magnetic polarity stratigraphy of siwalik group sediments of tinau khola section in west central nepal revisited summary remanent magnetization of sandstones sampled at 127 levels sites throughout a 1710 m thick siwaliks molasse sequence exposed along the tinau khola river in nepal has been studied using thermal demagnetization the magnetic remanence consists of a secondary low temperature component of normal polarity unblocked mostly below 400 500 c close to the present day field and a high temperature characteristic remanence chrm unblocked mostly between 600 and 685 c represented by both normal and reverse polarities demagnetization behaviour and isothermal remanence acquisition indicate that the secondary component resides on goethite and maghemite magnetite whereas specular haematite carries chrm anisotropy of magnetic susceptibility data reveals primary depositional magnetic fabric as judged by oblate ellipsoids and subvertical tilt corrected minimum susceptibility axes a magnetic polarity sequence established using tilt corrected chrm directions from 124 levels reveals more details not found in the polarity stratigraphy worked out by munthe et al 1983 comparison of the sequence with a standard polarity time scale harland et al 1989 suggests a depositional time range between c 5 9 ma older than chron 3r and 11 ma younger than chron 5r 2 for the section considered according to new data the horizon of sivapithecus punjabiecus fells close to the reversal boundary at c 8 54 ma the lower age limit of the normal polarity chron 4ar 1 hence the hominoid should be regarded as of c 8 5 ma rather than 9 0 9 5 ma as suggested by munthe et al 1983

1600 1500 ma hotspot track in eastern australia implications for mesoproterozoic continental reconstructions mesoproterozoic a type magmatic rocks in the gawler craton curnamona province and eastern mount isa inlier form a palaeo curvilinear belt for reconstructed plate orientations the oldest igneous rocks in the gawler craton are the hiltaba granite suite c 1600 1575 ma the youngest in the mount isa inlier are the williams naraku batholiths c 1545 1500 ma the belt is interpreted as a segment of a hotspot track that evolved between c 1600 and 1500 ma this hotspot track may define a quasilinear part of australia s motion between 1636 and 1500 ma and suggests that australia drifted to high latitudes an implication of this interpretation is that australia and laurentia may not have been fellow travellers leading to the formation of rodinia a hotspot model for a type magmatism in australia differs from geodynamic models for this style of magmatism on other continents this suggests that multiple geologic processes may be responsible for the genesis of proterozoic a type magmas

causes of recent himalayan uplift deduced from deposited patterns in the ganges basin the hypothesis1 that late cenozoic uplift of many of the world s mountain ranges was a response to climate change rather than to tectonic processes is based on the premise that enhanced erosion in an altered climate led to a reduction in both the total mass of a mountain range and its mean elevation this reduction was coeval with increased regional relief and with passive isostatic uplift of the remaining summits the hypothesis is difficult to evaluate because both climate and tectonic forces affect uplift in many active ranges in the absence of sufficient denudational and uplift data from the mountains themselves depositional patterns in adjacent basins may sometimes be used to distinguish between uplift arising from erosional or from tectonic processes in foreland basins uplift of the proximal basin and a reduction or absence of asymmetric subsidence are predicted responses to erosionally driven uplift2 here i show that patterns of sediment accumulation and the position of major rivers in the neogene gangetic foreland basin seem to have changed markedly in plio pleistocene times these changes lend support to the hypothesis that the importance of erosional unloading has increased in the himalayas during the past 4 myr

plume modified orogeny an example from the western united states plate tectonic theory implies that orogeny at convergent margins results from several processes including subduction of oceanic crust subduction of aseismic ridges accretion of terranes and continental collision each of these processes involves the consumption of an oceanic tract and each has its own characteristic style of tectonothermal activity an additional potentially important orogenic process has been largely overlooked oceanic regions contain chains of volcanic islands formed at hotspots which are generally considered to reflect the sites of rising plumes in a hotspot reference frame where active continental margins advance and override a plume the plume9s buoyant swell may profoundly change the tectonothermal expression of subduction and hence orogenic processes at the continental margin the late cretaceous tertiary evolution of the western united states may be an example of this process which has been overlooked in the development of orogenic paradigms

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

plume overriding triggers shallow subduction and orogeny in the southern central andes abstract plate tectonic theory implies that mantle plumes may be eventually overridden by lithosphere during continental drift these events have particular tectonomagmatic consequences for active margins and hence related orogenic processes since the first documentation of plume overriding and the definition of the plume modified orogeny concept only few examples have been recognized in the geologic record in this study we analyze the neogene tectonic evolution of the southern central andes between 35 and 38 s and its potential relation to the subduction of the payenia plume as a recent analogue of this process through a series of tectonic reconstructions we show that progressive payenia plume overriding correlates with neogene arc front migrations linked to slab shallowing fold belt reactivation in the main cordillera and intraplate contraction in the san rafael block additionally nazca slab tear determined from tomographic analyses and subsequent diachronous steepening of the subducted plate may also be an aftermath of plume subduction as often described in the final stages of plume modified orogeny finally we propose a modern analogue for processes previously described dating back to the mesoproterozoic which provides further insights into these complex settings

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

chemistry isotopic composition and origin of a methane hydrogen sulfide hydrate at the cascadia subduction zone abstract although the presence of extensive gas hydrate on the cascadia margin offshore from the western u s and canada has been inferred from marine seismic records and pore water chemistry solid gas hydrate has only been found at one location at ocean drilling program odp site 892 offshore from central oregon gas hydrate was recovered close to the sediment water interface at 2 19 m below the seafloor mbsf at 670 m water depth the gas hydrate occurs as elongated platy crystals or crystal aggregates mostly disseminated irregularly with higher concentrations occurring in discrete zones thin layers and or veinlets parallel or oblique to the bedding a 2 to 3 cm thick massive gas hydrate layer parallel to bedding was recovered at 17 mbsf gas from a sample of this layer was composed of both ch4 and h2s this sample is the first mixed gas hydrate of ch4 h2s documented in odp it also contains ethane and minor amounts of co2 measured temperatures of the recovered core ranged from 2 to 1 8 c and are 6 to 8 degrees lower than in situ temperatures these temperature anomalies were caused by the partial dissociation of the ch4 h2s hydrate during recovery without a pressure core sampler during this dissociation toxic levels of h2s 34s 27 4 were released the 13c values of the ch4 in the gas hydrate 64 5 to 67 5 pdb together with d values of 197 to 199 smow indicate a primarily microbial source for the ch4 the 18o value of the hydrate h2o is 2 9 smow comparable with the experimental fractionation factor for sea ice the unusual composition ch4 h2s and depth distribution 2 19 mbsf of this gas hydrate indicate mixing between a methane rich fluid with a pore fluid enriched in sulfide at this site the former is advecting along an inclined fault into the active sulfate reduction zone the facts that the ch4 h2s hydrate is primarily confined to the present day active sulfate reduction zone 2 19 mbsf and that from here down to the bsr depth 19 68 mbsf the gas hydrate inferred to exist is a 99 ch4 hydrate suggest that the mixing of ch4 and h2s is a geologically young process because the existence of a mixed ch4 h2s hydrate is indicative of moderate to intense advection of a methane rich fluid into a near surface active sulfate reduction zone tectonically active faulted margins with organic rich sediments and moderate to high sedimentation rates are the most likely regions of occurrence the extension of such a mixed hydrate below the sulfate reduction zone should reflect the time span of methane advection into the sulfate reduction zone

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

magnetostratigraphy of the miocene pleistocene surai khola siwaliks in west nepal summary phic age dating has been carried out on sandstones of a wellexposed continuous 5500 m thick miocene pleistocene section in the surai khola area west nepal the section is situated in the siwalik range which consists of molasse sediments from the himalaya progressive demagnetization and isothermal remanence experiments prove that haematite is the carrier of the characteristic remanent magnetization chrm throughout the entire section the final polarity sequence is based on 436 samples of which 233 have normal and 203 reverse polarity antiparallel mean directions of the normally and reversely polarized samples indicate that the chrm is of primary origin comparison of the polarity sequence with a standard polarity time scale yields a time period for the section between chron lr or chron 2r matuyama and probably chron 5ab between 1 2 myr and about 13 myr

asthenospheric buoyancy and the origin of high relief topography along the cascadia forearc abstract the forearc topography of the cascadia subduction zone varies systematically along strike with high relief in the north and south the olympic and klamath ranges separated by relatively low relief in its central region this systematic topographic variability reflects the long term pattern of uplift and erosion however the underlying cause of uplift and the mechanisms by which current topography is supported are unclear here we synthesize results from seismic imaging geodetic decadal and geomorphic millennial uplift rates erosion rates topographic analysis and characteristics of the megathrust interface with a mechanical model to infer that buoyancy in the subslab asthenosphere influences the development and longevity of cascadia s forearc topography the cascadia margin can be divided into three broad segments with the northern and southern segments characterized by rapid geodetic and geomorphic uplift rates rapid erosion rates high coseismic subsidence of great megathrust ruptures shallower slab dip angles and increased plate locking compared to the central segment tomographic images show low velocity anomalies beneath the slab in the northern and southern segments which are inferred to be regions of partial melt resulting from localized upwellings and regions of positive mantle buoyancy modeling suggests that these buoyant regions can locally increase the total shear force at the megathrust by either shallowing the slab dip thereby increasing the area of the seismogenic zone or increasing mechanical plate coupling along the megathrust by increasing normal stress and or the effective coefficient of friction we propose that 1 sub slab buoyancy influences topographic development by modulating along strike patterns of strain within the over riding plate during the seismic cycle 2 permanent development of surface topography occurs due to unrecovered strain over thousands of seismic cycles and 3 variations in cascadia s forearc topography are laterally supported by changes in the total shear force at the megathrust interface using independent estimates for slab dip and plate locking we predict the first order variations in cascadia forearc topography with local maxima in the north and south as observed given our results variations in subslab buoyancy may be critical to explaining forearc topography in other subduction systems

foraminifera as indicators of methane rich environments a study of modern methane seeps in santa barbara channel california methane hydrates have been implicated as a possible forcing mechanism for rapid climate change during the late quaternary via the release of methane from marine reservoirs carbon isotopic excursions of foraminifera observed in high resolution sediment records from santa barbara basin odp site 893 have been interpreted as evidence of episodes of methane release during the late quaternary however the potential relationship between methane in the modern environment and its effect on the stable isotopic composition of benthic and planktonic foraminifera is not well understood short cores taken inside a 500 m diameter pockmark with active methane seeps in santa barbara channel ca usa yield information on the effect of high concentrations of environmental methane on foraminiferal assemblages and their stable isotopic composition these seeps reside at 120 m water depth in the eastern portion of the channel associated with faults and fractures in the miocene monterey formation planktonic foraminiferal assemblages and radiocarbon ages show that surface sediments are of late glacial age 25 300 calendar years this finding suggests an absence of holocene and deglacial sedimentation in the seep area likely due the formation of the pockmarks and subsequent winnowing or lack of sedimentation benthic foraminiferal assemblages are marked by high abundance of bolivina tumida a species previously observed during interstadial episodes and the early holocene in santa barbara basin interpreted to include episodes of high methane flux from the basin while oxygen isotopic values of both benthic and planktonic foraminifera appear normal for cool glacial conditions negative carbon isotopic values are seen in benthic foraminifera primarily in the near surface intervals in one interval 1 2 cm below surface both benthic and planktonic species record highly negative carbon isotopic values implying that isotopically light methane influenced the entire water column this spike is of similar or larger magnitude than several earlier observed spikes in the late quaternary record from odp site 893 single specimen isotopic analyses give an indication of the total range of carbon isotopic values recorded 0 01 to 25 23 for benthics 0 07 to 22 22 for planktonics in the spike interval since modern methane seeps are commonly associated with authigenic carbonate precipitation scanning electron microscopy was used to assess the possibility that post depositional alteration may have affected the isotopic composition of the test these experiments indicate that authigenic carbonate is absent and therefore unable to account for the highly negative carbon isotopic signatures these data improve our understanding of methane associated environments by showing that foraminiferal isotopic composition and assemblages may be used to discern the flux of methane from the oceanic reservoir in the geologic record

neogene magnetic polarity stratigraphy of some fluviatile siwalik sections nepal summary the magnetostratigraphy of five new sections through the neogene siwalik group of nepal is presented rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary drm or a pdrm after stepwise thermal demagnetization of specimens from all sections directions of characteristic remanent magnetization were obtained the results show a positive reversal test and exhibit inclination errors of about 20d due to gaps in exposures and extremely variable demagnetization behaviour of the haematite bearing sediments the resulting polarity sequences are poorly defined for some parts of the sections however double sampled parts of some sections yield similar results a correlation with the surai khola section appel rosler corvinus 1991 is proposed based on rock magnetic parameters ams results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the surai khola section

upper crustal shortening and forward modeling of the himalayan thrust belt along the budhi gandaki river central nepal a balanced cross section along the budhi gandaki river in central nepal between the main central thrust including displacement on that fault and the main frontal thrust reveals a minimum total shortening of 400 km minimum displacement on major orogen scale structures include 116 km on the main central thrust 110 km on the ramgarh thrust 95 km on the trishuli thrust and 56 km in the lesser himalayan duplex the balanced cross section was also incrementally forward modeled assuming a generally forward breaking sequence of thrusting where early faults and hanging wall structures are passively carried from the hinterland toward the foreland the approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible in the balanced cross section the trishuli thrust is the roof thrust for the lesser himalayan duplex the forward model and reconstruction emphasize that the lesser himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the main himalayan thrust as the duplex developed the lesser himalayan ramp migrated southward the movement of lesser himalayan thrust sheets over the ramp pushed the lesser himalayan rock and the overburdens of the greater and tibetan himalayan rock toward the erosional surface this vertical structural movement caused by footwall collapse and duplexing in combination with erosion exhumed the lesser himalaya

the source of a type magmas in two contrasting settings u pb lu hf and re os isotopic constraints the sources of post orogenic a type magmas from two distinct geodynamic settings are compared the end of the ca 514 480 ma delamerian orogeny southeastern south australia was marked by 10 myr of bimodal a type magmatism driven by convective removal of thickened lithosphere initial os and hf isotope ratios record a heterogeneous lithospheric mantle source with some input from aesthenospheric mantle mafic parental melts fractionated to produce the granites in contrast initial os isotope ratios of the a type magmas that comprise the ca 1598 1583 ma mesoproterozoic gawler felsic large igneous province central south australia record a dominant evolved lower crust component however initial hf isotope ratios from these samples are depleted indicating a mantle source for lithophile elements this voluminous bimodal magmatism lasted for 15 myr and ended the wartakan orogeny in both cases the homogenisation of chemical rheological heterogeneities inherited from terrain amalgamation and orogenic thickening strengthened the lithosphere the contemporaneous fusion of heterogeneous mantle crust may represent a common stabilising influence on the lithospheric column regardless of tectono magmatic setting

the fate of the juan de fuca plate implications for a yellowstone plume head beneath the pacific northwest the juan de fuca plate a remnant of the farallon plate continues subducting beneath the north american continent to the east of the cascadia subduction zone lies the yellowstone hotspot track the origins of this track can be traced back to the voluminous basaltic outpourings in the columbia plateau around 17 ma if these basalts are the result of a large melting anomaly rising through the mantle to the base of the north american continent such as a mantle plume head the anomaly would need to punch through the subducting juan de fuca slab here we use teleseismic body wave travel time tomography to investigate the fate of the subducted slab and its possible interaction with a plume head our dataset is derived from the oregon array for teleseismic study oats deployment in oregon and all other available seismic data in this region during the same period in our jdf07 models we image the subducted juan de fuca plate in the mantle east of the cascades beneath oregon where the slab has not been imaged before to a depth of 400 km but no deeper the slab dips similar to 50 degrees e and has a thickness of similar to 75 km immediately beneath the slab we image a low velocity layer with a similar geometry to the slab and extending down to at least similar to 575 km depth in the v model the total length of the high velocity slab is similar to 660 km about 180 km longer than the estimated length of slab subducted since 17 ma assuming similar slab geometry to today this 180 km length of slab would reach similar to 60 km depth comparable to the thickness of continental lithosphere we propose that the absence of the slab below 400 km today is due to the arrival of the yellowstone plume head similar to 17 ma which destroyed the juan de fuca slab at depths greater than the thickness of the continental lithosphere given this scenario the low velocity anomaly beneath the slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate the amplitude of the observed low velocity anomaly is comparable with that expected for plume head material 100 300 degrees c hotter than the surrounding asthenosphere c 2007 elsevier b v all rights reserved

the influence of microhabitats on the carbon isotopic composition of deep sea benthic foraminifera we report 13c and 18o values for rose bengal stained benthic foraminifera from a set of box cores from continental margin environments in the atlantic and pacific oceans these isotopic results are compared with foraminiferal distribution data and pore water 13c profiles to evaluate the importance of environmental microhabitat effects on the carbon isotopic composition of benthic foraminifera the 13c values of infaunal taxa are consistently lower than those of epifaunal taxa suggesting that microhabitat effects on test composition do exist the 13c differences between foraminiferal carbonate and bottom water dissolved inorganic carbon are not correlated with the 18o differences between benthic foraminifera tests and equilibrium calcite but they do correlate with variations in the chemistry of sediment pore waters however interspecific 18o differences as well as 13c differences between species with similar vertical distributions in the sediments indicate that taxon specific vital effects also influence test composition

is the ancestral yellowstone hotspot responsible for the tertiary carlin mineralization in the great basin of nevada we propose a genetic relationship between the 43 34 ma magmatism extensional tectonics and gold mineralization event centered in the great basin of nevada and the development of the ancestral yellowstone hotspot the model is compatible with cenozoic regional tectonics and provides a plausible explanation of the spatial coincidence of the eocene oligocene magmatic events and the carlin type gold deposits these features are centered in the battle mountain region of the great basin coincident with the inferred ca 40 30 ma position of the yellowstone hotspot the yellowstone hotspot is probably a plume that ascended from the core mantle boundary a region thought to be anomalously rich in gold and in the moderately siderophile elements associated with gold deposits as the hotspot was progressively overridden by the north american plate after ca 60 ma a magmatically quiescent period related to subhorizontal subduction gave way to the generation of ca 43 34 ma voluminous intracrustal melts and metamorphic devolatization as the hotspot broke though the subducted farallon plate coeval crustal extension and convective circulation of hydrothermal fluids in the upper crust facilitated the exploitation of the structural and lithologic traps that characterize the classic carlin type deposits

isotopic evidence for the incorporation of methane derived carbon into foraminifera from modern methane seeps hydrate ridge northeast pacific the presence of modern methane seeps at hydrate ridge offshore oregon provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera a series of cores were collected from the southern summit of hydrate ridge in 2002 samples were preserved and stained to determine the 13c composition of three species of live stained and dead benthic foraminifera uvigerina peregrina cibicidoides mckannai and globobulimina auriculata specimens were examined under light and scanning electron microscopy sem and exhibit no evidence of diagenesis or authigenic carbonate precipitation individual living foraminifera from seep sites recorded 13c values from 0 4 to 21 2 indicating the isotopic influence of high methane concentrations average 13c values calculated from single specimens range from 1 28 to 5 64 at seep sites and 0 81 to 0 85 at a control off seep site two distinct seep environments distinguished by the presence of microbial mats or clam fields were studied to determine environmental influences on 13c values individual foraminifera from microbial mat sites exhibited more depleted 13c values than those from clam field sites we interpret these differences as an effect of food source and or symbiotic microbes on foraminiferal carbon isotopic values acting to magnify the negative 13c values recorded via the dic pool no statistical difference was found between 13c values of live vs dead specimens this suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions however a few dead specimens with extremely negative 13c composition 12 do indicate potential evidence for an authigenic influence on the recorded 13c composition

new paleomagnetic constraints on syntectonic growth strata in the western qaidam basin ne tibetan plateau abstract accompanied by crustal shortening and mountain uplift caused by the large magnitude south north directed shortening under the collision between india and eurasia syntectonic growth strata in the western qaidam basin document important information about intense intracontinental deformation of the ne tibetan plateau to better address the development of the qaidam basin and the propagation of the growth strata associated with multiple tectonic uplifts of the tibetan plateau we conducted a detailed paleomagnetic study on the late neogene fluvio lacustrine sedimentary sequence exposed at two outcrop sections and a drill core in the western qaidam basin and integrated the results with previous magnetostratigraphic studies based on well determined ages of the centers and limbs of the two anticlines we found an episodic late cenozoic crustal shortening in the center of the western qaidam basin as evidenced by syntectonic offlap growth strata deposited beginning at 3 2 ma combined with data from sedimentary facies and seismic profiles we suggest that offlap growth strata progressively became younger from southwest to northeast in the center of the western qaidam basin due to strong basinward propagation of the eastern kunlun fault and quick short lived pulses of multiple tectonic uplifts of the ne tibetan plateau

multiple frequency sh wave tomography of the western us upper mantle summary we estimate the sh wave velocity and attenuation structures of the western us upper mantle using the dense network of the usarray and new techniques we observe a multiple frequency data set of both traveltime and amplitude anomalies and interpret these with full 3 d finitefrequency sensitivity kernels amplitudes show stronger frequency dependence than traveltimes we perform a joint inversion on the measured traveltime and amplitude anomalies interpreting them in terms of velocity and attenuation heterogeneities aside from the expected clear division between the slow tectonically active region in the west and the fast craton in the east several interesting smaller velocity anomalies are observed the subduction along the cascades at 100 300 km depths shows lateral discontinuity with a slab hole absence of fast anomalies observed around 45 n the delaminated sierra nevada mountains root is observed to have sunk to 200 km depth the yellowstone plume seems to have an origin weak slow velocity anomalies near 1000 km depth but the plume conduit seems to be interrupted by a fast anomaly which is identified as a fragment of the farallon slab the s velocity model shows a trench perpendicular slab gap absence of fast anomalies at almost the same location as in the p model recently published by sigloch et al 2008 the methodological improvements described in the first paragraph have several benefits amplitude data help to sharpen the edges of narrow velocity heterogeneities in the shallow upper mantle the focusing effect from velocity heterogeneity dominates over that of attenuation and must be considered when interpreting amplitude anomalies in general velocity and attenuation heterogeneities correlate positively suggesting that temperature plays a major role in forming the anomalies

palaeoclimatic significance of magnetic properties from late quaternary lacustrine sediments at pithoragarh kumaun lesser himalaya india abstract mineral magnetic studies were carried out on lacustrine deposits from pithoragarh palaeolake kumaun lesser himalaya india these deposits have already provided valuable information for the reconstruction of palaeoclimatic changes and tectonic uplift in the himalayan region according to radiocarbon chronology the lake existed from 35 to 10 kyr bp using magnetic data radiocarbon ages and lithology the top of the well studied 13 m thick wadda section can be correlated to the basal part of the 4 7 m thick riyasi section fuzzy c means cluster analysis using only magnetic parameters enables a subdivision of the riyasi section into five magnetic zones based on three clusters these clusters are related to the relative contribution of the various magnetic constituents i e coarse hem c and ultrafine hem f grained hematite both showing relatively soft magnetic behaviour for the lowermost three zones existing pollen data from the wadda section helps to establish the climatic implications of the magnetic clusters and zones the model is as follows during humid periods magnetic mineralogy is mainly controlled by hem f formed in situ whereas during colder and more arid conditions detrital input of hem c and also hem f from the catchment area predominates climatic interpretation for the upper part of the section is attempted on the basis of the magnetic data but has to be confirmed by further pollen studies

three dimensional laramide deformation of the colorado plateau competing stresses from the sevier thrust belt and the flat farallon slab 1 kinematic analysis of laramide basement cored uplifts on the northern colorado plateau indicate that four of the uplifts were constructed by northeast directed shortening while at least two were constructed by southeast directed shortening available timing data suggest that all of the uplifts rose at approximately the same time 72 50 ma this paper proposes that the different shortening directions are a predictable result of deformation in a three dimensional stress field where the magnitude of 2 approaches that of 1 the concept is supported by both angelier stereographic analysis and elastic modeling tectonically 1 is may have been the result of coupling between the horizontally subducting farallon slab and the north american plate the elevated values of 2 are attributed to far field stresses generated by the topographically high sevier thrust belt which wraps around the western edge of the colorado plateau and was active at the time of deformation in the plateau

paleomagnetic study of cenozoic sediments from the zaisan basin se kazakhstan and the chuya depression siberian altai tectonic implications for central asia abstract this paper presents new paleomagnetic results on cenozoic rocks from northern central asia eighteen sites were sampled in pliocene to miocene clays and sandy clays of the zaisan basin southeastern kazakhstan and 12 sites in the upper oligocene to pleistocene clays and sandstones of the chuya depression siberian altai thermal demagnetization of isothermal remanent magnetization irm showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the zaisan basin stepwise thermal demagnetization up to 640 660 c isolated a characteristic chrm component of either normal or reverse polarity at nine sites at two other sites the great circles convergence method yielded a definite direction measurements of the anisotropy of magnetic susceptibility showed that the hematite bearing sediments preserved their depositional fabric these results suggest a primary origin of the chrm and were substantiated by positive fold and reversal tests the mean paleomagnetic direction for the zaisan basin d 9 i 59 k 19 95 11 is close to the expected direction derived from the apw path of eurasia j geophys res 96 1991 4029 and shows that the basin did not rotated relative to stable asia during the tertiary in the upper pliocene pleistocene sandstones of the chuya depression a very stable chrm carried by hematite was found its mean direction d 9 i 46 k 25 95 7 is characterized by declination close to the one excepted for early quaternary whereas inclination is lower in the middle miocene to lower pliocene clays and sandstones a stable chrm of both normal and reverse polarities carried by magnetite was isolated its mean direction d 332 i 63 k 31 95 4 is deviated with respect to the reference direction and implies a neogene 39 8 counterclockwise rotation of the chuya depression relative to stable asia these results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style predominantly compressive in the zaisan basin and transpressive in the siberian altai at a larger scale the pattern of vertical axis rotations deduced from paleomagnetic data in northern central asia is consistent with the hypothesis of a large left lateral shear zone running from the pamirs to the baikal heterogeneous rotations however indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations

relationships between the distribution and stable isotopic composition of living benthic foraminifera and cold methane seep biogeochemistry in monterey bay california 1 as part of an ongoing effort to explore the use of foraminifera as a means to assess modern and ancient methane release we compared ambient pore water chemistry with the distribution and stable isotopic composition of living rose bengal stained foraminifera in mbari rov ventana tube cores taken from modern seepage areas about 1000 m water depth in monterey bay california benthic foraminiferal isotopic differences between sites clearly indicate that methane influenced pore waters affect foraminiferal distributions and carbonate isotope geochemistry carbon isotope signatures of living benthic foraminifera did not conform to the very negative 30 to 48 methane influenced carbon isotope values of the pore waters they live in instead the influence of methane seep pore waters was reflected in the greater range and carbon isotopic variability of living seep foraminifera compared with published 13c values of foraminifera living in nonseep habitats it is not clear what relative influences biological ecological and physical factors have on the carbon isotopic signatures observed in seep foraminifera substantial carbon isotope differences can exist between individuals of the same seep species for instance 13c values of living globobulimina pacifica varied by as much as 2 9 between seeps within 8 km of each other whereas 13c values of living uvigerina peregrina varied by as much as 1 95 within the same seep provided there is no diagenetic alteration of the test carbonate isotopic results of individual seep foraminifera support the hypothesis that foraminifera can be used to assess past and present methane seepage

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

properties of chemical remanence in synthetic hematite testing theoretical predictions we synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization crm predicted on the basis of theories developed to explain thermoremanent magnetization trm we restrict this investigation to the remanence acquired during mineral crystallization grain growth crm and do not consider chemical remanence acquired during mineral alteration the material produced by our technique is hematite up to 10 m in diameter thermal demagnetization to 200 c removes the viscous component acquired under identical field temperature conditions to those prevalent during synthesis as predicted by analogy to trm the crm acquired parallels the orientation of the magnetic field applied during growth the intensity of chemical remanence is a linear function of the field in which it grew bcrm up to 7 5 mt the maximum growth field investigated furthermore the ratio of intensities of crm to trm acquired in the same field is independent of the strength of that field the intensity of bcrm controls the growth and orientation of hematite crystals a result not predicted from theory derived by analogy to trm the control on crystal growth by the field is reflected in the dependence on bcrm of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence

the effect of slab gaps on subduction dynamics and mantle upwelling abstract gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub slab mantle to flow through the gaps and into the mantle wedge this through slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and or dynamic uplift in the overriding plate while the altered mantle flow patterns affect the trench evolution numerous geodynamic models have investigated the processes that form slab gaps but few studies have examined the dynamics of slab gap altered mantle flow its effects on trench morphology and kinematics or the controlling parameters on these processes here laboratory subduction models with a pre cut gap in a subducting silicone plate are used to explore how slab gap size and slab gap depth influence the surrounding mantle flow field and trench dynamics results suggest that both the vertical extent and the depth of the top trailing edge of the slab gap are crucial parameters for modulating overall subduction dynamics they show that a slab gap which occurs near the surface and initially comprises 30 of the subducting plate width can extend enough vertically in the slab to produce significant vertical flow through the gap changes to the trench geometry and kinematics are also evident in the models such that double and triple arc geometries are formed during subduction of a shallow slab gap all of these results are consistent with observations of slab gaps and their induced surface expressions or the lack thereof in eastern anatolia east java italy and argentina

deglacial methane emission signals in the carbon isotopic record of lake baikal changes in the concentrations of atmospheric greenhouse gases constitute an important part of global climate forcing here we present the first continental evidence for climatically caused changes in a methane gas hydrate reservoir the organic carbon stable isotope record from lake baikal during the past 130 000 years registers regular emissions of isotopically light carbon by the occurrence of distinct negative shifts of 3 5 at every major orbitally forced cold to warm climatic transition during the past 130 000 years including marine oxygen isotope stage boundaries 6 5e 5d 5c 5b 5a and 2 1 we conclude that these emissions were associated with decomposition of sedimentary clathrates widespread in the baikal basin among potential hypotheses to account for these methane episodes the most probable appears to be hydrate dissociation due to deglacial warming of lake water we estimate that as much as 12 33 tg of methane could have been released with each episode by recording the systematically recurring episodes of massive methane clathrate decomposition closely linked with the northern hemisphere temperatures during major orbital warmings the new baikal 13c record provides further evidence for the potential involvement of clathrate reservoir in rapid deglacial rises of atmospheric methane levels

depositional chronology and fabric of siwalik group sediments in central nepal from magnetostratigraphy and magnetic anisotropy abstract magnetostratigraphic research undertaken within the past 15 years in the siwaliks distributed along 400 km of the sub himalaya in central nepal has proved that the sediments possess highly reliable hematite based primary detrital remanent magnetization suitable to determine depositional chronology in order to bring out the polarity sequences in a common chronological frame all available data are newly correlated to the latest global magnetic polarity time scale of cande and kent s c cande d v kent 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic journal of geophysical research 100 6093 6095 chronological data presented are referred in relation to the diverse lithological nomenclature to the formations whose ages are not constrained by isotopic or paleontologic ages the age of the sections dated by magnetostratigraphy ranges between 14 and

the role of a mantle plume in the formation of yellowstone volcanism the origin of the yellowstone volcanic province remains debated proposed hypotheses involve either a mantle plume or not recent tomographic images allow a quantitative evaluation of the plume hypothesis and its interaction with the farallon slabs using 4 d geodynamic models with data assimilation we find that the slab is always in the way of the initially rising plume and that the plume could reach the surface only through the broken slab hinge at 15 ma for most of the time the sinking slabs dominate the mantle flow and prohibit upwelling we find that a plume that satisfies the present mantle image beneath yellowstone fails to predict both voluminous hot materials at shallow depths beneath the western u s and the age migration of the hot spot tracks we suggest that a plume is likely to have much less influence on the yellowstone volcanism than previously thought

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

provenance of the early mesoproterozoic radium creek group in the northern mount painter inlier correlating isotopic signatures to inform tectonic reconstructions abstract new in situ zircon la icpms geochronologic and hf isotope data from the radium creek group within the mount painter inlier provide important temporal constraints on the early mesoproterozoic palaeogeography of eastern proterozoic australia the entire radium creek group was deposited in a single basin forming phase and has a maximum depositional age of 1595 3 7 ma detrital zircon from these metasedimentary rocks have u pb age populations at ca 1595 ma 1660 1680 ma 1710 1780 ma ca 1850 ma and ca 2500 ma these grains are characterised by isotopically diverse and evolved sources and have crystallised within predominantly felsic igneous host rocks the relative age spectra and isotopic character has more similarity with the gawler craton than the arunta block curnamona province or the mount isa inlier these observations suggest that the mount painter province was adjacent to the gawler craton in the early mesoproterozoic our data supports a coherent south australian craton at ca 1595 ma and a contiguous continental mass that included the north and south australian cratons the mount painter inlier occupied a complex plate tectonic setting in the overriding plate of two convergent margins

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

the magnetic polarity stratigraphy of the siwalik group at haritalyangar india and a new last appearance datum for ramapithecus and sivapithecus in asia abstract the haritalyangar india local fauna contains type specimens of several hominoid primates which are assigned a new last appearance datum age based on a magnetic polarity stratigraphy this youngest assigned age for sivapithecus indicus s sivalensis and ramapithecus punjabicus in asia is about 7 0 7 5 m y the age of the holotype specimen of gigantopithecus bilaspurensis is about 6 3 m y the nahan sandstone formation is at least in part contemporary with the nagri formation lithofacies of the potwar plateau pakistan

isotopic preservation of himalayan tibetan uplift denudation and climatic histories of two molasse deposits two distinctive molasse deposits within the indo asian collision zone have been investigated to help understand the post oligocene evolution of the himalaya and southern tibetan plateau the siwalik group predominantly fluvial sandstones and siltstones is widespread throughout the foothills of the himalaya from pakistan to eastern india paleomagnetic analysis of a measured section in the bakiya khola southeastern nepal constrains depositional ages t dep to between 10 8 and 4 9 ma the average accumulation rate during this interval was 0 4 0 5 mm yr 40 ar 39 ar dating of 126 detrital k feldspars from nine horizons in the bakiya khola section t dep 8 25 to 2 2 ma provide snapshots in time of the cooling ages exposed at the earth s surface in this drainage basin the minimum age in the spectra from seven of the nine horizons averages only 3 m y older than t dep indicating exceptionally rapid unroofing in the himalaya throughout that interval half of the grains fro

late paleo mesoproterozoic plate margin deformation in the southern gawler craton insights from structural and aeromagnetic analysis abstract the integration of structural analysis of outcrop and aeromagnetic interpretation has revealed a complex tectonic evolution for the southern gawler craton that reflects the regional tectonic signal during the late paleo to early mesoproterozoic results suggest that exposures at yarlbrinda hill comprise the margin of a highly deformed st peter suite pluton emplaced within the southwestern gawler craton at ca 1611 ma d 1 at outcrop records east to northeast directed ductile thrusting regionally this deformation overprints north south trending dextral yarlbrinda shear zone fabrics within ca 1622 ma st peter suite granite and ca 1690 1670 ma tunkillia suite granite progressive east to northeast directed thrust movements led to exhumation of the deformed ca 1611 ma granite d 2 deformation is recorded by the brittle reactivation of first generation mylonitic fabrics accompanied by cataclasis alteration and pseudotachylite development d 3 records a switch to northwest southeast shortening which led to the folding and tear faulting of the d 1 2 shear zone a switch to extension and subsequent rotations of the principle stress axes led to hiltaba suite granite emplacement and contemporaneous faulting under east west extension during d 4 and west northwest east southeast extension during d 5 deformation of ca 1622 ma st peter suite granite is consistent with movement on the yarlbrinda shear zone and requires the st peter suite to have been emplaced into or adjacent to the ca 1690 1670 ma tunkillia suite prior to shear zone activity and on the overriding plate during subduction d 1 d 3 deformation is constrained by the emplacement of ca 1580 ma hiltaba suite granite and correlates with the wartakan orogeny in the southeastern gawler craton and the olarian orogeny in the curnamona province a 52 disparity between the shortening directions in these respective areas suggests different degrees of mesoproterozoic rotation which likely aided the formation of an intervening basin province the remnants of which are presently preserved in the mount painter inlier the record of convergent deformation is geochronologically constrained to a 30 myr period and places this region of the central gawler craton near a late paleoproterozoic convergent plate margin prior to a period of transient extension beginning at ca 1580 ma the continental scale signal of tectonic events can be recognized at the outcrop scale highlighting the importance of detailed outcrop studies in sparsely exposed areas

the slot technique for rock magnetic sampling abstract we have developed a new technique for paleomagnetic sampling that allows one to sample extremely friable poorly to moderately lithified sedimentary and volcanic materials the method uses a portable electric drill and a dual blade masonry wheel or two sharp steel cutting blades with a 2 cm separation which cuts two parallel slots in the material being sampled a second set of cuts at right angles to the first produces a fixed cubic pedestal around which a plastic sample box can be inserted oriented and extracted the method has allowed us to take samples at localities where we were not able to sample using other methods to test the method we have sampled the bandelier tuff in northeast new mexico and have measured the anisotropy of magnetic susceptibility ams on boxes taken using the new method and on cylinders drilled at the same sites using standard techniques results show better ams within site precision for box samples taken using the new method we conclude that the method is convenient reliable and may be preferred in certain types of paleomagnetic studies

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

massive dissociation of gas hydrate during a jurassic oceanic anoxic event in the jurassic period the early toarcian oceanic anoxic event about 183 million years ago is associated with exceptionally high rates of organic carbon burial high palaeotemperatures and significant mass extinction heavy carbon isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon which is isotopically light in contrast examples of light carbon isotope values from marine organic matter of early toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12c versus 13c refs 1 2 5 6 here however we report carbon isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic biospheric and atmospheric carbon reservoirs and that this occurred despite the enhanced burial of organic carbon we propose that as has been suggested for the late palaeocene thermal maximum some 55 million years ago the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental margin sediments

use of anisotropy to determine the origin of characteristic remanence in the siwalik red beds of northern pakistan it is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone however anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains this in turn may reflect the development of the magnetic fabric of the sediment over time so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence in this work we make a study of the anisotropy of magnetic susceptibility ams in siwalik red beds of miocene age from northern pakistan comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it a coherent magnetization furthermore it should be possible to use ams fabric information to quantify the degree of pedogenesis in these miocene soils we attempted to determine the anisotropy of isothermal remanence air but found ams to be the technique of choice because of apparent changes in coercivity during air experiments we interpret the air data as resulting from metastable domains in hematite grains which change domain state during the air experiment

a biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of northern pakistan abstract two biostratigraphic reference sections are designated for the middle and upper siwalik formations of northern pakistan the stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard magnetic polarity time scale by the magnetic polarity stratigraphy the resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval zones the proposed interval zones are bounded by five important faunal events which are identified in the reference sections the four proposed interval zones and the approximate ages in the reference sections of their lower and upper boundaries are 1 the hipparion s l interval zone 7 4 9 5 m y b p 2 the selenoportax lydekkeri interval zone 5 3 7 4 m y b p 3 the hexaprotodon sivalensis interval zone 5 3 2 9 m y b p and 4 the elephas planifrons interval zone 2 9 1 5 m y b p the paleomagnetic and biostratigraphic evidence indicates that the tatrot beds from which pilgrim developed his tatrot faunal zone are between 2 5 and 3 4 m y b p with a preferred maximum age of 2 9 m y b p the biostratigraphic evidence indicates the sethi nagri locality y311 the primary source of pilgrim s nagri faunal zone is in the hipparion s l interval zone the paleomagnetic evidence suggests an approximate age of 9 m y b p a major faunal event at 9 5 m y b p introduced equids suids and large giraffes from eurasia into the indain subcontinent a second more diffuse faunal event between 5 3 and 2 9 m y b p introduced several african taxa as well as equus and cervids from eurasia

reconstructing the ancestral yellowstone plume from accreted seamounts and its relationship to flat slab subduction recent geodynamic analyses have emphasized the relationship between modern flat slab subduction zones and the overriding of buoyant oceanic crust although most models for the evolution of the late mesozoic cenozoic laramide orogeny in the southwestern united states involve flat slab subduction the mechanisms proposed are controversial an examination of the geological evolution of the 60 50 ma crescent terrane of the coast ranges indicates that it was formed in a shallowing upward loihi type oceanic setting culminating in the eruption of subaerial lavas plate reconstructions indicate that the crescent terrane was emplaced into ca 20 ma crust and the presence of subaerial lavas implies an uplift due to the plume of ca 4 2 km which we use to calculate a minimum buoyancy flux of 1 1 mg s 1 similar to that of the modern yellowstone plume published paleomagnetic data indicate that the crescent terrane was formed at a paleolatitude similar to that of the yellowstone plume the crescent seamount was accreted within 5 my of the cessation of plume magmatism plate reconstructions indicate that it would have originated about 750 km to the west of the north american plate margin if it developed above a fixed yellowstone plume and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement we interpret the crescent terrane as due to the ancestral yellowstone plume such a plume would have generated an elongate swell and related plateau that would have been overridden by the north american margin taken together the relationship between flat slab and overriding of oceanic plateau in laramide times would have been analogous to the relationship between modern andean flat slab subduction zones and the juan fernandez and nazca oceanic plateaus d 2003 elsevier science b v all rights reserved

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

late paleozoic subduction and collision processes during the amalgamation of the central asian orogenic belt along the south tianshan suture zone abstract the provenance of late paleozoic siliciclastic sedimentary strata on the northern margin of the tarim craton and the south tianshan orogenic belt provides important insights into subduction and collision processes during the formation of the southern central asian orogenic belt detrital zircons from carboniferous and permian sedimentary rocks in the south tianshan belt show two predominant age populations of 500 400 and 305 270 ma and three subordinate clusters around 2 5 ga 2 0 1 7 ga and 1 2 0 6 ma such age patterns are similar to major magmatic episodes in the tarim craton but are distinct from those in the central tianshan yili block implying that the carboniferous permian strata in the south tianshan belt were deposited on the northern margin of the tarim craton these data in combination with carboniferous passive margin deposition along the south tianshan and northern tarim regions and intense arc magmatism in the central tianshan area support the northward subduction of the south tianshan oceanic crust the abrupt decrease of zircon e hf t values at 310 ma indicates the tectonic transformation from oceanic subduction to continental collision however syn and post collisional sediments in the south tianshan and northern tarim regions did not contain detritus from the central tianshan yili block as evidenced by the absence paucity of 380 310 ma detrital zircons in the late paleozoic strata and by the stratigraphic record in the regions insignificant surface erosion and uplifting during the collision and ultra high pressure rock exhumation require divergence in the frontal wedge we propose that the switch from a convergent to a divergent regime was triggered by the arrival of the tarim mantle plume in the latest carboniferous which possibly had profound effects on regional sedimentation and exhumation of ultra high pressure rocks in the orogenic belt

on the influence of diagenesis on the original petrographic composition of miocene pliocene fluvial sandstone in the himalayan foreland basin of western central nepal abstract the petrologic composition of synorogenic sediments provides information on source rock lithologies and can be used for reconstructing the erosional history of mountain belts however diagenetic processes such as compaction dissolution of unstable framework grains and formation of authigenic minerals may significantly alter the original sediment composition the question is how important are the diagenetic alterations to address this question we review published and new petrologic and diagenetic data of fluvial sandstones from the miocene pliocene siwalik group of the himalayan foreland basin in western central nepal the new data include petrographic analyses of 18 samples collected along the tinau khola and surai khola sections in these samples evidence for early burial and late diagenesis have been recognized early and burial diagenetic calcite cement is the most important authigenic phase which in addition to moderate compaction and authigenic clay minerals occludes primary intergranular porosity patchy secondary porosity probably formed during late diagenesis related to exhumation standard point counting and polyquartz type analysis of detrital grains support previously published petrologic data high grade metamorphic lithic grains have a higher himalayan provenance and low grade metamorphic and sedimentary lithic grains being derived from lesser himalayan sources which became more important during the late miocene in the case of the himalayan foreland basin it seems that diagenetic alteration of the original sediment composition is limited to feldspars and certain sedimentary lithic grains which as a consequence may be under represented in point counting analyses because of replacement dissolution and compaction

terrestrial record of methane hydrate dissociation in the early cretaceous reconstruction of changing c isotopic composition of early cretaceous atmospheric co2 from fossilized c3 vascular land plant tissue revealed a brief and striking negative excursion d 25 in atmospheric d 13 c followed by a rapid positive compensation d 15 during the aptian ca 117 ma mass balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion this would also result in an increased co2 o2 mixing ratio as o2 is consumed during ch4 oxidation to co2 spurring the exponential phase of angiosperm biogeographic expansion

carbon isotope anomaly and other geochemical changes at the triassic jurassic boundary from a marine section in hungary most mass extinctions are linked with carbon isotope excursions implying that biotic crises are coupled with changes in the global carbon cycle the isotopic evolution during the end triassic extinction is far less documented than that for the other major phanerozoic extinctions here we report a sharp and short lived 23 5 excursion in carbon isotope values for carbonate d 13 ccarb corresponding to a 22 excursion in the isotopic composition of marine organic matter d 13 corg and other geochemical

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

the influence of a mantle plume head on the dynamics of a retreating subduction zone the geological record and seismic tomography together contain multiple examples of mantle plumes and subducting oceanic slabs interacting however consequences of these interactions and a mechanism for plumes to pass from the subducting to the overriding plate are uncertain we present three dimensional numerical simulations of a retreating subduction zone encountering a mantle plume head we assess the impact a plume head has on the geometry of the trench and whether it can create a window within the subducted slab in our models we consider slab plume head interactions to be a natural consequence of plume head entrainment by plate motion combined with slab retreat strong slabs prevent plume heads from reaching the surface for weak slabs strongly buoyant plume heads cause local trench advance slab windowing and accelerated slab retreat at the margins of the plume head for large plume heads this trench advance may contribute to crustal shortening in the overriding plate opening a slab window creates a potential conduit for transfer of plume material from the subducting plate to the overriding plate without terminating an active magmatic arc away from the point where the plume head tears the slab

evidence for a far traveled thrust sheet in the greater himalayan thrust system and an alternative model to building the himalaya the galchhi shear zone underlies the kathmandu klippe in central nepal and has emerged as a key structure for discriminating competing models for the formation of the himalayan orogenic wedge new chronologic data from the galchhi area suggest a new structural and orogenic interpretation zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 7 10 ma and 570 ma with metamorphic zircon growth at 23 29 ma comparable data from the greater himalayan sequence ghs at the intra ghs langtang thrust north of galchhi similarly restrict ghs deposition to between 475 7 3 and 660 ma undeformed pegmatites near galchhi constrain movement of the galchhi shear zone to 22 5 2 3 ma long before slip of the main central thrust in the region 17 ma shear sense indicators in the galchhi area indicate both top to the south and top to the north shears the old age of movement neoproterozoic youngest detrital zircons occurrence of top to the south shear sense indicators and intrusive paleozoic granites all suggest that the galchhi shear zone is an intra ghs top to the south thrust rather than either a thrust involving lesser himalayan rocks or a top to the north shear zone that juxtaposed tethyan and ghs rocks during coeval movement of the main central thrust the ghs in nepal was not emplaced as a single body of rock but consists of at least two ductile thrust sheets present in both the hinterland at langtang and toward the foreland at galchhi ghs thrust sheets sequentially underplated during southward propagation of the thrust belt

deep structure of the japan subduction zone abstract we determined 3 d p wave velocity structure down to 700 km depth under the japan islands using a large number of arrival time data from local and teleseismic events simultaneously we collected 207 000 arrival times from 7743 shallow and deep earthquakes occurred in and around japan and 34 148 data from 333 teleseismic events which were recorded by over 1000 seismic stations on the japan islands our tomographic model revealed some new features the philippine sea slab is found to subduct down to 500 km depth under southwest japan though the seismicity within the slab ends at 150 200 km depth significant low velocity anomalies are found to exist in the deep portion of the mantle wedge above the pacific slab which may be caused by the deep dehydration process of the slab slow anomalies are detected in the mantle beneath the pacific slab which may be caused by a mantle plume or upwelling of hot mantle materials associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

a time framework based on magnetostratigraphy for the siwalik sediments of the khaur area northern pakistan the magnetostratigraphy of eleven new sections in the khaur area of northern pakistan is presented all specimens have been subjected to thermal demagnetization the sections taken in adjacent ravines incising the siwalik formations nearly penpendicular to strike form a temporal framework in which to interpret the biological and lithological information a composite section is constructed from several long sections and is correlated to the magnetic polarity time scale chron 6 to chron 15 providing absolute age information for the biostratigraphic zonations of barry et al 1982 detailed sections along the strike of major stratigraphic marker beds document an isochronous horizon and form the basis for paleoenvironmental reconstructions the preservation of a polarity transition in fluvial sediments suggests continuity of sedimentation on a time scale of 103 104 years

relationships between the stable isotopic signatures of living and fossil foraminifera in monterey bay california 1 fossil foraminifera are critical to paleoceanographic reconstructions including estimates of past episodes of methane venting these reconstructions rely on benthic foraminifera incorporating and retaining unaltered the ambient isotopic compositions of pore fluids and bottom waters comparisons are made here of isotopic compositions of abundant live and fossil foraminifera uvigerina peregrina epistominella pacifica bulimina mexicana and globobulimina pacifica collected in monterey bay ca from two cold seeps clam flats and extrovert cliffs and from sediments 5 m outside of the clam flats seep clam flats has steep d 13 cdic gradients to 45 but dic at extrovert cliffs is less enriched in 12 c to approximately 22 oxygen isotope values of fossil foraminifera at clam flats are 1 5 enriched in 18 o over the living foraminifera as well as those of both live and fossil foraminifera at extrovert cliffs suggesting they may have lived during the last glacial maximum statistical comparisons student s t and kolmogorov smirnov tests of d 13 c and d 18 o values indicate that live and fossil foraminifera come from different populations at both clam flats and extrovert cliffs at clam flats the difference appears to result from alteration enriching some fossil foraminifera in 12 c over live foraminifera at extrovert cliffs the fossil foraminifera are enriched in 13 c over the live foraminifera suggesting they lived prior to the onset of venting and thus that venting began recently the short time of venting at extrovert cliffs may be responsible for the less alteration there compared with clam flats these results indicate that preservation of foraminifera is likely to be poor within long lived cold seeps but that foraminifera living in the surrounding sediment may incorporate and preserve broad basin wide changes in isotopic compositions of the ambient water

self reversed magnetization held by martite in basalt flows from the 1 1 billion year old keweenawan rift canada in some basalt flows of the 1 1 billion year old keweenawan rift exposed at mamainse point ontario there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession the paleomagnetic and geological context of this component demonstrates that it is not a chemicaloverprint whoseremanence is attributableto a subsequently reversed geomagnetic field but that the component is a self reversal of the primary magnetization here we use rock magnetic experiments and mossbauer spectroscopy to show that this phase occurs in the most oxidized flowsandisheldbya fine grainedpopulationofhematitethatacquiredits self reversedremanencethrough interactions with a phase of lower blocking temperature we propose that this self reversed hematite is a manifestation of the self reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and thathas been attributed to negative exchange coupling across the crystal lattices we suggest that hematite formed in association with iron silicates in the basalt flows carries a remanence reflective of the field in which it formed but that martite hematite pseudomorphed after magnetite which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self reversed remanence this study marks the second time that a naturally occurring self reversed magnetization has been attributed to this mechanism and marks the oldestreportedinstanceof anytypeof self reversedremanence themartitethatformsthrough thisprocess has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

magnetostratigraphic dating of the prime time sedimentary record of himalayan tectonics and climate new age constraints 13 10 ma from the siwaliks of the tinau khola north section nepal summary a molasse sequence comprising 1 5 km thick sediments of the lower and middle siwaliks group in the sub himalayan north belt along the tinau khola river in west central nepal was studied for the low field magnetic susceptibility and anisotropy using single core specimens of siltstone sandstone from 127 stratigraphic levels the magnetic fabric comprises primary sedimentary compactional and secondary tectonic components it is characterized by i predominantly oblate magnetic susceptibility anisotropy ellipsoids ii low anisotropy p mostly 1 1 and low magnetic susceptibility ca 10 7 m3 kg 1 contributed mainly by paramagnetic and diamagnetic minerals and iii wnw ese magnetic lineations subparallel to the fold axes bedding strikes thrust front and hence normal to the direction of palaeotectonic compression thermal demagnetization of single core specimens from 103 levels across 1120 m of the lower part revealed a characteristic remanence of the high unblocking temperature 600 c in hematite remanence ratios derived from demagnetization data allowed the first order estimation of remanence contributions from magnetic minerals goethite maghemite magnetite and hematite and discrimination of rockmagnetic zones correlatable with distinct lithofacies which will facilitate objective mapping we correlated a magnetic polarity sequence constructed from normal and reverse polarity directions from 77 levels that passed the reversal test and represented primary remanences with the standard geomagnetic polarity timescale to constrain the depositional age between ca 13 2 ma base of chron c5aan 13 015 13 183 ma and the middle of chron c5n 2n 9 987 11 040 ma we calculated the sediment accumulation rate for polarity zones from the chronologically better constrained part below chron c5n 2n i e below 11 040 ma to be 25 61 cm kyr 1 average 39 cm kyr 1 which is consistent with the value of 32 50 cm kyr 1 reported from siwaik sections in nepal the notable increase in accumulation rate after 12 1 ma probably reflects the peak of the earlier phase of uplift and or unroofing of the himalayan source region followed by rapid accumulation in the foredeep and a link to monsoon initiation intensification compared to the expected remanence from the latest apwp for the indian plate 10 13 ma the best defined mean 351 8 20 9 is rotated anticlockwise by 9 2 and records an inclination shallowing of 25 constraining the base of the tinau khola north section to 13 2 ma i e older than the tinau khola south by 1 7 myr should open up new horizons for multidisciplinary and multiproxy research targeting geotectonic climatic environmental palaeoreconsructions of himalaya wide events

biostratigraphy and geology of the neogene siwalik group of the surai khola and rato khola areas in nepal abstract an almost complete sequence of neogene molasse sediments of the siwalik group in the surai khola khola means small river in nepali area in nepal which was divided into five lithological units includes the upper part of the lower siwaliks the middle and the upper siwaliks it discloses a tectonically uncomplicated southern belt i e the surai khola sequence and a tectonically much disturbed northern belt the upper siwaliks at surai and rato khola yield a rich vertebrate fauna contained within thick bodies of multistoried sandstones the data from the pollen and plant fossils indicate vegetational changes from tropical evergreen forests in the lower and middle siwaliks to dry deciduous forests and grasslands by the beginning of the upper siwaliks biostratigraphically the upper siwalik sediments can be compared with the hexaprotodon sivalensis and the elephas planifrons interval zones of pakistan and with the tatrot and pinjor formations of northwest india palaeomagnetic dating of the surai khola sequence indicate a time scale from 13 to ca 1 million years

three dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones abstract we have investigated potential effects on a subduction zone during oceanic plateau trench collision using three dimensional numerical models an oceanic plateau of varied density was embedded in the slab which was pinned at one end the plateaus strongly influenced the shape of the trench for a plateau with a higher density the trench retreat rate was reduced in the region surrounding the plateau and the plateau subducted along with the slab for lower density plateaus the trench in the region of the plateau advanced and the plateau compressed and resisted subduction spreading laterally along the trench with a weaker slab rheology the arcuate shape of the trench towards the free end of the trench was enhanced beneath the most buoyant plateaus a tear formed in the subducted portion of the slab soon after the slab tip reached the top of the lower mantle we compare the model results with a region in the northwest pacific where the ogasawara plateau meets the trench of the izu bonin mariana subduction zone

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region

evidence for late jurassic release of methane from gas hydrate four late jurassic carbonate successions deposited in the tethys atlantic ocean record a negative carbon isotope excursion of at least 2 the excursion is present in both organic and carbonate carbon records and is comparable in magnitude and duration to isotopic changes during the late paleocene thermal maximum our results indicate that during the late jurassic long considered a warm greenhouse time additional greenhouse gas was input to the atmosphere by a sudden release of methane from buried gas hydrate a potential triggering mechanism may have been the opening of an oceanic gateway through the early atlantic between the ancient tethys and pacific oceans