earliest sea floor spreading magnetic anomalies in the north arabian sea and the ocean continent transition summary magnetic and gravity data collected during a gloria survey of the indus fan provide new information on the earliest sea floor spreading history of the arabian sea a negative gravity anomaly correlates with the buried laxmi ridge this ridge is interpreted here to be a sliver of continental crust adjacent to the oceancontinent transition which bounds thinned probably intruded transitional crust to the ne the oldest sea floor spreading anomaly is anomaly 28 65 66 ma breakup occurring at the time of the deccan traps volcanic event the earliest oceanic crust formed from two phases of rift propagation which accommodates the angular disparity between the e w trending anomalies in the western arabian sea and the ne sw trending western part of the laxmi ridge flow line projection shows that the laxmi ridge forms the conjugate structure to the northern mascarene plateau margin

recurrent early cretaceous indo madagascar 89 86 ma and deccan 66 ma alkaline magmatism in the sarnu dandali complex rajasthan 40 ar 39 ar age evidence and geodynamic significance abstract the sarnu dandali alkaline complex in rajasthan northwestern india is considered to represent early pre flood basalt magmatism in the deccan traps province based on a single 40 ar 39 ar age of 68 57 ma rhyolites found in the complex are considered to be 750 ma malani basement our new 40 ar 39 ar ages of 88 9 86 8 ma for syenites nephelinite phonolite and rhyolite and 66 3 0 4 ma 2 melanephelinite provide clear evidence that whereas the complex has deccan age 66 ma components it is dominantly an older by 20 million years alkaline complex with rhyolites included basalt is also known to underlie the early cretaceous sarnu sandstone sarnu dandali is thus a periodically rejuvenated alkaline igneous centre active twice in the late cretaceous and also earlier many such centres with recurrent continental alkaline magmatism sometimes over hundreds of millions of years are known worldwide the 88 9 86 8 ma 40 ar 39 ar ages for sarnu dandali rocks fully overlap with those for the indo madagascar flood basalt province formed during continental breakup between india plus seychelles and madagascar recent 40 ar 39 ar work on the mundwara alkaline complex in rajasthan 120 km southeast of sarnu dandali has also shown polychronous emplacement over 45 million years and 84 80 ma ages obtained from mundwara also arguably represent post breakup stages of the indo madagascar flood basalt volcanism remnants of the indo madagascar province are known from several localities in southern india but hitherto unknown from northwestern india 2000 km away additional equivalents buried under the vast deccan traps are highly likely

evidence for seafloor spreading in the laxmi basin northeastern arabian sea abstract marine magnetic anomaly data from the laxmi basin for the first time reveal the presence of fairly correlatable nnw trending magnetic lineations these magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short wavelength free air gravity low the anomalies are interpreted as representing a two limbed seafloor spreading sequence which can be equated to the a28 a33 interval of the geomagnetic polarity reversal timescale these results suggest that the laxmi basin is underlain by an oceanic crust evidence of seafloor spreading in this basin possibly implies a pre a27 spreading episode in the evolutionary history of the arabian sea

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

architecture of a magma poor passive margin insights from the somali margin abstract the architecture of magma poor continental margins from break up to the establishment of an oceanic spreading centre has received significant recent interest a general consensus as to their characteristics exists and although it has been applied to margins globally is still dominated by the iberian newfoundland type margin additionally many studies rely upon sparsely distributed 2d seismic reflection profiles or single seismic sections questions remain as to the validity of the model across other magma poor margins in particular the role of magmatic additions during the rifting phase and their relationship to the creation of true oceanic crust at the onset of sea floor spreading our data comprise a 2d seismic reflection survey from the obbia basin northern somalia passive margin the data are extremely well imaged to over 13 s two way time providing insights into the crustal structure here we present 4 representative sections from this larger survey which combined with gravity modelling describe the character and crustal architecture along the margin from the proximal rifted continental crust to the oceanic domain we find that the somali margin is a poorly understood example of a magma poor margin exhibiting iberian type architecture the margin comprises a stretched proximal domain that passes seaward into a strongly thinned necking zone across which the lower crust is completely thinned we identify a high amplitude reflection har that corresponds to the decoupling level and into which crustal faults sole out seaward the har merges with the top of the exhumed mantle directly underlying post rift sediments in contrast to other magma poor margins the somali margin has a narrow necking zone and a wide zone of exhumed mantle in excess of 100 km on most sections true oceanic crust is never encountered with only the distal portion of one section exhibiting oceanic crustal character we place the continent ocean transition zone in excess of 200 km east of the shoreline in the north the basin is affected by transform faults where this occurs we note a presence of volcanics within the crust and post rift sediments gravity modelling supports the addition of magmatic material in these regions this addition of volcanic material in contrast to that proposed at transform faults in other passive margins is spatially restricted to the transform faults emplaced post kinematically and is not associated with oceanic crust formation the results of this study not only provide new insights into the architecture on non typical non magmatic margins but also reveal the importance of incorporating these findings into plate reconstructions

systematics of ridge propagation south of 30 s abstract new high resolution geosat altimetry data south of 30 s reveal numerous propagating ridge wakes along intermediate and slow spreading ridges these new examples provide a large enough database to establish systematics of ridge propagation almost all active propagating ridges propagate down a regional along axis gravity or bathymetry gradient the sense of the propagating ridge offset right lateral vs left lateral is related to recent changes in spreading direction we find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than 50 of their full spreading rate and ridges with a median valley morphology which usually propagate at 25 of their spreading rate the axial high propagators leave behind an asymmetric wake the outer pseudofault appears as a continuous linear trough step while the sheared zone appears as a chain of small gravity bumps while we clearly see the propagating ridge wakes from offsets greater than 10 km at slow and intermediate spreading ridges at ridges spreading faster than 75 mm yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in geosat altimetry data the systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation

azimuth variation in microwave scatterometer and radiometer data over antarctica while designed for ocean observation scatterometer and radiometer data have proven very useful in a variety of cryosphere studies over large regions of antarctica ice sheet and bedrock topography and the snow deposition drift and erosional environment combine to produce roughness on various scales roughness ranges from broad basin scale ice sheet topography at spl sim 100 km wavelengths to large spatially coherent dune fields at spl sim 10 km wavelength to erosional features on the meter scale known as sastrugi these roughness scales influence the microwave backscattering and emission properties of the surface combining to introduce azimuth angle dependencies in the satellite observation data in this paper the authors explore the use of nasa scatterometer nscat data european remote sensing ers advanced microwave instrument ami scatterometer mode data and special sensor microwave imager ssm i data to study surface roughness effects in antarctica all three sensors provide strong evidence of azimuth modulation which is correlated with the surface slope environment and results in a katabatic wind flow regime due to its broad azimuth coverage nscat data appears to be the best suited for azimuth angle observations a simple empirical model for the azimuth variation in the radar backscatter is developed and an algorithm for computing the parameters of the model from nscat data at a fine scale is presented results indicate relationships exist between the azimuthal variation of the data and the orientation of the surface slope and small scale roughness relative to the sensor look direction

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

motion between the indian antarctic and african plates in the early cenozoic we used a three plate best fit algorithm to calculate four sets of euler rotations for motion between the india capricorn africa somali and antarctic plates for 14 time intervals in the early cenozoic each set of rotations had a different combination of data constraints the first set of rotations used a basic set of magnetic anomaly picks on the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir and fracture zone constraints on the cir and seir but did not incorporate data from the carlsberg ridge and did not use fracture zones on the swir the second set added fracture zone constraints from the region of the bain fracture zone on the swir which were dated with synthetic flowlines based on the first data set the third set of rotations used the basic constraints from the first rotation set and added data from the carlsberg ridge the fourth set of rotations combined both the swir fracture zone constraints and the carlsberg ridge constraints data on the indian plate side of the carlsberg ridge arabian basin were rotated to the capricorn plate before being included in the constraints plate trajectories and spreading rate histories for the cir and swir based on the new rotations document the major early cenozoic changes in plate motion on the cir and seir there was a large but gradual slowdown starting around chron 23o 51 9 ma and continuing until chron 21y 45 3 ma followed 2 or 3 myr later by an abrupt change in spreading azimuth which started around chron 20o 42 8 ma and which was completed by chron 20y 41 5 ma no change in spreading rate accompanied the abrupt change in spreading direction on the swir there was a continuous increase in spreading rates between chrons 23o and 20o and large changes in azimuth around chrons 24 and 23 and again at chron 21 unexpectedly we found that the two sets of rotations constrained by the carlsberg ridge data diverged from the other two sets of rotations prior to anomaly 22o when compared to rotations for the cir that are simultaneously constrained by data from all three branches of the indian ocean triple junction there is a progressively larger separation of anomalies on the carlsberg ridge with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y these data require that there was previously unrecognized convergence somewhere in the plate circuit linking the indian capricorn and somali plates prior to chron 22o we quantify this motion by summing our new capricorn somalia rotations with previously published rotations for neogene india capricorn motion and for early cenozoic somali india motion based solely on carlsberg ridge data the most likely possibility is that there was motion within the somalia plate due to a distinct seychelles microplate as young as chron 22o the sense of the misfit on the carlsberg ridge is consistent with roughly 100 150 km of convergence across a boundary passing through the amirante trench and extending north to the carlsberg ridge axis between anomalies 26y and 22o alternatively there may have been convergence within the indian plate either along the western margin of indian or east of the cir in the region of the current capricorn indian diffuse plate boundary our work sharpens the dating of the two major eocene changes in plate motion recognized in the indian ocean

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

studies of the location of azimuth modulation minima for ku band ocean radar backscatter an extensive data set of radar backscatter measurements at 14 ghz was used to evaluate the locations of the azimuthal modulation minima relative to the cross wind directions it was found that the minima were significantly offset from the cross wind directions toward the downwind direction for the horizontal polarization data these offsets were substantially smaller for the vertical polarization data simple scattering model calculations based on the two scale scattering approach also show similar offsets implications of these results for the geophysical model function are discussed

the relationship between wind vector and normalized radar cross section used to derive seasat a satellite scatterometer winds the dependence of ocean normalized radar cross section nrcs or 0 on the 19 5 m neutral stability wind vector for the seasat a satellite scatterometer sass is described for the sass wind vector algorithm this dependence is specified in the form nrcs u where is the radar incidence angle is the angle between wind direction and radar azimuth and u is the neutral stability wind speed in meters per second at a height of 19 5 m the development of models to express this relationship and to provide the basis for inversion of nrcs to sass winds is traced from its initial form based on aircraft scatterometer measurements through the seasat field validation experiments goasex storms and jasin to its final form comparison of the model nrcs versus surface wind speed with sass data from these experiments are made

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

the dependence of ocean backscatter at k sub u band on oceanic and atmospheric parameters the jet propulsion laboratory nuscat k sub u band scatterometer successfully acquired ocean backscatter data over a wide variety of oceanic and atmospheric conditions during the surface wave dynamics experiment swade ten flights resulted in 30 h of data collection were conducted on the nasa ames c130 aircraft the swade experimental area was deployed with several buoys providing appropriate in situ measurements to correlate with the radar backscatter for incidence angles from 10 to 60 spl deg at both horizontal and vertical polarizations the nuscat swade backscatter data base was utilized in conjunction with buoy measurements to investigate ocean backscatter signatures over the range of conditions encountered during swade to account for modulations and fluctuations in incidence angles a backscatter analysis independent of a priori geophysical model functions was developed tested and implemented results for backscatter azimuth modulations in terms of upwind downwind and crosswind radar returns are compared to airborne radscat results and to sass i and ii geophysical model functions versus neutral wind speed u sub n nuscat swade results are closest overall to sass ii values fit best with sass i at 10 spl deg incidence angle and are significantly higher than radscat effects of friction velocity u wave age c sub o u and significant wave height h sub 1 3 on ocean backscatter are studied the backscatter shows a good positive sensitivity to u an inverse dependence on c sub o u and no systematic trend with h sub 1 3 excluding cases of large swells coefficients of empirical relations between backscatter and neutral wind speed friction velocity and wave age are derived for 10 40 spl deg incidence angles at both horizontal and vertical polarizations covariance studies of backscatter with the derived relations to winds show an overall deviation factor in the order of 1 db for ocean signatures including uncertainties in surface conditions

constraints on plate motions in southern pakistan and the northern arabian sea from the focal mechanisms of small earthquakes the focal mechanism and depth were determined for nine small earthquakes m0 1025 dyn cm m 5 5 that occurred in southern pakistan and the northern arabian sea from an analysis of the vertical component of rayleigh waves in combination with limited first motion data focal parameters were determined from the rayleigh waves by using an event pair method of analysis for earthquakes that are located very close to each other 50 km the event pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest 20 50 s for events separated by more than 100 km the propagation effects are reduced for only the longer periods 40 50 s the earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern pakistan triple junction the owen fracture zone is a transform fault that accommodates right lateral motion between the indian and arabian plates the plate boundary in the vicinity of the murray ridge is also partially made up of transform segments that strike subparallel to the owen fracture zone spreading centers may also exist in the vicinity of the murray ridge but were not documented by seismic or other evidence the slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion the arabian plate is being subducted beneath the eurasian plate along the southern coast of pakistan slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions left lateral motion is documented along the boundary between the indian and eurasian plates in southern pakistan the predicted direction of relative motion between these plates is not significantly different from that observed two of the earthquakes studied appear to be intraplate in nature the depth and focal mechanism of one intraplate event which may have occurred within the accretionary prism along the makran coast however are based on limited data the other intraplate earthquake occurred within the sedimentary deposits along the western edge of the indian subcontinent near the indian eurasian plate boundary appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 009 2 50 payment must accompany order

sedimentation rates from calcareous nannofossil and planktonic foraminifera biostratigraphy in the andaman sea northern bay of bengal and eastern arabian sea abstract in this study we determined calcareous nannofossil and planktonic foraminifera events in sediments recovered during the 2006 indian national gas hydrate program nghp expedition 01 selected bioevents permitted the assignment of orbitally calibrated ages derivative sedimentation rate estimates and identification of sedimentary discontinuities in the andaman sea at hole nghp 01 17a a late miocene to recent record was recovered but with a significant hiatus during the latest miocene and pliocene sedimentation rates here vary from 50 m myr during the pleistocene to 130 m myr for the late miocene in the northern bay of bengal at hole nghp 01 19a the base of the cored record also reaches the late miocene and extends to recent but the same hiatus missing the pliocene is present at this hole a the early pleistocene at hole nghp 1 19a shows sensible variation in sedimentation rates ranging between 130 m myr and 10 m myr during the last 0 5 ma for the late miocene the sedimentation rate was ca 50 m myr in the western bay of bengal at hole nghp 01 10b d calcareous nannofossils were sparse and only foraminifera datums are available the base of the section is latest early pliocene to late pliocene and the majority of the record is considered younger than late pliocene due to the scarcity of calcareous nannofossils and the few foraminifera datums sedimentation rates were not determined here however hole nghp 01 16a in the same region recorded the highest occurrence of pseudoemiliania lacunosa an event that confirms a pleistocene age allowing us to estimate a sedimentation rate between 110 m myr and 450 m myr along the western peninsular indian margin at hole nghp 01 01a a continuous record from the holocene to the early oligocene indicates sedimentation rates of ca 25 m myr until the middle miocene and a sensible reduction downward reaching 4 m myr in the early miocene to the early oligocene no hiatuses were observed the large changes in sedimentation rates observed in these cores reflect long timescale changes in the environment likely induced by changes in the strength of the indian monsoon system

the deep structure of the east oman continental margin preliminary results and interpretation abstract we describe preliminary results of a coincident normal incidence and wide angle seismic experiment across the east oman continental margin just north of the masirah island ophiolite this margin is affected by tectonic deformation and may be the site of margin parallel shear in a zone parallel to the owen fracture zone the reflection profile is used to define upper crustal structure and shows a deep offshore basin dammed oceanwards by a ridge interpreted here as a rotated fault block wide angle data were collected using ten digital ocean bottom seismometers and 110 explosive shots preliminary raytracing of a crustal model based on the gravity model of whitmarsh 1979 shows rapid changes in crustal thickness across the margin a steep landward dipping reflector probably the moho lies beneath the continental slope the orientation of this reflector is exactly opposite to the direction of tectonic fabric predicted by a simple overthrust model of ophiolite emplacement from the ocean basin to the east

owen fracture zone the arabia india plate boundary unveiled we surveyed the owen fracture zone at the boundary between the arabia and india plates in the nw indian ocean using a high resolution multibeam echo sounder owen cruise 2009 for search of active faults bathymetric data reveal a previously unrecognized submarine fault scarp system running for over 800 km between the sheba ridge in the gulf of aden and the makran subduction zone the primary plate boundary structure is not the bathymetrically high owen ridge but is instead a series of clearly delineated strike slip fault segments separated by several releasing and restraining bends despite an abundant sedimentary supply by the indus river flowing from the himalaya fault scarps are not obscured by recent deposits and can be followed over hundreds of kilometres pointing to very active tectonics the total strike slip displacement of the fault system is 10 12 km indicating that it has been active for the past 3 to 6 ma if its current rate of motion of 3 1 mm yr 1 has remained stable we describe the geometry of this recent fault system including a major pull apart basin at the latitude 20 n and we show that it closely follows an arc of small circle centred on the arabia india pole of rotation as expected for a transform plate boundary

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

evidence for long term diffuse deformation of the lithosphere of the equatorial indian ocean the presence of large earthquakes east west striking folds and thrust faults in sediments and east west striking undulations of wavelength 200 km in topography and gravity shows that the equatorial indian ocean is the locus of unusual deformation1 2 3 4 5 6 7 8 this deformation has been interpreted as a diffuse boundary between two tectonic plates9 10 11 12 13 seismic stratigraphy and deep sea drilling at two locations in the bengal fan indicate that the deformation began 7 5 8 0 myr ago3 14 15 here however we show using plate reconructions that motion across this diffuse oceanic plate boundary began more than 10 myr earlier than previously inferred and that the amount of north south convergence across the boundary through the central indian basin has been significantly greater than the convergence estimated from seismic profiles the relative plate velocity accommodated across the central indian basin has varied with time and has been as fast as 6 mm yr 1 about half the separation rate of earth s slowest spreading mid ocean ridge the earliest interval of measurable motion which began more than 18 myr ago may coincide with rapid denudation of the tibetan plateau from 21 myr to 15 17 myr ref 16 the present motion across the central indian basin began no earlier than 11 myr following an earlier interval of slower motion from 18 to 11 myr and may have begun at 8 myr when the tibetan plateau is thought to have attained its maximum elevation16 17

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

pseudofaults and associated seamounts in the conjugate arabian and eastern somali basins nw indian ocean new constraints from high resolution satellite derived gravity data abstract marine gravity data derived from satellite altimeters are effective tools in mapping fine scale tectonic features of the ocean basins such as pseudofaults fracture zones and seamounts particularly when the ocean basins are carpeted with thick sediments we use high resolution satellite generated gravity and seismic reflection data to map boundaries of pseudofaults and transferred crust related to the paleocene spreading ridge propagation in the arabian and its conjugate eastern somali basins the study has provided refinement in the position of previously reported pseudofaults and their spatial extensions in the conjugate basins it is observed that the transferred crustal block bounded by inner pseudofault and failed spreading ridge is characterized by a gravity low and rugged basement the refined satellite gravity image of the arabian basin also revealed three seamounts in close proximity to the pseudofaults which were not reported earlier in the eastern somali basin seamounts are aligned along ne sw direction forming 300 km long seamount chain admittance analysis and flexural model studies indicated that the seamount chain is isostatically compensated locally with effective elastic thickness te of 3 4 km based on the present results and published plate tectonic models we interpret that the seamounts in the arabian basin are formed by spreading ridge propagation and are associated with pseudofaults whereas the seamount chain in the eastern somali basin might have probably originated due to melting and upwelling of upper mantle heterogeneities in advance of migrating propagating paleo carlsberg ridge

alteration and submergence of basalts in kachchh gujarat india implications for the role of the deccan traps in the india seychelles break up

detailed structure and plate reconstructions of the central indian ocean between 83 0 and 42 5 ma chrons 34 and 20 the central indian ocean namely the central indian crozet and madagascar basins formed by rifting and subsequent drifting of india now capricorn antarctica and africa now somalia we gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between c34ny 83 0 ma and c20ny 42 536 ma using the objective analytic signal technique we present high resolution magnetic isochrons for 29 periods based on 1 400 magnetic anomaly picks from the conjugate sets of picks we derive two plate finite rotation parameters for both the capricorn antarctica and capricorn somalia motions these finite rotations are compared to three plate reconstructions of the plate boundaries between the capricorn antarctica and somalia plates constrained by the closure of the indian ocean triple junction in general the three plate reconstructions slightly overrotate the reconstructed isochrons with respect to the capricorn antarctica and capricorn somalia two plate reconstructions conversely the two plate reconstructions for somalia antarctica slightly underrotate the isochrons compared to the capricorn antarctica somalia three plate reconstructions we suspect that the discrepancies between the two plate and three plate methods result from the recent seafloor deformation in the capricorn india diffuse plate boundary and or from the contrasted nature and geometry of magnetic isochrons at different spreading rates i e magnetic structure of the three spreading centers three plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries when enough quality data are available two plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved

plagioclase breakdown and regeneration reactions in grenville kyanite eclogite at glenelg nw scotland a rare type of eclogite at glenelg contains finegrained white streaks about 1 mm across consisting of kyanite oligoclase quartz kfeldspar garnet and omphacite bordering the streaks are relatively enriched in grossular and jadeite components respectively the streaks are interpreted as former plagioclase which reacted with neighbouring garnet and omphacite during a period of shearing and compression p t conditions during the reaction are estimated as 740 c and 17 kbar subsequent reaction manifest by narrow rims of oligoclase on kyanite new growth of garnet and resorption of omphacite is interpreted as an effect of near isothermal decompression by about 2 kbar the compressive phase is attributed to dextral transpression oblique convergence along the grenville front in scotland

early tertiary seafloor spreading magnetic anomalies and paleo propagators in the northern arabian sea abstract study of closely spaced new marine magnetic profiles in conjunction with published magnetic data provide an updated identification of linear magnetic anomalies in the northern arabian sea from the owen fracture zone in the west to the laxmi and chagos laccadive ridges in the east for the first time the entire magnetic anomaly sequence 28 through 20 is mapped eastwards from 65 e up to about 70 e further eastward the post anomaly 28 oceanic crust appears to have been overlain by the volcanic material forming the chagos laccadive ridge contrary to earlier interpretations the identified magnetic lineations are observed to have been segmented mainly by oblique offsets representing the pseudofaults associated with 11 major paleo propagating ridges these ridge propagation events commenced around anomaly 28 time 63 ma and continued at least until anomaly 23 time 52 ma and during the process a large amount of crust was transferred from the african plate to the indian plate the pre anomaly 24 period was characterized by ridge reorientation whereas such a phenomenon was not apparent during the post anomaly 24 period almost all these propagating ridges appear to have been propagating westward probably to migrate away from the proximity of the reunion hotspot the accretionary process of the early tertiary oceanic crust in the arabian sea thus appears to have been greatly influenced by the propagating ridges and the nearby hotspot

present day motion along the owen fracture zone and dalrymple trough in the arabian sea all plate motion data available for describing the present day motion between the arabian and indian plates are analyzed to understand the present day tectonic role of the presumed plate boundary the owen fracture zone and dalrymple trough in the northern arabian sea all prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary in particular published global plate motion models predict some convergence across the owen fracture zone and dalrymple trough convergence along the owen fracture zone is contradicted by earthquake focal mechanisms which show right lateral strike slip convergence along the dalrymple trough is contradicted by observed normal faulting in the trough and by normal and strike slip focal mechanisms for earthquakes occurring along or near the trough the sense and rate of motion is further constrained by spreading rates along the sheba ridge northwest of the owen fracture zone and along the carlsberg ridge southeast of the owen fracture zone to estimate the present day motion we determine 43 3 m y average spreading rates from sheba ridge carlsberg ridge and central indian ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the arabia india boundary we determine an arabia india euler vector that predicts 2 mm yr of right lateral strike slip motion along the owen fracture zone with a 95 confidence interval of 0 7 mm yr which excludes the faster rates of slips predicted by several prior models the direction of motion is 35 50 counterclockwise from that predicted by published global plate motion models if we omit the data along the arabia india boundary the motion differs insignificantly from zero suggesting that magnetic profiles from the carlsberg and sheba ridges give spreading rates too imprecise to detect the slow arabia india motion if the owen fracture zone and dalrymple trough are regarded as a plate boundary it is the slowest slipping plate boundary known on earth with motion about 100 times slower than the fastest slipping faults along the east pacific rise

morphotectonic architecture of an india madagascar breakup related anomalous submarine terrace complex on the southwest continental margin of india abstract an anomalous lateral bathymetric protrusion alleppey trivandrum terrace complex attc in the form of two contiguous terrace like features exists in the mid continental slope off southwest coast of india this bathymetric protrusion and a bathymetric notch in the northern madagascar ridge nmr have been postulated as conjugate features related to india madagascar separation in late cretaceous however geoscientific studies aimed to understand the crustal structure and genesis of the attc or its postulated conjugate region are meagre and ambiguous in this context the present study based on recently acquired as well existing bathymetry gravity magnetic and multi channel seismic reflection data was carried out to understand the morphotectonic architecture of the attc the bathymetric data suggest an escarpment chain kairali escarpment cke to demarcate the westward limit of the attc and another escarpment quilon escarpment qe to demarcate the boundary between the southerly larger trivandrum terrace tt and northerly smaller alleppey terrace at the multichannel seismic reflection data suggests a block faulted basement and presence of a nearly n s trending wide basement high in the central part of the trivandrum terrace forward modelling of gravity data constrained by seismic reflection information suggests that the crustal structure of the attc region is comparable to thinned continental crust forward modelling of prominent high amplitude magnetic anomalies suggest that the thinned continental crust of the attc region also contains volcanic bodies as intrusives extrusives the cke is inferred to be a sheared continental margin segment along which the se coast of madagascar glided past india and the volcanic emplacements in the attc region perhaps relates to the marion hotspot volcanism

40ar 39ar dating of the mumbai tholeiites and panvel flexure intense 62 5 ma onshore offshore deccan magmatism during india laxmi ridge seychelles breakup

spreading history of the arabian sea some new constraints abstract based on a new identification of magnetic anomalies additional constraints are provided on the two episodes of spreading history of the arabian sea commencing at a27 the older phase ended at a21 and spreading of the younger phase started shortly before the formation of a11 the hiatus between these spreading episodes characterized by a magnetic quiet zone was much shorter 18 106 yr than previously believed after spreading restarted the younger phase continued at a very slow rate 0 7 cm yr till anomaly 6a this rate increased between anomalies 5c and 6a and the spreading continued to the present at about 1 2 cm yr the data also enabled identification of a new ne sw trending fracture zone which right laterally offsets the entire magnetic anomaly sequences 1 through 11 across the carlsberg ridge by about 50 km

probabilities and statistics for backscatter estimates obtained by a scatterometer methods for the recovery of winds near the surface of the ocean from measurements of the normalized radar backscattering cross section must recognize and make use of the statistics i e the sampling variability of the backscatter measurements radar backscatter values from a scatterometer are random variables with expected values given by a model a model relates backscatter to properties of the waves on the ocean which are in turn generated by the winds in the atmospheric marine boundary layer the effective wind speed and direction at a known height for a neutrally stratified atmosphere are the values to be recovered from the model the probability density function for the backscatter values is a normal probability distribution with the notable feature that the variance is a known function of the expected value the sources of signal variability the effects of this variability on the wind speed estimation and criteria for the acceptance or rejection of models are discussed a modified maximum likelihood method for estimating wind vectors is described ways to make corrections for the kinds of errors found for the seasat sass model function are described and applications to a new scatterometer are given

early oceanic opening off western india pakistan margin the gop basin revisited the deccan traps one of the best known examples of rapid flood basalt are considered as marking of the inception of a mantle plume on the indian continental lithosphere their emplacement may be associated with the continental break up of india and the seychelles block and later formation of a new spreading centre the carlsberg ridge while spreading progressively ceased in the mascarene basin whether rifting continental break up and seafloor spreading predated or were the consequence of the deccan traps emplacement is still a matter of debate this issue is further complicated by the presence of a continental sliver the laxmi ridge and large basins lying landward of the laxmi ridge such as the laxmi and gop basins where nature of the crust is still ambiguous the present study attempts to decipher the tectonic setting and the imprints of plume ridge interaction in the gop basin where the crust has been interpreted as either volcanics intruded thinned continental crust or oceanic crust formed by a now extinct spreading centre based on interpretation of an updated compilation of marine geophysical data the present study supports the oceanic nature of the crust underlying the gop basin and proposes the palitana ridge as the extinct spreading centre in this region the prominent but short sequence of fairly linear magnetic anomalies in the gop basin does not allow a unique identification it can be reasonably explained either as a31r a25r 69 3 56 4 ma or as a29r a25r 64 8 56 4 ma sequence the variations of the spreading rates assumed by both these models suggest that spreading in the gop basin significantly slowed around 65 ma contemporaneous with the magmatic outburst of the reunion plume on the adjacent western indian mainland subsequently the gop basin spreading centre was waning whereas a new spreading centre was developing further south close to the relatively southward migrating plume in this last stage the gop basin spreading centre was associated with an abundant magmatism probably supplied from the plume region

early spreading and continental to oceanic basement transition beneath the indus deep sea fan northeastern arabian sea abstract a seismic survey to study the indus deep sea fan penetrated into basement this enabled the mapping of basement structures and a description of the geodynamical evolution of a part of the northeastern arabian sea the main tectonic structure of the basement is an e w graben gop rift with a prominent central e w horst the palitana horst this rift is cut obliquely by several ne sw faults large e w magnetic anomalies are not the continuation of the magnetic anomaly sequence of the oceanic arabian basin however these anomalies are linked to inverse magnetisation in the crust suggesting that the rift is either underlain by stretched continental crust intruded by linear volcanic bodies related to the deccan traps or by oceanic crust older than chron 28 64 ma as proposed in the laxmi basin southeastwards accompanying these structures ne sw faults with a strike slip pattern may have played the role of transform direction during the separation of the laxmi ridge from the indian continent we propose that rifting and oceanic spreading have occurred around a triple junction connecting the oceanic laxmi basin the narmada son lineament and the gop rift this phase preceded oceanic spreading in the arabian basin this interpretation is in agreement with the previously proposed initial separation of india and the seychelles before chron 28 the laxmi ridge could be a part of the seychelles microplate subsequently left attached to india

ers 1 scatterometer measurements i the relationship between radar cross section and buoy wind in two oceanic regions extensive european remote sensing satellite ers 1 spacecraft scatterometer measurements were collocated within spl plusmn 25 km of buoy measurements at midlatitudes and in the equatorial pacific during 1992 1994 two different directional functional forms for the geophysical model were fit to these measurements described by cross section maxima aligned and offset with respect to the mean wind direction the two models exhibit fits with similar residuals for each of three years in the equatorial region thus neither of these two model functions can convincingly be identified as more accurate than the other unless additional factors are considered nevertheless the offset model results in an average directional offset near 10 spl deg for each season during the three year period whereas no such offset was inferred at midlatitudes the cross section dependence on wind direction also exhibits another unexpected average property a larger downwind compared to upwind cross section a sub 10 spl ap 0 25 db in the equatorial pacific this characteristic could be caused by a concentration of the short bragg waves on the backside of the long wave crests both of these unexpected characteristics are likely related to a dominant systematic swell traveling nearly perpendicular to the prevailing easterly winds across the equatorial pacific ocean the possible cross section dependence on wind stress and wind speed is due to long waves when adjusting the buoy measurements to a common height are discussed