a review of magmatism and deformation history along the ne asian margin from ca 95 to 30 ma transition from the izanagi to pacific plate subduction in the early cenozoic abstract previous plate reconstruction in the paleo pacific domain indicates that the izanagi and pacific plates influenced the ne asian continental margin during the mesozoic and cenozoic respectively however it remains unclear how and when the transition from izanagi to pacific plate subduction occurred because most evidence came from the oceanic crust however the onshore geological records are poorly understood so in order to solve this problem we review recently published data and identify three magmatic episodes a magma gap and four deformation stages from ca 95 to 30 ma margin parallel izanagi pacific ridge subduction is proposed to explain the changing magmatic and deformation signatures and other regional events in the early cenozoic from ca 95 to 65 ma the n or nnw directed subduction of the izanagi plate triggered sinistral shearing along the nne striking faults arc magmas at that time incorporated older crustal materials from ca 65 to 55 ma the nww striking transform faults and ridge segments were subducted developing small slab windows and facilitating the input of juvenile magmas local extension was active above these windows during ca 55 to 45 ma the ridge was totally subducted and a margin wide slab window was opened resulting in an arc magma gap and extension massive asthenosphere upwelling caused the extensive accretion of juvenile materials from ca 45 30 ma the dextral nne trending faults resulted from the w directed subduction of the pacific plate new arc magmas were generated from juvenile materials and thus are isotopically depleted

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

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

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

carboniferous fault reactivation at the northern margin of the metal rich gawler craton south australia implications for ore deposit exhumation and preservation abstract the formation of major palaeoproterozoic and mesoproterozoic cu au deposits at the metal rich margins of the gawler craton south australia has received a lot of attention however the relationship between metal occurrences the exhumation level of the crust and the structural architecture of the craton margins is less clear here we present results from apatite fission track thermochronology applied to basement rocks at the northern margin of the gawler craton revealing a differential cooling history with respect to the karari shear zone ksz the ksz is a major shear zone that extends to the moho in reflection seismic images and has a prolonged history of high temperature activity during the paleoproterozoic and mesoproterozoic new apatite fission track data show that samples taken to the north of the ksz record a significant pulse of carboniferous cooling in contrast to the phanerozoic monotonic slow cooling history documented for the area just south of the ksz this carboniferous cooling signal coincides with a sedimentary hiatus between the neoproterozoic devonian officer basin and the late carboniferous to early permian arckaringa basin to the north of the ksz therefore carboniferous cooling can be linked with exhumation and fault reactivation of the ksz at that time which is interpreted to be associated with far field compression caused by the alice springs orogeny 450 300 ma of central australia following carboniferous exhumation a localized thermal overprint was observed in locations associated with palaeogene palaeochannels the extent of phanerozoic exhumation shows a spatial relation with the location of au and or cu fe mineralization in the northern gawler craton areas that were significantly modified by mesoproterozoic mineralizing events such as the olympic iocg province and the central gawler gold province record post silurian exhumation histories related to the alice springs orogeny to the west of these two major mineral provinces archaean early palaeoproterozoic terranes in the northwestern gawler craton with abundant au and cu fe mineral occurrences were not affected by phanerozoic exhumation and denudation these relations suggest that the mesoproterozoic mineralized terranes were more susceptible to phanerozoic deformation compared to the archaean palaeoproterozoic terranes within the stronger parts of the gawler craton hence understanding the timing of fault reactivation and the associated relative exhumation level may provide valuable constraints for ore deposit preservation and mineral exploration within the gawler craton

tectono thermal evolution of the southwestern alxa tectonic belt nw china constrained by apatite u pb and fission track thermochronology abstract the central asian orogenic belt caob is regarded to have undergone multiple phases of intracontinental deformation during the meso cenozoic located in a key position along the southern caob the alxa tectonic belt atb connects the northernmost tibetan plateau with the mongolian plateau in this paper we apply apatite u pb and fission track thermochronological studies on varieties of samples from the southwestern atb in order to constrain its thermal evolution precambrian bedrock samples yield late ordovician early silurian 430 450 ma and late permian 257 ma apatite u pb ages the late paleozoic magmatic sedimentary samples yield relatively consistent early permian ages from 276 to 290 ma these data reveal that the atb experienced multiple paleozoic tectono thermal events as the samples passed through the apatite u pb closure temperature 350 550 c we interpret these tectonic events to record the long lived subduction accretion processes of the paleo asian ocean during the formation of the southern caob with possible thermal influence of the permian tarim mantle plume apatite fission track aft data and thermal history modelling reveal discrete low temperature thermal events for the atb inducing cooling reheating through the aft partial annealing zone 120 60 c during the permian the samples underwent rapid cooling via exhumation or denudation from deep crustal levels to temperatures

seasonal upper shelf circulation along the central western gulf of mexico a preferential upcoast flow reinforced by the recurrent arrival of loop current eddies

thermal and exhumation history of sakhalin island russia constrained by apatite u pb and fission track thermochronology abstract sakhalin island represents a key locality to study the tectonic evolution of the western pacific the island is located at the amur okhotsk plate margin and records a complex thermotectonic history apatite double dating u pb and fission track and thermal history modelling were applied to three late eocene granitoid massifs within central and southern sakhalin the aniva okhotsk and langeri complexes apatite u pb results yield consistent late eocene 40 37 ma ages suggesting rapid post magmatic cooling apatite fission track results reveal bimodal age distributions with late eocene early oligocene 38 33 ma and early miocene 20 17 ma age populations that can be correlated with variations in uranium and chlorine concentrations thermal history modelling translates the aft age bimodality into two phase cooling histories the timing of the early cooling phase 38 33 ma corresponds with the apatite u pb ages indicating rapid cooling to at least 100 c during the late oligocene the second cooling phase at 20 17 ma cooled the samples to near surface temperatures both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate suggesting that cooling was a response to rapid exhumation in addition our data suggests that the studied terranes record differential exhumation with respect to the structural architecture the miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the kuril basin

the low temperature thermo tectonic evolution of the western tian shan uzbekistan abstract the kyzylkum nurata region represents a key area in understanding the tectonic evolution of the western tian shan in this study we present new thermochronological data apatite fission track and apatite u th sm he and associated thermal history models for 45 igneous samples from the kyzylkum nurata segment of south tian shan on the territory of uzbekistan and tajikistan our data show that the kyzylkum nurata segment experienced a multi phase mesozoic thermal history that differs from previously studied segments of the tian shan a triassic 220 200 ma cooling signal is widespread throughout the tian shan and is interpreted as being associated with exhumation following the closure of the palaeo asian ocean following this period of fast cooling the kyzylkum nurata segment experienced a period of slow cooling and erosion in the early jurassic 190 160 ma however in contrast to other parts of the tian shan our study area preserves evidence for rapid cooling during the late jurassic early cretaceous 160 120 given that this rapid cooling signal was only recorded for samples in association with major structures e g relic suture shear zones we interpret this event as a period of fault reactivation related with tectonic processes at the eurasian margin of the tethys ocean subsequently the late cretaceous early palaeogene 100 50 ma is characterised by slow cooling and erosion since the late palaeogene the basement of the tian shan experienced cooling related to the india eurasia collision the thermal signal of this collision has been extensively recorded in the high altitude kyrgyz tian shan within the low relief kyzylkum nurata segment this cenozoic overprint is not recorded allowing for a detailed assessment of the mesozoic thermal and landscape evolution of the western tian shan our study demonstrates that the cretaceous uzbek tian shan was characterised by a series of parallel linear mountain belts that formed along suture zones during fault reactivation

late eocene granites in the central sakhalin island russian far east and its implication for evolution of the sakhalin hokkaido orogenic belt abstract the sakhalin hokkaido orogenic belt along the western margin of the okhotsk sea is an important early cenozoic accretionary orogen related to subduction of the paleo pacific oceanic plate in order to study the eocene evolution of this accretionary orogen and related magmatism we carried out zircon u pb dating whole rock elemental and sr nd isotopic analyses on the langeri and val za granitic plutons that intruded the east sakhalin accretionary complex in the central sakhalin island zircon u pb dating results revealed that the langeri and val za plutons were emplaced in the late eocene at 38 36 ma whole rock elemental data indicate that granitoids are mainly peraluminous s type with subordinate i type granites and all granitic samples show arc like geochemical affinity whole rock sr nd isotopic results show consistent low initial 87sr 86sr ratios 0 7042 to 0 7049 positive end t values 1 7 to 3 1 and young nd two stage model ages 611 to 717 ma elemental and isotopic data reveal that granitic rocks of the langeri and val za plutons were generated from remelting of metasediments and amphibolites in the accretionary complex from comprehensive analyses of regional tectonics we concluded that the late eocene granitic rocks were generated in a syn collisional tectonic setting marking the timing for formation of the sakhalin hokkaido orogenic belt the middle late eocene welding of the okhotsk sea plate and the eurasia plate is the most plausible mechanism that caused crustal thickening beneath the sakhalin island and anatexis of metasediments and oceanic crustal materials in the lower crust might be responsible for the formation of late eocene granitic rocks considering the eocene tectonic events in ne asia late paleocene to early eocene subduction of the kula pacific ridge beneath the hokkaido island and eocene initial rift of pull apart basins along strike slip fault system in the hokkaido and sakhalin islands may also contribute to the emplacement of eocene magmatism in the sakhalin hokkaido orogenic belt

the thermo tectonic evolution of the southern congo craton margin as determined from apatite and muscovite thermochronology abstract the southern irumide belt sib of zambia consists of predominantly mesoproterozoic terranes that record a pervasive tectono metamorphic overprint from collision between the congo and kalahari cratons in the final stages of gondwana amalgamation this study applies multi method thermochronology to samples throughout southern zambia to constrain the post collisional phanerozoic thermo tectonic evolution of the region u pb apatite and 40ar 39ar muscovite data are used to constrain the cooling history of the region following congo kalahari collision and reveal ages of c 550 450 ma variations in the recorded cooling ages are interpreted to relate to localised post tectonic magmatism and the proximity of analysed samples to the congo kalahari suture apatite fission track data are used to constrain the low temperature thermo tectonic evolution of the region and identify mean central ages of c 320 300 210 200 and 120 110 ma thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the phanerozoic carboniferous to permian triassic heating is suggested to relate to the development of karoo rift basins found throughout central africa and constrain the timing of sedimentation in the basin permian to jurassic cooling is identified in a number of samples reflecting exhumation as a result of the mauritanian variscan and gondwanide orogenies subsequent cooling of the majority of samples occurs from the cretaceous and persists until present reflecting exhumation in response to larger scale rifting associated with the break up of gondwana each model reveals a later phase of enhanced cooling beginning at c 30 ma that if not an artefact of modelling corresponds to the development of the east african rift system the obtained thermochronological data elucidate the previously unconstrained thermal evolution of the sib and provides a refined regional framework for constraining the tectonic history of central africa throughout the phanerozoic

radiation enhanced fission track annealing revisited and consequences for apatite thermochronometry apatite fission track aft analyses for granitoid and metamorphic bedrock samples from the western superior province ontario the churchill rae province melville peninsula and southampton islan

mesozoic reactivation of the beishan southern central asian orogenic belt insights from low temperature thermochronology abstract the beishan orogenic collage boc is located in the southeast of the central asian orogenic belt caob and formed during final consumption of the palaeoasian ocean in the late palaeozoic this study applies low temperature thermochronology to constrain the meso cenozoic history of the boc apatite fission track and u th sm he data obtained for granitoid samples along a north south transect through the boc indicate three distinct phases of exhumation during 1 the late triassic early jurassic 225 180 ma 2 early cretaceous 130 95 ma and 3 late cretaceous early palaeogene 75 60 ma samples from the northern boc reveal a more profound early cretaceous signal and a weaker late triassic early jurassic signal than those in the southern boc a possible explanation for this discrepancy is the presence of crustal scale fault zones in the northern boc that are interpreted to have undergone repeated reactivation throughout the mesozoic exposing deeper exhumed sections of the boc these faults may thus have acted as a major control on exhumation in the region this pattern is consistent with results from elsewhere in the caob such as in the tianshan and the altai where regional widespread exhumation occurred since the early cretaceous while major fault zones record localised exhumation during the late cretaceous and cenozoic late cretaceous early palaeogene cooling ages were found only in the south of the boc suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area the meso cenozoic cooling events described here are thought to be related to the progressive closure of the palaeo tethys ocean to the south associated collision and accretion of micro continental blocks and island arcs at the southern eurasian margin are interpreted to have induced more widespread reactivation and exhumation in central asia than previously anticipated extending to the northern margin of the tarim craton this observation hence refines the existing tectonic history models for central asia

statistics for la icp ms based fission track dating abstract despite the conceptual elegance and simplicity of the external detector method edm for fission track dating an increasing number of laboratories are switching to la icp ms as a means of measuring the uranium content of apatite zircon and sphene la icp ms based fission track laft dating offers shorter turnaround times removes the need to handle radioactive materials and hydrofluoric acid and facilitates double dating with the u pb method this paper aims to bring the statistical treatment of laft data on an equal footing with the edm by formulating four different analytical protocols depending on the accuracy and reproducibility of the uranium measurements under the absolute dating approach the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation assuming that both of these measured quantities have been determined accurately and that the fission decay constant and equivalent isotopic track length are known the zeta calibration approach avoids making these assumptions by normalisation to a reference material of known age uranium zoning has a detrimental effect on the accuracy of laft ages this effect can be removed by counting only those fission tracks located within the laser ablation pit alternatively the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains using a lognormal distributional assumption for the uranium concentration laft dating is arguably less well suited than the edm to young and u poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning such samples occasionally contain no fission tracks at all resulting in infinite analytical uncertainties this paper introduces a pragmatic solution to this problem in which the uranium measurement are converted to virtual induced fission track counts to produce strictly positive ages with finite standard errors with the age equation and zero track strategy in place laft ages can be subjected to more sophisticated statistical analysis using a logarithmic transformation these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures the methods introduced in this paper have been implemented in a software package called isoplotr that is available free of charge at http isoplotr london geochron com

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

meso cenozoic multiple exhumation in the shandong peninsula eastern north china craton implications for lithospheric destruction abstract the shandong peninsula in the eastern north china craton ncc forms part of the region that witnessed extensive tectonic reactivation with concomitant craton destruction and lithospheric thinning during the meso cenozoic previous studies concentrated mainly on the timing mechanism and tectonic setting of the meso cenozoic magmatism with inadequate evidence from low temperature thermochronology to constrain the thermo tectonic exhumation history of this region in this study we present new apatite u pb aupb and fission track aft data with corresponding thermal history models for igneous rocks from the two flanks of the tan lu fault zone tlfz that bisects the shandong peninsula with a view to gain insights into the meso cenozoic exhumation history of this region and to evaluate its implications on the lithospheric destruction of the ncc the newly obtained aupb ages of 2 5 1 5 ga for the precambrian intrusive rocks and of 162 112 ma for the mesozoic igneous suite are mainly used to constrain their thermal history models in addition the mesozoic aupb ages of 162 112 ma highly overlap with their corresponding zircon u pb ages 161 115 ma suggesting shallow granitoid emplacement and associated rapid post magmatic cooling in response to the westward subduction of the paleo pacific plate the aft dating results yield two groups of aft central ages at 122 113 ma and 98 59 ma and corresponding thermal history models also reveal two rapid cooling stages during the early cretaceous 130 105 ma and late cretaceous to paleogene 85 55 ma by integrating previous low temperature thermochronological studies with this study we interpret that the early cretaceous rapid exhumation corresponds to the peak timing of craton destruction resulting from the paleo pacific slab rollback within a back arc extensional setting the late cretaceous rapid exhumation is interpreted as a response to continuing craton destruction derived by the nnw directed pacific plate subduction the paleogene cooling might represent the termination of craton destruction of shandong peninsula associated with a dextral motion along the tlfz triggered by the change in direction of the pacific plate from nnw to wnw and or far field effect of the india eurasia collision

apatite geochronology and chemistry of luanchuan granitoids in the east qinling orogen china implications for petrogenesis metallogenesis and exploration abstract the luanchuan granitoids in the east qinling orogen central china host multiple genetic types of mo w pb zn ag polymetallic deposits here we present results from systematic geochronological and geochemical investigations on apatite with a view to gain insights into the petrogenesis metallogenesis and potential for mineral exploration we analyzed 388 apatite grains from eight late mesozoic granitic plutons in the luanchuan ore district apatite u pb geochronology yields 206pb 238u mean ages in the range of 151 5 102 3 ma representing the crystallization ages through integrating our results with those from previous studies on the multi stage magmatism 160 130 ma 111 108 ma and metallogeny 150 139 ma 138 135 ma 114 ma the newly obtained apatite ages can be correlated with coeval magmatic and metallogenetic events of this ore district in comparison except for the nannihu pluton which is ca 19 ma younger than the corresponding zircon u pb ages the other studied plutons show almost consistent apatite and zircon u pb ages within error ranges suggesting rapid post magmatic cooling below 450 c in response to shallow emplacement apatite chemistry displays ree concentrations in the range of 2379 15 925 ppm characterized by rather high lree concentrations depleted to flat hree profiles and distinctive negative eu anomalies suggesting crust mantle mixing in the magma source prior to apatite crystallization which incorporated different degrees of sediment melting and slab melting components triggered by the subducted paleo pacific slab derived fluids in conjunction with element concentrations and ratios e g sr mn cl y la y eu eu sm nd n la sm n eu eu n ce ce n la yb n in apatite the luanchuan granitoids are classified as moderately oxidized and mineralized i type granites we suggest that the huangbeiling pluton and other igneous rocks in its vicinity likely have relatively higher potential for future w mo mo w exploration and that different degrees of magmatic differentiation might have generated the porphyry skarn mo w deposit in the yuku ore field moreover our study also demonstrates that the apatite ree concentrations and ratios can be used as potential indicators for mineral exploration

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

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

thermal history of the northern olympic domain gawler craton correlations between thermochronometric data and mineralising systems abstract multi method thermochronology applied to the northern olympic domain of south australia reveals a complex thermal evolution with multiple thermal events apatite u pb closure temperature 550 350 c and muscovite 40ar 39ar ages 400 350 c record post magmatic cooling from 1850 to 750 ma for the 1850 ma donington suite and from 1560 1500 ma for the 1590 ma hiltaba suite and the gawler range volcanics potassium feldspar 40ar 39ar ages 350 150 c record disturbed spectra that are likely related to hydrothermal alteration at 1000 650 ma in the neoproterozoic adelaide rift complex apatite fission track 60 120 c zircon u th sm he 180 200 c and apatite u th sm he 45 75 c ages reveal regional low temperature thermal events at 1000 ma 430 400 ma 350 330 ma and 200 ma in addition to localised thermal events during the cretaceous the 1000 ma apatite fission track ages are amongst the oldest recorded for south australia and are only preserved near the margins of the olympic domain the 430 400 ma and 350 330 ma cooling events are interpreted to be driven by the alice springs orogeny the mesozoic thermochronometric ages are interpreted to record localised thermal perturbations possibly related with hydrothermal activity within the northern olympic domain the presence of abundant iocg deposits near these young thermal anomalies may indicate that these zones record more favourable thermal conditions and or exposure levels for iocg discoveries in the study area

late palaeozoic and meso cenozoic tectonic evolution of the southern kyrgyz tien shan constraints from multi method thermochronology in the trans alai turkestan alai segment and the southeastern ferghana basin abstract in this paper we present new geochronological and thermochronological data from the turkestan alai segment of the southern kyrgyz tien shan the ferghana basin transition and the trans alai pamir foreland in the southern and central parts of the alai range towards the alai valley mainly post collisional granitoids were targeted for zircon u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms apatite fission track aft and apatite u th sm he ahe thermochronology these granitoids and their thermally metamorphosed contact aureole yielded middle permian zircon crystallization ages of 284 4 to 264 5 ma aft ages both using the traditional external detector approach as well as using la icp ms for these samples are constrained to the miocene and range between 22 3 1 1 and 8 4 0 3 ma while the ahe ages are somewhat younger between 12 5 0 8 and 5 0 0 3 ma this low temperature thermochronological data suggests an early miocene onset of denudation in this area in the tien shan orogen continuing through the miocene and pliocene this denudation is interpreted as a response to reactivation of the mountain belt in the framework of india eurasia convergence and specific to this region of the pamir tien shan interaction on the northern slopes of the alai range at the edge of the ferghana basin a late palaeozoic conglomerate yielded a reset early cretaceous aft age of 118 7 ma pointing to an older denudation event with respect to those identified in the southern and central parts of the alai range a mesozoic sediment sample from the same region only contains a late triassic age component of 223 ma mesozoic and cenozoic age components on the other hand are all detected in the cenozoic sediments of the trans alai range at the transition between the southern alai valley and the pamirs revealing several meso cenozoic denudation events in the adjoining basement source rocks the age components in these detrital apatites are 207 ma 75 ma 18 ma and 3 ma

evidence of the contrasted fission track clock behavior of the apatites from in ouzzal carbonatites northwest hoggar the low temperature thermal history of an archean basement this fission track study is performed on the apatites from the in ouzzal northwest hoggar algeria carbonatite complex and permits us to present a complete thermal history of this archean core two types of apatites the pink ones early carbonatites and the green ones late carbonatites have been dated the temperature during the pan african orogeny would have reached 100 120 c in the in ouzzal core as both types of apatites from the carbonatites have recorded this event 500 630 m y a mild permian thermal event reset only the fission track clock of the green late apatites this permian thermal event is of quite a large extent in the hoggar shield it has also been detected in the pharusian belt and in central hoggar this event is related to the opening of the atlantic ocean the very high u th content of some of these apatites and consequently the amount of radiation damage in the mineral lattice has been shown to possibly influence the fission track clock behavior of these minerals the f cl content of the apatites does not show any correlation with the degree of age reductions

differential exhumation and crustal tilting in the easternmost tianshan xinjiang china revealed by low temperature thermochronology the easternmost tianshan forms the eastern extremity of the modern central asian orogenic belt and represents a key locality to investigate strain propagation from the meso cenozoic plate margins to the eurasian interior the tianshan as a whole has been reactivated multiple times throughout the meso cenozoic but the extent of these reactivation events is yet to be fully understood this study applies apatite fission track and apatite u th sm he thermochronology to the mountain ranges of the easternmost tianshan our new results suggest that the area experienced two phases of rapid cooling in the mesozoic during the early middle triassic and the late cretaceous these cooling phases are linked to tectonic events at the distant plate margins such as the permian mid triassic closure of the paleoasian ocean and the late jurassic early cretaceous mongol okhotsk orogeny fault controlled differential exhumation and block tilting are recorded in the distribution of apatite fission track ages across the region finally we show through a combination of multiple thermochronometers and the integration of structural analysis that the easternmost tianshan has experienced insufficient exhumation to constrain the timing of reactivation in response to the cenozoic collision of india with eurasia and instead records older mesozoic tectonic events

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

apatite fission track and u th he data from fennoscandia an example of underestimation of fission track annealing in apatite abstract apatite fission track aft data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation often in the absence of any remnant sedimentary rocks or other supporting geological evidence reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic also u th he ages from cratonic igneous rocks that are older than paired aft data are commonly discarded as too old even when the former are repeatable at the 2 confidence level for example aft data from central fennoscandia have been interpreted to reflect first burial by a caledonian foreland basin several kilometres in thickness and subsequently its denudation however throughout fennoscandia there is no direct evidence that such a deep extensive foreland basin ever existed and indeed some geological evidence points to the contrary additionally a significant number of u th he ages from fennoscandia are older than their aft pairs but have been interpreted by their authors as too old relative to the aft ages and not geologically meaningful despite repeatable results here we propose a new interpretation of aft data from fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history discarding the hypothesis of a thick caledonian foreland basin we suggest that 1 aft ages in finland are too young rather than that the u th he ages are too old 2 repeatable sequences of u th he ages are more likely to record the true denudation history of cratons 3 the importance of radiation enhanced annealing at low temperatures has been overlooked and 4 fennoscandian aft data may be divided into an eastern domain strongly affected by radiation enhanced annealing where current aft annealing models are invalid and a western domain much less affected by radiation enhanced annealing where current modelling methods may appropriately be used to extract thermal histories in principle radiation enhanced annealing is also pertinent to the interpretation of young aft ages from apatites rich in emitter actinides the construction of a properly calibrated annealing model that fully incorporates the effects of radiation enhanced annealing is fundamentally important

the importance of apatite composition and single grain ages when interpreting fission track data from plutonic rocks a case study from the coast ranges british columbia abstract fission track results determined from granitic rocks exposed within the coast ranges of british columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples to test ages determined earlier by parrish using the population method of fission track dating new apparent apatite ages were completed using the external detector method so that single grain ages could be generated in most cases the new apatite ages were similar to the earlier ones within 2 however in many cases it was found that individual samples contained significant variations in single grain apatite ages microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry such that chlorine rich apatites retained older ages than fluorine rich grains examination of the single grain age data suggested that the spread in single grain ages provided information which constrained the thermal history of the sample and that distributions of single grain ages reflected the temperatures at which they resided during cooling these results suggest that it is important to generate more than a few single grain fission track ages when dating apatite from granitic rocks since variations in the chlorine concentration in apatite grains may exist in kg sized granitic rock samples therefore it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results

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

involvement of old crustal materials during formation of the sakhalin island russian far east and its paleogeographic implication constraints from detrital zircon ages of modern river sand and miocene sandstone abstract in order to decipher crustal nature of the sakhalin island in russian far east we carried out detrital zircon u pb age analyses on miocene sandstone and river sand from the longest river poronay river of the sakhalin island the detrital zircon data from two river sand samples display similar age distribution patterns with a dominant mesozoic age group subordinate age peaks at 1 8 ga and 2 5 ga and a few paleozoic and neoproterozoic grains the miocene sandstone shows age peaks at 22 84 260 and 497 ma respectively and a few paleo proterozoic grains these age groups indicate that abundant old crustal materials have been involved in the crustal formation of the sakhalin island detrital zircon result reveals two episodes of post accretion magmatism from the sakhalin island in ages of 37 ma and 22 21 ma they can be correlated with coeval post accretion magmatic events in the hokkaido island supporting the geological correlation between the sakhalin island and the hokkaido island comparison of detrital zircon dating result from the sakhalin island with those from surrounding blocks and cratons in eastern asia allows us to propose two possible sources in eastern asia the bureya jiamusi khanka block with the sikhote alin orogenic belt to its west and the south china craton the detrital zircon result indicates that the formation of the sakhalin island should be close to the east asia continent rather than as an independent intro oceanic island arc within the pacific ocean similar to formation of the japanese islands the south china craton may have played an important role during formation of the sakhalin island

low temperature thermochronology of the chatkal kurama terrane uzbekistan tajikistan insights into the meso cenozoic thermal history of the western tian shan

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 fish canyon tuff a new look at an old low temperature thermochronology standard abstract the fish canyon tuff fct has served as an important source for geochronology standards particularly for fission track k ar and 40 ar 39 ar dating even though efforts to establish precise ages for its constituent minerals have proved to be unexpectedly complex to evaluate the suitability of fct apatite as a standard for apatite u th sm he ahe thermochronometry and to test underlying assumptions about its suitability for apatite fission track aft thermochronometry we analysed samples from a series of sites throughout the vertical and lateral extent of the host ignimbrite sheet samples were collected from the relatively lithic rich classic sampling location in the lower part of the thick proximal ignimbrite and a 330 m vertical section of fct immediately above it average weighted mean ahe ages from multiple analyses at five sites in this profile range from 20 8 0 4 ma from the classic site at the base to an average of 28 4 0 2 ma all 1 in the upper part of the section the ahe age at the classic site is substantially reduced at 20 8 0 4 ma relative to a reference age for ignimbrite emplacement of 28 2 ma corresponding zircon u th he zhe ages for these samples are all concordant at 28 3 0 4 ma by contrast apatite fission track aft ages from the same vertical section are all concordant at 27 4 0 7 ma with the central age of 28 8 0 8 ma at the classic site except for the uppermost sample 23 2 1 7 ma for which clear evidence for local probably fire disturbance is seen in the track length distribution the ahe data at the classic site thus provide evidence for substantial post eruptive early miocene cooling of the tuff consistent with its position at the bottom of a deeply incised valley with 800 m of local relief and probably 1000 m of removed section above it the aft age of the classic sampling site however is indistinguishable from ignimbrite emplacement and thus continues to be a useful standard for aft but not for ahe geochronology apatites from this site have the highest measured cl concentrations 0 82 wt of any of the fct apatites analysed contributing to this suitability aft and ahe ages at three distal localities some 35 45 km to the east of the classic site where the thickness of the fct is reduced to 100 m all yielded concordant ages with a weighted mean of 28 5 0 11 ma with no evidence for post emplacement thermal disturbance one of the distal sites a quarry in the upper part of the fct appears to be an ideal locality for a standard reference material that would be suitable for both aft and ahe low temperature thermochronometers u pb ages for zircons and 40 ar 39 ar step heating age spectra for sanidine at this new distal site are essentially identical to those found at the classic site

thermal history and differential exhumation across the eastern musgrave province south australia insights from low temperature thermochronology abstract multi method geo and thermochronological data obtained for palaeo and mesoproterozoic granitoids traversing the main structural architecture of the eastern musgrave province within south australia reveal multiphase cooling histories apatite u pb dating on six samples yield consistent ages of 1075 1025 ma suggesting a thermal reset coinciding with mantle derived magmatism of the greater warakurna large igneous province 1080 1040 ma apatite fission track aft analysis indicate that four discrete thermal events affected the study area inducing cooling through the aft partial annealing zone 60 120 c supported by apatite and zircon u th sm he data late neoproterozoic cooling from deep crustal levels to temperatures the results highlight a complex phanerozoic thermal history for the eastern musgraves and help to elucidate the poorly appreciated tectonic evolution of inland australia this study further demonstrates how high density sample transects across structural architecture can assess the relative crustal level and associated preservation of the thermal history record within fault reactivated terranes

tertiary tectonics of the sea of okhotsk russia far field effects of the india eurasia collision a new structural map of the western sea of okhotsk based on grids of multichannel seismic data provides several new insights into the tertiary deformation of asia especially in regard to the extension of extrusion tectonics linked to the india eurasia collision into northeastern asia the sedimentary basins in this offshore region are the result of two regional shear systems in the west the north trending sakhalin hokkaido dextral fault system transects sakhalin island accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts the accessory normal faults are predominantly eocene to lower miocene and indicate transtensional shear whereas the folds and thrusts are mostly younger and indicate late miocene and pliocene transpression in the north an east west trending sinistral shear zone enters the sea at its northwest corner this sinistral shear system links farther west to extensional faults of the early tertiary baikal rift extending northeastward from the terminus of this sinistral shear zone are a series of long predominantly northwest dipping listric normal faults that form a large lazy s shaped pull apart basin the shantar liziansky basin slb normal faults in this 500 140 km basin accommodate 15 to 20 extension extension and related sinistral shear appear to be largely eocene to oligocene in age with lesser later activity to the northeast of slb a further extension of the sinistral shear zone appears to bend northeastward transecting pustorets and penzhina basins and following the course of the older mesozoic mongol okhotsk chukotsk active margin and suture it possibly reaches as far as the arctic as suggested by previous workers the baikal rift and its associated sinistral shear zone may be interpreted as by products of the india eurasia collision in eocene time mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast the sinistral shear zone is transected by the dextral sakhalin hokkaido fault zone in the kashevarov region of central slb where a fan of north to northwest striking late tertiary transpressional fault splays marks the northern termination of the dextral fault zone older slb normal faults are locally deformed and half grabens are partly inverted by the slightly younger dextral splays the two regional fault systems are essentially conjugate and the pattern of intersection closely resembles that predicted by recent modelling studies of india eurasia collision tectonics

apatite fission track chronometry using laser ablation icp ms abstract comparison is made of apatite fission track ft ages measured by using conventional thermal neutron irradiation and laser ablation inductively coupled plasma mass spectrometry la icp ms to determine sample uranium content spontaneous track densities were determined on specified apatite grains from the durango fish canyon tuff and mount dromedary complex age standards neutron induced track densities and icp ms uranium values were determined for the same specified grains ages determined using the two approaches show reasonable consistency indicating the possibility of routine fission track analysis using la icp ms uncertainties and assumptions in the la icp ms approach include the laser ablation of chemically etched mineral surfaces the suitability of reference materials for calibration and the derivation of ft ages using the alternatives of selection of a f value or a revised zeta comparative approach the use of la icp ms in ft chronometry would dramatically increase the speed of analysis and hence sample throughput as well as avoiding the need for neutron irradiation and the production transport and handling of radioactive samples with the acknowledged dependency of ft annealing in apatite on composition such grain by grain multi element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

implications of incremental emplacement of magma bodies for magma differentiation thermal aureole dimensions and plutonism volcanism relationships abstract field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets sills emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation country rock metamorphism and assimilation and for the formation of large magma chambers in the upper crust when a magma body begins to grow by slow accretion of sills each successive intrusion solidifies before the injection of the next one when the system is thermally mature sill temperatures equilibrate above the solidus melts accumulate and older sills can re melt the time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body the transition from a mafic crystal poor magma to a partially molten rock that retains a highly differentiated melt is fast whereas the resulting evolved residual melt can reside in the crust for protracted periods as long as temperatures in the system are relatively low highly differentiated melts are generated which may explain the bi modal character and the absence of intermediate compositions in some magmatic provinces the level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth metamorphism partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock magma body boundary if the magma emplacement rate is low the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body understanding emplacement of magma bodies is fundamental for our understanding of the plutonism volcanism relationship magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions the time averaged emplacement rates of plutons are lower than this critical emplacement rate eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust this suggests that the emplacement of magma bodies is a multi timescale process with the development of large magma chambers corresponding to the highest magma fluxes because they control magmatic processes and the impact of magma intrusion on the country rock future studies should focus on magma emplacement rates and on magma emplacement geometries these studies should integrate field observation on plutons and geophysical data on active magmatic systems coupled with laboratory experiments and numerical simulations

u pb systematics of the mcclure mountain syenite thermochronological constraints on the age of the 40 ar 39 ar standard mmhb recent advances in u pb geochronology allow unprecedented levels of precision in the determination of geological ages however increased precision has also illuminated the importance of understanding subtle sources of open system behavior such as pb loss inheritance intermediate daughter product disequilibria and the accuracy of the model assumptions for initial pb deconvolution of these effects allows a much richer understanding of the power and limitations of u pb geochronology and thermochronology in this study we report high precision id tims u pb data from zircon baddelleyite titanite and apatite from the mcclure mountain syenite from which the 40 ar 39 ar hornblende standard mmhb is derived we find that excess 206 pb in zircon due to inclusions of high th minerals and ele vated th u in titanite and apatite jeopardize the utility of the 238 u 206 pb system in this rock strongly air abraded zircons give dates that are younger than chemical abraded zircons which yield a statistically robust 207 pb 235 u date of 523 98 0 12 ma that is interpreted as the crystallization age we explore the best method of pbc correction in titanite and apatite by analyzing the u pb isotopes of k feldspar and using 2 d and 3 d regression methods the latter of which yields the best results in each case however the calcu lated compositions of pbc for titanite apatite and k feldspar are different implying that using a single pbc correction for multiple u pb thermochronometers may be inaccurate the u pb thermochronological results are used to predict a closure time for ar in hornblende of 522 98 1 00 ma widely cited k ar and 40 ar 39 ar dates overlap with the u pb date and relatively large errors make it impossible to verify whether u pb dates are systematically 1 older than k ar and 40 ar 39 ar dates

late quaternary faulting along the western margin of the poronaysk lowland in central sakhalin russia abstract sakhalin island straddles an active plate boundary between the okhotsk and eurasian plates south of sakhalin this plate boundary is illuminated by a series of mw 7 8 earthquakes along the eastern margin of the sea of japan although this plate boundary is considered to extend onshore along the length of sakhalin the location and convergence rate of the plate boundary had been poorly constrained we mapped north trending active faults along the western margin of the poronaysk lowland in central sakhalin based on aerial photograph interpretation and field observations the active faults are located east of and parallel to the tym poronaysk fault a terrane boundary between upper cretaceous and neogene strata the active faults appear to have reactivated the terrane boundary at depth in quaternary time the total length of the active fault zone on land is about 140 km tectonic geomorphic features such as east facing monoclinal and fault scarps back tilted fluvial terraces and numerous secondary faults suggest that the faults are west dipping reverse faults assuming the most widely developed geomorphic surface in the study area formed during the last glacial maximum at about 20 ka based on similarities of geomorphic features with those in hokkaido island we obtain a vertical component of slip rate of 0 9 1 4 mm year using the fault dip of 30 60 w observed at an outcrop and trench walls a net slip rate of 1 0 2 8 mm year is obtained the upper bound of the estimate is close to a convergence rate across the tym poronaysk fault based on gps measurements a trenching study across the fault zone dated the most recent faulting event at 3500 4000 years ago the net slip associated with this event is estimated at about 4 5 m since the last faulting event a minimum of 3 5 m of strain close to the strain released during the last event has accumulated along the central portion of the active strand of the tym poronaysk fault

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

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