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31.
Basab Mukhopadhyay Manoj Mukhopadhyay Sujit Dasgupta 《Journal of the Geological Society of India》2011,78(2):131-146
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. 相似文献
32.
The western margin of Sundaland is affected by two major tectonic events, the India-Eurasia collision and subduction along the Burmese-Andaman-Sunda Arc. Geotectonic analysis of the Burmese-Andaman-Sunda Arc areas reveals that it represents an arc setting exposed both in land and sea environment. Within this broad arc setting, there are individual segments that illustrate discernible trends in terms of structural, seismic and geological characteristics. The Equatorial Region around Sumatra has some peculiar geological/geophysical characteristics that make it a tectonic hotspot. Here a relationship between the active volcanism and the development of forearc basins is established. It is also proposed that the downgoing oceanic plate is more strongly coupled to the overlying plate where it is youngest (~ 40 Ma), has the highest temperature and is topographically most elevated with highest seismic activity. In particular, an increase in convergence rate and presence of youngest oceanic crust appear to be the main controlling factor underpinning the tectonics and surge of recent seismic activity in Sumatra. 相似文献
33.
Rita Parai Sujoy Mukhopadhyay John C. Lassiter 《Earth and Planetary Science Letters》2009,277(1-2):253-261
High 4He/3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle. 相似文献
34.
Twenty-five drinking water samples collected from the household property and from the Sydney Regional Municipality well bores and lakes were analyzed to evaluate the various inorganic parameters, level of concentrations of the priority elements and polyaromatic hydrocarbons (PAHs). The pH of the majority of the water samples was below the guidelines adopted by Health and Welfare Canada (1996), although the drinking waters supplied by the Sydney Regional Municipality were within the guidelines. Only three water samples (13 and 14: Point Aconi area and 16: Port Morien fish plant) have elevated concentrations of various PAHs compared to the detection limit. Eight samples have higher concentrations of manganese and two samples (number 7: Sydney Airport and number 1: RCMP Office; Reserve Mines) have higher concentrations of priority elements (especially lead) than the recommended guidelines (>0.05 mg/l). These priority elements and the PAHs in the drinking water samples may have originated from the leaching of the individual coal seams within that part of the Sydney Basin. Other potential sources of these elements and PAHs (Power Plant disposal, Sydney Tar Pond, metalliferous rocks, hydrocarbon reservoir rocks) are not located close enough to the sampling sites of the water samples. Therefore, they are not considered the source of these elements and PAHs. 相似文献
35.
36.
Dilip K. Mukhopadhyay 《Journal of Earth System Science》1990,99(2):201-213
In the Kolar Schist Belt well-preserved small-scale diastrophic structures suggest four phases of folding (F1 — F4). The near coaxial F1 andF
2folds are both isoclinal with long-drawn out limbs and sharp hinges. The axial planes of bothF
1andF
2folds are subvertical with N-S strikes; these control the linear outcrop pattern of the Schist belt. The later folds (F
3and F4) are important in small-to-intermediate scales only and are accommodation structures formed during the relaxation period
of the early folding episodes. Mesoscopic shear zones, post-F2 but pre-F3 in age, are present in all the rock types in this area. The F1 and F2 folds and the mesoscopic shear zones were formed during a continuous E-W subhorizontal compression. Available geochemical
and isotopic data show that the Kolar Schist Belt with ensimatic setting is bounded by two granitic terrains of contrasting
evolutionary histories. This, together with E-W subhorizontal compression over a protracted period of time, strengthens the
recent suggestions that the Kolar Schist Belt represents a suture. This belt then marks the site of a continent-continent
collision event of late Archaean-early Proterozoic age. 相似文献
37.
The study of seamount parameters in the tectonically most-complicated and least-understood Indian Ocean assumes importance
since their properties vary as a function of tectonic setting, physics of lithosphere, conduit geometry and chemical composition
of magma. More than 100 such seamounts ranging in summit height (h) from 300 to 2870 m, are indentified in the oceanic crust between Indian continent and Mid-Indian Ridge (MIR) and South-East
Indian Ridge (SEIR). Most of the minor seamounts (h > 1000) are found in the southern part of the study area. Major seamounts (h < 1000 m) are roughly distributed in two groups—the northern group on Cretaceous Oceanic Crust and southern group on Pliocene-Miocene
Oceanic Crust. On an average northern group seamounts (SM 1 to 6) are taller, wider and flatter than those from the southern
group. These seamounts appear to be the result of continuous growth from tapped, moving magma chamber while stress depleted
magma and inconsistent Indian Plate movement during Mid-Tertiary are attributed to the origin of southern group of smaller
seamounts. Distribution and morphology of seamounts as a whole indicate their formation either from Reunion hotspot or from
two separate hotspots in the geological past. 相似文献
38.
Comparison of microbially induced sedimentary structures (MISS) and stromatolitic bearing horizons from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, has been scrutinised to understand the formative processes and controls on MISS and stromatolites in the context of sedimentary facies and response to sea level fluctuations. MISS structures recorded are wrinkle structures, Kinneyia ripples, load casts, domal structures, sand chips, palimpsest and patchy ripples with limited desiccation cracks. Stromatolitic morphotypes recorded are solitary, branching, wavy and domal forms of stromatolites associated with ooids, peloids and fenestral laminae. MISS structures flourished within tidal flats to shallow intertidal while stromatolites mushroomed in environments ranging from tidal to deep subtidal. MISS structures were favoured by resistant substratum, low energy conditions, consistent water supply and low terrigenous input. Stromatolites boomed when the volume of carbonate accumulation exceeded siliciclastic deposition. Fluctuating environmental conditions and sediment budget controlled morphology of stromatolites. Owing to limited siliciclastic input during deposition of dolomudstones (characterizes transgressive systems tract), microbial growth was enhanced. Calcareous shales were deposited over dolomudstones which marks the maximum flooding surface (MFS) indicating the culmination of transgression. Deposition of storm-dominated sandstone-siltstone (FA1), wave-rippled sandstones (FA2), tide-dominated sandstones (FA3), heteroliths (FA4), wackestone-packestone (FA6), boundstone (FA7) and ooid-peloid grainstone (FA8) on top of the MFS reflects initiation of highstand systems tract (HST) which is mainly characterized by stromatolitic horizons, alternation of carbonates and siliciclastics with flourishing microbial activity. Eventually, increased sedimentation in upper part of Kunihar Formation marks late stage of regression. 相似文献
39.