observations from the basin and range province western united states pertinent to the interpretation of regional detachment faults this paper summarizes the results of completed and ongoing research in three areas of the basin and range province of the western united states that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated given that these examples were influential in the development of ideas about low angle normal faults and particularly in making the case for frictional slip at dips of appreciably less than the 308 lock up angle for m 0 6 where m is the coefficient of friction we advocate a critical re examination of interpreted detachments elsewhere in the basin and range province and in other extensional and passive margin settings the sevier desert detachment of west central utah is reinterpreted as a palaeogene unconformity that has been traced to depth west of the northern sevier desert basin along an unrelated seismic reflection most probably a splay of the cretaceous age pavant thrust the absence of evidence in well cuttings and cores for either brittle deformation above or ductile deformation below is inconsistent with the existence of a fault with as much as 40 km of displacement the pavant thrust and the structurally higher canyon range thrust are erosionally truncated at the western margin of the southern sevier desert basin and are not offset by the detachment in the manner assumed by those inferring large extension across the basin the mormon peak detachment of se nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction a particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks the castle cliff detachment in the nearby beaver dam mountains of sw utah is similarly regarded as a surficial feature as originally interpreted and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin the middle miocene eagle mountain formation of eastern california interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ese from a location close to the jurassic age hunter mountain batholith of the cottonwood mountains is reinterpreted as having accumulated in a fluvial lacustrine rather than alluvial fan lacustrine setting with no bearing on either the amount or direction of tectonic transport the conglomeratic rocks upon which the provenance argument was based are pervasively channelized with erosional relief of less than 1 m to as much as 15 m fining upwards successions at the same scale and abundant trough cross stratification all characteristic features of fluvial sedimentation and not of alluvial fans the interpretation of the eagle mountain formation as having been deposited within a few kilometres of the hunter mountain batholith which depends strongly on assumptions about the dimensions of alluvial fans is therefore not required the result is important because the eagle mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of california and for the existence of detachment faults of regional

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

tracking paleozoic evolution of the south korean peninsula from detrital zircon records implications for the tectonic history of east asia abstract the southern part of the korean peninsula preserves important records of the paleozoic evolutionary history of east asia here we present shrimp u pb ages of detrital zircon grains from paleozoic metasedimentary successions okcheon and joseon supergroups yeoncheon group taean formation and pyeongan supergroup that are incorporated into the major phanerozoic mountain belts okcheon and hongseong imjingang belts in south korea providing new insights for provenances and paleotectonic evolution of the south korean peninsula during paleozoic time the zircon ages from our samples display two distinct spectra patterns in their presence absence of neoproterozoic and or paleozoic populations our results together with the available data from the korean peninsula suggest that 1 the early to middle paleozoic successions in the okcheon belt were deposited in continental margin setting s formed by neoproterozoic intracratonic rifting 2 the middle paleozoic metasedimentary rocks in the imjingang belt can be interpreted as molasse and flysch sediments along an active continental margin 3 the late paleozoic to early triassic taean formation along the western gyeonggi massif represents a syn to post collision deltaic complex of a remnant oceanic basin and 4 the late paleozoic to possibly early triassic pyeongan supergroup in the okcheon belt might represent a wedge top and or foreland basin the spatial and temporal discrepancy between the south korean peninsula and the central china orogenic belt during paleozoic might reflect lateral variations in crustal evolution history along the east asian continental margin during the paleo tethyan ocean closure

850 790 ma bimodal volcanic and intrusive rocks in northern zhejiang south china a major episode of continental rift magmatism during the breakup of rodinia abstract shrimp u pb zircon ages and geochemical and nd isotopic data are reported for mid neoproterozoic igneous rocks in northern zhejiang province south china the shenwu dolerite dykes the daolinshan granite diabase complex and the shangshu bimodal basalt rhyolite volcanic rocks were dated at 849 7 ma 794 9 ma and 792 5 ma respectively all the studied mafic rocks are tholeiitic in compositions and exhibit overall moderate enrichments in most incompatible trace elements resembling intraplate basaltic rocks in continental rifts the 850 ma shenwu dolerites are the earliest anorogenic basaltic rocks identified so far after the ca 1 1 0 9 ga sibao orogeny but seem to predate the neoproterozoic rifting our results are clearly at variance with the alternative tectonic model that an active continental margin existed along the southern margin of the yangtze block until ca 800 ma the intensive 790 ma bimodal volcanic and plutonic rocks part of a major phase of mid neoproterozoic rifting are concurrent with the widespread mid neoproterozoic intraplate magmatism throughout south china they are most likely related to the proposed mantle superplume that led to the breakup of the supercontinent rodinia

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

evolution of miocene footwall derived coarse grained deltas gulf of suez egypt implications for exploration the miocene abu alaqa group exposed on the eastern side of the gulf of suez contains a series of coarse grained delta deposits located in the hanging wall of the northwest trending east margin fault zone seven major fan sequences have been identified the first three fan sequences are aggradational and cover an area of less than 2 sq km the sequences are dominated by pebble boulder conglomerates with predominantly sigmoidal high angle clinoforms between 15 and 25 degrees as much as 150 m high these fan sequences represent talus cones and fan deltas which were probably deposited during activity on the fault zone and are situated at transfer segments the other four fan sequences are predominantly progradational consisting of mixed carbonate siliciclastic braid d ltas with local reefs they are less than 30 m high have low angle clinoforms

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

mineralogy and geochemistry of metalliferous black slates in the okcheon metamorphic belt korea a metamorphic analogue of black shales in the south china block metalliferous black slates which were locally exploited for their low quality coal are distributed in the late proterozoic to paleozoic okcheon metamorphic belt omb of the korean peninsula the mineralogy of the fine matrix is dominated by either quartz ba v mica graphite or quartz ba feldspar graphite polycrystalline submillimeter ellipsoids and elongate lenses aligned along foliations and veinlets are scattered through the fine matrix both ellipsoids and veinlets include many minor minerals containing rare elements titanite apatite allanite rutile polycrase barite uraninite xenotime armenite zircon molybdenite and sphalerite large graphite apatite ellipsoids nodules with widths of several centimeters also occur in the highly carbonaceous black slates goldmanite occurs locally as porphyroblasts the maximum rare element contents are ba 9 7 v 2 04 mo 0 13 u 0 11 cr 0 33 cu 254 ppm ni 479 ppm zn 607 ppm y 255 ppm platinum group element pge au 309 ppb and carbon 57 the occurrence of the black slates and their elemental abundances suggests that most of the rare elements were accumulated from seawater in an oxygen poor environment however the high ba content of the omb black slates indicates some hydrothermal input into an organic rich basin although metamorphism and multiple deformations prevent a direct temporal and spatial correlation metal abundances and a close association with graphite apatite nodules and low quality coal suggest that the omb black slates are metamorphosed analogues of the early cambrian ba v deposits hosted by the black shales in the south china block

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

pb and nd isotopic constraints on paleoproterozoic crustal evolution of the northeastern yeongnam massif south korea abstract we report pb isotopic ages and nd isotopic signatures of paleoproterozoic basement rocks from the pyeonghae area northeastern yeongnam massif south korea the pbsl lead step leaching garnet data of the wonnam group precambrian metasediments yield a 207pb 206pb age of 1840 26 ma which can be regarded as the timing of amphibolite to upper amphibolite facies metamorphism and associated garnet growth whole rock data for the pyeonghae gneiss intruding the wonnam group give a 207pb 206pb age of 2093 86 ma denying the possibility of a direct link between the intrusion of the pyeonghae gneiss and the regional metamorphism of the wonnam group our results confirm the significance of the 2 1 ga and 1 8 ga episodes that have been broadly constrained in the yeongnam massif the depleted mantle nd model ages of metasedimentary rocks from the wonnam group 2 63 2 47 ga are slightly younger than those of the pyeonghae gneiss samples 2 71 2 57 ga this nd isotopic signature also precludes a direct derivation of the pyeonghae gneiss from the wonnam group instead implying the presence and involvement of the older probably late archean crustal materials during the 2 1 ga magmatism in the northeastern yeongnam massif compiled pb and nd isotope data from the yeongnam and gyeonggi massifs suggest a similar geologic history for them arguing against the conventional idea that the gyeonggi and yeongnam massifs are separate continental blocks respectively correlated to the south and north china blocks the whole rock pb isotope data of basement rocks from the two massifs form a well defined 207pb 206pb linearity of around 2 0 ga suggesting their common crustal evolution process for the past two billion years a broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data the nd model ages of basement rocks from the two massifs support a probable existence of archean crusts in south korea the nd model ages both archean and proterozoic of the gyeonggi and yeongnam massifs agree with neither those of the north china block predominantly archean nor those of the south china block predominantly proterozoic our compiled isotope data together with recent estimation for the age of the honam shear zone appear to refute the presence of suture zone between the two south korean precambrian massifs which leaves the imjingang belt as the possible suture zone

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

western frontal fault of the canyon range is it the breakaway zone of the sevier desert detachment geologic evidence developed from surface exposures demonstrates that the western frontal fault of the canyon range is a major structure representing the eastern breakaway zone of the sevier desert detachment the western frontal fault of the canyon range east of delta utah is a north trending west dipping low angle 18 24 normal fault that juxtaposes syntectonic conglomerates and large rock avalanches of the miocene oak city formation against footwall rocks that have been ductilely cataclastically and brittlely deformed the oak city strata dip variably to the east dips are shallow in eastern outcrops 5 15 steeper in central outcrops 20 60 and shallow in the western outcrops 10 20 in eastern exposures the shallow dips of the oak city formation 5 15 and the acute angle of 20 35 between the western frontal fault and oak city units limit past rotation of the fault to 15 the systematic variation in the dip of oak city units from east to west suggests that the western frontal fault probably flattens beneath present oak city outcrops linking the fault to the shallow reflector seen in the cocorp consortium for continental reflection profiling utah line 1 seismic line beneath the westernmost outcrops of the oak city formation upper plate movement is constrained by oak city clast compositions and rock avalanche lithologies fault geometry and lower plate reconstruction to a maximum of 6 7 km since oak city syntectonic deposition at 12 13 ma maximum upper plate displacement for the entire sevier desert detachment cannot be determined from these breakaway zone exposures

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

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

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

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

neoproterozoic bimodal volcanism in the okcheon belt south korea and its comparison with the nanhua rift south china implications for rifting in rodinia abstract a systematic geochemical examination and zircon u pb dating of neoproterozoic bimodal volcanic rocks from the chungju area in the northeast segment of the okcheon metamorphic belt omb south korea allow a comparison with similar rocks from the nanhua rift south china the bimodal metavolcanics of the chungju area comprise the subalkaline to alkaline basalts hereafter mafic metavolcanic rocks and trachytes hereafter felsic metavolcanic rocks the mafic metavolcanic rocks are characterized by light rare earth element lree enriched and humped trace element patterns with moderate depletions in sr and ti and variable but reasonably low end t values between 0 83 and 2 99 these geochemical features are consistent with the area s origin in an ocean island basalt oib mantle source with minor crustal contamination the felsic metavolcanic rocks are characterized by lree enriched patterns with a remarkable negative eu anomaly they display an overall enriched trace element pattern with

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

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

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

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

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

three dimensional model of the late cenozoic history of the death valley region southeastern california the accumulation of a large database on the timing and kinematics of late cenozoic deformation in the death valley region of southeastern california indicates a complex three dimensional history on the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull apart between two conjugate strike slip faults the garlock and furnace creek faults and evolved into a system characterized by distributed transtension related to the eastern california shear zone our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region the model may resolve 1 a long standing problem of the eastern termination of the garlock fault which is explained here as a complex system of splays that initially terminated in the pull apart between the furnace creek and garlock systems and 2 the complex architecture of the black mountains which is explained here in terms of initial extreme attenuation between the garlock and furnace creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension this model suggests much of the displacement field is taken up in rotations and translations and the actual crustal thinning in our model is relatively small 50 66 of original thickness

uranium isotopes as a tracer of sources of dissolved solutes in the han river south korea abstract the uranium u content and 234u 238u activity ratio were determined for water samples collected from korea s han river in spring summer and winter 2006 to provide data that might constrain the origin of u isotope fractionation in river water and the link between u isotope systematics in river waters and the lithological nature of the corresponding bedrock the large difference in the major dissolved loads between the two major branches of the han river the north han river nhr and south han river shr is reflected in the contrasting u content and 234u 238u activity ratio between the tributaries low u content 0 08 0 75 nm average 0 34 nm and small 234u 238u activity ratio 1 03 1 22 average 1 09 in the nhr and high u content 0 65 1 98 nm average 1 44 nm and large 234u 238u activity ratio 1 05 1 45 average 1 24 in the shr the large spatial differences in u content and 234u 238u activity ratio are closely related to both lithological differences between the two tributaries and groundwater input the low u content and small 234u 238u activity ratio in the nhr arise mainly from a combination of surface and meteoric weathering of the dominant silicate rocks in this branch and congruent dissolution of already weathered secular equilibrium materials in contrast the high u content and large 234u 238u activity ratio in the shr are ascribed to the dissolution of carbonates and black shales along with significant inputs of deep groundwater

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

crustal development of the hida belt japan evidence from nd sr isotopic and chemical characteristics of igneous and metamorphic rocks abstract chemical and sr nd isotopic investigations were performed on the igneous and metamorphic rocks in the hida belt central japan to clarify their ages and possible sources and to provide constraints on the crustal evolution of the eastern margin of the asian continent igneous rocks of three main stages in the hida belt represented by the late silurian amphibolites first stage the middle carboniferous mafic dikes second stage and the late triassic to early jurassic granite gabbro plutons third stage show alkaline to calc alkaline chemical affinity major and trace element characteristics show that the three stages of igneous rocks were emplaced in a continental margin or continental arc environment the mafic igneous rock suites have relatively high e nd 0 5 to 5 5 and low 87 sr 86 sr 0 7049 to 0 7061 values indicating their origin from upper mantle sources though the third stage granitoids contain recycled crustal components the hida gneisses of clastic origin show two groups of model ages t dm middle proterozoic 1 4 2 2 ga and late proterozoic to early paleozoic 0 55 0 74 ga consistent with geological geochemical and radiometric age data are that the sedimentation and basaltic volcanism occurred in early to middle paleozoic and the protoliths with middle proterozoic model ages are detritals from the proterozoic continental provenances a synthesis of available geological and geochemical data for the basement metamorphic and igneous rocks in eastern asia suggests two possibilities for the paleozoic to early mesozoic history of the hida belt 1 the hida belt was formed at the eastern margin of the north china block with later evolution as a suture zone between the north and south china blocks or 2 it evolved as a part of the east central asian orogenic belt ecaob to the north

tectonometamorphic evolution of the chuncheon amphibolite central gyeonggi massif south korea the chuncheon amphibolite part of the gubongsan group which overlies the yongduri gneiss complex is interlayered with calc silicate rock marble quartzite biotite schist and quartzofeldspathic gneiss in the central gyeonggi massif south korea metamorphic pressures and temperatures estimated from the amphibolite are 5 5 10 6 kbar and 615 714 c these p t conditions are close to those defined by the reaction curve between kyanite and sillimanite and suggest medium pressure type metamorphism of the chuncheon amphibolite for two metapelites intercalated with the amphibolite temperatures are estimated to be 607 699 c consistent with those obtained from the amphibolite on the other hand pressures estimated from these metapelites are significantly different 4 6 kbar and 9 13 kbar when rim and core compositions of garnet are respectively used these p t estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3 7 kbar during denudation rapid decompression is likely on the basis of the results of mineral chemistry phase equilibria and geothermobarometer moreover in conjunction with the occurrence of kyanite in the adjacent gyeonggi gneiss complex p t estimates of the chuncheon amphibolite and metapelite suggest a clockwise p t t path this evolutionary path may be related to the amalgamation of continents during the late proterozoic event which corresponds to the jinningian orogeny in the qinling belt of china

contrasting plate tectonic styles of the qinling dabie sulu and franciscan metamorphic belts the dabie mountains are part of the 2000 km long qinling dabie sulu suture zone juxtaposing the sino korean and yangtze cratons an eastern extension apparently crosses korea and lies along the japan sea side of honshu as the imjingang and sangun terranes respectively a northeastern segment may be present in sikhote alin russian far east this orogenic belt records late paleozoic ocean floor consumption and the triassic collision of two precambrian continental massifs in east central china coesite and microdiamond inclusions in strong refractory minerals of eclogite facies ultrahigh pressure uhp metamorphic rocks in the dabie sulu area attest to profound subduction of a leading salient of the old cold yangtze craton now recovered through tectonic exhumation and erosion northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high pressure hp and uhp metamorphism documented in the western alps in both regions subduction of narrow prongs of continental material uhp metamorphism and return toward midcrustal levels of relatively lower density buoyant microcontinental blocks resulted from delamination of these rocks from the descending higher density oceanic crust capped lithospheric plate such salients of continental crust represent an integral structural part of the downgoing slab remain intact and may be dragged to great depths before disengaging and rising differentially as coherent blocks uhp parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700 900 c and 28 35 kbar or more in contrast pacific type hp metamorphic belts as represented by the franciscan complex of western california recrystallized under physical conditions up to 200 500 c 10 3 kbar in this setting voluminous quartzo feldspathic and graywacke debris was carried downward on oceanic crust capped lithosphere choking the subduction zone with incompetent material sited between both plates and strongly adhering to neither this buoyant largely sedimentary complex decoupled at 25 30 km depth and ascended toward the surface in both alpine type intracontinental collision and pacific type underflow light sialic material displaced dense mantle thus the return to midcrustal levels was propelled dominantly by body forces

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

low angle normal fault earthquakes and triggering geologic evidence for low angle normal faulting is overwhelming and recent seismic geodetic and neotectonic studies have identified probable active examples thus apparent low levels of historical seismicity on such faults is enigmatic however one large mainshock was a low angle normal event and three others may have been one earthquake sequence had significant moment release from a triggered low angle normal fault and three earthquakes may have had normal subevents on low angle planes such sub events are hard to recognize may be underrepresented in catalogs helping to explain the apparent seismic enigma and pose increased seismic hazard due to delayed strong motions triggered sub events may be along strike or down dip from their triggering shock in cases where the triggered slip is downdip of the steep fault and under its hanging wall elevated pore fluid pressure or fault weakness may be required

the geology of north korea an overview abstract the korean peninsula kp occupies a junction area of three big tectonic domains that are the paleo central asian orogenic belt paleo tethyan orogenic belt and the western pacific orogenic belt therefore the kp is a key area to understand the northeast asian and even global geotectonic evolution in terms of geological unit division the kp has long been thought to be closely tied with the mainland of china with using collective term the sino korea craton for the north china craton ncc however such correlation suffered from the absence of relevant geological information in north korea nk to bridge this knowledge gap our group carries out a long term collaborative study between the two sides of the kp and china and organized united field investigation in the north korea south korea sk and china as possible this paper introduces general geology in the north korea and simply make comparison with the ncc and south korea these pioneering works culminated in the following summaries 1 the kp comprises three major precambrian massifs i e rangnim gyeonggi and yeongnam massifs the rangnim and gyeonggi massifs are separated by the imjingang structural belt while the gyeonggi and yeongnam massifs are separated by the ogcheon structural belt 2 the basement of the rangnim massif is correlated with that of the ncc the characteristic metamorphic rocks of high temperature high pressure granulite facies including meta sedimentary rocks kondalites and meta sandstones and marbles with 1 9 1 8 ga metamorphic ages occur in the three massifs similar to the ncc 3 the proterozoic rift sedimentary sequences are developed in the ncc and north korea but lower paleoproterozoic strata are rare and neoproterozoic strata are more developed in north korea relative to the ncc 4 two main paleozoic basins within the rangnim massif and within the gyeonggi and yeongnam massifs are the phyongnam basin and taebaeksan basin both of them have a similar paleozoic tectono stratigraphy to the ncc 5 although the nature and tectonic significance of the imjingang belt remains controversial it doesn t show characteristics and natures of collisional orogenic belt 6 the eclogite bearing hongseong complex hsc is located in the southwestern part of the gyeonggi massif in south korea but no similar rock unit has been found in north korea 7 mesozoic igneous rocks are widespread throughout the northern and southern parts of the kp triassic magmatism is related to indosinian orogeny jurassic and cretaceous igneous rocks are different and irregular from those in the ncc on age and spatial temporal distribution this paper also suggests several key and controversial academic issues that are worthwhile to study in the future

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

evolution of the qinling orogenic belt detailed field mapping geophysics and geochemistry show that the qinling orogenic belt is a southward vergent tectonic wedge consisting of six regional thrust sheets the lushan luanchun qinling douling wudang and zhuqi thrust sheets these regional thrusts underwent displacement at the end of the mid triassic and the end of the late jurassic due to the collision between the north and south china blocks and subsequent detachment the qinling orogenic belt is thus an indosinian yanshanian collision detachment zone the so called caledonian orogeny in the qinling orogenic belt records a succession of extensional and partial melting events in the continental crust the basic lavas in the lower paleozoic were erupted into rifting zones both in the northern qinling subzone and southern qinling subzone at the same time local melting of the transitional continental crust occurred and resulted in formation of s type granites

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

phanerozoic polyphase orogenies recorded in the northeastern okcheon belt korea from shrimp u pb detrital zircon and k ar illite geochronologies abstract we present the shrimp u pb detrital zircon and k ar illite 1m d 1m and 2m 1 ages suggesting new insight into the phanerozoic polyphase orogenies preserved in the northeastern okcheon belt korea since the initial basin formation during neoproterozoic rifting through several successive contractional orogens the u pb detrital zircon ages from the early paleozoic strata of the taebaeksan zone suggest a cambrian maximum deposition age and are supported by trilobite and conodont biostratigraphy although the age spectra from two sedimentary groups the yeongwol and taebaek groups show similar continuous distributions from the late paleoproterozoic to early paleozoic ages a grenville age hiatus 1 3 0 9 ga in the continuous stratigraphic sequence from the taebaek group suggests the existence of different peripheral clastic sources along rifted continental margin s in addition we present the k ar illite 1m d 1m ages of the fault gouges which confirm fault formation reactivation during the late cretaceous to early paleogene ca 82 62 ma and the early miocene ca 20 18 ma the 2m 1 illite ages at least those younger than the host rock ages provide episodes of deformation metamorphism and hydrothermal effects related to the tectonic events during the devonian ca 410 ma and permo triassic ca 285 240 ma these results indicate that the northeastern okcheon belt experienced polyphase orogenic events namely the okcheon middle paleozoic songrim late paleozoic to early mesozoic daebo middle mesozoic and bulguksa late mesozoic to early cenozoic orogenies reflecting the phanerozoic tectonic evolution of the korean peninsula along the east asian continental margin

detrital zircon u pb geochronology and tectonic implications of the paleozoic sequences in western south korea abstract the middle paleozoic metasedimentary sequences of the western gyeonggi massif taean formation and wolhyeonri complex imjingang belt and southwestern okcheon belt provide clues to the successive tectonic events that occurred during the evolution of the korean peninsula in this study we present shrimp u pb ages of detrital zircon grains from the metasandstones of the taean formation and yeoncheon group of the imjingang belt and from quartzites in three paleozoic formations of the southwestern okcheon belt the zircon grains from all of the samples yielded similar u pb age spectra of paleoarchean neoarchean to middle paleozoic with a dominant age population within the paleozoic the maximum ages of deposition estimated from the youngest dominant concordant detrital zircons were 431 457 ma for the western gyeonggi massif and imjingang belt and 380 355 ma for the southwestern okcheon belt permian metamorphic zircon overgrowths correlated with subduction prior to the triassic collision were also found middle paleozoic metamorphic zircon overgrowths that can be compared to those of the yunkai wuishan orogenic belt of the cathaysia block were indentified in the wolhyeonri complex the dominant zircon populations of this study and the available data for the korean peninsula compare well with those from the yangtze block cathaysia block and qilian qinling orogenic belt which were located at the northeastern margin of eastern gondwana during the early paleozoic

aspects of three dimensional strain at the margin of the extensional orogen virgin river depression area nevada utah and arizona the virgin river depression and surrounding mountains are neogene features that are partly contiguous with the little strained rocks of the structural transition to the colorado plateau province this contiguity makes the area ideally suited for evaluating the sense magnitude and kinematics of neogene deformation analysis along the strain boundary shows that compared to the adjacent little strained area large magnitude vertical deformation both uplift and depression locally exceeding 10 km greatly exceeds extensional deformation and that significant amounts of lateral displacement approximately parallel the province boundary extensional kinematics the directions of lateral displacements and the position and orientation of uplifts and one major depression are such that isostatic rebound following tectonic denudation is an unlikely direct cause of the strong vertical structural relief adjacent to the strain boundary instead the observed structures are first order features defining a three dimensional strain field produced by approximately east west extension vertical structural attenuation and extension normal shortening all major structural elements of the strain boundary strain field are also found in the adjacent basin and range especially evident is a discontinuous and inhomogeneous distribution of vertical structural relief resulting from uplift and tilting of the footwalls on convex upward normal faults combined with formation of extension parallel folds deformation resulted in a bewilderingly complex mixture of coeval folds normal faults strike slip faults and in some areas reverse faults thrust faults and thrust faults reactivated as extensional detachment faults these complexly mixed structures exhibit a tectonically important aspect of scale independence ranging from micro structures to mountain range scale many intensely extended domains appear to reflect a process of surface relief minimization above uplifted and tilted blocks some intensely extended zones are depth limited by relatively undeformed rocks above and intact tilted blocks below and are best viewed as subhorizontal zones of accommodation for the uplift and tilt of subjacent blocks whether or not the intense deformation extends to the surface its depth distribution commonly reflects lithologic or paleostructural controls whatever the cause for the concentration of extensional strain many gently dipping zones are more accurately viewed as local features recording small to moderate extension parallel translations than a regionally interconnected detachment faults recording enormous extension parallel translations although the total neogene strain field in the nevada utah arizona tricorner area has unique aspects other parts of the basin and range contain generally similar structures that presumably formed from similar processes these processes include lateral mass transfer probably by viscoelastic flow of the material needed to inflate areas of strong uplift and to translate blocks laterally by tectonic rafting directions of mass transfer vary but are commonly both parallel and normal to the direction of extension or parallel to the boundaries of extended areas much of the strong vertical structural relief probably reflects boundary condition or localized perturbations in the form of restraints or releases of the lateral flow system these conclusions are consistent with modern concepts of the flexural strength and rheological stratification of the crust in regions undergoing extension

discovery of a c 370 ma granitic gneiss clast from the hwanggangri pebble bearing phyllite in the okcheon metamorphic belt korea abstract the chemical th u total pb isochron method chime of dating was carried out on accessory minerals in samples from the okcheon metamorphic belt in korea dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the hwanggangri formation metamorphic allanite in garnet bearing muscovite chlorite schist of the munjuri formation and polycrase and monazite in post tectonic granite from the hwanggangri area overgrowth of mantles took place at 369 10 ma on c 1750 ma cores of xenotime and monazite in the granitic gneiss allanite occurring in textural equilibrium with peak metamorphic minerals yields a chime age of 246 15 ma that is discriminably older than the polycrase 170 6 ma and monazite 170 3 ma ages of the post tectonic granite these chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c 250 ma from post 370 ma sediments late permian age signatures have also been reported from the precambrian gyeonggi and yeongnam massifs that border the okcheon metamorphic belt and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

implications based on the first shrimp u pb zircon dating on precambrian granitoid rocks in north korea abstract precambrian basement rocks in north korea consist of supracrustals and granitoid rocks of which the former comprise the late archean nangrim group and the paleoproterozoic jungsan and machollyong groups whereas the latter include four major types grey ttg gneisses garnet sillimanite bearing s type granites massive porphyritic granites and hornblende bearing granites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the precambrian history of north korea and its relations to north china shrimp u pb zircon analyses on two grey gneiss samples po501 and k14 reveal that they were emplaced at 2538 14 ma and 2636 49 ma respectively and metamorphosed at 2462 2433 ma these data for the first time confirm the existence of late archean rocks in the korean peninsula moreover one grey gneiss sample k14 contains several zircons with ages of 3 1 3 4 ga suggesting possible middle archean crust in north korea rocks with such ages are widely distributed in north china implying that north china and north korea have similar archean basement one grey gneiss sample r53 yielded a shrimp zircon age of 1933 12 ma indicating that some grey gneisses in north korea formed during the paleoproterozoic two garnet and or sillimanite bearing s type granite samples gave 207 pb 206 pb ages of 1908 31 ma and 1903 49 ma interpreted to reflect the time of metamorphism of the paleoproterozoic jungsan and machollyong groups since these s type granites are considered to be derived from the partial melting of pelitic rocks from these groups the 1 9 ga metamorphic event has also been revealed in paleoproterozoic rocks in south korea and north china shrimp zircon data also show that massive porphyritic monzogranites in north korea evolved over a protracted interval from 1865 ma to 1843 ma similar aged porphyritic monzogranites are also found in north china and south korea and their origin is considered to be related to a post tectonic extensional event of regional extent the last phase of precambrian magmatism recognized in north korea is grenvillian aged represented by the hornblende bearing granite that yields a shrimp zircon age of 1195 4 ma and is considered to be the source rock of the adjacent yushulazi group north china that contains numerous 1 1 1 2 ga old detrital zircons

singularity analysis of global zircon u pb age series and implication of continental crust evolution abstract recently compiled global databases of igneous and detrital zircon u pb ages have been integrated with other types of isotope data e g neodymium hafnium and oxygen to characterize the episodic growth of the continental crust and the development of supercontinents however to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long standing question instead of analyzing amplitudes and periodicity of the age series here the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time series records the results indicate that zircon age distributions around peaks follow power law distribution regardless of the bin size used to measure the age distribution based on the commonly accepted mechanisms phase transition self organized criticality and multiplicative cascade processes for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to avalanches superplumes slab breakoff etc occurred during episodic convection of the mantle the exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity the values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature with a mean duration of approximately 600 800 myr both intensity of the zircon episodes and their duration for the interval from 3 ga to 0 5 ga depict a descending trend which may signify mantle cooling

permo triassic collision subduction zone metamorphism and tectonic exhumation along the east asian continental margin abstractconvergent plate motion at 320 210 ma generated the tongbai dabie sulu east central china imjingang gyeonggi central korea renge suo southwestern japan sikhote alin orogen along the paleo pacific edge of cratonal asia this amalgamated belt reflects collision between the sino korean and yangtze cratons on the sw portion and accretion of outboard oceanic arcs sialic fragments against the ne margin subducted proterozoic paleozoic continental and oceanic crustal complexes underwent high and ultrahigh pressure metamorphism at low to moderate temperatures tectonic slices of sialic crust episodically disengaged from the downgoing plate and driven by buoyancy ascended rapidly to midcrustal levels from depths exceeding 90 200 km after continental collision in east central china plus or minus korea and from 30 50 km after arrival of far traveled oceanic terranes in sw japan and the russian far east on achieving neutral buoyancy and stalling out at 10 20 km depth later doming gravitation

nd isotopic study of upper cambrian conodonts from korea and implications for early paleozoic paleogeography abstract correlation of the korean peninsula with the neighboring south and north china blocks has been one of the most important and long debated issues in east asian tectonics the eastward extension of the chinese collisional belt between the south and north china blocks into the korean peninsula is still in dispute the lower paleozoic sedimentary rocks in south korea the joseon supergroup accumulated in the okcheon belt which has been proposed as the suture zone or the block boundary between the south and north china blocks in korea the joseon supergroup comprises five different lithostratigraphic units and has been proposed to represent two tectonostratigraphic blocks bounded by the honam shear zone hsz in the okcheon belt among them the yeongweol unit in the western part of the hsz is correlated with south china and the duwibong unit in the eastern part of the hsz with north china to test the above geodynamic interpretation this study analyzed the nd isotopic composition of upper cambrian conodonts from the duwibong and yeongweol units the difference of the nd t values between the upper cambrian duwibong and yeongweol units is about 3 units compared to a wide range of the nd 0 values in the modern oceanic basins this slight difference can be regarded as a variation in a same paleo watermass the nd t values of the two units are consistent with coeval values from north china and this suggests that the duwibong and yeongweol units shared the same oceanic watermass with the north china considering current early paleozoic paleogeographic reconstructions the nd isotopic signatures of north china duwibong and yeongweol units may be independent of laurentia and baltica and thus can be indicative of another unique oceanic watermass in contrast to the previous tectonic interpretations the same paleoceanographic signatures shown by the duwibong and yeongweol units and north china and recent developments suggest that the yeongweol unit in the early paleozoic joseon supergroup is not correlated with south china but correlated with north china and the korean peninsula and north china block may have been included in a larger continental block the sino korean block the collisional belt between the south and north china blocks may not extend into the korean peninsula

is the ogcheon metamorphic belt of korea the eastward continuation of the nanhua basin of china abstract the ogcheon metamorphic belt in central korea has been interpreted to be the eastward extension of the nanhua basin aulacogen of southeast china in this paper comparisons are made between the two regions based on stratigraphic thermal tectonic and other considerations from this comparison correlation of geological events between the nanhua basin and ogcheon metamorphic belt are at best equivocal the closest similarity is the presence in both regions of two major diamictite units of glacial origin and neoproterozoic age 750 690 ma range in china but of controversial origin and uncertain age in korea volcanic rocks in both regions appear to have similar petrological and geochemical traits and are interpreted to be rift related however their ages are different mostly 795 780 ma in the nanhua basin and c 750 ma in central korea so correlation remains uncertain more isotopic data from both regions may shed light on this comparison correlation between other pre carboniferous stratigraphic units in the two regions is hampered by the uncertainty about the stratigraphic age and succession in the ogcheon metamorphic belt stemming mainly from the absence of fossils and the strong tectonic metamorphic overprint both regions appear to have undergone deformation and metamorphism during the middle triassic indosinian songrim in some uncertain way related to the collision between the north and south china plates in the ogcheon metamorphic belt there has been no confirmation of a mid palaeozoic event but in the nanhua basin that event is recorded by stratigraphic and palaeogeographic evidence in the nanhua basin there is no evidence for an early permian metamorphic event that appears on isotopic grounds to have affected the ogcheon metamorphic belt this apparent difference has been interpreted as a result of diachronous deformation during late palaeozoic early mesozoic plate collisions that took place earlier in the east in korea than in the west in china

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

tracing the precambrian tectonic history of east asia from neoproterozoic sedimentation and magmatism in the korean peninsula abstract the neoproterozoic metasedimentary successions and igneous suites in the central korean peninsula provide important insights into the tectonic evolution of east asia here we report detailed detrital zircon u pb age data from the neoproterozoic metasedimentary successions as well as geochemical data on the early neoproterozoic rift related plutonic rocks our results together with the available data from the central korean peninsula lead to the following inferences 1 regional sedimentation in the early neoproterozoic ca 1000 920 ma as a result of the late paleoproterozoic to early neoproterozoic intracratonic rifting 2 arc magmatism in the southern periphery of the central korean peninsula in the early neoproterozoic ca 913 895 ma and ca 865 770 ma 3 back arc magmatism in the inland area of the central korean peninsula in the early neoproterozoic ca 888 852 ma 4 magmatism and sedimentation in the early neoproterozoic ca 765 720 ma due to early neoproterozoic back arc and intracratonic rifting the early neoproterozoic geological and tectonic events in the central korean peninsula correlate with those in the southeastern part of the north china craton the spatial and temporal similarities and discrepancies in the early neoproterozoic geological records of the sino korean continent mark the tectonic evolution within a continental inland the southeastern part of the north china craton and a continental margin the central korean peninsula

kinematic evolution of a large offset continental normal fault system south virgin mountains nevada the south virgin mountains and grand wash trough comprise a mid miocene normal fault system that defines the boundary between the unextended colorado plateau to the east and highly extended crust of the central basin and range province to the west in the upper 3 km of the crust the system developed in subhorizontal cratonic strata in the foreland of the cordilleran fold and thrust system the rugged topography and lack of vegetation of the area afford exceptional three dimensional exposures compact stratigraphy and well defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1 12 000 shows that the fault system accommodated more than 15 km of roughly east west directed miocene extension extension was initially accommodated on moderately to steeply dipping listric normal faults as the early faults and fault blocks tilted steeply to moderately dipping faults initiated within the fault blocks soling into the early faults some of the early faults were active at dips of 20 isostatically driven tilting is superimposed on tilting due to active slip and domino style rotation of the fault blocks collectively these processes rotated originally steeply dipping faults to horizontal orientations the kinematics are inconsistent with the widely accepted view that many near horizontal normal faults were rotated to their present orientations by later crosscutting normal faults however reexamination of other areas suggests that the evolutionary sequence seen in the south virgin mountains may in fact be widely applicable

constraints on crust mantle transition zone with pn waveforms a case study of eastern china and southern korean peninsula abstract the seismic velocity structure of the crust mantle transition zone cmtz provides key constraints on crustal dynamics and can be studied with pn wave which propagates horizontally along the moho discontinuity in this study we first explore an effect of velocity gradient structures in the cmtz through synthetic pn waveforms and demonstrate that the shape of pn changes from step like to pulse like when the cmtz becomes from sharp to a gradient velocity structure respectively we then use pn waveforms of the regional earthquakes the july 2012 mw 4 8 earthquake in east china and the september 2016 mw 5 1 earthquake in south korea to examine the cmtz structures to the west and east of the yellow sea and thus to constrain tectonic affinity between the eastern china and southern korean peninsula the pn waveforms from the mw 4 8 event in east china show that the moho in southern yangtze craton is sharp whereas there may be a gradient structure in cmtz with a 6 10 km thickness in northern yangtze craton the pn waveforms from the mw 5 1 event in south korea show that the cmtz in south korea is also sharp similar to the southern yangtze craton observed difference in cmtz structures suggest that sulu orogenic belt may extend along the north of the gyeonggi massif in korea

recurrent rare earth element mineralization in the northwestern okcheon metamorphic belt korea shrimp u th pb geochronology nd isotope geochemistry and tectonic implications abstract rare earth element ree mineralization is hosted within neoproterozoic alkaline metaigneous rocks in the northwestern part of the okcheon metamorphic belt omb a polymetamorphosed fold and thrust belt transecting the paleoproterozoic gyeonggi and yeongnam massifs in the southern korean peninsula the principal carrier phase of rees is allanite allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz k feldspar biotite britholite apatite fergusonite andradite magnetite zircon titanite and fluorite electron microprobe analysis of allanite clearly distinguishes sample to sample variations in total rees ca al fe and y but the textural varieties from each rock sample do not show marked differences in those elements sensitive high resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization allanite grains from ree ores yielded late ordovician 444 6 8 0 ma permian to triassic ca 300 220 ma and early jurassic 199 183 ma 208pb 232th ages these multiple age components often coexist in single grains showing slight differences in backscattered electron brightness the ordovician components have distinctly higher th u than the younger domains in the same rock sample the cores and rims of zircon from a syenite hosting ree ore bodies yielded neoproterozoic 858 2 6 3 ma and early jurassic ca 190 ma 206pb 238u ages respectively the early jurassic ages 194 187 ma also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and ree mineralization the neoproterozoic zircon inheritance weighted mean 853 9 3 8 ma in these granites is in sharp contrast to the dominant paleoproterozoic inherited zircon from the widespread earliest middle jurassic granites enclosing the mineralized zone the geotectonic significance of the late ordovician event recorded in the allanite as well as in detrital zircon from the omb is still unclear but its temporal coincidence with intraplate volcanism and arc related igneous activity respectively reported from the southwestern edge of the adjacent taebaeksan basin and the southwestern gyeonggi massif is noteworthy the following permian triassic and early jurassic mineralization events are probably linked to the continental suturing between the north and south china blocks and subsequent post orogenic magmatism and arc magmatism resulting from the paleo pacific plate subduction respectively sub grain sm nd isotopic analyses of allanite by laser ablation multiple collector icpms yielded initial end values plotting along the nd isotopic evolution path of the neoproterozoic metaigneous rocks indicating that rees originating from the host rock have been recycled during multistage mineralization events the profound differences in inherited zircon ages and nd isotopic compositions between the early and middle jurassic granites may reflect the presence of a major thrust bounded crustal structure beneath the omb

nature and evolution of the cretaceous basins in the eastern margin of eurasia a case study of the gyeongsang basin se korea abstract the gyeongsang basin is the largest among the non marine cretaceous sedimentary basins in korea and japan formed by sinistral strike slip movement induced by the oblique subduction of the paleo pacific plate under the eastern asian continental margin and has an evolutionary history of about sixty million years shrimp and la mc icpms u pb analyses were performed on the detrital zircons separated from the constituent formations of the gyeongsang basin we determined the maximum depositional ages of the four formations based on their ages of the youngest groups as follows 118 0 2 6 ma n 4 for the hasandong formation 108 7 0 8 ma n 11 for the chilgok formation 106 0 0 2 ma n 63 for the silla conglomerate 105 4 0 4 ma n 14 for the haman formation and 99 9 0 7 ma n 5 for the jindong formation belonging to the period from aptian to cenomanian the pre cretaceous detrital zircons of the lower to middle gyeongsang supergroup indicate their derivation from the yeongnam massif and the okcheon metamorphic belt surrounding the gyeongsang basin at around 115 ma as reflected as enhanced proportion of such ages in the detrital zircons and occurrence of farther inland igneous activities the tectonic environment of gyeongsang basin changed to intra arc due to the more active reestablishment of subduction related igneous activity however the upper gyeongsang supergroup has a new age component of the mid permian along with changes in the direction of paleocurrent flowing from the northeast and east indicating sediment supply from sw japan with rocks of that age the changes in the direction of the paleocurrent flow seem to indicate that the center of igneous activity has moved in the direction of the subduction zone to the east as the intensity of igneous activity continued to increase the gyeongsang basin was closed by the filling of the basin by volcanic activity the cretaceous basins of the korean peninsula and southwest japan show similar evolutionary histories suggesting a type model of the creation and the evolution of basin around the continental margin subduction zone creation of sedimentary basin by pull apart or rifting in the backarc region evolution to intra arc basin by the landward progress of subduction related igneous activity and finally the closure of the basin due to the advent of highly enhanced igneous activity

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

initial constraints on the timing of granitic magmatism in north korea using u pb zircon geochronology abstract the geology of north korea is very poorly known in this paper twenty seven granitic samples from the tumangang belt and the rangnim massif including diorite granodiorite and monzogranite were selected for in situ zircon u pb isotopic dating in the tumangang belt granites formed mainly in the jurassic with one sample giving a triassic age this is contrary to previous views that considered the granites to be permian to triassic in age in the rangnim massif precambrian granitic magmatism mainly took place at 2 1 and 1 9 1 8 ga no archean emplacement ages have been documented in this study although some inherited discordant zircon may be of this age mesozoic magmatism in this area can be divided into three stages with ages of 234 213 173 and 114 109 ma the triassic granites are mostly exposed in the north and the early cretaceous ones in the west the distribution of jurassic granites is less well documented and only one sample was identified in this study however previous data indicate they are more widely distributed in the area the 2 1 and 1 9 1 8 ga precambrian ages are comparable to paleoproterozoic magmatism in northeastern china indicating that the rangnim massif was most likely part of the north china craton during that time however the absence of 2 5 ga granites and the occurrence of 1 2 ga granites indicate that the rangnim massif was probably at the margin of the north china craton and affected by mesoproterozoic magmatism the similarity in age patterns of the mesozoic granites in north korea to surrounding areas of south korea and the liaodong peninsula of northeastern china indicate that they were controlled by subduction of the pacific ocean under eastern asia however roll back or retreat of the subduction zone resulted in a younging trend from northwest to southeast during the cretaceous

linking the sulu uhp belt to the korean peninsula evidence from eclogite precambrian basement and paleozoic sedimentary basins abstract considerable debate on whether and how the sulu orogenic belt extends eastward to the korean peninsula has remained over the past decade new results reported here include the following 1 an eclogite and retrograded eclogite bearing complex hongseong complex is discovered in south korea in which the eclogite occurs as lenses in circa 810 820 ma granitic gneiss shrimp zircon dating of the eclogite yields 230 ma for the metamorphic age and 880 ma for the protolith age 2 the basement of the rangnim gyeonggi and yeongnam massifs have affinities to the basement of the north china block ncb however the gyeonggi massif encloses a minor amount of large or small slabs of the hongseong complex that are similar to the rocks of the sulu belt 3 two main paleozoic basins within the rangnim and gyeonggi massifs have a similar paleozoic tectono stratigraphy to the ncb 4 the imjingang and ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts based on these facts we propose a crustal detachment and thrust model and suggest that the collision belt between the yangtze block yb and ncb sino korea craton is preserved along the western margin of the korean peninsula the lower part of the uhp metamorphosed lithosphere of the yb was subducted under the korean peninsula and not uplifted to the surface the lower crust of the yb the hongseong complex was detached from the subducted lithosphere and thrust over the korean peninsula and inserted into the basement rocks of the gyeonggi massif the upper crust of the yb possibly was detached from the lower crust and overthrusted along the honam and chugaryong shear zones the imjingang and ogcheon belts possibly represent the detached upper crust of yb and their present occurrences are controlled by a mesozoic strike slip shear structure all these detached lower and upper crustal slabs were strongly deformed during the late jurassic and early cretaceous tectonic event leading to their present geological distribution and characteristics

crustal evolution of the gyeonggi massif south korea nd isotopic evidence and implications for continental growths of east asia abstract we investigated the sm nd isotopic characteristics of the gyeonggi massif korea in order to delineate its crustal growth history nd isotopic depleted mantle model ages t dm obtained from 32 new measurements of basement and supracrustal samples and from the literature suggest that the gyeonggi massif is as old as middle archean 3 0 ga and was primarily extracted from the mantle during the late archean 2 9 2 5 ga in addition minor crustal formation occurred during the early proterozoic ca 1 9 1 8 ga late archean early proterozoic crustal growth of the gyeonggi massif is comparable with that of the sino korean or yangtze craton the t dm distribution pattern alone is not sufficient to determine the crustal affinity of the gyeonggi massif with either of the chinese cratons however late proterozoic crustal evolution of the gyeonggi massif appears to favor its tectonic correlation with the yangtze craton

neoproterozoic bimodal magmatism in the cathaysia block of south china and its tectonic significance shrimp u pb zircon age geochemical and sm nd isotopic results are reported for the mamianshan volcanic rocks in the cathaysia block of southeastern south china the mamianshan volcanic rocks are bimodal in composition and are dominantly transitional to mildly alkaline basalts and subordinate alkaline rhyolite with an eruption age of 818 9 ma the basaltic samples are characterized by lree enriched and humped trace element patterns similar to many alkali basalts in continental rifts variablend t values between 3 33 and 4 35 indicate that the primary magma of these basalts was derived from an oib like mantle source and underwent fractional crystallization plus crustal contamination the rhyolitic rocks are highly enriched in th ta nb ree zr hf and y and depleted in sr p eu and ti sharing affinity to a 1 type granites combined with their slightly positive nd t values 0 22 to 0 92 the mamianshan felsic rocks were most likely generated by partial melting of the regional paleoproterozoic mayuan amphibolites the mamianshan bimodal volcanic rocks in the cathaysia block are coeval with the widespread intraplate magmatism around the yangtze block our results support the idea that a coherent south china craton was formed during the ca 1 0 ga sibao orogeny and it subsequently underwent extensive continental rifting related to mantle plume or superplume activities beneath rodinia since ca 825 ma 2004 elsevier b v all rights reserved

a middle ordovician drowning unconformity on the northeastern flank of the okcheon ogcheon belt south korea abstract the maggol limestone of ordovician age was deposited in the taebaeksan taebacksan basin which occupies the northeastern flank of the okcheon ogcheon belt of south korea carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the maggol limestone probably culminated in subaerial exposure of platform carbonates in the early middle ordovician earliest darriwilian elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the sauk tippecanoe sequence boundary beneath the middle ordovician succession and its equivalents most in notably north america and north china due to its global extent this paleokarst unconformity has been viewed as a product of second or third order eustatic sea level fall during the early middle ordovician the sauk tippecanoe sequence boundary in south korea however appears to be a discrete marine flooding surface in the upper maggol limestone strata beneath this surface represent by a thinning upward stack of exposure capped tidal flat dominated cycles that are closely associated with multiple occurrences of paleokarst related solution collapse breccias this marine flooding surface is onlapped by a thick succession of thin bedded micritic limestone that is eventually overlain by a middle ordovician condensed section this physical stratigraphic relationship suggest that second and third order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of maggol deposition the tectonic subsidence is also evidenced by the occurrence of coeval off platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias i e the yemi breccia in the basin with continued tectonic subsidence a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform forming a discrete marine flooding surface that may be referred to as a drowning unconformity this tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested thus it is proposed that the taebaeksan basin in the northeastern flank on the okcheon belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early middle ordovician

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

743 17 ma granite clast from jurassic conglomerate kamiaso mino terrane japan the case for south china craton provenance korean gyeonggi block abstract the polymict kamiaso conglomerate mino terrane japan contains jurassic to palaeoproterozoic clasts probably derived from korean basement that lay nearby to the northwest at time of deposition clast k2 broke cleanly into two halves during sampling but the halves were recombined for zircon separation a third of the k2 zircons are colourless euhedral prisms with oscillatory zoning with no inheritance and yielded a shrimp u pb date of 743 17 ma two thirds of k2 zircons are brown oscillatory zoned corroded prisms with a date of 1860 8 ma with inherited cores up to 2460 ma a likely explanation for this could be that clast k2 might have been composite and contained undistinguished 743 ma and 1860 ma granites kamiaso granitic clast k3 igneous zircons gave a date of 179 3 2 1 ma toarcian early jurassic with 2100 2300 ma and 1860 ma inherited cores 740 ma a type magmatism related to the extension and break up of rodinia occurs in both korea gyeonggi block and the main part of the south china craton but is unknown in the sino korean craton thus from recognition of a 743 ma clast the kamiaso detritus was probably derived from the northernmost part of the south china craton in korea

structural characteristics of the central ogcheon belt south korea orogen parallel tectonic transport model abstract the rocks in the central ogcheon belt south korea underwent three deformational phases d1 d2 and d3 during the mesozoic era in the study area nw and ne trending geological structures such as thrusts and folds are juxtaposed the nw and ne trending folds deform isoclinal folds f1 with axial planar slaty cleavage s1 and have axial planar crenulation cleavage all of these folds are overprinted by brittle faults d3 it is inferred from field data that the nw and ne trending structures are products of a single deformational phase d2 a very large scale inclined ne vergent isoclinal fold f1 produced an irregular boundary of precambrian basement and a basement promontory which acted as a structural obstacle against subsequent deformational phases during the e vergent phase d2 deformation partitioning occurred due to the irregular block boundary and orogen parallel and orthogonal structures were produced the d3 phase is recognized as large scale e w trending structures including folds and faults thus the structural evolution of the central ogcheon belt is related to the clockwise rotation of the maximum compressive stress axis from ne sw to n s during the mesozoic era this study shows that the shape of colliding boundary is a very important factor in controlling the structural pattern and evolution in the study area

neoproterozoic 900 ma sariwon sills in north korea geochronology geochemistry and implications for the evolution of the south eastern margin of the north china craton article the sariwon sills are distributed in the pyongnam basin at the center of the korean peninsula eastern part of the north china craton these sills are up to 150 m in thickness and up to more than 10 km in length baddeleyite grains separated from a 50 m thick sill give a sims 206 pb 207 pb age of 899 7 ma mswd 0 34 n 14 which is interpreted to be the crystallization age of this sill zircon grains from the same sill gives a lower intercept u pb age of 400 ma which is likely a close estimation of the greenschist facies metamorphism of this sill the sariwon sills are dolerites and have tholeiitic compositions they show enrichment of light rare earth element concentrations la ybn 1 4 2 8 and are slightly depleted in high field strength elements e g nb zr and ti in comparison to neighboring elements on the primitive mantle normalized spidergram the whole rock endt t 900 ma values are around 2 whereas in situ ehft t 900 ma values from zircon grains vary from 25 to 8 they are similar to the coeval sills in other parts of the north china craton e g the chulan sills xu huai basin shandong peninsula and the dalian sills lv da basin liaodong peninsula these sills possibly originated from a depleted mantle source e g asthenosphere rather than from the ancient lithospheric mantle of the north china craton and have experienced significant assimilation of lithospheric materials the strata and sills in the xu huai lv da and pyongnam basins are comparable moreover the three basins are geographically correlatable based on neoproterozoic geographical reconstruction we therefore propose that there is a xu huai lv da pyongnam rift system along the south eastern edge of the north china craton during neoproterozoic 900 ma with the closure of the rift at 400 ma as a result of a continent margin process it is possible that this southeastern margin of the ncc did not face the inland in the configuration of the supercontinent rodinia

thrust timing growth of structural culminations and synorogenic sedimentation in the type sevier orogenic belt western united states most of the regional shortening in the type area of the sevier orogenic belt in central utah was accommodated by displacement on the canyon range neocomian aptian pavant aptian albian paxton cenomanian campanian and gunnison late campanian paleocene thrust systems inception of each thrust system generated synorogenic sediment associated with frontal thrust tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland the sevier culmination a large antiformal duplex cored by crystalline basement rocks developed during paxton and gunnison thrusting west of and structurally beneath the canyon range and pavant thrusts growth of the sevier culmination was coeval with reactivation of the canyon range and pavant thrust systems and produced a second culmination in proterozoic lower cambrian rocks in the canyon range these structural highs provided much of the sediment to the adjacent foreland basin from late cenomanian to late paleocene time and may have helped to maintain critical taper in the thrust belt