phosphatised limestones and associated sediments from the western continental shelf of india abstract the phosphatised limestones and associated sediments of the outer continental shelf of the west coast of india between goa and ratnagiri at depths of between 70 and 120 m have been studied the highest p 2 o 5 content 9 ranging from 2 to 9 depending on location occur in the algal nodules polished and thin sections show that the algal nodules grew with the occasional microbial mat these mats apparently formed by trapping and binding of sediment particles which were subsequently phosphatised phosphate also occurs as pellets and rounded structures high magnesium calcite and carbonate fluorapatite are the major minerals aragonite low magnesium calcite and quartz are also present in some samples microprobe geochemistry of the phosphatised material of the algal nodules reveals the presence of land derived siliciclastic detritus and the possibility of more than one source for ca and p phosphate also occurred as cavity fillings in coral and bryozoan limestones but not in oolitic limestones regardless of their occurrence at the same depths and in the vicinity of the other limestones the associated sediments are mostly sandy and show smaller concentrations of p 2 o 5 average 0 3 and organic carbon average 0 93 as compared to the regions of upwelling where phosphorites are forming the correlation exhibited between the p 2 o 5 and the organic carbon is negative the environmental conditions for the formation of phosphatised limestones and possible sources of phosphate are discussed sheath organic matter associated with filaments appears to be the major source for phosphorus the phosphatisation is a microbial process which takes place at shallow depths the impact of present day upwelling is insignificant in the phosphatisation of the limestones

curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central aeolian ridge southern tyrrhenian sea italy the salina lipari and vulcano volcanic ridge and the surrounding sea sectors aeolian archipelago southern tyrrhenian sea italy are characterized by vents responsible for a recent 40 ka 1889 1890 ad effusive and explosive subareal activity and repeated 56 to 7 ka in age submarine explosive eruptions from source areas located between lipari and vulcano a spectral depth estimation of the magnetic bottom using a fractal method on aeromagnetic data from vulcano lipari and salina volcanic ridge allows us to constrain the curie isotherm depth the elevated portion of the isotherm is between 2 and 3 km below salina and vulcano and about 1 km below lipari the curie depth results in the context of other geological and geophysical evidence suggest that the rise of the curie isotherm is mainly due to the occurrence of shallow heat sources such as magma ponds and associated hydrothermal systems the short wavelength magnetic anomaly field reflects magnetic contrasts from highly magnetized volcanic bodies low magnetization sediments and hydrothermally altered rocks borehole temperature data verify the curie temperature derived from the magnetic methods on the island of vulcano we conclude that the whole vulcano lipari and salina volcanic ridge is active and should be monitored

heat flow in indian gondwana basins and heat production of their basement rocks abstract temperatures have been measured in eight boreholes ranging from 260 to 800 m in depth in five gondwana basins of the damodar and son valleys with the aid of about 250 thermal conductivity determinations on core samples from these holes heat flow has been evaluated measurements of radioactive heat generation have been made on samples of precambrian gneisses constituting the basement for the sonhat son valley and chintalapudi godavari valley basins heat flow values from all of the damodar valley basins are within the narrow range of 69 79 mw m 2 the value from the sonhat basin 107 mw m 2 is significantly higher the generally high heat flows observed in gondwana basins of india cannot be attributed to the known tectonism or igneous activity associated with these basins the plots of heat flow vs heat generation for three gondwana basins jharia sonhat and chintalapudi are on the same line as those of three regions in the exposed precambrian crystalline terrains in the northern part of the indian shield this indicates that the crust under exposed regions of the precambrian crystalline rocks as well as the gondwana basins form an integral unit as far as the present day geothermal character is concerned

modelling of magnetic data for scaling geology interpretation of magnetic data can be carried out either in the space or frequency domain the interpretation in the frequency domain is computationally convenient because convolution becomes multiplication the frequency domain approach assumes that the magnetic sources distribution has a random and uncorrelated distribution this approach is modified to include random and fractal distribution of sources on the basis of borehole data the physical properties of the rocks exhibit scaling behaviour which can be defined as p k ak where p k is the power spectrum as a function of wave number k and a and are the constant and scaling exponent respectively a white noise distribution corresponds to 0 the high resolution methods of power spectral estimation e g maximum entropy method and multi taper method produce smooth spectra therefore estimation of scaling exponents is more reliable the values of are found to be related to the lithology and heterogeneities in the crust the modelling of magnetic data for scaling distribution of sources leads to an improved method of interpreting the magnetic data known as the scaling spectral method the method has found applicability in estimating the basement depth curie depth and filtering of magnetic data

using fractal crustal magnetization models in magnetic interpretation1 evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three dimensional distribution of magnetization within the crust can be described as fractal this property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization specifically we address the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging when magnetic data are continued downwards the depth at which the power spectrum flattens out the white depth can be taken to be an estimate of the top of the source distribution this procedure assumes that individual sources are uncorrelated with each other taking into account the correlation of the magnetization using a fractal description leads to a reduction in this depth estimate gridding of randomly distributed magnetic measurements using kriging requires an estimate of the covariance of the data compared with the assumption of a white uncorrelated magnetization distribution using fractal covariances for kriging produces gridded estimates which more closely reflect the statistics of the underlying magnetization process and produce maps with a justifiable degree of smoothness

omission surface incipient phosphate crusts on early diagenetic calcareous concretions and their possible origin upper campanian southern israel abstract flattened calcareous concretions enveloped by a 2 4 mm thick phosphate bearing crust occur in a single level in the lower part of the phosphorite unit upper part of mishash formation in nahal zinim southern israel negev the crust is associated with various ichnofossils penetrating an originally incompletely lithified surficial zone at the periphery of the hardened part of the concretions in thin section the phosphate fraction appears structureless scanning and backscattered electron microscopy shows that this fraction occurs as walls and envelopes of polyhedral cell like units of about 5 m in diameter these structures are interpreted as synsedimentary phosphate coated possible remnants of chasmolithic and epilithic colonial unicells which colonized the pre hardened surficial zone around the concretions during exposure on the sea floor this thin microbially constructed phosphate coat is believed to be an incipient equivalent of more intensely developed phosphate crusts on many omission surfaces in the geological record

curie point depths in northeast china and their geothermal implications for the songliao basin abstract this work integrates processing and interpretation of the combined geomagnetic anomaly model of ngdc 720 v3 1 to better constrain thermal structures of the northeast china lithosphere which shows intensive extension and active volcanism during the mesozoic and cenozoic numerical tests demonstrate that magnetic anomalies calculated at 20 km altitude can be used for accurate curie depth estimation averaged curie depths using a 3d fractal exponent of 3 0 vary from 13 2 0 to 33 4 1 km with a mean of 22 2 4 km in northeast china with temperature dependent thermal conductivity models constrained by incorporated surface heat flow and curie depths in different geological blocks the thermal lithospheric thicknesses are estimated mostly between 45 and 90 km based on 1d steady state thermal conduction equation beneath the songliao basin the moho temperatures are abnormally high ranging from ca 700 to 1000 c and the mantle contributes about 65 of the total surface heat flow the shallow curie points and thermal lithosphere asthenosphere boundary together with high moho temperatures and mantle heat flow support abnormally high geotherms in the songliao basin originated from hot upper mantle upwelling triggered likely by lithospheric delamination

gravity evidence for mid crustal domal structure below delhi fold belt and bhilwara super group of western india a scheme to interpret nonstationary gravity profile is proposed in two steps 1 divide the entire nonstationary gravity profile into piecewise stationary sub profiles and 2 apply the scaling power spectral analysis to each sub profile the scheme is applied to gravity data along a well studied nagaur jhalawar transect in western india the profile has been subdivided into three sub profiles each sub profile corresponds to a geological unit gravity data delineated a 2 d mid crustal domal structure below the delhi fold belt and bhilwara super group further forward gravity modelling is carried out to model the subsurface along the entire profile

stromatolites the challenge of a term in space and time abstract several definitions of stromatolites are discussed and kalkowsky s most important statements about stromatolites are translated from german to english a new definition for stromatolites is proposed namely stromatolites are laminated rocks the origin of which can clearly be related to the activity of microbial communities which by their morphology physiology and arrangement in space and time interact with the physical and chemical environment to produce a laminated pattern which is retained in the final rock structure unconsolidated laminated systems clearly related to the activity of microbial communities and often called recent stromatolites or living stromatolites are defined as potential stromatolites the main microbial activities which are important in the formation of potential stromatolites and stromatolites are described and examples of stromatolites of various types including iron stromatolites are also described

microbial processes in the negev phosphorites southern israel microscopic and sem observations indicate that cyanobacterial activity was important during the formation of the campanian negev phosphorites one aspect of such activity is the development of phosphorites through the binding of phosphate particles these sediments often demonstrate a conspicuous smooth cryptalgal lamination the algal binder was fossilized as intertwined phosphatic sheaths that coat and seal off the phosphate grains different types of sheaths representing the remnants of an algal community were found thick apatite overgrowth commonly deforms the microbial fossils converting them into barely recognizable forms fabrication of phosphate particles is another aspect of the microbial activity phosphate algal coated grains are a common constituent of the negev phosphorites the algal coating consists of stacked phosphatic tubes and colonies of coccoid unicells with an apatite infilling fragmented phosphatic sheath bundles form another type of phosphate particle produced by algae obliteration of algal structures is common resulting in undifferentiated groundmasses of cryptocrystalline apatite the extremely close connection between microbial structures and phosphatization processes suggests cyanobacterial participation in phosphorus enrichment

the eastern ghats mobile belt india a collage of juxtaposed terranes abstract examination of landsat tm images reconnaissance field traverses and the published geological maps from the eastern ghats mobile belt egmb india reveal a network of major ductile shear zones both within and at the margins these shear zones are characterized by mylonitic foliation grain size reduction metamorphic retrogression stretching lineations and distinct signatures of alkaline anorthositic and granitic magmatism these shear zones divide the egmb into distinct terranes which are heterogeneously deformed with extensive tracts of foliated mylonitic gneisses and ultramylonites the main gneissic foliation in all the terranes is refolded in near non coaxial manner generally about the axis subparallel to the elongation of the terrane structural history in each terrane is distinct in the orientation of stretching lineations attitude of gneissosity and early fold axial planes lithological assemblages and available geochronological data it is possible to recognise nine large terranes within the egmb and the characteristics of each terrane have been described the terrane distribution in the egmb could well fit a thrust tectonic allochthonous model of amalgamation and accretion different terranes could be different thrust nappes or allochthonous tectonic sheets representing tectono stratigraphic terranes

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

terrestrial heat flow in the malawi rifted zone east africa implications for tectono thermal inheritance in continental rift basins abstract rift basins show the widest range of heat flow values compared to passive continental margins and intracratonic sag basins elevated heat flow is often used to infer asthenospheric upwelling or subsurface magmatic activity for magma assisted rifts despite the lack of surface volcanism for magma poor rifts it is possible that magmatic bodies will be present at lower and mid crustal levels but yet to breach the surface thus necessitating consideration of heat flow from tectono thermal inheritance in addition to more recent magmatism we use aeromagnetic data to investigate the terrestrial heat flow distribution in the malawi rifted zone mrz in east africa defined as the neogene malawi rift and surrounding permo triassic karoo rift basins we use the two dimensional power density spectrum technique to estimate the curie point depth cpd geothermal gradient and heat flow beneath the mrz our results reveal predominance of shallow cpds 18 20 km high geothermal gradients 29 32 c km and elevated heat flow 70 82 mw m 2 within the karoo rift basins along the malawi rift geothermal gradient 25 27 c km and heat flow 60 66 mw m 2 are generally lower except pronounced high heat flow 70 82 mw m 2 at the rungwe volcanic province rvp in the north and at the central rift segment where the rift overprints karoo rift basins the surrounding cratonic blocks show deeper cpds 24 27 km lower geothermal gradients 22 24 c km and lower heat flow 53 63 mw m 2 we interpret that the elevated heat flow in the karoo rift basins are related to residual heat flow from the permo triassic rifting phase replenished by later jurassic cretaceous diking events apart from the thermal anomaly beneath the rvp which is due to neogene magmatism other important targets for geothermal explorations in the mrz are the residual thermal anomalies in the karoo rift basins we infer that in areas of active magma poor rifting pronounced heat flow may not only occur in regions of asthenospheric upwelling but may concentrate in the ancient magmatic rift segments our findings have implications for crustal stretching tectono thermal inheritance and geothermal energy potentials in east africa and other segments of magma poor rifting

seismic evidence for significant crustal thickening beneath jabalpur earthquake 21 may 1997 source region in narmada son lineament central india 1 we investigate crustal properties beneath the deep crustal 35 km jabalpur earthquake of 21 may 1997 narmada son lineament nsl central india in search of a possible cause of stress accumulation in the region teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests moho downwarp to 52 km across the width of the lineament in contrast with an average 40 km depth elsewhere in addition the crust beneath the nsl has higher vp vs of 1 84 compared to 1 73 in the surrounding suggestive of a high density mafic mass at depth that compensates the crustal root also supported by small topographic variation 200 m across the lineament presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre existing narmada son fault leading to the earthquake in deep crust

the nature and thickness of lithosphere beneath the archean dharwar craton southern india a magnetotelluric model abstract the dharwar craton in the southern indian shield region occupies a key position in reconstructions of the earliest supercontinent ur as well as in the global neoarchean configurations of continental assembly here we synthesize the available geophysical data that include heat flow seismic tomography temperature profile derived from xenoliths and magnetotellurics mt from the dharwar craton a thickness of more than 200 km is identified for the lithosphere beneath this craton by magnetotelluric model our inference lends support to the models from recent seismological investigations that argue for the presence of a thick lithosphere as well as the upper mantle structure derived from the analysis of mantle xenoliths a synthesis of recent geophysical and geochemical data together with our magnetotelluric model indicate the existence of a thick lithospheric root beneath the dharwar craton and suggest that not much of the lithosphere has been eroded since precambrian in this region leading to the conclusion that the dharwar craton preserves an archean tectosphere an increase in the lithospheric thickness from east to west in the dharwar craton and the lithosphere asthenosphere boundary lab migration to shallower levels observed from geophysical data towards the east is interpreted as a probable reflection of the mesoproterozoic subduction erosion of the archean cratonic keel

a preliminary study of satellite derived gravity and magnetic anomalies of india and contiguous regions abstract the residual satellite gravity anomaly map in and around the indian region has been prepared these residual anomalies are interpreted for crustal inhomogeneities using the magsat anomalies as one of the constraints while a strong correlation between these anomalies is observed over the dharwar aravalli marwar and singhbhum regions inverse correlation prevails over the eastern ghats and cambay basin the positive gravity and magnetisation strong correlation over the stable blocks of the peninsula is related to the alternating layers of basic and ultrabasic materials the positive gravity and low magnetisation poor correlation over the eastern ghats and cambay basin is interpreted as thinning of the crust the residual gravity map identifies the signatures of west central indian lineament and the submarine topographic features ridges in the indian ocean such as the chagos laccadive 85 e and ninety east ridges which are not so obvious in the unprocessed anomaly map at ground level an attempt is also made to explain the anomaly over the indo ganga brahmaputra basin using the ground and 400 km level satellite gravity anomaly maps

bacterial origin of east australian continental margin phosphorites many models of the origin of marine phosphorites require a sediment rich in organic matter which by decomposition releases phosphorus for the precipitation of carbonate fluorapatite1 7 this concept is useful in explaining contemporary phosphorite in areas of very high productivity off the coasts of peru chile and south west africa8 10 where there are high organic matter fluxes to the sea floor4 7 11 it does not explain the origin of marine phosphorite deposits which formed in regions of limited oceanic upwelling and productivity12 14 the east australian continental margin an area of phosphogenesis throughout the late pleistocene and holocene13 14 is a modern analogue of an east coast phosphogenic province15 17 with low productivity over the upper slope region where the most recent phosphorites have been found18 evidence is reported here for the carbonate fluorapatite in the east australian phosphorites being located within bacterial cellular structures and a model proposed for the origin of these deposits through the slow bacterial assimilation of phosphorus from seawater in an area of restricted sedimentation

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

probing magnetic bottom and crustal temperature variations along the red sea margin of egypt abstract over 50 magnetic bottom depths derived from spectra of magnetic anomalies in eastern egypt along the red sea margin show variable magnetic bottoms ranging from 10 to 34 km the deep magnetic bottoms correspond more closely to the moho depth in the region and not the depth of 580 c which lies significantly deeper on the steady state geotherms these results support the idea of wasilewski and coworkers that the moho is a magnetic boundary in continental regions reduced to pole magnetic highs correspond to areas of younger granites that were emplaced toward the end of the precambrian other crystalline precambrian units formed earlier during the closure of ocean basins are not strongly magnetic in the north magnetic bottoms are shallow 10 15 km in regions with a high proportion of these younger granites in the south the shoaling of the magnetic bottom associated with the younger granites appears to be restricted to the aswan and ras banas regions complexity in the variation of magnetic bottom depths may arise due to a combination of factors i regions of younger precambrian granites with high magnetite content in the upper crust leaving behind low curie temperature titanomagnetite components in the middle and lower crust ii rise in the depth of 580 c isotherm where the crust may have been heated due to initiation of intense magmatism at the time of the red sea rifting 20 ma and iii the contrast of the above two factors with respect to the neighboring regions where the moho and or curie temperature truncates lithospheric ferromagnetism estimates of fractal and centroid magnetic bottoms in the oceanic regions of the red sea are significantly below the moho in places suggesting that oceanic uppermost mantle may be serpentinized to the depth of 15 30 km in those regions

depth estimation from the scaling power spectrum of potential fields summary depth estimation from potential field power spectra requires a realistic assumption of the statistical properties of the source distributions density and susceptibility distributions in the earth s crust exhibit a long range dependence which is adequately described by scaling random fields with a spectral density proportional to some power of the wavenumber the theoretical power spectrum for a half space model of scaling sources explains the shape of observed power spectra of real potential field data very well minimizing the misfit between the model and the observed power spectrum yields an estimate for the depth to the top of the sources after demonstrating this approach on synthetic magnetic data we reinterpret power spectra of gravity and aeromagnetic data from utah hawaii and saskatchewan finding depth values that differ significantly from earlier interpretations all three power spectra are best explained by source distributions starting at surface level even the power spectrum from an aeromagnetic survey of a sedimentary basin with virtually non magnetic basin fill in the latter case a priori information on the intensity and the scaling exponent of the field caused by the basement had to be included to obtain an approximate estimate of the basin depth in general potential field power spectra are dominated by scaling properties of their source distributions and contain only limited depth information

crustal magmatism and lithospheric geothermal state of western north america and their implications for a magnetic mantle abstract the western north american lithosphere experienced extensive magmatism and large scale crustal deformation due to the interactions between the farallon and north american plates to further understand such subduction related dynamic processes we characterize crustal structure magmatism and lithospheric thermal state of western north america based on various data processing and interpretation of gravimetric magnetic and surface heat flow data a fractal exponent of 2 5 for the 3d magnetization model is used in the curie point depth inversion curie depths are mostly small to the north of the yellowstone snake river plain hotspot track including the steens mountain and mcdermitt caldera that are the incipient eruption locations of the columbia river basalts and yellowstone hotspot track to the south of the yellowstone hotspot track larger curie depths are found in the great basin the distinct curie depths across the yellowstone snake river plain hotspot track can be attributed to subduction related magmatism induced by edge flow around fractured slabs curie depths confirm that the great valley ophiolite is underlain by the sierra nevada batholith which can extend further west to the california coast range the curie depths thermal lithospheric thickness and surface heat flow together define the western edge of the north american craton near the roberts mountains thrust rmt to the east of the rmt large curie depths large thermal lithospheric thickness and low thermal gradient are found from the differences between curie point and moho depth we argue that the uppermost mantle in the oceanic region is serpentinized the low temperature gradients beneath the eastern great basin montana and wyoming permit magnetic uppermost mantle either by serpentinization metasomatism or in situ magnetization which can contribute to long wavelength and low amplitude magnetic anomalies and thereby large curie point depths

microbially influenced formation of phosphate nodules and megafossil moulds negev southern israel abstract phosphate nodules and megafossil moulds occur at the top of the late cretaceous mishash formation in southeastern israel their palaeogeographic distribution as well as the microstructure of the moulds and the nodules suggests that the former represent rapid lithification of the infilling sediment by apatite precipitating bacteria before the dissolution of the calcareous skeletons the nodules on the other hand are partly lithified portions of microbial mats formed in a lagoonal environment disrupted rolled and embedded in granular phosphorite carbonate replacement by apatite does not seem to be involved in the formation of this pebbly phosphate

curie point depth geothermal gradient and heat flow estimation and geothermal anomaly exploration from integrated analysis of aeromagnetic and gravity data on the sabalan area nw iran prospecting the geothermal resources in northwest of iran conducted in 1975 revealed several promising areas and introduced the sabalan geothermal field as a priority for further studies the sabalan mt representing the sabalan geothermal field is a large stratovolcano which consists of an extensive central edifice built on a probable tectonic horst of underlying intrusive and effusive volcanic rocks in this study curie point depth cpd geothermal gradient and heat flow map were constituted from spectral analysis of the aeromagnetic data for the nw of iran the top of the geothermal resource i e the thickness of the overburden was evaluated by applying the euler deconvolution method on the residual gravity data the thickness of the geothermal resource was calculated by subtracting the euler depths obtained from the cpds in the geothermal anomalous region the geothermal anomalous region was defined by the heat flow value greater than 150 mw m2 cpds in the investigated area are found between 8 8 km in the sabalan geothermal field and 14 1 in the northeast the results showed that the geothermal gradient is higher than 62 c km and the heat flow is higher than 152 mw m2 for the geothermal manifestation region the thickness of the geothermal resource was also estimated to vary between 5 4 and 9 1 km these results are consistent with the drilling and other geological information findings indicate that the cdps agree with earthquake distribution and the type of thermal spring is related to the depth of the top of the geothermal resource

magnetic anomalies india and antarctica abstract magnetic anomalies over the continental shelf off the east coast of india orissa suggest the presence of a highly magnetic rock type magnetized with an intensity of 900 nt in a direction azimuth a 150 and inclination i 65 this suggest the occurrence of igneous volcanic rocks which is confirmed from samples found below tertiary sediments from a few boreholes in this region the depth of this rock type as estimated from magnetic anomalies varies from approximately 1 2 km near the coast to 4 4 5 km towards the shelf margin this direction of magnetization is the reverse of the reported direction of magnetization for the rajmahal traps of the cretaceous period 100 110 m y a small strip of the body near the continental shelf margin appears however to possess normal magnetization suggesting the occurrence of normal and reversed polarities side by side a characteristic typical for oceanic magnetic anomalies the reversed polarity of the rocks on the continental shelf suggests that they correspond probably to the mo reversal 115 m y on world magnetostratigraphic scale and provide a paleolatitude of 47 s for the land mass of india which agrees with the palaeoreconstruction of india and antarctica in this reconstruction the mahanadi gondwana graben on the indian subcontinent falls into line with the lambert rift in antarctica suggesting a probable common ancestry the volcanic rocks on the continental shelf off the east coast of india might represent a missing link that is rocks formed between india and antarctica at the time of the break up of gondwanaland satellite magnetic anomalies magsat recorded over the indian shield and interpreted in terms of variations in the curie point geotherm provide a direction of magnetization which also places this continent close to antarctica as such magsat anomalies recorded over eastern antarctica are found compatible with those recorded over the indian shield

quaternary phosphorites from the continental margin off chennai southeast india analogs of ancient phosphate stromatolites abstract pleistocene phosphorites occur on the continental margin off chennai abundantly in the depth range of 186 293 m they are associated with outer shelf glauconites and carbonate skeletals including large shells of molluscs and rhodoliths these are primary phosphate deposits and are similar to phosphate stratiform stromatolites this is the first report of phosphate stratiform stromatolites during the quaternary and modern times and they provide a quaternary analog for ancient phosphorites thin sections of phosphorites exhibit laminated phosphatized microbial mats consisting of mechanically deposited clastic particles or of phosphatic molds clay particles are scattered throughout the matrix sem studies reveal that the laminated phosphate matrix is made of thin sheets of tightly packed apatite globules densely packed intertwined microfilaments or filament molds phosphate with obscured laminae is composed of phosphate tubules cell like structures resembling coccoid cyanobacteria or their botryoidal aggregates are common carbonate fluorapatite and low magnesium and high magnesium calcites are the major mineral phases the structural co2 content of apatite ranges from 4 to 5 5 quartz feldspar and goethite are accessory minerals the microbial mats were formed on the outer shelf most probably during conditions of low sea level in the quaternary and microbial processes played a major role in direct phosphatization of these mats the primary phosphorus source seems to have been the continental supply and degradation of organic matter associated with benthic microbial communities the phosphate stratiform stromatolites thus formed were subsequently reworked into a shelf margin depression and resulted in the occurrence of condensed phosphorites

cambrian phoscrete profiles coated grains and microbial processes in phosphogenesis georgina basin australia abstract middle cambrian phoscrete profiles occurring within phosphatic carbonates that surround the phosphate deposits of the georgina basin australia are described in detail the profiles occur within a mixed mineralogy suite of rocks and are characterized by fabrics and textures identical to those developed in calcretes phoscrete profiles are subdivided into an overlying laminar phoscrete crust and an underlying phoscrete modified substrate diagenesis in the substrate results in the elimination of primary structures producing a coated to vaguely pelleted texture micritic phosphate cements dominate the phosphate profiles these cements occur as irregular coatings on 1 grains and 2 thread like structures which criss cross interparticle spaces scanning electron microscope examination f these cements indicates that they are coatings on tubular structures herein interpreted as chasmolithic and epilithic filaments the preferential occurrence of phosphate on organic substrates suggests that organic matter plays a dual role in phosphogenesis it provides both a source of phosphate to the pore waters via bacterial degradation and it also provides a substrate on which phosphate preferentially nucleates phosphate particles derived from the erosion of phoscrete profiles vary from friable aggregates and lumps to peloids and coated grains whereas the composite particles were unlikely to survive sediment transport peloids and coated grains were coherent particles able to survive transport and sorting this suggests that some of these particles may occur in the phosphate d posits

curie point depths of the island of kyushu and surrounding areas japan as part of a comprehensive nationwide evaluation of geothermal resources for japan the first of the curie point depth maps covering the island of kyushu has been prepared the map was created by inverting gridded regional aeromagnetic data two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles the first algorithm estimates x0 y0 and z0 the coordinates of the centroid of the distribution by computing a least squares fit to the radial frequency of the fourier transform the second algorithm estimates centroid depth only by computing a least squares fit to the squared amplitude of the frequency estimates the average depth to the top zt of the collection of point dipoles was estimated by a variation of the second algorithm the depth to the bottom of the dipoles inferred curie point depth is zb 2z0 zt the depth estimates are hand contoured to produce the final map the curie point depth map is then compared to regional geology and heat

the role of boundary layer convergence zones and horizontal rolls in the initiation of thunderstorms a case study abstract the initiation of thunderstorms is examined through a combined observational and modeling case study the study is based on doppler radar aircraft mesonet balloon sounding and profiler and photographic data from the convection initiation and downburst experiment cinde conducted near denver colorado the study examines the initiation of a line of thunderstorms that developed along a preexisting quasi stationary boundary layer convergence line on 17 july 1987 the storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present the primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection the critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line the effect was to cause the updrafts in the convergence line

phosphorites hardgrounds and syndepositional solution subsidence a palaeoenvironmental model from the miocene of the maltese islands abstract two major phosphorite conglomerate horizons occur throughout the central mediterranean maltese islands within the lower middle miocene globigerina limestone formation the lower conglomerate lies directly upon a hardground and both conglomerates include lithoclasts of hardground material variation in clast size and bed thickness indicates a source area to the west and north of the present day islands of malta and gozo particularly thick phosphorite conglomerates occur in solution subsidence structures in western gozo being developed as a result of synsedimentary seafloor collapse these structures indicate that phosphorite clasts were deposited during several discrete pulses of turbulence punctuated by periods of non depositional hardground formation or normal marine pelagic sedimentation geochemical analysis of the contained phosphorite clasts within the conglomerates shows that p 2 o 5 generally is concentrated around the outer margins of the pebbles and contributes up to 17 by weight of some samples some 654 580 m 3 and 837 870 m 3 of phosphorite bearing strata are contained within the xlendi and qawra subsidence structures respectively the maltese islands represent part of a miocene carbonate shelf whose margin lay nearby to the west and which was subject to periodic erosion by strong currents transporting phosphorite pebbles eastwards to their present sites these currents further contributed phosphorus to semi autochthonous phosphorite developments located within the present maltese area the maltese phosphorites are related to other phosphorite areas in sicily a major province of miocene phosphogenesis is proposed to account for these occurrences it extends along the western margins of the malta ragusa rise

early diagenetic phosphate cements in the albian condensed glauconitic limestone of the tatra mountains western carpathians early diagenetic phosphate cements are described from the albian condensed glauconitic limestone of the tatra mountains western carpathians with regard to their macro and micromorphology distribution classification and genesis the cements occur within stratigraphically condensed semi pelagic foramini feral glauconitic layers and are associated with mature hardgrounds within the tatra albian limestone phosphate cement fabrics consist of crypto to microcrystalline carbonate fluorapatite and they occur as i rim envelopes ii infillings of intraparticle porosity iii rim cement iv multiple rim cement v palisade fabric and vi cluster cement micromorphological variability of the cement fabrics results from varying texture of the cemented sediment the nature of original porosity as well as from presence of associated microbial fabrics the microbial fabrics are interpreted as fossilized coccoid cyanobacteria phosphate cementation developed under peculiar early diagenetic conditions within semi closed microenvironments rich in organic matter in the marine phreatic environment the cementation contributed to the formation of phosphatic fossils and hardgrounds the accretion of the cements was due to concentration of biologically uptaken phosphorus near the sediment water interface enrichment of pore fluids with respect to phosphate and its precipitation within restricted microenvironments phosphate cementation post dated seafloor formation of pelletal glauconite but predated partial decomposition of organic matter as well as dissolution or neomorphism of aragonite and high mg calcite phosphate cementation occurred on a carbonate platform following the submersion of urgonian reefal build ups episodes of phosphate cementation were repeated during the sedimentation of the tatra albian limestone as a response to rapid relative sea level rises and increased influence of nutrient rich tethyan waters

curie isotherm map of indian subcontinent from satellite and aeromagnetic data abstract estimating depth to the bottom of the magnetic crust synonymously called the curie isotherm on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations there is still no consensus on the minimum survey area required to arrive at a reliable estimate of the curie isotherm depth from the available aeromagnetic data over india depth to the curie isotherm is estimated using spectral techniques we found that the significant spectral maxima required to calculate the depth to the curie isotherm existed only for 4 4 blocks in southern peninsular india up to 18 n latitude and for 5 5 blocks for the rest of the region in central india as aeromagnetic data coverage over india is incomplete we also calculate the magnetic crustal thickness from the lithospheric model of the champ satellite data for the whole country using an iterative forward modeling approach the calculated curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations as the curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques match reasonably well it lends credence to the methodology adopted the derived curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the indian subcontinent a comparison is made of the calculated curie isotherm depth with moho depths along available dss profiles over india and it is found that the curie depth is generally shallower than the moho depth implying that it possibly represents a thermal boundary rather than a compositional change further we find that high magnitude earthquakes are associated with high gradients in curie depth

crustal velocity inhomogeneities along the hirapur mandla profile central india and its tectonic implications abstract wide angle seismic and gravity data across the narmada son lineament nsl in central india are analyzed to determine crustal structure velocity inhomogeneities and hence constrain the tectonics of the lineament we present the 2 d crustal velocity structure from deep wide angle reflection data by using a ray trace inverse approach the main result of the study is the delineation of fault bounded horst raised to a subsurface depth 1 5 km and the moho upwarp beneath the nsl the crust below the basement consists of three layers with velocities of 6 45 6 7 6 2 6 5 and 6 7 6 95 km s and interface depths of about 5 5 8 7 14 17 and 18 23 km along the profile the low velocity 6 2 6 5 km s layer goes up to a depth of 5 km and becomes the thickest part 13 km while the overlying high velocity 6 45 6 7 km s layer becomes the thinnest 3 km and upper boundary lies at a depth of 1 5 km beneath the nsl the overall uncertainties of various velocity and boundary nodes are of the order of 0 12 km s and 1 40 km respectively the up lifted crustal block and the up warping moho beneath the nsl indicate that the north and south faults bounding the nsl are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap the lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the narmada south fault during the 1997 jabalpur earthquake

potential field power spectrum inversion for scaling geology we propose a method for inverting the power spectrum of gravity and magnetic data the method is demonstrated on aeromagnetic and bore well data from the german continental deep drilling project ktb density and susceptibility distributions in the earth s crust exhibit scaling behavior with power spectra proportional to f where f is the wavenumber and is the scaling exponent of the source distribution we model the sources of the potential field by a random function with scaling properties defined on a half space with its top at a specified depth beneath the observation plane comparing the theoretical power spectrum for this model with the power spectrum of the measured data we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function despite the simplicity of the model it offers a new understanding of the factors influencing the shape of the potential field power spectrum in particular the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement the scaling exponent of the field varies with the type of surface geology the question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion

a study of spectral methods of estimating the depth to the bottom of magnetic sources from near surface magnetic anomaly data summary based on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations 1 using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows 300 500 km 2 avoiding filtering to remove arbitrary regional fields accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 earth s main field models e g recent international geomagnetic reference field models igrf or comprehensive models cm 3 ascertaining the near circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends and 4 avoid determining the slopes from the exponential low wavenumber part of the spectra in the cases of layered magnetization we also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results we found that despite all these precautions in some cases the results can still be erroneous and therefore we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible in the southcentral us east of the rockies where the surface heat flow ranges between 40 and 65 mw m 2 w e obtained the magnetic bottom depth of 40 10 km using the approach of the forward modelling of the spectral peak this range is similar to the seismically derived crustal thickness of 45 50 km suggesting therefore that the entire crust may be magnetic in this region because of the uncertainties in the various heat flow contributing parameters such as the variations in thermal conductivity radiogenic heat and hydraulic regime we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km

study of aeromagnetic data over part of eastern ghat mobile belt and bastar craton abstract a portion of the aeromagnetic anomaly map of india from 17 0 to 20 0 n and 78 o to 84 o e has been analysed to understand the tectonics of the region the distribution of magnetic sources in the study region are clearly brought out in the analytic signal map and found to be associated with charnockitic rocks iron formation and trap flows the godavari graben is devoid of any magnetic sources high grade charnockitic rocks on surface and sub surface flank the shoulders of the godavari graben on either side from the analysis of magnetic data sileru shear zone ssz is identified as the contact of the bastar craton and the eastern ghat mobile belt egmb the eastern ghat is divided into two blocks block n north of srikakulam is devoid of magnetic sources while the charnockitic rocks are the main magnetic carriers in block s the difference in magnetic characteristics of the two blocks has been attributed to the difference in metamorphic history block n has an over print of amphibolite facies metamorphism while block s to the south depicts granulite facies metamorphism the euler solutions within the egmb shows that the magnetic sources along ssz is shallower than the south east implying that the exhumation process in the egmb has a differential rate

crustal structure from deep seismic soundings along the koyna ii kelsi loni profile in the deccan trap area india abstract the crustal depth section obtained from deep seismic soundings along the koyna ii kelsi loni profile which lies near latitude 18 n roughly in the east west direction in that part of the deccan trap maharashtra state india shows a number of reflection segments below the deccan traps down to the moho discontinuity a deep fault below the deccan traps 13 km east of mahad divides the entire cross section including the moho boundary into two crustal blocks the reflection segments show updip towards the west coast in the western block the moho discontinuity which is at a depth of 39 km near the deep fault starts rising towards the coast reaching a depth of 31 5 km at the west coast the eastern block is thrown up by 1 5 km with respect to the western block along the deep fault a structural contour map of the moho discontinuity for the koyna reservoir area has been prepared from the present results and the crustal information obtained along the koyna i profile kaila et al 1979a shows that the deep fault in the koyna area is aligned in the nnw sse direction refraction seismic data analysis by the wave front method reveals that the thickness of the deccan trap increases towards the west coast the deccan trap is 600 700 m thick in the eastern region between nira sp 130 and loni sp 200 and attains a thickness of 1500 m at 10 km east of the west coast the longitudinal wave velocity in the deccan traps along the profile varies from 4 8 to 5 0 km sec and in the crystalline basement from 6 0 to 6 15 km sec a tentative isopach contour map of the deccan traps and a tentative structural contour map of the pre deccan trap contact have been prepared for the koyna reservoir area from the results along the koyna ii and koyna i profiles a flexure aligned in a nnw sse direction in the pre deccan trap contact which is an expression of the deep fault into the basement has been clearly brought out the flexure coincides in general with the orientation of the deccan volcanic scarp in this area

ultrastructure of a microbial mat generated phosphorite optical and sem observations of phosphorites reveal that mineralization is concentrated on ooids and micro oncoids the coated grains occur within microbial mats microbial mats represent the formational environment of the ooids and oncoids both coated grains and mats exhibit similar filamentous micro organisms the mat filaments show no fixed orientation and they merge with the concentrically oriented filaments of the coated grains the branching nature and chlamydospore like structures of filaments suggest that both mat and coated grains have been formed by fungi some coated grains appear to have been slightly disturbed and sometimes mobilized from their sites of formation due to separation from the parent mat resulting perhaps from contraction fragmentation the voids so created within the mat had been later filled with either micrite or sparite

depth to the bottom of magnetic sources dbms from aeromagnetic data of central india using modified centroid method for fractal distribution of sources abstract the depth to the bottom of the magnetic sources dbms has been estimated from the aeromagnetic data of central india the conventional centroid method of dbms estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on scaling distribution has been proposed shallower values of the dbms are found for the south western region the dbms values are found as low as 22 km in the south west deccan trap covered regions and as deep as 43 km in the chhattisgarh basin in most of the places dbms are much shallower than the moho depth earlier found from the seismic study and may be representing the thermal compositional petrological boundaries the large variation in the dbms indicates the complex nature of the indian crust

an investigation of the spectral properties of primary reflection coefficients this paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world in all but one case densities are taken as constant due to the paucity of suitable density data the reflection sequences are pseudo white only above a corner frequency below which their power spectrum falls away according to a power law f where is between 0 5 and 1 5 this spectrum can be adequately modelled in practice very simply with an arma 1 1 process which acts on a white innovation sequence the corollary of this is that before wavelet estimation methods are applied all of which except those based on synthetic seismograms presuppose white reflection sequences or deconvolution filters estimated seismic traces should be filtered with the inverse of this process interestingly the estimated arma processes group themselves into two clearly differentiated categories having very different indices of predictability or strictly indices of linear determinism the two categories apparently correlate precisely with two kinds of sedimentation one which consists largely of sequences of rocks with repeating properties called repetitive in this paper but perhaps loosely describable as cyclic and the other which is randomly bedded with no apparent pattern of components the former has indices of predictability which are two to four times as great as those of the latter another probably related property is that for the repetitive sequences tends to be greater than that for non repetitive rock columns the observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of time scale parameters is reciprocal detailed effects of block averaging and sampling the logs are shown to depend on the type of sequence under examination

evidence of underplating from seismic and gravity studies in the mahanadi delta of eastern india and its tectonic significance 1 we have imaged the crustal configuration of the mahanadi delta of eastern india by modeling both wide angle seismic and bouguer gravity data this delta has undergone several stages of rifting subsidence sedimentation and uplift it is one of the important gondwana sedimentary basins of india where widespread volcanic activity occurred during the early cretaceous along the rift zones this volcanic activity corresponds to the breakup of greater india from east gondwana e g present antarctica and australia we have derived a five layer crustal model with velocities of 6 0 6 5 6 0 7 0 and 7 5 km s having estimated densities of 2 7 2 8 2 65 2 9 and 3 05 g cm3 respectively the velocity density relation along with heat flux and other geological geochronological information indicates typical rift related evolution of the delta with a midcrustal low velocity 6 0 km s and low density 2 65 g cm3 zone and a 10 km thick high velocity 7 5 km s and high density 3 05 g cm3 layer at the base of the crust presumably due to underplating the moho upwarping or crustal thinning in the rift zone and emplacement of thick high velocity material at the base of the crust strongly suggests basaltic underplating probably due to the kerguelen hot spot activity these activities are synchronous with 117 ma rajmahal volcanism in northeast india and the lambert graben in east antarctica and are closely associated with the gondwana breakup in the india antarctica sector the rifting stages are closely correlated with the sedimentation phases of the lower and upper gondwana deposits in the mahanadi delta which evolved parallel to the lambert graben in east antarctica

hot spots magma underplating and modification of continental crust most common mantle derived magmas have densities at moho pressures that exceed the density of average continental crust thus unless the crust is tightly coupled with the mantle many such magmas should penetrate at or near the moho interface the scale of such a phenomenon can be impressive recent hot spots may have perturbed more than 5 of the planet s surface the details of the process will depend on the ease of melting of the base of continents underplating can explain continental construction from below moho structures or lack thereof delamination of the continental basement partial subduction and density cleaning of crust the formation of massive anorthosite complexes and major changes of elevation of continental blocks in the thermally turbulent archean the very dense and hot komatiite magmas must have had even more profound influences modern studies of the subduction process show that the mantle must be heterogeneous and that volatiles can be carried to great depths in significant volumes

nannostructure and genesis of phosphorites from odp leg 112 the peru margin abstract a sem based petrographic study of miocene to quaternary phosphorites recovered during odp leg 112 shows that they contain a variety of apatitic nannostructures in porous types apatite appears as ovoid dumbbell like botryoid micron sized and inframicron apatite particles and flakes these structures are probably due to microbially mediated apatite precipitation in compact varieties diagenesis reduced the porosity obscuring the original apatite microparticles macroscopic aspects of phosphorites colour brightness hardness density are essentially linked to the porosity of apatitic structures but independent of the apatite particles and structures generally they reflect diagenetic stages in evolution the variety and distribution of the apatite particles and structures suggest that phosphorites mainly formed in three different ways phosphatization of 1 fish excrements forming copronodules 2 diatom muds producing nodules and 3 faecal pellets resulting in rounded grains these phosphorite particles formed in low energy conditions many of them were subsequently reworked

convergence zones over the greek peninsula and associated thunderstorm activity summary in the frame of this study the initiation of summer storm activity over the greek peninsula during a prevailing weak synoptic flow is investigated using the colorado state university regional atmospheric modelling system on 10 july 1994 thunderstorm activity was observed along a convergence zone which had developed following the main axis of the peninsula the convergence zone first deepened the moist layer locally providing a region potentially favourable to deep convection while terrain variability and diurnal differential heating forced vertical motions the cold air intrusion aloft associated with the low level convergence resulted in deep convection and significant storm activity the origin of the moist air masses which fed this activity has been investigated through simulations with a lagrangian particle dispersion model moist air masses which originated from marine boundary layer of the ionian sea on the west and north aegean sea on the east of the greek peninsula were transported over the land through the sea breeze mechanism and later at the time of the storm activity were deeply injected into the troposphere over the convergence line at heights up to 5 h

the nature of mesoproterozoic central indian tectonic zone with exhumed and reworked older granulites abstract the ese wnw trending central indian tectonic zone citz divides the indian subcontinent into southern and northern crustal provinces its narrow northern belt consists of closed early proterozoic mahakoshal rift zone which is presently confined between two moho reaching faults that characterize the son narmada sona lineament the mesoproterozoic sausar mobile belt smb the chhotanagpur granite gneiss complex cggc and the gneissic complex of northeast india represent the wider southern belt the mahakoshal rift belt has a thick pile of mafic ultramafic rocks and possibly oceanic components the basin closed due to a south directed subduction creating a magmatic arc that was sutured to its southern boundary during 1 8 1 7 ga a south dipping ductile shear zone known as the son narmada south fault affects both its supracrustals and intrusive granitoids the son narmada south fault generally delimits the northern boundary of the gondwana basins whereas the son narmada north fault delineates the northern boundary of the mahakoshal mobile belt and the southern boundary of the meso neoproterozoic vindhyan basin the smb representing the southern belt of the citz in the central sector comprises strongly folded and metamorphosed non volcanic and mn rich sausar sediments intermixed with reworked basement gneisses migmatites and granulites representing the pre sausar assemblage the sausar orogeny closed during terminal mesoproterozoic ca 1 0 ga on the other hand metamorphism in the basement gneiss migmatites and northern and southern granulite belts pre dates the sausar orogeny the earliest structure of the smb truncates and rotates the n s oriented structures of the southward located sakoli and the dongargarh belts of neoarchaean palaeoproterozoic age the northern granulite belt of the smb and the cggc and the high grade supracrustals from the sonapahar area in ne india all indicate very high p t conditions during 1 7 1 5 ga peak metamorphism which was followed by decompressional cooling the setting resembles continental collision which was followed by tectonic exhumation and denudation on the other hand the southern granulite belt from the smb documents a late archaean 2 6 ga event and a later 1 4 ga event the dalma volcanic belt located close to the southern tectonized boundary of the cggc belt and possibly the assemblage of copious mafic enclaves within the ca 550 ma granite plutons from ne india represent back arc like setting during 1 6 1 5 ga the setting is caused due to the southward subduction of the north indian block beneath the southern block leading to continental collision the metasedimentary belt located further to the south of the dalma volcanic belt and flanking the archaean singbhum craton to the north on the other hand is palaeoproterozoic in age but has been metamorphosed during ca 1 6 ga because of the northern collision related orogeny reworking and incorporation of basement gneisses and granulites from the mid crust level to the younger sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny the tectonic transport was from the north to the south with north dipping thrusts thus there was a reversal in the direction of convergence from the preceding orogeny the sausar orogeny closed during ca 1 0 ga it caused anatectic event producing granitoid plutons migmatites and charnockites in the cggc belt and migmatites in the gneissic complex in ne india

depth estimation from the scaling power spectral density of nonstationary gravity profile a technique to estimate the depth to anomalous sources from the scaling power spectra of long nonstationary gravity profiles is presented the nonstationary profile is divided into piecewise stationary segments based on the criterion of optimum gate length in which the time varying and time invariant autocorrelation functions are similar the division of a nonstationary into piecewise stationary allows identification of the portion of the crust with different geological histories and using the stationary portion of the gravity profiles more consistent depths to the anomalous sources have been obtained the technique is tested with the synthetic gravity profile and applied along the jaipur raipur geotransect in western and central india the geotransect has been divided into four stationary parts vindhyan low bundelkhand low narmada rift and chhattisgarh basin each section corresponding to a different geological formation forward modeling of gravity data using results of each stationary section is carried out to propose the subsurface structure along the jaipur raipur transect

on the mechanism and source parameters of the deep crustal jabalpur earthquake india of 1997 may 21 constraints from aftershocks and changes in static stress summary the 1997 may 21 jabalpur earthquake mb 6 0 occurred in the lower crust of the ene wswtrending narmada son palaeorift zone in central india known as the narmada son lineament nsl we correlate the aftershock distribution of this earthquake with the computed changes in static stress caused by slip on the earthquake rupture in order to constrain some of the source parameters of the main event we estimate that the downdip edge of the earthquake rupture extended up to a depth of about 38 km and that the majority of the aftershocks occurred due to unclamping in the hangingwall near the downdip edge of the rupture considering the intermediate to moderately high heat flow in the region we suggest that 1 high pore pressure in the lower crust which developed due to the dehydration of the serpentinite body during previous episodes of thermo tectonic processes 2 a lower frictional coefficient possibly arising due to fractured rocks at depth 3 a higher strain rate and 4 the favourable influence of n s compression due to the motion of the indian plate are responsible for the infrequent earthquakes in the lower crust of the nsl zone

large variation of curie depth and lithospheric thickness beneath the indian subcontinent and a case for magnetothermometry summary a first order estimate of curie depth and lithospheric thickness variation beneath the indian region has been obtained using the available geothermic data the curie depth is found to vary from 18 to 68 km and the lithospheric thickness from 38 to 186 km these estimates conform well with the mags at and seismological findings both the parameters exhibit covariancy and suggest the possibility of approximating heat flow values and lithospheric thickness in geothermally uncharted areas using curie point geotherm depths derived from air satellite borne magnetic anomalies interestingly the areas with shallow curie depth and lithospheric thickness are found to be associated with a thin crust higher heat flow higher geothermal gradients and positive gravity anomalies and vice versa large thickness estimate of magnetic crustal layer extending below the moho demands a new conceptual understanding the study has brought out a good case for wider use of magneto thermometry and provides understanding of the geodynamic evolution of the indian subcontinent

geothermal energy reconnaissance of southeastern nigeria from analysis of aeromagnetic and gravity data given the daunting energy challenges currently bedeviling nigeria with demand for electrical power exceeding availability alternative energy sources preferentially renewable are vigorously considered this is the first study to examine geothermal energy prospects in southeastern nigeria with the ultimate aim of locating and mapping regions of shallow curie point depths cpds crustal thickness and geologic structures supportive of a geothermal system we estimated cpd using the spectral analysis technique on magnetic field anomalies gravity anomalies were subjected to a series of procedures to assess geological structures result reveals deeper cpds within central and southern regions of okposi afikpo and biase towns in an approximate range of 18 4 to 19 3 km shallow cpds 9 8 17 4 km have also been obtained within obubra and abakaliki regions estimated crustal thickness moho ranges between 26 5 and 35 8 km part of the region with shallow cpd correlates with regions of shallow moho depths particularly in abakaliki and deeper moho depths coincide with deeper cpd estimates at the afikpo location the estimated geothermal gradient and heat flow values range between 29 0 and 45 0 oc and 52 2 101 5 mw m2 respectively and fall within values evaluated from deep wells drilled for oil exploration within the adjoining anambra basin region this study recommends the regions with shallow depths for possible geothermal power plant location gravity data evaluations also reveal significant subsurface intrusions within ugbodo obubra and abakaliki locations overall this study is a step forward toward a better evaluation of the geothermal energy resources potential of southeastern nigeria

combined use of the centroid and matched filtering spectral magnetic methods in determining thermomagnetic characteristics of the crust in the structural provinces of central brazil abstract spectral analysis of data acquired in twelve different aeromagnetic surveys of central brazil has led to new insights in the magnetic characteristics of the lithosphere in the structural provinces of tocantins and sao francisco since centroid based spectral magnetic methods do not model the spectrum explicitly they are used for the first time in conjunction with the matched filtering spectral modeling method to verify that appropriate slope segments are fit when determining source depths the tocantins province is characterized by subdued variations in the thickness of magnetized layers most of which may be associated with structural discontinuities and subcrustal intrusions in contrast variations in thickness of magnetized crustal layers are more prevalent in the sao francisco province the depths to the bottom of magnetized crust in the tocantins province are in general less than 30 35 km whereas they reach depths greater than 40 km in the sao francisco province in parts of sao francisco craton the depths to the bottom of magnetized crust are greater than the crustal thickness determined from seismic refraction and receiver function studies however these are indistinguishable in terms of resolution of the two methods analysis of heat flow data and results of thermal modeling indicate depths of curie isotherms consistent with estimates from spectral analysis of aeromagnetic data hence the possibility that the uppermost part of mantle under the sao francisco craton may be ferromagnetic cannot be dismissed

power law scaling behavior of crustal density and gravity 1 the scale invariant nature of crustal heterogeneity has been established through direct measurements of rock properties and inferences from geophysical observations we examine the behavior of crustal density variations over length scales from centimeters to hundreds of kilometers using well logs surface rock samples and observed gravity data the three data types support a simple model of an isotropic scaling crust with density characterized by a power law spectral behavior

magsat data and curie depth below deccan flood basalts india ground and airborne magnetic data are severely disturbed due to random susceptibility variations in deccan flood basalts however magnetic satellite magsat data over the deccan flood basaltic region of the indian subcontinent exhibit filtering of surficial noise three passes over deccan traps show a low at about 20 n latitude and a high at about 23 n latitude spectral analysis of these passes and an available 2 d magsat vertical intensity map indicate a deep 40 4 km magnetic interface it is interesting to note that the determination of curie depth from magsat matches and confirms the geothermal data model the estimates correspond to the moho depth derived from gravity and deep seismic sounding studies the study suggests a continental shield like geothermal gradient of about 14 c km below the area

evaluation of curie point depths geothermal gradients and near surface heat flow from high resolution aeromagnetic hram data of the entire sokoto basin nigeria abstract an evaluation of curie point depths geothermal gradients and near surface heat flow has been carried out from the spectral analysis of the recently acquired high resolution aeromagnetic hram data of the entire sokoto basin in northwestern nigeria the hram data was divided into twenty two 22 overlapping blocks and each block analyzed using the spectral centroid method to obtain depth to the top centroid and bottom of magnetic sources the depth values were subsequently used to evaluate the curie point depth cpd geothermal gradient and near surface heat flow in the study area the result shows that the cpd varies between 11 13 and 27 83 km with an average of 18 57 km the geothermal gradient varies between 20 84 and 52 11 c km with an average of 33 99 c km and the resulting heat flow varies between 52 11 and 130 28 mw m 2 with an average of 84 97 mw m 2 such heat flow values are suggestive of anomalous geothermal conditions and are recommended for detailed geothermal exploration in the basin

u pb data on granulite facies rocks from fold island kemp coast east antarctica abstract the northern tip of fold island 67 16 s 59 20 e on the kemp coast is composed largely of quartzofeldspathic gneisses charnockitic and enderbitic and pyroxene granulite garnet biotite sillimanite gneiss garnet clinopyroxene plagioclase calc silicate rock magnetite bearing rocks and ultramafic rocks are very subordinate the gneisses were deformed by the two phases of isoclinal folding and were subsequently intruded by mafic dikes a second stage of deformation involving tight ductile folding post dates the dikes this deformation culminated with the emplacement of hypersthene bearing pegmatites in discordant planar veins and irregular masses and with the hornblende granulite facies metamorphism that resulted in the present day mineral assemblages u pb isotope data on zircon and monazite from two pegmatites define a chord intersecting concordia at 0 94 0 08 ga and 0 21 0 21 ga the 0 94 ga age dates deformation granulite facies metamorphism and the pegmatite emplacement of stage ii u pb data on a total of five fractions of highly rounded zircons from two quartzofeldspathic gneisses define a chord with intersections at 3 08 0 17 ga and 0 82 0 48 ga our results suggest that the gneiss zircons are xenocrysts and do not provide any direct indication of the age of the host gneisses the fold island gneisses are interpreted to be older napier complex rocks that were reworked during the late proterozoic 1000 ma rayner event a similar sequence of events has been reported for a coastal portion of the eastern ghats province of south india suggesting that these two areas were part of the same metamorphic complex in gondwanaland

curie temperature depth estimation using a self similar magnetization model summary the earth s crust is magnetized down to the curie temperature depth at about 10 to 50 km this limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of south africa and central asia at short wavelengths the power increases as rapidly towards longer wavelengths as expected for a self similar magnetized crust with unlimited depth extent above wavelengths of about 100 km the power starts increasing less rapidly indicating the absence of deep seated sources to quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self similar magnetization distribution this model power spectrum matches the power spectra of south africa and central asia for a self similarity parameter of 4 and curie temperature depths of 15 to 20 km

severe downslope windstorm calculations in two and three spatial dimensions using anelastic interactive grid nesting a possible mechanism for gustiness abstract the clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model application of the conservation condition of kurihara and others is shown to result in significant improvements in the treatment of interactive nesting a significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow this result for the anelastic system is in agreement with the earlier results of phillips and shukla who considered the hydrostatic shallow water system of equations the interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 january 1972 in boulder using both two and three spatial dimensions the three dimensional simulation results in a gustiness signature in the

deep seismic sounding in the godavari graben and godavari coastal basin india abstract deep seismic sounding investigations along two profiles one along and the other across the godavari graben delineate the northeastern extension of the bapatla ridge which separates the godavari graben from the godavari coastal basin the godavari graben is filled with a maximum of 2 8 km thick lower gondwana upper permian to lower triassic sediments it is inferred on the basis of p wave velocities that the upper precambrian pakhal formation does not continue south of paloncha the upper gondwana upper jurassic to lower cretaceous rocks with marine incursions appear to continue towards the southeast for about 15 km from the exposures below the rajahmundry sandstone mio pliocene two dimensional velocity modelling of the upper crust indicates a major interface at 3 5 km depth in the upper crust where the velocity increases from 5 4 5 5 km s 1 to 6 2 6 4 km s 1 this interface shows a dpmal upwarp across the godavari graben which may be an indication of ne sw crustal extension in this region the deeper part of the crust consists of a large number of almost horizontal small reflector segments two dimensional crustal velocity modelling for some correlatable wide angle reflections yielded a three layered structure below 10 km depth with the moho at about 41 to 43 km

reconstructing the eruption magnitude and energy budgets for the pre historic eruption of the monogenetic 5 ka mt gambier volcanic complex south eastern australia understanding explosive volcanic eruptions especially phreatomagmatic eruptions their intensities and energy budgets is of major importance when it comes to risk and hazard studies with only a few historic occurrences of phreatomagmatic activity a large amount of our understanding comes from the study of pre historic volcanic centres which causes issues when it comes to preservation and vegetation in this research we show that using 3d geometrical modelling it is possible to obtain volume estimates for different deposits of a pre historic complex monogenetic centre the mt gambier volcanic complex south eastern australia using these volumes we further explore the energy budgets and the magnitude of this eruption vei 4 including dispersal patterns eruption columns varying between 5 and 10 km dispersed towards north east to south to further our understanding of intraplate monogenetic eruptions involving phreatomagmatic activity we also compare which thermodynamic model fits best in the creation of the maar crater of mt gambier the major explosion dominated model or the incremental growth model in this case the formation of most of the craters can best be explained by the latter model

phosphate stromatolites from condensed cephalopod limestones upper jurassic southern spain upper jurassic phosphate stromatolites of the almola sierra southern spain encrust macrofossils and hardgrounds and form oncoids included within pelagic condensed fossiliferous limestones their accretion was determined by bacterially mediated precipitation of phosphate by the trapping and binding of fine siliciclastics and pelagic biomicrite and by the encrustation of benthonic foraminifera phosphorous trace elements and rare earth elements were concentrated from degraded organic matter and seawater by stromatolite building communities which mediated the formation of phosphate rich and fe al si rich organic gels under oxic conditions favouring the precipitation of amorphous mineral precursors acp and fe al si oxyhydroxides the observed ce enrichment for some stromatolites is explained by oxidative scavenging of ce4 from seawater by fe mn oxyhydroxides the bacterially mediated gels were able to migrate and fill the voids of the stromatolite structure and later changed to carbonate fluorapatite haematite and poorly crystalline fe rich clays under postoxic conditions phosphatization of trapped carbonate particles also occurred the phosphate stromatolites formed on a sediment starved pelagic swell during periods of no carbonate sedimentation and hardground development stromatolite lamination provides evidence for rhythmic alternation between bacterially mediated phosphogenesis sedimentation and erosion suggesting episodic changes in the sedimentary environment although some of the parameters that controlled phosphate precipitation associated with the stromatolites local high organic productivity sediment starvation moderate depth of deposition and physicochemical conditions were similar to those found in modern and ancient phosphogenic settings the palaeogeographical framework and the intensity of sedimentary processes were different to those of the world s major phosphorite deposits