triassic bedded cherts in central japan are not pelagic although the origin of bedded radiolarian cherts in orogenic belts has been widely debated1 15 their geochemistry is still poorly understood triassic bedded radiolarian cherts occur extensively in the japanese islands and are generally not associated with ophiolitic rocks but rather with clastic rocks we present here chemical data on the triassic bedded cherts of kamiaso in central japan the relationship of sio2 with al2o3 tio2 and zr shows that the bedded cherts have been formed by a simple mixing of biogenic silica derived from radiolarians and sponge spicules with lithogenic materials the ratios of several elements relative to tio2 strongly suggest that the shale partings and lithogenic portions of the cherts are consanguineous sedimentary structures as well as the geochemical evidence suggest that most kamiaso cherts are turbidity current deposits our geochemical and geological data compared with those of recent marine sediments from various environments demonstrate that the triassic cherts and associated shale partings are not deep pelagic in origin but formed in an offshore or marginal sea environment

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

northern larsen ice shelf antarctica further retreat after collapse changes of larsen ice shelf antarctica and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery with emphasis onchanges in the glaciers which previously nourished the ice shelf north of seal nunataks the large glaciers retreated several kilometres inland of the previousgrounding line the velocity field of drygalski glacier the largest glacier in this area was mapped by means of interferograms derived from pairs of ers syntheticaperture radar images from 1995 and 1999 the main part of the glacier showed significant acceleration of flow over these 4 years with an increase of veocity upto three fold at the terminus similar accelerations were observed by means of interferometry on several other grounded glaciers suggesting that the removal ofice shelves could lead to an effect on eustatic sea level for larsen b the northernmost surviving part of larsen ice shelf the retreat of the ice front to october2000 is documented

mesozoic cenozoic rift drift sequence of asian fragments from gondwanaland abstract the university of cambridge atlas map plotting computer program has been used to plot a reconstruction of the dispersal of continental blocks from eastern gondwanaland from the late jurassic 160 ma until the late miocene 10 ma the geological basis for the reconstruction rests on the concept of south tibet burma western thailand malaya and sumatra having been part of the eastern gondwana continent during the late palaeozoic and early mesozoic the reconstruction is also based on the view that these asian continental blocks were rifted from eastern gondwanaland in the late jurassic this rifting episode gave rise to the continental margin of central new guinea and northern australia it was associated with the sea floor spreading anomalies preserved in the floor of the northeastern indian ocean the model considers the indication of crustal uplift in the middle jurassic followed by profound subsidence in the cretaceous and palaeogene exposed in the islands of the outer banda arc from seram to timor and the similar jurassic cretaceous facies in eastern sulawesi implies that these areas formed part of this rifted northern margin of australia in the jurassic subsequent tectonic collision and associated major strike slip movements have deformed and disrupted these rock sequences during the cainozoic the date of the rifting that removed south tibet burma western thailand and malaya from gondwanaland is controversial the indications in favour of the rifting episode having occurred in the jurassic rather than in the permo triassic are stratigraphie structural observations on the northwestern australian shelf in the banda arc and in central new guinea the identification of the continental blocks removed by this rifting is based partly on stratigraphie correlation with the banda arc and new guinea and partly on indications that the age of collision of tibet burma western thailand and malaya with asia was a late cretaceous event and not triassic jurassic as is generally held there are very few reliable land based palaeomagnetic measurements to constrain the movements of these dispersed continental fragments from gondwanaland the computer generated map reconstructions put forward here fit the available indian ocean sea floor spreading data although they are not unique solutions much of the critical ocean floor data associated with the rifting and dispersal of these continental fragments has been destroyed in the tectonic collision processes that occurred at the asian continental margin

grounding line retreat of the west antarctic ice sheet from inner pine island bay ice loss from the marine based potentially unstable west antarctic ice sheet wais contributes to current sea level rise and may raise sea level by 3 3 m or even 5 m in the future over the past few decades glaciers draining the wais into the amundsen sea embayment ase have shown accelerated ice flow rapid thinning and fast retreat of the grounding line gl however the long term context of this ice loss is poorly constrained limiting our ability to accurately predict future wais behavior here we present a new chronology for wais retreat from the inner continental shelf of the eastern ase based on radiocarbon dates from three marine sediment cores the ages document a retreat of the gl to within 100 km of its modern position before ca 10 000 calibrated cal yr b p this early deglaciation is consistent with ages for gl retreat from the western ase our new data demonstrate that in contrast to the ross sea wais retreat from the ase shelf was largely complete by the start of the holocene our results further suggest either slow gl retreat from the inner ase shelf throughout the holocene or that any episodes of fast gl retreat must have been short lived thus today s rapid retreat may be exceptional during the holocene and may originate in recent changes in regional climate ocean circulation or ice sheet dynamics

rise and fall of the eastern great indonesian arc recorded by the assembly dispersion and accretion of the banda terrane timor abstract new age petrochemical and structural data indicate that the banda terrane is a remnant of a jurassic to eocene arc trench system that formed the eastern part of the great indonesian arc the arc system rifted apart during eocene to miocene supra subduction zone sea floor spreading which dispersed ridges of banda terrane embedded in young oceanic crust as far south as sumba and timor in timor the banda terrane is well exposed as high level thrust sheets that were detached from the edge of the banda sea upper plate and uplifted by collision with the passive margin of nw australia the thrust sheets contain a distinctive assemblage of medium grade metamorphic rocks overlain by cretaceous to miocene forearc basin deposits new u pb age data presented here indicate igneous zircons are less than 162 ma with a cluster of ages at 83 ma and 35 ma 40ar 39ar plateau ages of various mineral phases from metamorphic units all cluster at between 32 38 ma these data yield a cooling curve that shows exhumation from around 550 c to the surface between 36 28 ma after this time there is no evidence of metamorphism of the banda terrane including its accretion to the edge of the australian continental margin during the pliocene these data link the banda terrane to similar rocks and events documented throughout the eastern edge of the sunda shelf and the banda sea floor

radiocarbon constraints on the age of the maximum advance of the british irish ice sheet in the celtic sea reconstructions of the british irish ice sheet biis during the last glacial maximum lgm in the celtic sea and southern ireland have been hampered by a paucity of well dated stratigraphic records as a result the timing of the last advance of the largest outlet of the biis the irish sea ice stream to its maximum limit in the celtic sea has been variously proposed as being pre last glaciation early devensian and lgm the irish sea till was deposited by the irish sea ice stream during its last advance into the celtic sea we present 26 stratigraphically well constrained new ams radiocarbon dates on glacially transported marine shells from the irish sea till in southern ireland which constrain the maximum age of this advance the youngest of these dates indicate that the biis advanced to its overall maximum limit in the celtic sea after 26 000 20 000 14c yr bp thus during the last glaciation the most extensive phase of biis growth therefore appears to have occurred during the lgm at least along the celtic sea and irish margins these data further demonstrate that the uppermost inland glacial tills from the area of supposed older drift in southern ireland a region previously regarded as having been unglaciated during the lgm also date from the last glaciation thus most of southern ireland was ice covered at the lgm advance of the biis to its maximum southern limit in the celtic sea may have been a short lived glaciodynamic response facilitated by subglacial bed conditions rather than a steady state response to climate forcing alone

maximum extent and dynamic behaviour of the last british irish ice sheet west of ireland a complete reconstruction of the last british irish ice sheet biis is hindered by uncertainty surrounding its offshore extent and dynamic behaviour this study addresses this problem by reconstructing the depositional history of four sediment cores taken from a series of sinuous glacigenic sediment ridges on the continental shelf west of ireland we present new geomorphic sedimentary and micropaleontological data that record a maximum westward biis extent that was at least 80 km farther offshore from any previous estimates the data suggests that a large ice shelf formed over parts of the shelf prior to retreat this new data increases the areal extent of grounded biis ice by 6700 km2 from previous estimates which represents a 3 increase in the irish sector of the ice sheet three new ams radiocarbon dates demonstrate for the first time that the biis advanced to the shelf edge during last glaciation late midlandian late devensian with ice advance onto the porcupine bank occurring after 24 720 260 yr cal bp deglaciation was complete by 19 182 155 yr cal bp thus constraining biis occupation over the porcupine bank to less than 5500 years estimated retreat rates of marine terminating ice across the shelf range from 70 to 180 myr 1

melange versus forearc contributions to sedimentation and uplift during rapid denudation of a young banda forearc continent collisional belt abstract new sedimentary geochemistry and petrographic analyses provide the most extensive sedimentary documentation yet of the rapid denudation of the young timor orogen the data from three basins including two widely separated well dated sections of the synorogenic megasequence of timor leste and a re dated dsdp 262 constrain the source and timing of detrital sediment flux during forearc continent collision along the timor sector of the banda arc the exhumed synorogenic piggy back basins formed above a melange unit that developed at the expense of a weak stratigraphic horizon in the mesozoic stratigraphy and was exhumed to the sea floor in latest messinian time following an interval of deep marine chalky marl sedimentation an increasingly muddy sediment flux indicates that the island of timor became emergent and shed sediment by 4 5 ma comparison of exhumed sections with similar patterns in the dsdp262 chemistry suggests that the sediment source was probably located some 50 60 km distant from the basin which is consistent with the aileu region of timor leste that shows an appropriate exhumation history all sedimentation between 4 5 and 3 2 ma was probably derived from a low relief rapidly eroding and mudstone dominated landscape with geochemical affinities to the triassic mudstone derived synorogenic melange the melange unit overlies and surrounds the banda terrane and was presumably structurally emplaced by propagation of a decollement through the triassic rocks during the collision after 3 2 ma sedimentation was dominated by hard rock lithologies of the banda terrane consisting of forearc cover and basement the latter including elements of metamafic rocks and metapelites this phase of sedimentation was accompanied by rapid uplift which may have been partly driven by a transient imbalance between rock uplift and denudation as resistant lithologies emerged from below melange like mudstone previous work has suggested that the timing of collision in timor leste and west timor was substantially different our reevaluation of dsdp 262 facies migration history in the context of the re dating presented here favours a relatively synchronous onset of uplift in both halves of the island but with different partitioning of strain between the foreland and hinterland in each half of the island

deep water geomorphology of the glaciated irish margin from high resolution marine geophysical data abstract the continental margin offshore of western ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid latitude margin high resolution multibeam bathymetry and backscatter data combined with shallow seismic and tobi deep towed side scan sonar profiles provide the basis for this study and allow a detailed geomorphological interpretation of the northwest irish continental margin several features including submarine mass failures canyon systems and escarpments are identified in the rockall trough for the first time a new physiographic classification of the irish margin is proposed and linked to the impact of glaciations along the margin correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the british irish ice sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems the glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons these lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the rockall trough during the last glacial maximum none of the sedimentary lobes demonstrates notable relief indicating that the amount of glaciogenic sediment delivered by the british irish ice sheet into the rockall trough was limited their southward disappearance suggests a more restricted biis which did not reach the shelf edge south of 54 23 n the various slope styles observed on the irish margin represent snapshots of the progressive stages of slope development for a glacially influenced passive margin and may provide a predictive model for the evolution of other such margins

tertiary evolution of the eastern indonesia collision complex abstract eastern indonesia is the zone of interaction between three converging megaplates eurasia the pacific and indo australia the geological basis for interpretations of the tertiary tectonic evolution of eastern indonesia is reviewed and a series of plate tectonic reconstructions for this region at 5 million year intervals covering the last 35 million years is presented the oldest reconstruction predates the onset of regional collisional deformation at this time a simple plate configuration is interpreted consisting of the northward moving australian continent approaching an approximately e w oriented southward facing subduction zone extending from the southern margin of the eurasian continent eastwards into the pacific oceanic domain beginning at about 30 ma the australian continental margin commenced collision with the subduction zone along its entire palinspastically restored northern margin from sulawesi in the west to papua new guinea in the east from this time until ca 24 ma the australian continent indented the former arc trend with the northward convergence of australia absorbed at the palaeo northern boundary of the philippine sea plate the present day palau kyushu ridge at ca 24 ma the present day pattern of oblique convergence between the northern margin of australia and the philippine sea plate began to develop at about this time a large portion of the palaeogene colliding volcanic arc the future eastern philippines began to detach from the northern continental margin by left lateral strike slip from ca 18 ma oblique southward directed subduction commenced at the maramuni arc in northern new guinea at ca 12 ma the sorong fault zone strike slip system developed effectively separating the philippines from the indonesian tectonic domain the sorong fault zone became inactive at ca 6 ma since which time the tectonics of eastern indonesia has been dominated by the anticlockwise rotation of the bird s head structural block by some 30 40 contemporaneously with post 18 ma tectonism the banda arc subduction collision system developed off the northwestern margin of the australian continent convergence between indo australia and eurasia was accommodated initially by northward subduction of the indian ocean and subsequently since ca 8 ma by the development of a second phase of arc continent collision around the former passive continental margin of nw australia

rates of neogene and quaternary tectonic movements in the southern banda arc based on micropalaeontology it is generally considered that the outer banda arc islands are composed in part of strongly deformed sediments that accumulated at the north australian continental margin the onset and completion of folding nappe emplacement imbrication and uplift that produced the islands of the outer banda arc have been dated by means of planktonic foraminifera the dating of the onset of uplift of timor from a deep submarine position at the end of the neogene nappe emplacement to mountains now reaching nearly 3 km high indicates the rate of post collision uplift to have been initially 3 mm yr then it slowed to about 1 5mm yr where the australian continental margin meets the eastern end of the present java trench there are indications that the australian margin has overridden the trench in the timor region by 240 km micropalaeontological dating of the neogene collision between the australian continental slope proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions 7 5cm yr against the rates based on the tectonic model 7 4cm yr after nappe emplacement possibly faster than 6 25 cm yr shortening of the continental crust appears to have migrated towards the australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 ma

continental margin origin for cretaceous radiolarian cherts in western timor in timor fig 1 widespread outcrops of cretaceous cherts belonging to the wai bua and cretaceous kolbano facies have been interpreted as deep sea deposits that probably accumulated on the australian continental rise1 i consider here another but less well known group of cretaceous cherts exposed in western timor at the base of the palelo group which outcrops in mountain massifs with metamorphic rocks of the mutis complex although the observed contacts between the two units are faulted2 3 the palelo group contains fragments of metamorphic rocks similar to those of the mutis complex4 5 suggesting an original unconformable relationship geochemical data6 7 from metasedimentary gneisses in the boi massif suggest that metamorphism occurred at pressures and temperatures likely to prevail deep in the continental crust i suggest that the cretaceous radiolarian cherts were deposited on a continental margin as the first sediments of a forearc basin this is the first reported occurrence of radiolarian cherts deposited in this tectonic sedimentary environment and contrasts strongly with other cretaceous radiolarian cherts exposed on the island the evidence is crucial to understanding the tectonic evolution of the region because it is clear that the two groups of cherts were derived from different sides of the continent island arc collision zone which makes up the banda arcs the palelo group and the rocks it is associated with were derived from the south east asian side which has had an active margin since at least the early cretaceous

stratigraphic constraints on suture models for eastern indonesia abstract although collision in eastern indonesia is now accreting the australian continent to southeast asia the small north and south banda oceanic basins within the suture zone are interpreted as late cenozoic extensional features stratigraphic columns from the surrounding islands conform to one of three generalised patterns two of which can be related to the margins of se asia sundaland and the australian continent respectively the third system which is dominant in the outer banda arc and eastern sulawesi is associated with a microcontinent that was rifted from australia in the jurassic drifted northwards ahead of australia in the cretaceous and collided with the sundaland margin in the paleogene subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the banda sea behind the outer banda arc thrust belt eastern indonesia thus duplicates a pattern familiar in the mediterranean the tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision which began only in the pliocene

middle jurassic radiolaria from a siliceous argillite block in a structural melange zone near viqueque timor leste paleogeographic implications abstract thin bedded siliceous argillite forming a large block within a structural melange zone at viqueque timor leste has yielded a middle jurassic late bathonian early callovian radiolarian assemblage belonging to unitary association zone 7 fifty five species are recognized and illustrated forming the most diverse radiolarian fauna yet documented from the jurassic of timor the fauna shows little similarity in species content to the few other assemblages previously listed from the middle or late jurassic of timor and also has few species in common with faunas known elsewhere in the region from rotti sumatra south kalimantan and sula based on lithofacies similarities and age the siliceous argillite succession in the melange block at viqueque is included in the noni group originally described as the lower part of the palelo series in west timor in terms of lithofacies the noni group is distinct from other stratigraphic units known in timor it may be associated with volcanic rocks but age relationships are uncertain although some of the radiolarian cherts in the noni group in west timor have been reported to include tuffaceous sediment the deep water character of the siliceous hemipelagite pelagite facies the probable volcanic association and an age close to that of continental breakup in the region suggest deposition in a newly rifted indian ocean in timor s tectonostratigraphic classification scheme the noni group is here placed in the indian ocean megasequence

the sumba enigma is sumba a diapiric fore arc nappe in process of formation abstract the anomalous updomed morphological expression of sumba island its enigmatic lack of strong neogene deformation and the northward morphological indentation of southern sumbawa and flores require explanation the stratigraphy of sumba may be correlated with the cretaceous to miocene part of the timor allochthon the sedimentary and eruptive rock succession in sumba shows remarkable similarities with the allochthonous palelo wiluba and cablac deposits of timor in both islands the cretaceous parts of these sequences are regarded as characteristic of fore arc deposits built on thin continental crust the timor nappe is interpreted as a 5 km thick tectonic flake of the banda fore arc thrust onto the australian continental margin in the mid pliocene collision the postulated sumba nappe has not yet been thrust onto the australian margin which in the sumba region has not yet converged as close to the arc as in the timor area the postulated sumba nappe is interpreted as a diapiric elongated dome of the sunda fore arc that is being squeezed by the converging margin of australia against the volcanic islands of sumbawa and flores the absence of indications on the seismic reflection profiles for the presence of the thrust fault of the sumba nappe may perhaps be explained by the thrusts being nearly horizontal within flat lying strata the savu thrust is correlated with the probably older pre late pliocene wetar suture as a major southward dipping lithospheric rupture east of 124 e this suture does not seem to have moved much since the mid pliocene collision that emplaced the nappes on timor however microearthquake data suggest some activity is continuing

the structural setting and tectonic significance of the lolotoi laclubar and aileu metamorphic massifs east timor abstract the lolotoi and laclubar metamorphic massifs of east timor form a part of the lolotoi metamorphic complex based on stratigraphic relationships with the australian affinity parautochthonous stratigraphic succession and structural arguments the lolotoi complex is interpreted as representing australian continental basement entrained into the timor arc continent collision complex by basement involved thrusting the lolotoi complex is not synonymous with the mutis complex defined from west timor which represents allochthonous metamorphic basement derived from the pre collisional banda forearc the aileu complex is a syn collisional metamorphic series with an inverted metamorphic gradient developed beneath overthrust allochthonous peridotite

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

the effects of quartz recrystallization and reaction on weak phase interconnection strain localization and evolution of microstructure abstract we conducted axial compression and general shear experiments at t 900 c and p 1 5 gpa on samples of banded iron formation bif and synthetic aggregates of quartz hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization and the role of weak second phases oxides on the rheology and microstructural evolution of the aggregates experiments showed strain localization into oxide rich layers and that the oxide content and oxide distribution are key factors for the strength of the aggregate only 2 10 wt hematite leads to pronounced weakening and increasing hematite content above 10 has only a minor additional effect where oxide grains are dispersed the initial strength contrast with quartz induces stress concentrations at their tips promoting high stress recrystallization accommodated dislocation creep of quartz fine recrystallized quartz reacts with oxide forming trails of fine reaction product ferrosilite fayalite leading to the interconnection percolation of a weaker matrix the strength contrast between the quartz framework and these fine grained trails promotes strain localization into micro shear zones inducing drastic strain weakening thus dynamic recrystallization of quartz promotes syn deformational reactions leading to a microstructurally controlled evolution of phase strength contrast it results in a rheologic transition from load bearing framework to a matrix controlled rheology with transition from s c to s c fabric with increasing strain