coseismic offsets due to the 11 april 2012 indian ocean earthquakes mw 8 6 and 8 2 derived from gps measurements 1 the 11 april 2012 earthquake mw 8 6 in the indian ocean about 100 km west off the sumatra subduction zone is the largest intraplate strike slip earthquake in the known history two hours later it triggered another great earthquake of mw 8 2 in its vicinity the earthquakes reflect the internal deformation of the diffused plate boundary between india and australia caused by the differential plate motion between them the slip occurred on conjugate planes and the presence of some of them has been reported from the swath bathymetry and satellite magnetic anomalies we estimate coseismic offsets due to these earthquakes at continuous gps sites in the andaman nicobar region and at other international gnss service igs sites around the earthquakes source region the sites on the andaman islands which are about 900 1200 km to the north of the earthquake epicenters experienced predominantly southward coseismic offset of up to 3 cm the nearest site campbell bay on great nicobar island about 500 km to the north of the earthquake documented an ese offset of about 4 cm the coseismic offsets are consistent with the finite fault slip models derived from back projection of the seismic waves recorded by the global networks

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

timing of intraplate deformation northeastern indian ocean abstract seismic stratigraphy and dsdp and odp drilling have demonstrated that two unconformities of early eocene and latest miocene age can be traced over much of the northeastern indian ocean the eocene event was a major hiatus in sedimentation following collision of india with asia and was the time of termination of deposition of a continental rise off the east side of india and the initiation of proximal bengal fan deposition the miocene event was the onset of north south compression in the indian plate during rapid uplift in tibet and the himalayas

stratigraphy and reservoir potential of glacial deposits of the itarare group carboniferous permian parana basin brazil drilling in the parana basin of brazil in the mid 1980s discovered gas and condensate in the itarare group and showed that glacial deposits in brazil can contain hydrocarbons the reservoir potential of the carboniferous permian itarare group of the basin is analyzed using new subsurface data from 20 deep wells drilled in the early to middle 1980s central to the analysis was the construction of over 3000 km of cross sections based on more than 100 wells the description of more than 400 m of core and study of 95 thin sections subsurface exploration and mapping of the itarare are greatly aided by the recognition of three recently defined and described formations and four members which are traceable for hundreds of kilometers these units belong to three major glacial cycles in which the pebbly mudstones and shales are seals and glacially related sandstones are reservoirs the best sandstone reservoirs in the deep subsurface belong to the rio segredo member the uppermost sandy unit of the itarare the rio segredo member is the best petroleum target because it is overlain by thick seals and massive pebbly mudstones and shales and because it is shallower and less compacted than underlying more deeply buried sandstones this member has little detrital matrix and much of its porosity is secondary developed by carboxylic acid and co 2 generated when jurassic cretaceous basalts sills and dikes were intruded into the parana basin as gondwana broke up

the thermal history of the bowen basin queensland australia vitrinite reflectance and clay mineralogy of late permian coal measures abstract the thermal history of the bowen basin queensland australia has been investigated using vitrinite reflectance data and clay mineralogy vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the bowen basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest middle triassic earliest late triassic as previously believed the results of the present study indicate that the development of a zone of high heat flow in the latest late triassic had a major control on the thermal history of the bowen basin high palaeogeothermal gradients estimated in the northern bowen basin are interpreted to result from convective heat transfer during a hydrothermal event variable heat distributions due to localised fracture enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures the estimated palaeogeothermal gradients in the southern bowen basin also indicate high heat flow in the lower sections of the stratigraphy sections in the southern bowen basin however are believed to reflect a rock dominated semi closed system with low water rock ratio where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction the correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite smectite i s mixed layer clays in the northern bowen basin this phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks the relationship between i s expandability and vitrinite reflectance for the bowen basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the bowen basin the effect of thin igneous intrusions on clay mineral reactions is very limited intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies this further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration in contrast to organic maturity indicators these differences between mineral and organic parameters aid in the identification of local contact metamorphic effects

regional gps data confirm high strain accumulation prior to the 2000 june 4 mw 7 8 earthquake at southeast sumatra summary site velocities derived from repeated measurements in a regional gps network in southeast asia help to constrain the motion of tectonic blocks as well as slip rates along major faults in the area using 3 d forward dislocation modelling the influence of seismic elastic loading and unloading on the measured site motions are approximated results suggest that the northwestern sunda arc is fully coupled seismogenically whereas its eastern part along java shows localized deformation higher horizontal velocity gradients than expected from the modelling of a fully coupled plate interface west of manila in the philippines suggest that deformation may be localized there assuming that geodetically derived convergence represents long term rates accumulated geodetic moments are compared to those derived using seismic data from 1977 to 2000 harvard cmt catalogue if areas displaying localized deformation are dominated by creep processes the largest difference between accumulated and seismically released deformation is located where the 2000 june 4 mw 7 8 sumatra earthquake occurred

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

distribution of shortening between the indian and australian plates in the central indian ocean we analyze a single channel seismic scs reflection profile that completely crosses the zone of deformed oceanic lithosphere in the central indian ocean at 78 8 e by summing the apparent shortening on all seismically resolvable faults throws 10 m we find that 11 2 2 km of shortening has occurred at this longitude during the past 7 5 m y this estimate together with the 27 f 5 km of shortening previously estimated from a multichannel seismic mcs profile farther east at 81 5 e are consistent with the west to east increase in shortening predicted by euler poles which treat the indian and australian plates as separate tectonic units our result therefore provides direct evidence from the deformation itself that the compression of oceanic lithosphere in the central indian ocean originally regarded as intraplate is better described as constituting part of a broad boundary zone between distinct indian and australian plates we also examine the size statistics of faults revealed in scs and mcs profiles running nearly normal to the deformation trends in the longitude band 78 8 e 81 5 e the n s extent of the deformation does not change appreciably over these longitudes we find that the average fault spacing remains constant at about 7 km whereas the mean throw increases systematically from west to east ff 74 m to n 177 m basically the contribution of small faults those with throws of lo 50 m decreases systematically across the deforming region i e with increasing amount of shortening this suggests that the deformation occurs by reactivation of a select fault population that these faults continue to add displacement with time and that relatively few new faults are initiated we also infer from the fault size statistics that the contribution to the deformation of faults below the resolution of the seismic methods 10 m for scs and 50 m for mcs is likely to be quite limited

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

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

diagenesis and organic maturation of sedimentary rocks under volcanic strata oregon 1 in the pacific northwest diagenesis of cretaceous strata mantled by extrusive volcanic rocks occurred at temperatures of 150 200 degrees c due to heat conducted downward from a perched confined geothermal aquifer this aquifer produced a homogeneous temperature field within the underlying sedimentary rock authigenic minerals such as sphene chlorite illite and quartz developed throughout a vertical section composed of andesites basalts and both silicic and mafic sedimentary rocks the geologically rapid heating also resulted in nearly constant organic maturation profiles within the sedimentary rock and produced unknown quantities of hydrocarbons the contemporaneous diagenesis and maturation took place before extrusion of the columbia river basalt group and was p rhaps associated with earlier intrusive activity such hydrothermal maturation accounts for both local diagenesis and other organic maturation anomalies in the pacific northwest the mechanism has ramifications for the petroleum potential of geologically active basins because it provides a means by which volcanism and associated geothermal activity can increase maturation and generate hydrocarbons

high temperature hydrothermal origin for fractured carbonate reservoirs in the blackburn oil field nevada geologic note 1 preliminary petrographic and fluid inclusion studies of drill cuttings and core from the blackburn oil field reveal that many of the oil saturated fractures veinlets and breccias in the dolomite reservoir rock may be of high temperature hydrothermal origin textures of the earliest formed fracture veinlet networks and breccias suggest development by explosive hydrothermal fracturing the veinlets themselves preserve evidence that high temperature fluids favorable for such natural hydrothermal rock rupture once circulated in these rocks primary fluid inclusion homogenization temperatures indicate that early vein dolomite precipitated from hydrothermal brines at temperatures exceeding 350 degrees c 662 degrees f younger quartz sphalerite galena veinlets developed from dilute aqueous solutions at temperature of more than 225 degrees c 437 degrees f the high paleotemperatures recorded by early vein dolomite in the now much cooler about 120 degrees c or 248 degrees f blackburn reservoir rock imply an ancient magmatic heat source in this case almost certainly a small granodiorite stock tertiary adjoining and possibly underlying the field other carbonate hosted petroleum reservoirs in eastern nevada are also intimately associated with felsic intrusives we propose that igneous related high temperature hydrothermal fracturing could have created reservoir porosity and permeability at some or all of these fields and that future petroleum exploration in the basin and range might profitably incorporate the search for concealed plutons

en echelon and orthogonal fault ruptures of the 11 april 2012 great intraplate earthquakes the magnitude 8 7 earthquake that occurred off the coast of the sumatra subduction zone on 11 april 2012 is shown to have had an extraordinarily complex four fault rupture these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates on 11 april 2012 two of the largest strike slip earthquakes ever recorded at magnitudes of 8 7 and 8 2 occurred in the northeastern indian ocean a few hundred kilometres off the coast of sumatra three groups now report the analysis of seismic data from the days and months before and after these events as well as the events themselves matthias delescluse and co authors show that these earthquakes are part of the ongoing boost of intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes they conclude that the australian plate driven by slab pull forces at the sunda trench is gradually detaching from the indian plate han yue and colleagues show that the 11 april event involved a complex four fault rupture lasting several minutes followed two hours later by a magnitude 8 2 aftershock these great ruptures on a lattice of strike slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the indian and australian plates fred pollitz and colleagues show that in the six days following 11 april the global rate of remote earthquakes with magnitudes greater than 5 5 increased nearly fivefold and events up to magnitude 7 seem to have been triggered the unprecedented delayed triggering power of this earthquake may arise from its strike slip source geometry or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure the indo australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary from an environment of continental collision to an island arc subduction zone1 2 on 11 april 2012 one of the largest strike slip earthquakes ever recorded seismic moment magnitude mw 8 7 occurred about 100 200 kilometres southwest of the sumatra subduction zone occurrence of great intraplate strike slip faulting located seaward of a subduction zone is unusual it results from northwest southeast compression within the plate caused by the india eurasia continental collision to the northwest together with northeast southwest extension associated with slab pull stresses as the plate underthrusts sumatra to the northeast here we use seismic wave analyses to reveal that the 11 april 2012 event had an extraordinarily complex four fault rupture lasting about 160 seconds and was followed approximately two hours later by a great mw 8 2 aftershock the mainshock rupture initially expanded bilaterally with large slip 20 30 metres on a right lateral strike slip fault trending west northwest to east southeast wnw ese and then bilateral rupture was triggered on an orthogonal left lateral strike slip fault trending north northeast to south southwest nne ssw that crosses the first fault this was followed by westward rupture on a second wnw ese strike slip fault offset about 150 kilometres towards the southwest from the first fault finally rupture was triggered on another en echelon wnw ese fault about 330 kilometres west of the epicentre crossing the ninetyeast ridge the great aftershock with an epicentre located 185 kilometres to the ssw of the mainshock epicentre ruptured bilaterally on a nne ssw fault the complex faulting limits our resolution of the slip distribution these great ruptures on a lattice of strike slip faults that extend through the crust and a further 30 40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates

location of the africa australia india triple junction and motion between the australian and indian plates results from an aeromagnetic investigation of the central indian and carlsberg ridges accepted 1994 june 7 received 1994 may 23 in original form 1993 may 24 summary prior studies have proposed and examined the hypothesis that india and australia are separate rigid plates separated by a wide near equatorial e w striking plate boundary attempts to place narrow limits on the location of the africa australiaindia triple junction have been hindered however by the lack of useful magnetic profiles crossing the eastern carlsberg ridge and northern central indian ridge herein we present near ridge portions of new profiles from an aeromagnetic survey of the carlsberg ridge east of 66 e and of the central indian ridge north of 19 s these new data are used to estimate 35 new spreading rates averaged from the middle of chron 2a 3 03 ma to the present all other plate motion data along the central indian and carlsberg ridges are also analysed to investigate the present kinematics of the indian ocean especially the motion and boundary between the indian and australian plates unlike prior efforts we objectively estimate uncertainties in the strikes of transform faults along the carlsberg and central indian ridges

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

thermal history of the permian formations from the breccia pipes area grand canyon region arizona the thermal history of the permian formations from the breccia pipes area in the colorado plateau has been assessed using different methods burial reconstruction indicates maximum temperatures between 100 and 140 c for a 30 c km geothermal gradient a temperature of 140 5 c integrated over 10 to 50 ma has been deduced from fission track analysis this range of temperatures is consistent with trapping temperatures obtained by fluid inclusions study 75 140 c fission track and burial hypotheses have been introduced in a kinetic model for organic matter maturation the computed tmax average 445 and 421 c respectively a comparison with organic geochemistry data from the permian toroweap formation is carried out and demonstrates that the two above hypotheses constitute a thermal framework for the organic matter evolution main conclusions referring to beginning of oil window and regional uplift ages laramide orogeny as well as maximum temperature ranges are deduced from this study

interpretation of discontinuous vitrinite reflectance profiles time temperature modeling was used to examine the effects of reburial on vitrinite reflectance profiles associated with stratigraphic uncomformities offsets in reflectance data for shallow unconformities can in many cases be explained by erosion and may provide a minimum estimate of the amount of missing section as reburial progresses the offset in reflectance values at the unconformity decreases in accordance with the thermal energy required to activate reaction pathways in strata above and below the unconformity until no statistically significant difference exists in vitrinite reflectance across the stratigraphic boundary the reflectance profile is thus annealed when observed at great depths significant offsets in reflectance values typically require geologic lly unreasonable amounts of erosion alternate explanations for offsets at great burial depths include igneous and metamorphic events and complications associated with editing and interpreting vitrinite reflectance data

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

the tectonic stress field evolution of india since the oligocene abstract a multitude of observations suggest neotectonic deformation in and around india but its causes and history are unknown we use a 2 dimensional finite element model with heterogeneous elastic strengths in continental regions to model the regional stress field orientation and relative magnitudes since the oligocene the large scale geological structure of india is embedded in our model by using published outlines of cratons fold belts and basins associated with estimates of their relative strengths enabling the modelling of stress field deflections along interfaces between relatively strong and weak tectonic elements through time at 33 ma a roughly nnw sse oriented band of relatively high maximum horizontal compressive stress s hmax straddled india s west coast while india s east coast and the adjacent wharton basin were characterized by relatively low intraplate stresses between 20 ma and the present growing collisional boundary forces combined with maturing mid ocean ridge flanks result in the establishment of an arcuate belt with anomalously high intraplate stress that stretches from india to the wharton basin intersecting the continental shelf roughly orthogonally and crossing the 85 east and ninetyeast ridges this results in a compressive tectonic regime favouring folding and inversion northeast of the godavari graben on india s east coast as observed in seismic reflection data whereas no tectonic reactivation is observed on the continental margin further north closer to the mahanadi graben or further south our stress models account for these differences via spatial variations in modelled horizontal stress magnitudes and intersection angles between margin paralleling pre existing basement structures and the evolving neogene stress field the models further account for fracture zone strike slip reactivation offshore sumatra and lithospheric folding along india s west and southeast coast and can be used to estimate the onset of these deformation episodes to at least the oligocene and miocene respectively

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

strain rate dependency of oceanic intraplate earthquake b values at extremely low strain rates we discovered a clear positive dependence of oceanic intraplate earthquake oceq b values on the age of the oceanic lithosphere oceq b values in the youngest 20 ma oceanic lithosphere exceed 1 5 which is significantly higher than the average worldwide earthquake b value around 1 0 on the other hand the b value of intraplate earthquakes in the ninety east sumatra orogen where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere is 0 93 which is significantly lower than the oceq b value about 1 9 with the same age 50 110 ma thus the variation in b values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure we revealed a negative strain rate dependency of the b value at extremely low strain rates 1 5 in oceanic lithosphere 20 ma old imply that future improvement in seismic observation will capture many smaller magnitude oceqs which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics

kinematic constraints on distributed lithospheric deformation in the equatorial indian ocean from present motion between the australian and indian plates from an expanded accurate and up to date data set comprising 110 spreading rates 46 transform azimuths and 151 earthquake slip vectors from the indian ocean and gulf of aden we determine a new rigid plate model describing the motion since 3 ma between india and australia the euler vector 0 313 m y about 5 s 78 e lies near the middle of the equatorial diffuse plate boundary dividing the indian from the australian plate and predicts a rate of north south shortening along 85 e of 4 3 mm yr only 30 as fast as predicted by our prior model the new model also predicts north south extension of 6 2 mm yr at 68 e along the western segment of the diffuse plate boundary where our prior model predicted north south contraction using data only along the carlsberg and central indian ridges and no other plate boundaries we show that plate motion data cannot be fit by a single euler vector however the data are well fit by two euler vectors when an east west striking india australia plate boundary is assumed to intersect the central indian ridge near the equator the best location along the central indian ridge for this triple junction is 6 s 3 s with 95 confidence limits of 9 s 4 n just west of a region of intense seismicity the sense of deformation as recorded in earthquake focal mechanisms reverse faults mapped with reflection seismic data and undulations in basement topography surface gravity and the geoid agrees well with the north south extension predicted in the western part and the north south shortening predicted in the eastern part of the india australia boundary the predicted rate of north south shortening between 79 e and 86 e is consistent with the rate of shortening inferred from observed faulting and folding shortening taken up by faulting is 6 to 100 times greater than that taken up by the spectacular basement folds little of the rapid north south shortening predicted east of 86 e is taken up by crustal thickening instead lithosphere is transported northeastward toward the sumatra trench through strike slip faulting and possible clockwise rotation the failure of the convergent segment of the diffuse plate boundary to form a subducting trench provides some new observations that must be satisfied by models for the initiation of subduction a model of separate rigid indian and australian plates divided by a diffuse plate boundary appears to be valid and useful because the predictions of the model agree with independent data and because the predicted velocity of relative motion is statistically significant and comparable with that across other plate boundaries

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

seismic reflection images of intraplate deformation central indian ocean and their tectonic significance multichannel seismic reflection profiles collected from the intraplate deformation area in the central indian ocean basin are used to describe brittle structures produced under compressive stress reflectors within oceanic basement are divided into four types north dipping and south dipping reverse faults that are interpreted as reactivations of structures formed at the spreading centre as outward and inward dipping faults respectively lower angle north dipping reflectors that probably represent new faults or faults just initiating and sub horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts within the sedimentary cover upwards fault propagation shows the faults steepening from c 40 just above basement to near vertical and is preceded by sediment folding a fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure measurements of north south crustal shortening indicate a shortening rate of 2 5 0 9 mm a 1 which is at the lower end of predictions from plate motions but significant enough to recognize this area as a diffuse plate boundary the formation of long wavelength basement undulations and the reactivation of fracture zones and ridge parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area there is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 ma

diagenesis and reservoir potential of volcanogenic sandstones cretaceous of the surat basin australia abstract the mesozoic surat basin is an important hydrocarbon producing basin in australia the principal productive reservoirs there are the quartzose sandstones of early jurassic age although oil and gas shows have been reported from the cretaceous part of the succession it does not contain any known economic hydrocarbons the sandstones of the cretaceous section are characterized by abundant volcanogenic detritus in the form of rock fragments and feldspars these compositionally immature sandstones are not regarded as favourable exploration targets for hydrocarbons because of their lithic nature their shallow burial depths and hence the low thermal maturity of the intercalated mudrocks which might have constituted hydrocarbon source rocks however petrographic and petrophysical examinations show that significant primary and diagenetic secondary dissolution porosity and permeability exist in some of these stratigraphic units which under certain circumstances could be the host for hydrocarbons and may become the future exploration targets shallow burial depth and early interstitial precipitation of various authigenic minerals have retarded compaction and thereby contributed to the preservation of moderate to good primary porosity additionally flushing by co 2 charged meteoric water after the inception of the great artesian basin in the tertiary has created significant amounts of secondary dissolution porosity in these sandstones

motion between the indian capricorn and somalian plates since 20 ma implications for the timing and magnitude of distributed lithospheric deformation in the equatorial indian ocean summary approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the carlsberg ridge and central indian ridge are used to estimate the rotations of the indian and capricorn plates relative to the somalian plate for 20 distinct points in time since 20 ma the data are further used to place limits on the locations of the northern edge of the rigid capricorn plate and of the southern edge of the rigid indian plate along the central indian ridge data south of and including fracture zone n the fracture zone immediately south of the vema fracture zone which intersects the central indian ridge near 10 s are well fit assuming rigid capricorn and somalian plates while data north of fracture zone n are not in agreement with prior results data north of fracture zone h which intersects the central indian ridge near 3 2 s are well fit assuming rigid indian and somalian plates while data south of and including fracture zone h are not resulting in a smaller rigid indian plate and a wider diffuse oceanic plate boundary than found before the data are consistent with capricorn somalia motion about a fixed pole since 8 ma but require rotation about a pole 15 farther away from the central indian ridge from 20 to 8 ma the post 8 ma pole also indicates capricorn somalia displacement directions that are 7 clockwise of those indicated by the pre 8 ma stage pole in contrast india somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 ma india somalia motion has changed little during the past 20 myr nonetheless astronomically calibrated ages for reversals younger than 12 9 ma allow resolution of the following small but significant changes in spreading rate india somalia spreading slowed from 31 to 28 mm yr 1 near 7 9 ma and later sped up to 31 mm yr 1 near 3 6 ma capricorn somalia spreading slowed from 40 to 36 mm yr 1 near 11 0 ma later sped up to 38 mm yr 1 near 5 1 ma and further sped up to 40 mm yr 1 near 2 6 ma the motion between the indian and capricorn plates is estimated by differencing india somalia and capricorn somalia rotations which differ significantly for all 20 pairs of reconstructions india has rotated relative to the capricorn plate since at least 20 ma if about a pole located near 4 s 75 e the rate of rotation was slow 0 11 0 01 myr 1 95 per cent confidence limits from 20 to 8 ma but increased to 0 28 0 01 myr 1 95 per cent confidence limits at 8 ma the onset of more rapid rotation coincides within uncertainty with the inferred onset at 7 8 ma of widespread thrust faulting in the central indian basin and with the hypothesized attainment of maximum elevation and initiation of collapse of the tibetan plateau at 8 ma the plate kinematic data are consistent with steady india capricorn motion since 8 ma and provide no evidence for previously hypothesized episodic motions during that interval the convergence since 8 ma between the indian and capricorn plates significantly exceeds by 13 to 20 km the convergence estimated from three north south marine seismic profiles in the central indian basin where and how the additional convergence was accommodated is unclear

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

localized rifting at chagos bank in the india capricorn plate boundary zone chagos bank is a region of thick crust in the central indian ocean where a concentration of seismicity indicates that extension is ongoing we have modeled bathymetry and gravity data to show that the crust mantle boundary shallows near a seabed trough consistent with a total extension of 15 km over a zone 50 km broad this estimate is consistent with 5 10 m y of deformation at the present rate of relative india capricorn motion the 15 km of extension is toward the lower limit of the total extension expected from magnetic anomaly reconstructions but would be accumulated in 1 3 m y in a regime having the present day seismicity we attribute the localization of seismicity beneath chagos bank to a weak rheology caused by its thick crust

reconciling plate kinematic and seismic estimates of lithospheric convergence in the central indian ocean the far field signature of the india asia collision and history of uplift in tibet are recorded by sediment input into the indian ocean and the strain accumulation history across the diffuse plate boundary between the indian and capricorn plates we describe the history of india capricorn convergence from updated estimates of india somalia capricorn plate rotations and observations derived from seismic reflection data new india capricorn plate rotations for the past 20 m y are consistent with slow north south convergence from 18 ma about a stationary or nearly stationary pole near the eastern edge of the chagos laccadive ridge simpler than predicted by previous models based on many fewer data the new rotations suggest that convergence began between 18 and 14 ma consistent with marine seismic evidence for an onset of deformation at 15 4 13 9 ma they further show that convergence rates doubled at 8 ma in agreement with a sharp increase in fault activity at 8 7 5 ma seen on seismic reflection profiles a discrepancy between the total strain estimated from kinematic and seismic reflection data can be reconciled if pervasive reverse faulting within the diffuse plate boundary is accompanied by block rotations of 1 3

hydrocarbon exploration in western oregon and washington continuing development of the mist gas field and stepout discovery wells affirm the hydrocarbon prospectivity of western oregon and washington understanding the factors controlling the mist gas field accumulation helps define an exploration strategy for the pacific northwest reservoir sandstones in the mist area are in the cowlitz formation of middle to late eocene age reservoir quality sandstones have average porosities of 25 and average permeabilities of 200 md the reservoir sands are well sorted feldspathic quartzose sandstones and are less susceptible to diagenetically formed pore filling authigenic minerals than are the more lithic sandstones of other horizons and less well sorted depositional environments potential hydrocarbon source rocks consist of marine shale to coaly facies organic matter is predominately terrestrially derived mist gas field pools are small and have variable gas types suggesting to some workers that the gas is generated from rocks immediately adjacent to the reservoir gas wetness and dgr13c values indicate that gas from the bruer flora and newton pools is probably thermally generated shales encasing the mist gas field sandstone reservoirs are thermally immature having vitrinite reflectance values less than 0 4 thermal gas most likely would have been generated downdip within nearby depocenters and migrated into the reservoir the integration of paleogeographic models for mineralogic provenance well sorted sand accumulation and thermal maturation within cenozoic depocenters provides an exploration strategy for defining areas of highest hydrocarbon potential in western oregon and washington each of the many eocene formations in oregon and washington with relatively lithic free feldspathic quartzose mineralogy should be evaluated as a potential target

relating rapid plate motion variations to plate boundary forces in global coupled models of the mantle lithosphere system effects of topography and friction abstract recent high resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short term variations in global plate velocities over the past 10 myrs such variations serve as powerful probe into the nature and magnitude of plate boundary forces because they are unlikely to originate from changes in mantle buoyancy forces which evolve on longer time scales here we explore the constraints of the velocity record using a novel coupled modeling approach of global neo tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models mcms to arrive at simple global budgets of mantle lithosphere and plate boundary forces we focus on three plate boundary systems along the nazca south america plate margin the aleutian trench and the india australia plate boundary to show that gravitational spreading from high topography in the andes and tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35 of recent velocity changes over the earth s surface including among others the observed 30 convergence reduction between the nazca south america plates our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench parallel gravity and bathymetry anomalies and the occurrence of large earthquakes as well as differences by as much as a factor of five from margin to margin they also support the notion of a relatively young plate boundary separating the india and australia plates which has been previously suggested based on independent observations importantly we find that the modeled nazca south america convergence reduction explains recent spreading rate variations in the south atlantic and south pacific which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins our numerical results demonstrate a that detailed budgets of forces acting upon plates can be obtained and b support the notion of strong forcing along weak plate boundaries

magma production rate along the ninetyeast ridge and its relationship to indian plate motion and kerguelen hot spot activity the ninetyeast ridge a linear trace of the kerguelen hot spot in the indian ocean was emplaced on a rapidly drifting indian plate magma production rates along the ridge track are computed using gravity derived excess crustal thickness data the production rates change between 2 and 15 m3 s over timescales of 3 16 myr major variations in magma production rates are primarily associated with significant changes in the indian plate velocity with low production phases linked to high plate velocity periods the lowest magma production rate 2 m3 s at 62 ma is associated with the rapid northward drift of indian plate under the influence of the reunion mantle plume the contemporaneous slowing of the african plate coincides with increase in magma production rate along the walvis ridge in the atlantic ocean the present study suggests that variations in the indian plate motion and frequent ridge jumps have a major role in controlling the magma production particularly on long period cycles 16 myr short period variations 5 myr in magma productions may be associated with intrinsic changes in the plume possibly due to the presence of solitary waves in the plume conduit

geochemical reconnaissance of cretaceous inliers in north central oregon cretaceous inliers in north central oregon represent isolated exposures of potential source rocks in the region interpreted neritic to upper bathyal mudstones of the hudspeth formation represent late deposition within the mesozoic fore arc terrane accretionary terranes that are represented by the blue mountain province are in part inundated by voluminous tertiary volcanic sequences most notably the columbia river group both pyrolysis and solvent extraction source rock data from the hudspeth formation present in local cretaceous inliers support a mature oil prone mixed marine source rock environment a trend reflected in these data indicates that the western part of the study area is more mature if these same strata and conditions prevail beneath the volcanic cover the potential for the presence of paraffinic oil accumulations exists near surface soil gas investigations have shown the occurrence of light hydrocarbon microseep gases which cluster in the vicinity of the mitchell inlier the soil gas signal received in the near surface far removed from the inliers is identical to the soil gas signature measured on the cretaceous sediments it is suggested that the application of reconnaissance surface geochemical methods may offer additional insight in defining potentially favorable source rocks peripheral to the columbia river plateau as well as projecting their presence into the interior of the plateau

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

shallow subsurface morphotectonics of the nw sumatra subduction system using an integrated seismic imaging technique in subduction zones shallow subsurface structures are the manifestation of the plate interactions at depth however significant water depths rough bathymetry and presence of heavily deformed accretionary wedge materials hamper imaging of the near surface features to a great extent using conventional imaging techniques in this study we show results using an integrated processing technique to a multichannel seismic data set acquired in 2006 from the northwestern offshore sumatra we start with first downward continuing the 12 km long surface streamer data to the seafloor followed by a high resolution traveltime tomography of refracted phases to determine a detailed velocity depth model of subsurface which in turns is used for pre stack depth migration in order to delineate the shallow subsurface structures beneath the trench subduction front and outer accretionary wedge our velocity depth model and the depth migrated image depict variation of sediment properties across the front and structures of uppermost sedimentary sequence with an unprecedented high resolution providing the precise location of the frontal and conjugate thrusts highly folded sedimentary sequences which in turns describe their relationship with the top of the subducting plate and factors that control rupture propagation to the trench furthermore we estimate the porosity distribution across the front where we find a 12 and 18 per cent decrease in porosity beneath the deformation front and the inner accretionary plateau at 500 m below the seafloor respectively which we interpret to be due to the compaction a significant decrease in porosity at the plate interface below 5 6 km thick sediments near the deformation front would increase the coupling leading to the rupture propagation up to the trench uplifting 4 5 km water and producing large tsunami

indian ocean floor deformation induced by the reunion plume rather than the tibetan plateau the central indian ocean is considered the archetypal diffuse oceanic plate boundary data from seismic stratigraphy and deep sea drilling indicate that the contractional deformation of the indian ocean lithosphere commenced at 15 4 13 9 ma but experienced a sharp increase at 8 7 5 ma this has been maintained through to the present day with over 80 of the shortening accrued over the past 8 myr here we build on previous efforts to refine the form timing and magnitude of the regional plate motion changes by mitigating the noise in reconstructed indian and capricorn plate motions relative to somalia our noise mitigated reconstructions tightly constrain the significant speed up of the capricorn plate over the past 8 myr and demonstrate that the history of the indian ocean floor deformation cannot be explained without this plate motion change we propose that the capricorn plate motion change is driven by an increase in the eastward directed asthenospheric flow associated with the adjacent reunion plume and quantitatively demonstrate the viability of this hypothesis our inference is supported by volcanic age distributions along the reunion hotspot track the anomalously high residual bathymetry of the central indian ridge full waveform seismic tomography of the underlying asthenosphere and geochemical observations from the central indian ridge these findings challenge the commonly accepted link between the deformation of the indian ocean floor and the tibetan plateau s orogenic evolution and demonstrate that temporal variations in upwelling mantle flow can drive major tectonic events at the earth s surface deformation of the indian ocean floor over the past 8 million years was caused by a change in plate motions linked to flow of the reunion mantle plume according to analyses of forces upon plates

the dynamics of plate tectonics and mantle flow from local to global scales plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries but observationally constrained high resolution models of global mantle flow remain a computational challenge we capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions with individual plate margins resolved down to a scale of 1 kilometer back arc extension and slab rollback are emergent consequences of slab descent in the upper mantle cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high viscosity slabs altering the velocity of oceanic plates viscous dissipation within the bending lithosphere at trenches amounts to 5 to 20 of the total dissipation through the entire lithosphere and mantle

a complete structural model and kinematic history for distributed deformation in the wharton basin abstract the equatorial eastern indian ocean hosts a diffuse plate boundary where widespread deformation accommodates the relative motion between the indian australian and capricorn sub plates we integrate iodp expedition 362 borehole data which for the first time provides an accurate ground truthed chronostratigraphy of the sedimentary sequence east of the ninety east ridge ner with 2d seismic reflection profiles and multibeam bathymetry to assess the styles of faulting between the ner and the sunda subduction zone timing of activity and comparison with physical and rheological properties we identify four distinct fault sets east of the ner in the northern wharton basin n s 350 010 orientated faults associated with the n s fracture zones formed at the now extinct wharton spreading centre are still active and have been continuously active since at least 10 ma nne and wnw trending fault fabrics develop between the fracture zones the orientations and likely sense of displacement on these three sets of faults defines a riedel shear system responding to nne ssw left lateral strike slip activity at depth demonstrated by the recent 2012 great intraplate earthquakes we also find evidence of ne sw reverse faults similar in style to e w reverse faults observed west of the ner where reverse faulting is more dominant we show that the activity of this strike slip system increased ca 7 9 ma contemporaneous with reverse faulting and intraplate deformation west of the ner

structure and kinematics of the indo burmese wedge recent and fast growth of the outer wedge 1 the northern sunda subduction zone offshore burma and the associated indo burmese wedge mark the active eastern boundary of the burma platelet jammed between the india plate and the sunda plate the aim of this paper is to provide a structural and kinematics analysis of the indo burmese wedge based on seismic reflection geodetic and geological field data we found out that the indo burmese wedge is the place of diffuse strain partitioning right lateral shearing in the innermost part and e w shortening in the outermost part in the outer wedge thick skinned deformation overprints thin skinned deformation it can be explained by the required preservation of the critical taper after the fast westward propagation of the outer wedge above a very efficient clayey decollement layer the thick skinned deformation is here characterized by major right lateral strike slip faults the kaladan fault and the chittagong coastal fault and yet a typical internal deformation of the indo burmese wedge we suggest that the westward migration of internal right lateral shear deformation allows preservation of the strain partitioning ratios between internal n s trending right lateral shearing and external e w shortening some seismic lines show that the outer wedge deformation is not older than 2 ma its propagation could have been enhanced by the large amount of sediments filling up the sylhet flexural basin formed after the pliocene tectonic uplift of the shillong plateau

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

use of biological marker distributions to study thermal history of the permian kupferschiefer of the lower rhine basin in the central european zechstein basin the permian kupferschiefer has been deposited under marine anoxic conditions from a lagoon at the southwest border of the zechstein sea today part of the lower rhine basin 30 core samples have been studied by means of organic geochemical methods within the area investigated the depth of the kupferschiefer horizon increases from about 350 m in the south to about 1000 m in the north furthermore the kupferschiefer has been thermally affected in the western part by the intrusive body of the so called krefeld high this geological situation opens the possibility to investigate differences of short term and long term temperature effects on the biological marker distribution within a sediment layer of a largely uniform facies type depth related changes in the hopane sterane and diasterene diasterane distributions are compared to those changes induced by the krefeld high the composition of aromatic steroid hydrocarbons is used to discuss time temperature effects it is shown that depth related values of maturation parameters can only be interpreted by taking into account post depositional tectonic events samples from tectonic horst structures show higher maturation values than one would expect from the present day depth

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

monsoon speeds up indian plate motion article i nfo short term plate motion variations on the order of a few myr are a powerful probe into the nature of plate boundary forces as mantle related buoyancies evolve on longer time scales new reconstructions of the ocean floor spreading record reveal an increasing number of such variations but the dynamic mechanisms producing them are still unclear here we show quantitatively that climate changes may impact the short term evolution of plate motion by linking explicitly the observed counter clockwise rotation of the indian plate since 10 ma to increased erosion and reduced elevation along the eastern himalayas due to temporal variations in monsoon intensity by assimilating observations into empirical relations for the competing contributions of erosion and mountain building we estimate the first order decrease in elevation along the eastern himalayas since initial strengthening of the monsoon furthermore we show with global geodynamic models of the coupled mantle lithosphere system that the inferred reduction in elevation is consistent with the indian plate motion record over the same period of time and that lowered gravitational potential energy in the eastern himalayas following stronger erosion is a key factor to foster plate convergence in this region our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the euler pole falls within its associated plate

the columbia river basalts between 17 million and 6 million years ago 200 000 square kilometers of the american northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long larger individual eruptions covered over a third of the columbia plateau in a few days the lavas represent partial melts of the earth s mantle that were only slightly modified by near surface upper crustal processes the abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earth s crust

thermal alteration of cretaceous black shale by diabase intrusions in the eastern atlantic ii effects on bitumen and kerogen abstract the thermal effects of three one major and two minor miocene diabase intrusions on cretaceous black shales from dsdp site 41 368 have been analyzed a concentration gradient was observed especially for the hydrocarbons decreasing towards the major intrusion and between the three sills the thermally altered samples in the proximity of and between the sills contained elemental sulfur and an excess of thermally derived pristane over phytane whereas the unaltered sediments contained no elemental sulfur and more phytane than pristane a maximum yield of the extractable hydrocarbons was observed at a depth of 7 m below the major sill two classes of molecular markers were present in this bitumen suite the first was sesqui di and triterpenoids and steranes which could be correlated with both terrigenous and autochthonous sources they were geologically mature and showed no significant changes due to the thermal stress the second class was found in the altered samples which contained only polynuclear aromatic hydrocarbons with low alkyl substitution and sulfur and oxygen heterocyclic aromatic compounds these compounds were derived from pyrolytic reactions during the thermal event kerogen was isolated from all of these samples but only traces of humic substances were present the h c n c 13 c 34 s and d all exhibit the expected effects of thermal stress the kerogen becomes more aromatized and richer in 13c 34s and d in the proximity of and between the sills maturation trends were also measured by the vitrinite reflectance and electron spin resonance where the thermal stress could be correlated with an elevated country rock temperature and an increased degree of aromaticity the effects of in situ thermal stress on the organic rich shales resulted in the generation and expulsion of petroliferous material from the vicinity of the sills

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

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

synrift magmatism in a cenozoic rift basin from 3d seismic data wichianburi sub basin phetchabun basin thailand part 1 distribution and timing of intrusions and forced folds the late oligocene pliocene wichianburi sub basin phetchabun rift thailand has undergone three phases of development late oligocene early miocene c 16 ma east west extension on predominantly nnw sse to north south striking faults 16 ma c 3 ma ne extension on nnw sse north south and ne sw trending faults and 3 ma to present post rift subsidence the basin was also affected by intrusive and extrusive igneous activity from 24 to 2 ma the relationships among the structure rift basin stratigraphy and sheet intrusions are unusually well documented as a result of extensive hydrocarbon exploration drilling and 3d seismic data together with the production of hydrocarbons from igneous sills the oldest igneous activity c 24 13 ma lies sw of the wichianburi sub basin where extrusive rocks exposed at the surface have been dated by k ar and 40ar 39ar methods the overall igneous activity youngs from the south of the basin to the north 11 9 ma numerous domal folds occur across the basin the sill and laccolith dimensions the rate of fold development the variable timing of fold formation and the fold geometry all indicate that the folds are related to magma emplacement not tectonic inversion fold formation occurred at c 16 15 and 12 11 ma in single or multiple stages the timing of folding coupled with the radiometric dating of some sills k ar 39ar 40ar indicates that the greatest amount of intrusive activity occurred between 16 and 11 ma the 16 ma date coincides with the switch from east west to nw se extension which promoted dilatancy within the upper crust along a range of synrift fault orientations and some pre rift trends the burial history of the main depocentre shows that miocene sill emplacement occurred at depths

numerical simulation of present day tectonic stress across the indian subcontinent in situ measurements of maximum horizontal stress shmax in the indian subcontinent are limited and do not present regional trends of intraplate stress orientation the observed orientations of shmax vary considerably and often differ from the plate velocity direction we have simulated orientation and magnitude of shmax through finite element modeling incorporating heterogeneities in elastic property of the indian continent and plain stress approximation to understand the variability of shmax four different scenarios are tested in simulation 1 homogeneous plate with fixed plate boundary 2 homogeneous plate with boundary forces 3 heterogeneous plate with fixed boundary 4 heterogeneous plate with boundary forces the estimated orientation and magnitude of shmax with a heterogeneous plate with boundary forces in the himalayan region and an eastern plate boundary comprising the indo burmese arc and andaman subduction zone are consistent with measured maximum horizontal stress this study suggests that plate boundary force varies along the northern indian plate margin and also provides a constraint on the intraplate stress field in the indian subcontinent

intraplate shortening in the central indian ocean determined from a 2100 km long north south deep seismic reflection profile although intraplate deformation is commonly observed on the continents there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary the central indian ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere reflecting an exceptionally high stress level however the strain pattern derived from indirect and incomplete observations remains poorly known a more complete data set was obtained during the phedre cruise 1991 by means of deep seismic reflection profiling one 2100 km long profile runs along the 81 e meridian from lat 14 s to the coast of sri lanka and provides us with the first long cross section through the region of observed crustal level faults fault analysis offers new estimates of the amount of shortening which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry the contribution of the seismically resolvable faults amounts to 22 37 km of shortening or 2 5 4 3 this is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles

current global plate kinematics from gps 1995 2007 with the plate consistent reference frame 1 we present the vectors of rotation of 10 major lithospheric plates estimated from continuous gps observations at 192 globally distributed stations 71 stations were selected as representing stable plate regions all days for the period 1995 0 2007 0 were included in the analysis in contrast to previous gps plate models our model is independent of international terrestrial reference frames itrf the origin of our plate consistent reference frame is the center of plate rotation cp rather than the center of mass of the entire earth s system cm as in recent versions of itrf we estimate plate rotations and cp by minimizing the misfit between the horizontal velocities predicted by the plate model and the observed gps velocities if any version of itrf is used as the reference frame the drift of the itrf origin relative to cp cannot be neglected in estimation of plate rotation vectors and plate residual station velocities the model of the plate kinematics presented here addresses the problem debated since the beginning of the space geodesy how big are disagreements between the current plate motions and the motions averaged over several million years we compare the vectors of relative plate rotations estimated here with the published vectors from gps and geologic models we also discuss the integrity of individual plates as exhibited by plate residual station velocities for seven largest plates the rms value of plate residual station velocities in stable plate interiors is 0 5 0 9 mm a this value is an upper bound on deviation of real plates from infinite stiffness

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

an integrated geophysical study east of the southern chagos laccadive ridge complex central indian ocean basin implications for the rodriguez triple junction dynamics in the late cretaceous abstract the central indian ocean basin east of the southern chagos laccadive ridge complex has evolved due to seafloor spreading in the n s direction since the late cretaceous the trace of the rodriguez triple junction rtj has been inferred in this basin but its exact location and dynamics prior to c29o are not known about 20 000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the rtj in the late cretaceous synthetic seafloor spreading models endorse the presence of late cretaceous to early palaeogene magnetic anomalies c34y to c24o in the study area half spreading rates are initially low 32 mm yr between c34y to c33y and increase up to 78 mm yr by c28y thereafter a decreasing trend is seen 57 mm yr towards the west in the study area higher spreading rates 120 mm yr between c33y and c32ay and distorted anomalies between chrons c32ay and c30y are observed detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation plate reconstruction models suggest that the rtj at c34y time lay southwest of comorin ridge which coincided with the northern edge of the conrad rise until c33y the rtj left a linear trace parallel to the fracture zone trend on both the indian and antarctic plates while it gradually moved eastwards on the african plate the rtj migrated westwards until c31y causing the transfer of a considerable amount of crust from the african plate to both the indian and antarctic plates thereafter the rtj rapidly reversed its migration direction towards the east until c29o causing the formation and rotation of a microplate on the indian plate and lengthening of the southwest indian ridge the present study suggests that this episodic migration of the rtj may be due to ridge propagation under the influence of the marion hotspot which was in the vicinity during that period this rtj migration resulted in microplate formation lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the pacific ocean thus the origin of anomalous structures such as the crozet bank may be related to triple junction dynamics under the influence of marion hotspot further the counter clockwise rotation of the southwest indian ridge between c33y and c29y may have a causal link with the formation and rotation of the microplate which occurred during the same period the present study thus facilitated the inference of the dynamics of the rtj trace between c34y and c29o on the three plates

hydrothermal petroleums from yellowstone national park wyoming u s a abstract in yellowstone national park oil is actively discharging with thermal waters at several widely separated thermal areas samples from two locations were studied petroleum from calcite springs occurs as vapor condensates in steam vents detailed geochemical analyses show a complex origin for the petroleum fluid deeply buried hydrothermally altered sedimentary rocks are believed to have contributed a high temperature pyrolysate consisting largely of polynuclear aromatic hydrocarbons and heterocyclic compounds shallow sedimentary rocks which are also hydrothermally altered have contributed large amounts of aromatic and polar compounds as well as minor amounts of hydrocarbons the permian phosphoria formation is suggested as a likely source rock for the bulk of the oils autochthonous pyrolysates and lipids from thermally altered surface and near surface bacterial and land plant debris have also been introduced to the vents the latter may have been introduced by downward percolating meteoric water the oil from rainbow springs is a highly paraffinic low s crude oil depleted in light hydrocarbons unlike the calcite springs samples the oil from rainbow springs is believed to have originated from a single sedimentary source rock with little or no hydrocarbon contributions from surface biota the oil contains a unique molecular marker assemblage that suggests both gymnosperm and angiosperm input and places the age of the source unit in the tertiary consideration of the local stratigraphy and water temperatures limits the probable source zone to the eocene aycross formation an intra volcanic lacustrine mudstone calculation of deep water temperatures indicates that this unit is exposed to thermal waters in an extensive near surface aquifer such conditions are consistent with the widespread surface manifestations and biomarker kinetic models consideration of the petroleum chemistry and regional hydrodynamics indicates that the yellowstone park oils did not migrate into the park from distant sources the chemical data are also inconsistent with the oils being generated and trapped prior to tertiary volcanism at both calcite springs and rainbow springs oil generation appears to be contemporaneous and is attributed to thermal alteration of relatively shallow sedimentary rocks by hydrothermal fluids

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

a new type of reservoir rock in volcaniclastic sequences development of pronounced secondary porosity and permeability accompanied by dramatic changes in wave propagation velocity and other physical properties in laumontite tuffs occurs in the oil fields of eastern georgia soviet union these rocks originated during intense hydrothermal alterations of andesite tuffs and comprise local few meters thick commonly lens shaped bodies hydrothermal alteration was lithologically and structurally controlled resulting in the formation of specific reservoir rocks identifiable on geophysical logs and capable of producing oil and gas the considerable relief of the in situ stress within these bodies was estimated from differential velocity analysis using sonic log and laboratory data this stress relief as well as borehole enlargem nts accompanied by the development of zones of nonelastic deformation around the hole tends to enhance near well permeability and hence the productive potential of these uncommon and poorly studied reservoirs

casto ring zone a 4 500 km2 fossil hydrothermal system in the challis volcanic field central idaho 18 o analyses of granitic and volcanic rocks reveal the largest hydrothermal system ever documented around a single granite pluton occupying a 4 500 km 2 area in central idaho the remains of this meteoric hydrothermal system are principally preserved within a sharply bounded 15 km wide 70 km diameter annulus of low 18 o rock 2 0 to 8 8 termed the casto ring zone the zone centered on a less depleted 4 5 core zone consisting of granitic rocks of the casto pluton this 700 km 2 eocene subvolcanic batholith has intruded domed and hydrothermally metamorphosed a thick sequence of challis volcanics the stratigraphically low rocks in the 2 000 km 2 van horn peak and the 1 000 km 2 thunder mountain cauldron complexes being most strongly altered less extreme 18 o depletions occur in the youngest major ash flow sheets of these complexes indicating a vertical 18 o gradient water rock ratios of geothermal systems are surprisingly insensitive to the circulation scale

rapid early miocene acceleration of uplift in the gangdese belt xizang southern tibet and its bearing on accommodation mechanisms of the india asia collision abstract five samples from a biotite hornblende granodiorite phase of the 42 5 ma quxu pluton gangdese batholith southern tibet have been collected at 250 m vertical intervals biotite from these rocks yields monotonically decreasing40ar 39ar isochron ages with decreasing elevation of 26 8 0 2 23 3 0 5 19 7 0 3 18 4 0 4 and17 8 0 1ma tc 335 c coexisting k feldspars have virtually identical minimum apparent40ar 39ar ages of 17 0 0 4ma tc 285 c these data indicate parts of southern tibet experienced a pulse of uplift in the early miocene with the rate of uplift rising from 0 07 to 4 4 mm year in the interval 20 to 17 ma an apatite fission track age of 9 9 0 9ma from this locality constrains the average uplift rate at this site to about 0 81 mm year between 17 and 9 9 ma and 0 30 mm year from 9 9 ma to present k feldspar from the dagze granite 30 km to the east near lhasa yields a minimum apparent40ar 39ar age of 35 9 0 9ma tc 227 c which indicates an average uplift rate there of 0 21 mm year since then the marked pulse of uplift of the quxu granodiorite and the difference in uplift history between the dagze and quxu plutons suggests southern tibet has experienced discrete pulses of uplift variable in both space and time these data are not consistent with models which require a large proportion of uplift of the tibetan plateau to have occurred in the last 2 ma the data support the suggestion that convergence between india and asia was largely accommodated by tectonic escape during the opening of the south china sea 32 to 17 ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the tibetan plateau subsequent to 20 ma

slow downs and speed ups of india eurasia convergence since 20 ma data noise uncertainties and dynamic implications india somalia and north america eurasia relative motions since early miocene 20 ma have been recently reconstructed at unprecedented temporal resolution o 1 myr from magnetic surveys of the carlsberg and northern mid atlantic ridges these new datasets revamped interest in the convergence of india relative to eurasia which is obtained from the india somalia nubia north america eurasia plate circuit unless finite rotations are arbitrarily smoothed through time however the reconstructed kinematics i e stage euler vectors appear to be surprisingly unusual over the past 20 myr in fact the euler pole for the india eurasia rigid motion scattered erratically over a broad region while the associated angular velocity underwent sudden increases and decreases consequently convergence across the himalayan front featured significant speed ups as well as slow downs with almost no consistent trend arguably this pattern arises from the presence of data noise which biases kinematic reconstructions particularly at high temporal resolution the rapid and important india eurasia platemotion changes reconstructed since early miocene are likely to be of apparent nature because they cannot result even from the most optimistic estimates of torques associated for instance with the descent of the indian slab into earth s mantle our previous work aimed at reducing noise in finiterotation datasets via an expanded bayesian formulation which offers several advantages over arbitrary smoothing methods here we build on this advance and revise the india eurasia kinematics since 20 ma accounting also for three alternative histories of rifting in africa we find that india eurasia kinematics are simpler and most importantly geodynamically plausible upon noise reduction convergence across the himalayan front overall decreased until 10 ma but then systematically increased albeit moderately towards the present day we test with global dynamic models of the coupled mantle lithosphere system how a previously proposed uplift of the tibetan plateau by 1 2 km as well as the break up of the indo australian plate into the india and australia units impact the kinematic record from 20 to 10 ma our modelling results indicate that the convergence slow down until 10 ma resulted chiefly from the indo australian break up however the tectonic scenario yielding the best fit to kinematic reconstructions is indeed one where the tibetan mean elevation increased by 1 2 km

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

origin of marginal basins of the nw pacific and their plate tectonic reconstructions abstract geometry of basins can indicate their tectonic origin whether they are small or large the basins of bohai gulf south china sea east china sea japan sea andaman sea okhotsk sea and bering sea have typical geometry of dextral pull apart the java makassar celebes and sulu seas basins together with grabens in borneo also comprise a local dextral transform margin type basin system similar to the central and southern parts of the shanxi basin in geometry the overall configuration of the philippine sea resembles a typical sinistral transpressional pop up structure these marginal basins except the philippine sea basin generally have similar or compatible rift history in the cenozoic but there do be some differences in the rifting history between major basins or their sub basins due to local differences in tectonic settings rifting kinematics of each of these marginal basins can be explained by dextral pull apart or transtension these marginal basins except the philippine sea basin constitute a gigantic linked dextral pull apart basin system formation of the gigantic linked dextral pull apart basin system in the nw pacific is due to nne to ene ward motion of east eurasia this mainly was a response to the indo asia collision which started about 50 ma ago the displacement of east eurasia can be estimated using three aspects 1 the magnitude of pull apart of the dextral pull apart basin system 2 paleomagnetic data from eastern eurasia and the region around the arctic and 3 the shortening deficits in the large tibetan plateau all the three aspects indicate that there was a large amount 1000 to 1200 km of northward motion of the south china block and compatible movements of other blocks in eastern eurasia during the rifting period of the basin system such a large amount of motion of the eastern eurasia region contradicts any traditional rigid plate tectonic reconstruction but agrees with the more recent concepts of non rigidity of both continental and oceanic lithosphere over geological times based on these three estimates the method developed for restoration of diffuse deformation of the eurasian plate and the region around the arctic and the related kinematics of the marginal basins we present plate tectonic reconstructions of these marginal basins in global plate tectonic settings at the four key times 50 35 15 and 5 ma the plate tectonic reconstructions show that the first order rift stage and post rift stage of the marginal basins are correlated with the first order slow uplift stage and the rapid uplift stage of the tibetan plateau respectively the proto philippine sea basin was trapped as a sinistral transpressional pop up structure at a position that was 20 south of its present position at about 50 ma ago or earlier while the japan arc migrated eastward during the rifting period of the japan sea basin the shikoku basin opened and the parece vela basin widened

space geodetic test of kinematic models for the indo australian composite plate the existence of a distinct capricorn component plate within the composite indo australian plate has previously been questioned if there is no capricorn plate the global positioning system site at bangalore on the indian plate is predicted to move relative to the australian plate at a velocity of 10 mm yr 1 if there is a distinct capricorn plate bangalore is predicted to move relative to the australian plate at a velocity of 17 mm yr 1 space geodetic data from australia and india are accurate enough to provide quantitative tests of this prediction and to discriminate between alternative composite plate models the data best fit a composite plate model that assumes the existence of a distinct capricorn component plate moving relative to both the indian and australian component plates

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

evidence for multiphase folding of the central indian ocean lithosphere long wavelength 100 300 km folding in the central indian ocean associated with the diffuse plate boundary separating the indian australian and capricorn plates is earth s most convincing example of organized large scale lithospheric deformation to test the timing and mechanics of this deformation as implied by plate kinematic and deformation models we present a new analysis of the seismic stratigraphy of the bengal fan sediments this analysis shows that the folding of the oceanic lithosphere was multiphase with major events occurring in the miocene 8 0 7 5 ma pliocene 5 0 4 0 ma and pleistocene 0 8 ma the miocene phase was the most intense and involved deformation of an area south of 1 s whereas in the pliocene the activity shifted northward in the final phase pleistocene the activity was focused in the equatorial region no evidence was found for deformation prior to 8 0 7 5 ma the spatial extent of the pleistocene folding event overlaps the pliocene and or miocene folding events and coincides with both the area of most active faulting and the zone of greatest historical seismicity the seismic data show that the timing of reverse faulting and thus more significant shortening of the lithosphere generally coincided with the phases of folding but there are examples of folding of the oceanic lithosphere without associated reverse faulting

rapid sea level changes at the close of the last interglacial substage 5e recorded in bahamian island geology rapid and abrupt relative sea level changes within the last interglacial substage 5e are recorded in the island geology of the tectonically stable bahamas from 132 to 118 ka reef growth reached a maximum elevation near 2 m as indicated by fossil reef elevation across the platform whereas bioeroded notches are incised in coastal cliffs as high as 6 m the end of the interval is characterized by voluminous eolianites exhibiting palm tree and frond impressions it is inferred that sea level for most of the interval remained near 2 m restraining reef growth and that the notch at 6 m represents a rapid and brief excursion just before the close of the substage the subsequent fall must have been rapid in order to leave the notch profile intact and mobilize windward lagoon ooids into dunes before cementation could anchor them in order to explain the rapid rise to 6 m glacial surging is invoked the subsequent fall also rapid may be a consequence of the surge flooding high latitudes and providing enough moisture to initiate reglaciation and drawdown

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

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

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