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1.
The statistical relationship between the summer monsoon rainfall over all India, northwest India and peninsular India, onset
dates of monsoon and the index of mid latitude, (35° to 70°N) meridional circulation at 500 hPa level over different sectors
and hemisphere based on 19 years (1971–1989) data, have been examined. The results indicate that (i) the summer monsoon rainfalls
over all India, northwest India and peninsular India show a significant inverse relationship with the strength of meridional
index during previous January over sector 45°W to 90°E. (ii) The summer monsoon rainfalls over all India and peninsular India
show a significant inverse relationship with the strength of meridional index during previous December over sector 90°E to
160°E, (iii) The summer monsoon rainfall over northwest India shows a significant direct relationship with the meridional
index during previous May over sector 160°E to 45°W.
Significant negative relationships are also observed between the meridional circulation indices of previous October (sector
3 and 4), previous December (sectors 1, 3 and 4), previous winter season (sector 3 and 4) and the onset dates of summer monsoon
over India. The meridional circulation index thus can have some possible use for long range forecasting of monsoon rainfall
over all India, northwest India and peninsular India, as well as the onset dates of monsoon. 相似文献
2.
Two magnetometer array experiments were conducted in India during 1978–1980, under an Indo-Australian collaboration project, using 21 Australian three-component magnetometers of the Gough-Reitzel type. The first array study was made in the northwestern region covering the Aravallis, the Punjab, and the lesser Himalaya, while the second experiment was carried out in the southern peninsular shield area. Both these sets of geomagnetic deep sounding (GDS) observations yielded valuable results on the crustal and upper mantle structure in the two geologically and geophysically important regions of India.Geomagnetic induction patterns observed in northwest India have revealed a variety of electrical conductivity structures. The primary conductivity structure providing paths for induced currents is found to be striking at right-angles to the Himalayan Mountains. The conductivity structure is indicated to be a northward continuation of the Aravalli belt and, thus, suggesting the continuation of the Indian shield at depth into the base of the Himalayan foothills under the Ganga basin.The induction effects observed in the southern tip of peninsular India are by far the most complex geophysical phenomenon due to the simultaneous occurrence of the sea coast, the crustal and upper mantle conductivity anomalies between India and Sri Lanka under the sea, and the day-time equatorial electrojet as part of the external heterogeneous inducing field. It is further complicated by the existence of a conductive step, structure along the coastline at the Moho boundary and a “graben” structure in the Palk Strait, as revealed by the array observations. 相似文献
3.
F.H. Chamalaun S.N. Prasad F.E.M. Lilley B.J. Srivastava B.P. Singh B.R. Arora 《Tectonophysics》1987,140(2-4):247-255
The geomagnetic variation data from the 1979 Indian array experiment have been reanalyzed and reexamined using the hypothetical event analysis technique. The contour map of the |Z/H ratio replicates distinctive anomaly in northwest India previously delineated in maps of the Fourier coefficients. The anomaly reveals the presence of a significant conductor under the Ganga basin. The contour map has been used to derive a response profile perpendicular to the strike of the anomaly, for comparison with 2-D numerical models. An excellent fit was found for a conductor at a depth of 32 km, with a width of 110 km and a conductivity contrast of 1000. This result places the conductor deep within the lithosphere. In the absence of supporting data the origin of the conductor is difficult to resolve. However, it is thought to be related to pressure-released partial melting, caused by fracturing of the Indian crust during the collision of India with Asia. 相似文献
4.
5.
An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. Mass balance estimations to characterize temporal and spatial variations in the frontal accretion, or underplating and subduction of sediments since the Late Miocene, were made using seismic and drill-hole data. At 200 km north of the triple junction, almost 80% of the sediment on the downgoing Nazca plate are subducted. Sediment subduction rate decreases towards the triple junction because of a low in sedimentation rates as the flank of the spreading ridge approaches the trench. At the triple junction, the forearc is almost completely destroyed by spreading ridge collision and subduction erosion. Less than 12% of the available sedimentary input is accreted. South of the triple junction, where the spreading ridge passed 6 Ma ago, a large fraction (>60%) of the sediment on the incoming Antarctic plate has been scraped off and was frontally accreted to the Chile forearc. Spreading ridge subduction leaves a distinctive geological fingerprint, and has a large impact on the mass balance of the subduction zone. However, the high rates of change in the process may make this fingerprint hard to detect in fossil convergent orogens. In the ridge collision zone the sediment supplied to the trench, and the amount of sediment subducted, show strong and distinctive variations on a 1- to 5-million-year time scale. On a 10-million-year time scale, sediment subduction to the Earth's mantle is reduced by spreading ridge collision, caused by the need of the overriding forearc to regain a low angle of taper by frontal accretion. 相似文献
6.
This paper examines the variability of seismic activity observed in the case of different geological zones of peninsular India (10°N–26°N; 68°E–90°E) based on earthquake catalog between the period 1842 and 2002 and estimates earthquake hazard for the region. With compilation of earthquake catalog in terms of moment magnitude and establishing broad completeness criteria, we derive the seismicity parameters for each geologic zone of peninsular India using maximum likelihood procedure. The estimated parameters provide the basis for understanding the historical seismicity associated with different geological zones of peninsular India and also provide important inputs for future seismic hazard estimation studies in the region. Based on present investigation, it is clear that earthquake recurrence activity in various geologic zones of peninsular India is distinct and varies considerably between its cratonic and rifting zones. The study identifies the likely hazards due to the possibility of moderate to large earthquakes in peninsular India and also presents the influence of spatial rate variation in the seismic activity of this region. This paper presents the influence of source zone characterization and recurrence rate variation pattern on the maximum earthquake magnitude estimation. The results presented in the paper provide a useful basis for probabilistic seismic hazard studies and microzonation studies in peninsular India. 相似文献
7.
Some allochthonous terranes form along active continental margins when slivers of forearc crust (or more extensive crust) are displaced along arc-parallel strike-slip faults. Such faults can be generated or reactivated in response to either oblique subduction or ridge collision (collision between an oceanic spreading ridge and the leading edge of the forearc). The mechanical and thermal effects of ridge collision are important factors in the origin crustal development of some forearc sliver terranes. Some of the effects of ridge collision are well illustrated in the South American forearc near the Chile triple junction (46° S) where the Chile Rise is colliding today. Impingement of the Chile Rise, in conjuction with oblique subduction, has caused an elongate forearc sliver terrane to move northward away from an extensional zone at the collision site. The terrane is bounded on the east by the arc-parallel Liquiñe-Ofqui fault system (LOF) which coincides roughly with the forearc-arc boundary, and on the south by the Golfo de Penas extensional basin. Fault fabrics, recent seismicity, and paleomagnetic results indicate a component of right-lateral strike-slip movement on the LOF. Neotectonic geomorphology and pre- and post-seismic vertical strain data from the 1960 Concepcíon earthquake indicate a west-down dip-slip component of movement. Three-dimensional finite element models of ridge collision in this region substantiate these shear strains and development of an arc-parallel fault at about 150–200 km from the trench.Development of the forearc crust during Miocene and younger collision also involved intrusion of silicic magmas and emplacement of the Pliocene(?) Taitao ophiolite within about 15 km of the trench. The ophiolite and the silicic magmas constitute anomalous additions to the forearc crust, and record tectonic events leading to the origin of the allochthonous terrane carrying them. Similar ophiolite/silicic plutonic associations may help unravel the origin of other allochthonous terranes. 相似文献
8.
9.
J. R. Kayal V. K. Srivastava P. Kumar Rima Chatterjee P. K. Khan 《Journal of the Geological Society of India》2011,78(1):76-80
A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus, Dhanbad.
We have used the broadband (BB) seismograms of 17 teleseismic events (M ≥ 5.8) recorded by this single BB station during 2008–09
to estimate the crust and upper mantle discontinuities in Dhanbad area which falls in the peninsular India shield. The converted
wave technique and the Receiver function analysis are used. A 1-D velocity model has been derived using inversion. The Mohorovicic
(Moho) discontinuity (crustal thickness) below the ISM observatory is estimated to be ∼41 km, with an average Poisson ratio
of ∼0.28, suggesting that the crust below the Dhanbad area is intermediate to mafic in nature. The single station BB data
shed new light to the estimate of crustal thickness beneath the eastern India shield area, which was hitherto elusive. Further,
it is observed that the global upper mantle discontinuity at 410 km is delayed by ∼0.6 sec compared to the IASP-91 global
model; this may be explained by a slower/hotter upper mantle; while the 660 km discontinuity is within the noise level of
data. 相似文献
10.
P. Sengupta S. K. Nath K. K. S. Thingbaijam S. Mistri 《Journal of the Geological Society of India》2011,78(3):226-232
The seismicity of a region is implicit of the causal faulting mechanisms and geodynamic diversity of the subsurface regime
nucleating earthquakes of different magnitudes, several of which may be as devastating as ones historically reported in global
perspective of tectonic complexity as in the case of India. Fractal analysis using box-counting method for the major fault
networks across the country estimates fractal dimension, Df, values to be varying between 0.88 and 1.36. The fault segments in parts of northwest Himalayas, northeast India and Indo-Gangetic
plains, are observed to be associated with higher Df values implicating high seismicity rates. On the other hand, low Df values in the peninsular India indicate isolated pattern of the underlying faults. The fractal dimension is observed to be
indicative of predominant faulting types — higher values conforming to thrust faulting mechanism while lower to strike slip
tectonism. 相似文献
11.
12.
L.N. Kailasam 《Tectonophysics》1979,52(1-4)
Recent crustal movements have been observed and studied in several parts of India including the Himalayan and sub-Himalayan regions, the Precambrian shield of peninsular India and also the coastal tracts. The results of studies of Holocene deformation and crustal movements in two type areas are presented, one in the extreme southeastern part of the peninsula and the other in northeastern India.The Precambrian shield in the extreme southeastern part is characterised by a major NE—SW trending fault zone in the Tirupattur—Mattur areas of Tamil Nadu with some major extended faults, one of which apparently cuts through the entire crust and Moho as indicated by gravity data and which is associated with occurrences of alkaline and basic intrusions and carbonatite complex. Evidence of Recent crustal movements in this zone is afforded by geomorphic features and recent and current seismicity of a mild nature which is apparently to be attributed to slow movements along the fault plane.The Shillong plateau in northeastern India occurs as block-uplifted horst, comprising for the most part Archaean crystalline rocks with plateau basalts and Cretaceous and Tertiary sediments occurring on its southern margin. The plateau is bounded by major faults and is located in a zone of high seismicity lying astride and parallel to the eastern Himalayas intervened by the alluvium of the Brahmaputra Valley. Geomorphic features such as raised terraces, straight-edged scarps, etc., provide evidence for Recent crustal movements with dominant vertical movements along the fault planes which have continued through Tertiary and Recent times. Repeated precision levelling measurements conducted by the Survey of India indicate a rate of uplift of 4–5 cm per 100 years during the period 1910–1975.The gravity data pertaining to this region are also discussed in relation to the crustal movements. 相似文献
13.
The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate. 相似文献
14.
P Guhathakurta Preetha Menon P M Inkane Usha Krishnan S T Sable 《Journal of Earth System Science》2017,126(8):120
Meteorological drought during the southwest monsoon season and for the northeast monsoon season over five meteorological subdivisions of India for the period 1901–2015 has been examined using district and all India standardized precipitation index (SPI). Whenever all India southwest monsoon rainfall was less than ?10% or below normal, for those years all India SPI was found as ?1 or less. Composite analysis of SPI for the below normal years, viz., less than ?15% and ?20% of normal rainfall years indicate that during those years more than 30% of country’s area was under drought condition, whenever all India southwest monsoon rainfall was –15% or less than normal. Trend analysis of monthly SPI for the monsoon months identified the districts experiencing significant increase in drought occurrences. Significant positive correlation has been found with the meteorological drought over most of the districts of central, northern and peninsular India, while negative correlation was seen over the districts of eastern India with NINO 3.4 SST. For the first time, meteorological drought analysis over districts and its association with equatorial pacific SST and probability analysis has been done for the northeast monsoon over the affected regions of south peninsular India. Temporal correlation of all India southwest monsoon SPI and south peninsular India northeast monsoon SPI has been done with the global SST to identify the teleconnection of drought in India with global parameters. 相似文献
15.
A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m3 representing an oceanic crust. 相似文献
16.
S. Naseema Beegum K. Krishna Moorthy Vijayakumar S. Nair S. Suresh Babu S. K. Satheesh V. Vinoj R. Ramakrishna Reddy K. Rama Gopal K. V. S. Badarinath K. Niranjan Santosh Kumar Pandey M. Behera A. Jeyaram P. K. Bhuyan M. M. Gogoi Sacchidanand Singh P. Pant U. C. Dumka Yogesh Kant J. C. Kuniyal Darshan Singh 《Journal of Earth System Science》2008,117(1):303-313
Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Ångström parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Ångström coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from tshe adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Ångström exponent (α) remained significantly lower (～1) over the Arabian Sea compared to Bay of Bengal (BoB) (～1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB. 相似文献
17.
Robert Tenzer Pavel Novák Peter Vajda Vladislav Gladkikh Hamayun 《Computational Geosciences》2012,16(1):193-207
We developed and applied a novel numerical scheme for a gravimetric forward modelling of the Earth’s crustal density structures
based entirely on methods for a spherical analysis and synthesis of the gravitational field. This numerical scheme utilises
expressions for the gravitational potentials and their radial derivatives generated by the homogeneous or laterally varying
mass density layers with a variable height/depth and thickness given in terms of spherical harmonics. We used these expressions
to compute globally the complete crust-corrected Earth’s gravity field and its contribution generated by the Earth’s crust.
The gravimetric forward modelling of large known mass density structures within the Earth’s crust is realised by using global
models of the Earth’s gravity field (EGM2008), topography/bathymetry (DTM2006.0), continental ice-thickness (ICE-5G), and
crustal density structures (CRUST2.0). The crust-corrected gravity field is obtained after modelling and subtracting the gravitational
contribution of the Earth’s crust from the EGM2008 gravity data. These refined gravity data mainly comprise information on
the Moho interface and mantle lithosphere. Numerical results also reveal that the gravitational contribution of the Earth’s
crust varies globally from 1,843 to 12,010 mGal. This gravitational signal is strongly correlated with the crustal thickness
with its maxima in mountainous regions (Himalayas, Tibetan Plateau and Andes) with the presence of large isostatic compensation.
The corresponding minima over the open oceans are due to the thin and heavier oceanic crust. 相似文献
18.
Collision tectonics of the Central Indian Suture zone as inferred from a deep seismic sounding study
The Central Indian Suture (CIS) is a mega-shear zone extending for hundreds of kilometers across central India. Reprocessing of deep seismic reflection data acquired across the CIS was carried out using workstation-based commercial software. The data distinctly indicate different reflectivity characteristics northwest and southeast of the CIS. Reflections northwest of the CIS predominantly dip southward, while the reflection horizons southeast of the CIS dip northward. We interpret these two adjacent seismic fabric domains, dipping towards each other, to represent a suture between two crustal blocks. The CIS itself is not imaged as a sharp boundary, probably due to the disturbed character of the crust in a 20 to 30-km-wide zone. The time sections also show the presence of strong bands of reflectors covering the entire crustal column in the first 65 km of the northwestern portion of the profile. These reflections predominantly dip northward creating a domal structure with the apex around 30 km northwest of the CIS. There are a very few reflections in the upper 2–2.5 s two-way time (TWT), but the reflectivity is good below 2.5 s TWT. The reflection Moho, taken as the depth to the deepest set of reflections, varies in depth from 41 to 46 km and is imaged sporadically across the profile with the largest amplitude occurring in the northwest. We interpret these data as recording the presence of a mid-Proterozoic collision between two micro-continents, with the Satpura Mobile Belt being thrust over the Bastar craton. 相似文献
19.
By compiling wide-angle seismic velocity profiles along the 400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, 25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of 26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels. 相似文献
20.
Ravi P. Srivastava Nimisha Vedanti O. P. Pandey V. P. Dimri 《Journal of the Geological Society of India》2009,73(5):715-723
Vindhyan Basin of Central India situated just north of SONATA rift zone, forms one of the major geotectonic segment of the
Indian subcontinent which is associated with complex thermo-tectonic history. Southern part of this basin is known to contain
favorable conditions for hydrocarbon entrapment. Keeping this in view, a detailed gravity survey network comprising 40 gravity
bases and 1500 data points in an area of about 110 × 100 km2 was planned in and around Jabera-Damoh region. Analysis of Bouguer and free air gravity anomaly maps, prepared using fractal
based gridding method, indicates presence of two sedimentary basins (Jabera and Damoh) faulted on either sides beside ridge
like features. However, well-known Jabera domal structure appeared to be a shallow feature only. Inversion of gravity data
further reveals presence of 5 to 6 km thick Vindhyan sediments in the Jabera basin which are underlain by Mahakoshal/Bijawar
group of rocks, resting directly over the lower crust, thereby indicating almost total absence of granitic crust from this
region. It appears that due to an underlying thermal anomaly, the entire region may have been subjected to sustained uplift,
deformation, erosion and consequent crustal extension during early to mid Proterozoics which brought high velocity mafic crust
to such shallow levels. 相似文献