deformation history of tectonic melange and its relationship to the underplating process and relative plate motion an example from the deeply buried shimanto belt sw japan map to microscopic scale structural analysis of the hanazono assemblage of the shimanto belt in sw japan an excellent example of a deeply buried accretionary complex indicates a detailed process from underthrusting to underplating the earliest underthursting process is recorded in fabric of melange which has deformed by shear along the decollement characterized by thinning strain due to extensional breakage of blocks of sandstone and exotic materials such as basalt and pelagic sediments of oceanic affinities the deformation mechanism for melange formation for domain i is plastic flow concordant with metamorphic grade which is higher than that of domain ii the deformation mechanism for domain ii is predominantly pressure solution partly with grain scale brittle breakage for the less metamorphosed part of the hanazono assemblage the posterior underplating process recorded is contraction and thickening due to thrust stacking and folding locations of the folds developed beneath thrusts are emphasized because they are a line of evidence of thickening the main deformation mechanism of the second stage is plastic flow for domain i and cataclasis for domain ii the cataclasis in domain ii may be a result of change in strain rate and seems to be related to seismicity at the time of underplating kinematics deduced from fabric analysis of the earliest deformed melange indicates a good consistency with a relative plate motion between the eurasian and oceanic plates that is estimated from a hot spot reference plate circuit model by engebretson et al 1985 while that of the subsequent thrusting is not this fact suggests that strain partitioning in association with oblique subduction of oceanic plate was minimum during subduction another possibility for strain partitioning is the requirement of coupling between plates that are recorded by the deformation mechanisms

tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin an example from the cretaceous early tertiary mugi melange the shimanto belt japan abstract a tectonic melange exposed on land is examined to reveal relationships between melange formation underplating and deformation mechanisms focusing on the deformation of basaltic rocks the studied mugi melange of the shimanto belt is composed of a shale matrix surrounding various blocks of sandstone pelagic sediments and basalts the melange was formed during late cretaceous to early tertiary times in a subduction zone under p t conditions of 150 200 c and 6 7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions the melange represents a range of deformation mechanisms pressure solution with micro scale cataclasis in the shale matrix brittle tension cracking in the blocks and ubiquitous strong cataclasis in the basal portion of basaltic layers the cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

structural evolution of the shimanto belt accretionary complex in the area of the gokase river kyushu sw japan the shimanto belt of sw japan is an accretionary complex which developed between late cretaceous and late miocene the deeper levels of this complex are exposed along the gokase river of eastern kyushu deformation in this region took place as a result of southeasterly directed accretionary thrusting resulting in the development of folds fabrics and melanges all these structures may be attributed to progressive top to the se shearing and the melanges show many similarities to other rock types in fault or shear zones it is also possible to apply shear zone models to explain extensional structures developed perpendicular to thrust transport direction it is suggested that deformation took place during underplating beneath the developing prism

a community based geological reconstruction of antarctic ice sheet deglaciation since the last glacial maximum a robust understanding of antarctic ice sheet deglacial history since the last glacial maximum is important in order to constrain ice sheet and glacial isostatic adjustment models and to explore the forcing mechanisms responsible for ice sheet retreat such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and sub antarctic islands here we report a new synthesis of those datasets based on an accompanying series of reviews of the geological data organised by sector we present a series of timeslice maps for 20 ka 15 ka 10 ka and 5 ka including grounding line position and ice sheet thickness changes along with a clear assessment of levels of confidence the reconstruction shows that the antarctic ice sheet did not everywhere reach the continental shelf edge at its maximum that initial retreat was asynchronous and that the spatial pattern of deglaciation was highly variable particularly on the inner shelf the deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small antarctic contribution to meltwater pulse 1a we discuss key areas of uncertainty both around the continent and by time interval and we highlight potential priorities for future work the synthesis is intended to be a resource for the modelling and glacial geological community

offscraping accretion of jurassic chert clastic complexes in the mino tamba belt central japan abstract detailed structural and biostratigraphical analysis of the jurassic inuyama sequence a coherent chert clastic complex in the mino tamba belt central japan clarifies the evolution of accretionary processes at shallow structural levels the inuyama sequence is characterized by a series of stacked thrust sheets each sheet consists of an early triassic to middle jurassic oceanic plate stratigraphy composed of four lithologic units which are in ascending order siliceous claystone ribbon chert siliceous mudstone and clastic rocks the structural features of the inuyama sequence demonstrate a four stage progressive deformation 1 a decollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front clastic dikes and sills formed at the horizon just above the decollement at this time 2 the stratigraphic section above the decollement was imbricated by in sequence thrusting in the frontal part of the wedge this initial stage of thrusting and imbrication was followed by 3 the formation of duplex structures with fault related folds within the lower stratigraphic section as the decollement stepped down section to the lowest siliceous claystone interval finally 4 these thrust packages were overprinted by secondary prism thickening in the form of out of sequence thrust faulting

the mid quaternary climatic transition as the free response of a three variable dynamical model abstract a simplified version of a previously described dynamical model governing global ice mass atmospheric carbon dioxide and mean ocean temperature that may also be a proxy for some other co2 controlling oceanic variable e g nutrient supply is shown to possess solutions in a realistic parameter range that can replicate the main features of the climatic variations implied by the full two million years quarternary 18o record these variations include a major transition between a low ice 18o low variance mode before roughly 900 kyr bp to a high ice near 100 kyr period variance mode after that time to the present the model contains only three free parameters in this simplified form no external earth orbital forcing is prescribed i e the model represents only internal dynamics from the previous studies it seems clear that additional variance representing such features as the rapid deglaciation and the phase of the major quaternary oscillation can be largely explained with no more

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

intra arc deformations with vertical rotation axes the case of the pre middle miocene terranes of southwest japan abstract various folds bends with vertical or subvertical rotation axes occur in the pre middle miocene terranes of southwest japan they are variable in shape conical folds accordion folds megakink bands and so on and some are accompanied with strike slip faults they range from several kilometers to more than 100 km in width and some of their hinge surfaces are traceable more than a few tens of kilometers their shapes which resemble megakink band or buckle folds indicate that they were produced by horizontal compression with vertical or subvertical 2 axes in a shallow level of the crust they were superimposed on the pre middle miocene fold and thrust structures or nappe melange complexes and are regarded to have been formed mostly during the middle miocene contemporaneous with the clockwise rotation of southwest japan related to the fan shaped opening of the sea of japan geometrical analysis clearly suggests that the drifting southwest japan behaved as a non rigid body and the formation of these structures resulted from the differential rotation of the terranes and the collision of the izu bonin arc with eastern southwest japan

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

surface water temperature changes in the high latitudes of the southern hemisphere over the last glacial interglacial cycle a set of numerical equations is developed to estimate past sea surface temperatures sst from fossil antarctic diatoms these equations take into account both the biogeographic distribution and experimentally derived silica dissolution the data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments factor analysis of 166 samples 124 holocene core top and 42 artificial samples resolved four factors three of these factors depend on the water mass distribution one subantarctic and two antarctic assemblages factor 4 corresponds to a dissolution assemblage inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and gives accurate estimates of sst over a wide range of biosiliceous dissolution a transfer function dtf 166 34 4 is derived from the distribution of these factors versus summer sst its standard error is 1 c in the 1 to 10 c summer temperature range this transfer function is used to estimate sst changes in two southern ocean cores 43 s and 55 s which cover the last climatic cycle the time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios the reconstructed sst records present strong analogies with the air temperature record over antarctica at the vostok site derived from changes in the isotopic ratio of the ice this similarity may be used to compare the oceanic isotope stratigraphy and the vostok time scale derived from ice flow model the oceanic time scale if taken at face value would indicate that large changes in ice accumulation rates occurred between warm and cold periods

deformation process and kinematics of melange in the early cretaceous accretionary complex of the mino tamba belt eastern southwest japan 1 the kanayama unit of the mesozoic mino tamba accretionary complex in eastern southwest japan is a thick melange unit composed mainly of abundant muddy matrix and variously sized clasts of chert hemipelagic siliceous mudstone and sandstone two episodes of deformation formed the melange fabrics the first involved fragmentation of sandstone layers in response to mud injections the second involved layer parallel noncoaxial shear that resulted in the mixing of pelagic to terrigenous clasts and the formation of s c like asymmetric fabrics with scaly foliation the s c fabrics exhibit a predominantly sinistral shear sense in plan view after fold and tilt corrections the restored slip motion inferred from the s c fabrics indicates a bulk top to the south shear combining the unit s terminal fossil age and radiometric age with a regional plate reconstruction model these data suggest that the kanayama melange formed along a decollement during oblique subduction of the izanagi plate in the early cretaceous

kinematic analysis of shear zones in sandstone and mudstone of the shimanto belt shikoku sw japan abstract i describe kinematic criteria for sense of shear in unmetamorphosed interbedded sandstone and mudstone of the tertiary shimanto accretionary complex at ariigawa k chi prefecture in certain zones parallel to bedding and up to several meters wide sandstone layers with internal bedding parallel turbidite laminations are broken into blocky rectangular fragments or inclusions many inclusions rotated counterclockwise and so indicate a sinistral sense of shear with respect to the present orientation of bedding asymmetric folds corroborate this interpretation amounts of rotation are variable and are inversely proportional to the axial ratios of fragments fragments with axial ratios greater than about 4 6 did not rotate while those with ratios between 4 and 1 record progressively greater maximum rotations these observations accord with theoretical and experimental analyses describing the behavior of rigid inclusions in a matrix subjected to simultaneous simple shear and pure shear the data from the shimanto suggest that 1 the rotation inducing deformation included a coaxial component and departed from strictly non coaxial simple shear and 2 total shear strains were of the order of 3 5

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

criteria helpful in recognizing shear zone and diapiric melanges examples from the hoh accretionary complex olympic peninsula washington a study of the middle miocene hoh melanges exposed on the western olympic peninsula washington provides criteria for differentiating between shear zone and diapiric melanges the shear zone melange is characterized by a consistent foliation orientation throughout the outcrop intense pervasive scaly foliation uniform fold vergence directions exotic clasts predominantly phacoidal clasts and consistent clast long axis lineations the diapiric melange is characterized by radial foliation in map view well developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange opposite fold vergence directions across the melange and rare exotic clasts additionally clasts vary in shape from phacoidal at the contacts to angular in the center in the diapiric melange large clasts are found only in the center of the melange and clast long axes vary from tightly clustered at the contacts to near random orientations in the center the systematic variation in clast size in the diapir may be due to increased shear at the margins and or clast migration to the center of the diapir caused by a velocity viscosity contrast interpretation of field data indicates that some clasts rotate continuously in the matrix yet clast long axes have a definite preferred orientation this contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane thus although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane parasitic folds in the matrix indicate that clast long axes align parallel to transport direction small scale folds in the matrix clast long axis lineations and the mean foliation indicate that the hogsback section of the hoh melange was emplaced by a high angle oblique thrust directed to n10 e

asymmetrical melange fabrics as possible indicators of the convergent direction of plates a case study from the shimanto belt of the akaishi mountains central japan abstract scaly foliated melanges in the upper maastrichtian lowest paleogene strata occurring in a zone several kilometers wide in the shimanto belt of the akaishi mountains central japan often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry they are characterized by block in matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers these fabrics are geometrically very similar to those of foliated fault related rocks another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer parallel contraction the co existence of these extensional and contraction planar fabrics suggests that non coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation suggesting that a south trending overthrust shear about 40 70 counterclockwise with respect to the trend of strata prevailed during melange formation this direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age which indicates the oblique subduction of the pacific plate relative to the eastern margin of the eurasian continent thus the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates and may become excellent geological indicators for determining the direction of convergence of plates during melange formation since data on melange fabrics are directly and easily available in the field

atmospheric co2 decline during the pliocene intensification of northern hemisphere glaciations 1 several hypotheses have been put forward to explain the onset of intensive glaciations on greenland scandinavia and north america during the pliocene epoch between 3 6 and 2 7 million years ago ma a decrease in atmospheric co2 may have played a role during the onset of glaciations but other tectonic and oceanic events occurring at the same time may have played a part as well here we present detailed atmospheric co2 estimates from boron isotopes in planktic foraminifer shells spanning 4 6 2 0 ma maximal pliocene atmospheric co2 estimates gradually declined from values around 410 atm to early pleistocene values of 300 atm at 2 0 ma after the onset of large scale ice sheets in the northern hemisphere maximal pco2 estimates were still at 2 5 ma 90 atm higher than values characteristic of the early pleistocene interglacials by contrast pliocene minimal atmospheric co2 gradually decreased from 310 to 245 atm at 3 2 ma coinciding with the start of transient glaciations on greenland values characteristic of early pleistocene glacial atmospheric co2 of 200 atm were abruptly reached after 2 7 ma during the late pliocene transition this trend is consistent with the suggestion that ocean stratification and iron fertilization increased after 2 7 ma in the north pacific and southern ocean and may have led to increased glacial co2 storage in the oceanic abyss after 2 7 ma onward

carbon cycle instability as a cause of the late pleistocene ice age oscillations modeling the asymmetric response a dynamical model of the pleistocene ice ages incorporating many of the qualitative ideas advanced recently regarding the possible role of ocean circulation chemistry temperature and productivity in regulating long term atmospheric carbon dioxide variations has been constructed this model involves one additional term and free parameter beyond that included in a previous model b saltzman and a sutera 1987 providing the capacity for an asymmetic for example saw toothed response it is shown that many of the main features exhibited by the 18o derived ice record and the vostok core 13c derived carbon dioxide record in the late pleistocene can be deduced as a free oscillatory solution of the model including a rapid deglaciation during which a spike of high co2 and a rapid surge in north atlantic deep water production occurs it is expected that the addition of reasonable levels of external for example earth orbital forcing will enable the model to account for a significant amount of the remaining observed variance and covariance of the slow response climatic variables over the full pleistocene including the mid pleistocene transition

highly variable latest pleistocene holocene incremental slip rates on the awatere fault at saxton river south island new zealand revealed by lidar mapping and luminescence dating geomorphic mapping using high resolution lidar imagery and luminescence dating reveal highly variable incremental holocene latest pleistocene slip rates at the well known saxton river site along the awatere fault a dextral strike slip fault in the marlborough fault system south island new zealand using lidar and field observations we measured seven fault offsets recorded by fluvial terraces and bedrock markers improved dating of the offsets is provided by post ir irsl225 luminescence ages incremental slip rates varied from 15 mm yr over intervals of thousands of years and tens of meters of slip demonstrating order of magnitude temporal variations in rate at a single site these observations have basic implications for earthquake fault behavior lithospheric mechanics discrepancies between geodetic and geologic slip rates and probabilistic seismic hazard assessment

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

kinematic analysis of sinistral cataclastic shear zones along the northern margin of the mino belt central japan abstract in this paper cataclastic shear zones along the northern margin of the mino belt central japan are described and the significance of the shearing in the tectonic evolution of sw japan is examined the mino belt in sw japan is composed of accretionary complexes of jurassic to early cretaceous age field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the mino belt closely spaced cleavage is developed in these shear zones lineation on the cleavage plunges at shallow to moderate angles deformation structures e g composite planar fabric and asymmetric structure of clasts in the sheared rocks clearly indicate a sinistral sense of shear the shearing ceased by latest cretaceous time because the sheared rocks are overlain by unsheared upper cretaceous volcanic rocks the sinistral shearing may be closely related to cretaceous sinistral movement along the eastern margin of asia sinistral shearing along the northern margin of the mino belt can be considered as a key for re examining the tectonic development of sw japan

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

quaternary glaciations from observations to theories abstract ice ages are known since the mid nineteenth century from the beginning they have been at the center of theories of climate and climate change still the mechanisms behind these large amplitude oscillations remain poorly understood in order to position our current knowledge of glacial interglacial cycles it is useful to present how the notion of climate change appeared in the xixth century with the discovery of glacial periods and how the two main theories the astronomical one and the geochemical one emerged progressively both from sound physical principles but also from extravagant ideas major progresses in geochemistry in the xxth century led first to the firm evidence of an astronomical pacemaker of these cycles thanks to the accumulation of paleoceanographic data still the milankovitch s theory predicts an ice age cyclicity of about 41 000 years while the major periodicity found in the records is 100 000 yr besides ice cores from antarctica proved unambiguously that the atmospheric carbon dioxide was lower during glacial periods even more importantly during the last termination the atmospheric pco 2 increases significantly by about 50 ppm several millenia before any important change in continental ice volume this fact together with many other pieces of information strongly suggests an active role of greenhouse gases in the ice age problem at least during deglaciations since terminations are precisely at the heart of the 100 ka problem we need to formulate a new synthesis of the astronomical and geochemical theories in order to unravel this almost two century old question of ice ages the foundations of such a theory have already been put forward and its predictions appear in surprisingly good agreement with many recent observations

the antarctic ice sheet and the triggering of deglaciations a new physical mechanism involving the antarctic ice sheet extent is able to link climatic and co2 glacial interglacial changes it is furthermore able to explain many features of the glacial cycles like the 100 kyr oscillations or the peculiarities of stage 11 about 400 kyr bp indeed from recent results the glacial ocean bottom waters were possibly much more saline and consequently may have an unsuspected large density this glacial deep stratification could account for a significant part of the glacial interglacial co2 difference the formation of these waters around antarctica involves brine rejection over the continental shelves and is directly linked to changes in sea ice formation and antarctic ice sheet extent using this new scenario we formulate a conceptual model that reproduce the succession and the amplitude of glaciations over the last few million years this new mechanism furthermore provides the clue to understanding the changes in the main climatic frequency from 23 to 41 ka about 3 myr ago and from 41 to 100 ka about 1 myr ago the antarctic ice sheet influence on bottom water formation could provide the bmissing pieceq to complete the astronomical theory of quaternary climates d 2004 elsevier b v all rights reserved

paleomagnetic evidence for the clockwise rotation of southwest japan abstract over 500 oriented samples of felsic rocks of cretaceous to middle miocene age were collected along the go river in the central part of southwest japan in an attempt to detect the process of tectonic rotation of southwest japan from the paleomagnetic view point thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities the paleomagnetic results establish that southwest japan began to rotate clockwise through58 14 later than 28 ma and ceased its motion by about 12 ma southwest japan has undergone no detectable north south translation since 28 ma these results imply that southwest japan was rotated about the pivot around 34 n 129 e between 28 ma and 12 ma in association with the opening of the japan sea

off scraping accretionary process under the subduction of young oceanic crust the shimanto belt of okinawa island ryukyu arc the eocene kayo formation exposed at the southwestern end of the shimanto belt in the ryukyu arc records the progressive development of off scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint the formation consists almost wholly of coherent trench turbidites supplied by axial currents detailed structural analysis has revealed that two successive deformation stages are recognized in the formation the early deformation stage 1 occurred by horizontal shortening related to the initial stage of off scraping accretion in this stage sands were water rich and nearly unlithified and the competence contrast between sandstone and shale was low the late deformation stage 2 is marked by the formation of seaward verging imbricate fold and thrust system within the accretionary wedge compared with stage 1 sands in stage 2 were more dewatered and lithified and the competence contrast between sandstone and shale increased in accordance with this deformation history structures caused by high pore fluid pressure indicate increased cohesion of sands during off scraping accretion studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the formation became buried and suffered thermal imprint pressure solution cleavage developed as an axial planar to the stage 2 folds thermal imprint on the formation is characterized by high paleotemperatures at approximately 250 c 300 c in a region of the prehnite pumpellyite facies metamorphism this thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate

evolution and kinematics of an ancient decollement zone melange in the shimanto accretionary complex of okinawa island ryukyu arc detailed structural analysis of the melange in the shimanto accretionary complex of okinawa island in the ryukyu arc elucidates the spatial distribution and temporal progression of decollement related deformation early deformation took place in the footwall of the decollement and is characterized by subhorizontal layer parallel extension and subsequent heterogeneous shear of partially lithified sediments resulting in alternation of asymmetric e g s c fabrics and layer parallel extensional boudined layers fabrics mud intrusions suggest that high fluid pressure was locally generated in the footwall of the decollement late deformation is marked by the partitioning of deformation in the melange the lower structural level of the melange was pervasively sheared along microfaults whereas the upper structural level was flattened with development of planar pressure solution cleavage because shear is concentrated in the lower structural level late deformation is considered to have occurred in the hanging wall of the decollement kinematic indicators suggest that shear directions during decollement related deformation reflect the relative plate motion during early tertiary after the melange was incorporated into the accretionary prism crenulation cleavage and cylindrical upright folds were formed under subhorizontal shortening

last glacial maximum co2 and δ13c successfully reconciled 1 during the last glacial maximum lgm 21 000 years ago the cold climate was strongly tied to low atmospheric co2 concentration 190 ppm although it is generally assumed that this low co2 was due to an expansion of the oceanic carbon reservoir simulating the glacial level has remained a challenge especially with the additional 13c constraint indeed the lgm carbon cycle was also characterized by a modern like 13c in the atmosphere and a higher surface to deep atlantic 13c gradient indicating probable changes in the thermohaline circulation here we show with a model of intermediate complexity that adding three oceanic mechanisms brine induced stratification stratification dependant diffusion and iron fertilization to the standard glacial simulation which includes sea level drop temperature change carbonate compensation and terrestrial carbon release decreases co2 down to the glacial value of 190 ppm and simultaneously matches glacial atmospheric and oceanic 13c inferred from proxy data lgm co2 and 13c can at last be successfully reconciled

structural evolution of underthrusted sediments kodiak islands alaska the late cretaceous to early tertiary sediments of the kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin because of the thick trench fill this early deformation resulted in the formation of two distinct structural terranes 1 coherent terranes of layered turbidites and 2 melange terranes the earliest deformation d1 in the coherent terranes is marked by normal faults mineralized layer perpendicular veins in sandstones and microfaults in shales these features suggest that 1 was vertical and that 3 was horizontal and nw trending during d1 we interpret d1 structures as hydrofractures that formed in flat lying sediments with high fluid pressures caused by rapid tectonic loading in contrast the melanges appear to record a more protracted and complex history that culminated in structures similar to the d1 structures above we have recognized three successive transitional stages in the melanges i breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix resulting in the blocks in matrix character of the melange ii development of ductile shear bands that crosscut the melange fabric at a low angle and iii hydrofracturing with the formation of calcite and quartz filled veins in sandstone inclusions and microfaults in the shales the sequence of microstructures associated with stages i iii records increased cohesion of the sediments during deformation dominated by layer parallel shear intense mineralization and microfaulting during stage iii suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability stage iii structures are texturally indistinguishable and contemporaneous with the less pervasive d1 structures in the coherent terranes based on these observations we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement the coherent terranes represent the structurally and stratigraphically lowest sediments and they bypassed the stage i and ii deformations of the melanges because they were the greatest distance from the decollement when they were deformed moreover both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism