern Dharwar block has rotated relative to the southern block. By restoring a 30° counter clockwise rotation of the northern Dharwar block relative to the southern block, we show that pre-2.08 Ga arcuate and fanning dyke swarms consistently become approximately linear. Two possible tectonic models for this apparent bending, and concomitant dyke rotations, are discussed. Regardless of which deformation mechanisms applies, these findings reinforce previous suggestions that the radial patterns of the giant ca. 2.37 Ga Bangalore-Karimnagar dyke swarm, and probably also the ca. 2.21 Ga Anantapur-Kunigal swarm, may not be primary features.

Development of the intracontinental, continuous, narrow transpressional zone along the Sinnyeong Fault in the Cretaceous Gyeongsang Basin, SE Korea

The WNW-ESE-striking Sinnyeong Fault, the most conspicuous fault of the Gaeum Fault System in the Cretaceous Gyeongsang Basin of Korea, provides an opportunity to understand the architectures and evolution of an intracontinental transpressional fault zone. We focus on the structural characteristics of the Sinnyeong Fault based on detailed field observations and magnetic fabric analysis. Its main movement is interpreted as sinistral slip with a small reverse component, although it could also have been active before the main movement. The deformation zone is asymmetric about the fault core. Sedimentary strata of the southern deformation zone are more extensively folded and deformed than those of the northern part, and the southern zone is much wider than the northern zone. NW-SE-trending en-echelon folds are limited to the southern periphery of the fault zone, where it is narrow, and underwent rotation toward the fault surface. These contractional deformations along the entire length of the fault are interpreted to have formed approximately coevally with the sinistral faulting. Newly observed WNW-ESE-striking mappable faults show a similar structural feature. Our results suggest that the damage asymmetry resulted from the eastward transport direction and relative uplift of south blocks of the faults under NE-SW compressive stress, which oriented at high angle (60–70°) to pre-existing strike of the faults. During progressive deformation, continuing slip accumulation along vertically tilted and fault-parallel sedimentary strata located directly south of the fault cores was responsible for the larger motions of the southern blocks of the faults rather than the relatively fixed northern blocks. We highlight that the sinistral reactivation of the Sinnyeong Fault formed a continuous, narrow transpressional zone during its long-term evolution.

Electromagnetic survey to constraints ore mining exploration in the eastern Anti-Atlas belt (case of Imiter inlier, Morocco):Application of frequency electromagnetic helicopter-borne method

The present work is based on data of a geophysical survey of the Imiter inlier that was carried out by DIGHEMV multi-coil, multi-frequency helicopter-borne electromagnetic system. Its main goals are (i) to interpret the results of this survey in light of regional geological settings; (ii) to define how geological factors controlled mineralization in the world-class Imiter silver mine, which is located in the central western part of the survey area; (iii) to correlate the modalities of this control with the electromagnetic response observed in the Imiter mine district; and (iv) to extend the model thus obtained to the whole study area in view of locating other potentially mineralized ore bodies.The obtained results showed that the Imiter mine district is covered by two important electromagnetic anomalies and three conductor axes, which are interpreted as directly reflecting the presence of metallic deposits in the subsurface. The most intense of these anomalies (B2) is located in the northern bloc of the main Imiter Fault, in an area which is highly fractured by secondary faults, is covered by Ediacaran volcanic and has never been mined. All mineralized bodies exploited so far are localized in the southern block of this important fault and are all essentially hosted by the Cryogenian black shales. Since we have established that there is a direct relation between faulting and mineralization, and that the intensity of the electromagnetic anomaly is directly related to ore concentration in hosted rocks; we conclude that the area covered by the electromagnetic anomaly B2 would be even more interesting to explore that the southern bloc presently in exploitation. Extending these results to the rest of the survey area, it was found that two other areas to the south-east of the Imiter mine district are also two potentially mineralized zones meriting future exploration.

Formation and Evolution of Microcontinents of the Kerguelen Plateau, Southern Indian Ocean

The paper considers geological and geophysical data on the crustal structure of the volcanic Kerguelen Plateau and nearby Indian Ocean, as well as evolution of the Kerguelen Plateau during the Gondwana breakup. It is assumed that three isolated continental blocks (microcontinents) evolved within the Kerguelen Plateau. In order to provide insight into the mechanisms for the microcontinent formation, physical modeling under three different conditions has been carried out. The first experiment simulates a homogeneous lithosphere with two rift fractures propagating toward each one other; in the second experiment, the propagating rift fractures collide with a structural “barrier” having a stronger lithosphere; the third experiment simulated the effect of a local heat source (a hot spot). Based on these experiments and taking into account the available structural and geophysical data, it is suggested that isolation of continental block in the southern Kerguelen Plateau could have begun due to counterpropagation of rifting branches: oceanic from east to west and continental from west to east. The isolation of the southern block with attachment to the Antarctic Plate took place 120 Ma ago after extinction of the spreading ridge in the Princess Elizabeth Trough. Because of the structural inhomogeneity in the prebreakup lithosphere, which is represented by a zone of intracontinental Lambert–Mahanadi rifts, opening of the ocean could have developed in a more complex regime. Under the effect of a hot spot, a jump of the spreading axis occurred at the moment of its emplacement into the lithosphere.

Early Paleozoic or Early-Middle Triassic collision between the South China and Indochina Blocks: The controversy resolved? Structural insights from the Kon Tum massif (Central Vietnam)

In Central Vietnam, the Kontum massif is subdivided from North to South into: (i) Kham Duc, (ii) Ngoc Linh, (iii) Kan Nack complexes. The Kham Duc complex consists of metapelite, metapsammite, paragneiss, metagabbro, amphibolite, serpentinized ultramafic, and orthogneiss blocks. The southern part of this complex experienced an Early Paleozoic crustal melting event responsible for migmatites and anatectic granites. The Ngoc Linh complex, the largest one, is formed by metatexite and granitoid enclosing blocks of eclogite, high pressure (HP) granulite, orthogneiss, metagabbro, amphibolite, granodiorite, and migmatite. The Kan Nack complex is formed by migmatites with (ultra) high temperature (U)HT granulite, charnockites and enderbites. The three metamorphic complexes are intruded by per-aluminous granite, and subordinate mafic plutons. In spite of local variations, the foliation of the Ngoc Linh complex defines a bulk domal structure, and the stretching lineation consistently trends NW-SE. The NW-SE lineation is also observed in the tectonically overlying the Kan Nack complex. The Ngoc Linh and Kan Nack complexes are interpreted here as parts of a metamorphic core complex (MCC), or “Ngoc Linh MCC”, exhumed by a top-to-the-NW detachment fault. Along the northern margin of the Ngoc Linh MCC, upright folds with E-W striking axes, and E-W stretching lineation deform the foliation. These structures are developed in a transpressive regime coeval with the activity of plurikilometer-scale dextral strike-slip faults. Zircon and monazite U/Pb radiometric datings document a 250–240 Ma age for the crustal melting, and the top-to-the-NW ductile shearing coeval with the formation of the Ngoc Linh MCC. This extensional tectonics is followed by a dextral strike-slip faulting, at ca 240–230 Ma, before the emplacement of the two-mica granitic plutons at ca 240–224 Ma. Furthermore in the Ngoc Linh and Kan Nack complexes, zircon and monazite yield U-Pb Early Paleozoic ages. In the Kham Duc complex, a ca 460 Ma MP/MT, garnet-biotite-staurolite-kyanite metamorphism, followed by migmatites at ca 450 Ma, is also recognized. The 420–400 Ma age of the Dai Loc plutonic suite, and the 450–425 Ma age of the Dien Binh calc-alkaline granodiorite document an Early Paleozoic event. The Early Paleozoic mafic and ultramafic masses of the Hiep Duc complex are ophiolites included into a metasedimentary matrix. Thus, the Kham Duc complex is interpreted as a tectonic mélange formed during a collisional orogeny that subsequently underwent crustal melting giving rise to the migmatites. A part of the Early Paleozoic rocks are enclosed as xenoliths in the Triassic migmatite of the Ngoc Linh MCC. These petrologic, structural, and chronological features allow us to propose the following geodynamic evolution for the Kontum massif. During the Early Paleozoic, a continental collision, which was accommodated by a north-directed subduction of a southern block (S. Vietnam, or Viet-Cambodia block) below a northern block (N. Vietnam or Viet-Lao block), occurred along the Tam Ky-Phuoc Son suture. During the Early Triassic, at ca 250–245 Ma, the S. Vietnam block experienced a crustal melting giving rise to the Ngoc Linh MCC that reworked the Early Paleozoic orogen. Lastly, during the Middle Triassic, the dextral faulting superimposed upon the Ngoc Linh migmatite. The geodynamic significance of the Kontum massif is discussed in the general framework of Indochina- South China relationships. The Early Paleozoic collision in Vietnam is viewed as the driving force responsible for the development of the intracontinental orogeny of S. China. In order to account for the important heat flow required for the formation of UHT metamorphic rocks, a possible interaction between the SCB-Indochina collision, and the Emeishan mantle plume is proposed.

Seismic anisotropy in central North Anatolian Fault Zone and its implications on crustal deformation

We investigate the crustal seismic structure and anisotropy around the central portion of the North Anatolian Fault Zone, a major plate boundary, using receiver function analysis. The characterization of crustal seismic anisotropy plays a key role in our understanding of present and past deformation processes at plate boundaries. The development of seismic anisotropy in the crust arises from the response of the rocks to complicated deformation regimes induced by plate interaction. Through the analysis of azimuthally-varying signals of teleseismic receiver functions, we map the anisotropic properties of the crust as a function of depth, by employing the harmonic decomposition technique. Although the Moho is located at a depth of about 40 km, with no major offset across the area, our results show a clear asymmetric distribution of crustal properties between the northern and southern blocks, divided by the North Anatolian Fault Zone. Heterogeneous and strongly anisotropic crust is present in the southern block, where complex intra-crustal signals are the results of strong deformation. In the north, a simpler and weakly anisotropic crust is typically observed. The strongest anisotropic signal is located in the first 15 km of the crust and is widespread in the southern block. Stations located on top of the main active faults in the area indicate the highest amplitudes, together with fault-parallel strikes of the fast plane of anisotropy. We interpret the origin of this signal as due to structure-induced anisotropy, and roughly determine its depth extent up to 15–20 km for these stations. Away from the faults, we suggest the contribution of previously documented uplifted basement blocks to explain the observed anisotropy at upper and middle crustal depths. Finally, we interpret coherent NE-SW orientations below the Moho as a result of frozen-in anisotropy in the upper mantle, as suggested by previous studies.

Petrology, Geochemistry and Zircon U‐Pb Geochronology of the Xiangshan Group in the Eastern Hexi Corridor Belt: Implications for Provenance and Tectonic Evolution

AbstractThe eastern Hexi Corridor Belt (HCB) is located in the transitional belt among the Alxa Block, the Qilian Orogenic Belt and the North China Block. Because of its unique tectonic location, the tectonic setting, provenance, and even the age of the sedimentary strata in the eastern HCB during the Early Paleozoic remain controversial. This study analyzes the provenance of the poorly studied Xiangshan Group, discusses its age of development and tectonic setting in the eastern HCB using a combination of petrological, geochemical and LA‐ICP‐MS U‐Pb zircon dating methods. Based on the youngest age peaks and the fossil evidence, we suggest that the Xiangshan Group is Middle Cambrian to Late Ordovician in age. The complexity of the geochemical characteristics and associated diagrams suggests that the early stage of the Xiangshan Group developed in a passive continental margin environment, late in the back‐arc basin of the eastern HCB. Based on the sandstone detrital composition, whole‐rock geochemistry and detrital zircon ages, we conclude that the Xiangshan Group had an early provenance that was mainly from the Qilian Block and a late provenance from the Qilian Block and the western Alxa Block. The eastern HCB and its northern and southern blocks have similar palaeontology, lithology and basement age characteristics to the South China Block. This indicates that the eastern HCB might not have formed in the intra‐continental aulacogen of the North China Block during the Early Paleozoic but has a close affinity to eastern Gondwana.