paleomagnetic evidence for block rotations in central hokkaido south sakhalin northeast asia abstract the eocene to lower miocene strata in the bykov region of south sakhalin island far east russia show a mean paleomagnetic direction of dec 28 7 inc 55 7 95 6 7 and n 22 and a paleomagnetic pole at 66 3 n 245 7 e and a95 8 8 which shows that south sakhalin rotated 25 15 clockwise with respect to northeast asia since the early miocene this pole agrees with that of the eocene to middle miocene sedimentary rocks in central hokkaido japan it suggests that both south sakhalin and central hokkaido have rotated clockwise with respect to northeast asia since the early miocene these clockwise rotations can be interpreted as a vertical axis rotation of the blocks distributed in the north south direction more than 1000 km along the dextral hokkaido sakhalin shear zone considering the development of an en echelon structure of the basins in the shear zone we recognized five blocks each of which had rotated since the early miocene we conclude that this dextral shear is responsible not only for these block rotations but also for the opening of the kuril basin and the japan sea and that the shear motion was caused by oblique convergence between the eurasian and the okhotsk plates since the early miocene

extrusion tectonics of the okhotsk plate northeast asia based on focal mechanism geologic and lineament data faulting in the chersky range northeast asia changes from right lateral transpression between north america and eurasia to left lateral strike slip between north america and the okhotsk plate the location of the okhotsk eurasia boundary is poorly constrained available data suggest this boundary is right lateral the okhotsk plate is being compressed and extruded from between north america and eurasia resulting in high levels of microseismic activity along the southern flanks of the chersky range

evolution of collision related basins in the eastern end of the kurile basin okhotsk sea northwestern pacific abstract a densely meshed 9400 km long high resolution seismic survey revealed the evolution of sedimentary basins in the southeastern rim of the kurile basin offshore of eastern hokkaido nw pacific hokkaido is situated in a collision zone not only of okhotsk and eurasian plates and also of the kurile and ne japan arcs offshore eastern hokkaido is overlain by n s trending basins hamatonbetu oki monbetu oki and kitami yamato sedimentary basins hosb mosb and kysb respectively and abasiri oki trough aot separated by zones of uplift depositional patterns of the hosb and deformation of the adjacent uplift zone suggest the pull apart origin of the basin related to dextral motions along the central sakhalin fault which extends to hokkaido and binds the okhotsk and eurasian plates the uplift zone adjacent to the mosb suggests the initiation of subsidence in the middle miocene within a splay fault basin with subsequent transformation into a 40 km wide graben in the late miocene and into a half graben in the pliocene the pre late miocene unconformity is widespread the kysb and the aot began as a concatenated basin in the south of mosb in the middle miocene in the pliocene the kitami yamato bank kyb was uplifted and divided the kysb aot basin into kysb and aot the kyb uplift was probably induced by the kurile forearc sliver movement the interpretation of structures suggests that the kurile basin may have opened in a n s direction from dextral shearing along a collision zone in the middle miocene in the pliocene the kinematics changed to a ne se spreading direction because of southwestward migration of the kurile arc with respect to the ne japan arc compressional tectonics after the pliocene may suggest the destruction of the kurile basin

high resolution 87sr 86sr chemostratigraphy of the silurian implications for event correlation and strontium flux analyses of 87sr 86sr in silurian conodonts recovered from localities in north america and europe representing 13 of the 14 defined silurian conodont zones provide a high resolution record of seawater chemistry for the silurian period these data which are characterized by little or no scatter depict several high frequency cycles superimposed on a gradual longer term rise in 87sr 86sr for the silurian high frequency cycles have a duration of about one conodont zone and many correlate with sequence boundaries recognized around the world these data provide a much higher resolution image of secular changes in 87sr 86sr during the silurian and may require a rethinking of models of strontium isotope flux in marine basins

drowning unconformities palaeoenvironmental significance and involvement of global processes abstract drowning unconformities are stratigraphic key surfaces in the history of carbonate platforms they mostly consist in the deposition of deep marine facies on top of shallow marine limestones although large scale depositional geometries mimic lowstand systems track architecture these sedimentary turnovers are developed in relation with major sea level rise inducing an increase in the rate of creation of accommodation space that outpaces the capacity of carbonate to keep up this so called paradox of carbonate platform drowning implies that parameters other than purely eustatic fluctuations are involved in the demise of shallow marine ecosystems worldwide and at different times during earth history in depth studies of drowning unconformities revealed that changes in nutrient input clastic delivery temperature or a combination of them may be responsible for a decrease in light penetration in the water column and the progressive suffocation and poisoning of photosynthetic carbonate producers the examination of such case examples from various stratigraphic intervals and palaeogeographical settings thus helps in identifying and hierarchizing potential triggering mechanisms for drowning unconformities this is complemented by new data from early cretaceous successions from the helvetic alps during this time period the helvetic carbonate platform developed along the northern tethyan margin using both photozoan and heterozoan communities phases of healthy production were interrupted by several drowning episodes the latter are marked in the sedimentary record by condensation and associated phosphogenesis and glauconitisation from the earliest valanginian to the early to late barremian three drowning unconformities reflect the intermittent installation of a more humid climate and subsequent enhanced trophic conditions which first induced a switch from photozoan to heterozoan communities and then to long lasting drowning phases the latter encompass several sea level rise and fall cycles and may be linked to strengthened upwelling currents with the return to more oligotrophic conditions during the late barremian photozoan urgonian type communities took up again their development has been abruptly stopped at the end of the early aptian by a major emersion phase the subsequent drowning is documented in various peritethyan areas this initial crisis is followed by three other drowning phases that ultimately led to the replacement of shallow ecosystems by a deeper marine sedimentation in the cenomanian this long term trend in the evolution of the helvetic carbonate platform and of other peritethyan ecosystems may have been driven by more global phenomena in particular the progressive opening of the northern and equatorial atlantic may have impacted sea level by creating new oceanic basins the emplacement of submarine volcanic plateaus may have triggered sea level rise and fertilized deep oceanic waters through hydrothermal processes drowning unconformities thus record the interplay of local with long term processes and constitute regional sedimentary archives of global phenomena

nonmonotonic variation of seawater 87 sr 86 sr across the ivorian chadian boundary mississippian osagean evidence from marine cements within the irish waulsortian limestone abstract detailed analysis of compositionally unaltered marine fibrous cements mfc from a single core through the mississippian irish waulsortian limestone indicates that the variation of seawater 87sr 86sr is nonmonotonic across the ivorian chadian boundary this nonmonotonic variation has not been recognized by previous studies furthermore marine cements yielded 87sr 86sr ratios lower than previously reported values for the ivorian chadian osagean marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope c o composition comparable to previous estimates of mississippian marine calcite the isotope chemistry c o sr and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high mg calcite to low mg calcite microdolomite a conversion that probably took place in marine water during precipitation of zone 1 calcite cement the oldest non mfc cement high stratigraphic resolution was obtained by restricting the sample set to a single core 429 m long thereby eliminating chronostratigraphic correlation errors the mfc cements are pre arundian based on previous study and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones the core is estimated to represent about 9 8 million years of waulsortian limestone deposition 87sr 86sr ratios of pristine mfc samples range from a high of 0 707908 in the early ivorian to a low of 0 707650 in the late ivorian and mid chadian with an early chadian maximum at 0 707800 the maximum rate of change in seawater 87sr 86sr is 0 00012 ma comparable in magnitude to tertiary values our data document the presence of fine scale seawater 87sr 86sr modulations for the ivorian chadian in contrast to the previously published monotonic seawater 87sr 86sr curve for this interval and emphasize the importance of well characterized intraformational isotopic baselines

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

bivalve molluscs of the abyssal zone of the sea of okhotsk species composition taxonomic remarks and comparison with the abyssal fauna of the pacific ocean abstract sokhobio expedition until recently hardly anything was known about the abyssal bivalve fauna of the sea of okhotsk which communicates with the pacific ocean via many straits between the kuril islands with a maximum depth of 2318 m in bussol strait and about its similarity with the deep water fauna of the pacific investigation of the materials collected by the sokhobio russian german deep sea expedition rv akademik m a lavrentyev 2015 and two russian expeditions rv toporok 1948 rv vityaz 1949 from the bottom of the kuril basin of the sea of okhotsk 2850 3366 m depth revealed a rich fauna of bivalves including 25 species belonging to 12 families of which only 12 species 48 0 were determined to the species level nine species poroleda extenuata katadesmia vincula dacrydium rostriferum bathyarca imitata catillopecten squamiformis channelaxinus excavata vesicomya pacifica cuspidaria cf abyssopacifica myonera garretti were first records for the sea of okhotsk the richest families were thyasiridae 7 species and cuspidariidae 6 species the high diversity and richness of the bivalve fauna on the bottom of the kuril basin are probably caused by the favorable trophic conditions the kuril basin is characterized by very high rates of sedimentation and high content of organic matter in the sediments for estimation of connections between deep water faunas of the sea of okhotsk and the pacific ocean the sokhobio expedition performed 1 station at the maximum depth of the bussol strait and 2 stations 3342 3432 and 4679 5013 m on the pacific slope of the kuril islands opposite to the bussol strait where 24 species belonging to 13 families were found among the 25 species found in the deep part of the kuril basin more than half of species 17 species 68 occur in the pacific ocean most of them are widespread in the northern pacific and are eurybathic bathyal abyssal species this probably allows them to penetrate into the sea of okhotsk through deep straits between the kuril islands photographs and taxonomic remarks are provided for all identified species 15 that were found in the sea of okhotsk and or the pacific ocean taxonomic decisions herein nuculana aikawai habe 1958 and nuculana sagamiensis okutani 1962 are synonymized with nuculana leonina dall 1896 arca bathyarca nucleator dall 1908 is synonymized with bathyarca imitata smith 1885 for leda extenuata dall 1897 a new combination is suggested poroleda extenuata dall 1897

incipient subduction and deduction along the eastern margin of the japan sea the eastern margin of the japan sea has been under convergent tectonics since sometime after the pliocene the suture line is on the ocean continent boundary between the japan yamato basins and the tohoku northeast japan arc incipient subduction and obduction of the oceanic and sub oceanic crust are observed along the suture line with the occurrence of numerous n s trending fault bounded ridges and troughs focal mechanism solutions of several compressional type earthquakes with magnitudes of m 6 9 to 7 7 which have occurred along the zone are consistent with the observation of geological thrust faults the convergent stresses in the japan sea may be due to india eurasia collision and its associated intra plate or inter microplate movements in east asia the eastward movement of the amurian plate causes baikal extension along its western margin and the japan sea convergence along its eastern margin

late ordovician mass extinction in the selwyn basin northwestern canada geochemical sedimentological and paleontological evidence we present a detailed study of the trace element and stable isotope geochemistry sedimentology and fossil distributions in two avalanche lake av4b av1 ordovician silurian boundary sections in the selwyn basin trilobites and conodonts indicate a profound extinction at the end of the ordovician which is constrained stratigraphically within a 3 in magnitude is recorded in the extinction interval iridium abundances 0 051 ppb in the extinction interval are low and fail to provide evidence for an impact the highest ir abundance is found to be associated with reduced sedimentation in a condensed horizon cerium anomalies indicate a s

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

87sr 86sr ratios in permo carboniferous sea water from the analyses of well preserved brachiopod shells abstract sr isotopic analyses of well preserved portions of permo carboniferous brachiopods distributed globally confirm the general shape of the sr isotope age curve established by previous workers for this time interval there is little variation between the sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered based on cathodoluminescence elemental and stable isotopic data however the sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic the sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

america eurasia plate boundary in eastern asia and the opening of marginal basins abstract the opening of the arctic ocean during the past 55 ma resulted in relative rotation of america with respect to eurasia about a pole located in eastern siberia near the plate boundary the extensional plate boundary enters deep inside the eurasian continent up to the rotation pole on the opposite side of the pole on the pacific side of the plate boundary compressive tectonics are recorded along sakhalin and hokkaido from the oligocene to middle miocene the relative movement was accommodated by strike slip motion along sakhalin and hokkaido although the rotation pole was not located at a significatively different position from now in this paper we explain this by independent motion of the southernmost tip of the american plate towards the pacific margin which behaves as a free boundary this oceanward motion resulted in an extension of the american plate giving rise to the wedge structure of the okhotsk sea the japan sea opened as a pull apart basin along the strike slip boundary finally the increasing extension in the okhotsk sea led to the opening of the oceanic kuril basin

geochemistry of mercury in surface sediments of the kuril basin of the sea of okhotsk kuril kamchatka trench and adjacent abyssal plain and northwest part of the bering sea abstract mercury concentrations in surface sediments collected from the northwestern pacific were analyzed by mercury zeeman atomic absorption spectrometer with high frequency modulation of light polarization and a pyrolysis attachment the range of total hg concentrations in sediments was 19 158 g kg 1 with a mean of 77 g kg 1 n 50 the variation in mercury concentrations in sandy deposits of the slopes was 19 79 g kg 1 hg in clayey deposits from the kuril basin was 84 130 g kg 1 hg in clayey deposits from slopes of the kuril basin was 44 84 g kg 1 hg in clayey deposits from the abyssal area was 46 116 g kg 1 hg and in clayey deposits from the slopes and bottom of the kuril kamchatka trench was 42 143 g kg 1 hg within the distribution of mercury across the study area high mercury concentrations were observed in clayey sediments which are enriched in organic matter and the remains of silicate phytoplankton the hg content of sandy deposits was minimal the level of mercury in our deep water sediments is somewhat overestimated by comparison to representative background values in clayey sediments from the impact areas subject to anthropogenic pollution anomalies of mercury in bottom sediments near a hydrothermal source piip volcano have not been observed the enrichment factor values at the stations range between 0 3 and 4 3 ef values of the most samples were generally more than 1 indicating that enrichment was by biogeochemical processes based upon four guideline values erl erm isqg and pel we can assume a minimal toxic mercury effect on the benthic marine biota

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

seawater sr isotope variations through time a procedure for constructing a reference curve to date and correlate marine sedimentary rocks a new curve of seawater 87 sr 86 sr variation through the past 450 m y enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties we discuss the method by which this curve was constructed strictly standardized 87 sr 86 sr ratios and biostrat igraphic data for 1300 87 sr 86 sr analyses of marine minerals were input into a data base objective criteria were used to assign each data point a reliability score these scores were used as initial weights in a robust nonparametric regression lowess to obtain a curve of seawater 87 sr 86 sr variation with rigorous 95 confidence intervals for prediction of age uncertainty precise dating and stratigraphic correlation are feasible for much of the phanerozoic the main problem with future sr isotope stratigraphy is likely to be sample quality we discuss guidelines for sample selection

strontium isotopes in early jurassic seawater abstract the analysis of well preserved well dated belemnites and oysters from the jurassic of great britain has resulted in a well constrained detailed seawater strontium isotope curve for the early jurassic the preservation of fossil low mg calcite was monitored using mn fe 13 c and 18 o iron was the most useful indicator with about 75 of the samples containing more than 150 ppm fe showing 87 sr 86 sr ratios elevated relative to adjacent points on the curve high mn concentrations less often correlated with elevated 87 sr 86 sr ratios however low mn concentrations 87 sr 86 sr 13 c and 18 o proved to be insensitive to diagenesis as it affects 87 sr 86 sr the principal features of the strontium isotope curve include a rise to about 0 70772 in the latest triassic and earliest jurassic hettangian from the hettangian the curve begins a roughly linear descent through the sinemurian and pliensbachian following a small levelling off and increase in the late pliensbachian the curve falls rapidly to its early jurassic minimum of 0 70706 it then gently increases through the toarcian until the falciferum zone where it shows an apparently abrupt increase to 0 70719 before continuing its slow increase to 0 70728 in the aalenian bajocian middle jurassic this reversal of the downwards trend established in the sinemurian and pliensbachian had not been previously identified the sinemurian and pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones 0 5 to 1 ma

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of etna crystallization paths of basaltic 1763 eruption and hawaiitic 1865 and 1329 eruptions scoria from etna were deduced from mineralogy and melt inclusion chemistry the volatile behaviour was investigated through the study of melt inclusions trapped in the phenocrysts and those of the whole rocks and the matrix glasses the results from the 1763 eruption point to the early crystallization of olivine fo 81 7 from a water rich alkaline basalt with high cl 1750 2000 ppm and s 2100 2400 ppm concentrations the hawaiitic melt inclusions trapped in olivine fo 74 salite and plagioclase are characterized by a decrease in cl k2o and s k2o ratios in each investigated system there is good correlation between k2o and p2o5 in the whole rocks cl ranges from 980 to 1680 ppm from basaltic to hawaiitic lavas whereas s 110 136 ppm remains low cl and s behaviour in the 1763 magma suggests an early degassing stage of cl and s with co2 and a water rich gaseous phase for a pressure close to 100 mpa consistent with a permanent outgassing at the summit craters of etna during the eruption the sulphur remaining in the hawaiitic liquid is lost and the degassing of chlorine is limited such a degassing model can be extended to the 1865 and 1329a d eruptions

trace elements in anorthite megacrysts from the kurile island arc a window to across arc geochemical variations in magma compositions abstract anorthite megacrysts from twelve volcanic centers of the kurile island arc were analyzed by the ion microprobe for concentrations of li be b f mg p cl k ti fe co rb sr y zr nb cs ba la ce pr nd sm eu and pb these 1 2 cm megacrysts are mainly found as early phenocrysts in basalts but are also present in andesites rhyolites and hybrid rocks from almost every studied volcanic center both subaerial and submarine electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned whereas rims often result from overgrowth of sodium richer plagioclase in equilibrium with its host silica richer melt the cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a window into an earlier basaltic stage of magmatic evolution petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second after olivine liquidus phase in high al basalts regardless of varying k2o contents across the arc given the narrow compositional range of anorthite megacrysts the across arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc the concentration of k in megacrysts increases linearly by a factor of 11 ba 13 sr eu 4 be ti p 3 li 2 ree 5 2 whereas concentrations of pb b and halogens and their ratios to hfse either remain constant or decrease with deepening of the benioff zone from 110 to 220 km we used ion microprobe derived plagioclase basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values the reconstructed concentrations of several key trace elements and their correlation with benioff zone depth are consistent with whole rock basalt data but all give systematically more primitive compositions therefore anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing they can be used to assess delicate variations in primary basaltic magma chemistry as a function of benioff zone depth we also report new results for sr nd and pb isotopes for anorthite bearing rocks and other rocks from the same volcanoes across arc zoning with respect to trace element concentrations ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes trace element transfer by fluids variation in degree of partial melting and progressive change of the melting source

neogene strike slip faulting in sakhalin and the japan sea opening we describe structural data from a 2000 km n s dextral strike slip zone extending from northern sakhalin to the southeast corner of the japan sea satellite images field data and focal mechanisms of earthquakes in sakhalin are included in the interpretation since miocene time the deformation in sakhalin has been taken up by n s dextral strike slip faults with a reverse component and associated en echelon folds narrow en echelon neogene basins were formed along strike sup faults and were later folded in a second stage of deformation we propose a model of basin formation along extension al faults delimitating dominos between two major strike slip faults and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening the association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in sakhalin we present fault set measurements undertaken in noto peninsula and yatsuo basin at the southern end of the sakhalin east japan sea strike slip zone early and middle miocene formations recorded the same transtensional regime as observed along the west coast of ne honshu during the early and middle miocene the strike slip regime was transpressional to the north in sakhalin and hokkaido and transtensional to the south along the west coast of ne honshu as far as noto peninsula and yatsuo basin dextral motion accommodated the opening of the japan sea as a pull apart basin with the tsushima fault to the west the opening of the japan sea ceased at the end of the middle miocene when transtension started to change to e w compression in the japan arc subduction of the japan sea lithosphere under the japan arc started 1 8 ma ago the evolution of the stress regime from transtensional to compressional in the southern part of the strike slip zone is related to the inception of the subduction of the young philippine sea plate lithosphere under the japan arc during the late miocene subduction related extension is a necessary condition for the opening of the japan sea two possible mechanisms can account for dextral shear in this area 1 counterclockwise rotation of crustal blocks due to the collision of india with asia 2 extrusion of the okhotsk sea block squeezed between the north america and eurasia plates

construction of the mississippian pennsylvanian and permian seawater 87sr86sr curve abstract the estimated path of seawater 87 sr 86 sr during the mississippian pennsylvanian and permian is based on 228 analyses the samples are largely from the southern interior of the u s a where rocks of this age are widely exposed multiple analyses from single rock units are used to define the seawater ratio for different time periods the seawater 87 sr 86 sr declines from 0 70812 sw 95 mississippian to an estimated low in the early meramecian of near 0 70755 sw 152 the value rises to near 0 70812 sw 95 in latest mississippian the ratio rises to 0 70830 sw 77 in mid pennsylvanian and decreases to near 0 70809 sw 98 during virgilian time the earliest permian value is near 0 70793 sw 114 after rising to 0 70804 sw 103 in the early leonardian the seawater 87 sr 86 sr falls sharply to reach a low of 0 70672 sw 235 in ochoan time the end of the paleozoic is estimated to be near 0 70707 sw 200 based on samples from china analysis of the paleozoic curve shape shows an alternating dominance of oceanic and continental sr contributions the extended and precipitous decline in the 87 sr 86 sr during mid permian is unmatched during the phanerozoic the contrast of the paleozoic seawater sr curve shape to that in the mesozoic and cenozoic indicates a potentially fundamental change in the balance of oceanic and continental sr contributions at the end of the permian

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

the strontium isotopic composition of brigantian late dinantian seawater abstract strontium isotopic determinations of different fossiliferous biominerals from brigantian late dinantian carbonates in northern england reveal that many are affected by diagenetic overprinting of their original 87 sr 86 sr ratios only pristine non luminescent articulate brachiopod shell material can be used to gain reliable estimates of carboniferous and by deduction palaeozoic ocean chemistries conodonts are found to be the only type of phosphatic biomineral capable of preserving original palaeoseawater chemistries however this only applies to specimens subjected to minimal burial temperatures other phosphatic biominerals are conclusively shown to be readily susceptible to diagenetic overprinting of original sr isotopic ratios a brigantian strontium curve compiled from pristine brachiopod calcite shows a relatively complex evolution with time but it can be resolved into two separate components operating on different time scales a cyclic sinusoidally shaped component with a frequency of 1 ma appears to fluctuate about a longer term more gradual linear increase in the brigantian seawater isotopic composition

sr and nd isotopes at the prmian triassic boundary a record of climate change abstract we present a detailed curve of seawater 87sr 86sr for the middle permian to triassic based on analyses of conodonts from overlapping sections in the u s a pakistan correlated using conodont biostratigraphy the isotope ratio decreased in the middle permian at an average rate of 0 000062 ma 1 reached a minimum in the capitanian 257 258 ma and increased in the late permian at an average rate of 0 000097 ma 1 the late permian rate of increase was roughly two and a half times greater than the average increase over the past 40 ma and approximately equal to the highest cenozoic rates which occurred over much shorter time intervals modeling results suggest that decreasing middle permian 87sr 86sr ratios were driven by changes in the riverine sr flux to the oceans while increasing ratios in the late permian triassic are attributed to both increased riverine 87sr 86sr and flux the reduced middle permian riverine flux coincides with extreme continental aridity associated with the formation of pangea and recorded by massive evaporite deposits in addition mountains in the equatorial region of pangea may have created a rain shadow thereby minimizing precipitation in regions that currently contribute the bulk of chemical weathering products to the ocean increasing riverine 87sr 86sr in the late permian is suggested by the observation that 143nd 144nd values decrease at the same time however the source of radiogenic sr is not known frequently cited mechanisms for increasing 87sr 86sr in runoff such as glaciations and continent to continent collisions coincide instead with decreasing seawater 87sr 86sr in the middle permian one possible source may have been deep erosion into older orogens associated with a dramatic increase in chemical weathering in the late permian the cause of enhanced weathering appears to have been increased levels of atmospheric co2 with associated global warming and increased humidity proposed sources of co2 include dissociation of gas hydrates and oxidation of organic matter during extreme sea level regression as well as volcanic emissions from siberian traps eruptions continental floral and faunal distributions are consistent with this interpretation as are oceanic 13c patterns and variations in shallow water sediment lithologies

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

evidence for compressionally induced high subsidence rates in the kurile basin okhotsk sea a combined volcanological geochemical paleo oceanological geochronological and geophysical study was undertaken on the kurile basin in order to constrain the origin and evolution of this basin very high rates of subsidence were determined for the northeastern floor and margin of the kurile basin dredged volcanic samples from the geophysicist seamount which were formed under subaerial or shallow water conditions but are presently located at depths in excess of 2300 m were dated at 0 84 0 06 and 1 07 0 04 ma with the laser 40ar 39ar single crystal method yielding a minimum average subsidence rate of 1 6 mm year for the northeast basin floor in the quaternary trace element and sr nd pb isotope data from the volcanic rocks show evidence for contamination within lower continental crust and or the subcontinental lithospheric mantle indicating that the basement presently at 6 km depth is likely to represent thinned continental crust average subsidence rates of 0 5 2 0 mm year were estimated for the northeastern slope of the kurile basin during the pliocene and quaternary through the determination of the age and paleo environment depth of formation of sediments from a canyon wall taken together the data from the northeastern part of the kurile basin indicate that subsidence began in or prior to the early pliocene and that subsidence rates have increased in the quaternary similar rates of subsidence have been obtained from published studies on the sakhalin shelf and slope and from volcanoes in the rear of the kurile arc the recent stress field of the kurile basin is inferred from the analysis of seismic activity focal mechanism solutions and from the structure of the sedimentary cover and of the alaid back arc volcano integration of these results suggests that compression is responsible for the rapid subsidence of the kurile basin and that subsidence may be an important step in the transition from basin formation to its destruction the compression of the kurile basin results from squeezing of the okhotsk plate between four major plates the pacific north american eurasian and amur we predict that continued compression could lead to subduction of the kurile basin floor beneath hokkaido and the kurile arc in the future and thus to basin closure

strontium isotope stratigraphy of the middle devonian brachiopods and conodonts a set of 145 strontium isotope ratios based on skeletal components of emsian to frasnian age reveals a detailed structure of the seawater 87sr 86sr curve that can ultimately serve as a tool for high resolution isotope stratigraphy in the devonian brachiopod and conodont samples were collected in the eifel region of germany the area around the global stratotype for the middle devonian with an apparent average temporal resolution in the 105 y range preservation of the brachiopod shell material has been assessed by optical microscopy sem and cathodoluminescence and only the better preserved internal secondary layer of the shell has been utilized for strontium isotope measurements the 87sr 86sr curve shows a short decline from 0 7081 to 0 7078 through the late emsian followed by short term fluctuations with an amplitude of up to 10 4 around a mean of 0 7078 during the eifelian and givetian and a rise from late givetian into the late devonian the above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records due to erosional and nondepositional events but a direct confirmation for such a scenario can be claimed only for the latest eifelian otomari event insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes conodont skeletal elements even if well preserved at cai 2 5 yielded results that are mostly enriched in radiogenic 87sr if compared to coeval brachiopods although this enrichment is usually within the 10 5 range conodonts despite their superior stratigraphic resolution should be utilized only as a material of second choice in cases where brachiopods are rare or absent

the 190pt 186os decay system applied to dating platinum group element mineralization of the bushveld complex south africa abstract we evaluate the 190pt 186os decay system measured by laser ablation mc icpms as a useful geochronometric tool for direct dating of platinum group minerals pgm in large mafic ultra mafic intrusions by analysing mineralised rocks from the geochronologically well constrained bushveld complex s africa mixed pgm laurite cooperite laurite platarsite sperrylite and pt fe alloy from the merensky reef yield a pt os isochron age of 1995 50 ma 2 n 48 mswd 1 16 initial 186os 188os 0 119819 0 000006 while this is 59 myr younger than the u pb zircon age for the merensky reef scoates and friedman 2008 it is consistent with recent re os ages for the platreef considering the younger age recorded by both re os and pt os systems we propose that the pt os isochron age records a late low temperature hydrothermal event affecting pgm and sulphides in the platreef and merensky reef pt rich phases may also yield single grain 190pt 186os model ages since initial 186os 188os can be well constrained a pt os model age of 2024 ma 101 ma was calculated for a merensky cooperite a pt fe alloy from the onverwacht pipe and a tweefontein hill sperrylite give pt os model ages of 2113 106 ma and 2042 102 ma respectively a pt os isochron derived from all bushveld units gives an age of 2012 47 ma 2 186os 188osi 0 119818 0 000006 with an mswd of 1 19 hence pge mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the pt os chronometer nonetheless the potential for the pt os system in dating events in pge mineralised systems is evident and the agreement of single grain model ages in high pt os phases with the pt os and re os isochron ages indicates their usefulness in the geochronology of these systems

distribution of diatoms and geochemical features of holocene sediments from the kuril basin sea of okhotsk abstract sediments from the kuril basin were collected during the russian german deep sea expedition sokhobio sea of okhotsk biodiversity studies concentrations of biogenic and trace elements quantitative content and species composition of diatom assemblages in surface sediments and core samples were analyzed distribution in the surface sediments depending on different environmental conditions as well as changes in the diatom frustule content the ratio of dominant diatom species and the ecological structure of diatom assemblages were studied the collected sediments were silt and clayey silt with sio2am and corg contents ranging from 15 85 to 36 61 and 0 84 1 92 respectively the spatial distribution patterns of the elements studied and the diatoms as well as vertical profiles of the sediment cores revealed that the contents of biogenic components and trace elements such as amorphous silica organic carbon barium and phosphorus changed in parallel with the change in the number of diatoms and depended mainly on the biological productivity of the sea the change in water mass parameters and in the hydrological regime also affected the chemical composition of the bottom sediments over time a predominance of oceanic species is observed in the sediments at the bottom of the basin and the mixed oceanic neritic composition of diatoms is characteristic of slope deposits moreover the assemblages reflect the influence of pacific ocean water masses at stations close to the kuril straits as well as in the warm waters of the sea of japan at the station in the soya current zone a nearly synchronous increase in the impact of okhotsk water masses and the entry of the sakhalin current waters into the kuril basin as one moved deeper into the core at stations with different climatic conditions was observed the productivity and species diversity of diatoms and the chemical composition of sediments are reflections of the regional features of climatic processes

tectonics of the south okhotsk deep sea basin abstract the south okhotsk deep sea basin is a morphologically simple fault bounded basin located in the kuril back arc region the structure of the crust is distinguished by three principal units layer 1 v p 2 4 km sec is a layer of sediments 3 4 km thick layer 2 v p 4 5 5 km sec has a thickness of 0 5 1 5 km layer 3 v p 6 5 7 km sec is about 5 km thick and has a probable basaltic to gabbroic composition to a depth of 30 km the upper mantle beneath the basin consists of alternating layers having velocities of 8 0 and 7 0 km sec starshinova 1980 which may be indicative of localized partial melting heat flow values are 2 2 5 times higher than normal sedimentation rates of 5 15 m ma for a thick transparent sedimentary unit probably consisting of clays and argillites and immediately overlying basement suggest that sedimentation in the basin began in pre cretaceous time there is no evidence of a ridge or rift in the basin and the overlying sediments are undeformed it is thus difficult to relate the formation of the south okhotsk basin to subduction of the pacific plates along its eastern margin

the temperatures and oxygen isotope composition of early devonian oceans the oxygen isotope composition of pre carboniferous 360 myr old marine carbonates cherts and phosphates has been reported1 8 to be lower than that of later post devonian samples according to one explanation1 3 this is a reflection of warmer generally 40 c pre carboniferous oceans relative to modern oceans alternatively it has been proposed5 7 that the pre carboniferous oceans were depleted in 18o by 2 smow relative to modern oceans here i report high 18o values 1 9 to 2 9 pdb in normal shallow marine limestones and three brachiopod shells from the lower devonian haragan and bois d arc formations in south central oklahoma i interpret these values as near primary and therefore constraining the temperature and oxygen isotope composition of early devonian sea water to 25 7 c and 0 1 smow respectively in conjunction with similarly high 18o values obtained from older ordovician and silurian samples8 these results imply that the temperature and oxygen isotope composition of pre carboniferous oceans may at least during some time intervals have been similar to those of modern oceans

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

opening of the kuril basin deduced from the magmatic history of central hokkaido north japan abstract the kuril basin is a fan shaped back arc basin in the northwestern pacific rim the basin is separated by the kuril arc from the pacific ocean to the southeast and is bounded by the subsided okhotomorsk block to the north and by the late eocene to early miocene hidaka magmatic arc to the west the clastic sediments constituting the late cretaceous to early eocene proto kuril arc which underlies the modern kuril arc were transported from the okhotomorsk block between them the kuril basin now intervenes this suggests that the kuril basin had not opened and that the proto kuril arc had been located in the southern margin of the okhotomorsk block at least until early eocene time it also indicates that the kuril basin was not entrapped by the proto or modern kuril arc the tertiary hidaka magmatic arc in central hokkaido was formed along a n s trending subduction zone where the pacific plate was subducting beneath the eurasian plate therefore the kuril basin was not yet present on the east of central hokkaido during the period when the hidaka magmatism was taking place based on the fact that the hidaka arc magmatism ceased at 17 16 ma and coevally along its n s trending extension it is suggested that the kuril basin opened just after 16 ma by the southward retreat of the proto kuril arc trench system in central hokkaido an extensional stress regime during the middle miocene is inferred from the occurrence of a type magmatism and graben like sedimentary basins this stress field can be attributed to subsidence of the pacific slab covered by the retreated proto kuril arc during the opening of the kuril basin the occurrence of an undeformed and horizontal sedimentary cover on the rugged basement of the kuril basin is consistent with rather rapid opening at 16 15 ma during the middle miocene the kuril basin and japan basin were formed by a similar process i e rotation and oceanward retreat of an arc trench system opening of the two back arc basins along different plate boundaries i e the pacific north american boundary for the kuril basin and the pacific eurasian boundary for the japan basin may have been caused by the migration of a hot region across the overriding plate boundary