gravity anomalies and associated tectonic features over the indian peninsular shield and adjoining ocean basins abstract 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

attenuation of p waves in oceanic crust multiple scattering from observed heterogeneities we extend a model for seismic attenuation that includes scattering and intrinsic components q 1 qs 1 qi 1 over five decades in experimental wavelength core images and downhole logs from ocean drilling program hole 735b near the atlantis ii fracture zone in the indian ocean were used to construct vertical profiles of heterogeneity and to compute a spectrum of qs 1 using a multiple scattering model measurements of velocity dispersion suggest that qi 1 is small compared to the measured attenuation q 1 measurements of q 1 made at this site by laboratory sonic log and vsp experiments are compared to the computed qs 1 spectrum peaks in the qs 1 spectrum occur at six major correlation lengths scales 0 006 m 0 02 m 0 16 m 0 63 m 8 0 m and 25 0 m which can be attributed to formation heterogeneity compared to typical seismic estimates of q 1 in oceanic layer 3 the measurements in hole 735b are high but can be explained by large qs 1 values in shallow fractured ocean crust based on this comparison we conclude that the proximity of the experimental wavelength in band limited studies to the length scale of heterogeneity is critical in the measurement of q 1

gravity field structure and tectonics of the eastern ghats abstract the eastern ghats are a prominent topographic feature on the indian peninsula stretching from the southern tip of the peninsula to near bhubaneswar 20 n 86 e along the east coast the belt is characterised by occurrences of high grade metamorphic rocks such as pyroxene granulites sillimanite gneisses charnockites and gabbro anorthosite masses the gravity field over the eastern ghats is appreciably positive as compared to the surrounding low grade gneissic terrain analysis of the gravity field along the coastal and southern granulite terrain comprising the eastern ghats shows that a large number of gravity highs are associated with charnockites of basic and intermediate nature as well as gabbro anorthosite masses the lows appear to be associated with acid charnockites syenite masses or granitic intrusives the boundary between the eastern ghats terrain and the adjoining dharwar bastar cratons appears to be a faulted one the crust underneath the eastern ghats is inferred to be of a higher density than that of the dharwar bastar cratons to its west the gravity field over the eastern ghats is compared to that of similar terrains in other parts of the world it is inferred that the eastern ghats are characterised by a crust of higher than normal density

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

long wavelength gravity and topography anomalies it is shown that gravity and topography anomalies on the earth s surface may provide new information about deep processes occurring in the earth such as those associated with mantle convection two main reasons are cited for this the first is the steady improvement that has occurred in the resolution of the long wavelength gravity field particularly in the wavelength range of a few hundred to a few thousand km mainly due to increased coverage of terrestrial gravity measurements and the development of radar altimeters in orbiting satellites the second reason is the large number of numerical and laboratory experiments of convection in the earth including some with deformable upper and lower boundaries and temperature dependent viscosity the oceans are thought to hold the most promise for determining long wavelength gravity and topography anomalies since their evolution has been relatively simple in comparison with that of the continents it is also shown that good correlation between long wavelength gravity and topography anomalies exists over some portions of the ocean floor

the first direct measurements of upper oceanic crustal compressional wave attenuation the first direct measurement of compressional wave attenuation of the uppermost 650 m of oceanic crust was performed using data recorded by seafloor hydrophones and large 56 116 kg deep explosive sources the site was 13 km east of the southernmost juan de fuca ridge on crust 0 4 m y old spectral ratios were performed between bottom refracting waves and direct water waves adjusted for spreading losses and transmission coefficient losses several tests of the data were performed demonstrating that attenuation is linearly related to frequency between 15 and 140 hz but frequency independent components of attenuation are also evident values of compressional wave q cluster between 20 and 50 and do not show any systematic variation with depth over 650 m the attenuation results also indicate the presence of heterogeneities within the crust as the solutions for each receiver s data set are significantly different no evidence for azimuthal variations of attenuation are supported by the data although the data do not optimally sample a wide variation of azimuths our attenuation values are judged to be normal to higher than expected for the whole oceanic crust based upon comparisons to results from synthetic seismogram modeling by others and by modeling signal to noise ratios of typical seismic refraction profiles the results are consistent with recent laboratory measurements at ultrasonic frequencies for dry and saturated basalts at seafloor pressures and temperatures

marine geophysical study of the comorin ridge north central indian basin marine geophysical data are used to interpret the continental margin structure south of india and west of sri lanka in the northern central indian basin free air and isostatic anomaly maps are presented they show the prominent indian ocean gravity low with values more negative than 100 mgal south west of sri lanka a steep gradient in the free air and isostatic anomaly forms a nnw sse trending line extending south and striking parallel to the southwest coast of india this line marks the eastern boundary of a topographic and basement high here named the comorin ridge which is also clearly seen on a sediment isopach map the ridge seems to act as a barrier to the passage of ganges cone sediments to the west a comparison of the eastern landward edge of the comorin ridge with similar structures of other passive continental margins such as those off southern south africa agulhas fracture zone off the falkland plateau and off western norway is made it is suggested that the eastern edge of the comorin ridge may mark a significant structural crustal boundary

attenuation of refracted seismic waves in young oceanic crust abstract it is commonly observed in seismic refraction experiments over young oceanic crust that refracted energy is concentrated in the 10 to 15 hz band even though the source may have far greater bandwidth we apply spectral ratio methods to data from a midwater hydrophone array towed 10 km from the juan de fuca ridge and confirm that the impulse response of the crust to refracted energy has a peak near 10 hz this peak appears to result from a combination of two factors that produce attenuation one is related to the large velocity gradient in the upper crust caused by crack closure with increasing depth the other is related to anelastic attenuation which is probably also related to cracks and voids in the upper crust at low frequencies wavelengths greater than or equal to the thickness of the high gradient zone it is shown that attenuation is dominated by the reflectivity of the upper crust at high frequencies attenuation is dominated by anelastic effects associated with upper crust q 9s of between 10 and 50 the wave propagation is dispersive causing observable phase shifts in the response function and q may be frequency dependent this model of the propagation explains why refraction data over young oceanic crust have energy concentrated near 10 hz

structure of the northern bay of bengal offshore bangladesh evidences from new multi channel seismic data abstract new multi channel seismic data were acquired in the northern part of the bay of bengal and at the northernmost termination of the 90 e indian ridge offshore bangladesh this survey was coupled with a seismic refraction experiment indicating this offshore basin is here floored by a thinned 15 km thick continental crust injected by mesozoic volcanism this attenuated continental crust is interpreted as formed during gondwana super continent fragmentation during a syn rift period the dominant tectonic pattern is marked by ne sw trending tilted blocks filled by syn rift sediments clearly identified on seismic profiles the uppermost part of this continental crust 3 4 km thick shows a complex assemblage of dipping reflectors and west facing tilted blocks injected by volcanic build ups the lower crustal sequence 11 12 km thick does not reveal significant reflectors this syn rift fabric is attributed to the mesozoic up to the early cretaceous by correlation with published seismic data along the eastern coast of india opposite normal faults vergency on the indian and burma sides indicate an asymmetrical rifting simple shear creating a wide cot on the burma side and a short cot on the opposite indian side a geometry typical of continental crust stretching this crustal fabric is overlain disconformably by a thin reflector attributed to the late cretaceous paleocene pelagic sequence deposited during india bay of bengal drift phase before the cenozoic india asia collision marked by thick clastic sedimentation associated with the ganges delta southward progradation below this delta this mesozoic rift closes axially and is affected by the incipient late miocene shortening of the shillong plateau the ne sw fabric of this attenuated crust might be traced southward to 15 n close to magnetic chron 34 where steady state spreading of the central indian ocean occurred

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

geophysical signatures over and around the northern segment of the 85 e ridge mahanadi offshore eastern continental margin of india tectonic implications abstract 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

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

gravity field and deep structure of the bengal fan and its surrounding continental margins northeast indian ocean abstract a revised gravity anomaly map for the northeast indian ocean shows that the shelf edge underlying the eastern continental margin of india is a rather narrow but extensively linear gravity low minimum free air 149 mgal the bengal fan seaward of the shelf has a depressed gravity field average free air 20 to 30 mgal in spite of the enormous thickness of sediments of as much as 10 15 km the two buried ridges below the bengal fan the 85 east and 90 east ridges have a large negative 75 mgal and a substantial positive 40 mgal free air anomaly respectively the andaman and burmese arcs lying along the east margin of the bengal fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mgal gravity interpretation for three regional traverses across the central and northern parts of the bengal fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of india the 85 east ridge that was created when the indian ocean lithosphere was very juvenile appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere while the 90 east ridge submarine topography buried load below the bengal fan is probably isostatically compensated by a low density mass acting as a cushion at the base of the crust the bengal fan crust with its thick sediment layer is carried down the andaman subduction zone to a depth of about 27 km where possibly phase transition takes place under higher pressure the maximum sediment thickness at the andaman burmese subduction zone is of the order of 10 12 km the gravity model predicts a low density zone about 60 km wide below the andaman burmese volcanic arc penetrating from crustal to subcrustal depths in the overriding burma plate a more complex density distribution is however envisaged for the andaman volcanic arc that is split by the neogene back arc spreading ridge the ocean continent crustal transition possibly occurs farther east of the volcanic arc below the shan plateau margin in burma or below the mergui terrace at the malayan continental margin east of the andaman sea

insights into the lurking structures and related intraplate earthquakes in the region of bay of bengal using gravity and full gravity gradient tensor comprehension of subsurface structures buried under thick sediments in the region of bay of bengal is vital as structural features are the key parameters that influence or are caused by the subsurface deformation and tectonic events like earthquakes here we address this issue using the integrated analysis and interpretation of gravity and full gravity gradient tensor with few seismic profiles available in the poorly known region a 2d model of the deep earth crust mantle is constructed and interpreted with gravity gradients and seismic profiles which made it possible to obtain a visual image of a deep seated fault below the basement associated with thick sediments strata gravity modelling along a ne sw profile crossing the hypocentre of the earthquake of 21 may 2014 m w 6 0 in the northern bay of bengal suggests that the location of intraplate normal dip fault earthquake in the upper mantle is at the boundary of density anomalies which is probably connected to the crustal fault we also report an enhanced structural trend of two major ridges the 85 e and the 90 e ridges hidden under the sedimentary cover from the computed full gravity gradients tensor components

structure and evolution of the afanasy nikitin seamount buried hills and 85 e ridge in the northeastern indian ocean abstract geophysical data of the afanasy nikitin seamount ans partly buried hills and 85 e ridge in the northeastern indian ocean were studied together with published seismic refraction results to understand genesis and evolution of the structures the ans joins the 85 e ridge through isolated buried hills and intervening subsurface structures and together they form a linear ridge system the gravity signature of the ridge system changes from negative to positive towards south of 5 n which seems to coincide with termination thinning of pre collision continental sediments in the bay of bengal thick pile of bengal fan sediments had great impact on underneath pre collision sediments as well as on basement rocks the process resulted in attaining higher velocities up to 6 6 and 7 1 km s respectively gravity model studies suggest that structures of the ridge system are compensated in different modes the ans is underlain by an 8 km thick deep crustal body of magmatic rocks while beneath other structures oceanic crust is down flexed up to 2 5 km the presence of metasediments more dense than volcanic rocks and flexure of the lithosphere would explain the negative gravity anomaly over the 85 e ridge whereas lack of metasediments and magmatic rocks at depth would explain the compensated positive anomaly over the ans the width of the 85 e ridge the wavelength 190 km and amplitude 2 5 km of the flexed oceanic crust and the intersection of the ridge with the mesozoic fracture zones suggest that the ridge was formed in intraplate position when the lithosphere underneath was approximately 35 myr old initial emplacement of the ans was coeval with the formation of oceanic lithosphere at 80 73 ma the hotspot forming the 85 e ridge had reactivated the ans during the paleocene and brought it to the sea surface then it underwent erosion and subsidence processes the deformation activity had converged the north and south parts of the ans at late miocene and again at late pleistocene and northward buried hills at early pliocene

seismicity gravity anomalies and lithospheric structure of the andaman arc ne indian ocean abstract the andaman arc in the northeastern indian ocean defines nearly 1100 km long active plate margin between the india and burma plates where an oblique benioff zone develops down to 200 km depth several east trending seismologic sections taken across the andaman benioff zone abz are presented here to detail the subduction zone geometry in a 3 d perspective the slab gravity anomaly computed from the 3 d abz configuration is a smooth long wavelength and symmetric gravity high of 85 mgal amplitude centering to the immediate east of the nicobar island where a prominent gravity high follows the nicobar deep the slab residual gravity anomaly srga and mantle bouguer anomaly mba maps prepared for the andaman plate margin bring out a double peaked srga low in the range of 150 to 240 mgal and a wider cum larger mba low having the amplitude of 280 to 315 mgal demarcating the andaman arc trench system the gravity models provide evidences for structural control in propagating the rupture within the lithosphere the plate margin configuration below the andaman arc is sliced by the west andaman fault waf as well as by a set of sympathetic faults of various proportions often cutting across the fore arc sediment package some of these fore arc thrust faults clearly give rise to considerably high post seismic activity but the seismic incidence along the waf further east is comparatively much less particularly in the north although the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults tectonic correlation of the gravity derived models presented here tends to favour the presence of oceanic crust below the andaman nicobar outer arc ridge

break up of australia india madagascar block opening of the indian ocean and continental accretion in southeast asia with special reference to the characteristics of the peri indian collision zones abstract linear belts of gondwana basins developed in the indian continent since late palaeozoic along favoured sites of precambrian weak zones like cratonic sutures and reactivated mobile belts the tibetan and sibumasu west yunnan continental blocks that were located adjacent to proto himalayan part of the indian continent rifted and drifted from the northern margin of the east gondwanic indo australian continent during late palaeozoic when the said northern margin was under glacial or cool climatic condition and rift drift tectonic setting the indo burma andaman iba sikule lolotoi blocks were also rifted and drifted from the same northern margin during late jurassic this was followed by the break up of the australia india madagascar continental block during the cretaceous the activity was associated with hot spot related volcanism and opening up of the indian ocean the late cretaceous and tertiary phases of opening of the arabian sea succeeded the early cretaceous phase of opening of the bay of bengal part of the indian ocean the palaeo and neo tethyan sutures in tibet yunnan laos thailand and vietnam reveal the complex opening and closing history of the tethys the iba block rotated clockwise from its initial e w orientation because of 90 e and adjacent dextral transcurrent fault movements caused due to faster northward movement of the indian plate relative to that of australia the india tibet terminal collision during early middle eocene initiated himalayan orogenesis and contemporaneously there was foreland basin development that was accompanied with sporadic but laterally extensive continental flood basalt cfb type and related volcanism the paleogene rocks of the himalayan foreland basin are involved in tectonism and are mostly concealed under older rocks the mesozoic early eocene ophiolite terrane on iba does not represent the eastern suture of the indian plate but occurs as klippe on iba caused due to oblique collision between sibumasu and iba during late oligocene post collisional indentation of y shaped indian continent into the asian collage produced himalayan syntaxes clockwise rotation of the sibumasu block which was then sutured to the tibetan and se asian blocks and tectonic extrusion of the indochina block along the ailao shan red river asrr shear zone highly potassic magmatic rocks were emplaced during late palaeogene at the oroclinally flexed marginal parts of the south china continental lithosphere these magmatic bodies were dislocated by the asrr left lateral shear zone soon afterwards petrogenetic and tectonic processes that generated the eocene cfb volcanics at the himalayan foreland basin may have also produced late palaeogene magmatism from outer parts of the namche barwa syntaxis their site specific location and time sequence suggest them to be genetically related to the india asia collision process and indian continent s indentation induced syntaxial buckling deep mantle reaching fractures were apparently produced during india asia terminal collision at the strongly flexed leading brittle edge of the indian continental lithosphere and possibly later in time at the outer oroclinally bent marginal parts of the rigid south china continental lithosphere generating typical magma the subduction zone that developed along the western margin of iba due to oblique convergence between the iba and the indian plate is still active the northern end of iba ultimately collided with the ne prolongation of the indian continent and was accreted to it during mio pliocene the shillong massif was uplifted and overthrust over the bengal basin located over its passive margin to the south whereas the eocene distal shelf sediments of iba were overthrust over the tertiary shelf of the indian continent

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

the negative gravity field over the 85 e ridge an isopach map made from seismic reflection and refraction data in the bay of bengal shows two prominent n s trending features in the basement topography one is the northernmost portion of the ninetyeast ridge which is totally buried by sediments north of 10on the other buried ridge trends roughly n s for 1400 km at 85oe to the latitude of sri lanka and then curves toward the west it has basement relief up to 6 km two free air gravity anomaly profiles across the region show a strong gravity low 60 mgal over the 85oe ridge while the ninetyeast ridge shows a gravity high to understand the negative free air gravity anomaly over the 85oe ridge we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load a plausible formation history for a buried ridge consists of at least two major episodes the first is the formation of the ridge on a lithosphere with a flexural rigidity of d i at some later time the ridge is buried by an influx of sediments the lithosphere is cooler and the flexural rigidity has increased to d 2 the character of the gravity field depends primarily upon the initial and final values of flexural rigidity these d1 and d2 values are varied to obtain good agreement between the model and observed gravity anomalies best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial an approximate relationship between flexural rigidity and crustal age shows that the 85oe ridge was formed on relatively young lithosphere 5 15 m y old and that it was buried when the lithosphere was 40 80 m y old

seismicity and plate deformation below the andaman arc northeastern indian ocean abstract the seismic activity originating below the andaman arc sea region is generally discernible into fore and back arc seismic zones which are traceable for nearly 1500 km in a n s direction at the junctures between the indian burma and se asia plates the fore arc seismicity displays an east dipping 40 55 benioff zone upto about 200 km focal depths details of the benioff zone in correspondence to the observed gravity field are discussed in four n s sectors which suggest some significant variations in the configuration of the benioff zone the back arc seismicity affects only the top 40 45 km of the lithosphere below the andaman sea where the back arc spreading ridge splits the volcanic arc stress distribution and faulting due to earthquakes below the andaman west sunda arc are studied here using 68 focal mechanism solutions their most significant results are low angle thrust events occur along the upper edge of the descending indian plate downdip tensional events have steeply dipping 60 nodal planes and normal faulting takes place in most parts of the benioff zone along moderately dipping 30 45 planes downdip compressional events high angle reverse fault nodal plane dip 60 or reverse faulting along moderately dipping 30 45 nodal planes also occur below the andaman arc the compressive earthquakes dominate the shallower level of the subducting slab and the tensional stress observed locally in north part of the andaman sea may be an outcome of the weak coupling between the descending and overriding plates generally a more or less complete sequence of faulting i e thrusting below the trench normal faulting below the fore arc and strike slip motion along the inner edges of the fore arc characterize the andaman west sunda arc in the southern andaman region a rather oblique convergence between the indian ocean and the se asia plates is needed to explain the existence of a somewhat contorted benioff zone in which compressional stress dominates in deeper lithosphere oceanward the ninetyeast ridge also impinges on the subduction zone in this region left lateral shear motion along the east margin of the ninetyeast ridge is further inferred by the results of focal mechanism solutions

thin oceanic crust below bay of bengal inferred from 3 d gravity interpretation abstract the bay of bengal bob lithosphere evolved due to rifting between india and antarctica and subsequent seafloor spreading process since the early cretaceous is known for the presence of thick bengal fan sediments covering the entire bob with maximum sediment accumulation of 22 km in the offshore bangladesh three dimensional crustal structure of the bob is investigated by integrating the sediment isopach map seismic reflection refraction and wide angle reflection data with the 3 d gravity modelling large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function 3 d gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from 5 mgal over the ninetyeast ridge near the equator to as low as 210 mgal at the apex of bengal fan in the bangladesh shelf region the residual crustal bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to moho in the bob region varying from 11 km in the south to as much as 35 km in the bangladesh shelf with moho depressed locally below both 85 e and ninetyeast ridges the total crust map obtained after subtracting the water and sediment layers from the moho depth map shows an unusually thin oceanic crust of 2 4 km in the deep basinal areas of bob and 10 12 km of thick crust below 85 e and ninetyeast ridges due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85 e and ninetyeast ridges we infer its formation during plume spreading interactions in the eastern indian ocean further higher flexural rigidity of lithosphere and cold mantle in the bob region indicates that huge amount of bengal fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the amazon fan

gravity field and structures of the rajmahal hills example of the paleo mesozoic continental margin in eastern india abstract a narrow strip of gondwana basins separates the rajmahal traps from the peninsular shield in eastern india this part of the shield margin is associated with a conspicuous gravity high of 100 km wavelength and 48 mgal amplitude over an area of 25 000 km 2 second order residual anomalies due to gondwana sediments and traps are superposed on this wider gravity high gravity interpretation partly constrained by seismic data suggests that the wider high is caused by a denser metamorphic layer amphibolite and granulite up to 3 5 km thick the metamorphic layer also extends below the eastern rajmahal hills where the gondwanas traps and younger sediments have covered it the gondwanas are downfaulted against the shield edge and are preserved over an irregular basin floor whose deepest part underlies the eastern flank of the rajmahal hills adjacent to the bengal basin it is inferred that the gondwanas were deposited over a rifted and highly faulted shield margin that was intruded by the rajmahal traps nearly 100 m y ago high grade metamorphism along the shield edge presumably preceded the continental rifting perhaps occurring in the precambrian as a part of the eastern ghats orogeny along the east coast of india

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

phosphate authigenesis in the aramachay formation lower jurassic of peru abstract the aramachay formation lower jurassic of the peruvian central cordillera is made up of 300 m of organic rich mudstones ichpachi member which grade up into 100 m of phosphatic siltstones chart and arenites alata member the organic rich mudstones are slightly enriched in p2o5 0 5 p2o5 but do not contain visible authigenic phosphate the organic maturity of the rocks as evidenced from kerogen elemental analysis may serve to explain this radiolaria in this facies are generally calcitized in contrast radiolaria in the phosphatic siltstones are phosphatized occurring within apparently authigenic blabs and stringers of collophane 2 p2o5 whole rock in addition this facies although with a lower organic ca bon content contains the remains of sponges benthic blue green algae and a few epibenthic bivalves monospecific it also contains more silt and some glauconite it is inferred that the site of phosphate authigenesis lay wholly within this phosphatic siltstone facies which records low or fluctuating oxygenation rather than fully anoxic conditions this hypothesis is supported by studies of other similar sequences and by published carbon and sulfur isotope data it is proposed that the facies sequence represents the fossilized upper margin of an oceanic o2 minimum zone comparable with that observed on the modern peruvian shelf such an environment has been suggested for phosphate authigenesis before but has not previously been identified in the stratigraphic record the f ne grained sequence is sharply overlain by phosphatic lithoclastic arenites 8 p2o5 whole rock in which the rounded phosphate grains contain radiolaria and are therefore inferred to have been eroded from the underlying siltstone these are in turn overlain by shallow marine carbonates completing a sequence interpreted to represent a substantial marine regression

geophysical studies over the continental margins of the east coast of india abstract results of six profiles of continuous echosounding magnetic and gravity data collected across the continental margin of the east coast of india between 10 and 20 n in the bay of bengal aboard r v chain in 1973 are presented continuity of surface and subsurface features from profile to profile is indicated two major structural elements viz the marginal high situated at the foot of the continental slope and extending along the east coast of india and the marginal basin wide in the south becoming narrower towards the north have been delineated based on the topography and the gravity and the magnetic anomalies the eastern margin of india may be divided into two parts viz the southern and northern margins the southern margin is broader and more stable than the northern margin over which lava flows occur

internal friction in tholeiitic basalts internal friction in two tholeiitic basalts from deccan traps india was measured at 1 3 hz in the temperature range 500 900 c the results gave two attenuation peaks one at approximately 620 c and the other near 830 c the peaks are attributed to bimodal grain size distribution in contrast poly crystalline alumina with a single grain size distribution gave one attenuation peak at 810 c grain boundary sliding in the presence of an intergranular fluid phase is suggested as the possible mechanism of anelasticity in the basalts with this assumption we estimate the viscosity of the palagonite glass in the basalts to be 1 7 1013 p in the temperature range 600 640 c and 2 6 5 0 109 p in the temperature range 820 840 c these estimates are in general agreement with direct measurement of viscosity in tholeiite basalt by scarfe 1977

seismotectonics of subduction and back arc rifting under the andaman sea abstract active underthrusting of the indian plate below the burma plate produces seismic activity under the andaman and burmese arcs it is suggested that significant differences exist in the configuration of the benioff zone underlying the two arcs several parallel and linearly extensive fault zones are outlined by epicentral distribution in the andaman arc the andaman back arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients nne to sse in north and south parts of the andaman sea respectively the spreading ridge is offset by active transform faults on the far north the spreading ridge appears to connect to a major transform fault in burma across the burmese coast focal mechanism study provides evidence for tensional stress near the inner wall of the andaman trench dominant compressive deviatoric stress within the overriding burma plate and tensional stress under the back arc spreading ridge

three dimensional compressional wave attenuation tomography for the crust and uppermost mantle of northern and central california i present a frequency independent three dimensional 3 d compressional wave attenuation model indicated by quality factor qp for the crust and uppermost mantle of northern and central california the tomographic inversion used t values measured from amplitude spectra of 80 988 p wave arrivals of 3247 events recorded by 463 network stations through a 3 d seismic velocity model the model has a uniform horizontal grid spacing of 15 km and the vertical node intervals range between 2 and 10 km down to 45 km depth in general the resulting qp values increase with depth and agree with the surface geology at shallow depth layers the most significant features observed in the qp model are the high qp values in the sierra nevada mountains and low qp anomalies in the western fault zones low qp values are also imaged in owens valley and long valley at shallow depths and the cape mendocino region in the lower crust 25 km depth an overall contrast of qp values across the fault is observed in the creeping parkfield and cholame carrizo sections of the san andreas fault the new 3 d qp model provides an important complement to the existing regional scale velocity models for interpreting structural heterogeneity and fluid saturation of rocks in the study area

compressional wave attenuation in oceanic basalts to understand better the seismic attenuation in the upper volcanic regions of the oceanic crust compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse echo spectral ratio technique capable of measuring attenuations to pressures of 500 mpa seven basalts five from deep sea drilling project cores and two from dredge samples have wide ranges of densities porosities and alterations making possible an analysis of the parameters influencing basalt attenuation attenuation increases with the volume of secondary minerals present and with increasing porosity thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation with the application of hydrostatic pressure cracks close thereby reducing attenuations this pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth an inverse relationship between velocity and attenuation is observed at high hydrostatic pressures water saturation increases attenuation at pressures below 200 mpa and enhances the sensitivity of attenuation to pressure thus making the state of saturation important in the 40 to 100 mpa range generally found in layer 2 these results provide a framework for interpreting marine attenuation data

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

some structural features of the bay of bengal abstract continuous bathymetric magnetic and gravity data covering the bay of bengal from 10 n to 19 30 n and between the east coast of india and the western margins of the andaman islands and bangladesh were collected on board r v chain in 1973 these data have brought out several features that show correspondence from profile to profile in topography and in magnetic and gravity values the magnetic basement computed from the magnetic anomalies and the crustal structure models deduced to fit the observed free air gravity anomalies have revealed that the sediment thickness ranges from about 4 5 km to more than 10 km between 10 n and 19 30 n five major structural zones trending nearly n s have been delineated in the bay of bengal these are the marginal high that extends along all the east coast of india the marginal basin identified only on the southern margins of the east coast of india the graben abutting the continental slope and shelf near visakhapatnam the central basin in the central bay of bengal and the extension of ninetyeast ridge as a subsurface feature up to 17 n

the shallow attenuation structure of the fast spreading east pacific rise near 9 30 n direct measurements of q values in the shallow crust are difficult to obtain due to the lack of energy turning in this region we analyze a unique data set consisting of seven refraction experiments carried out with both the source and receiver near the seafloor this experimental geometry allows us to resolve the shallow compressional wave attenuation structure of young crust at the east pacific rise all models depict a low q layer 2a apparent q 11 22 and several models resolve a layer 2b with a much higher q apparent q 70 this structure is consistent with a layer 2a composed of extrusives overlying a layer 2b composed of sheeted dikes

structure and tectonics of 85 e ridge from analysis of geopotential data abstract the bay of bengal bob sedimentary basin is one of the largest offshore sedimentary basins of the world having the thickest accumulation of sediments which mask the underlying crust and it is the geopotential data that offers the unique opportunity of seeing below the thick sediments satellite derived free air gravity fag data marine magnetic data bathymetry available seismic profiles and isopach maps over the bay of bengal are utilized in the present paper to throw light on the nature and tectonics of the bob with particular emphasis on the 85 e ridge combined analysis of gravity and magnetic data along six ew profiles of length varying from 325 to 440 km is undertaken incorporating seafloor spreading anomalies in the model of the several theories put forth for the evolution of the 85 e ridge the present study supports the sagging of the crust and rules out the hotspot trace and magmatic under plating theories the crustal model derived from the magnetic profile analysis suggests that the 85 e ridge does not extend to the north of 15 n latitude and this is supported by the euler solutions of the fag and the long wavelength filtered fag however the intermediate wavelength filtered fag suggests that the 85 e ridge extends up to the ocean continent boundary thereby suggesting that the 85 e ridge thins out dramatically to the north of 15 n latitude analysis of gravity data along 17 n latitude profile predicts that in the northern segment the 85 e ridge is a geo morphological feature within the sediments above the basement whereas to the south of 15 n latitude it is a feature within the oceanic crust below the basement further the reverse magnetization associated with the seafloor spreading anomalies lying on the 85 e ridge belongs either to the albian period or the anomaly a34 a33 the 85 e ridge would be younger than these seafloor spreading anomalies and would have formed due to horizontal compressional forces of the lithosphere preceding development of the subduction zone at the andaman trench

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

effects of pore pressure on compressional wave attenuation in a young oceanic basalt laboratory data are reported for ultrasonic compressional wave attenuation p as a function of pore pressure in a juan de fuca ridge dredge basalt four experiments have been made to determine the relationships between attenuation and quality factor qp and confining and pore pressures in the shallow ocean crust attenuation was measured at 1 a constant differential pressure of 40 mpa 2 confining pressures to 120 mpa and atmospheric pore pressures 3 a constant confining pressure of 50 mpa while varying pore pressures 4 a constant confining pressure of 100 mpa while varying pore pressures for atmospheric pore pressures p ranges from 1 64 db cm to 7 08 db cm qp 24 to 10 in addition attenuation increases systematically with increasing pore pressure and decreasing differential pressure confining pressure pore pressure results from dsdp and odp downhole packer experiments suggest that the hydrostatic pore pressure regime may best approximate in situ conditions for young oceanic crust hydrostatic pore pressures 5000 meters depth reduce qp as much as 35 from normal atmospheric pressure conditions therefore pore pressures generated in the upper oceanic crust may be responsible in part for the observed low seismic qp within layer 2a qp measurements at elevated pore pressures agree well with seismic qp data

pore pressure and oceanic crustal seismic structure summary marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust in many localities compressional and shear wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth it is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability this is supported by compressional and shear wave velocity measurements as functions of confining pressure and pore pressure for basalt from the juan de fuca ridge and dolerite from the samail ophiolite oman within the oceanic crust regions of overpressure and underpressure will possess anomalous velocities the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic the influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low of significance poisson s ratio is shown to be dependent on pore pressure as well as confining pressure at constant confining pressure poisson s ratio increases with increasing pore pressure thus overpressured regions within the upper oceanic crust are likely to have relatively high poisson s ratios as well as low compressional and shear wave velocities

continent like crustal thickness beneath the bay of bengal sediments observed dispersion of fundamental mode rayleigh and love waves across the bay of bengal to wwssn and indian seismograph stations suggests increasing crustal thickness northward from an approximate 15 km thick oceanic crust south of the latitude of the southern tip of india to a more continent like thickness of approximately 25 km at 20 n and over 35 km at the northernmost part of the bengal fan the dispersion data are not consistent with models which simply add the increasing sedimentary thickness on top of a normal oceanic crust the sediment thicknesses and crust velocities are controlled by extensive seismic refraction results and are in agreement with clearly observed sedimentary higher mode waves recorded at the indian seismograph stations madras and vishakapatnam several hypotheses might explain this surprising result 1 1 the blanketing effect of the sediments with consequent temperature rise has brought about differentiation of basalt to increase the crustal thickness r n singh personal communication 2 2 the change in moho depth represents dynamic isostatic adjustment such as a phase change boundary which is lowered because of temperature or pressure perturbations from the sedimentary blanket or from pressure perturbations of plate collision 3 3 the collision has underthrust a wedge of low velocity material beneath the oceanic crust 4 4 the northern bay of bengal is actually fortuitiously continental or continent like possibly as a result of the influence of the 90 ridge especially in the northern part if the explanation is 1 2 or 3 the result is important to our understanding of crust and upper mantle dynamics it is suggested that further controlled studies be performed to verify the result

effects of rock mechanical properties on local stress field of the mahanadi basin india results from finite element modelling 1 two dimensional 2 d finite element models fem are generated using two available seismic sections across the mahanadi basin mb at the east coast of india this permits to study the interactive effect between applied stress in the direction of maximum horizontal compressive stress sh and variations in rock mechanical properties of several litho units present in the basin the sh orientation for six offshore wells in the mb known from our previous studies chatterjee and mukhopadhyay 2000 have been used as constraints for fem local stress modifications due to contrast in the young s modulus between sedimentary in fill and the crust down to 20 km depth are investigated for two more orthogonal seismic profiles traversing the basin from onland to offshore fem derived stress vectors clearly respond to variations in elastic properties of rocks corresponding to lithological interfaces in the basin and its underlying crust rotation of the principal stress is prominent where the stress concentration is high further the stress concentration reflects the basinal configuration

tectonics of the bay of bengal new insights from satellite gravity and ship borne geophysical data abstract recently released satellite derived free air gravity anomalies and the existing ship board geophysical data provide new insights into the tectonics of the bay of bengal with respect to the structure and regional extension of the buried 85 e ridge and the tectonics of the eastern continental margin of india ecmi the 85 e ridge can be visualized extending inland via the mahanadi basin volcanics to the rajmahal traps a large volcanic province in eastern india encompassing the rajmahal and sylhet traps and volcanics in the bengal and mahanadi basins almost on the scale of the deccan volcanic province along the west coast can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 ma the 85 e ridge represents the deep ocean volcanic trace of this magmatic activity towards the south the ridge continues in an arcuate manner to the afanasy nikitin seamount at equatorial latitudes in the central indian ocean gravity models of the ridge are indicative of hotspot related crustal underplating processes beneath the ridge the ecmi can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data which bear similarities with the conjugate east antarctica margin

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

lithosphere structure and upper mantle characteristics below the bay of bengal the oceanic lithosphere in the bay of bengal bob formed 80 120 ma following the breakup of eastern gondwanaland since its formation it has been affected by the emplacement of two long n s trending linear aseismic ridges 85oe and ninetyeast and by the loading of ca 20 km of sediments of the bengal fan here we present the results of a combined spatial and spectral domain analysis of residual geoid bathymetry and gravity data constrained by seismic reflection and refraction data self consistent geoid and gravity modeling defined by temperature dependent mantle densities along a n s transect in the bob region revealed that the depth to the lithosphere asthenosphere boundary lab deepens steeply from 77 km in the south to 127 km in north with the greater thickness being anomalously thick compared to the lithosphere of similar age beneath the pacific ocean the geoid topography ratio gtr analysis of the 85 e and ninetyeast ridges indicate that they are compensated at shallow depths effective elastic thickness te estimates obtained through admittance coherence analysis as well as the flexural modeling along these ridges led to the conclusions i 85 e ridge was emplaced in off ridge environment te 10 15 km ii the higher te values of 25 km over the afanasy nikitin seamount ans reflect the secondary emplacement of the seamount peaks in off ridge environment iii that the emplacement of the ninetyeast ridge north of 2 n occurred in an off ridge environment as indicated by higher te values 25 30 km furthermore the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths 800 km are compensated by processes related to upper mantle convection

attenuation differences in layer 2a in intermediate and slow spreading oceanic crust in situ seismic attenuationq 1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate and slow spreading oceanic crust q 1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in ocean drilling program holes 504b and 896a and in hole 395a located in 5 9 7 3 ma crust on the pacific and atlantic plates respectively our results strongly tie crustal properties to seismic measurables and observed geological structures we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity we interpret a distinct decrease in the q 1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2a 2b boundary the boundary is located at 465 m depth below the sea floor in both hole 395a and 504b although this is likely to be a coincidence of the sediment thickness at these sites layer 2a is estimated to be approximately 150 m thick in hole 504b and 300 m thick in hole 395a cyclic sequences of high porosity pillows and low porosity massive units in the uppermost 100 m of volcanics in hole 395a result in large velocity heterogeneities which cause 5 times more attenuation in this layer than in hole 504b in hole 896a by contrast fewer pillows more massive flows and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from hole 504b we conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration the observed differences inq 1profiles and layer 2a thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid ocean spreading centers for these pacific and atlantic ridge segments

velocity and attenuation in sandstone at elevated temperatures and pressures new measurements of seismic velocity and attenuation have been made in the kilohertz frequency range to temperatures of 120 c confining and pore pressure to 200 bars in berea sandstone with increasing temperature shear velocity and attenuation decrease at all pressures in a fully saturated rock the data suggest that thermal relaxation is not a significant loss mechanism under these conditions we propose that dissipation is controlled by a viscous fluid flow mechanism in which a sharp frequency peak in attenuation is shifted from 2khz at room temperature to 8 khz at 120 c as the pore fluid viscosity is decreased with increasing temperature however the velocity decrease is too great to be accounted for by a change of relaxation times a non dispersive temperature softening in shear may control the velocities

40 ar 39 ar geochronology of subaerial lava flows of barren island volcano and the deep crust beneath the andaman island arc burma microplate little was known about the nature and origin of the deep crust beneath the andaman island arc in spite of the fact that it formed part of the highly active indonesian volcanic arc system one of the important continental crust forming regions in southeast asia this arc formed as a result of subduction of the indian plate beneath the burma microplate a sliver of the eurasian plate contains only one active subaerial magmatic center barren island volcano whose evolutional timeline had remained uncertain in this work we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island by incremental heating 40ar 39ar method in an attempt to understand the evolutionary histories of the volcano and its basement based on concordant plateau and isochron ages we establish that the oldest subaerial lava flows of the volcano are 1 58 0 04 2 ma and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 3 2 ma mineralogy anorthite cr rich diopside minor olivine and isotopic compositions 87sr 86sr 7 0 of xenoliths not only indicate their derivation from a lower oceanic crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the andaman accretionary prism we speculate that the basements of the forearc and volcanic arc of the andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the eurasian plate

regional conductance map of andaman and nicobar region abstract to image the electrical conductivity distribution fluxgate magnetometers are operated at five sites in andaman and nicobar region transfer functions are estimated for the period range 8 128 min from nighttime transient geomagnetic variations using robust regression analysis the observed induction arrows in andaman islands are found to point towards east despite deep sea located towards its west this indicates that fore arc basin andaman nicobar deep is more conducting than the region of outer non volcanic island arc thin sheet model requires the conductance of 10 000 35 000 s with increase conductivity towards the south for explaining the observed induction pattern the observed induction pattern at andaman nicobar stations can be explained in terms of high conducting cretaceous tertiary sediments filling the andaman nicobar deep high conductivity over invisible bank has been attributed to the partial melts volatile fluids derived from the subducting indian plate that are intruding into the eastern margin of fore arc basin through the west andaman fault waf the induction pattern at great nicobar station campbell bay may be related to the highly conducting sediments filling the mergui basin along with mafic intrusions also crustal transition occurs below the mergui terrace at the malayan coast contributing to the enhanced conductivity anomaly

structural lineaments from the magnetic anomaly maps of the eastern continental margin of india ecmi and nw bengal fan abstract three major structural lineaments are inferred from the magnetic and bathymetry data collected over the eastern continental margin of india ecmi and nw bengal fan covering the area between 10 55 n to 20 15 n and 80 e to 88 e analysis of magnetic data associated with these trends suggests that 1 trend i located at the foot of the continental slope around 3000 m water depth represents the ocean continent boundary ocb of ecmi 2 trend ii represents the northern extension of 85 e ridge abutting the continental shelf off chilka lake and 3 trend iii located over the continental shelf slope between visakhapatnam and paradip represents a folded ridges and depressions nature of the continental basement the study also reveals the occurrence of dyke intrusions within the continental crust landward of ocb which might be related to the rift phase volcanism along the ecmi

acoustic attenuation in oceanic gabbro summary compressional wave velocity and attenuation were measured in oceanic layer 3 from nearly 0 5 km of recovered core at ocean drilling program site 735 in the southern indian ocean high frequency 400 khz acoustic transmission experiments were conducted on 117 water saturated minicore samples under room conditions of temperature and pressure on board the drillship joldes resolution although experimental errors were large in some deformed samples the average of our measurements of attenuation qp on these samples is 0 049 f 0 032 agreement between spectral ratio and inversion estimates of qp indicates that acoustic noise in the data does not significantly reduce the measurement accuracy ultrasonic tests on a 1 per cent porosity sample at elevated effective pressures from 10 to 500mpa showed an increase in v from 6 8 to 7 0kms and a decrease in qp from 0 026 to 0 023 the small pressure dependence of qp in this low porosity gabbro suggests that it is possible to use measurements at room conditions for relative interpretation as a function of depth the measurements of qp show 1 an overall decrease as a function of depth 2 a decrease in the average variance in the measurement with depth and 3 an inverse relationship between average v and average qp although qp shows greater variation in these rocks than v our explanation for these results is that rock fabric has a significant effect on v and qp a simple deduction from these results is that seismic data will be strongly influenced by the degree of alteration and tectonism in similar crustal rocks particularly in fracture zones seismic reflections from lithologic contacts may be attenuated and deep crustal reflectors identified as faults or as the mohorovic discontinuity may result from v and qp contrasts caused by alteration and tectonism within layer 3 not from the lithologic contact itself

deep structure and tectonics of the burmese arc constraints from earthquake and gravity data abstract active subduction of the indian plate is currently occurring beneath the burmese arc along an east dipping benioff zone which extends to a depth of about 180 km the overriding burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the benioff zone a crustal seismic zone some 60 80 km east of the benioff zone correlates to backarc activity a triangular aseismic wedge in the top part of the crust outlines the central belt molasse basin east of the burmese foldbdt fault plane solutions show that the burmese benioff zone is characterized by shallow angle thrusting at its upper edge whereas down tip tensional events dominate its lower edge most of the backarc seismicity is accounted for by the sagaing transform or by the activity of the shan scarp normal fault zone at the margin of the asian plate a gravity anomaly pair with amplitude of 175 mgal coincides with the 1100 km long burmese arc lying in a north south direction the gravity anomalies along a profile in central burma and in adjacent areas of the bengal basin are interpreted in terms of plate subduction as well as near surface mass anomalies this suggests that sediments below the central belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone the oceanic crust underlying deeper parts of the bengal basin experiences phase transition at about 30 km depth in a benioff zone environment east of the burmese foldbelt several thrust planes are present within the folded and deformed cretaceous tertiary sediments of the fold belt these are often associated with ophiolites and basic to ultrabasic rocks a low density zone at least 60 km wide underlies the andesitic volcanic axis in the overriding plate

crustal structure of the sunda forearc region west of central sumatra from gravity data free air gravity anomalies near nias island include a 100 mgal low centered 10 20 km landward of the trench axis and a 80 mgal high over the outer arc ridge but also show a few large anomalies unrelated to topography a 90 mgal low is found on the structural high between the banyak islands and sumatra north of nias an 80 mgal rise at a gradient of 8 mgal km is modeled by a near surface body of high density material oceanic crust 4 km wide and tens of kilometers long this slab may be exposed on the southwest coast of nias where ultramafic bodies have been mapped and its northerly trend compared with northwesterly bathymetric trends may be related to other obliquely trending structures near here to a proposed strike slip fault which crosses nias and or to the basement high topped by the batu islands a 30 mgal free air low over the forearc basin can be modeled best if pre miocene melange or continental crust underlies the basin and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust