strain localization due to a positive feedback of deformation and myrmekite forming reaction in granite and aplite mylonites along the hatagawa shear zone of ne japan abstract with increasing mylonitization of aplite veins and their adjacent granite in the hatagawa shear zone of ne japan the modal content and grain size of feldspar porphyroclasts decrease while the modal content of fine grained 50 m polymineralic aggregate of plagioclase k feldspar quartz and biotite increases up to 60 80 in the granite and aplite ultramylonites deformation induced myrmekite lobes are ubiquitously developed around k feldspar porphyroclasts the occurrence and chemical composition of plagioclase and k feldspar in fine grained polymineralic aggregates indicate that the plagioclase is mostly derived from myrmekite while the interstitial k feldspar originates from isolated porphyroclast rims between growing myrmekite lobes and or precipitations from solution modal and bulk chemical compositions of granite and aplite indicate that the mylonites are preferentially developed in aplite veins richer in k feldspar than the surrounding granite deformation induced k feldspar replacement by myrmekite is then more favored during deformation to produce more abundant fine grained polymineralic aggregates subsequent reaction softening leads to more enhanced deformation which promotes further myrmekite forming reaction such a positive feedback of deformation and myrmekite forming reaction in aplite veins and their adjacent granite may be responsible for strain localization in the granite and aplite mylonites along the hatagawa shear zone

40ar 39ar dating of shear zones in the variscan basement of greater kabylia algeria evidence of an eo alpine event at 128 ma hauterivian barremian boundary geodynamic consequences a ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the alpine maghrebides in algeria dating of shearing movement is carried out by 40ar 39ar laser probe experiments performed on synchronous neoformed micas the proposed age at 128 ma is constrained by a two component mixing model for the saddle shaped age spectra on phengites and by a plateau age at 128 3 0 3 ma on muscovite this hauterivian barremian boundary age records a strong reworking of the alboran plate input of detritals in the flysch basin and surrounding areas it can be correlated with the j magnetic anomaly in the atlantic and with the distensive events in the pyrenees and external alps therefore isotopic ages 80 130 ma obtained on the alpine mediterranean basement could reflect interferences between this event at 128 ma and late alpine stages

early middle paleozoic intraplate orogeny in the ogcheon belt south korea a new insight on the paleozoic buildup of east asia the polycyclic ogcheon belt of south korea represents the boundary between two precambrian blocks investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the asian continent we report here structural evidence showing that the earlier tectonic event dates back to early middle paleozoic the ogcheon tectonism or orogeny the ductile piling up of nappes of the ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area the nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement field evidence includes the presence of s1 and s1 2 foliations bearing an extensive stretching lineation perpendicular to the belt associated with a strongly noncoaxial deformation regime the f1 isoclinal folds are commonly a type and sheath folds whose axes are parallel to the stretching lineation whereas the southeast facing f2 recumbent folds have subhorizontal axes parallel to the belt all lines of evidence unconformity superposed deformations age of the metamorphism etc imply that the d1 2 ogcheon tectonism is pre middle carboniferous late silurian early devonian the subsequent indosinian middle triassic f3 upright synfolial folds that trend nne ssw and later structures clearly overprint the middle paleozoic ductile thrusts and isoclinal folds the early paleozoic ogcheon supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin on the basis of the geochemical characteristics of the metavolcanics it appears that oceanization did not occur and ogcheon rift aborted soon after it was formed we suggest that middle paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features during the ductile stacking episode the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event middle carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks a tentative correlation with early paleozoic imjingang qinling and cathaysian belts within the asian continent reveals that ogcheon rift was emplaced within the south china plate and that imjingang belt should represent the limit between north and south china blocks in the korean peninsula

a newly recognized type of slickenside striation abstract this note summarizes current views on the origin of slickenside striations and points to the existence of a type of striation not readily explained by established genetic ideas such striations have been seen on fault surfaces in naturally deformed shale and in experimentally deformed paraffin wax they are defined by arrays of shallow slip parallel ridges and grooves of millimetric width and below on any given slip surface the ridges are as common as the grooves and display similar dimensions and profile shapes across any pair of slip surfaces in contact at the end of faulting ridges protruding from one surface fit into or nest with grooves on the other surface the origin of these features is not yet understood

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

early permian peak metamorphism recorded in u pb system of black slates from the ogcheon metamorphic belt south korea and its tectonic implication abstract we report pb pb whole rock and uraninite chemical th u total pb isochron method chime ages of carbonaceous black slates from the ogcheon metamorphic belt south korea the pb isotopic data of whole rock samples yield 207 pb 206 pb ages of 283 33 and 291 13 ma for two outcrops from the northeastern part of the belt the uraninite chime age is estimated at 283 26 and 281 27 ma for the northeastern and the middle part of the belt respectively all of the above ages are identical within error ranges and represent the timing of peak metamorphism after the late precambrian intraplate rifting on the other hand the 207 pb 206 pb whole rock age for the southwestern part of the belt is estimated at 194 27 ma probably representing the timing of contact thermal metamorphism associated with the intrusion of jurassic granitic plutons rb sr isotopic data of the black slates do not define any meaningful isochron the early permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the silurian devonian or the triassic time instead it corroborates the probability that the two zones in the ogcheon belt the ogcheon metamorphic belt and the taebaegsan zone were separated from each other before the development of major structural framework in the former our data do not support an idea that the ogcheon belt corresponds to the continuation of the triassic collision belt in east central china

stepwise expansions of c4 biomass and enhanced seasonal precipitation and regional aridity during the quaternary on the southern chinese loess plateau abstract the expansion of c 4 plants is one of the most prominent vegetation changes in the global ecosystem during the cenozoic era although c 4 plant expansions in the latest miocene have been widely reported factors driving the expansions are still in debate and the details of vegetation changes during the quaternary have not been well studied here we present high resolution carbon isotope time series of both organic matter and bulk carbonates covering the past 2 58 ma derived from the loess soil successions on the southern chinese loess plateau the organic matter 13 c values indicate stepwise c 4 plant expansions initiated at 1 6 and at 0 43 ma respectively we conclude that such tectonic time scale c 4 plant expansions are controlled by enhanced seasonality of precipitation relatively more precipitation in the warm growing season as well as regional aridity and this long term fluctuation superimposes on the orbital scale variations of c 4 plants while the latter appears phase locked with cyclical changes of summer monsoon circulations

subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

geodynamics of the ogcheon belt south korea abstract the ogcheon belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early palaeozoic rift which was affected by polyphase tectonics its pre cretaceous structure results from three main orogenic events the ogcheon late silurian devonian indosinian late permian triassic and yenshanian middle jurassic early cretaceous tectonic phases the external autochthonous or para autochthonous units represent platform zones with shallow water deposits whereas rift facies characterize the internal domain the yeongweol sequence forming an outer shelf on the southeastern margin of the rift displays affinities with the jiangnan basin of south china it is in wrench fault contact with the easternmost duwibong sequence correlated with the north china north korea platform in the internal metamorphic units post cambrian early ordovician pelitic and turbiditic rift metasediments overlie a thin cambrian or cambrian ordovician early platform sequence two subdomains are distinguished the upper units chung ju and pibaryeong units representing the deep axial part of the basin contain distal flyshoids and large olistoliths derived from the northwestern narrow margin the lower units turung san poeun and iwharyeong are related to the wide margin located between the deep rift domain and the stable southeastern platform the latter transitional domain is characterized by thick submarine debris flows and volcanics of alkali or transitional tholeiitic affinity the volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the cambrian ordovician platform sequence in one single basin the post middle silurian pre mid carboniferous ogcheon ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area it gave rise to an intracontinental fold thrust belt without ophiolites this tectonic event may have been contemporaneous with early palaeozoic tectonism in the quinling belt in central china and in the southeast china fold belt the definitive constitution of the sino korean craton took place before the mid carboniferous and molasse sedimentation was initiated on a single platform located on the western edge of the yakuno oceanic basin of southwest japan in late permian and early triassic times the closure of the oceanic area and the subsequent westward collision of the honshu block are marked in korea by the intracratonic indosinian tectonism since late triassic times predominantly transcurrent movements have occurred along nne ssw faults associated with syntectonic intermontane troughs and important granitization

retrieval of p t t d paths in a syn metamorphic shear zone implications for p t calculations and the permian triassic orogeny on the korean peninsula abstract crustal scale shear zones developed on the boundaries of the precambrian gyeonggi massif gm can play key role on understanding collisional orogeny during the late paleozoic to early mesozoic in the korean peninsula garnet schist formed on the boundary between the gm and okcheon metamorphic belt omb experienced contractional dn and extensional dn 1 shearing combined with a clockwise pressure temperature deformation p t d path two distinct mineral assemblages were found in the schists 1 garnet pre and or syn dn staurolite syn dn and or inter dn and dn 1 and syn dn 1 andalusite syn dn 1 bearing and 2 garnet pre and or syn dn epidote bearing assemblages depending on bulk rock compositions various compositions an10 40 in the former assemblage and an45 70 in the latter one and texture of plagioclase resulted from bulk rock compositions heterogeneous strain and dissolution and precipitation during the multiple shearing the p t conditions estimated for the early contractional shearing top to the north reverse shear are 560 c 600 c and 5 5 7 0 kbar based on the intersections between compositional isopleths for pre and or syn dn garnet and plagioclase an30 40 an45 55 calculated in the mno na2o cao k2o feo mgo al2o3 sio2 h2o and mno na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 systems subsequent extensional shearing top to the south normal shear was characterized by recrystallization of fine grained syn dn 1 staurolite with growth of syn dn 1 andalusite porphyroblasts during cooling and decompression to 560 c and 3 5 kbar the contractional and extensional shearing probably occurred at the permian and the late triassic to early jurassic respectively based on the previous geochronological data consequently inferred p t t d paths on the gm omb boundary probably suggest not only crustal thickening followed by thinning but also multiple amalgamations of several terranes

δ13c values of loess total carbonate a sensitive proxy for asian summer monsoon in arid northwestern margin of the chinese loess plateau abstract we measured carbon isotope ratio of loess total carbonates 13 cic from the jingyuan loess profile in the arid northwestern portion of the chinese loess plateau clp in order to examine the effectiveness and sensitivity of loess total carbonate 13 cic values as a potential paleoenvironmental indicator the 13 cic record in the loess profile shows large variations ranging from 1 to 7 during the past 70 ka the most negative 13 cic peaks coincide with paleosol formation s 0 l 1 ss 1 and l 1 ss 2 which occurred under relatively wetter climatic conditions during middle holocene and marine 18 o stages 3 on the other hand the least negative 13 cic values responded to loess accumulation during relative drier phases of lgm and marine 18 o stages 4 while 13 cic variations in the loess profile in arid area demonstrate a clear correlation with changes of total organic carbon isotope 13 coc of bulk sediments which has been widely used as a proxy for the past monsoon precipitation intensity the 13 cic values in the loess profile are more sensitive to the changes of paleohabitat controlled by the asian monsoon variation in the arid area we propose that carbon isotope compositions of total carbonates in loess profile can serve as a sensitive and reliable proxy for the asian summer monsoon in the arid northwestern chinese loess plateau

muscovite recrystallization and saddle shaped 40 ar 39 ar age spectra example from the blond granite massif central france muscovites from the blond granite west french massif central were dated by the 40ar 39ar single grain method the 40ar 39ar ages obtained vary from 305 5 0 3 ma to 311 3 0 5 ma and most of the age spectra are slightly saddle shaped the analyzed muscovites show phengitic recrystallization under optical microscope observations sem images and electron microprobe chemical analyses it is proposed that the saddle shaped age spectra result from a partial recrystallization which produced three different isotopic reservoirs in the analyzed white mica single grains domains of early muscovite domains of neocrystallized muscovite formed by phengitic and al fe substitutions and low activation energy sites

neoproterozoic bimodal volcanism in the central ogcheon belt korea age and tectonic implication abstract u pb zircon ages were determined for a metavolcanic rock from the munjuri formation in the central ogcheon belt korea four zircon fractions were analyzed of which three are nearly concordant the data define a discordia with an upper intercept at 755 8 1 3 ma and a lower intercept at 160 19 ma mswd 0 72 the upper intercept age is interpreted to indicate the timing of bimodal volcanism and the lower intercept age is interpreted to reflect jurassic thermal metamorphism this is the first definite evidence supporting the existence of a formation in the ogcheon belt that is at least in part late precambrian nd isotope and trace element geochemistry of the amphibolites and felsic metavolcanics suggests that they originated from a mantle source in a continental rift environment the rift does not appear to have matured enough to develop oceanic crust since rocks indicating deep sea environments are absent the possibility that the ogcheon belt is an extension of the east chinese collision belt as suggested by several authors is not supported by the available evidence

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

winter spring precipitation as the principal control on predominance of c3 plants in central asia over the past 1 77 myr evidence from δ13c of loess organic matter in tajikistan abstract although changes in atmospheric co2 levels are thought to be the major factor driving long term c3 c4 vegetation evolution recent studies tend to emphasize the effect of regional climate conditions on c3 c4 variations the middle latitudes 30 45 in which c3 c4 plants are highly sensitive to environmental changes provide an optimal basis for the investigation of the relative impacts of climate and pco2 on shifts in c3 c4 cover in order to assess the factors controlling these shifts as well as the complex interactions between environmental factors the carbon isotopic composition of bulk organic matter from the chashmanigar loess section southern tajikistan was measured for the past 1 77 myr in general the 13c record shows mostly negative values throughout the sequence almost all 13c values falling between 23 and 26 indicating a predominance of c3 plants in central asia over this time period despite the presence of numerous glacial interglacial cycles from 0 85 myr to the present the 13c values become increasingly positive reflecting a growing c4 signal however this c4 component is not detectable prior to 0 25 myr after which minor peaks are evident at 228 171 and 18 kyr the 13c record from chashmanigar indicates that winter spring precipitation i e mediterranean climatic conditions have characterized central asia throughout the past 1 77 myr leading to the predominance of c3 vegetation in the context of glacial interglacial scale changes in atmospheric co2 therefore it is climate rather than pco2 that controls c3 c4 variations in asia s middle latitudes the gradual increase in the c4 component since 0 85 myr especially the notable peaks after 0 25 myr may have been caused by an increase in summer precipitation due to an enhanced southward shift of the climate zones

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

multiple expansions of c4 plant biomass in east asia since 7 ma coupled with strengthened monsoon circulation the expansion of plants using the c4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the cenozoic era a significant late miocene expansion is well documented however the existence and cause of subsequent expansions are still not clear owing in part to the lack of long continuous climate vegetation records here we present two high resolution carbon isotope time series covering the past 7 m y derived from eolian deposits on the chinese loess plateau the data indicate three intervals of significant c4 plant expansions within the semiarid monsoonal region of east asia ca 2 9 2 7 ma 1 3 0 9 ma and 0 6 ma present these expansions covary with strengthened east asian summer monsoon circulation we conclude that in east asia large scale late miocene c4 c3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation

tectonometamorphic evolution of the southern quebec appalachians 40ar 39ar evidence for middle ordovician crustal thickening and silurian early devonian exhumation of the internal humber zone published age data from the humber zone of the southern quebec appalachians indicate that deformation and metamorphism occurred during middle to late ordovician time this paper presents the results of a regional scale geochronological study of 70 amphibole and mica single grains by the 40 ar 39 ar laser step heating method which brings forward a more complete middle ordovician early devonian tectonometamorphic evolution of the quebec appalachian humber zone biotites have plateau ages varying between 420 and 411 ma amphiboles are often compositionally rimmed and or contain biotite inclusions their age spectra are generally disturbed but may suggest the occurrence of two distinct events silurian 429 424 ma and middle ordovician 462 460 ma two main groups of muscovite ages have been obtained middle ordovician 469 461 ma and silurian early devonian 431 411 ma but middle to late devonian 387 376 ma ages are also locally present middle ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional taconian metamorphism silurian early devonian ages are attributed to consequences of backthrusting and normal faulting a statistical analysis of silurian early devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal humber zone this age decreasing trend is interpreted to result from the temporal ca 431 411 ma and possibly spatial northwest to southeast progression of deformation recrystallization and fluid circulation during tectonic exhumation of the internal humber zone in southern quebec tectonic models for the northern appalachian internal humber zone should take into account the strong body of evidence for silurian early devonian hinterland directed and extensional deformation as documented in southern quebec

termination of the permo triassic songrim indosinian orogeny in the ogcheon belt south korea occurrence of ca 220 ma post orogenic alkali granites and their tectonic implications abstract the indosinian orogeny is defined in a broad sense as encompassing all the late paleozoic to early mesozoic tectonic events associated with the closure of the eastern parts of the paleo tethys ocean it includes the permo triassic continent continent collision between the north china and south china blocks that generated the qinling dabie sulu orogen containing the largest ultrahigh pressure uhp terrane in the world the korean peninsula situated at the easternmost end of the tethyan tectonic regime was also tectonically influenced by the indosinian orogeny often known as the songrim orogeny in korea the songrim orogeny characterized by intense regional medium to high pressure metamorphism extends from the imjingang belt through the gyeonggi massif to the ogcheon belt it was initiated as early as the early permian period ca 290 ma and peaked in the triassic period ca 255 to 225 ma this paper presents new geochemical and shrimp u pb zircon ages for two alkali granites that postdated the songrim orogeny in the ogcheon belt both granites are contemporaneous 219 3 3 3 ma and 219 6 1 9 ma and are geochemically post orogenic a type granites intruded in an extensional setting subsequent to a major collision the data provide a critical time constraint on the termination of the indosinian orogeny in the korean peninsula despite the absence of an uhp terrane in the korean peninsula similarities in the duration metamorphic characteristics and post orogenic magmatism between the qinling dabie sulu and songrim orogens as parts of the broad indosinian orogenic regime are clearly alike suggesting a genetic linkage in their time space characteristics

metamorphic evolution of the northwestern ogcheon metamorphic belt south korea the ogcheon metamorphic belt omb comprises late proterozoic to paleozoic metasedimentary and metavolcanic sequences which are intruded by mesozoic granitoid plutons to delineate the metamorphic evolution of the omb we have investigated mineral parageneses metamorphic reactions and p t conditions of pelitic and mafic schists in the chungju area northwestern part of the omb the regional metamorphic grade increases north westward from biotite to garnet zones although low pressure assemblages have developed in contact aureoles around jurassic granites garnet crystals show chemical zoning typical for prograde metamorphism with decreasing mn and increasing fe and mg from core to rim compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite and garnet zones documented in the medium pressure metabasites p t conditions of the garnet zone estimated from garnet biotite garnet chlorite and amphibole plagioclase geothermometers together with garnet plagioclase biotite muscoviterquartz geobarometers are in the range of 5 8 kbar and 520 5908c retrograde p t path based on fluid inclusion studies suggests that the exhumation of the omb has passed through the p trange of 1 3 kbar and 350 5008c following the isochore curves of the co inclusions in conjunction with structural and geochronologic data we conclude 2 that the omb has experienced a polycyclic p t evolution characterized by 1 crustal thickening during the middle paleozoic time and 2 regional retrograde metamorphism in the triassic our result further suggests that the triassic collision belt in east central china does not pass through the omb q 1998 elsevier science b v

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

dating mylonitic deformation by the 40ar 39ar method an example from the norumbega fault zone maine strongly mylonitic rocks associated with the regionally extensive norumbega fault zone in south central maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite 40ar 39ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined early carboniferous plateau ages in contrast to these nonmylonitized samples all age spectra for muscovite from the mylonites are highly discordant they are characterized by young ages at low extraction temperatures which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older relict muscovite porphyroclasts and fine grained white mica aggregates that recrystallized during mylonitic deformation total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size indicating a larger component of the recrystallized grains in the finer grain size fractions although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra their initial increments do coincide at approximately 290 ma this indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation these data combined with kinematic analysis reveal that the segment of the norumbega fault zone studied the sandhill corner fault is a late carboniferous early permian dextral strike slip fault a lack of significant offset in regional early carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km this study demonstrates that 40ar 39ar dating methods can be used to date deformational events effectively as long as several important criteria are met first and foremost samples must be well characterized prior to analysis dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral finally the effects of excess argon must be negligible

the geounri shear zone in the paleozoic taebaeksan basin of korea tectonic implications abstract the songrim and daebo orogenies represent two major phanerozoic tectonic events that are well preserved across much of the present day korean peninsula the songrim orogeny corresponds to the late permian triassic collision of the north and south china cratons whereas the jurassic daebo tectonic event represents a thin skinned contractional deformation in a continental arc setting it is well established that the songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the korean peninsula e g the pyeongnam basin and the imjingang belt with only a minor impact on the geology of southern korea e g the okcheon and taebaeksan basins it was the daebo tectonic event however which generated most of the deformational structures observed within the okcheon and taebaeksan basins the deokpori thrust in the taebaeksan basin is a significant fault structure that formed during the daebo tectonic event no other regional structures related to the songrim orogeny have been found in the taebaeksan basin in the vicinity of the deokpori thrust we have identified in this study a previously undocumented reverse slip shear zone which is named the geounri shear zone microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at 400 c in contrast with the nearby deokpori thrust which formed in a brittle deformation regime a geochronological analysis of muscovite isolated from phyllonite samples of the geounri shear zone yielded two 40 ar 39 ar age spectra with a combined weighted mean age of 209 5 2 ma thermal modeling of the ar data combined with structural interpretations suggests that this muscovite 40 ar 39 ar age represents a robust lower age limit for the timing of the geounri shearing event therefore linking formation of the shear zone with deformational events caused by the songrim orogeny these results imply that structural features caused by songrim deformation were developed at higher temperatures than structures caused by daebo deformation as found within the taebaeksan basin

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

pretectonic and posttectonic emplacements of the granitoids in the south central okchon belt south korea implications for the timing of strike slip shearing and thrusting structural analyses of three granitic plutons in the south central okchon belt of the korean peninsula reveal that the baegnok and cheongsan plutons are pretectonic with respect to right lateral strike slip ductile shearing along the cheongsan shear zone and later top to the ese thrusting whereas the boeun pluton is posttectonic with respect to these two deformation events u pb sphene age data from the three plutons indicate that the intrusion ages of the baegnok cheongsan and boeun plutons are 222 7 2 1 ma 216 9 2 2 ma and 171 7 1 4 ma respectively microstructural evidence preserved in deformed rocks from the ne striking cheongsan shear zone and adjacent nne striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ese directed thrusting since these thrusts crosscut the late triassic early jurassic taedong group we estimate that the thrusts developed at 180 ma during the daebo tectonic event just prior to the intrusion of the boeun pluton we also constrain the age of the cheongsan shear zone at middle late triassic time corresponding to the songrim orogeny so that the shear zone is not related to deformation along the middle jurassic honam shear zone 176 ma the honam shear zone is not a simple tectonic boundary between the okchon belt and the yongnam massif but an anastomosing ductile shear zone not only affecting the okchon belt and the yongnam massif but also crosscutting the boundary between them at a low angle we suggest that the right lateral cheongsan shear zone is a major boundary between the okchon zone and the yongnam massif with paleozoic cover rocks the kyonggi massif and the okchon zone south china block were juxtaposed against the yongnam massif and taebaeksan basin north china block along the cheongsan shear zone during a late stage of the middle triassic songrim orogeny probably related to the late paleozoic early mesozoic collision between the north and south china blocks

saddle shaped 40ar39ar age spectra from young microstructurally complex potassium feldspars abstract a suite of young potassium feldspars show markedly saddle shaped 40 ar 39 ar age spectra as a result of incorporating 10 10 to 10 9 mol g of excess 40ar the minima of these age spectra record reasonable cooling ages based on the known thermal history and geology of the samples acid etching of one sample indicates that excess 40ar is concentrated near grain margins the release of a substantial portion of this excess ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40ar anion vacancies have been proposed to act in this role in plagioclase and we speculate that this is so in k feldspar as well such a mechanism would explain the observation that relative to radiogenic 40ar excess 40ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory oxygen diffusion provides an appropriate analogy for this phenomenon being relatively fast under natural hydrothermal conditions but extremely slow in anhydrous environments such as an ar extraction system tem observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns due to the presence of such microstructures as incoherent exsolution lamellae dislocations and stepped twins tem observations also reveal the presence in one sample of orthoclase enclaves in a microcline host

extensional deformation along the southern boundary of the gyeonggi massif south korea structural characteristics age constraints and tectonic implications the permo triassic collision of the north and south china blocks caused the development of the dabie sulu orogen in china and songrim orogen in the korean peninsula extension after this collision is known from the dabie sulu orogen but post orogenic extension is not well defined in the korean peninsula extensional deformation along the southern boundary of the gyeonggi massif in korea is characterized by top down to the south ductile shearing and subsequent brittle normal faulting and was predated by regional metamorphism and north vergent contractional deformation extension occurred between 220 and 185 ma based on the ages of pre extensional regional metamorphism and post extensional pluton emplacement 40ar 39ar dating of syn extensional muscovite in quartz mica mylonite yields an age of 187 8 5 6 2 ma in agreement with constraints from structural relationships together with the extensional deformation identified along the northern boundary of the gyeonggi massif 226 ma the extension along the southern boundary is probably related to the exhumation of the massif during late orogenic or post orogenic extension associated with the songrim orogeny of the korean peninsula and forms an important event in the phanerozoic crustal evolution of east asia

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

shrimp u pb ages of detrital zircons in metasedimentary rocks of the central ogcheon fold thrust belt korea evidence for tectonic assembly of paleozoic sedimentary protoliths abstract we analyzed detrital zircons in four metasandstones and two metadiamictites of the central ogcheon metamorphic belt omb in order to provide geochronologic constraints on the time of deposition and protolith age distributions shrimp u pb ages of the newly analyzed zircons range from archean to early paleozoic and their age patterns permit the definition of four lithotectonic slices upper and lower pibanryeong units consisting primarily of quartzite and metasandstone an upper poeun unit of black slate containing early permian plant fossils and a lower poeun unit made up of metasandstone and metadiamictite a quartzite of the upper pibanryeong unit yielded two major zircon age populations at 965 11 ma and 447 5 ma whereas two lower pibanryeong metasandstones show major age concentrations at 2 5 ga 1 0 0 9 ga and 545 535 ma the youngest components of these units suggest maximum ages of sedimentation as late ordovician and early cambrian respectively these upper and lower pibanryeong zircon age distribution patterns are consistent with those of ordovician and devonian sedimentary strata respectively from the south china craton scc on the other hand three samples of the lower poeun unit yielded two distinct age populations at 1 87 ga and 755 7 ma in addition a metadiamictite contains abundant mesoproterozoic zircons with two broad peaks centered at 1 3 and 1 6 ga protolith ages and distribution patterns are profoundly different between the pibanryeong and poeun lithotectonic packages suggesting the presence of a major thrust fault juxtaposing allochthons that are also characterized by large p t contrasts our detrital zircon results for the pibanryeong unit are compatible with derivation from the scc in apparent contrast with the poeun occurrence of early permian plant fossils most likely correlative with the north china craton ncc taken together the suturing of paleozoic metasedimentary terranes in the omb is a product of permo triassic orogeny reflecting collision of the north and south china cratons

ages of deformation from k ar and 40ar 39ar dating of white micas a structural and isotopic dating study of white micas from mylonites in the ruby gap duplex off central australia shows that mylonitic deformation occurred during the paleozoic alice springs orogeny deformation of the white micas took place under greenschist facies conditions in the approximate temperature range 250 350 c two distinct populations of white mica have been found 1 porphyroclasts of original muscovite and 2 neocrystallized phengites that define the fabric associated with deformation 40ar 39ar age spectra of neocrystallized phengites yield plateau like spectra of paleozoic age in contrast 40ar 39ar age spectra of muscovite porphyroclasts exhibit proterozoic apparent ages indicating that white micas were not open to significant argon diffusion during formation of the ruby gap duplex we conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system

post orogenic evolution of the dabie shan eastern china from u th he and fission track thermochronology the dabie shan of eastern china is a 200 kilometers wide mountain range with nearly 2 kilometers of relief and is an archetype of deep ultrahigh pressure metamorphic rock exhumation despite its regional and petrologic importance little is known about the low temperature and post orogenic evolution of the dabie shan here we present apatite and zircon u th he ahe and zhe respectively and apatite fission track aft cooling ages from the dabie shan that constrain the patterns and history of exhumation over the last 115 myr on the scale of the whole orogen zhe and ahe ages are inversely correlated with mean elevation and are systematically younger in the core of the range these cooling ages were converted to exhumation rates assuming steady state erosion and accounting for topographic effects these results indicate that since the eocene flanks of the range have eroded at rates as low as 0 02 km myr while the range core has eroded at about 0 06 km myr even in the core of the range these recent exhumation rates are at least 10 to 20 times slower than those estimated for the initial stages of exhumation in the triassic jurassic in a 1 4 kilometer vertical transect in the core of the range all ages are positively correlated with elevation with zhe ages increasing from 76 to 112 ma aft from 44 to 70 ma and ahe from 24 to 43 ma we present a simple model for topographic correction of thermochronometric ages in vertical transects using the admittance ratio ratio of isotherm relief to topographic relief applied to the ahe age elevation relationship this yields tertiary exhumation rates of 0 05 to 0 07 km myr in the core of the dabie shan in good agreement with regional exhumation rate patterns finally age elevation relationships for all three chronometers in the vertical transect are consistent with a constant exhumation rate of 0 06 0 01 km myr since the creta ceous with a possible modest increase in exhumation rates as high as 0 2 km myr between 80 to 40 ma these data show no evidence for significant variations in exhumation rates over the last 115 myr as might be expected for decay of old topography or tectonic reactivation of old structures

strontium isotopic equilibration during metamorphism of tillites from the ogcheon belt south korea the ne sw trending ogcheon belt formed during the mesozoic by deformation and metamorphism of a sedimentary sequence which includes tillite formations of late precambrian or lower palaeozoic age in this study rb sr isotopic data are reported from cm scale sampling of clasts and matrix in these tillites which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone sr isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons mswd s 12 the mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix which were probably accompanied by a substantial fluid flow the matrix isochrons for three of the rocks together with separated white micas from associated schists suggest that metamorphism took place in the trias approximately 200 myr ago metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190 150 myr these results suggest that where high fluid rock ratios can be demonstrated on independent petrological grounds the analysis of small scale whole rock samples may allow effective rb sr dating of metamorphism

the kyonggi shear zone of the central korean peninsula late orogenic imprint of the north and south china collision abstract the crustal scale kyonggi shear zone of central korea has been identified as a major boundary between the precambrian kyonggi massif in the south and the imjingang belt in the north the latter is an eastward extension of the qinling dabie sulu collisional belt of china field observations and microstructural analysis indicate that the extensional shear zone evolved from a deep crustal ductile regime to a shallow crustal brittle regime associated with a rapid uplift of the kyonggi massif following the late permian early triassic collision between the sino korean and yangtze cratons a rb sr muscovite age documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 norm

on the loss of 40ar from muscovite during polymetamorphism abstract recent laser studies have documented 40ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years a fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or alternatively loss during chemical reactions and deformation new data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40ar via lattice diffusion during overprinting events than has been inferred previously kyanite muscovite assemblages in proterozoic schist along the eastern flank of the green mountain massif central vermont appalachians formed during proterozoic metamorphism at ca 1 1 ga and were remetamorphosed in the greenschist facies during both taconian ca 465 ma and acadian ca 390 ma events the overprinting events resulted in deformation of earlier kyanite and muscovite growth of chemically and isotopically distinct rims on earlier muscovite and growth of neoblastic muscovite and chloritoid with each event at inferred minimum temperatures of ca 425 c laser spot analyses of the grenville muscovite range from 988 11 ma in the centers of porphyroclasts to 397 7 ma along rims ages younger than 750 ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals parts of early formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40ar laser spot analyses along the outer 100 m of an undeformed growth face yield ages close to the timing of proterozoic metamorphism and cooling the present study suggests episodic heating events are ineffective for producing measurable diffusive argon loss along the edges of large muscovite crystals ca 1 mm in diameter particularly in comparison with deformation and reaction mechanisms and are unlikely to result in complete 40 ar 39 ar age resetting of such crystals the results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work

shrimp u pb zircon ages of pyroclastic rocks in the bansong group taebaeksan basin south korea and their implication for the mesozoic tectonics abstract the bansong group daedong supergroup in the korean peninsula has long been considered to be an important time marker for two well known orogenies in that it was deposited after the songnim orogeny permian triassic collision of the north and south china blocks but was deformed during the early to middle jurassic daebo tectonic event here we present a new interpretation on the origin of the bansong group and associated faults on the basis of structural and geochronological data shrimp sensitive high resolution ion microprobe u pb zircon age determination of two felsic pyroclastic rocks from the bansong group formed in the foreland basin of the gongsuweon thrust in the taebaeksan basin yielded ages of 186 3 1 5 and 187 2 1 5 ma respectively indicating the deposition of the bansong group during the late early jurassic inherited zircon component indicates ca 1 9 ga source material for the volcanic rocks agreeing with known basement ages the bansong group represents syntectonic sedimentation during the late early jurassic in a compressional regime during the daebo tectonic event the northeast trending regional folds and thrusts including the deokpori gakdong and gongsuweon thrusts with a southeast vergence developed in the taebaeksan basin this is ascribed to deformation in a continental arc setting due to the northwesterly orthogonal convergence of the izanagi plate on the asiatic margin which occurred immediately after the juxtaposition of the taebaeksan basin against the okcheon basin in the late stage of the songnim orogeny thus the deokpori thrust is not a continental transform fault between the north and south china blocks but an intracontinental thrust that developed after their juxtaposition

inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations can it be reliably done abstract the timing of porphyroblast inclusion trails can be confidently interpreted relative to surrounding external foliations only where there is continuity between the two where this continuity is broken timing is ambiguous where single or multiple growths of two or more different porphyroblastic minerals have occurred during a relatively complex deformation history the risk of misinterpreting the relative timing of porphyroblast growth is high and can lead to wrong inferences about pressure temperature time deformation p t t d paths misinterpreting porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions the effects of porphyroblast rotation vs non rotation relative to an externally fixed reference frame on inferred p t t d paths are poorly understood however the d part of the path can differ considerably depending on whether or not porphyroblasts are inferred to have rotated the effect on the p t t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred sequence of porphyroblast growth

the effect of allanite inclusions on u pb step leaching ages and sm nd isotope systematics of garnet from the ogcheon metamorphic belt south korea abstract the ogcheon metamorphic belt consists primarily of metasedimentary and metavolcanic rocks that have experienced polyphase tectonometamorphism since the neoproterozoic peak metamorphism reaching up to lower amphibolite facies produced ubiquitous garnet porphyroblasts in pelitic and mafic schists to determine the timing of their formation step leaching experiments were undertaken for five garnet fractions separated from pelitic and quartz hornblende garnet schists the u pb ages from three samples are identical within 2 errors ranging from 291 41 ma to 276 29 ma the quasi linearity of leachates in 238u 206pb and 208pb 206pb diagrams suggests that u and pb are released from a single mineral phase and that minor chemical fractionation between u and pb may have occurred during the leaching experiment deviations of residues and bulk garnet fractions from the linear trend are attributed to partial dissolution of refractory inclusions of detrital zircon th u ratios of leachates are in the range of 3 4 12 much higher than those of pure garnet and suggest the contribution of allanite negative relationships in the sm nd isochron diagram and similar 147sm 144nd ratios between whole rock and garnet corroborate the influence of light rare earth element lree rich allanite on the sm nd isotopic system simple mass balance calculations indicate that only a trace amount 0 35 modal of allanite inclusions should govern the u th pb systematics of garnet petrographic evidence together with the consistency in u pb ages suggests that allanite is a product of prograde metamorphism thus peak metamorphism responsible for the growth of allanite bearing garnet porphyroblasts in the ogcheon metamorphic belt is best estimated to be early permian

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

postcollisional extension of the caledonide orogen in scandinavia structural expressions and tectonic significance the scandinavian caledonides are much more affected by postcontractional devonian extension than previously thought the extensional deformation is not restricted to the internal parts of the orogen but can be traced for more than 200 km from the west coast of norway toward the foreland it involved significant reactivation of the basal thrust zone as a low angle extensional detachment whic consequently contains abundant top to the north west sense of shear indicators the extensional deformation involved substantial backsliding of the orogenic wedge eduction of the baltic shield together with stretching of the basement by west dipping extensional shear zones after 30 km of backsliding the extensional detachment splayed to penetrate the overlying orogenic edifice in the coastal region in southern norway which led to inactivation of the main portion of the basal thrust zone as an extensional detachment

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

the timing of sediment transport down monterey submarine canyon offshore california while submarine canyons are the major conduits through which sediments are transported from the continents out into the deep sea the time it takes for sediment to pass down through a submarine canyon system is poorly constrained here we report on the first study to couple optically stimulated luminescence osl ages of quartz sand deposits and accelerator mass spectrometry 14 c ages measured on benthic foraminifera to examine the timing of sediment transport through the axial channel of monterey submarine canyon and fan offshore california the osl ages date the timing of sediment entry into the canyon head while the 14 c ages of benthic foraminifera record the deposition of hemipelagic sediments that bound the sand horizons we use both single grain and small 2 mm area single aliquot regeneration approaches on vibracore samples from fining upward sequences at various water depths to demonstrate relatively rapid decadal scale sand transport to at least 1 1 km depth and more variable decadal to millennial scale transport to a least 3 5 km depth on the fan significant differences between the time sand was last exposed at the canyon head osl age and the timing of deposition of the sand from 14 c ages of benthic foraminifera in bracketing hemipelagic sediments are interpreted as indicating that the sand does not pass through the entire canyon instantly in large individual events but rather moves multiple times before emerging onto the fan the increased spread in single grain osl dates with water depth provides evidence of mixing and temporary storage of sediment as it moves through the canyon system the ages also indicate that the frequency of sediment transport events decreases with distance down the canyon channel system the amalgamated sands near the canyon head yield osl ages that are consistent with a sub decadal recurrence frequency while the fining upward sand sequences on the fan indicate that the channel is still experiencing events with a 150 250 year recurrence frequency out to 3 5 km water depths

possible eastward extension of chinese collision belt in south korea the imjingang belt structural petrological and geochronological data from the middle korean peninsula indicate that the qinling dabie sulu collisional belt of east central china crosses the yellow sea and extends into the imjingang belt the yeoncheon complex first identified as the western imjingang belt comprises primarily north dipping metamorphic sequences 1 the northern jingok unit consisting of barrovian type metapelites and 2 the southern samgot unit consisting of calc silicate and amphibolitic rocks south vergent structures with reverse sense shearing are dominant in the jingok unit whereas late normal sense shearing is pervasive in the samgot unit and the deformed granitoid to the south these structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt pressure temperature p t estimates from the amphibolites suggest a high p amphibolite facies metamorphism 8 13 kbar and 630 790 c possibly evolving from eclogite facies conditions along a clockwise p t path sm nd and rb sr geochronological data suggest that the amphibolites emplaced in late proterozoic time were metamorphosed during permian triassic time

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

geochronology and geochemical and nd sr isotopic characteristics of triassic plutonic rocks in the gyeonggi massif south korea constraints on triassic post collisional magmatism abstract sensitive high resolution ion microprobe shrimp zircon u pb ages and whole rock chemical and isotopic sr nd compositions have been measured on some middle triassic plutonic rocks associated with post collisional magmatism in the gyeonggi massif hongseong namyang and yangpyeong areas south korea plutonic rocks from the southwest hongseong and central yangpyeong gyeonggi massif and central okcheon belt cheongsan have ages of ca 227 226 231 and 225 ma respectively the plutonic rocks from hongseong are dominated by granite and quartz syenite from namyang by granite and from yangpyeong by quartz monzonite to quartz syenite and gabbro three rocks from the gyeonggi massif have the depletions in ta nb p and ti and enrichment in large ion lithophile elements lile characteristic of plutonic rocks that are subduction related or derived from an arc related crustal source the hongseong quartz syenite and yangpyeong quartz monzonite to quartz syenite have features of shoshonitic and high ba sr granitoids with lree enrichment and no significant eu anomaly the plutonic rocks from the gyeonggi massif have high initial 87 sr 86 sr ratios 0 7083 0 7144 and strongly negative e nd t 20 3 to 10 4 indicating derivation of the magmas from sources in the middle to upper crust these high k calc alkaline and shoshonitic plutonic rocks formed during periods of relaxation that separated the culminations of post orogenic uplift collapse in the transitions from orogenic to anorogenic events after the early middle triassic continent continent collision 249 229 ma between the north and south china blocks in south korea the plutonic rocks of the gyeonggi massif support therefore a possible eastern extension of the early middle triassic dabie sulu collision belt in china

kinematics of exhumation of high and ultrahigh pressure rocks in the hong an and tongbai shan of the qinling dabie collisional orogen eastern china

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

paleoceanographic history of the northwest pacific ocean over the past 740kyr discerned from radiolarian fauna abstract the northwest pacific ocean is characterized by a strong mixing of water masses due to the interplay of three distinct currents i e the kuroshio the tsugaru and the oyashio the c9001c drill core site located east of the shimokita peninsula and directly influenced by the tsugaru warm current and the oyashio subarctic current was used here to reconstruct the paleoceanographical history of this region especially focusing on the mid brunhes event mbe and its consequences this core provides a continuous record from marine isotope stage mis 18 740 ka to the present polycystine radiolarian assemblages were analyzed to highlight paleoceanographic and sea surface temperature changes at this site based on the radiolarian fauna seven time periods are defined which coincide with changes in the dynamics of the tsugaru and oyashio currents respectively the oldest time interval covered by our analysis i e vii 740 621 ka was marked by generally sluggish ocean circulation the tsugaru current influence increased during the following interval vi 621 478 ka which encompasses interglacials mis 15 and 13 while the oyashio current strengthened during the interval v 478 337 i e from glacials mis 12 to 10 these latter intervals vi to v constitute a long climatic transitional period where tsugaru current and oyashio current influences are strengthened in the time period from interval iv to interval i 0 337 ka the warming intensity of interglacials mis 9 mis 5 appears to be close to the modern one however several unusually warm glacials associated with a relatively strong tsugaru current flow were identified during this interval e g mis 8 and mis 6 radiolarian productivity data suggest that during intervals vii to v deep water masses are rich in nutrient content however a high vertical mixing event is recorded after interval iv when high nutrient concentrations appear to shift to the surface layer

vegetation variation of loess deposits in the southeastern inner mongolia ne china over the past 1 08 million years abstract the stable carbon isotopic composition of organic matter of aeolian silt deposits is regarded as an appropriate proxy index of paleovegetation especially in the chinese loess plateau in central china in this study a loess paleosol sequence in the southeastern inner mongolia autonomous region in northeastern ne china which is located outside the chinese loess plateau is chosen to reconstruct the vegetation history since 1 08 ma temperature exhibits a threshold value which determines the growth of c4 plants in this study area the organic matter of the samples is derived from two different vegetation types namely the mixed c3 and c4 plants and the pure c3 plants the 13c of the organic matter shows negative values in loess units and higher values in paleosol units this finding reflects the influence of temperature and summer monsoon intensity on the vegetation dynamics over glacial interglacial cycles on a longer time scale the 13c values are higher between 1 1 and 0 9 ma and after 0 35 ma and lower between 0 9 and 0 35 ma which may be attributed to a long term temperature variation our analysis shows that regional temperature is the most important limiting factor that forces vegetation changes at the glacial interglacial time scale in ne china

organic stable carbon isotopic composition reveals late quaternary vegetation changes in the dune fields of northern china abstract vegetation changes during the late quaternary in the dune fields of northern china are not well understood we investigated organic carbon stable isotopic composition of surface soils related mainly to the ratio of c3 and c4 plants across a range of arid to subhumid climates in this region isotopic composition is weakly related to both temperature and moisture multiple r2 0 53 with the highest 13c greatest c4 abundance in the warm subhumid horqin dune field in late quaternary eolian stratigraphic sections of the mu us and horqin dune fields but not in the much colder otindag dune field 13c is higher in organic carbon from paleosols than in eolian sands this contrast most evident for paleosols recording a major early to middle holocene phase of dune stabilization is interpreted as evidence for expansion of c4 plants due to increased effective moisture high temperature because of high insolation and decreased disturbance related to eolian erosion and deposition

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

polyorogenic 40ar 39ar mineral age record within the kalak nappe complex northern scandinavian caledonides the kalak nappe complex is composed of variably retrogressed middle proterozoic basement and imbricated polydeformed metasedimentary cover successions which at least in part appear to have originated within the late proterozoic early palaeozoic baltoscandian miogeocline mafic alkaline plutons seiland igneous suite occur within uppermost structural units of the nappe complex 40 ar 39 ar incremental release mineral ages suggest local differences in tectonothermal evolution within the kalak nappe complex post metamorphic cooling dates of c 490 ma are locally recorded by hornblende within the structurally highest tectonic unit suggesting significant orogenic activity occurred during and or immediately prior to the arenig a distinctly later metamorphic overprint occurred in the silurian and was of sufficient magnitude to everywhere rejuvenate ar systems in muscovite and nepheline and locally in hornblende this was followed by relatively rapid post metamorphic cooling through hornblende and muscovite ar closure temperatures between c 425 and 415 ma the complex tectonic history suggested for the kalak nappe complex is generally similar to that previously outlined for the central swedish caledonides together 40 ar 39 ar mineral ages from these widely separated areas indicate that significant tectonothermal activity occurred along the baltoscandian margin prior to the silurian this activity appears at least in part to have been associated with formation of a late cambrian early ordovician accretionary wedge over a westward dipping subduction zone during initial closure of the iapetus ocean

alpine deformation on naxos greece a detailed structural study of naxos attic cycladic massif greece reveals two major deformation events the first one is associated with large scale thrusting and high pressure low temperature metamorphism during an early alpine subduction episode the second event occurred during continental extension and the associated development of localized thermal domes where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone we agree with a model of naxos as a cordilleran type metamorphic core complex however our observations show that the sense of shear was upper plate moving north during the second event calling for a reinterpretation of existing tectonic models of the cyclades

phanerozoic high pressure eclogite and intermediate pressure granulite facies metamorphism in the gyeonggi massif south korea implications for the eastward extension of the dabie sulu continental collision zone abstract petrological analysis zircon trace element analysis and shrimp zircon u pb dating of retrogressed eclogite and garnet granulite from bibong hongseong area sw gyeonggi massif south korea provide compelling evidence for triassic 231 4 3 3 ma high pressure hp eclogite facies m1 metamorphisms at a peak pressure temperature p t of ca 16 5 20 0 kb and 775 850 c this was followed by isothermal decompression itd with a sharp decrease in pressure from 20 to 10 kb and a slight temperature rise from eclogite facies m1 to granulite facies m2 followed by uplift and cooling granitic orthogneiss surrounding the baekdong garnet granulite and the ophiolite related ultramafic lenticular body near bibong records evidence for a later silurian 418 8 ma intermediate high pressure ihp granulite facies metamorphism and a prograde p t path with peak p t conditions of ca 13 5 kb and 800 c k ar ages of biotite from garnet granulites amphibolites and granitic orthogneisses in and around the bibong metabasite lenticular body are 208 219 ma recording cooling to about 310 c after the early triassic metamorphic peak neoproterozoic zircon cores in the retrogressed eclogite and granitic orthogneiss provide evidence that the protoliths of these rocks were 800 and 900 ma old respectively similar to the ages of tectonic episodes in the central orogenic belt of china this and the evidence for triassic hp uhp metamorphism in both china and korea is consistent with a regional tectonic link within northeast asia from the time of rodinia amalgamation to triassic continent continent collision between the north and south china blocks and with an eastward extension of the dabie sulu suture zone into the hongseong area of south korea

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

fault zone weakening processes along the reactivated outer hebrides fault zone scotland the outer hebrides fault zone is a major reactivated structure cutting amphibolite grade lewisian basement gneisses in nw scotland during a regionally important phase of sinistral strike slip movements the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist facies phyllonitic shear zones later ese directed extensional strain was preferentially focused into these pre existing zones of weakness the syn tectonic alteration of a relatively strong feldspar hornblende dominated load bearing framework microstructure to an interconnected weak layer microstructure of fine grained strongly aligned phyllosilicate aggregates leads to the long term weakening in the fault zone comparison with experimental data suggests that this produces a shallowing of the frictional viscous creep 9brittle ductile9 transition and a substantial reduction in total crustal strength similar processes may account for the apparent weakness of many long lived fault zones

folds with axes parallel to the extension direction an experimental study abstract experimental studies of single and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction x i e layers parallel to yz or in a more limited number of cases oblique layers still containing the intermediate y axis few experimental studies have considered the case where the extension direction x lies within the layer itself although in nature folds with axes parallel to x are quite commonly seen these folds have often been ascribed to passive rotation of fold axes during continued shear but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to x single and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate y axis and with the extension direction x lying within the layer itself all experiments were conducted in plane strain either in pure shear or simple shear paraffin waxes of different melting ranges were used as analogues for rocks with a power law rheology stress exponent around 2 3 with a viscosity ratio of ca 30 1 no measurable fold amplification was discernible for shortening of 36 or shear strain of 3 6 neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified only at much higher viscosity ratios ca 600 1 did active buckle folding develop this folding at high viscosity ratio was associated with flow of the matrix in the x direction around the layer developing a strong linear fabric parallel to x in the matrix immediately adjacent to the layer the development of this flow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to xz with fold axes parallel to x

rb sr microchrons of synkinematic mica in mylonites an example from the dav fault of the eastern alps abstract a technique for texturally controlled microsampling from rock thick sections and subsequent rb sr dating of g sized samples has been developed it allows direct dating of minerals unequivocally related to deformation the technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the defereggen antholz vals dav fault south of the tauern window eastern alps control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral rb sr the field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub mm scale geochronology the ages obtained establish that sinistral strike slip shearing associated with the dav fault lasted from at least 33 ma to 30 ma followed by an abrupt change to dextral transpression related to the adjacent periadriatic fault

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

effect of mica on the grain size of dynamically recrystallized quartz in a quartz muscovite mylonite abstract quartz ribbons in a quartz muscovite mylonite from the sunchang shear zone of south korea show a variation in the average size 25 107 m of dynamically recrystallized quartz depending on the fraction and dispersion of muscovite within the ribbons micas within the quartz ribbons occur both inside quartz grains and along their boundaries having a strong preferred orientation with the basal planes parallel to the mylonitic foliation micas or mica aggregates inside the quartz show a size range of 1 25 m whereas those along the quartz grain boundaries range from 2 to 98 m the average quartz grain size d calculated from three dimensional grain size distribution decreases drastically with increasing mica volume fraction f m up to f m 3 in the ribbons and further increases in the mica fractions above 3 result in only a little decrease with some fluctuation in the quartz grain size generating the equation of d 20 11 ln f m 9 64 with almost the same fraction of mica however the quartz ribbons with dispersed mica show a smaller quartz grain size than those with clustered mica indicating the importance of dispersion as well as fraction of second phase particles to the grain size of dynamically recrystallized quartz quartz c axes tend to have a random orientation with an f m 3 presumably due to the greater contribution to the total deformation of grain boundary sliding for quartz with a smaller grain size d

high resolution δ13ctoc and magnetic susceptibility data from the late early pleistocene southern margins of the chinese loess plateau abstract on the chinese loess plateau stable carbon isotope values for total organic matter 13ctoc from more than 10 loess paleosol sequences since the last glacial interglacial show generally consistent variations with enriched values normally occurring in paleosol layers the only exceptions are records from the westernmost and northwestern chinese loess plateau however both asian summer monsoon intensity and temperature have been suggested as the principal factor affecting these consistently similar 13ctoc variations until now 13ctoc records covering the whole quaternary have been relatively rare with problems including low resolution and conflicts in both the c4 expansion history and with related climatic interpretations in this paper we report on high resolution late early pleistocene low frequency magnetic susceptibility lf and 13ctoc data from yushan section on the southern margins of the chinese loess plateau our 13ctoc data present an abstruse relation with the glacial interglacial cycles represented by loess paleosol sequences with 13c enriched excursions in some paleosol layers and 13c depleted excursions in others furthermore our 13ctoc data shows negative and positive correlations with lf data using a threshold lf value of 70 10 8 m3 kg if the lf data are taken as an indicator of pedogenetic intensity and therefore an indicator of asian summer monsoon intensity there are two possible relations between chinese loess 13ctoc and asian summer monsoon intensity i e the chinese loess 13ctoc cannot simply be adopted as the sole indicator of asian summer monsoon intensity at least during the late early pleistocene on the southern margins of the chinese loess plateau our results demonstrate that c4 plants became a significant proportion of local vegetation cover only when asian summer monsoon intensity exceeded a certain degree for example in the surface vegetation for the s15 and s16 paleosol layers of the yushan section the relation between climate and chinese loess 13ctoc requires further detailed study

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

a new concept on tectonic correlation between korea china and japan histories from the late proterozoic to cretaceous abstract the dabie sulu collision belt in china extends to the hongseong odesan belt in korea while the okcheon metamorphic belt in korea is considered as an extension of the nanhua rift within the south china block the hongseong odesan belt divides korea s gyeonggi massif into northern and southern portions the southern gyeonggi massif and the yeongnam massif are correlated with china s yangtze and cathaysia blocks respectively while the northern gyeonggi massif is part of the southern margin of the north china block the southern and northern gyeonggi massifs rifted from the rodinia supercontinent during the neoproterozoic to form the borders of the south china and north china blocks respectively subduction commenced along the southern and eastern borders of the north china block in the ordovician and continued until a triassic collision between the north china and south china blocks while subduction was occurring on the margin of the north china block high p t metamorphic belts and accretionary complexes developed along the inner zone of southwest japan from the ordovician to the permian during the subduction the hida belt in japan grew as a continental margin or continental arc collision between the north and south china blocks began in korea during the permian 290 260 ma and propagated westwards until the late triassic 230 210 ma creating the sinistral tanlu fault in china and the dextral fault in the hida and hida marginal belt in japan phanerozoic subduction and collision along the southern and western borders of the north china block led to formation of the qinling dabie sulu hongseong hida yanji belt

two phase contractional deformation of the jurassic daebo orogeny chungnam basin korea and its correlation with the early yanshanian movement of china 1 the daebo orogeny is a major orogenic event in korea that affected the early to middle jurassic daedong supergroup our investigation of the nampo group part of the supergroup in the chungnam basin reveals the occurrence of two phases of shortening the majority of the nampo group and a basement slab were folded and displaced toward the wnw along the ocheon thrust during the first phase middle jurassic early late jurassic field relationships suggest a ramp flat geometry for the ocheon thrust mechanical stratigraphy within the slice is consistent with a detachment fold model geometric modeling suggests 5 5 km and 3 3 km as the minimum amount of shortening accommodated by the ocheon thrust and its slice for the ramp slopes of 15 and 30 respectively the second phase late late jurassic was a n s shortening which produced roughly e w trending reverse faults and folds to form a dome and basin structure these shortening events occurred over the entire chungnam basin and coincide in orientation and timing with those reported from the ogcheon belt and the gyeonggi massif the first and second phases of the daebo orogeny are respectively correlated with the shortening in eastern china and the sinistral transpression along the tan lu fault both belonging to the early yanshanian movement subsequently in the early cretaceous a regional extension commenced over korea and eastern china these remarkable correlations suggest that two stages of crustal thickening occurred throughout the eastern asia during the middle to late jurassic followed by regional thinning in the early cretaceous

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

climatically driven glacial interglacial variations in c3 and c4 plant proportions on the chinese loess plateau high resolution 13c records of bulk organic matter om from three loess paleosol successions from the central chinese loess plateau document systematic fluctuations in c3 and c4 plant abundance for certain intervals during the past 620 k y measured 13c values of om in paleosols are less negative by 0 5 4 and total organic carbon toc contents are higher by up to 0 5 wt relative to corresponding values in loess deposits the 13com and toc decrease from peak values in paleosols progressively into overlying and underlying loess deposits integrated 13com toc and magnetic susceptibility rec ords indicate an increase in c4 vegetation to 50 during peak paleosol formation and a decrease in c4 vegetation to 30 during loess deposition for three of the past glacial interglacial cycles these results indicate that c4 plant abundance declined during glacials in this mid latitude region driven primarily by a shift to enhanced winter precipitation and lower temperatures and further contribute to the rapidly growing body of evidence that low p co2 during pleistocene glacials was insufficient to trigger c4 plant expansion without coincident favorable climatic conditions

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed

dating polyphase deformation across low grade metamorphic belts an example based on 40ar 39ar muscovite age constraints from the southern quebec appalachians canada geochronologic 40ar 39ar data of fabric forming metamorphic minerals in conjunction with structural and metamorphic studies are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low grade orogens careful data interpretation is needed to extract meaningful age constraints from neo and recrystallized minerals affected by isotopic disturbances in the southern quebec appalachians the sutton mountains anticlinorium exposes the metamorphic core of the early paleozoic continental margin of laurentia orogenesis in this part of the appalachians was the result of tectonic events that have been classically attributed to the combined effects of the middle to late ordovician taconian and the middle devonian acadian orogenies however evidence of separate and distinct silurian early devonian tectonism has also been recently documented laser step heating 40ar 39ar data on single grains of muscovite from polydeformed greenschist facies samples of the sutton mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde taconian event at ca 456 ma and an acadian overprint at ca 390 ma the integration of 40ar 39ar age spectra analyses with structural relationships provides precise age constraints on the duration propagation and evolution of silurian early devonian hinterland directed deformation as it migrated across the anticlinorium including back thrusting from ca 433 ma to ca 420 ma and extensional faulting from ca 417 ma to ca 405 ma along the laurentian margin of the northern appalachians such silurian early devonian tectonism is currently attributed either to the collapse of the taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri gondwanan terranes during the salinic orogeny