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.     

Tectonic analysis of an oceanic transform fault zone based on fault-slip data and earthquake focal mechanisms: the Húsavík–Flatey Fault zone, Iceland

The Húsavík–Flatey Fault (HFF) is an oblique dextral transform fault, part of the Tjörnes Fracture Zone (TFZ), that connects the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We carry out stress inversion to reconstruct the paleostress fields and present-day stress fields along the Húsavík–Flatey Fault, analysing 2700 brittle tectonic data measured on the field and about 700 earthquake focal mechanisms calculated by the Icelandic Meteorological Office. This allows us to discuss the Latest Cenozoic finite deformations (from the tectonic data) as well as the present-day deformations (from the earthquake mechanisms). In both these cases, different tectonic groups are reconstructed and each of them includes several distinct stress states characterised by normal or strike-slip faulting. The stress states of a same tectonic group are related through stress permutations (σ₁−σ₂ and σ₂−σ₃ permutations as well as σ₁−σ₃ reversals). They do not reflect separate tectonic episodes. The tectonic groups derived from the geological data and the earthquake data have striking similarity and are considered to be related. The obliquity of the Húsavík–Flatey Fault implies geometric accommodation in the transform zone, resulting mainly from a dextral transtension along an ENE–WSW trend. This overall mechanism is subject to slip partitioning into two stress states: a Húsavík–Flatey Fault-perpendicular, NE–SW trending extension and a Húsavík–Flatey Fault-parallel, NW–SE trending extension. These three regimes occur in various local tectonic successions and not as a regional definite succession of tectonic events. The largest magnitude earthquakes reveal a regional stress field tightly related to the transform motion, whereas the lowest magnitude earthquakes depend on the local stress fields. The field data also reveal an early extension trending similar to the spreading vector. The focal mechanism data do not reflect this extension, which occurred earlier in the evolution of the HFF and is interpreted as a stage of structural development dominated by the rifting process.     

Landslides and neotectonic activities in the Main Boundary Thrust (MBT) zone: Southeastern Kumaun,Uttarakhand

Main Boundary Thrust (MBT) zone is constituted of some of the landslide prone areas in southeastern part of Kumaun Sub-Himalaya. Role of landslides as natural hazard and hill slope modifying agent is well documented from various part of Himalayan region and southern hills of Kumaun particularly in the MBT zone, which are susceptible to various type of mass movement. The rocks making up the slopes has been put to a number of brittle deformation phases during the movement along the MBT, and are traversed by number of joint sets. In the open slope these intersecting joint sets forms wedges and are the most favorable site for initiation of rockfalls and other types of landslides. Landslides are taking place primarily due to high angle slopes, formation of structural wedges along the free steep slopes, sheared nature of the rocks due to proximity to the MBT and neotectonic activities along the MBT and other transverse faults. Wedge failure is a common type of landslides in rock slopes characterized by multiple joints and acts as sliding planes for the failed blocks. Field observations and wedge failure analysis indicates most of the landslides taking place in MBT zone of Kumaun Sub-Himalaya are joint controlled. Safety Factor analysis suggests MBT zone of Kumaun Sub-Himalayan region is prone to landslides and related mass movements. This zone is also neotectonically active as indicated by various geomorphic signatures such as structurally controlled drainage pattern, offsetting of fan by MBT and formation of number of small lakes.     

Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions.During winter: the soil is only partly saturated in water and the CO₂ partial pressure is low (Ca 10^?3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and ¹³C content ($\text{δ}{}_{\mathrm{tidc}}{}^{13}\text{C = ?12,2\%0}$) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: $\text{δ}{}_{\mathrm{G}}{}^{13}\text{C = ?21,5\%0}$; $\text{A}{}_{\mathrm{G}}{}^{14}\text{C = 118\%}$; $\text{δ}{}_{\mathrm{M}}{}^{13}\text{C = +2,9\%0}$; $\text{A}{}_{\mathrm{M}}{}^{14}\text{C = 28\%}$).During spring and summer: the soil moisture decreases, the input of biogenic CO₂ induces an increase of the soil CO₂ partial pressure (Ca from 3.10^?3 atm to 7.10^?3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO₂ with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO₂ dissolution and CO₂ effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δ_{L mes} = from ?9,4%0 to ?11,5%0, δ_{l calc} = from ?9,8%0 to ?13,9%0 A_{L mes} = 119%, A_{L calc} = from 119% to 125%).The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite.     

The metallogenic relationship between Birimian and Tarkwaian gold deposits in Ghana

The traditional concept of the Early Proterozoic gold deposits in Ghana — i.e. gold-bearing shear zones overlain by Tarkwaian paleoplacers containing reworked gold derived from the shear-zones — needs to be reconsidered in the light of recent research in Ghana, the Ivory Coast and French Guiana. This research has revealed a consistent pattern of geostructural and metallogenic evolution in which both the Birimian and the Tarkwaian rocks were deformed by a major Eburnean compression (D2). It has shown that the NE-SW faults controlling the Gold Coast Range shear-zone mineralization (Ashanti-Prestea) were formed during the Eburnean D2 episode of thrusting that was followed by hydrothermal activity with the emplacement of auriferous arsenopyrite and then by the development of quartz veinlets and native gold; thus the shear-zone mineralization could only have appeared during the D2 late-orogenic stage. It has also shown evidence of post-depositional D2 deformation in the gold sites examined in the Tarkwa gold-bearing conglomerate, although the effects are limited and primary lithological controls have been preserved that reveal these deposits to be modified paleo-placers. Thus, the Tarkwaian gold could not be derived from the gold-bearing shear-zones.     

Effect of water on the heat capacity of polymerized aluminosilicate glasses and melts

The effect of water on heat capacity has been determined for four series of hydrated synthetic aluminosilicate glasses and supercooled liquids close to albite, phonolite, trachyte, and leucogranite compositions. Heat capacities were measured at atmospheric pressure by differential scanning calorimetry for water contents between 0 and 4.9 wt % from 300 K to about 100 K above the glass transition temperature (T_g). The partial molar heat capacity of water in polymerized aluminosilicate glasses, which can be considered as independent of composition, is (J/mol K). In liquids containing at least 1 wt % H₂O, the partial molar heat capacity of water is about 85 J/mol K. From speciation data, the effects of water as hydroxyl groups and as molecular water have tentatively been estimated, with partial molar heat capacities of 153 ± 18 and 41 ± 14 J/mol K, respectively. In all cases, water strongly increases the configurational heat capacity at T_g and exerts a marked depressing effect on T_g, in close agreement with the results of viscosity experiments on the same series of glasses. Consistent with the Adam and Gibbs theory of relaxation processes, the departure of the viscosity of hydrous melts from Arrhenian variations correlates with the magnitude of configurational heat capacities.     

Retrograde mineral and fluid evolution in high-pressure metapelites (Schistes lustrés unit, Western Alps)

Fluid inclusions have been analysed in successive generations of syn-metamorphic segregations within low-grade, high-pressure, low-temperature (HP–LT) metapelites from the Western Alps. Fluid composition was then compared to mass transfer deduced from outcrop-scale retrograde mineral reactions. Two types of quartz segregations (veins) occur in the `Schistes lustrés' unit: early blueschist-facies carpholite-bearing veins (BS) and retrograde greenschist-facies chlorite-bearing veins (GS). Fluid inclusions in both types of segregations are aqueous (no trace of dissolved gases such as CO₂, CH₄, N₂), with significant differences in density and composition (salinity). BS fluids are moderately saline fluids (average 9.1 wt% eq. NaCl) characterized by a chronological trend towards more dilute composition (from 15 down to 0 wt% eq. NaCl), whereas GS fluids have a very constant salinity of ∼3.7 wt% eq. NaCl. Both types of inclusions were continuously reset to lower densities along the retrograde path, until a temperature of ∼300 °C. Mass-balance calculations, together with fluid inclusion data, suggest that GS fluids result from the mixing between two fluid sources: one initial, early metamorphic, moderately saline HP fluid and a second nearly pure water fluid provided by the breakdown of carpholite. Estimates of the amount of water released by carpholite breakdown result in a dilution of the interstitial fluid phase (from 10 to 2.5–4 wt% eq. NaCl) consistent with the actual shift of the fluid composition. Alkali elements required for the formation of the GS chlorite + phengite assemblage after carpholite could be locally provided by HP phengite. This is taken as an indirect evidence that, during the generation of both BS and GS fluids, mixing with externally derived fluids may have been very limited. The location, amount and constant composition of the less saline GS fluids appear to be related to an interconnected porosity at the time of inclusion formation. Received: 19 October 1998 / Accepted: 19 July 2000     

The structure of the guinean continental margin: Implications for the connection between the central and the south atlantic oceans

The results of a recent geological-geophysical survey, conducted off Guinea, are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the West African continental margin. Three sectors are distinguished:

A northwestern portion of the margin comprises a wide and deeply submerged plateau — the Guinean Marginal Plateau underlain by a thick sedimentary sequence and facing westward toward the Gambia Abyssal Plain. Scismic stratigraphy and structures show clear analogies to the Jurassic margins of the central Atlantic. Including the presence of a Cretaceous paleoslope covered by Cenozoic deposits.

A southern area of the margin comprises a series of aligned (W-E trending), acoustic basement features extending along the slope and bounding the Guinean Plateau to the south. These features, basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data. The bordering deep Sierra Leone abyssal plain, also dissected by E-W-trending oceanic fracture zones, contains a sedimentary cover apparently not older than middle Cretaceous.

Between both sectors and between two NW-SE trending scarps lies an intermediate area. The seismic profiles show that here, the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the S-E and covered by untectonized (but partly eroded) Upper Cretaceous to Cenozoic sediments.

The overall structure of the Guinean Margin is interpreted as the result of two major events. During a first phase the margin was created at the southern extremity of the central Jurassic Atlantic and developed like other comparable margins. During a s econd phase (beginning in Early Cretaceous times) the margin was progressively submitted to the opening of the equatorial South Atlantic. This process gave rise to the margin of the southern Guinean plateau (locally injected by volcanics) and generated the tectonic features of the intermediate zone. This protion may thus represent a part of the rifted Jurassic margin discordantly dissected by the oblique opening of the south Atlantic in the area. The oceanic crust of the central and south Atlantic were definitively connected only during Late Albian times as indicated by the end of the tectonic activity and the early Upper Cretaceous unconformity.