Seismic clusters and their characteristics at the Arabian Sea Triple Junction: Supportive evidences for plate margin deformations

The plate margin features defining the Arabian Sea Triple Junction (ASTJ) are: the Aden Ridge (AR), Sheba Ridge (SR) with their intervening Alula-Fartak Transform (AFT), Carlsberg Ridge (CR) and Owen Fracture Zone (OFZ). Exact nature of ASTJ is presently debated: whether it is RRF (ridge-ridge-fault) or RRR (ridge-ridge-ridge) type. A revised seismicity map for ASTJ is given here using data for a period little more than a century. “Point density spatial statistical criterion” is applied to short-listed 742 earthquakes (mb ≥ 4.3), 10 numbers of spatio-temporal seismic clusters are identified for ASTJ and its arms. Relocated hypocentres help better constraining the cluster identification wherever such data exist. Seismic clusters actually diagnose the most intense zones of strain accumulation due to far field as well as the local stress operating at ASTJ. An earthquake swarm emanating from a prominent seismic cluster below SR provides an opportunity to investigate the pore pressure diffusion process (due to the active source) by means of “r-t plot”. Stress and faulting pattern in the active zones are deduced from 43 CMT solutions. While normal or lateral faulting is characteristic for these arms, an anomalous thrust earthquake occurs in the triangular ‘Wheatley Deep’ deformation zone proximal to ASTJ. The latter appears to have formed due to a shift of the deformational front from OFZ towards a transform that offsets SR. Though ASTJ is still in the process of evolution, available data favour that this RRF triple junction may eventually be converted to a more stable RRR type.     

Satellite detection of earthquake thermal infrared precursors in Iran

Stress accumulated in rocks in tectonically active areas may manifest itself as electromagnetic radiation emission and temperature variation through a process of energy transformation. Land surface temperature (LST) changes before an impending earthquake can be detected with thermal infrared (TIR) sensors such as NOAA-AVHRR, Terra/Aqua-MODIS, etc. TIR anomalies produced by 10 recent earthquakes in Iran during the period of Jun 2002–Jun 2006 in the tectonically active belt have been studied using pre- and post-earthquake NOAA-AVHRR datasets. Data analysis revealed a transient TIR rise in LST ranging 2–13°C in and around epicentral areas. The thermal anomalies started developing about 1–10 days prior to the main event depending upon the magnitude and focal depth, and disappeared after the main shock. In the case of moderate earthquakes (<6 magnitude) a dual thermal peak instead of the single rise has been observed. This may lead us to understand that perhaps pre-event sporadic release of energy from stressed rocks leads to a reduction in magnitude of the main shock. This TIR temperature increment prior to an impending earthquake can be attributed to degassing from rocks under stress or to p-hole activation in the stressed rock volume and their further recombination at the rock–air interface. A precise correlation of LST maps of Bam and Zarand with InSAR-generated deformation maps also provides evidence that the thermal anomaly is a ground-related phenomenon, not an atmospheric one.     

Contrasting Episodes of Regional Granulite-Facies Metamorphism in Enclaves and Host Gneisses from the Aravalli Delhi Mobile Belt, NW India

The Aravalli–Delhi Mobile Belt in the northwestern partof India demonstrates how granulite enclaves and their hostgneisses can be utilized to unravel multistage metamorphic historiesof orogenic belts, using three suites of metamorphic rocks:(1) an enclave of pelitic migmatite gneiss–leptynite gneiss;(2) metamorphosed megacrystic granitoids, intrusive into theenclave; (3) host tonalite–trondhjemite–granodiorite(TTG) gneisses associated with an interlayered sequence of garnetiferousmetabasite and psammo-pelitic schist, locally migmatitic. Basedon integrated structural, petrographic, mineral compositional,geothermobarometric studies and P–T pseudosection modellingin the systems NCKFMASH and NCFMASH, we record three distincttectonothermal events: an older, medium-pressure granulite-faciesmetamorphic event (M₁) in the sillimanite stability field, whichis registered only in the enclave, a younger, kyanite-gradehigh-pressure granulite-facies event (M₂), common to all thethree litho-associations, and a terminal amphibolite-faciesmetamorphic overprint (M₃). The high-P granulite facies eventhas a clockwise P–T loop with a well-constrained prograde,peak (M₂, P 12–15 kbar, T 815°C) and retrograde (M_2R,6·1 kbar, T 625°C) metamorphic history. M₃ is recordedparticularly in late shear zones. When collated with availablegeochronological data, the metamorphic P–T conditionsprovide the first constraint of crustal thickening in this belt,leading to the amalgamation of two crustal blocks during a collisionalorogeny of possible Early Mesoproterozoic age. M₃ reactivationis inferred to be of Grenvillian age. KEY WORDS: Northwestern India; polycyclic granulite enclave; pseudosection; high-pressure metamorphism; P–T path     

Paleomagnetism of a paleocene pluton on jamaica

A Paleocene granodiorite pluton on Jamaica has been subject to extensive weathering caused by the tropical marine environment of the island. The natural remanence of 29 samples obtained from relatively fresh rock in two localities was found to consist of two components with overlapping coercivity ranges. Alternating field treatment proved ineffective for removing the secondary component without destroying the primary one. Thermal demagnetization of samples from the two localities was more effective and yielded paleomagnetic poles at 14.7°N, 11.6°W and 58.9°N, 15.9°E respectively. These pole positions are different from those available from contemporaneous North American rocks and from poles derived from Jamaican Cretaceous and Upper Miocene rocks. Mineralogical studies showed that the granodiorite has undergone an extensive maghemitization superposed on earlier class 2 deuteric oxidation and related to the weathering process. Some of the titanomaghemite has, however, been converted to titanohematite. Hence although the secondary remanence carried by the former was removable by thermal treatment at 500°C, its part carried by the latter could not be removed without simultaneously destroying the primary remanence carried by the residual titanomagnetite. The observed paleopole positions do not, therefore, represent the true Paleocene geomagnetic field, but suggest that the direction of magnetization of the pluton has been approximately equatorial and was probably acquired in a reversed geomagnetic field. This could be interpreted as having been caused by the behavior of the geomagnetic field during a polarity transition, but a more favorable interpretation appears to be a large anticlockwise tectonic rotation of the islands since the Paleocene.     

Electron delocalization and magnetic behavior in a single crystal of ilvaite,a mixed valence iron silicate

Ilvaite, Ca(Fe²⁺, Fe³⁺)Fe²⁺Si₂O₈(OH), a black mixed valence iron silicate shows considerable Fe²⁺?Fe³⁺ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P2 ₁/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe²⁺ and Fe³⁺ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400?21 K. Below 343±1K, a continuous rotation of the molar susceptibility K _∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. X _a, X _b and X _c increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe²⁺ and Fe³⁺ spins on A sites was suggested in previous Mössbauer and neutron powder diffraction studies. In addition, 1/X _a shows an antiferromagnetic maximum at 50±3 K, whereas 1/X _b and 1/X _c at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe²⁺ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe²⁺(A)-Fe³⁺(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K _∥-K_⊥) suggests a singlet ground State ⁵A₁ for the Fe²⁺ ions, in agreement with previous Mössbauer studies.     

Suprathermal grains: On intergalactic magnetic fields

Charged dust grains of radiia3×10^–63×10^–5 cm may be driven out of the galaxy due to radiation pressure of starlight. Once clear of the main gas-dust layer, dust grains may then escape into intergalactic space. Such grains are virtually indestructible-being evaporated only during galaxy formation. The dust grains, once injected into the intergalactic medium, may acquire suprathermal energy, thus suprathermal grains in collision with magnetized cloud by the Fermi process. In order to attain relativistic energy, suprathermal grains have to move in and out (scattering) of the magnetic field of the medium. It is now well established that high energy cosmic rays are of the order 10²⁰ eV or more. We have speculated that these high energy (>-10¹⁸ eV) cosmic ray particles are charged dust grains, of intergalactic origin. This is possible only if there exists a magnetic field in the intergalactic medium.     

Relative abundance of elements in quasi-stellar objects and evolution of galaxies

Abundance ratios in QSOs Q 1246-057, 0453-423, PKS 0528-250, Q 0002-422, and PKS 2126-158 have been calculated by the method of curve of growth. Relative abundance of carbon with respect to silicon has been calculated in the QSOs Q 1246-057, PKS 0528-250, Q 0002-422, and PKS 2126-158. Relative abundance of aluminium with respect to silicon has been calculated in the QSOs PKS 0528-250 and PKS 2126-158. Relative abundance of iron with respect to magnesium has been also calculated in the QSOs Q 0453-423 and Q 0002-422. The ratiosN(C)/N(Si) andN(Al)/N(Si) andN(Fe)/N(Mg) in QSOs considered as a class are 3.45±2.33, 0.063±0.054, and 1.08±0.70, respectively.The ratiosN(C)/N(Si) andN(Al)/N(Si) show a mild trend of increase with decreasing emission redshift. On the other hand, the ratioN(Fe)/N(Mg) shows a mild trend of decrease with decreasing emission redshift. This suggests the possibility of the chemical evolution of QSOs.The comparison ofN(C)/N(Si), the relative abundance of carbon-one of the four most plentiful elements — with respect to silicon in QSOs and normal galaxies suggests that QSOs might belong to an early phase in the evolutionary sequence of galaxies and that they evolve chemically into normal galaxies in course of time.Mrs. Alpana Gangopadhyaya after marriage.     

Effective medium theory and multiple linear regression-based velocity estimation in syn-rift clastic sequence of the Krishna–Godavari basin,India

An inclusion model, based on the Kuster–Toksöz effective medium theory along with Gassmann theory, is tested to forward model velocities for fluid-saturated rocks. A simulated annealing algorithm, along with the inclusion model, effectively inverts measured compressional velocity (V_P) to achieve an effective pore aspect ratio at each depth in a depth variant manner, continuously along with depth. Early Cretaceous syn-rift clastic sediments at two different depth intervals from two wells [well A (2160–2274 m) and well B (5222–5303 m)], in the Krishna–Godavari basin, India, are used for this study. Shear velocity (V_S) estimated using modelled pore aspect ratio offers a high correlation coefficient (>0.95 for both the wells) with measured data. The modelled pore aspect ratio distribution suggests the decrease in pore aspect ratio for the deeper interval, mainly due to increased effective vertical stress. The pore aspect ratio analysis in relation to total porosity and volume of clay reveals that the clay volume has insignificant influence in shaping the pore geometry in the studied intervals. An approach based on multiple linear regression method effectively predicts velocity as a linear function of total porosity, the volume of clay and the modelled pore-space aspect ratio of the rock. We achieved a significant match between measured and predicted velocities. The correlation coefficients between measured and modelled velocities are considerably high (approximately 0.85 and 0.8, for V_P and V_S, respectively). This process indicates the possible influence of pore geometry along with total porosity and volume of clay on velocity.     

Predicting dam-related downstream geomorphic response with widely available stream gauge data: A case study of the Godavari River Basin,India

Dam-related downstream adjustments of riverbeds are normally investigated by analysing the trend in sediment supply and high flow events during the pre- and post-dam periods. The required data for existing predictive models is not measured at river gauges, which limits the application of these tools. We derived the frequency of sediment-transporting streamflow events (T*) and upstream sediment supply (S*) in the pre- and post-dam periods with widely available gauged records and predicted changes in the downstream riverbed by adapting an existing model. Ten gauging stations in the Godavari River Basin, India, located downstream of dams, were chosen as study sites. Annually surveyed cross-sections at each site validated the accuracy of the predicted dam-related downstream changes. Then, a regression equation (R² = 0.75) was established between T*/S* (independent variable) and changes in the downstream bed elevation (dependent variable) for the Godavari Basin. We recommended that similar local empirical equations be formulated for larger river basins. Models of large-scale rainfall-runoff and sediment transport processes that can account for the influence of dams, such as the Soil & Water Assessment Tool, can be paired with the proposed regression equation to estimate dam-related downstream erosion and deposition with globally available data.     

Facies characteristics of the basal part of the Talchir Formation,Talchir Basin,India — Depositional history revisited

The lowest unit of the Talchir Formation of Talchir Basin, Orissa, was described by pioneer workers as the ‘basal boulder bed’. In an attempt to explain the co-existence of gravel and clay, materials of contrasting hydraulic properties, a probable situation resembling the effects of the action of ground-ice enabled boulders to be carried down by sluggish currents resulting in an intermixture of large boulders and fine mud was conceived. Misinterpretation of this conclusion led to a general tendency to describe the ‘basal boulder bed’ as ‘glacial tillite’. However, the unit described as ‘basal boulder bed’ is actually represented by a matrix rich conglomerate with pockets of normally graded silty clay. The present study reveals that the depositional imprints preserved in this part of the sedimentary succession indicate emplacement of successive debris flows generated through remobilization of pre-existing unconsolidated sediments. Small pockets of fine-grained turbidites presumably deposited from the entrained turbidity currents associated with the debris flows suggest the composite character of the debris flow deposit.