western pacific interannual variability associated with the el nino southern oscillation observations of sea surface temperature sst sea level pressure slp surface wind and outgoing longwave radiation olr show that the el nino southern oscillation enso displays western pacific anomaly patterns in addition to eastern pacific anomaly patterns during the warm phase of enso warm sst and low slp anomalies in the equatorial eastern pacific and low olr anomalies in the equatorial central pacific are accompanied by cold sst and high slp anomalies in the off equatorial western pacific and high olr anomalies in the off equatorial far western pacific also while the zonal wind anomalies over the equatorial central pacific are westerly those over the equatorial far western pacific are easterly the nearly out of phase behavior between the eastern and western tropical pacific is also observed during the cold phase of enso but with anomalies of opposite sign these western pacific interannual anomaly patterns are robust features of enso independent of data sets it is argued that equatorial easterly westerly wind anomalies over the far western pacific during the warm cold phase of enso are initiated by off equatorial western pacific cold warm sst anomalies and that these winds are important for the evolution of enso an atmosphere model is employed to demonstrate that small off equatorial western pacific cold warm sst anomalies compared to those in the east are sufficient to produce equatorial easterly westerly wind anomalies as observed over the far western pacific the coupled ocean atmosphere model of zebiak and cane is then modified to investigate the evolution of the western pacific interannual anomaly patterns in a coupled ocean atmosphere system by including a meridional structure to the subsurface temperature parameterization in the western pacific the modified model produces both western and eastern pacific interannual anomaly patterns

structure and isostatic compensation of the comorin ridge north central indian ocean summary bathymetry gravity and magnetic data about 9200 lkm of the comorin ridge north central indian ocean were investigated using the transfer function and forward model techniques to understand the mode of isostatic compensation and origin of the ridge the ridge extends for about 500 km in nnw sse direction and associates with low amplitude gravity anomalies ranging from 25 to 30 mgal compared to the ridge relief suggesting that the anomalies are compensated at deeper depths from admittance analysis an airy model or local compensation with an elastic plate thickness t e of about 3 km and crust thickness t of 15 20 km are suggested for the southern part of the comorin ridge south of 5 n whereas for the northern part a flexural plate model with an elastic thickness of about 15 km is obtained admittance analysis together with the results from gravity forward modelling reveal that the south part was emplaced on relatively weak oceanic crust with both surface and subsurface loading while the north part was emplaced on the continental crust based on present studies and published plate kinematic models we interpret that the comorin ridge was evolved at about 90 ma during the rift stage of madagascar from the southwest of india we have also demarcated the continent ocean boundary cob west of sri lanka and southern tip of india which runs across the strike of the ridge placing the northern part of the ridge on continent and southern part on oceanic crust on the southern part of the ridge eastern flank is steep faulted up to 0 6 km and is controlled by the 79 e fz and then by cob

crustal structure and tectonics of the ninetyeast ridge from seismic and gravity studies seismic reflection and refraction gravity and bathymetric data across and along the central part of the ninetyeast ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics the ridge in the study area rises to 3 5 km above the seafloor of adjacent ocean basins basement is covered by sediments up to 0 8 km thick but exposed at some locations seismic refraction and gravity models of the ridge show that the crustal thickness is 22 km greater than the average thickness 7 km of oceanic crust estimated in the central indian and wharton basins the ridge is compensated by flexure of layers 2a 2b and 3a to the extent of 2 km and by a 12 km thick deep crustal body layer 3b of underplated material upper crustal velocities layer 2a beneath the ridge are lower up to 3 4 km s 1 than in adjacent basins because of weathering process and hydrothermal circulation of the waters in the tectonized blocks while velocities in deep crustal body layer 3b are higher up to 7 8 km s 1 and seem to be resulted from magmatic rocks accreted within the lithosphere steep down ward faulting of 2 km along the eastern flank of the ninetyeast ridge possibly originated because of compressional and extensional stresses applied along the 89 e fracture zone fz the stresses are interpreted as resulting from multiple ridge jumps of wharton spreading centers during the late cretaceous and middle eocene the velocity depth model suggests that the rocks of layers 2a and 2b and layer 3a are exposed on the eastern flank of the ninetyeast ridge and 90 e fz respectively they are consistent with basalt dolerite and gabbro rocks collected from the fault scarps the central part of the ridge between 1 n and 11 s has uniformly deepened by 0 7 km and is bounded by major sublatitudinal deep seated fractures we interpret that the deepened structural block and associated fractures resulted from the activity of the intense deformation of the central indian ocean

el nino southern oscillation related ocean atmosphere coupling in the western equatorial pacific using 43 years of comprehensive ocean atmosphere data set and related data for the period 1950 1992 an examination is made into the regional dependence of ocean atmosphere coupling in relation to the el nino southern oscillation enso the cross correlation between sea surface temperature sst and sea level pressure slp anomalies over the global tropics shows two patterns of significant negative correlation consistent with a local hydrostatic response of slp to sst 1 the eastern pacific where the correlation is symmetric about and largest on the equator and 2 the western pacific where symmetric regions of negative correlation are found off the equator separated by a region of positive correlation on the equator anomalies within these two patterns vary out of phase with each other while the slp anomalies on both sides of the basin are of similar magnitude the sst anomalies in the east are much larger than those in the west despite this disparity in the sst anomaly magnitudes between the eastern and western pacific we argue that the ocean atmosphere couplings in the western and west central pacific are important for enso the off equator sst anomalies in the west enhance the slp anomalies there and they appear to initiate easterly wind anomalies over the far western pacific during the peak el nino phase of enso as these easterlies evolve their effect upon the ocean tends to oppose that of the westerly wind anomalies found over the west central pacific these competing effects suggest a mechanism that may contribute to coupled ocean atmosphere system oscillations the west central equatorial pacific the region separating the eastern and western patterns while exhibiting large momentum and heat flux exchanges shows minimum correlation between sst and slp thus neither the sst and slp anomaly magnitudes nor the correlation between them is alone indicative of ocean atmosphere coupling and the regional dependence for such coupling in relation to enso appears to be more complicated than mechanistic interpretations of enso would suggest

abandoned paleocene spreading center in the northeastern indian ocean evidence from magnetic and seismic reflection data abstract magnetic data from the northeastern indian ocean reveal east west trending magnetic lineations 28 through 34 an abandoned spreading center asc and part of the cretaceous magnetic quiet zone cmqz which are used to reconstruct past tectonic processes involved in the evolution of the ocean offsets in the magnetic lineations outline the existence of four n s trending fracture zones 80 e indira 84 5 e and 86 e the 84 5 e fracture zone fz acts as a boundary separating oceanic crust with different magnetic isochron patterns the pairs of magnetic lineations 30 through 32n 2 between the 86 e fz and the ninetyeast ridge reveal an asc of about 65 ma age parallel to 0 5 s latitude the structure of the asc is well identified in seismic reflection data as an undulating basement topographic rise covered by up to 2 km of bengal fan sediments the spreading center might have initiated its activity along with other spreading centers of the wharton ridge and india antarctica ridge after the first major plate reorganization of the indian ocean about 95 5 ma but ceased shortly after formation of anomaly 30 about 65 ma then it jumped southward between anomalies 32n 2 and 33 the jump captured the oceanic crust of anomalies 30 through 32n 2 that initially formed on the antarctica plate but were later transferred to the indian plate the spreading centers in the vicinity of the ninetyeast ridge jumped several times southward to maintain a close proximity to the kerguelen hotspot

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

structure of large scale convective anomalies over tropical oceans abstract geographical variations in the variance and cross correlation of monthly mean sea surface temperature sst outgoing longwave radiation olr a proxy for deep convection and vertical motion and convergence of winds at the surface and at 850 mb across the tropical indian and pacific oceans are examined within about 10 of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle largest variances in the sst and surface convergence data occur across the eastern near equatorial pacific whereas olr and 850 mb convergence variances are largest across the western pacific olr anomalies are significantly correlated with collocated sst and surface convergence anomalies from the date line eastward to the south american coast but are uncorrelated west of the date line the olr and 850 mb convergence anomalies are significantly correlated from about 120 w westward but are uncorrelated

atmospheric convergence feedback in a simple model for el nino abstract a parameterization is developed for the feedback between dynamics and heating associated with moisture convergence in the tropical atmospheric boundary layer the feedback improves the ability of a simple model to simulate observed anomalies of the tropical atmosphere during el nino events in particular two features of the observations are reproduced by including the feedback process the smaller scale of atmospheric anomalies as compared to sst anomalies and the focusing of the anomalies in the vicinity of the mean convergence zones the principal remaining shortcomings of the model are discussed

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

pressure change and surface expansion in salt marshes due to tidal inundation infiltration of water across the surface water interface of a marsh in areas flooded due to tides has been known to cause swelling of sediments the sudden tidal inundation in an unflooded soil creates a head gradient at the surface of the unsaturated soil above the water table when water infiltrates into the soil under this gradient the suction head in the unsaturated domain decreases and the pore pressure in the saturated zone increases with time resulting in swelling of highly compressible marsh soils a one dimensional mathematical model which consists of a mass conservation equation quasi static equilibrium equations stress strain relations for an assumed perfectly elastic solid matrix and a variable total stress expression has been employed to obtain a numerical solution for pore pressure change and swelling due to tidal inundation numerical results are presented graphically at the spatial and temporal points of interest for a constant depth of ponding of water at the marsh surface resulting from tidal inundation results show that swelling of soil is primarily governed by the initial head gradient across the marsh surface water interface

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

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

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

internal structure of the 85 e ridge bay of bengal evidence for multiphase volcanism abstract the 85 e ridge located in the bay of bengal of the northeastern indian ocean is an enigmatic geological feature as it possesses unusual geophysical signatures the ridge s internal structure and mode of eruptions are unknown due to lack of deep seismic reflection and borehole data control here we analyze 10 km of long streamer seismic reflection data to unravel the ridge s internal structure and thereby to enhance the understanding of how the ridge was originated and grew over a geologic time seismic facies analysis reveals the ridge structure consisting of volcanic vent and several stratigraphic units including packs of prograding clinoforms the clinoform sequences are interpreted as volcanic successions and led to the formation of lava delta fronts interpreted features of lava fed deltas and intervening erosional surfaces and mass flows along ridge flanks suggest that the 85 e ridge is a volcanic construct and was built by both subaqueous and multiphase sub marine volcanism during the late cretaceous approximately from 85 to 80 ma at later time from oligocene miocene 23 ma onwards the ridge was buried under the thick sediments of the bengal fan system

diagnostic studies of pacific surface winds abstract analyses are performed using surface winds derived from the monthly mean fsu pacific pseudo stress fields vorticity budget calculations reveal the relative contributions of various terms and allow simplification of the full momentum equations based on the simplified equations a procedure is developed for estimating surface pressure and then adjusting the winds the wind field adjustments fall well within the uncertainty of the data while allowing generally large imbalances in the vorticity budget to be removed completely further validation of the procedure is provided by comparisons between estimated and observed pressure anomalies the adjusted winds and pressure are then used to infer boundary layer and upper level forcing within the context of particular model formulations these forcing fields an compared with sea surface temperature outgoing longwave radiation and highly reflective cloud data in order to infer the dominant mechanisms contributing to observed surface circulation anom

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

tectonics of the ninetyeast ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge 1 analysis of new and existing geophysical data for the central indian and wharton basins of the indian ocean were used to understand the formation and evolution of the ninetyeast ridge ner especially its relationship to the kerguelen hot spot and the wharton spreading ridge satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking n5 e one of these at 89 e crosses the n10 e trending ner impacting the ner morphology from 77 to 43 ma the ner lengthened at a rate of 118 km myr twice that of the 48 58 km myr accretion rate of adjacent oceanic crust this difference can be explained by southward jumps of the wharton spreading ridge toward the hot spot which transferred portions of crust from the antarctic plate to the indian plate lengthening the ner magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the ner especially two leaving observable 65 and 42 ma fossil spreading ridges in contrast complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the ner comparison of the ner dates and magnetic anomaly ages implies that the hot spot first emplaced ner volcanoes on the indian plate at a distance from the wharton ridge but as the northward drifting spreading ridge approached the hot spot the two interacted keeping later ner volcanism near the spreading ridge crest by spreading center jumps

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

chemical dilation and the dual porosity of humified bog peat abstract hydrochloric acid and calcium and sodium chloride salt solutions were passed through humified bog peat cores of 1 m length to determine values for the material properties governing solute transport hydraulic conductivity effective porosity and dispersivity chloride passed through the bulk peat cores about twice as fast as predicted from chloride breakthrough curves measured by selective ion probes inserted into the peat 30 cm downgradient from the source with continued experimentation chloride dispersion increased chloride also passed through the peat up to 12 times slower than predicted by conservative advection the retardation of the chloride fronts the faster bulk transport and the asymmetry of the breakthrough curves all indicate that humified bog peat is a dual porosity medium solutes both move preferentially through active macropore spaces and diffuse into dead pore space in the peat matrix neither the effective porosity nor the dispersivity of the peat could be precisely determined from the breakthrough curves because the hydraulic conductivity and discharge rates through the peat increased as much as five times when the chloride tracer solutions passed through the cores in initial experiments the shapes of the breakthrough curves were generally consistent with dispersivities of the order of centimeters similar to that observed for granular soils at the same length scale meters whereas they flattened considerably during later experiments suggesting dispersivities in the range of tens of centimeters we hypothesize that the interaction of the chloride tracer solutions with organic acid functional groups may cause macropores to dilate and micropores to close off

tectonic model for the evolution of oceanic crust in the northeastern indian ocean from the late cretaceous to the early tertiary bathymetry and magnetic studies part of the trans indian ocean geotraverse investigations in the northeastern indian ocean revealed seafloor topographic features magnetic lineations 19 through 32b and abandoned spreading centers the seafloor topography of the ninetyeast ridge is relatively wider and shallower south of 15 s the magnetic anomalies indicate nine fracture zones two of them are newly identified some of the fracture zones are reflected in the bathymetry abandoned spreading centers between 86 e fracture zone fz and 92 e fz are interpreted as the western extensions of the wharton ridge they ceased spreading along with other spreading centers in the wharton basin soon after the formation of magnetic anomaly 19 around 42 ma and merged the indian and australian plates as single indo australian plate the pattern of magnetic lineations between 86 e fz and 90 e fz indicate a series of southerly ridge jumps at anomalies 30 26 royer et al 1991 and other workers and 19 these ridge jumps transferred portions of the antarctic plate to the indian plate the captured portions and offset along 86 e fz between india antartica ridge and wharton ridge resulted in an anomalous extra oceanic crust between 86 e fz and ninetyeast ridge spanning 11 in latitude

the 1997 98 el nino evolution relative to previous el nino events abstract the evolution of the 1997 98 el nino is described using ncep sst and olr data ncep ncar reanalysis sea level pressure slp fields and the florida state university surface wind data from november 1996 to january 1997 the eastern pacific is characterized by equatorial cold sst and high slp anomalies while the western pacific is marked by off equatorial warm sst anomalies and off equatorial anomalous cyclones corresponding to this distribution are high olr anomalies in the equatorial central pacific and low olr anomalies in the off equatorial far western pacific the off equatorial anomalous cyclones in the western pacific are associated with a switch in the equatorial wind anomalies over the western pacific from easterly to westerly these equatorial westerly anomalies then appear to initiate early sst warmings around the date line in january february 1997 and around the far eastern pacific in march 1997 subsequently both the westerly wind and warm sst anomalies along with the low olr ano

mid ocean ridge jumps associated with hotspot magmatism abstract hotspot ridge interaction produces a wide range of phenomena including excess crustal thickness geochemical anomalies off axis volcanic ridges and ridge relocations or jumps ridges are recorded to have jumped toward many hotspots including iceland discovery galapagos kerguelen and tristan de cuhna the causes of ridge jumps likely involve a number of interacting processes related to hotspots one such process is reheating of the lithosphere as magma penetrates it to feed near axis volcanism we study this effect by using the hybrid finite element code flac to simulate two dimensional 2 d cross section viscous mantle flow elasto plastic deformation of the lithosphere and heat transport in a ridge setting near an off axis hotspot heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width to determine the conditions necessary to initiate a ridge jump we vary four parameters hotspot magmatic heating rate spreading rate seafloor age at the location of the hotspot and ridge migration rate our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non linearly with increasing spreading rate and seafloor age models predict that magmatic heating itself is most likely to cause jumps at slow spreading rates such as at the mid atlantic ridge on iceland in contrast despite the higher magma flux at the galapagos hotspot magmatic heating alone is probably insufficient to induce a ridge jump at the present day due to the intermediate ridge spreading rate of the galapagos spreading center the time required to achieve a ridge jump for fixed or migrating ridges is found to be on the order of 105 106 years simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back arc settings and ridge segment propagation along the mid atlantic ridge

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

nature of the crust in the laxmi basin 14 20 n western continental margin of india 1 the nature of the crust in the laxmi basin western margin of india is an uncertain issue more importantly this has implications on paleogeographic reconstructions of the western indian ocean we have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust basement of the laxmi basin includes numerous highs which make the basement uneven and shallower compared to the western basin the laxmi basin is characterized by a broad gravity high and a narrower prominent gravity low within it while within the basin the broad anomaly gradually increases toward north the panikkar ridge is associated with the gravity low which is comparable at least in sign to known negative gravity anomaly of the laxmi ridge intrusive structures mapped in the laxmi basin coincide with significant magnetic anomalies which were earlier interpreted as seafloor spreading anomalies model studies reveal that the laxmi basin consists of 14 km thick stretched continental crust in which magmatic bodies have been emplaced whereas the panikkar ridge remains less altered stretched continental crust the crust of the laxmi basin is mostly thinner than crust under the laxmi ridge and continental margin in addition to the rift drift related stretching of the continental margin the laxmi basin possibly has undergone extra stretching in e w direction during the pre tertiary period at 68 ma deccan volcanism on western india may have disrupted the initial conditions that were leading to onset of spreading in the basin subsequently the reunion hot spot had emplaced the volcanic material within the stretched thinned continental crust we interpret the laxmi basin as a failed rift undergone stretching following intraplate kinematics prior to deccan volcanism

periodic deformation of oceanic crust in the central indian ocean new seismic reflection profiles of 5370 km running through the ocean drilling program leg 116 sites and deep sea drilling project sites 215 and 218 were obtained to investigate the spatial extent timing and nature of the tertiary deformation of the equatorial central indian ocean analyses of the data revealed basement and sedimentary structures and structural unconformities that resulted from the release of compressional forces long wavelength 150 300 km anticlinal basement structures with 1 2 km relief and tight folding and high angle faulting 5 20 km long of oceanic basement and overlying sediments exist between the afanasy nikitin seamount and the ninety east ridge the deformation zone extends from 10 s to the north up to 7 n latitude the changes in the regional basement trend along 87 e longitude between latitudes 11 s and 15 n coincide with the significant tectonic events of the first major plate reorganization occurred in the eastern indian ocean at 90 5 ma and changes in the indian plate motion at 65 5 ma the basement deformation at selected places a widespread unconformity of the upper miocene and subsequent younger unconformities lower pliocene and upper pleistocene indicate that the deformation activity might have begun earlier than the generally believed age of 7 5 ma and appears to be periodic the upper miocene and upper pleistocene deformational unconformities are in general observed south of 1 s while the basement deformation and lower pliocene deformational unconformity are mostly present in the area north of 1 s it is surmised that the compressional stresses built up since the hard collision of india with eurasia may have released for a short period prior to the early miocene time and deformed the oceanic basement in the northeastern indian ocean thereafter the stress regime seems to have transferred to the north of the indian shield and caused deformation of the continental lithosphere later the activity reoccurred during the late miocene 7 5 ma early pliocene 4 ma and late pleistocene 0 8 ma with a cyclicity of 3 5 m y and deformed the oceanic crust and sedimentary strata in the central indian ocean

spacing of faults at the scale of the lithosphere and localization instability 2 application to the central indian basin 1 tectonic deformation in the central indian basin cib is organized at two spatial scales long wavelength 200 km undulations of the basement and regularly spaced faults the fault spacing of order 7 11 km is too short to be explained by lithospheric buckling we show that the localization instability derived by montesi and zuber 2003 provides an explanation for the fault spacing in the cib localization describes how deformation focuses on narrow zones analogous to faults the localization instability predicts that localized shear zones form a regular pattern with a characteristic spacing as they develop the theoretical fault spacing is proportional to the depth to which localization occurs it also depends on the strength profile and on the effective stress exponent ne which is a measure of localization efficiency in the brittle crust and upper mantle the fault spacing in the cib can be matched by ne 300 if the faults reach the depth of the brittle ductile transition bdt around 40 km or ne 100 if the faults do not penetrate below 10 km these values of ne are compatible with laboratory data on frictional velocity weakening many faults in the cib were formed during seafloor spreading the preexisting faults near target locations separated by the wavelength of the localization instability were preferentially reactivated during the current episode of compressive tectonics the long wavelength undulations may result from the interaction between buckling and localization index terms 8010 structural geology fractures and faults 8020 structural geology mechanics 8159 tectonophysics evolution of the earth rheology crust and lithosphere keywords faults central indian basin fault spacing folds strength envelope diffuse plate boundary

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

repeat ridge jumps associated with plume ridge interaction melt transport and ridge migration 1 repeated shifts or jumps of mid ocean ridge segments toward nearby hot spots can produce large long term changes to the geometry and location of the tectonic plate boundaries ridge jumps associated with hot spot ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off axis volcanism to study how these processes influence ridge jumps we use numerical models to simulate 2 d in cross section viscous flow of the mantle viscoplastic deformation of the lithosphere and melt migration upward from the asthenospheric melting zone laterally along the base of the lithosphere and vertically through the lithosphere the locations and rates that magma penetrates and heats the lithosphere are controlled by the time varying accumulation of melt beneath the plate and the depth averaged lithospheric porosity we examine the effect of four key parameters magmatic heating rate of the lithosphere plate spreading rate age of the seafloor overlying the plume and the plume ridge migration rate results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate the time required to complete a ridge jump decreases with larger values of magmatic heating rate younger plate age and faster spreading rate for cases with migrating ridges models predict a range of behaviors including repeating ridge jumps much like those exhibited on earth repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere the tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the mid atlantic ridge model results also suggest that magmatic heating may significantly thin the lithosphere as has been suggested at hawaii and other hot spots

a unified oscillator model for the el nino southern oscillation the delayed oscillator the western pacific oscillator the recharge discharge oscillator and the advective reflective oscillator have been proposed to interpret the oscillatory nature of the el nino southern oscillation enso all of these oscillator models assume a positive ocean atmosphere feedback in the equatorial eastern and central pacific the delayed oscillator assumes that the western pacific is an inactive region and wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate the western pacific oscillator emphasizes an active role of the western pacific in enso the recharge discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate the advective reflective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries motivated by the existence of these different oscillator models a unified oscillator model is formulated and derived from the dynamics and thermodynamics of the coupled ocean atmosphere system consistent with enso anomaly patterns observed in the tropical pacific this oscillator model considers sea surface temperature anomalies in the equatorial eastern pacific zonal wind stress anomalies in both the equatorial central pacific and the equatorial western pacific and thermocline depth anomalies in the off equatorial western pacific if the western pacific wind forced response is neglected thermocline and zonal wind stress anomalies in the western pacific are decoupled from the coupled system and the unified oscillator reduces to the delayed oscillator if wave reflection at the western boundary is neglected the unified oscillator reduces to the western pacific oscillator the mathematical form of the recharge discharge oscillator can also be derived from this unified oscillator most of the physics of the advective reflective oscillator are implicitly included in the unified oscillator and the negative feedback of wave reflection at the eastern boundary is added to the unified oscillator with appropriate model parameters chosen to be consistent with those of previous oscillator models the unified oscillator model oscillates on interannual timescales

behavior of deep convective clouds in the tropical pacific deduced from isccp radiances abstract we analyze the properties of deep convection over the equatorial pacific and its relationship to sea surface temperature sst and surface wind divergence using isccp radiance data for july 1983 and january 1984 deep convective clouds dcc are diagnosed with both a combined visible infrared threshold method and an infrared only threshold method the visible channel is important in diagnosing deep convection in different regions with different surface and atmospheric properties because of the existence of large scale variations of dcc top altitudes the itcz and spcz exhibit changes in these two months that are characteristic of both the el nino and seasonal cycles deep convection is latitudinally confined to a much smaller spatial scale than that suggested by maps of outgoing longwave radiation diurnal variations of dcc cover and associated mesoscale cirrus anvil cloud cac cover are out of phase with deep convection peaking in the early morning throughout the equatorial pacific the diurna

a western pacific oscillator paradigm for the el nino southern oscillation a data based hypothesis is presented on the mechanism of the el nino southern oscillation enso a major determinant of interannual global climate variability the hypothesis emphasizes the importance of off equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western pacific these winds compete with westerly winds over the equatorial central pacific enabling the coupled ocean atmosphere system to oscillate consistent with this hypothesis an analogical oscillator model is constructed that produces enso like oscillations the proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean atmosphere system to oscillate

early cretaceous fracture zones in the bay of bengal and their tectonic implications constraints from multi channel seismic reflection and potential field data abstract due to the lack of clearly discernible magnetic anomaly identifications and fracture zones in the bay of bengal bob the early cretaceous plate reconstruction history of eastern gondwanaland remains an unresolved issue with regard to the separation of the east coast of india from the conjugate enderby basin of antarctica based on the correlation of oceanic basement information obtained from a vast amount of deep reflection seismic data we identified 16 nw se trending fracture zones fz1 fz16 in the bob region the trends of these fracture zones at the ecmi indicate two distinctly different sets one set in the direction n25 w to n35 w in the southern part fz1 fz7 and the other set in n50 w fz8 fz15 direction along the northern part these two sets of fracture zones further support the earlier inferences about the segmented nature of the ecmi into southern sheared oblique rifted margin and the northern normal rifted margin while the southern part of the ecmi had formed under slow asymmetric spreading and mixed transform setting with its conjugate western enderby basin the northern part of the ecmi formed after the breakup of the elan bank from india at m2 considering this scenario we infer the presence of late mesozoic younger to m4 anomalies along the ecmi and the subdued nature of magnetic anomalies in the western bob can be explained by the obliquity of fracture zones with respect to the coast as observed in many marginal smooth zones of the world oceans the disposition of the 85 e ridge with respect to the pre evolved bob crust and early cretaceous volcanic episodes in the region together indicate an emplacement of the ridge from 105 ma onwards as a result of a hotspot source related to the kerguelen super plume during its intense eruption activity

zonal momentum balance in the tropical atmospheric circulation during the global monsoon mature months abstractin this paper zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months january and july are analyzed in three dimensions based on the ecmwf interim re analysis era interim it is found that the dominant terms in the balance of the atmospheric boundary layer abl in both months are the pressure gradient force the coriolis force and friction the nonlinear advection term plays a significant role only in the asian summer monsoon regions within the abl in the upper troposphere the pressure gradient force the coriolis force and the nonlinear advection are the dominant terms the transient eddy force and the residual force which can be explained as convective momentum transfer over open oceans are secondary yet cannot be neglected near the equator zonal mean equatorial upper troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross equatorial hadley circulation equatorial upper troposphere ea

a mathematical model for regional land subsidence due to pumping 3 integrated equations for a phreatic aquifer a mathematical model is developed for the areal distribution of drawdown land subsidence and horizontal displacements due to groundwater pumping from a deformable phreatic aquifer this paper is a continuation of earlier works published by the authors bear and corapcioglu 1981a b on the subject of regional land subsidence due to pumping in confined and leaky aquifers all the basic assumptions underlying these previous works are also valid here however in this case the same methodology is extended to the case of a phreatic aquifer the main additional feature in the present paper is that due to the decline of the water table the total stress at points within the flow domain changes continuously following the development of a three dimensional mathematical model consisting of a mass conservation equation quasi static equilibrium equations and stress strain relations for an assumed perfectly elastic solid matrix a regional horizontal two dimensional model is derived by averaging the three dimensional model over the vertical thickness of the aquifer taking into account the continuous variation in total stress as a result of water table fluctuations the separation of an initially coinciding phreatic surface which serves as the upper boundary of the aquifer and a surface consisting of a set of solid particles during the lowering of the water table and the compaction of the aquifer has been incorporated into the averaging procedure an analytical solution is given for the case of a single pumping well the solution which is expressed in terms of the classical well function for a confined aquifer but with a modified storativity provides estimates of drawdown vertical subsidence and horizontal displacement the displacements for a sample problem are compared with those of a confined aquifer under similar conditions the results indicate that under the conditions of the studied case of radial flow vertical and horizontal displacements due to pumping from a phreatic aquifer with the same discharge rate would be less than those of a confined one furthermore the same averaging procedure is extended for a system of phreatic and leaky confined aquifers and for the unsaturated zone above the water table an additional change in total stress is obtained in terms of moisture content of the zone above the phreatic surface

influence of rift jump and excess loading on the structural evolution of northern iceland 1 new structural data combined with published structural and geochronological data allow reconstruction of the structural evolution that followed the last rift jump across northern iceland tertiary lava flows erupted along the skagafjordur paleo rift have been down bent under the weight of and in the direction of plio pleistocene lava flows emitted from the northern volcanic zone and the central part of iceland this down bending process involved development of local flexure zones and a flexural extension along the resulting monoclines this structural reorganization explains the existence of the hunafloi skagi synform without need for a paleo rift axis along it in agreement with previous radiometric dating the large amount of plio pleistocene lava flows erupted in central iceland may have been enhanced by ice cap loading

modeling tropical pacific sea surface temperature 1970 87 abstract an attempt is made to model the sea surface temperature sst of the tropical pacific ocean between january 1970 and august 1987 the sst is computed using the model and heat flux parameterization discribed in the climatological study of seager et al the model is forced with the observed winds as given by the florida state university analysis the results indicate that while short period variability in the model and observations is largely uncorrelated variability with periods greater than about one year is well represented in the model each el nino that occurred in this time period is captured by the model the main discrepancies in the evolution of the 1982 83 and 1986 87 model el nino events are the inability of the model equatorial sst anomaly to cool in early 1983 and the disappearance of the 1986 87 anomaly during 1987 the results suggest a number of conclusions in the east pacific equatorial kelvin waves excited by variations in the trade wind strength in the central and west pacific

spatial variations in isostatic compensation mechanisms of the ninetyeast ridge and their tectonic significance the ninetyeast ridge ner one of the longest linear volcanic features on the earth offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid ocean ridge bathymetry geoid and gravity ship borne and satellite data along 72 closely spaced transects and 17 overlapping grids on the ner are analyzed and modeled to determine the effective elastic thickness te beneath the entire ridge the results of 2 d and 3 d flexural modeling of the ner show large spatial variations in te values ranging from 4 to 35 km suggesting that the ridge was compensated along its length by different isostatic mechanisms the southern south of 22 s latitude and northern north of 2 n latitude parts of the ner have te values of 10 and 23 km respectively revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the indian plate whereas the northern part was emplaced in an intraplate setting in contrast in the central part of the ner between latitudes 22 s and 2 n highly variable te values 4 22 km are estimated the scattered te values in the central ner suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of kerguelen hot spot with rapid northward migration of the wharton spreading ridge residual mantle bouguer anomaly rmba map of the ner and adjacent basins reveals that the entire length of the ner is associated with a significant negative anomaly up to 200 mgal indicating the presence of thickened crust or less dense mantle beneath the ridge 3 d crustal thickness map of the ner generated by inversion of the rmba data shows a thick crust ranging from 15 to 19 km the present study clearly shows that ner possesses a highly segmented isostatic pattern with the occurrence of sub crustal underplating or sub surface loading

the influence of cross equatorial pressure gradients on the location of near equatorial convection tomas and webster note that in regions where there is a substantial cross equator surface pressure gradient there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus the flow in this region meets the parcel criterion for inertial instability they hypothesize that the atmospheric response to this absolute vorticity distribution is a convergence divergence doublet in the boundary layer the convergence centre results in strong convection and thus is very important in determining the strength and location of convection i e the intertropical convergence zones whereas the divergence centre results in suppressed convection in this work a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further an analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability it does not satisfy the linear stability criteria because of the stabilizing influence of dissipation and the finite vertical scale much greater shear than that observed is required for linear instability it is noted however that when the parcel criterion is met one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow several experiments are performed integrating the full nonlinear model relaxing back to a pressure distribution having a cross equatorial gradient to test whether the observed vorticity wind and convergence distributions can be well simulated it is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator this absolute vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence divergence doublet similar to that observed by tomas and webster this response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity the response of the meridional wind to the strength of the pressure gradient is quasi linear for small and somewhat larger values of forcing but between these values there is a range where the response is highly nonlinear i e large increases in the strength of the meridional wind result from small changes in the pressure gradient it is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations the nonlinear model is also forced using pressure distributions taken from observations the resulting steady state simulated vorticity wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east pacific and east atlantic during july when forced using the pressure distribution from the indian ocean region during july the model produces an unrealistically strong meridional wind response some improvement is obtained by including the latitudinally dependent zonal pressure gradient force which is moderately strong in this region finally the model is forced using a pressure distribution taken from observations in the central pacific ocean during july in this case the cross equator pressure gradient is near zero

observations of ridge hotspot interactions in the southern ocean the evolution of ridge hotspot systems is not well understood in this investigation satellite derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge hotspot interaction at four sparsely explored systems in the southern ocean these systems illustrate three different stages of ridge hotspot interaction in which a migrating spreading center approaches a hotspot pacific antarctic louisville passes over or is captured by the hotspot mid atlantic shona discovery and ultimately migrates away from the hotspot southeast indian kerguelen all of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading interestingly these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge hotspot systems a simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump the presence of previously uncharted obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered the azimuths of the aseismic ridges cannot be explained by plate kinematics alone they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure the features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets

slow variability in the equatorial west central pacific in relation to enso abstract six years of upper ocean velocity temperature and surface wind data collected in the west central pacific at 0 170 w reveal a slow ocean dynamical mode associated with the el nino southern oscillation enso latent and sensible heat flux calculations using the basin wide tropical atmosphere ocean tao array data show a coincident slow ocean atmosphere thermodynamical mode beginning with the la nina conditions in 1988 through the peak el nino conditions in 1992 the equatorial undercurrent euc speed decreased along with the surface zonal wind stress and the zonal pressure gradient simultaneous with these were increasing trends in the richardson number above the euc core and in sea surface temperature sst after peak warming was achieved the variations in all of these quantities reversed in a movement toward their previous la nina conditions as this evolved within the ocean the sensible and latent heat fluxes increased with large values emanating eastward from the western pacific the

extended wind stress analyses for enso abstract surface wind stresses are fundamental to understanding el nino yet most observational stress products are too short to permit multidecadal enso studies two exceptions are the florida state university subjective analysis fsu1 and the ncep ncar reanalysis ncep1 which are widely used in climate research here the focus is on the aspects of the stress most relevant to enso namely the climatological background anomaly spectrum response to sst changes subannual noise forcing and seasonal phase locking and how these differ between fsu1 and ncep1 over the tropical pacific for 1961 99 the ncep1 stress climatology is distinguished from fsu1 by weaker equatorial easterlies stronger off equatorial cyclonic curl stronger southerlies along the peruvian coast and weaker convergence zones with weaker seasonality compared to fsu1 the ncep1 zonal stress anomalies x are weaker less noisy and show less persistent westerly peaks during el nino events ncep1 also shows a more stationary spec

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka

the april 2012 indian ocean earthquakes seismotectonic context and implications for their mechanisms the 11 april 2012 earthquakes m w 8 6 and m w 8 2 were sourced within the northern wharton basin in the northeastern part of the indo australian diffuse plate boundary this unusually active oceanic intraplate region has generated many large earthquakes in the past most of which are believed to have occurred by strike slip motion triggered by the nw se oriented compressional stresses acting across the indian and australian plates in the aftermath of the 2004 megathrust earthquake along the nearby sunda trench increased seismicity in the northern wharton basin is attributed to the stress transfer from the sumatra andaman plate boundary models proposed for the april 2012 earthquakes differ somewhat in details but partly attribute their complex rupture to the reactivation of pre existing structures these structures include previously mapped n s trending fracture zones within the northern wharton basin and e w lineations across the ninetyeast ridge in this paper we review the regional tectonics and past seismicity on the indo australian plate in order to understand the seismotectonic setting of the april 2012 indian ocean earthquakes c 2014 elsevier b v all rights reserved