Deep-seated Fractures Research Articles

Geochemical processes influencing stream water chemistry: a case study of Ala River, Akure, Southwestern Nigeria

Ala River is the main river that drains Akure metropolis in southwestern Nigeria. The river takes its sources from deep-seated fractures in granite and charnockite rock bodies at the North and North-western regions of Akure town, flowing through quartzite and granite gneiss exposures. This study was carried out to assess the geochemical processes influencing the quality of Ala river water. The dendritic river system, covering an area of about 427 km2, was studied and water was sampled at thirty-seven (37) points for in-situ determination of some physicochemical parameters and dissolved concentrations of the major ions. pH values obtained varied between 7.16 and 7.75, electrical conductivity ranged from 123 to 520 µs/cm, total dissolved solids varied between 62 and 260 mg/l-CaCO3 while average temperature value was 26.65 °C. Relative concentration of cations and anions were in the order of Ca2+ > Mg2+ > Na+ > K+ and HCO3− > SO42− > Cl− > NO3−. Mean values of dissolved Na+, K+, Ca2+, Mg2+, Fe2+, Cl−, HCO3−, SO42−, NO3−, and PO42− were 34.028, 8.872, 34.175, 18.289, 0.428, 16.543, 56.41, 12.503, 0.495 and 2.029 mg/l, respectively. Two hydochemical facies namely Calcium-Bicarbonate facies and mixed calcium–magnesium–chloride facies dominated the chemistry of the water. CaHCO3 facies revealed the contribution of geogenic processes while the mixed Ca(Mg)-Cl type reflected domestically induced anthropogenic activities influencing chemistry of the water. Gibbs diagrams identified rock–water interaction as an important geochemical process in the study area. Silicate weathering of rocks and carbonate weathering were identified as the important geochemical processes which influence the water chemistry of the study area.

Influence of Geo-environmental and Chemical Factors on Thermotolerant Coliforms and E. coli in the Groundwater of Central Kerala

ABSTRACT Geo-environmental studies in parts of Ernakulam district (Kerala, India) were carried out. The study area comprises of sedimentary (Limestone, sandstone, Clay and Lignite) and crystalline (Charnockite and gneisses) rocks. The sedimentary terrain is characterized by confined and unconfined aquifers. In hard crystalline formations groundwater occurs under phreatic conditions in the shallow weathered portions and under semi confined to confined conditions in the deep-seated fractures. The study area is demarcated into five areas based on soil types. Climate is hot humid to hot summer with heavily raining monsoon. Two major rivers Periyar and Muvattupuzha drain through the area. Agricultural and industrial activities are prevalent in Ernakulam district. The major cause of pollution in the study area is due to the presence of coliforms. Detailed investigations carried out to identify the coliforms indicated spatial and seasonal changes in the distribution pattern. Monsoon and post monsoon showed significantly high total coliforms compared to pre-monsoon. Thermo-tolerant coliforms is high during monsoon season. This seasonal change could be due to the effect of rainfall, overland flow, nutrient load and temperature change. Wide variations in the coliform counts are observed in wells situated near rivers, canals, paddy fields and in water bodies lying close to pilgrimage center, fertilizer industry, and public places. It is noticed that the chemical characteristics of the groundwater influence the coliform survival. pH, nitrite, bicarbonate, hardness, and alkalinity play a significant role in controlling coliform count. At the outset, the study highlighted the impact of anthropogenic activities on ground water in a coastal district of Kerala state.

Thematic interpretation of heliborne magnetic data for delineation of concealed regional structural fabric in the Narnaul-Palsana tract, North Delhi Fold Belt, India

The North Delhi Fold Belt (NDFB) exposure of the Delhi Supergroup of rocks is significant for its structurally controlled uranium mineralization. The Narnaul-Palsana tract within the Khetri subbasin of the NDFB comprises the arenaceous Alwar and argillaceous Ajabgarh Groups of the Delhi Supergroup. The metasedimentary sequence has been subjected to polyphase deformation and igneous intrusion. We used heliborne magnetic data to enhance our geologic understanding of the area. Total magnetic intensity data are gridded and enhanced to resolve the magnetic anomalies. The regional magnetic signature reveals a deep-seated fracture. Varying concentrations of magnetic minerals in different lithologies are reflected in the magnetic response and provide clues to the formational trends. Trend lines and breaks are extracted from the magnetic signature. Thematic analysis of trend lines reveals formational trends that indicate an antiformal and synformal fold pattern in different sectors of the study area. The spatial correlation of the fold patterns is used to decipher the tectonic sequence. Superimposition of antiformal folding over earlier antiform-synform structure and displacement due to later faulting is inferred. Magnetic data analysis is used as a tool to unravel the regional structural fabric of the area that is widely concealed below soil cover.

Rare Earth Elements (REE) Deposits Associated with Great Plain Margin Deposits (Alkaline-Related), Southwestern United States and Eastern Mexico

W.G. Lindgren in 1933 first noted that a belt of alkaline-igneous rocks extends along the eastern edge of the Rocky Mountains and Basin and Range provinces from Alaska and British Columbia southward into New Mexico, Trans-Pecos Texas, and eastern Mexico and that these rocks contain relatively large quantities of important commodities such as, gold, fluorine, zirconium, rare earth elements (REE), tellurium, gallium, and other critical elements. In New Mexico, these deposits were called Great Plain Margin (GPM) deposits, because this north-south belt of alkaline-igneous rocks roughly coincides with crustal thickening along the margin between the Great Plains physiographic province with the Basin and Range (including the Rio Grande rift) and Rocky Mountains physiographic provinces, which extends into Trans-Pecos Texas and eastern Mexico. Since 1996, only minor exploration and development of these deposits in New Mexico, Texas, and eastern Mexico has occurred because of low commodity prices, permitting issues, and environmental concerns. However, as the current demand for gold and critical elements, such as REE and tellurium has increased, new exploration programs have encouraged additional research on the geology of these deposits. The lack of abundant quartz in these systems results in these deposits being less resistant to erosion, being covered, and not as well exposed as other types of quartz-rich deposits, therefore additional undiscovered alkaline-related gold and REE deposits are likely in these areas. Deposits of Th-REE-fluorite (±U, Nb) epithermal veins and breccias are found in the several GPM districts, but typically do not contain significant gold, although trace amounts of gold are found in most GPM districts. Gold-rich deposits in these districts tend to have moderate to low REE and anomalously high tungsten and sporadic amounts of tellurium. Carbonatites are only found in New Mexico and Mexico. The diversity of igneous rocks, including alkaline-igneous rocks, and associated mineral deposits along this boundary suggests that this region is characterized by highly fractionated and differentiated, multiple pulses of mantle-derived magmas evolving to lower crustal magmas related to the subduction of the Farallon plate. The differences in incompatible trace elements, including REE and beryllium, between the different granitic to rhyolite rocks are likely related to either differences in the crustal rocks that were assimilated during magmatic differentiation or by potential minor contamination from crustal sources and/or magma mixing. Deep-seated fracture systems or crustal lineaments apparently channeled the magmas and hydrothermal fluids. Once magmas and metal-rich fluids reached shallow levels, the distribution and style of these intrusions, as well as the resulting associated mineral deposits were controlled by local structures and associated igneous rock compositions.

Investigation of Hydrogeological Structures of Paiko Region, North-Central Nigeria Using Integrated Geophysical and Remote Sensing Techniques

Aeromagnetic data coupled with Landsat ETM+ data and SRTM DEM have been processed in order to map regional hydrogeological structures in the basement complex region of Paiko, North Central Nigeria. Lineaments were extracted from derivative maps of aeromagnetic, Landsat ETM+ and SRTM DEM datasets. Ground geophysical investigation utilizing Radial Vertical Electrical Sounding (RVES) was established in nine transects comprised of four sounding stations which are oriented in three azimuths. Source Parameter Imaging (SPI) was employed to map average depths of structures from aeromagnetic dataset. Selected thematic layers which included lineaments density, lithologic structure, slope, drainage density and geomorphologic maps were integrated and modeled using ArcGIS to generate a groundwater potential map of the area. Groundwater zones were classified into four categories: Very good, good, moderate and poor according to their potential to yield sustainable water to drilled wells. The results from RVES survey reveal a close correlation to lineaments delineated from surface structural mapping and remotely sensed datasets. Hydrogeological significance of these orientations suggest that aeromagnetic data can be used to map relatively deep-seated fractures which are likely to be open groundwater conduits while remotely sensed lineaments and orientations delineated from the RVES survey may indicate areas of recharge. Regions with high lineament density attributable to thick regolith and highly fractured basement have better groundwater potential. Drill depths in this area should target a minimum of 80 m to ensure sufficient and sustainable supplies to drilled wells. The outcome of this study would act as information framework that would guide the siting of productive water wells while providing needed information for relevant agencies in need of data for the development of groundwater resources.

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of Felsic Magmatism in the Son Valley

ABSTRACT Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intra-basinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.

Hydrochemistry and gas geochemistry of the northeastern Algerian geothermal waters

This study focuses on the water and gas chemistry of the northeastern Algerian thermal waters. The helium gas was used to detect the origin of the geothermal fluid. In the Guelma Basin, the heat flow map shows an anomaly of 120 ± 20 mW/m2 linked to the highly conductive Triassic extrusion. The chemical database reveals the existence of three water types, Ca-SO4/Na-Cl, which are related to evaporites and rich in halite and gypsum minerals. The third type is Ca (Na)-HCO3, which mostly characterizes the carbonated Tellian sector. The origin of thermal waters using a gas-mixing model indicates a meteoric origin, except for the El Biban hot spring (W10), which shows a He/Ar ratio of 0.213, thus suggesting the presence of batholith. The helium distribution map indicates a lower 3He/4He ratio between 0 Ra and 0.04 Ra in the W10 and W15 samples, which is compatible with the crustal ratio. Reservoir temperatures estimated by silica geothermometers give temperatures less than 133 °C. The geothermal conceptual model suggests that a geothermal system was developed by the deep penetration of infiltrated cold waters to a depth of 2.5 km and then heated by a conductive heat source (batholith for El Biban case). The thermal waters rise up to the surface through the deep-seated fractures. During their ascension, they are mixed with shallow cold groundwater, which increase the Mg content and cause the immature classification of the water samples.

Large Deformation Analysis of a High Steep Slope Relating to the Laxiwa Reservoir, China

The unstable rock slope in the Laxiwa reservoir area of the Yellow River upstream, China, shows the signs of gravitational and water-impounding induced large deformations over an area of 1.15 × 105 m2. Slope movements have been measured daily at more than 560 observation points since 2009, when the reservoir was first impounded. At two of these points, an average daily movement of around 60–80 mm has ever been observed since the beginning of the impounding. Based on the observed deformations and the geology of the site, a fluid–solid coupling model was then adopted to investigate the existing rockslide activity to better understand the mechanism underlying the large deformations. The results from the field observation, kinematic analysis and numerical modeling indicate that the slope instability is dominated by the strong structurally controlled unstable rock mass. Based on an integrated overview of these analyses, a new toppling mode, i.e. the so-called ‘conjugate block’ mode, is proposed to explain the large deformation mechanism of the slope. The conjugate block is formed by a ‘dumping block’ and toppling blocks. The large deformation of the slope is dominated by (1) a toppling component and (2) a subsiding bilinear wedge induced by planar sliding along the deep-seated faults. Following a thorough numerical analysis, it is concluded that small collapses of rock blocks along the slope will be more frequent with the impounding process continuing and the water level fluctuating during the subsequent operation period. Based on a shear strength reduction method and field monitoring, four controlling faults are identified and the instability of the loose structure in the surface layer is analyzed and discussed. The factor of safety against the sliding failure along the deep seated fractures in the slope is 1.72, which reveals that (1) the collapse of the free-standing fractured blocks cannot be ruled out and the volume of the unstable blocks may be greater than 100,000 m3; (2) the collapse of the whole slope, i.e. with the volume being greater than 92 million m3, or a very large collapse involving several million m3, is considered to be of very low likelihood, unless there are extreme conditions, such as earthquakes and exceptionally heavy rain.

Carpentaria Conductivity Anomaly revisited with preliminary magnetotelluric results from the SE Mt Isa Survey 2014

SUMMARY The region to the east of Mt Isa has complex electrical conductivity, with conductive basin sediments overlying the deeper Carpentaria Conductivity Anomaly (CCA). Early magnetotelluric (MT) model results show alignment of the CCA with aeromagnetic, gravity and seismic features, together implying that they define the major structural edge of the Mt Isa Block. Profile MT data acquired during the previous 20 years have helped refine the position and depth of the CCA. New MT and deep seismic reflection data have recently been acquired in 2014 along a NW to SE profile, funded by the Geological Survey of Queensland’s Greenfields 2020 Program in conjunction with Geoscience Australia. These new data provide further evidence of the complex nature of the crustal conductivity in this region. Induction vectors indicate that the CCA itself is braided into several zones which may define deep-seated fracture systems.

Characteristics and geological significance of particles on fractures from the Dongshengmiao polymetallic pyrite deposit, Inner Mongolia, China

Fault gouge samples were collected from deep-seated fractures in the Dongshengmiao polymetallic pyrite deposit. Fault gouge particles were analysed by transmission electron microscopy. Many particles were identified with sizes of 200 nm or less, such as carbon-bearing, gold, Pb-bearing or Zn-bearing mineral particles. Carbon-bearing particles included graphite, carbonate, organic matter and carbonate mixed with organic matter. Gold particles were found in fault gouge samples in the form of natural gold, which had not been previously observed. Other particles, such as Pb- and Zn-bearing particles, directly reflect the ore-forming elemental characteristics of the Dongshengmiao deposit. Based on analysis of these particles, the following conclusions made: (1) carbon-bearing particles were deduced to be able to provide additional information related to the deposit formation; (2) fracture-induced stress acted on particles based on the observed lattice deformations in the gold, graphite, dolomite and Fe-bearing particles and the uneven edges of many particles; and (3) information about concealed ore bodies can be more easily obtained through the study of fault gouge particles, which will make ore body exploration more efficient.

Effects of geological structures on groundwater flow and quality in hardrock regions of northern Tirunelveli district, southern India

Geological and structural influences on groundwater flow and quality were evaluated in the present study in the hardrock regions of Tirunelveli District, southern India. Groundwater is a major source of freshwater in this region to cater to the requirements of domestic and agricultural activity, as there are no surface water resources. Geologically, the area is characterized by charnockites and garnetiferous biotite gneiss. Groundwater in this region is found to occur in the weathered portion under unconfined condition and in fractured/fissured portions under unconfined to semi-confined condition. Existence of deep-seated fractures are minimal. Lineaments/dykes play a major role in the occurrence and movement of groundwater in the region. Lineaments/dykes of the study area can be broadly divided into two types: north–south and west–east oriented structures. Analysis and field observations revealed that the north–south dykes act as a barrier of groundwater while the west–east oriented structures behave as a carrier of groundwater. Both quality and quantity of groundwater is different on the upstream and downstream sides of the dyke. Hence, it is conclusive that the west–east oriented dykes in this region are highly potential and act as a conduit for groundwater movement from recharge areas to the discharge area.

Geochemistry and petrogenesis of Rajahmundry trap basalts of Krishna-Godavari Basin, India

The Rajahmundry Trap Basalts (RTB) are erupted through fault-controlled fissures in the Krishna-Godavari Basin (K-G Basin) of Godavari Triple Junction, occurring as a unique outcrop sandwiched between Cretaceous and Tertiary sediments along the east coast of India. Detailed geochemical studies have revealed that RTB are mid-Ti (1.74–1.92) to high-Ti (2.04–2.81) basalts with a distinct quartz tholeiitic parentage. MgO (6.2–13.12 wt.%), Mg# (29–50) and Zr (109–202 ppm) suggest that these basalts evolved by fractional crystallization during the ascent of the parent magma along deep-seated fractures. Moderate to high fractionation of HREE, as indicated by (Gd/Yb)N ratios (1.71–2.31) of RTB, suggest their generation through 3–5% melting of a Fe-rich mantle corresponding to the stability fields of spinel and garnet peridotite at depths of 60–100 km. Low K2O/P2O5 (0.26–1.26), high TiO2/P2O5 (6.74–16.79), La/Nb (0.89–1.45), Nb/Th > 8 (8.35–13), negative anomalies at Rb reflect minimum contamination by granitic continental crust. (Nb/La)PM ratios (0.66–1.1) of RTB are attributed to endogenic contamination resulted through recycling of subducted oceanic slab into the mantle. Pronounced Ba enrichment with relative depletion in Rb indicates assimilation of Infra- and Inter-trappean sediments of estuarine to shallow marine character. Geochemical compositions such as Al2O3/TiO2 (3.88–6.83), medium to high TiO2 (1.74–2.81 wt.%), positive Nb anomalies and LREE enrichment of these RTB attest to their mantle plume origin and indicate the generation of parent magma from a plume-related enriched mantle source with EM I signature. Ba/Th (46–247), Ba/La (3.96–28.51) and Th/Nb (0.08–0.13) ratios suggest that the source enrichment process was marked by recycling of subduction-processed oceanic crust and lithospheric components into the mantle. Zr/Hf (37–41) and Zr/Ba (0.51–3.24) indicate involvement of an asthenospheric mantle source. The Rajahmundry basalts show affinity towards FOZO (focal zone mantle) and PSCL (post-Archaean subcontinental lithosphere) which reflect mixing between asthenospheric and lithospheric mantle components in their source. Origin of RTB magma is attributed to plume-lithosphere interaction and the upward movement of melt is facilitated by intrabasinal deep-seated faults in the K-G Basin.

Late Triassic subduction-related ultramafic–mafic magmatism in the Amasya region (eastern Pontides, N. Turkey): Implications for the ophiolite conundrum in Eastern Mediterranean

The eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the geology, petrology, geochemistry, Nd–Sr–Pb isotopes and U–Pb zircon ages from a ultramafic–mafic complex in Aksalur which forms part of a series of Alaskan-type ultramafic–mafic bodies in the Amasya region located at the south-western corner of the eastern Pontides orogenic belt. These ultramafic–mafic intrusions occur as minor (30–300 m in diameter), scattered, roughly circular bodies within the low-grade metamorphic rocks of the Tokat metamorphic massif. Petrographic studies characterise the rock types as gabbro and wehrlite, with the latter showing typical cumulate texture and mainly comprising serpentinized olivine, clinopyroxene, hornblende, mica and opaque minerals. The gabbroic rocks are characterized by relatively high initial ratios of 143Nd/ 144Nd (0.512618–0.512633) and low initial ratios of 87Sr/ 86Sr (0.70420–0.70435). Their initial Pb isotope ratios range from 17.306 to 17.771. The gabbros and cumulate wehrlites display geochemical distribution patterns analogous to the Alaskan-type intrusions that were derived from a subduction-related, high-alumina, hydrous basaltic parental magma in a convergent plate setting. LA-ICPMS analyses of zircons separated from the gabbros show high Th/U ratios (1.35–5.26) typical of magmatic origin. The U–Pb isotopic data define a weighted mean 206Pb/ 238U age of 203.3 ± 4.9 Ma. Detailed evaluation of the magma tectonics suggests that the Alaskan-type Aksalur ultramafic–mafic intrusions were accumulated in magma chamber(s) located at relatively shallow crustal levels. Two distinct magmatic pulses are identified: the first phase produced the gabbroic intrusions and the second phase was characterized by numerous small-scale werhlitic cumulate intrusions. In contrast to the previous models which assigned a mid-oceanic ridge-derived ophiolite affinity for ultramafic–mafic complexes in the eastern Pontides orogenic belt, our study reveals that these intrusions were derived from subduction-related high-alumina hydrous basaltic magmas transferred into the overlying relatively thin continental crust along deep-seated fractures.

Low grade uranium deposits of India – a bane or boon

Abstract Uranium resources of the world is estimated to be 5.5 million tonnes and the proven resources in India forms 3% of the world resources. The biggest uranium deposit is the Olympic dam deposit in Australia, which contains nearly 2.45 million tonnes of 0.023% U3O8, while the highest average grade of nearly 21% is established in the McArthur river deposit, Canada. Another very high grade deposit, the Cigar lake deposit, is also in Canada with an average grade of nearly 18%. Most of the uranium deposits established in India so far falls under the category of low grade. These low grade uranium deposits are distributed mainly in Singhbhum Shear Zone, Jharkhand; in parts of Chhattisgarh; Southern parts of Meghalaya; Cuddapah Basin, Andhra Pradesh; in parts of Karnataka and Aravalli- and Delhi Supergroups, Rajasthan and Haryana. These deposits are mainly hydrothermal vein type, stratabound type and unconformity related. The Singhbhum Shear Zone, Jharkhand hosts seventeen low grade vein type uranium deposits, aggregating about 30% of Indian uranium resources. The stratabound uranium mineralisation hosted by Vempalle dolostone extends over 160 km belt along southwestern margin of Cuddapah Basin in Andhra Pradesh and accounts 23% of the Indian resources. Though the dolostone hosted Tummalapalle uranium deposit was established in the early nineties, because of techno -economic constraints, the deposit remained dormant. As a consequence of the development of an innovative pre ssure alkali leaching process, the deposit became economically viable and a mine and mill are being constructed here. Recent exploration inputs are leading more such strata-bound low grade uranium deposits in the extension areas of Tummalapalle. Nearly 10 blocks have been identified within a 30 km belt which are being actively explored and a large uranium deposit has already been proved in this province. The deposit at Tummalapalle and adjoining areas is likely to emerge as one of the larger deposits in the world. The northern parts of Cuddapah basin in Andhra Pradesh is the host for unconformity related low grade uranium deposits. Four low grade low to medium tonnage deposits viz. Lambapur, Peddagattu, Chitrial and Koppunuru have been established in this province along the contact between basement granite and Proterozoic sedimentary rocks. This province also holds promise for large tonnage-high grade mineralization in the deeper unexplored parts of the basin. The North Delhi Fold Belt in the states of Rajas than and Haryana is the host for albitite/vein type uranium mineralization. A 320 km long NE-SW trending zone of deep seated fractures is the locus of mineralization and the mineralization is caused by soda metasomatism. A low grade uranium deposit is already established at Rohil associated with albitites and vast areas in this zone is open for exploration. The Cigar lake deposit, one of the two very high grade deposits in the world, though discovered in the year 1991 remains unexploited because of it high grade and requirement of remote handling and high end mining technology. The Mc Arthur deposit is being mined by raisebore mining method and the entire operation is carried out by 750 employees. Because of the high grade nature of the ore, it is diluted with barren rocks to bring the grade into manageable level before the beneficiation. The Rossing uranium deposit in Namibia, the biggest opencast mine for uranium, is of low grade and produces nearly 8% of the global output. As a result of the massive minin g and beneficiation operation, the employment of manpower is also very high compared to the high grade deposits. India, being a developing country with a high rate of unemployment, should treat the low grade deposits as an opportunity for employment. The operation of Rossing mine in Namibia is a shining example of as to how a low grade deposit could be economically exploited in the national interest and still be among the top five countries in terms of production. Inspite of mining the low grade resources, uranium mining industry in India provides direct and indirect employment to thousands of people and still operates in profit mode. Environmental concerns and radiation exposure to mining and milling personnel is negligible in the case of low grade deposits. The new mines, which are going to be opened in Tummalapalle, Andhra Pradesh; KPM (Domiasiat), Meghalaya; Gogi, Karnataka; Rohil, Rajasthan etc. will create substantial direct and indirect employment opportunities and transform the economy of these regions.

Seismic b-Value and the Assessment of Ambient Stress in Northeast India

Seismicity data of northeast India, recorded between 1986 and 1999 by a local network, are analysed for estimation of b-values. Based on the obtained values, viz. low (b ≤ 0.5), moderate (0.5 < b ≤ 0.7) and high (b > 0.7), the study area is classified into different seismic-domains. An assessment of stress level is also carried out in identifying seismic-domains. Seismic activities, though mostly confined in some sectors, are presumably triggered by mutual interaction of the Shillong Plateau, Mikir Hills, Indo-Burman Ranges and the easternmost part of the Himalayas, and the contributions from deep-seated fractures cannot be ignored. The results resemble the seismic character of a foreland setting adjacent to a convergent margin. The b-values estimated for 240 square grids of dimension 0.6° × 0.6° over five seismic domains indicate wide variation. An analysis of cumulative seismic moment release (M O) in different layers also indicates an anomaly in reference to the total seismic-energy budget of the five zones. The lower b-value and higher M O recorded at relatively lower depth (~30 km) towards the southwest of the study area might be associated with upward bulging of a strong lithosphere. The bulging is perhaps regionally compensated by the downward flexing of the descending Indian lithosphere beneath the Upper Assam area; features unequivocally observed in any foreland setup. Towards the north and east of the study area, random variations of in both b-value and M O along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas.

Provenance of Lower Cretaceous Wölong Volcaniclastics in the Tibetan Tethyan Himalaya: Implications for the final breakup of Eastern Gondwana

Abstract Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wolong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wolong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ∼ 140 Ma to ∼ 119 Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust. The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (∼ 102 Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent.

Topographic ellipses in Finney and Sedgwick counties, Kansas may signal deep structures in Precambrian basement

Hillshade maps based on digitized topography and capable of detecting subtle changes in slope that are otherwise imperceptible, reveal two large and heretofore unrecognized topographic ellipses in Kansas here designated the Garden City ellipse (10 by 14 km dimensions) in Finney County and the Goddard ellipse (11 by 13 km) in Sedgwick County. Cheyenne Bottoms in Barton County, although long known, is a larger ellipse (22 by 32 km). All three are bounded by outlines that are remarkably elliptical, suggesting that they may represent the surface expressions of deeply buried bodies in the Precambrian basement perhaps the result of meteorite impact. Cheyenne Bottoms is controversial because it has been ascribed to subsidence caused by solution of the underlying Permian Hutchinson Salt. The Garden City and Goddard ellipses are not known to involve faulting, but it is hypothesized that fractures that bound deeply buried elliptical bodies that lie beneath them are propagated to the surface and perceptibly affect the topography. Support for the hypothesis that deep-seated fractures affect the landscape in the region also is provided by the pervasive influence of a “fine-scale fracture system” that may extend through the crust and influence the development of reticulate linear drainage patterns. Recurrent small up and down movements, perhaps in response to tectonic movements elsewhere and maintained by earth tides, may be responsible for the fine-scale fracture system. Paradoxically the fractures of the fine-scale system seem to be distinct from those that bound the ellipses, but they strengthen the hypothesis that deep-seated fractures of various configurations are basic aspects of the crust and are capable of revealing deep-seated features heretofore unrecognized.

The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts

AbstractThe Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20%), clinopyroxene (30–35%), plagioclase (45–55%) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO238.2–45.5 wt%, MgO 8.7–13.0 wt% and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N= 19.6), indicative of a garnet-bearing source. The143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 (εNd= 4.5 to 5.9), and87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10%), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1%) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.

New K–Ar age determinations of Kilimanjaro volcano in the North Tanzanian diverging rift, East Africa

The Kilimanjaro is the African highest mountain and culminates at 5895 m high. This huge volcanic edifice is composed of three main centres along a N110°E-striking axis (Shira, Kibo and Mawenzi from W to E), and emplaced in a key area where a major N80°E-oriented volcanic lineament intersects a first-order NW–SE basement fault-like discontinuity. Seventeen K–Ar ages (on microcrystalline groundmass) acquired on lavas and intrusive facies from the three eruptive centres confirm that the Plio-Quaternary volcanicity of Kilimanjaro is clearly polyphased. The oldest phases of volcanic activity begun at ~ 2.5 Ma in the Shira vent and our data suggest that the latest important phases occurred around 1.9 Ma, just before the collapse of the Northern part of the edifice. Magmatic activity then shifted eastwards in the Mawenzi and Kibo twin centres where initial volcanism is dated at ~ 1 Ma. Two K–Ar ages obtained for the most recent Mawenzi rocks from the Neumann Tower-Mawenzi group (492 ka) and Mawenzi eruptive centre (448 ka), near the present summit, are linked to the final stage of edification for this centre. Whereas the eruptive activity ceased in the Mawenzi, it still continued on Kibo since sub-actual time. The oldest dated rocks from Kibo (482 ka) is obtained on a dyke from the Lava Tower group cropping out at 4600 m high. The main phase of magmatism on Kibo is recorded by two lava formations with a great spatial extension – the Rhomb Porphyry group and the Lent group – that have been emplaced in a short time interval at ~ 460–360 ka (including two erosive stages) and 359–337 ka, respectively. Based on the dating of Caldera rim group lavas, it is shown that the edification of the present cone was accomplished in a period ranging from 274 to 170 ka. The new ages obtained for the main episodes of volcanic activity on Kibo appear to roughly coincide with the oldest known Quaternary glaciations. The interaction between eruptive phenomena and the ice cover is assumed to have played an important role in triggering collapse processes and associated lahars deposits. The last volcanicity, around 200–150 ka, is marked by the formation of the present summit crater in Kibo and the development of linear parasitic volcanic belts, constituted by numerous Strombolian-type isolated cones on the NW and SE slopes of Kilimanjaro. These belts are likely to occur above deep-seated fractures that have guided the magma ascent, and the changes in their directions with time might be related to the rotation of recent local stress field.

Pyroxenite xenoliths from the Rio Puerco volcanic field, New Mexico: Melt metasomatism at the margin of the Rio Grande rift

Mantle xenoliths from the Pliocene Rio Puerco volcanic field, New Mexico, record chemical modification of the subcontinental lithosphere in the transition zone between the Colorado Plateau and the Rio Grande rift. The Rio Puerco necks brought a wide variety of mantle xenoliths to the surface, including abundant spinel ± garnet pyroxenites. Garnet pyroxenites equilibrated at pressures within the spinel lherzolite stability field (16–18 kbar) but at higher temperatures (1020–1100 °C) than lherzolite xenoliths (900–1000 °C). Calcite-bearing pyroxenites record temperatures as high as 1145 °C. Textural observations combined with the high equilibration temperatures are consistent with pyroxenite formation in response to melt interaction with a lherzolite precursor in the approximate ratio of 35% lherzolite to 65% melt. Bulk chemical data and the presence of both carbonatitic and basaltic glass inclusions indicate that the infiltrating melt(s) included both carbonatite and silicate components. Different mantle xenolith populations in volcanic centers of different ages in the Rio Grande rift constrain the likely age of melt infiltration and pyroxenite formation beneath the Puerco necks to Miocene–early Pliocene. Orthopyroxene + spinel + clinopyroxene + silicate glass symplectites around relict garnets record continued rift-related decompression ± heating following pyroxenite formation. Carbonatite + silicate melt generation was likely concentrated in areas affected by earlier metasomatism related to devolatilization of the subducted Farallon slab. Deep-seated fractures associated with the Jemez Lineament may have focused melts beneath the Puerco necks and resulted in wholesale conversion of lherzolite into pyroxenite.