seismic imaging of salt structures in the gulf of mexico in this article we discuss seismic imaging of salt structures in the gulf of mexico with emphasis on the recognition preservation and migration of salt signals we describe three salt imaging technologies

a thermoconvective interpretation of heat flow data in the area of ocean drilling program leg 116 in a distal part of the bengal fan we present the results of a numerical simulation study of the thermo convective circulation in a sedimentary pile in the distal part of the bengal fan the occurrence of faults and the variability of surface heat flow data led previous workers to suggest hydrothermal circulation in this area according to the purpose of ocean drilling program survey leg 116 data were collected which are adequate to constrain a model for the comprehension and the quantification of the relationship between ocean basin heat flow and pore fluid circulation in this paper the sedimentary pile is modeled as an anisotropic and inclined porous layer interrupted by faults with a periodic distribution an important observation the model tries to reproduce is the shift of approximately 2 3 km between the surface expression of the heat flow maximum and the fault zone near site 718 the influence of permeability anisotropy and of permeability within those fault zones is analyzed when the maximum vertical permeability in the fault zones is 5 times greater than the vertical permeability of the porous block the circulation consists of longitudinal rolls i e rolls with their axis oriented downslope with more vigorous convection in fault zones when the maximum permeability of the fault zone is 10 or more times greater than the vertical permeability of the porous layer convection is primarily restricted to the fault plane in all cases maximum heat flow values are located directly above faults this paper shows that the best agreement between those data suggests a 1 m yr 1 forced convection and a permeability of the sediments of 0 5 darcy 5 10 13 m2 and 5 mdarcy respectively in the horizontal and vertical derictions

some new observations on the intra plate deformation in the central indian basin cib abstract single channel airgun seismic reflection data from equatorial region of the central indian basin cib revealed some new information on the structure and stratigraphy of the deformed blocks in this area the data from five n s and e w traverses and from a detailed grid over a 45 45 km block covering the area between 2 n and 2 s and 81 and 84 e indicate 1 a ne sw trend for the faulted deformed blocks 2 presence of fracture zones older than and unrelated to the tectonics associated with the process of deformation 3 an older age 17 ma for the base of the bengal fan turbidites and 4 a thin layer of transparent probably pre fan pelagic sediments immediately above the acoustic basement

an investigation of the regional variations and frequency dependence of anelastic attenuation in the mantle under the united states in the 0 5 4 hz band summary studies of teleseismic p and s wave amplitudes and spectra in the 0 5 4 hz band show large variations in the attenuative properties of the upper mantle under the united states the data indicate that attenuation is greatest under the south western united states including but not confined to the basin and range province the lowest attenuation prevails under the north central shield regions the north eastern part of the country consisting of new england and possibly including a larger area along the eastern seaboard is characterized by moderate attenuation in the mantle the level of the high frequency energy in short period seismic waves and the differences between q values derived from short and long period data indicate that q is frequency dependent the form of frequency dependence of t compatible with the data in the 0 5 4 hz range does not allow a rapid decrease of t with increasing frequency rather it supports a gradual decrease covering the broader 0 1 4 hz range the curves of t versus frequency for shield to shield and mixed shield to western united states type paths are parallel with an average difference of 0 2 s in t in the short period band but may diverge towards the long period band for both curves t p is below 1 s for shield to shield paths t p must be below 0 5 s at 1 hz

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

seismic seafloor observations a study of anomalous intraplate seismicity in the northeastern indian ocean abstract present day intraplate tectonic activity in the northeast indian ocean which manifests itself in high seismicity is unusual for oceanic lithosphere plates the contradictory points of the various tectonic hypotheses proposed to explain this anomalous seismicity may be due to sparsity of ocean intraplate earthquakes of m 4 5 which are recorded by means of networks of land based permanent seismic stations seismological observations on the ocean floor by means of ocean bottom seismographs may provide additional and important data some results of such observations carried out in the central indian basin simultaneously with deep seismic sounding surveys are analysed in this paper about 118 seismic events were recorded at a site near 3 45 s and 80 e for a period of 7 3 days and 24 events were recorded at a site near 0 50 s and 83 53 e for a period of 3 3 days magnitudes of these events range approximately from 2 8 to 4 0 estimates of epicentre locations based on the jeffreys bullen travel time table show that over 90 of the earthquakes recorded at the first site were related to the chagos trough during a period when no event was recorded there by land based seismic stations the most probable reason for these events is the chagos bank earthquake of 30 november 1983 m s 7 6 which caused many aftershocks extending eastward from the main shock thus the high aftershock activity east of the chagos bank earthquake revealed by ocean bottom seismic observations complements the data on epicentres of larger earthquakes which in turn provide the scope to extend the known seismicity of the chagos bank further east in general our results are in accordance with the most popular tectonic model of the central indian ocean in which a diffuse plate boundary divides the australian plate from the indo arabian or the indian plate initiating a new subduction zone

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

tectonic aspects of intraplate seismicity in the northeastern indian ocean abstract since the late cenozoic the northeastern indian ocean region of the indo australian plate has been characterized by tectonic activity that is unusual in a mid plate setting this activity including deformation of the basement and the sediments and a high level of seismicity is typical of plate boundaries the characteristics of the seismicity and other phenomena there bears evidence to the initial stage of diffuse intraplate deformation of oceanic lithosphere it would be reasonable to suggest that the intraplate deformation in the northeastern indian ocean is episodic in behaviour the previous pulse of deformation seems to have occurred in the late miocene when the recent deformation of the sediments and the basement observed on the seismic reflection profiles was generated modern tectonic activity manifesting itself in high seismicity can be related to a new pulse of intraplate deformation a near latitudinal equatorial seismic zone comprising the northern segment of the ninetyeast ridge and northern central indian and cocos basins was suggested on the basis of analysis of seismological data for the eastern indian ocean from 1907 to 1983 the intraplate earthquake epicenters are concentrated in this broad band without distinct physiographic boundaries from nearly 80 e in the central indian basin to the sunda trench northeastern trends of earthquake epicenters that seem to be related to reactivation of some old faults can be observed within this band the orientation of the intraplate seismic zone is approximately similar to that of the continental collision zone at the northern indo australian plate margin the himalaya the possible cause of this high intraplate seismicity might be the southward migration or jumping of the himalayan subduction zone after the subduction there was stopped by the indian continental block due to continental collision the episodic intraplate deformation in the northeastern indian ocean region may manifest a future site of subduction along the southern margin of the indian subcontinent

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

continental lithosphere folding in central asia part i constraints from geological observations abstract geological observations support the occurrence of long wavelength surface undulations in central asia which can be treated in terms of folding of the whole lithospheric plate the characteristic wavelength of the whole lithosphere folding is 360 km in the western part of the gobi sub plate superimposed on the long wavelength deformation two shorter wavelength scales of spatially periodical deformation are also observed an intermediate wavelength of 30 50 km of the intraplate deformation reflects the response of the mechanically strong upper crustal part of the central asian lithosphere small wavelength 4 9 km deformation corresponds to faulting in the uppermost crustal layer and folding and faulting in the sedimentary cover folding of the lithosphere controls the location and geometry of compressional basin formation in the western part of the gobi sub plate a number of folding and faulting characteristics of continental intraplate deformation in central asia presented in this study are strikingly similar to observed styles of intraplate deformation in oceanic lithosphere in the northeastern indian ocean

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

long term seismic observations along myanmar sunda subduction margin insights for 2004 m w 9 0 earthquake the present study investigates the temporal variation of few seismic parameters between the myanmar zone i andaman nicobar northwest sumatra zone ii southeast sumatra west indonesia zone iii and east indonesia zone iv converging boundaries in reference to the generation of 26 december 2004 mw 9 0 off sumatra mega earthquake event the four segments are distinguished based on tectonics parameters distinct geological locations great earthquake occurrences and the wadati benioff zone characteristics two important seismic parameters such as seismic energy and b values are computed over a time window of 6 month period during the entire 1976 2013 period for these segments the b values show a constant decrease in zones ii iii and iv whereas the zone i does not show any such pattern prior to the 2004 mega event the release of seismic energy was also gradually decreasing in zones ii and iii till the 2004 event and little similar pattern was also noted in zone iv this distinct observation might be indicating that the stress accumulation was dominant near the sumatra java area located towards southeast of zone ii and northwest of zone iii the released strain energy during the 2004 event was subsequently migrated towards north rupturing 1300 km of the boundary between the northwest sumatra and the north andaman the occurrence of 2004 mega event was apparently concealed behind the long term seismic quiescence existing near the sumatra and nicobar margin a systematic study of the patterns of seismic energy release and b values and the long term observation of collective behaviour of the margin tectonics might have had given clues to the possibility of the 2004 mega event

last interglacial reef and subsidence of the cocos keeling islands indian ocean abstract poorly consolidated holocene sands and coralline limestones beneath reef islands on the cocos keeling islands are underlain by a well lithified coralline limestone 8 11 m below sea level uranium series ages of 118 000 7000 yrs b p bulk sample and 123 000 7000 yrs b p sample from which secondary cement was removed on a coral from within this lithified unit indicate that it was formed during the last interglaciation oxygen isotope substage 5e the depth at which the reef is encountered supports darwin s hypothesis that this atoll is undergoing gradual subsidence elevation of a reef of this age varies on other islands on the indo australian plate but an average rate of subsidence of 0 1 mm yr 1 is implied for the cocos keeling islands this is significantly faster than the subsidence rate expected from thermal contraction of the lithosphere at this location and suggests that additional factors have to be considered to account for the subsidence

dip moveout by fourier transform the conventional normal moveout nmo and common midpoint cmp stacking process enhances reflections having a particular moveout velocity while attenuating events such as multiple reflections having different moveout velocities unfortunately this process also acts as a dip filter applied to the cmp stack in other words nmo and stacking enhances reflections having a particular slope in the cmp stack while attenuating reflections having different slopes nmo and stacking like any dip filter degrades lateral resolution fortunately this dip filtering action can be suppressed by applying in addition to nmo a prestack process known variously as devilish prestack partial migration and dip moveout as the latter term implies this process is a dip dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same nmo velocity stated another way nmo velocities estimated from dip moveout corrected seismograms are independent of the dips o

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

historical seismicity near chagos a complex deformation zone in the equatorial indian ocean abstract over the last twenty years chagos bank has a seismicity rate disproportionate to its supposed intraplate location earthquake relocation also shows a high seismicity rate in pre wwssn time 1912 1963 with seven events located off of the central indian ridge including large events in 1912 m 6 8 and 1944 m 7 2 this study uses the moment variance technique a systematic search for the mechanism which best fits p pp sh love and rayleigh amplitudes to determine the focal mechanisms of two pre wwssn earthquakes a test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available moment variance analysis shows a thrust faulting mechanism for the 1944 event northeast of chagos bank near the chagos laccadive ridge and a strike slip focal mechanism for a smaller 1957 event west of chagos bank the 1944 event one of the largest oceanic intraplate earthquakes known moment 1 4 1027 dyne cm indicates that the chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought but rather regional tectonic deformation extending northeast of chagos bank and including thrust normal and strike slip events this seismicity and previously studied seismicity near the ninetyeast ridge and central indian basin suggest a broad zone of deformation stretching across the equatorial indian ocean this zone contains all known magnitude seven oceanic intraplate earthquakes not associated with subduction zones or continental margins suggesting that elsewhere such extensive deformation occurs only along plate boundaries this study proposes that a slow diffuse plate boundary extends east from the central indian ridge to the ninetyeast ridge and north to the sumatra trench a recent plate motion study confirms this boundary and suggests that it separates the australian plate from a single indo arabian plate

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

the stratigraphy and structure of the chitradurga region an illustration of cover basement interaction in the late archaean evolution of the karnataka craton southern india abstract the succession in the late archaean supracrustal belts in the chitradurga region begins with basal quartz pebble conglomerates and quartzites which were deposited on a basement of tonalitic granitic gneisses ca 3000 ma containing tracts and enclaves of older supracrustal rocks the basal beds are overlain by basic metalavas and amphibolites interbedded with cross bedded quartzites and siliceous phyllites this series passes up into siliceous phyllites locally containing a large lenticular mass of polymict conglomerate talya conglomerate overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks the latter being more abundant at higher levels with progressive deepening of the whole basin fine grained graded greywackes and local polymict conglomerates aimangala conglomerate were deposited the turbidite sequence forms the youngest sedimentary rocks exposed estimated total stratigraphic thickness is at least 10 km in deeper parts of the basin deformation led to a series of upright anticlines and synclines and related ls fabrics with extreme variations in plunge of the coaxial fold axes and l fabrics within axial surfaces which maintain a steep orientation low grade metamorphism outlasted the deformation sedimentation and volcanism in the chitradurga supracrustal belt is linked to variable subsidence of the basement foundation probably as a result of rifting the structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history variable displacement of the basement during these late archaean events in the karnataka craton took place on shear zones and related fractures that acted as channels for water and co 2 as indicated by vein minerals and alteration products uplift of basement segments and their distension by granitic diapirs led to deformation of the supracrustal cover made ductile by its high water content

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

earthquake source mechanisms from body waveform inversion and intraplate tectonics in the northern indian ocean double couple point source parameters for 11 of the largest intraplate earthquakes in the northern indian ocean during the last 20 y were determined from a formal inversion of long period p and sh waveforms nine of the events have centroid depths at least 17 km below the seafloor well into the upper mantle two have centroid depths as great as 39 km using the source mechanisms of these earthquakes we distinguish two major intraplate tectonic provinces in the northern indian ocean to the west of the ninetyeast ridge in the southern bay of bengal intraplate earthquakes have thrust faulting mechanisms with p axes oriented n s the centroid depths of these earthquakes range from 27 to 39 km below the seafloor lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding in the immediate vicinity of the ninetyeast ridge earthquakes display strike slip mechanisms with left lateral motion on planes parallel to the ridge this type of faulting occurs from at least 10 s to the northern end of the ninetyeast ridge near 10 n where the ridge meets the sunda arc seismic activity diminishes to the east of the ninetyeast ridge but is also characterized by strike slip faulting despite these variations in deformational style the inferred orientation of greatest compressive stress in the northern indian ocean displays a consistent long wavelength pattern over a large portion of the indian plate varying smoothly from nearly n s in the bay of bengal to nw se in the northeastern indian ocean this plate wide stress pattern and the high level of intraplate seismicity in the northern indian ocean are likely the results of substantial resistance along the himalayan continental collision zone to the continued northward motion of the western portion of the indian plate oceanic intraplate earthquakes in other regions where the level of deviatoric stress associated with the long wavelength part of the stress field is likely to be smaller need not be comparably reliable indicators of the plate wide stress field

crustal fault reactivation facilitating lithospheric folding buckling in the central indian ocean high quality normal incidence seismic reflection data confirm that tectonic deformation in the central indian ocean occurs at two spatial scales whole lithosphere folding with wavelengths varying between 100 and 300 km and compressional reactivation of crustal faults with a characteristic spacing of c 5 km faults penetrate through the crust and probably into the upper mantle both types of deformation are driven by regional large intraplate stresses originating from the indo eurasian collision numerical modelling of the spatial and temporal relationships between these two modes of deformations shows that in agreement with geophysical observations crustal faults are reactivated first with stick slip behaviour subsequent lithospheric folding does not start until horizontal loading has significantly reduced the mechanical strength of the lithosphere as predicted by elasto plastic buckling theory modelling suggests that lithospheric folding does not develop in the absence of fault reactivation crustal fault reactivation therefore appears to be a key facilitating mechanism for oceanic lithospheric buckling in the central indian ocean

fluid inclusions in high grade gneisses of the kapuskasing structural zone ontario metamorphic fluids and uplift erosion path fluid inclusions in quartz grains from five samples of high grade rocks two paragneisses an amphibolite a mafic gneiss and a tonalite dike from the 2 7 ga kapuskasing structural zone ksz ontario were examined with petrographic microthermometric and laser raman techniques three types of fluid inclusions were observed co2 rich h2o rich and mixed co2 h2o co2 rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure co2 a few contain varying amounts of ch4 h2o rich fluid inclusions are secondary in nature contain variable amounts of dissolved salts and generally contain daughter crystals mixed co2 h2o fluid inclusions occur where trails of h2o rich inclusions intersect trails of co2 rich inclusions isochores for high density p 1 03 g cm3 pseudosecondary pure co2 inclusions intersect the lower pressure portion of the estimated p t field for high grade metamorphism implying that pure co2 was the peak metamorphic fluid the variable ch4 content of co2 inclusions within graphite bearing samples suggests that ch4 was introduced locally after the formation of the co2 inclusions however the origin of the ch4 remains problematic an aqueous fluid clearly penetrated the gneisses after the peak metamorphism during uplift erosion forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks the presence of the pseudosecondary high density co2 inclusions in quartz crystals in the ksz rocks constrains the uplift erosion path for the ksz to one of simultaneous decrease in pressure and temperature

the 2014 mw 6 1 bay of bengal indian ocean earthquake a possible association with the 85 e ridge oceanic intraplate earthquakes are known to occur either on active ridge transform structures or by reactivation of their inactive counterparts generally referred to as fossil ridges or transforms the indian ocean one of the most active oceanic intraplate regions has generated large earthquakes associated with both these types of structures the moderate earthquake that occurred on 21 may 2014 m w 6 1 in the northern bay of bengal followed an alternate mechanism as it showed no clear association either with active or extinct ridge transform structures its focal depth of 50 km is uncommon but not improbable given the similar to 90 ma age of the ocean floor with 12 km thick overlying sediments no tectonic features have been mapped in the near vicinity of its epicenter the closest being the 85 degrees e ridge located similar to 100 km to its west hitherto regarded as seismically inactive the few earthquakes that have occurred here in the past are clustered around its southern or northern limits and a few are located midway at around 10 degrees n the 2014 earthquake sourced close to the northern cluster seems to be associated with a northwest southeast oriented fracture located on the eastern flanks of the 85 degrees e ridge if this causal association is possible we believe that reactivation of fossil hotspot trails could be considered as another mechanism for oceanic intraplate seismicity

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

angle domain common image gathers for migration velocity analysis by wavefield continuation imaging we analyze the kinematic properties of offset domain common image gathers cigs and angle domain cigs adcigs computed by wavefield continuation migration our results are valid regardless of whether the cigs were obtained by using the correct migration velocity they thus can be used as a theoretical basis for developing migration velocity analysis mva methods that exploit the velocity information contained in adcigs we demonstrate that in an adcig cube the image point lies on the normal to the apparent reflector dip that passes through the point where the source ray intersects the receiver ray the image point position on the normal depends on the velocity error when the velocity is correct the image point coincides with the point where the source ray intersects the receiver ray starting from this geometric result we derive an analytical expression for the expected movements of the image points in adcigs as functions of the traveltime perturbation caused by velocity errors by applying this ana

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

moment tensor solutions of earthquakes in the indian ocean from surface waves and their tectonic implications rayleigh and love waves generated by sixteen earthquakes which occurred in the indian ocean and were recorded at 13 wwssn stations of asia africa and australia are used to determine the moment tensor solution of these earthquakes a combination of thrust and strike slip faulting is obtained for earthquakes occurring in the bay of bengal thrust strike slip or normal faulting or either of the combination is obtained for earthquakes occurring in the arabian sea and the indian ocean the resultant compressive and tensional stress directions are estimated from more than 300 centroid moment tensor cmt solution of earthquakes occurring in different parts of the indian ocean the resultant compressive stress directions are changing from north south to east west and the resultant tensional stress directions from east west to north south in different parts of the indian ocean the results infer the counterclockwise movement of the region 0 33 s and 64 e 94 e stretching from the rodriguez triple junction to the intense deformation zone of the central indian ocean and the formation of a new subduction zone island arc beneath the intense deformation zone of the central indian ocean and another at the southern part of the central indian basin the compressive stress direction is along the ridge axis and the extensional stress manifests across the ridge axis the north south to northeast south west compression and east west to northwest southeast extension in the indian ocean suggest the northward underthrusting of the indian plate beneath the eurasian plate and the subduction beneath the sunda arc region in the eastern part the focal depth of earthquakes is estimated to be shallow varying from 4 to 20 km and increasing gradually in the age of the oceanic lithosphere with the focal depth of earthquakes in the indian ocean

prestack kirchhoff time migration of 3d coal seismic data from mining zones conventional seismic data processing methods based on post stack time migration have been playing an important role in coal exploration for decades however poststack time migration processing often results in low quality images in complex geological environments in order to obtain high quality images we present a strategy that applies the kirchhoff prestack time migration pstm method to coal seismic data in this paper we describe the implementation of kirchhoff pstm to a 3d coal seam meanwhile we derive the workflow of 3d kirchhoff pstm processing based on coal seismic data the processing sequence of 3d kirchhoff pstm includes two major steps 1 the estimation of the 3d root mean square rms velocity field 2 kirchhoff prestack time migration processing during the construction of a 3d velocity model dip moveout velocity is served as an initial migration velocity field we combine 3d kirchhoff pstm with the continuous adjustment of a 3d rms velocity field by the criteria of flattened common reflection point gathers in comparison with post stack time migration the application of 3d kirchhoff pstm to coal seismic data produces better images of the coal seam reflections

kirchhoff time migration for transversely isotropic media an application to trinidad data using a newly developed nonhyperbolic offset mid point traveltime equation for prestack kirchhoff time migration instead of the conventional double square root dsr equation results in overall better images from anisotropic data specifically prestack kirchhoff time migration for transversely isotropic media with a vertical symmetry axis vti media is implemented using an analytical offset midpoint traveltime equation that represents the equivalent of cheop s pyramid for vti media it includes higher order terms necessary to better handle anisotropy as well as vertical inhomogeneity application of this enhanced kirchhoff time migration method to the anisotropic marmousi data set demonstrates the effectiveness of the approach further application of the method to field data from trinidad results in sharper reflectivity images of the subsurface with the faults better focused and positioned than with images obtained using isotropic methods the superiority of the anisotropic time migration is evident

migration moveout analysis and depth focusing prestack depth migration still suffers from the problems associated with building appropriate velocity models the two main after migration before stack velocity analysis techniques currently used depth focusing and residual moveout correction have found good use in many applications but have also shown their limitations in the case of very complex structures to address this issue we have extended the residual moveout analysis technique to the general case of heterogeneous velocity fields and steep dips while keeping the algorithm robust enough to be of practical use on real data our method is not based on analytic expressions for the moveouts and requires no a priori knowledge of the model but instead uses geometrical ray tracing in heterogeneous media layer stripping migration and local wavefront analysis to compute residual velocity corrections these corrections are back projected into the velocity model along raypaths in a way that is similar to tomographic reconstruction while this approa

heat transfer and intraplate deformation in the central indian ocean nineteen new heat flow measurements made across deformed oceanic lithosphere in the central indian ocean and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region many of the temperature depth profiles are nonlinear upward convection of water is the most likely explanation for the curvature of the temperature profiles since other possible causes including variations in bottom water temperatures conductivity changes with depth in the sediments and experimental error can be eliminated this interpretation requires water velocities of the order of 7 x 10 sup 8 m s which is unusual because the lithosphere is relatively old 72 82 m y and a thick sedimentary cover 1 2 5 km is present these observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment water interface we infer that extra heat is being generated at shallow depths perhaps less than 35 km in the plate although the specific mechanism by which deformational energy is converted into heat is difficult to determine

proterozoic transcurrent movements along the kapuskasing lineament superior province canada and their relationship to surrounding structures abstract analysis of large scale geological and geophysical lineaments provide evidence for a new model to explain the kapuskasing structural zone ksz and james bay james bay is interpreted to have formed due to rifting related to lateral strike slip along the ksz the sutton arc is interpreted to be an extension from hudson bay arc which has been displaced by rifting of james bay magnetic anomalies which terminate at the ksz on the west have correlated magnetic anomalies on the east which are displaced left laterally it is proposed that these displacements and the rifting of james bay and the strike slip along the ksz were caused as a result of 160 km of left lateral motion between east and west superior terranes with an accompanying 5 of anticlockwise rotation of the west superior terrane additional support for this model comes from structural evidence drawn from previous reports from the last twenty six years

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

magnetic mineralogy in an archean crustal cross section implications for crustal magnetization a traverse across precambrian rocks from the kapuskasing uplift in ontario provided 31 samples from a crustal cross section that is now exposed at the surface about half the samples are dominantly paramagnetic magnetite accompanied in a few samples by minor amounts of pyrrhotite is the only significant magnetic mineral in the other half magnetic susceptibilities of these samples are constant between room temperature and about 540 c above which temperature they drop rapidly to zero magnetic susceptibility does not depend on rock type apparent original depth in the crust or degree of metamorphism the average induced magnetization of the most magnetic units in this section is less than 1 a m which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

the early opening between broken ridge and kerguelen plateau abstract reconstructions to total closure of the australia antarctic continents causes an unacceptable overlap of broken ridge and kerguelen plateau this has been partially resolved in the past by supposing that the northern part of kerguelen that is principally involved in the overlap is younger than the remainder we have revised the early reconstructions using a newly proposed breakup chronology of australia and antarctica which suggests that opening began at least 90 m y b p at an initially slow rate this eliminates the overlap problem without invoking major age differences within the kerguelen plateau we also suggest that the northeastern flank of kerguelen plateau may be underlain by the missing westward continuation of the diamantina zone it may have been isolated on the antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the ob trench

geochemical evolution and metamorphic development of the early precambrian in eastern hebei china abstract within the archaean qianxi group in eastern hebei china two roughly coeval tectonic units can be recognized the qian an granulite complex and the zunhua high grade greenstone belt both belong to the high grade terranes and have recorded almost the same history of metamorphism the granulite facies metamorphism was generally followed by a retrograde metamorphism of amphibolite facies and multistage anatexis despite the similar metamorphic histories the geochemical features of the two units are different 1 1 the protoliths of the qian an granulite complex consist of a calc alkaline volcanic series dominated by andesitic composition and a shallow water sedimentary series whereas that of the zunhua greenstone belt clearly show the character of a bimodal magmatic series 2 2 most metabasalts of the qian an complex belong to the continental basalt type and grade into andesite whereas in the zunhua belt they are characterized by a suite of high magnesian series low k2o tholeiites and a large amount of ultramafic mafic layered complexes 3 3 in terms of regional geochemistry the qian an complex is rich in potassium and potassium charnockites of autochthonous anatectic and intrusive origin are widespread in contrast the zunhua belt is rich in sodium and tonalitic gneisses of deep seated mixed origin are widespread finally a model for archaean crustal evolution of high grade terranes is suggested

strain deformation in the northern indian ocean abstract seismicity focal mechanisms and moment tensors are used to estimate the strain deformation for earthquakes which occurred during the last 80 yrs in the northern indian ocean up to 38 s the shear strain deformation values are 0 47 10 8 near the owen s fracture zone 0 26 10 8 in the north carlsberg ridge 0 51 10 8 in the south carlsberg ridge 4 92 10 8 in the chagos laccadive ridge 0 21 10 8 in the rodriguez fracture zone 0 9 10 8 in the southwest mid indian ocean ridge 0 16 10 8 in the southeast mid indian ocean ridge 0 15 10 8 in the ninety east ridge and 2 11 10 8 in the sunda arc the high shear strain obtained in the area 6 s 10 s and 65 e 72 e suggests that the earthquakes here are characterized by high creep strength 20 kbars of material in the vicinity of the earthquake epicentral regions the slip rates obtained from the seismic moment are found to vary from 0 30 to 9 85 mm yr in different parts in the northern indian ocean the maximum slip rate is obtained for the chagos laccadive ridge moment tensor values obtained in different parts of the northern indian ocean suggest that the e w extension and the n s compression are predominant vertical compression crustal thinning is observed in the western areas whereas vertical extension crustal thickening occurs in eastern parts of the northern indian ocean the tension and compression strain deformation values estimated in different parts of the northern indian ocean support the hypothesis of ocean floor spreading and continental collision of the indian and eurasian plates the principal stress directions obtained from the moment tensor of earthquakes show that the compressive stress field is acting approximately in the alignment of the ridge and that the extensional stress is perpendicular to the ridge axis vertical compressions are more prominent near the owen s fracture zone in the chagos laccadive ridge and in the southwest mid indian ocean ridge whereas vertical extension is prominent in the sunda arc and the ninety east ridge

sensitivity analysis of seismic depth migrations depth migration is the processing step that can position seismic reflectors in their proper spatial locations beneath the earth9s surface accurate seismic imaging assists in structural interpretation and determination of optimum drilling locations there are many migration algorithms to perform this depth imaging step many of these algorithms and their applications to foothills data have been summarized by lines et al 1999 as mentioned by parkes and hatton 1987 most depth migration algorithms and seismic time measurements are quite accurate but to improve the migrated image greater emphasis needs to be placed on improving the accuracy of the input velocity field in addition a greater awareness of depth imaging errors due to velocity inaccuracy is crucial parkes and hatton 1987 state that depth migration uncertainty is primarily a result of the velocity field uncertainty used to map seismic data from time to space

paleomagnetism of the hearst dike swarm and implications for the tectonic history of the kapuskasing structural zone northern ontario prominent north to northwest trending dikes herein termed the hearst swarm west of the kapuskasing structural zone ksz in northern ontario yield a paleomagnetic pole position of 48 6 n 69 0 e dm 6 9 dp 3 6 60 samples from 12 dikes using paleomagnetic petrographic and structural criteria the hearst swarm is correlated with the 2 6 ga matachewan swarm east of the ksz a positive correlation is observed between the trend and paleomagnetic declination of hearst matachewan dikes and may result from an 15 post 2 6 ga rotation of terranes across the ksz paleomagnetic and structural data from the hearst dikes are consistent with the regional westward tilting of the crust near the southern ksz as first proposed in 1983 by percival and card

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

parallel evolution of archaean low and high grade terrane a view based on relationships between the abitibi wawa and kapuskasing belts abstract the kapuskasing structural zone ksz is a region of high grade archaean gneiss transverse to the regional east west structural trends of the adjacent lower grade abitibi wawa and quetico belts the structural zone is made up of granulite and amphibolite facies paragneiss tonalitic gneiss and anorthosite suite gneiss whereas adjacent regions are metavolcanic metasedimentary belts with gneissic and plutonic components foliations in the ksz strike northeast and dip moderately northwest to account for the development of contrasting lithological structural and metamorphic characteristics in adjacent areas a unique archaean history is postulated for the ksz psammitic sedimentary rocks possibly deposited in a fault bounded basin were intruded by anorthosite which may have had its erruptive equivalents in the tholeiitic volcanic rocks in the adjacent abitibi belt during the kenoran orogeny the ksz was metamorphosed and intruded at infrastructural levels while low grade regions outside underwent sub greenschist to amphibolite metamorphism at higher levels late ductile shear in the ksz reoriented gneissic structural elements into northeast directions proterozoic fault movements juxtaposed low and high grade regions

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

inverse age stratification in the archaean crust of the superior province evidence for infra and subcrustal accretion from high resolution u pb zircon and monazite ages abstract new u pb mineral ages from the agawa migmatite terrain in the southern superior province canada together with published ages from neighbouring high and low grade terrains reveal a pattern of progressive younging of plutonic and metamorphic processes with depth and provide insights into the mechanisms controlling late archaean cratonisation the following ages are reported 1 about 2670 ma for a first zircon generation about 2660 ma for a second zircon generation and 2649 2 ma for monazite all from a leucocratic layer in migmatites 2 2666 2 ma for a first zircon generation 2663 1 5 ma for a second zircon generation and 2642 3 ma for titanite all from an amphibolite 3 2654 4 ma for monazite from a granodiorite and 4 2637 2 ma for monazite from a granite together with petrographic observations and chemical and morphological features of the zircons the ages indicate the occurrence of a protracted period of magmatism and metamorphism commencing with migmatisation at 2670 2665 ma followed by injection of calc alkalic melts and metasomatism at 2663 2660 ma and concluded by the intrusion of granites and pegmatites and extensive hydrothermal activity between 2660 and 2635 ma the timing and the style of plutonism and metamorphism in the agawa migmatite terrain contrast with the situation observed in the adjacent but tectonically higher level batchawana greenstone belt where major metamorphism occurred about 2675 ma ago and plutonism was concluded at 2668 ma ago on the other hand it resembles the evolution of the high grade portion of the kapuskasing zone where the orogenic activity continued until after 2630 2620 ma ago these age relationships are interpreted as reflecting downward vertical accretion of the archaean crust the lower crust experienced magmatism and metamorphism long after conclusion of these processes at shallower crustal levels the late tectonic regime produced granulites at depth transferring granitic material rich in h 2 o and heat producing elements to intermediate crustal levels these processes were probably accompanied by crustal thickening through the addition of mantle derived material