strength of plate coupling in the southern ryukyu subduction zone abstract understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones the interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy here we calculate the mantle lithosphere buoyancy across the northern portion of the southern ryukyu subduction zone based on gravity modeling with the constraints from a newly derived p wave seismic velocity model the result indicates that the strength of the plate coupling in the study area is relatively strong which is consistent with previous observations in the southernmost ryukyu subduction zone because few large earthquakes mw 7 have occurred in the southern ryukyu subduction zone a large amount of energy is locked and accumulated by plate coupling that could be released in the near future

displacement partitioning and arc parallel extension in the aleutian volcanic island arc abstract convergence between the pacific and north american plates along the aleutian volcanic island arc is generally right oblique the obliquity increases from 0 in the east to nearly 90 in the west where the plate boundary becomes a transform fault zone preliminary results of a structural geology study of attu island the most westerly u s island in the aleutian chain and part of the near islands group shows that the deformational structures are not directly related to the relative plate convergence rate vector but to the plate boundary normal and parallel components of this vector the plate boundary sub normal component is responsible for conjugate sets of arc sub parallel thrust faults and the plate boundary parallel component resulted in arc parallel dextral strike slip faults and their conjugates however the most penetrative structures measured in outcrop are conjugate sets of strike slip faults and arc perpendicular normal faults and fractures veins and dikes which have formed by arc parallel extension this extension is most likely the result of the increase of obliquity from east to west and thus the increase of the arc parallel component of the convergence rate vector along the aleutian arc these structures require that attu island migrated westward possibly for as many as 1150 km

alkenone and coccolith records of the mid pleistocene in the south east atlantic implications for the u37k index and south african climate abstract successful application of the alkenone palaeothermometer the u 37 k index relies upon the assumption that fossil alkenone synthesisers responded to growth temperature changes in a similar manner to the modern producers chiefly the coccolithophores emiliania huxleyi and gephyrocapsa oceanica we compare coccolith and u 37 k data from odp site 1087 in the south east atlantic between 1500 and 500 ka and show that evolutionary events and changes in species dominance within the coccolithophore populations had little impact on the u 37 k record the relative abundances of the c37 and c38 alkenones also closely resembled those found in modern populations and suggest a similar temperature sensitivity of u 37 k during the early and mid pleistocene to that found at present these results support the application of the u 37 k index to reconstruct sea surface temperatures ssts throughout the quaternary the u 37 k record at odp site 1087 contains an sst signal that documents the emergence of the 100 kyr cycles that characterise the late quaternary ice volume records this is preceded by significant cooling at odp site 1087 marked by a negative shift in ssts and a positive shift in the planktonic 18o some 250 kyr earlier at ca 1150 1000 ka this results in a permanent fall in average ssts of around 1 5 c the predicted increase in aridity onshore as a result of this cooling can be identified in a number of published records from southern africa and may have played a role in some important evolutionary events of the mid pleistocene

changes in sand content of loess deposits along a north south transect of the chinese loess plateau and the implications for desert variations geological records have shown that the deserts east of the helan mountains in northern china were covered by grass during the holocene optimum whereas during marine oxygen isotope stages 2 and 4 distribution of the deserts was almost the same as at present the wide advance retreat cycles of the deserts may have exerted an important control on grain size changes in the loess of the loess plateau by altering the distance between the source and the accumulation zone of the loess this challenges the widely accepted model that winter monsoon winds were the sole factor responsible for spatial and temporal changes in loess texture to observe spatial changes in sedimentological characteristics of loess during the last glacial interglacial cycle the texture of loess was measured along a north south transect of the loess plateau this transect consists of nine loess sections starting at yulin in the transitional region between the loess plateau and the mu us desert and ending at weinan in the southernmost part of the loess plateau southward changes in sand 63 m content along the transect suggest that variations in desert extent have indeed played a significant role in loess grain size distributions particularly in the northern part of the loess plateau it is proposed that sand content 63 m of loess in the loess desert transitional zone may be used as a proxy indicator for proximity to the desert margin

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

high altitude aeolian research on the tibetan plateau aeolian processes and their role in desertification have been studied extensively at low elevations but have been rarely studied at high elevations in areas such as the tibetan plateau where aeolian processes were active in the geologic past and remain active today in this review we summarize research that improves our understanding of aeolian processes on the tibetan plateau including the distribution characteristics and provenance of aeolian sediments the history of aeolian activity aeolian geomorphology and wind driven land degradation contemporary aeolian processes primarily occur in dry basins in wide river valleys on lakeshores on mountain slopes and on gravel pavements sediment characteristics suggest a local origin and provide interesting contrasts with those of china s loess plateau the history of aeolian activity and its paleoclimatic implications reconstructed based on aeolian archives is short mostly since the late glacial and shows wide regional differences aeolian geomorphology is simple and suggests short formation time wind driven land degradation is less severe than previously thought driven by different factors in different areas and exhibited complex interactions with freeze thaw processes aeolian research has been conducted within the general framework of aeolian science but addresses issues specific to the tibetan plateau that arise due to the low air temperature low air density and the presence of a cryosphere we propose six priorities for future research aeolian physics the effect of freeze thaw cycles comparisons with other areas regional differences effects of wind driven land degradation and integrated observation and monitoring

late quaternary aeolian activity in the mu us and otindag dune fields north china and lagged response to insolation forcing 1 dune fields in parts of northern china contain important stratigraphic records of late quaternary change in the east asian monsoon in this study 33 new optically stimulated luminescence osl ages and other measurements from aeolian sediment sections are used to reconstruct the timing of wet dry climate variation in the mu us and otindag dune fields of north china the results indicate dune activity and dry climate in the last few hundred years 14 ka to about 7 8 ka and 50 ka to 60 ka the dunes were mainly stable implying a wetter climate between about 7 8 ka and 2 4 ka these results imply a lag of several thousand years between peak summer insolation at 10 11 ka and high summer monsoon rainfall after 7 8 ka in the investigated regions the monsoon climate may not respond directly to orbital forcing over millennial time scales land surface feedbacks may account for lagged dune field response

holocene climate changes over the desert loess transition of north central china a 2 m trench section at the desert loess transition of north central china was analyzed at 2 cm intervals for grain size distribution md m and total organic carbon toc ams 14c time series of the md and toc records provides direct information on variations of the east asian monsoon climate during the last 10 000 yr md is considered to represent the wind strength and high md reflects strong winter monsoon winds toc denotes the degree of vegetation cover a function of the intensity of summer monsoon circulations from ca 10 000 to 7000 cal yr bp the climate was dominated by a strong winter monsoon and a very weak summer monsoon as indicated by high md and low toc values subsequently climatic conditions became warmer gradually from ca 7000 to 5500 cal yr bp the period between ca 5500 and 2700 cal yr bp displayed the highest toc and lowest md values over the last 10 000 yr marking a stable state of a warm humid climate i e the holocene climatic optimum of china during the interval of ca 2700 1500 cal yr bp the expansion and intensification of agrarian activities could have led to desertification and acceleration of deposition of coarse grained particles resulting in a decreased organic carbon accumulation rate however the toc during this interval still has values as high as during the episode of ca 7000 5500 cal yr bp reflecting a relatively stronger summer monsoon circulation since ca 1500 cal yr ago high amplitude fluctuations of toc that fluctuate in phase with the clay size fraction imply that the climate experienced three major oscillations with a prominent peak of enhanced summer monsoon circulation at ca 1300 cal yr ago and with two intervals of increased winter monsoon strength occurring around ca 1000 and 300 cal yr ago the md and toc records over the past 5000 yr demonstrate a good correlation with the archaeological phenological and historical records of china climate changes in the east asian monsoon region were not only related to the changing seasonality of solar insolation resulting from progressive changes in the earth s orbital parameters but were also controlled by the pattern and intensity of the atmospheric circulation

stepwise expansion of desert environment across northern china in the past 3 5 ma and implications for monsoon evolution abstract a systematic study of the last glacial cycle along three transects across the chinese loess plateau shows that sand sized particle content within loess decreases rapidly from north to south and that markedly high sand particle contents in loess horizons occur only in the northern part of the plateau this suggests that variation in the sand sized particle fraction within loess near the desert margin is closely linked to migration of the southern desert border in northern china where sand grains move mainly in saltation or modified saltation mode near the ground surface as desert margin shift is essentially controlled by the amount of monsoon precipitation the sand sized particle content within loess near desert margin is regarded as a new and readily applied proxy for variations in the strength of the east asian summer monsoon a continuous record of sand content in loess along the loess desert transitional zone shows that the mu us desert migrated southward at 2 6 1 2 0 7 and 0 2 ma suggesting a stepwise weakening of the east asian summer monsoon during the past 3 5 ma this evolutionary pattern is significantly different from that previously inferred from loess magnetic susceptibility records a widely used monsoon proxy our results further suggest that changes in global ice volume may have been an essential factor in controlling plio pleistocene monsoon evolution and that the anticipated future melting of polar ice cover may lead to a northward migration of the monsoon rainfall belt in northern china

identifying eolian dust in the geological record abstract recognition of terrestrial dust in geological records is essential for reconstructing paleoenvironments and quantifying dust fluxes in the past however in contrast to eolian sands silt sized dust is difficult to recognize in pre quaternary records due to a lack of macroscopic features indicating eolian transport and mixing with alluvial sediments windblown dust deposits are commonly identified by comparing their sedimentological and petrological features with quaternary examples of dust known as loess here we review the characteristics of terrestrial dust deposits and conclude that most of these features are not exclusively windblown and may be formed by alluvial deposits as well we therefore synthesize a set of criteria which enable a reliable identification and quantification of dust while acknowledging potential contributions of alluvial components these methods include quartz grain surface morphology analysis to distinguish eolian and alluvial transport modes provenance studies to identify local and extrabasinal sources grain size shape end member modelling to quantify the various sedimentary contributions to the record and a basin scale stratigraphic approach to derive regional patterns and avoid interpretation of local phenomena we reassess the eocene to pliocene records of the chinese loess plateau and conclude that these strata represent both alluvial and eolian sediments deposited in extensive mudflat systems quaternary loess by contrast is almost exclusively composed of windblown dust the early pleistocene shift from mudflat to loess deposits is associated with a significant increase in accumulation rates likely due to increased dust production upwind overwhelming and blanketing the local mudflat systems in central china

deposits and soils of the past 130 000 years at the desert loess transition in northern china the desert loess transitional zone in north central china has long been thought sensitive to quaternary climatic change however reconstruction of quaternary climates in this area has been hindered by incompleteness of geological sections here we report the analytical results of two recently found sand loess soil sections both sections have thick eolian deposits from the last interglacial glacial cycle and can be correlated with one another field observations thermoluminescence dating and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers loess from the last glacial period is interbedded with three sand horizons that represent desert extension the expansion and contraction of desert in northern china may have been forced by the east asia monsoon

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

mars sedimentary rock erosion rates constrained using crater counts with applications to organic matter preservation and to the global dust cycle abstract small crater counts on mars light toned sedimentary rock are often inconsistent with any isochron these data are usually plotted then ignored we show using an 18 hirise image 104 crater dataset that these non isochron crater counts are often well fit by a model where crater production is balanced by crater obliteration via steady exhumation for these regions we fit erosion rates we infer that mars light toned sedimentary rocks typically erode at 102 nm yr when averaged over 10 km2 scales and 107 108 yr timescales crater based erosion rate determination is consistent with independent techniques but can be applied to nearly all light toned sedimentary rocks on mars erosion is swift enough that radiolysis cannot destroy complex organic matter at some locations e g paleolake deposits at sw melas but radiolysis is a severe problem at other locations e g oxia planum the data suggest that the relief of the valles marineris mounds is currently being reduced by wind erosion and that dust production on mars

evolution of the southern taiwan sinzi folded zone and opening of the southern okinawa trough abstract recent interpretation of seismic sections and free air gravity anomalies in offshore northern taiwan reveals that the southern taiwan sinzi folded zone began to form in late middle miocene though it was mainly constructed in the late pliocene with strong reverse faulting and folding two westward progradational sequences were deposited in the shelf basin with sediments supplied from the southern taiwan sinzi folded zone and the southern ryukyu arc these two structures are displaced by several northwest striking dextral strike slip faults that were active in the early quaternary when the clockwise rotated southern ryukyu arc and the folded southern taiwan sinzi folded zone were broken it is believed that recent extension in the southern okinawa trough started in the early quaternary because uplift on the southern taiwan sinzi folded zone continued to latest pliocene early quaternary paleogene miocene sediments of the east china sea shelf in the western part of the southern okinawa trough basin are interpreted to indicate that the east china sea shelf basin extended to the east of the southern taiwan sinzi folded zone

sunda strait and central sumatra fault sunda strait marks the limit between java trench frontal subduction and sumatra trench oblique subduction the right lateral central sumatra fault accommodates the oblique subduction it does not pass sunda strait but ends between sumatra and java in a complex pattern of dominantly normal faults associated with subsidence seismicity and volcanism we examine the implications of this phenomenon in sunda strait and in the adjacent fore arc basin and accretionary prism which underwent north south compression due to subduction as well as east west extension we propose that the strait is a consequence of the northwestward motion of the southwestern sumatra block along the central sumatra fault we also propose that the extensional zone in the sunda strait widens considerably to the southwest toward the trench and changes into a composite zone of deformation affected by strike slip as well as normal faulting

holocene glaciation and climate evolution of baffin island arctic canada abstract lake sediment cores and cosmogenic exposure ce dates constrain the pattern of deglaciation and evolution of climate across baffin island since the last glacial maximum lgm ce dating of erratics demonstrates that the northeastern coastal lowlands became ice free ca 14 ka as the laurentide ice sheet lis receded from its lgm margin on the continental shelf coastal lakes in southeastern baffin island started to accumulate sediment at this time whereas initial lacustrine sedimentation was delayed by two millennia in the north reduced organic matter in lake sediment deposited during the younger dryas chron and the lack of a glacial readvance at that time suggest cold summers and reduced snowfall ice retreated rapidly after 11 ka but was interrupted by a widespread readvance of both the lis and local mountain glaciers 9 6 ka cockburn substage a second readvance occurred just before 8 ka during a period of unusually cold summers corresponding to the 8 2 ka cold event in the greenland ice sheet most local glaciers were behind their present margins before 7 ka and in some instances much earlier although the foxe dome of the lis continued to slowly retract toward the present day barnes ice cap throughout the holocene pollen in lake sediments is rare and dominated by exotic sources prior to 12 ka subsequently grass tundra became established followed by modern tundra vegetation ca 8 ka with subtle changes in pollen assemblages in the late holocene lake primary productivity peaked in the early holocene before terrestrial vegetation or marine surface waters reached their apparent thermal maxima lacustrine marine and glacial proxies all reflect significant late holocene cooling the onset of neoglaciation is well dated in lacustrine records at ca 6 ka with intensification after 2 5 ka the expansion of local glaciers during the little ice age represents the most extensive advance since 7 ka we suggest that the replacement of atlantic surface waters by cold low salinity arctic ocean water coupled with the steady reduction of summer insolation resulted in a significant positive sea ice feedback that produced a larger late holocene summer temperature depression over the baffin region than in the pacific sector of the arctic

binary sources of loess on the chinese loess plateau revealed by u pb ages of zircon the age distribution of detrital zircon has been used to trace sediment sources existing datasets show great similarity of zircon ages between the loess on the chinese loess plateau clp and the sediments from the north tibetan plateau ntp implying that eolian dust is delivered from the ntp to the clp by westerly winds or via the yellow river however ntp dust can also be transported by northwesterly winds from the alxa arid lands aals where materials are received from both the ntp and the gobi altay mountains gams here we report u pb zircon ages for aals sands and ntp and clp loess the results show that the zircons in the aals are mixed from ntp and gams zircons ntp loess is mainly derived from local sources mixing of materials from the ntp and gams defines the zircon ages of the loess on the clp better than the pure ntp source no temporal and spatial heterogeneities of zircon ages have been observed for the loess on the clp which suggests that the well mixed materials in the aals likely have an eolian source

luminescence dating of sand loess sequences and response of mu us and otindag sand fields north china to climatic changes the sand loess transition zone in north china is sensitive to climate change and is an ideal place to investigate past environmental changes however past climate change at millennial centennial timescales in this region has not been well reconstructed because of limited numerical dating alternations of sandy loam soils with aeolian sand layers in the mu us and otindag sand fields which lie along the sand loess transition zone indicate multiple intervals of dune activity and stability this change is probably a response to variations of the east asian monsoon climate during the late quaternary the single aliquot regeneration sar optically stimulated luminescence osl dating protocol which has been successfully applied to aeolian deposits worldwide is applied to these two sand fields in this study the osl ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale sections in both sand fields contain aeolian sand beds recording millennial scale episodes of dry climate and widespread dune activation including episodes at about the same time as heinrich event 5 and the younger dryas in the north atlantic region these results demonstrate the potential of aeolian sediments in semi arid north china to record millennial scale climatic events and also suggest that dry wet climate variation at the desert margin in china may be linked to climatic change elsewhere in the northern hemisphere through atmospheric circulation this article was published online on 27 november 2008 an error was subsequently identified this notice is included in the online and print versions to indicate that both have been corrected 16 december 2008 copyright 2008 john wiley sons ltd

a magnetostratigraphic record of landscape development in the eastern ordos plateau china transition from late miocene and early pliocene stacked sedimentation to late pliocene and quaternary uplift and incision by the yellow river abstract a continuous fluviolacustrine record provides the geochronological framework to reconstruct the fluvial landscape evolution of the middle reach of the huanghe yellow river magnetostratigraphic records from six sections at the north of the jinshaan gorge indicate that an isolated fluviolacustrine system occupied this gorge from 8 3 to 3 7 ma local fluvial incision initiated at the lake margin around 5 3 4 9 ma and concomitant fluvial deposits intruded laterally into fine lacustrine strata increasing the sedimentation rate ca 3 1 cm ky within the isolated lake the temporal coincidence of the onset of uplift driven valley incision at 3 7 ma along the yellow river and the onset of basin excavation suggest that uplift possibly played an important role in the late pliocene quaternary drainage evolution consequently incision of the yellow river upstream in the jinshaan gorge initiated at 3 7 ma

spatial variation of the crustal stress field along the ryukyu taiwan luzon convergent boundary 1 we applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the philippine sea plate our results indicate that the crustal stress along the ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction including the tokara channel an extensional stress regime is observed along the entire ryukyu back arc implying that back arc rifting may have extended northward to kyushu a triangular area near the southernmost terminus of the ryukyu arc is characterized by a unique stress signature the eastern boundary of this ryukyu taiwan stress transition coincides with the 123 e meridian where the gagua ridge intercepts the ryukyu trench whereas its western boundary agrees remarkably well with the border between the postcollision and waning collision domains in northern taiwan the taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the lukang magnetization high lmh suggesting that the lmh may be critical in controlling the local stress distribution the stress signature of the luzon arc taiwan collision reaches as far south as 19 5 n the tectonic stress along the manila trench luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in slab membrane stress since this extensional regime is observed only south of 22 n it probably marks the northern limit of the contemporary boundary between the subduction along the manila trench and the collision in taiwan

sedimentation and diagenesis of chinese loess implications for the preservation of continuous high resolution climate records chinese loess has been extensively utilized to produce continuous and high resolution climate records of the late cenozoic such work assumes uninterrupted loess deposition and limited diagenesis here closely spaced optically stimulated luminescence osl dates are used to characterize the holocene and late pleistocene sedimentation histories of three sites across a nw se transect of the chinese loess plateau the results suggest that sedimentation is episodic at subglacial interglacial time scales with rates rapidly varying within units and between sites unconformities noneolian deposition and mixing of sediments also appear to be common existing understanding of loess deposition therefore requires reexamination while previous reconstructions of rapid climate change not dated using absolute methods should be regarded with caution loess deposits may still yield detailed climate records from specific high sedimentation rate strata and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating the rapid changes in sedimentation presented here indicate the east asian monsoon has the capacity to vary on millennial scales

a new model of topographic effects on the distribution of loess a model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed based on the critical role played by saltating eolian sand in the entrainment and long distance transport of finer grained suspended dust in this model long term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand and associated re entrainment of dust regional and local patterns of distribution of late wisconsinan loess in parts of the upper mississippi river basin are closely related to the distribution of topographic obstacles to sand transport these cases of topographic influence occur in a region where evidence exists for sparse tundra like vegetation during the deposition of loess in the late wisconsinan similar topographic effects may not be evident further south in the missouri and mississippi river basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys thus topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions

dune sand transport as influenced by wind directions speed and frequencies in the ordos plateau china the ordos plateau in china is a region with extensive wind erosion severe desertification and various aeolian sand hazards in order to determine aeolian sand transport in this region the relationship between the sand transport rate and wind speed at 10 min frequencies was established by field observation in both the qubqi sand desert and the mu us sandy land threshold wind speeds 2 m above the ground for mobile semi fixed and fixed dune surfaces were estimated by field observations the sand transport rate increased with the increase of the bare land ratio and near bed wind speed high resolution meteorological 10 min average wind velocity data at 10 m above the ground were converted into velocity values at a height of 2 m to calculate sand transport potential based on three specific parameters decisive for sand transport wind speed duration and direction the quantity of aeolian sand transported was calculated for various wind speed levels and directions and the overall characteristics of sand transport on different dune surface types were determined by vector operation techniques sand transporting winds took place mainly in springtime the prevailing wind directions were w wnw and nw with a frequency of more than 60 in total and sand transport in these directions made up more than 70 of the total transport corresponding to a general southeastward encroachment of aeolian sand in the study area the relationship between wind frequency and speed can be expressed by a power function high magnitude strong winds had a low frequency but they played a dominant role in aeolian sand transport

the southwest ryukyu arc is a migrating microplate forearc sliver abstract we propose a new interpretation for the tectonism of the southwest ryukyu arc based on inland stress field analyses and the interpretation of seismic reflection profiles the recent stress field in the southwest ryukyu arc is under tension the axial direction of which is parallel to the trench axis but an e w dextral slip shear zone is developing at its western extremity yonaguni and westwards to taiwan these features are closely related to the degree of the oblique subduction of the philippine sea plate and the strength of coupling between it and southwest ryukyu arc we regard the southwest ryukyu arc as a migrating microplate the neotectonics of the southwest ryukyu arc are attributed to the westward migration of the microplate which is bounded by the miyako depression to the east and by taiwan to the west the microplate obducting onto the eurasian plate at taiwan started to migrate westward at about 4 ma when the relative motion between two plates philippine sea and eurasian changed direction from s n to se nw in region

limited change in dune mobility in response to a large decrease in wind power in semi arid northern china since the 1970s abstract the climatic controls on dune mobility especially the relative importance of wind strength remain incompletely understood this is a key research problem in semi arid northern china both for interpreting past dune activity as evidence of paleoclimate and for predicting future environmental change potential eolian sand transport which is approximately proportional to wind power above the threshold for sand entrainment has decreased across much of northern china since the 1970s over the same period effective moisture ratio of precipitation to potential evapotranspiration has not changed significantly this natural experiment provides insight on the relative importance of wind power as a control on dune mobility in three dunefields of northern china mu us otindag and horqin although poorly understood and potentially large effects of human land use complicate interpretation dune forms in these three regions are consistent with sand transport vectors inferred from weather station data suggesting that wind directions have remained stable and the stations adequately represent winds that shaped the dunes the predicted effect of weaker winds since the 1970s would be dune stabilization with lower sand transport rates allowing vegetation cover to expand large portions of all three dunefields remained stabilized by vegetation in the 1970s despite high wind power since the 1970s trends in remotely sensed vegetation greenness and change in mobile dune area inferred from sequential landsat images do indicate widespread dune stabilization in the eastern mu us region on the other hand expansion of active dunes took place farther west in the mu us dunefield and especially in the central otindag dunefield with little overall change in two parts of the horqin dunes better ground truth is needed to validate the remote sensing analyses but results presented here place limits on the relative importance of wind strength as a control on dune mobility in the study areas high wind power alone does not completely destabilize these dunes a large decrease in wind power either has little short term effect on the dunes or more likely its effect is sufficiently small that it is obscured by human impacts on dune stability in many parts of the study areas

amino acid geochronology of raised beaches in south west britain abstract based on 1 the epimerization of l isoleucine to d alloisoleucine d l ratios in patella vulgata littorina littorea l littoralis l saxatilis littorina species and nucella lapillus from raised beaches in south west britain 2 statistical analysis of the d l ratios and 3 lithostratigraphic and geomorphic evaluation three d l stages are proposed the d l ratios for all the species measured are converted to a patella vulgata standard the three d l stages are 1 1 the minchin hole d l stage d l ratios 0 175 0 014 defined at a stratotype in minchin hole cave gower wales 2 2 a provisionally defined but as yet unamed d l stage because of the current unavailability of a suitable stratotype with d l ratios of 0 135 0 014 3 3 the pennard d l stage d l ratios 0 105 0 016 defined at a stratotype in minchin hole cave gower wales two geochronological models of the three high sea level events representing the d l stages are constrained by uranium series age determinations on stalagmite interbedded with marine beds in minchin hole and bacon hole caves gower wales a potential fixed point in model evaluation is an age determination which is equivalent to oxygen isotope sub stage 5e 122 ka the two models are oxygen isotope stage and age ka bp d l stage d l ratio model 1a model 1b model 2 pennard 0 105 0 016 5a ca 80 5e 122 5e 122 5c ca 100 unnamed 0 135 0 014 5e 122 7 194 minchin hole 0 175 0 014 7 186 245 7 186 245 7 216 the ages of the raised beaches are used to constrain the timing and extent of glaciation in the area the extent of the late devensian glaciation is fixed by reference to localities where raised beaches of the pennard d l stage outcrop as well as by using unpublished data on molluscs incorporated into glacial deposits two pre devensian glaciations antedate the minchin hole d l stage the irish sea glaciation of the bristol channel irish sea older drift of south wales fremington till of north devon is ascribed to oxygen isotope 10 or earlier while the paviland welsh glaciation is ascribed to oxygen isotope stage 8 or earlier an earlier glaciation berwyn glaciation is inferred from erratics of known welsh provenance in essex

seesaw pattern in dust accumulation on the chinese loess plateau forced by late glacial shifts in the east asian monsoon recent chronological studies have revealed significant site specific variations in loess sedimentation which challenge the use of loess deposits as continuous easily analyzable paleoclimate and d

evidence for three north sea tsunamis at the shetland islands between 8000 and 1500 years ago abstract coastal fen and lake deposits enclose sand layers that record at least three holocene tsunamis at the shetland islands the oldest is the well known storegga tsunami ca 8100 cal yr bp which at the shetlands invaded coastal lakes and ran up peaty hillsides where it deposited sand layers up to 9 2 m above present high tide level because sea level at ca 8100 cal yr bp was at least 10 15 m below present day sea level the runup exceeded 20 m in two lakes we also found deposits from a younger tsunami dated to ca 5500 cal yr bp the sediment facies are similar to those of the storegga tsunami rip up clasts sand layers re deposited material and marine diatoms runup was probably more than 10 m yet another sand layer in peat outcrops dates to ca 1500 cal yr bp this sand layer thins and fines inland and was found at two sites 40 km apart and traced to ca 5 6 m above present high tide the oldest tsunami was generated by the storegga slide on the norwegian continental slope we do not know what triggered the two younger events

types of cenozoic sedimentary basins of the east asia and pacific ocean junction area abstract a rather wide class of structures has been distinguished in marginal basins the marginal basins in the modern pacific can be classified based on the type of crust and their position relative to the structures of active continental margin sedimentary basins of the philippine sea east china sea yellow sea and bohai sea comprise oceanic and transitional crust and form trenches and back arc inter arc forearc and intra arc basins basins with continental crust consists of marginal continental shelf and intracontinental basins in the first type basins the crust thickness ranges from 4 2 to 24 km they are sea basins rather deep only partially filled with marine sediments with substantial volcaniclastics and lavas their basement is generally made up of volcanics in inter and intra arc basins of the group polymetallic sulphide ores have been found treches have favourable conditions for gas accumulation the second type of basin was fonmed at the expense of rifting of continental crust with a thickness of 28 44 km they are completely filled with alluvial deltaic and lacustrine sediments up to 11 km thick basins of this type are rich in oil and gas

land sea correlations in the pleistocene based on isoleucine epimerization in non marine molluscs the correlation between the oxygen isotope stratigraphy of the oceans and continental stratigraphic records is one of the main challenges for quaternary research most continental classifications however are based on techniques and hypotheses that predate recent advances nevertheless the deep sea oxygen isotope global stratigraphic framework has been extended to the continents by means of some long sequences1 but these are exceptional and regional geology is unsatisfactorily classified for correlation with the oxygen isotope signal because successive geological events produced similar evidence homotaxis geochronometric dating is essential for correlation but available methods are highly site regional or sample specific fortunately non marine molluscs are ubiquitous in time and space and here we use the time dependent epimerization of l isoleucine in these fossils to subdivide the pleistocene of the british isles and to identify more events than recognized by the existing classification2 by calibrating the relative aminostratigraphic scale with independent dating methods we have set up a geochronology which is the basis for land sea correlations back to oxygen isotope stage 15

onshore offshore correlation of pleistocene rhyolitic eruptions from new zealand implications for tvz eruptive history and paleoenvironmental construction abstract taupo volcanic zone tvz in the north island new zealand is arguably the most active quaternary rhyolitic system in the world numerous and widespread rhyolitic tephra layers sourced from the tvz form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences in deep sea cores from ocean drilling program odp leg 181 sites 1125 1124 1123 and 1122 located east of new zealand ca 100 tephra beds are recognised post dating the plio pleistocene boundary at 1 81 ma these tephras have been dated by a combination of magnetostratigraphy orbitally tuned stable isotope data and isothermal plateau fission track ages the widespread occurrence of ash offshore to the east of new zealand is favoured by the small size of new zealand the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field although some tephras can be directly attributed to known tvz eruptions there are many more tephras represented within odp cores that have yet to be recognised in near source on land sequences this is due to proximal source area erosion and or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near source welded ignimbrites despite these difficulties a number of key deep sea tephras can be reliably correlated to equivalent aged tephra exposed in uplifted marine back arc successions of wanganui basin where an excellent chronology has been developed based on magnetostratigraphy orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra significant pleistocene tephra markers include the kawakawa omataroa rangitawa onepuhi kaukatea kidnappers b potaka unit d ahuroa ongatiti rewa sub rewa pakihikura ototoka and table flat tephras six other tephra layers are correlated between odp core sites but have yet to be recognised within onshore records the identification of pleistocene tvz sourced tephras within the odp record and their correlation to wanganui basin and other onshore sites is a significant advance as it provides 1 an even more detailed history of the tvz than can be currently achieved from the near source record 2 a high resolution tephrochronologic framework for future onshore offshore paleoenvironmental reconstructions and 3 well dated tephra beds correlated from the offshore odp sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore plio pleistocene sedimentary sequences e g wanganui basin cf naish et al 1998 quaternary science reviews 17 695 710

rapid fluvial incision along the yellow river during headward basin integration the onset of fluvial erosion in an area of tectonic uplift is thought to reflect the timing of the uplift geomorphological data from the yellow river in tibet indicate that the rapid incision of this river channel occurred as a result of climate change at least six million years after the onset of plateau uplift

extension in the graben systems around the ordos china and its contribution to the extrusion tectonics of south china with respect to gobi mongolia the graben basins around the ordos block are major tectonic features of late cenozoic extension in north china this paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben bounding faults this view is based on the available geological data spot imagery analysis field observations and focal mechanisms of earthquakes from this region these graben basins have been grouped into two major systems the yinchuan hetao graben system along the northern and northwestern margins of the ordos and the weihe shanxi graben system along the southern and eastern margins of the ordos crustal deformation along these graben systems involves normal slip on ne sw striking graben bounding faults normal right lateral and normal left lateral slip on nne ssw and wnw ese striking faults respectively this fault pattern is consistent with nw se trending extension the amount and rate of deformation resulting from this extension have been estimated from geological data using a cross sectional area balance and the extensional factor method deduced from the tilt of faulted blocks the results of these estimates show that the uppermost pliocene quaternary extension rate in a nw se direction is in the order of 1 6 mm yr 1 across the weihe graben 0 5 mm yr 1 across the central shanxi graben on the southeastern margin of the ordos block and 3 1 mm yr 1 across the linhe graben on the northwestern margin of the ordos block the extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north china ordos taihang shan north china plain a kinematic model is proposed in which the late cenozoic extension and block rotation in north china are interpreted as the effect of the eastward termination of the left lateral slip haiyuan fault and kunlun taibai fault system the ordos block rotates counterclockwise as a result of the push of tibet through the left lateral strike slip haiyuan fault and the liupan shan thrust and fold belt the extension along the weihe shanxi graben system may represent the effect of the left lateral slip on the kunlun taibai qinling fault system the rough estimates of the extensional deformation around the ordos block indicate that this contributes to about 5 2 mm yr 1 of left lateral movement in a n125 e direction between south china and gobi mongolia this motion adds to the left lateral slip on the qinling fault zone about 7 2 mm yr 1 in a n90 e direction between the qinling shan and the hua shan north china thus the eastward motion of south china with respect to gobi mongolia is partitioned between normal faulting around the ordos and strike slip faulting in the qinling shan it is estimated to be in the order of 12 4 mm yr 1 in a n105 e direction during the quaternary

from dust to dust quaternary wind erosion of the mu us desert and loess plateau china the ordos basin of china encompasses the mu us desert in the northwest and the chinese loess plateau to the south and east the boundary between the mostly internally drained mu us desert and fluvially incised loess plateau is an erosional escarpment up to 400 m in relief composed of quaternary loess linear ridges with lengths of 10 2 10 3 m are formed in cretaceous quaternary strata throughout the basin ridge orientations are generally parallel to near surface wind vectors in the ordos basin during modern winter and spring dust storms our observations suggest that the loess plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes the linear topography mu us desert internal drainage and escarpment retreat are all attributed to wind erosion the aerial extent of which expanded southeastward in china in response to quaternary amplification of northern hemisphere glaciation

pollen event stratigraphy of the varved sediment of lake suigetsu central japan from 15 701 to 10 217 sg vyr bp suigetsu varve years before present description interpretation and correlation with other regions high resolution pollen data average interval between samples 15 years are reported on part of a varved sediment core from lake suigetsu japan spanning the interval 15 701 to 10 217 sg vyr bp suigetsu varve years before present this new record is compared with a previously proposed event stratigraphy based on pollen based reconstructed changes of mean annual temperature the deglacial climate history reconstructed at lake suigetsu resembles that observed in the north atlantic although the major boundaries of pollen zones are asynchronous with those in the north atlantic event stratigraphy by several centuries the onset of the late glacial interstadial occurred earlier in japan than in the north atlantic this demonstrates that the climate in japan was closely linked to the low latitude pacific sea surface temperatures that first reacted to orbital forcing conversely the onset of the subsequent cold reversal phase in japan lagged that of the north atlantic younger dryas by several centuries the duration of this cold phase was about the same as the younger dryas event but the amplitude was much reduced 4 2 c in mean annual temperature these findings support the hypothesis that this pan hemispheric cooling event was triggered by north atlantic forcing most probably by a meltwater pulse and an associated change in the north atlantic thermohaline circulation however the mechanism which transmitted the change in the north atlantic to the far east is unknown

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

dependence of near surface magnetic susceptibility on dust accumulation rate and precipitation on the chinese loess plateau magnetic susceptibility ms of surface sediment varies systematically across the loess plateau in central china decreasing exponentially from 200 10 8 m3 kg at the northern margin of the qinling shan to 30 10 8 m3 kg near the southern margin of the mu us desert ms correlates highly with loess median grain size r2 0 79 which decreases south southeastward across the plateau it also correlates with mean annual temperature mat and mean annual precipitation map r2 0 58 and 0 60 respectively and with their product mat map r2 0 83 which is considered a measure of potential pedogenic activity because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel it is difficult to determine their relative influence on ms a simple ms model based on the observed spatial variation in loess thickness permits quantitative assessment of the effect of the dust accumulation rate on the ms signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals the model suggests that 84 of the loess ms variance is dictated by the diluting effect of dust and 10 11 is associated with meteorological factors primarily precipitation the observed and modeled relationships support hypotheses that attribute variations in ms in the loess paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils both of which are controlled by monsoon climate

detailed volcanic morphology of daisan miyako knoll in the southern ryukyu arc abstract high resolution bathymetric mapping using an autonomous underwater vehicle and a survey vessel was conducted at daisan miyako knoll in the southern ryukyu arc this paper presents for the first time the detailed volcanic morphology of a submarine edifice that previously was poorly explored and discusses its formation process and relation to arc volcanism and tectonics bathymetry shows morphological evidence that daisan miyako knoll is a stratovolcano with three caldron like depressions 1 km in diameter a central cone and multiple lava flows at the summit the main edifice is conical with a basal diameter of 8 10 km and relief of 1000 1100 m with a summit depth of 779 m the central cone that stands inside the depressions has a circular crater numerous blocks up to 10 m wide are distributed around the crater indicating a previous explosive eruption lava flows erupted from the central cone have a maximum thickness of 46 m at the flow margins and a maximum flow length of 1 9 km the total flow area is 1 5 km2 and the estimated volume is 0 03 km3 the relatively thick lava flows up to 46 m with steep sided margins 30 40 and curtain folded surface textures suggest that silicic magmas erupted and their fresh appearance indicates that lava flow eruptions the most recent volcanic activity occurred during the quaternary the volcanic features at daisan miyako knoll are aligned mainly in the nw se direction suggesting that volcanism here may be controlled primarily by arc parallel extension no volcanic edifices have been confirmed on the trench side of daisan miyako knoll therefore this edifice defines the location of the arc volcanic front in this region from the larger rhyolitic edifices 10 km wide around daisan miyako knoll along the front it is inferred that silicic arc volcanism is extending from the central to southern submarine ryukyu arc

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

the quaternary coral reef tracts of hateruma ryukyu islands japan the island of hateruma is located at the southwestern end of the ryukyu island chain and covers an area of about 15 km2 with its highest point at 59 5 m a s l the morphology of hateruma is characterized by an eight step staircase i to viii after ota et al 1982 most of which eroded into the previously formed coral reefs ii and iv to viii this study presents 100 age estimations on 43 coral samples using electron spin resonance esr alpha and mass spectrometric u series methods the dating results imply that the ryukyu limestone was predominantly formed during oxygen isotope stages 7 and 5e two age estimates of around 300 ka suggest an earlier reef building period the main problem for establishing the quaternary history of hateruma lies in the fact that the coral ages and the staircase morphology do not correlate additionally it is difficult to distinguish autochthonous from allochthonous samples the latter having possibly been relocated by tsunamis corals relating to substages 5a and 5c around 75 and 105 ka respectively are found in the immediate vicinity of much older samples the results of this study prove the disappointing fact that hateruma cannot yet be used as a tool for reconstructing palaeo sea levels

early holocene palaeoceanography and deglacial chronology of disko bugt west greenland two new cores from disko bugt a large marine embayment in west greenland are used to assess the deglacial chronology and palaeoceanography of the area during the early holocene deglaciation of the main part of disko bugt is directly dated for the first time to a minimum of ca 10 2 ka cal bp by this time the jakobshavn isbrae ice stream had retreated to the eastern margins of disko bugt initial deglaciation took place before the relatively warm west greenland current wgc was initiated a strengthening in the wgc is identified from ca 9 2 ka cal bp accompanied by retreat of the ice stream to a shallow bank immediately west of the present day jakobshavn isfjord while in this position the calving and melting ice stream produced large volumes of sediment and meltwater the large volumes of meltwater prevented the relatively warm waters of the wgc reaching the eastern margins of disko bugt the ice stream remained grounded on this bank for over a thousand years until ca 7 9 ka cal bp when reduced sedimentation rates and an increase in boreal fauna point to its retreat inside the present day fjord system

extension in the southern ryukyu arc japan link with oblique subduction and back arc rifting compilation of earthquake focal mechanisms morphological analysis and examination of mesofractures help to clarify the cenozoic and recent stress history in the southern ryukyu arc between the ryukyu trench and the southern okinawa trough 1 an older pre miocene intermediate type stress field vertical stress component v 2 is attested by conjugate strike slip faults and shear joints in eocene rocks of ishigaki island the 1 axis trends n90 e to n110 e in present day coordinates 2 normal faults and extensional joints in mio pliocene strata of iriomote and yonaguni islands and fault scarps in pleistocene strata at yonaguni allow us to reconstruct a miocene to pleistocene stress field of tensional type v 1 the 3 axis trends about nw se the effects of this stress field can be traced back in eocene rocks of ishigaki island 3 evidence for the youngest holocene to present day stress field comes from the analysis of the harvard moment tensor catalog 1976 1997 for shallow earthquakes quaternary fault scarps visible on the two islands of miyako and yonaguni and extensional joints in holocene rocks of yonaguni this stress field is tensional two directions of extension are inferred a regional arc perpendicular n s one and a local arc parallel n60 e one in the easternmost part of the area near miyako the record of neogene to quaternary tensional stress fields demonstrates that the extension in the okinawa trough can be traced until the arc region arc oblique to arc perpendicular stretching can be directly correlated with oceanward rifting in the back arc basin while the arc parallel stretching reflects the increase of the arc curvature the predominance of extensional features the absence of shortening or transcurrent deformation structures and a weak interplate coupling do not support the forearc sliver model for the present day southern ryukyu arc

two phase opening model for the okinawa trough inferred from paleomagnetic study of the ryukyu arc tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central ryukyu arc and the southern ryukyu arc for a paleomagnetic and geochronological study in an attempt to understand the opening of the okinawa trough back arc basin stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central ryukyu arc and from 12 sites of two geological units of the southern ryukyu arc the paleomagnetic results were compared with the tertiary apparent pole wander path of eurasia in order to investigate the tectonic movement of the ryukyu arc with respect to the asian continent the comparison and the radiometric ages indicate that the southern ryukyu arc rotated clockwise through 25 between 10 ma and 6 ma in contrast the central ryukyu arc experienced little rotational motion since 17 ma we propose the following two phase opening model for the okinawa trough opening the first stage took place between 10 ma and 6 ma the southern part of the trough opened by means of a wedge mode to cause the clockwise rotation of the southern ryukyu arc in the central part the parallel opening occurred and the ryukyu arc drifted without any rotation the second phase started recently in the whole okinawa trough it is observed as the present activities in the trough

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone

new data for the last glacial maximum in great britain and ireland abstract understanding the history of the british and irish ice sheet biis at the last glacial maximum lgm has been advanced by new approaches in particular by cosmogenic nuclide surface exposure dating aminostratigraphy of shelly glacial deposits ams radiocarbon dating and the evidence from continental margin marine cores all of which supersede the previously weak geochronologic control it was formerly believed that great britain and ireland was largely ice free between the last interglacial oxygen isotope sub stage 5e and the late devensian when the lgm occurred as such the biis was effectively out of phase with laurentide and scandinavian ice sheets as well as inferences of ice volume from oxygen isotope stratigraphy the biis during the late devensian maximum was also perceived as having been relatively stationary new evidence shows that the lgm was an important event during the evolution of an earlier biis when the extent of ice was greater repeated iceberg rafting events over the past 50 ka are shown by marine cores while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36 cl ages on glaciated surfaces and glacial boulders that are indicative of deglacial events between 40 and 12 ka these appear to be associated with heinrich events the earliest being inferred as heinrich 4 at about 40 ka during this advance the biis and scandinavian ice sheet sis were in contact and all of ireland was glaciated the ice sheet appears to have fluctuated several times between 40 and 25 ka although evidence for this is poorly preserved but the 36 cl and 14 c evidence is clear that the biis reached its lgm maximum size about 22 ka soon after heinrich event 2 when the biis and sis were not in contact one cluster of 36 cl and 14 c ages at 21 4 1 3 ka records an initial pulse of deglaciation that was followed by extensive deglaciation about 17 4 0 4 ka just before heinrich event 1 when the ice sheet readvanced contrary to previous views the biis probably existed throughout much of devensian time as a mobile and sensitive ice sheet during which the lgm advance was but one important event in places glacial deposits of the earlier devensian glaciation have previously been incorrectly identified as products of the later lgm glaciation

tectonics of the hellenic trench a synthesis of sea beam and submersible observations abstract detailed bathymetric investigations were performed in four zones of the hellenic trench system in 1978 using the multi narrow beam echo sounder sea beam on r v jean charcot in 1979 a field study using the 3000 m submersible cyana was conducted in three of the four sites covered by sea beam we summarize here the principal tectonic results of these studies and publish the corresponding sea beam maps it is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall in addition there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the mediterranean ridge we suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall we show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m y ago

on the interpretation of chinese loess as a paleoclimate indicator the records of wind blown dust i e loess in china are some of the most important terrestrial records of past climate changes stretching back over the last 10 ma in the paleoclimate literature intervals of increased dust generation have been almost always interpreted as being associated with more intense or prolonged wintertime conditions here it is shown that in accordance with modern observations dust outbreaks in asia are predominantly springtime phenomena during spring frequent cyclogenesis in the lee of the mongolian altai and the passage of strong cold fronts produce the intense windstorms that loft and entrain dust into the air the meteorology governing such outbreaks is likely robust in past climates contrary to the common paleoclimate presumption it is actually the breakdown of the siberian high that permits the dust producing windstorms to occur the importance of cold fronts in generating such windstorms suggests that cooling of high latitude climate during the miocene or during glacial intervals might play a significant role in the signal recorded in the loess deposits the unique springtime factors that generate dust storms are an example of why the common partitioning of asian climate into a winter and summer monsoon is oversimplified and can be misleading

mechanisms of dust emission from pleistocene loess deposits nebraska usa 1 saltation bombardment is commonly believed to be the primary mechanism by which large quantities of dust can be generated direct entrainment of silt on the other hand is thought to generate minimal quantities of dust if so loess landscapes should rarely be major sources of dust unless disturbed by human activity to test the mechanisms by which loess can be eroded by the wind we used the portable in situ wind erosion lab pi swerl pi swerl tests were conducted on benches that were carved into loess at field locations coarse to fine loess was also collected and tested in a controlled setting using 1 dry unconsolidated loess and 2 crusted loess results indicate that in most cases the threshold friction velocity for silt is less than that for sand and that most dust generation occurs by direct entrainment of dust saltation even in coarse loess was nonexistent to intermittent in a few cases saltation of sand or soil aggregates preceded dust generation or enhanced emissions after direct entrainment began emission fluxes of loess are potentially high in both proximal and distal settings suggesting that loess is easily entrainable if vegetation density is low we hypothesize that an arid and windy climate of the late pleistocene paired with lower vegetation density facilitated large scale erosion of peoria loess in nebraska to generate wind aligned ridges and troughs more broadly our results indicate that loess landscapes have the potential to be major sources as well as sinks of dust

contrasting styles and rates of tectonic uplift of coral reef terraces in the ryukyu and daito islands southwestern japan abstract coral reef terraces on kikai hateruma and yonaguni in the ryukyu islands on the eurasian lithospheric plate and those of kita daito and minami daito islands on the philippine sea plate indicate three distinct patterns of coral reef terrace formation and preservation primarily reflecting different uplift rates the kikai type which includes many terraces from oxygen isotope stages 5e to 1 is a result of high uplift rate of ca 1 8 m kyr on the fore arc rise of a chilean type subduction zone arcward westward tilting and faulting are also characteristic of the kikai type the hateruma type is characterized by two coral reef terraces younger than the stage 5e terrace and they are arranged in a concentric annular pattern slow uplift rate of ca 0 3 m kyr adjacent to a mariana type subduction margin is typical of the hateruma type the emergence of kikai and hateruma islands is recent younger than isotope stage 5e and 7 respectively reflecting the active deformations of the leading edge at the overriding eurasian plate in the daito type atolls surrounded by fringing reefs have been uplifted very slowly less than 0 05 m kyr but over a time period of at least several million years slow uplift and long history of daito type islands is the result of up bulging of the philippine sea plate as it migrates toward the ryukyu trench

exhumation of triassic hp lt rocks by upright extrusional domes and overlying detachment faults ishigaki jima ryukyu islands abstract the tomuru formation of ishigaki jima in the southernmost part of the ryukyu arc comprises blueschist facies subduction complex rocks metamorphosed in triassic time d1 structures related to subduction blueschist facies mineral growth and possibly early stages of exhumation are deformed by d2 structures that appear to reflect the last stage of exhumation d2 structures define several anticlines with parasitic overturned folds verging away from anticlinal axes the shortening recorded by this deformation appears to reflect upward extrusion relative to flanking material the anticlines are flanked by detachment faults with normal sense of shear parallel to d2 vergence hanging wall rocks that include the fusaki formation an accretionary prism with early cretaceous metamorphic ages and late eocene limestone conglomerate and andesitic volcanics the eocene strata contain metamorphic detritus derived from the tomuru and fusaki formations indicating pre late eocene surface exposure of these units ultramafic rocks and gabbro blocks of the tomuru formation were incorporated by sedimentary sliding into the trench prior to subduction and high pressure metamorphism rather than being emplaced as diapirs along a post metamorphic fault as previously proposed geochronologic metamorphic and thermal considerations suggest exhumation of the tomuru formation to relatively shallow crustal depths prior to or concurrent with early cretaceous metamorphism of the fusaki formation arcward vergent thrusting may have placed the younger and formerly structurally lower subduction complex fusaki formation over the older one tomuru formation d2 extrusional doming began after the emplacement of the fusaki formation at high structural levels the d2 transport directions are subparallel to the strike of the orogen suggesting that the upright extrusion may have occurred along a forearc strike slip fault system this final stage of exhumation concluded in the late eocene with extensional collapse and the development of detachment faults the progression from initial exposure of the tomuru and fusaki formations deposition of late eocene strata extrusional doming and late detachment faulting may have been associated with migrating step overs rather than changes in regional tectonics

links between marine and atmospheric processes oscillating on a millennial time scale a multi proxy study of the last 50 000 yr from the alboran sea western mediterranean sea abstract the large dataset obtained from the extensive study of images core md95 2043 recovered from the alboran sea western mediterranean shows the periodicities and phase relationships of oceanographic and atmospheric processes on a millennial time scale the 1470 yr cycle is the most significant with the exception of the records reflecting climatic or environmental changes on land which show statistically significant 3300 and 8000 frequency bands the investigation of these core records on a millennial scale resolution allows us to establish the evolution of oceanographic and atmospheric mechanisms that influenced the western mediterranean region in the course of the dansgaard oeschger cycles accordingly possible land sea interactions can be identified and situated in the context of the temporal succession of the different climatic processes for instance saharan dust supply from northern africa appears to lead high latitude climate changes suggesting that low latitude feedback processes were involved in forcing the millennial climatic variability in the westernmost mediterranean

trench parallel stretching and folding of forearc basins and lateral migration of the accretionary wedge in the southern ryukyus a case of strain partition caused by oblique convergence detailed seafloor mapping in the area east of taiwan revealed trench parallel stretching and folding of the ryukyu forearc and lateral motion of the accretionary wedge under oblique convergence east of 122 40 e a steep accretionary wedge is elongated in an e w direction a major transcurrent right lateral strike slip fault accommodates the strain partitioning caused by an oblique convergence of 40 a spectacular out of sequence thrust may be related to the subduction of a structural high lying in the axis of the n s trending gagua ridge this asperity is likely responsible for the uplift of the accretionary wedge and forearc basement and may have augmented strain partitioning by increasing the coupling between the two plates west of 122 40 e the low taper accretionary wedge is sheared in a direction subparallel to the convergence vector with respect to the ryukyu arc the bayonet shape of the southern ryukyu arc slope partly results from the recent re opening of the southern okinawa trough at a rate of about 2 to 4 cm yr right lateral shearing of the sedimentary forearc with respect to the nonlinear ryukyu backstop generates trench parallel extension in the forearc sediment sequence at dilational jogs and trench parallel folding at compressive jogs the hoping basin lies above a diffuse trench trench fault ttf or tff unstable triple junction moving toward the south along a n s transform zone which accommodates the southward drift of the ryukyu arc with respect to eurasia

saalian supercycle mindel riss interglacial and milankovitch s dating abstract there are five pleistocene glacial interglacial cycles recorded in deep sea sediments from the last half million years but only two classic interglacials are recognized on land the dilemma stems from the different understanding and identification of glacials and interglacials in the geologic records on land and in the ocean only those intervals of warm climate that followed a major southward ice sheet advance and were accompanied by a deep penetrating oceanic transgression were recognized in their type areas of northern europe as true interglacials only those ice advances which left moraines farther south than younger ones were recognized as glacials in contrast all intervals reflecting exceptionally low global ice volume high sea level and warm sea surface are taken as interglacials in the deep sea record classic subdivisions are aligned with the glacial interglacial cycles of marine isotope stage stratigraphies by proposed higher order units called supercycles sc bracketed by superterminations st the saalian supercycle sc2 includes glacial cycles c d and e composed of marine isotope stages mis 6 7 8 9 10 and 11 the units mis 7 to mis 11 correspond to the great gross interglacial estimated from the early geologic considerations to be about 240 millennia long penck and bruckner 1909 die alpen im eiszeitalter vols 1 3 christian herman tauchnitz leipzig and determined from astronomic computations to last 235 millennia milankovitch 1941 kanon der erdbestrahlung und seine anwendung auf das eiszeitenproblem academie royale serbe editions speciales section des sciences mathematiques et naturelles tome cxxxiii stamparija mihaila curcica beograd 633pp the current high precision radiometric dating fully confirms the accuracy of milankovitch s determination and the pivotal role of orbital mechanisms in pleistocene climate change

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

tectonics of an arc arc junction an example from kyushu island at the junction of the southwest japan arc and the ryukyu arc abstract three distinctive geologic events occurred synchronously during the last 6 m y on kyushu island lying at the junction of the southwest japan arc and the ryukyu arc these events included 1 dextral fault displacement along a 100 km section of the median tectonic line 2 formation of a rectangular volcano tectonic depression 70 km long and 40 km wide and 3 approximately 30 of counterclockwise crustal rotation we propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the philippine sea plate which resumed at around 6 ma after a halt of more than 5 m y oblique subduction of the philippine sea plate beneath the southwest japan arc detached a fore arc sliver that was displaced along a dextral fault the median tectonic line north of the nankai trough a volcano tectonic depression the hohi volcanic zone was formed by the pull apart structure on the western margin of the median tectonic line by contrast normal subduction of the philippine sea plate under the ryukyu arc caused back arc spreading observed as crustal extension in the northern okinawa trough the crustal extension caused the counterclockwise rotation for the northernmost ryukyu arc including southern kyushu