provenance of jurassic sediments in the hefei basin east central china and the contribution of high pressure and ultrahigh pressure metamorphic rocks from the dabie shan abstract the provenance of the jurassic sediments in the hefei basin is constrained by compositions of the detrital k white micas and garnets and shrimp dating of the detrital zircons which can help to understand the evolution and to reconstruct the paleogeographic distribution of hp uhp rocks in the jurassic dabie shan 1 for the oldest mesozoic sediments at the bottom of the fanghushan formation j 1 the predominance of the early paleozoic and luliang 1700 1900 ma zircons indicates a major source from the north china block however neoproterozoic zircons as the major component in other jurassic sediments indicate that the source rocks were mainly derived from the exhumed yangtze block in the dabie shan 2 the co occurrence of high si phengites and triassic zircons provides stratigraphic evidence that the first exposure of the uhp rocks at the earth s surface in the dabie shan occurred in the early jurassic during deposition of the fanghushan formation 3 from the east to the west of the hefei basin there is a spatial variation in the compositions for detrital micas and garnets and in the u pb ages of detrital zircons evidently hp uhp rocks were widely distributed at outcrop in the eastern dabie shan in contrast they were less important in the western dabie shan during the jurassic

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

detrital zircon u pb dating of low grade metamorphic rocks in the sulu uhp belt evidence for overthrusting of the north china craton onto the south china craton during continental subduction the shiqiao pingshang low grade metasedimentary rocks sporadically crop out in the sulu ultrahigh pressure uhp belt in east central china major and trace element data indicate that they were deposited in a passive margin basin probably located at the southern margin of the north china craton sensitive high resolution ion microprobe shrimp u pb dating of detrital zircons from a quartzite sd53 from pingshang and a quartz schist sd54 from shiqiao records ages ranging from 2800 to 1900 ma three age populations are defined at 2660 2500 2350 2150 and 2100 1900 ma with peak ages at 2522 2212 and 2020 ma respectively muscovite ar ar dating of the quartz schist sd54 yields a plateau age of 265 9 1 2 ma these data support the view that the shiqiao pingshang low grade metasedimentary rocks originated from the north china craton and underwent deformation in the permian during subduction of the south china craton yangtze block beneath the north china craton this requires that the low grade metasedimentary rocks were overthrust for several tens of kilometres onto the south china craton explaining why source materials from the north china craton are common in triassic to jurassic sedimentary basins adjacent to the dabie sulu orogenic belt

pb isotopes of granitoids suggest devonian accretion of yangtze south china craton to north china craton feldspar separates of 30 granitoid intrusions ranging in age from neoproterozoic to mesozoic were analyzed for pb isotopic compositions together with associated precambrian basement whole rocks from the north and south qinling orogenic belts the two belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively proterozoic basement rocks from the north and south qinling belts are clearly distinguished in terms of both present day pb isotopes and calculated initials at the time of granite magmatism with the former having considerably more radiogenic pb granitoids of the south qinling belt irrespective of age and specific granitoid rock type show a fairly homogeneous pb isotopic composition that is similar to that of the south qinling basement in contrast in the north qinling belt the pb isotopic composition of granitoids is correlated with their age neoproterozoic and early paleozoic granitoids have radiogenic pb isotopic compositions like those of the north qinling proterozoic basement rocks but late paleozoic and mesozoic granitoids show significantly less radiogenic pb isotopic compositions similar to those of the south qinling basement rocks the unusual change is interpreted to reflect underthrusting of the south qinling belt beneath the north qinling belt at ca 360 ma the process resulted in a change of the source for younger north qinling granitoids from the radiogenic north qinling to the less radiogenic south qinling basement rocks thus the yangtze craton accreted to the north china craton no later than 360 ma

provenance of the beihuaiyang lower grade metamorphic zone of the dabie ultrahigh pressure collisional orogen china evidence from zircon ages the beihuaiyang zone of the dabie ultrahigh pressure collisional belt which was formed by the early mesozoic collision of the north and south china yangtze blocks is diversely interpreted as an active or a passive sedimentary sequence it comprises the luzhenguan and foziling complexes six granitoid rocks of the luzhenguan complex are dated at 770 ma to 720 ma and one schist sample also contains detrital zircons of 760 ma to 720 ma these data indicate that this complex was of the south china affinity and probably originated from the northern yangtze block two quartzite samples of the foziling complex contain abundant detrital zircons of archean to paleozoic ages clustering around 2 5 ga 1 9 1 8 ga 1 0 0 7 ga and 0 5 0 4 ga this age spectrum demonstrates a mixed source of these two blocks the youngest zircons suggest a paleozoic deposition along an active continental margin south of the north china block while these late proterozoic zircons are characterized for the yangtze block subsequently it is suggested that a terrain drifted from the northern yangtze block in early paleozoic must have been situated to the southern margin of the north china block to provide material source prior to the final collision in early mesozoic q 2003 elsevier ltd all rights reserved

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

tracking partial melting and protolith nature by zircon u pb and hf o isotope compositions of migmatites in the north dabie terrane with emphasis on paleozoic low δ18o magmatism

detrital zircon u pb age constraints on cretaceous sedimentary rocks of lingshan island and implications for tectonic evolution of eastern shandong north china abstract in order to clarify sedimentary environment and tectonic history of the sulu orogenic belt we date zircons from four sandstones and one interbedded rhyolite collected on lingshan island in east shandong north china u pb age distributions of detrital zircon populations from the newly named lingshandao formation resemble that of overlying fluvial sedimentary rocks age distributions exhibit four main populations of paleoproterozoic 2 5 2 3 ga mesoproterozoic 2 1 1 9 ga neoproterozoic 850 700 ma and early cretaceous 138 121 ma age accompanied by sporadic occurrences of archean paleozoic and early mesozoic aged grains data indicate a depositional age of 121 ma for the lingshandao formation which constrains the maximum depositional age of overlying fluvial sedimentary rocks the lingshandao formation thus more likely correlates to the uppermost laiyang group of the jiaolai basin proterozoic age populations indicate a mixture of material derived from the north china ncb and yangtze block yzb basement along with protolith material from the hp uhp metamorphic units of the sulu orogenic belt proterozoic age populations differ from those of the roughly coeval jiaolai basin which represents a terrestrial depositional environment to the north of the sulu terrane the yzb appears to be a significant source of detrital zircons consistent with field observations

destruction of the north china craton induced by ridge subductions abstractthe destruction of the north china craton ncc mainly occurred in the cretaceous and has been attributed to a top down rapid delamination bottom up long term thermal chemical erosions or hydration by subduction released fluids on the basis of the distribution of one jurassic and two early cretaceous adakite belts and the drifting history of the paleo pacific plate we propose that three ridge subduction events dominated the large scale decratonization in the ncc both physical erosion and magmatism induced by ridge subduction contributed to the destruction of the ncc the last ridge subduction at ma was the key driving force in the final destruction we present mineralogical geochemical and isotopic data in support of the ridge subduction model flat subduction of a spreading ridge resulted in stronger physical erosion on the thick lithosphere mantle of the ncc consequently slab melting occurred during ridge subduction forming adakites with mantle mg isotope compositions followed b

separation of u pb lu and hf from single zircons for combined u pb dating and hf isotope measurements by tims and mc icpms abstract the u pb system has long been used to precisely date zircons because the high u to common pb ratio in zircon produces very radiogenic pb isotope compositions over time in contrast to u pb zircon has very low lu hf and therefore unradiogenic hf making this mineral ideally suited for determining the initial hf composition of its original host rock a new chemical separation technique presented here enables the determination of both u pb age and initial hf isotope composition of individual zircon grains the acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies zircons are spiked with mixed 176 lu 180 hf and 233 u 205 pb tracers and then digested in hf hno 3 lead lu u and hf are sequentially separated from the zircon matrix on a single ion exchange column filled with eichrom ln spec resin using only 100 l of resin for the separation keeps pb blanks low 5 pg while achieving better than 90 yields for each of the four elements hafnium isotope compositions and lu concentrations are measured with multiple collector inductively coupled plasma mass spectrometry mc icpms whereas u and pb are analyzed by thermal ionization mass spectrometry tims the minimum grain size that can be processed is dictated by the amounts of pb and hf needed for an analysis the smallest grains we currently analyze as small as 50 m 12pg of pb and 3ng of hf can be analyzed with an external 176 hf 177 hf precision of 100 ppm 2 s d the utility of this method is demonstrated with a population of detrital zircons from a cambrian sediment of the takaka terrane new zealand in addition the technique has been used for 14 analyses of the standard zircon 91500 which yield a mean present day 176 hf 177 hf of 0 282305 12 2 s d i e an external reproducibility of 43 ppm the hf isotopic compositions and u pb ages presented here are in good agreement with those of previous studies e g wiedenbeck m alle p corfu f griffin w l meier m oberli f von quadt a roddick j c spiegel w 1995 three natural zircon standards for u th pb lu hf trace element and ree analyses geostandards newsletter 19 1 23 but our data suggest that the 91500 zircon is heterogeneous with respect to lu and hf contents and lu hf

evidence of early cretaceous transpression in the sulu orogenic belt eastern china abstract recent studies have documented marine turbidites with syn sedimentary deformation features in the central sulu orogenic belt of eastern china these units preserve essential information on the late mesozoic evolution of the sulu orogenic belt referred to as the baxiandun formation the turbidites exhibit similar lithologic characteristics to nearby units such as the lingshandao formation that have been well studied even though precise geochronologic constraints are lacking for a more precise correlation this study reports detrital zircon u pb age data that correlate the baxiandun formation turbidites of the central sulu orogenic belt to the early cretaceous lingshandao formation we also report al in hornblende emplacement depth estimates for granitic intrusions of the sulu orogenic belt s laoshan mountain a sharp contact between the laoshan granites and the marine baxiandun formation indicates that the baxiandun basin rapidly subsided to the emplacement depth of the laoshan plutons lateral correlation among the marine turbidites the lingshandao and baxiandun formations combined with information established by previous studies indicates initiation of transpressional tectonics at 122 121 ma transpression ceased with the emplacement of the laoshan granites whose a1 type composition indicates a return to extensional tectonics at ca 111 ma

zircon u pb ages of the cetaceous sedimentary rocks in the laiyang basin eastern china and their tectonic implications abstract the laiyang basin is located in the northern margin of the sulu high pressure and ultrahigh pressure hp uhp metamorphic belt the sediment source and depositional time of the laiyang basin can provide important insights into the exhumation mechanism of the sulu orogen and the formation of the tancheng lujiang tan lu fault but have not been well constrained in this study seven samples were collected from typical sections of the laiyang and qingshan groups in the southern margin of the laiyang basin in the sulu orogen la icpms u pb dating of detrital zircon from these samples yielded ages from 3145 ma to 115 ma and can be divided into four groups 1 3145 1728 ma which indicate a provenance from the precambrian basement rocks of the north china craton 2 908 522 ma which are consistent with the ages of the protolith of the sulu hp uhp rocks 3 240 220 ma which are identical to the ages of the hp uhp metamorphism this suggests that the hp uhp rocks of the sulu orogen had been exposed to the surface and contributed to the deposition of the laiyang group by the early cretaceous 4 148 115 ma weighted mean age 125 0 5 ma mswd 1 2 which as the main group suggests that the major source of the sediments in the laiyang group was from early cretaceous igneous rocks these young ages combined with the ages of 119 1 mswd 0 7 ma determined from the overlain qingshan group confine the depositional age of the laiyang group to 119 125 ma further comparison of detrital zircon u pb age spectrum between the laiyang basin and the hefei basin indicates that these two basins were developed independently rather than developed from single basin and then truncated by the tan lu fault as suggested by previous studies based on the formation age of the laiyang basin and other age constraints we further suggest that the tan lu fault was formed during the late triassic to early jurassic the mesozoic geological settings in the sulu orogen including the sediments of the laiyang basin early cretaceous magmatic intrusive and detachment activities of the wulian fault imply that the cretaceous exhumation of the sulu hp uhp belt was consistent with a metamorphic core complex model

the early cretaceous orogen scale dabieshan metamorphic core complex implications for extensional collapse of the triassic hp uhp orogenic belt in east central china the dabieshan massif is famous as a portion of the world s largest hp uhp metamorphic belt in east central china that was built by the triassic north south china collision the central domain of the dabieshan massif is occupied by a huge migmatite cored dome i e the central dabieshan dome cdd origin of this domal structure remains controversial synthesizing previous and our new structural and geochronological data we define the cretaceous dabieshan as an orogen scale metamorphic core complex mcc with a multistage history onset of lithospheric extension in the dabieshan area occurred as early as the commencement of crustal anatexis at the earliest cretaceous ca 145 ma which was followed by primary early stage detachment during 142 130 ma the central dabieshan complex in the footwall and surrounding detachment faults recorded a consistently top to the nw shearing it is thus inferred that the primary detachment was initiated from a flat lying detachment zone at the middle crust level removal of the orogenic root by delamination at ca 130 ma came into the extensional climax and subsequently isostatic rebound resulted in rapid doming along with exhumation of the footwall the mid crustal detachment zone had been warped as shear zones around the cdd after 120 ma the detachment system probably experienced a migration accommodated to the crustal adjustment which led to secondary late stage detachment with localized ductile shearing at ca 110 ma the migmatite gneiss with hp uhp relicts in the cdd i e the central dabieshan complex was product of the cretaceous crustal anatexis that consumed the deep seated part of the hp uhp slices and the underlying para autochthonous basement compared with the contemporaneous mccs widely developed along the eastern margin of the eurasian continent we proposed that occurrence of the dabieshan mcc shares the same tectonic setting as the destruction of the north china craton however geodynamic trigger of the cretaceous continental scale nw se extension in eastern asia is still a mystery

upb zircon age and geochemical constraints on tectonic evolution of the paleozoic accretionary orogenic system in the tongbai orogen central china abstract the tongbai orogen is located in a key tectonic position linking the qinling orogen to the west and the dabie sulu orogen to the east this orogen comprises a paleozoic accretionary orogenic system in the north and a permo triassic collisional orogenic system in the south hence it may serve as an ideal place to unravel the tectonic evolution from the initial oceanic subduction accretion to the final continental collision between the sino korean and yangtze cratons u pb zircon geochronology of metasedimentary and metaigneous rocks and geochemical characterization of metabasalts indicate a close genetic relationship between the individual tectonic units of the paleozoic accretionary orogenic system 1 the kuanping group as a neoproterozoic ordovician subduction accretion complex which received sediments from the qinling microcontinent 2 the erlangping group and huanggang diorite granodiorite complex as an ordovician silurian island arc evolving from backarc basin 3 the qinling group as a precambrian microcontinent converted into continental arc during the ordovician 4 the guishan complex as a mixture of the qinling continental arc and devonian forearc sediments and 5 the nanwan flysch as a devonian forearc sequence deposited on the newly accreted active margin of the sino korean craton consequently the northern and southern boundaries of the paleozoic accretionary orogenic system represent two sutures of the qinling microcontinent with the sino korean and yangtze cratons the new and previously published data reveal that the tectonic evolution of the tongbai orogen involved a series of events from ordovician early silurian ca 490 440 ma oceanic subduction and arc magmatism silurian early devonian ca 440 400 ma arc continent collision carboniferous ca 340 310 ma oceanic subduction and accretion late permian triassic ca 260 200 ma continental subduction and collision to cretaceous ca 140 90 ma extension and lateral eastward extrusion the entire accretionary orogenic processes until the final collision between the sino korean and yangtze cratons may have lasted more than 200 ma

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

induced seismicity in mines in canada an overview monitoring of mine induced seismicity in canada has improved with the expansion of regional seismograph networks into areas of active mining however the severity and in some cases the frequency of mine induced tremors has increased as mining extends to greater depths and at accelerated rates of extraction because of the complex design and large areal extent of many mines potash coal and metalliferous the most feasible and practical way to monitor these tremors at the present time is to deploy a network of seismometers in and on the surface above mines experiencing microearthquake activity a few of these mines already have a network of seismometers deployed around them and plans are under way to deploy seismograph networks around other mines that have experienced some rather severe tremors in recent times six possible mechanisms for mine induced tremors are described and the associated p and s wave radiation patterns presented a comparison of actual seismic radiation patterns with theoretical predictions is a quick way to diagnose the potential source mechanisms in addition recognizing the pattern of microearthquake activity preceding larger tremors can be used to mitigate the potential effects of severe tremors

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

thermobaric structure of a traverse across western dabieshan implications for collision tectonics between the sino korean and yangtze cratons as the best preserved high and ultrahigh pressure hp and uhp metamorphic terrane in the qinling dabieshan sulu orogen western dabieshan is divided into six lithotectonic units along a traverse across the orogen i e from north to south the nanwan balifan huwan xinxian hong an and mulanshan units in this terrane five eclogite bearing zones i v are developed the garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning suggesting that the rims record general peak temperature and pressure thermobarometric study indicates that the peak p t conditions of eclogite are 550 570 c and 21 kbar for zone i 470 500 c and 14 17 kbar for zone ii 620 670 c and 26 29 kbar for zone iii 530 560 c and 20 22 kbar for zone iv and 490 510 c and 19 20 kbar for zone v the symmetrical thermobaric pattern in conjunction with structural and geochronological data demonstrates that the huwan and hong an units belong to the same hp slice overlying the uhp slice this pattern together with the mulanshan lt hp blueschist greenschist belt in the south roughly constitutes a normal metamorphic zonation however clear metamorphic gaps occur between different slices it is inferred that the lt hp hp and uhp slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients the successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices whereas exhumation of the deepest uhp slice was effected by underthrusting of the lower crust of the yangtze craton

seismic hazard assessment and mitigation in india an overview the indian subcontinent is characterized by various tectonic units viz himalayan collision zone in north indo burmese arc in north east failed rift zones in its interior in peninsular indian shield and andaman sumatra trench in south east indian territory during the last about 100 years the country has witnessed four great and several major earthquakes soon after the occurrence of the first great earthquake the shillong earthquake m w 8 1 in 1897 efforts were started to assess the seismic hazard in the country the first such attempt was made by geological survey of india in 1898 and since then considerable progress has been made the current seismic zonation map prepared and published by bureau of indian standards broadly places seismic risk in different parts of the country in four major zones however this map is not sufficient for the assessment of area specific seismic risks necessitating detailed seismic zoning that is microzonation for earthquake disaster mitigation and management recently seismic microzonation studies are being introduced in india and the first level seismic microzonation has already been completed for selected urban centres including jabalpur guwahati delhi bangalore ahmadabad dehradun etc the maps prepared for these cities are being further refined on larger scales as per the requirements and a plan has also been firmed up for taking up microzonation of 30 selected cities which lie in seismic zones v and iv and have a population density of half a million the paper highlights the efforts made in india so far towards seismic hazard assessment as well as the future road map for such studies

inversion for parameters of tensile earthquakes the tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane the proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic iso compensated linear vector dipole clvd and double couple dc components in seismic moment tensors the most significant parameters inverted are the ratio at the fault denoted as the parameter and the inclination of the slip vector from the fault the parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes the inclination can be accurately determined from the dc component in the moment tensor because the dc component rapidly decreases with increasing for example the inclination of 20 causes dc being 50 60 only the inversion is applied to earthquakes which occurred in january 1997 in west bohemia czech republic it is shown that some of these earthquakes display tensile faulting the parameter is 0 1 the inclination of the slip from the fault attains values of up to 20 this inclination is a result of tensile traction and reduced shear traction along the fault and high fluid pressure in the region

geochemistry and sm nd and rb sr isotopic composition of eclogite in the lhasa terrane tibet and its geological significance abstract a new occurrence of eclogite has been discovered in the lhasa terrane northeast of lhasa tibet along the northern margin of the gangdese magmatic arc the eclogite occurs as a 500 1000 m wide belt that extends over 60 km in an e w direction and consists of coarse grained massive rutile eclogite quartz eclogite and phengite eclogite whole rock geochemistry and relatively high end t for the eclogites suggest that their protoliths were low k oceanic tholeiites similar to typical n morb derived from a depleted mantle source however all analyzed samples have negative nb anomalies in morb normalized trace element patterns suggesting a suprasubduction zone influence zircon shrimp u pb ages of the eclogite ranging from 242 15 ma to 291 13 ma are interpreted as the age of peak metamorphism a sm nd isochron age of 306 50 ma 2 likely reflects the protolith age these data suggest that the paleo tethyan ocean existed along the northern margin of the gangdese island arc until the early permian

automated full moment tensor inversion of coal mining induced seismicity seismicity induced by coal mining in the ruhr region germany has been monitored continuously over the last 25 yr in 2006 a dense temporary network hamnet was deployed to locally monitor seismicity induced by longwall mining close to the town of hamm between 2006 july and 2007 july more than 7000 events with magnitudes m l from 1 7 to 2 0 were detected the spatiotemporal distribution of seismicity shows high correlation with the mining activity in order to monitor rupture processes we set up an automated source inversion routine and successfully perform double couple and full moment tensor mt inversions for more than 1000 events with magnitudes above m l 0 5 the source inversion is based on a full waveform approach both in the frequency and in the time domain providing information about the centroid location focal mechanism scalar moment and full mt inversion results indicate a strong dominance of normal faulting focal mechanisms with a steeper plane and a subhorizontal one fault planes are oriented parallel to the mining stopes we classify the focal mechanisms based on their orientation and observe different frequency magnitude distributions for families of events with different focal mechanisms the overall frequency magnitude distribution is not fitting the gutenberg richter relation full mts indicate that non negligible opening tensile components accompanied normal faulting source mechanisms finally extended source models are investigated for largest events results suggest that the rupture processes mostly occurred along the subvertical planes

moment tensors of ten witwatersrand mine tremors ground motions recorded both underground and on the surface in two of the south african gold mining districts were inverted to determine complete moment tensors for 10 mining induced tremors in the magnitude range 1 9 to 3 3 the resulting moment tensors fall into two separate categories seven of the events involve substantial coseismic volumetric reduction v together with normal faulting entailing shear deformation ad where the summation is over fault planes of areaa and average slipd for these events the ratio v ad ranges from 0 58 to 0 92 with an average value of 0 71 for the remaining three events v is not significantly different from zero these events are largely double couple sources involving normal faulting surprisingly the two types of source mechanism appear to be very distinct in that there is not a continuous distribution of the source mix from v 0 to v ad presumably the coseismic closure indicates substantial interaction between a mine stope and adjacent shear failure in the surrounding rock under the influence of an ambient stress for which the maximum principal stress is oriented vertically

metamorphic evolution and 40ar 39ar geochronology of the wuguan complex eastern qinling area china implications for the late paleozoic tectonic evolution of the qinling orogen abstract the late paleozoic tectonic evolution of the qinling orogen is poorly constrained the wuguan complex which lies between the north qinling and south qinling tectonic belts represents a late paleozoic metamorphic belt of the qinling orogen in this contribution we report new metamorphic pressure temperature p t constraints and 40ar 39ar ages of garnet amphibolites and garnet bearing metapelites from the wuguan complex textural observations and mineral compositions especially garnets with two stages of growth indicate the wuguan complex experienced two distinct metamorphic events conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak p t conditions of 660 725 c and 8 0 12 0 kbar with a clockwise p t path whereas the later metamorphic overprint records p t conditions of 560 610 c and 7 5 9 0 kbar hornblendes from amphibolites yield 40ar 39ar ages of 306 299 ma whereas muscovites from garnet bearing metapelites yield 40ar 39ar ages of 252 251 ma the two groups of 40ar 39ar ages correspond to the cooling time of the two distinct metamorphic events respectively combining with previously published zircon u pb ages from the wuguan complex and two groups of metamorphic ages from the guishan complex and the xiongdian eclogite belt we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during carboniferous subduction of oceanic crust the later metamorphic overprint might have been related to the initial collision between the sino korean and yangtze cratons during the late permain

qinling gneiss domes and implications for tectonic evolution of the early paleozoic orogen in central china abstract the qinling arc terrane qat at the southern margin of the north qinling early paleozoic orogen central china is composed of gneiss domes overridden by cold allochthon nappes the gneiss domes are characterized by migmatitic and plutonic cores of granite or gabbro and overlying upper amphibolite to granulite facies metamorphic sequences with pervasive injection of leucogranitic veins the gneiss domes display a high temperature domal foliation dipping radially away the core and a down dipping stretching lineation cascading folds occur in the mantling metamorphic rocks the new data of zircon monazite u pb dating suggest that the gabbroic and granitic plutons formed at 514 476 ma and 495 441 ma respectively whereas the high grade metamorphism of the mantling rocks took place at 469 412 ma and the leucogranitic veins formed at two main episodes 472 440 ma and 416 413 ma mica 40ar 39ar dating yields 402 328 ma for the cooling ages of the leucogranitic veins and gneissic rocks the basaltic magma underplated into the qat lower crust resulted in partial melting producing granitic magma at the late cambrian to middle ordovician during northward subduction of the shangdan proto tethyan oceanic slab buoyant upwelling of the migmatitic and plutonic core was promoted by constriction in the lower crust and decompression extension in the upper crust during the silurian since the closure of the shangdan ocean and the collision between the north china block and south china block 400 ma the qinling gneiss domes experienced rapid uplift and were finally overthrust by the upper allochthon transported from the north qinling

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

an implosive component in the seismic moment tensor of a mining induced tremor in early 1988 a special study in one of the major south african gold fields yielded seismograms that indicate seismic moment tensors having substantial implosive components the moment tensor resulting from the inversion of the ground motion data from the best recorded event was decomposed into isotropic and deviatoric components from which both the coseismic volumetric closure v and the total shear deformation ad where a is fault area and d is average slip could be estimated the finding here that v ad is consistent with earlier analyses of how the tabular mine stopes interact with the surrounding rock mass to produce seismic deformation

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

microseismic monitoring of a controlled collapse in field ii at ocnele mari romania several decades of faulty exploitation of salt through solution mining led to the creation of an underground cavern containing several million cubic meters of brine to eliminate the huge hazard near a densely inhabited area a technical solution was implemented to resolve this instability concern through the controlled collapse of the roof while pumping the brine out and filling the cavern with sterile to supervise this an area of over 1 km2 was monitored with a staggered array of 36 one component 15 hz geophones installed in 12 boreholes about 160 360 m deep a total of 2 392 seismic events with mw 2 6 to 0 2 occurred from july 2005 to march 2006 located within an average accuracy of 18 m the b value of the frequency magnitude distribution exhibited a time variation from 0 5 to 1 and from there to 1 5 suggesting that the collapse initiated as a linear fracture pattern followed by shear planar fragmentations and finally a 3 d failure process the brunching ratio of seismicity is indicative of a super critical process except for a short period in mid february when temporary stability existed event relocation through the use of a collapsing technique outlines that major clusters of seismicity were associated with the main cavern collapse whereas smaller clusters were generated by the fracturing of smaller size nearby caverns it is shown that one component recordings allow for stable and reliable point source event mechanism solutions through automatic moment tensor inversion using time domain estimates of low frequency amplitudes with first polarities attached detailed analysis of failure mechanism components uses 912 solutions with conditional number cn 0 5 the largest pure shear dc components characterize the events surrounding the cavern ceiling which exhibit normal and strike slip failures the majority of mechanism solutions include up to 30 explosional failure components which correspond to roof caving under gravitational collapsing the largest vertical deformation rate relates closely to the cavern roof and floor as well as the rest of the salt formation whereas the horizontal deformation rate is most prominent in areas of detected collapses

high pressure metamorphic rocks from tongbaishan central china u pb and 40ar 39ar age constraints on the provenance of protoliths and timing of metamorphism abstract the tongbaishan area is situated at the junction between the paleozoic north qinling orogen and the mesozoic south qinling dabie sulu orogen the area contains an important geological section for understanding the assembly of the sino korean and yangtze cratons two eclogite zones were recently recognized in this area the peak metamorphic conditions are estimated to be 530 610 c and 17 20 kbar for the northern eclogite zone and 460 560 c and 13 19 kbar for the southern eclogite zone shrimp u pb analyses on zircons of eclogite metagabbro and orthogneiss indicate that their protoliths were formed during the neoproterozoic and experienced high pressure hp recrystallization during the permian triassic period therefore the tongbaishan area belongs to the western extension of the dabieshan hp uhp terrane in addition u pb ages of detrital zircons from quartzite paragneiss and schist show three age populations of 2 49 ga 1 93 ga and 1 85 1 82 ga suggesting that extensive paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the yangtze craton the shrimp dating and complementary 40ar 39ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent hp eclogite facies metamorphism at ca 255 ma slightly earlier than the timing of deep subduction of the uhp slice the hp rocks were later exhumed to mid to upper crustal levels and suffered from retrograde recrystallization at ca 238 ma the dating also discloses a still later phase of deformation and recrystallization at ca 215 ma the available age data throughout the tongbai dabie sulu orogen reflect diachronous exhumation from ca 238 ma for the hp slice to ca 213 ma for the uhp slice such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting yangtze slab

tracing exhumation of the dabie shan ultrahigh pressure metamorphic complex using the sedimentary record in the hefei basin china jurassic rocks in the hefei basin were deposited by braided fluvial and alluvial fan systems characterized by a general coarsening upward sequence multiproxy provenance analyses demonstrate that the sediment source areas for the hefei basin are composed of a variety of rocks including ultrahigh pressure uhp and high pressure hp metamorphic rocks and yangtze basement rocks of the axial dabie shan metamorphic complex the luzhenguan complex granite low and medium grade metamorphic rocks and the yangshan group sandstone in the north huaiyang fold and thrust belt a middle jurassic section in the western part of the basin is characterized by relatively high e nd values at 176 ma ranging from 13 8 to 11 3 whereas a section in the middle part of the basin has higher 147 sm 144 nd ratios from 0 1168 to 0 1266 and somewhat lower e nd values at 176 ma from 15 0 to 14 5 sediments in a section in the eastern part of the basin have the lowest e nd values at 176 ma ranging from 22 0 to 14 6 the highest t dm values from 1 8 to 2 4 ga and low 147 sm 144 nd ratios from 0 0937 to 0 1067 provenance analyses of detrital compositions and nd isotopic compositions of the sediments in the hefei basin clearly demonstrate that the depth of exhumation in the dabie shan orogen increases from the west to the east the unroofing ages of the uhp and hp metamorphic rocks change from early jurassic to late jurassic westward the exhumation rate during the late triassic and jurassic is inferred to have increased eastward from 1 4 mm a to 2 5 mm a on average the sediments in the basin record the episodic thrusting events and periodic unroofing in the orogen

coal mining induced seismicity in the ruhr area germany over the last 25 years mining induced seismicity in the ruhr area has continuously been monitored by the ruhr university bochum about 1 000 seismic events with local magnitudes between 0 7 ml 3 3 are located every year for example 1 336 events were located in 2006 general characteristics of induced seismicity in the entire ruhr area are spatial and temporal correlation with mining activity and a nearly constant energy release per unit time this suggests that induced stresses are released rapidly by many small events the magnitude frequency distribution follows a gutenberg richter relation which is a result from combining distributions of single longwalls that themselves show large variability a high b value of about 2 was found indicating a lack of large magnitude events local analyses of single longwalls indicate that various factors such as local geology and mine layout lead to significant differences in seismicity stress redistribution acts very locally since differences on a small scale of some hundreds of meters are observed a regional relation between seismic moment m0 and local magnitude ml was derived the magnitude frequency distribution of a single longwall in hamm was studied in detail and shows a maximum at ml 1 4 corresponding to an estimated characteristic source area of about 2 200 m2 sandstone layers in the hanging or foot wall of the active longwall might fail in these characteristic events source mechanisms can mostly be explained by shear failure of two different types above and below the longwall fault plane solutions of typical events are consistent with steeply dipping fracture planes parallel to the longwall face and nearly vertical dislocation in direction towards the goaf we also derive an empirical relation for the decay of ground velocity with epicenter distance and compare maximum observed ground velocity to local magnitude this is of considerable public interest because about 30 events larger than ml 1 2 are felt each month by people living in the mining regions our relations for example indicate that an event in hamm with a peak ground velocity of 6 mm s which corresponds to a local magnitude ml between 1 7 and 2 3 is likely to be felt within about 2 3 km radius from the event

shrimp u pb zircon dating of the wulian complex defining the boundary between the north and south china cratons in the sulu orogenic belt china abstract neoproterozoic metasedimentary and metavolcanic rocks of the wulian group are associated with gneissic granitoids at the northern margin of the sulu uhp belt adjacent to the wulian yantai fault and are collectively referred to as the wulian complex shrimp u pb dating of detrital zircon from the metasedimentary rocks of the wulian group indicates a range in age from 3478 to 625 ma based on zircons 3000 ma with the oldest age being 3339 ma 2700 2400 ma with a peak at 2436 ma 2200 1800 ma with a peak at 2156 ma and at 770 ma these results indicate that deposition of the wulian group was after 770 ma and that some detrital material most likely came from neoproterozoic sources in the south china craton scc shrimp u pb zircon dating of a metavolcanic rock defines a weighted mean 206 pb 238 u age of 761 8 ma which is consistent with other rocks of south china affinity and can be correlated with syn rift magmatism during the breakup of the supercontinent rodinia the wulian group together with neoproterozoic granites of the wulian complex constrains the location of the triassic suture zone between the north china and south china cratons based on our new data the suture must lie north of the wulian complex and is marked by the baichihe yantai fault

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

u pb zircon ages and geochemistry of the wuguan complex in the qinling orogen central china implications for the late paleozoic tectonic evolution between the sino korean and yangtze cratons abstract the tectonic evolution of the qinling orogen central china is the key to understanding the assembly of the sino korean and yangtze cratons the wuguan complex between the early paleozoic north qinling and mesozoic south qinling tectonic belts can provide important constraints on the late paleozoic evolutionary processes in the qinling orogen u pb zircon analyses using laser ablation multicollector inductively coupled plasma mass spectrometry reveal protolith ages of 446 2 ma for a garnet amphibolite 368 3 ma for a meta andesite and 351 2 ma for a mylonitized granitic dike from the wuguan complex elemental geochemistry indicates typical island arc affinities for all the above rocks but some amphibolites of unknown age have e morb signatures detrital zircons from a metaquartzite have an age spectrum with a major peak at 462 ma two subordinate peaks at 828 and 446 ma and a youngest weighted mean age of 423 5 ma this suggests that at least some of metasedimentary rocks from the wuguan complex belong to the part of the devonian turbidite sequence of the liuling group which was deposited in a fore arc basin along the southern accreted margin of the sino korean craton whereas the late ordovician precursors of the amphibolite might be derived from the danfeng group the occurrence of late devonian early carboniferous arc related rocks in the wuguan complex implies penecontemporaneous oceanic subduction and therefore the paleo qinling ocean was not finally closed until the early carboniferous on the other hand metamorphic zircon grains from two amphibolites yielded ages of 321 2 and 318 3 ma hence the wuguan complex in the qinling orogen and the guishan complex in the tongbai orogen constitute a medium pressure carboniferous metamorphic belt that is more than 500 km long and which was formed in the hanging wall of a subduction zone

moment tensor inversion of induced microseisnmic events evidence of non shear failures in the 4 m 2 moment magnitude range to assess objectively the source mechanism of mining induced seismic events we developed an algorithm to invert for the moment tensor components from this inversion the ratio of isotropic deviatoric component in the source is obtained we are applying this method to microseismic events 4 m 2 recorded at the underground research laboratory url in pinawa manitoba where a tunnel is being excavated at 420 m depth to monitor the seismic activity induced by this opening an array of 16 triaxial accelerometers was installed around the planned tunnel results from the moment tensor inversion on a fist data set provide evidence of non shear failure mechanisms furthermore the location of events displaying a tensile component corresponds to a breakout observed underground in the roof of the tunnel

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

shrimp u pb zircon dating of the neoproterozoic penglai group and archean gneisses from the jiaobei terrane north china and their tectonic implications abstract archean basement gneisses and supracrustal rocks together with neoproterozoic sinian metasedimentary rocks the penglai group occur in the jiaobei terrane at the southeastern margin of the north china craton shrimp u pb zircon dating of an archean ttg gneiss gave an age of 2541 5 ma whereas metasedimentary rocks from the neoproterozoic penglai group yielded a range in zircon ages from 2 9 to 1 8 ga the zircons can be broadly divided into three age populations at 2 0 1 8 ga 2 45 2 1 ga and 2 5 ga detrital zircon grains with ages 2 6 ga are few in number and there are none with ages

u pb ages and trace elements of detrital zircons from early cretaceous sedimentary rocks in the jiaolai basin north margin of the sulu uhp terrane provenances and tectonic implications abstract the dabie sulu orogen in east central china is the largest uhp belt in the world many studies have dealt with detrital minerals from sedimentary basins surrounding the dabie belt in order to constrain the tectonic unroofing of this terrane on the other hand still few are the works focused on detritals from the basins associated with the sulu belt the jiaolai basin adjacent to the sulu belt is made of mesozoic sedimentary rocks two sandstone samples collected from the early cretaceous laiyang group in the northern and southern margins of the jiaolai basin were selected for zircon u pb dating and trace element analysis the zircons show main u pb age clusters at ca 2500 2400 2000 1700 850 700 and 330 130 ma with a minor peak at ca 1100 ma and a few archean ages most zircons show texture features reflecting a magmatic origin whereas several late paleoproterozoic and triassic zircons show homogeneous cl images and low th u ratios indicating a metamorphic origin the neoproterozoic ages are consistent with the protolith ages of the metamorphic rocks in the sulu terrane implying that the sulu tarrane may be a major source for this component whereas most neoproterozoic zircons are euhedral to subhedral or angular in shape and are not surrounded by overgrowths this implies that low grade metamorphic neoproterozoic metaigneous rocks ocurring along the northern margin of the sulu terrane or metasediments overlying the hp uhp rocks in the sulu terrane might provide source materials to these detrital zircons the late and early paleoproterozoic age peaks are consistent with the thermal events occurred in the jiaobei terrane slightly younger than those in the yangtze block these suggest that the jiaobei terrane was a potential source for the paleoproterozoic to archean detrital zircon components only two early paleozoic zircon grains were found in the studied samples indicating the early paleozoic arc did not extend into the sulu terrane according to the distribution of different kinds of rock adjacent to the sulu terrane the early cretaceous detrital zircons may be potentially derived from cretaceous plutons from the sulu belt eight detrital zircons including six metamorphic grians and two metamorphic overgrowth rims yield the triassic ages which might be derived from hp uhp rocks from the sulu terrane a metamorphic overgrowth rim yields a metamorphic age of 246 3 ma probably recording the timing of the prograde metamorphism of the sulu terrane one metamorphic grain yields an age of 212 2 ma which is interpreted to record a retrograde metamorphic event based on the ree patterns the remaining six triassic analyses can be subdivided into two groups four of them record a weighted mean age of 230 6 ma and show nearly flat hree patterns and slight negative or positive eu anomalies the last two grains are enriched hree considering their ages and the ree patterns it is suggested that they may originate from other diverse uhp rocks the triassic metamorphic zircons were detected in the laiyang group indicating that the sulu uhp rocks were exposed to the surface before the early cretaceous

automated moment tensor inversion to estimate source mechanisms of hydraulically induced micro seismicity in salt rock abstract two methods for an automated moment tensor inversion are investigated the first is a single source inversion using amplitude spectra of observed body wave phases and theoretical green functions and the second a relative moment tensor inversion using spectral moments of body wave phases an optional general dislocation source constraint is introduced to stabilize the inversions for sparse data both methods are successfully tested on synthetic data and are applied to hydraulically induced micro seismicity in salt rock the estimated moment tensors of 187 events show similar radiation patterns with a major double couple component and a small or zero isotropic component we explain the double couple component with failure on horizontal preexisting planes of weakness within the process zone of circular tension fractures

u pb zircon ages of granitoid rocks in the north dabie complex eastern dabie shan china the north dabie complex ndc comprises a high temperature metamorphic suite dominated by amphibolite facies granitoid orthogneiss intruded by voluminous granites u pb zircon dating on these rocks yields the following new results 1 two of the granitoid orthogneiss samples have early cretaceous protolith emplacement ages of 133 7 2 3 ma and 134 0 2 8 ma 2 an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125 6 0 3 ma these early cretaceous ages entirely post date late triassic early jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high pressure metamorphic terrains to the south in the dabie shan hence they do not support previous tectonic models that infer paleozoic or triassic protolith emplacement ages for these granitoid orthogneisses 3 a mylonitic granite from the northern boundary of the ndc yields a concordant age of 756 6 0 8 ma which is interpreted as magma emplacement age the concordance of this late

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

petrogenesis of early cretaceous alkaline dolerite dykes in the dabie orogen china constraints on the timing of lithospheric thinning abstract the dabie orogen in central china has long been proposed to transform from triassic compression related to northward deep subduction to late jurassic to early cretaceous extension involving marked crustal thinning and doming processes zircon u pb ages mineral chemistry and whole rock geochemistry of alkaline dykes intruding early cretaceous intermediate mafic ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension these dykes of dominantly dolerites with minor monzodiorites have zircon u pb ages of 128 3 129 7 ma and are characterized by sr nd isotopic ratios around the enriched mantle i the dolerites can be divided into two groups with the first group having lower incompatible elements but more enriched sr nd isotopes than the second group equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation systematic normative minerals trace elements and isotopic contrasts of early cretaceous gabbros 125 130 ma dolerite dykes of this study and early cretaceous mafic dykes 111 115 ma indicate that the source regions for the dolerites are deeper than those for the host gabbros therefore the lithospheric mantle thickening probably continued until the formation of the dolerites and the lithospheric mantle thinning occurred significantly later than previously proposed the thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125 130 ma but changed to less than that for the source region of tholeiitic basalts before 115 ma suggesting a minimum decrease of 20 km within 10 m y this rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 ma

zircon u pb and lu hf isotope study of the neoproterozoic haizhou group in the sulu orogen provenance and tectonic implications abstract the neoproterozoic haizhou group crops out sporadically in the sulu orogen in east central china it is divided into the jinping and yuntai formations and consists of quartzite quartz schist marble and graphite and apatite bearing sequences major and trace element data for quartz schist from the two formations indicate that these rocks have a greywacke protolith and have been deposited during strong tectonic activity la icpms u pb dating of detrital zircon yields ages of 635 to 1074 ma for three samples from the jinping formation and 611 to 943 ma for two samples from the yuntai formation more than 78 of the detrital zircons from the two formations have u pb ages grouped between 700 and 890 ma with two clusters peaking at 758 ma and 828 ma respectively this indicates that their provenance is magmatic rocks of neoproterozoic age that have a tectonic affinity to the south china block scb a few older zircon populations with peak u pb ages at 943 and 1074 ma are also present a younger population shows peaks at 661 and 611 ma this suggests that deposition of the haizhou group was later than 611 ma rather than during the mesoproterozoic as previously thought zircon lu hf isotope data collected from the same u pb sites show negative ehf t values of 22 8 to 7 4 and hf model ages of 2341 to 3100 ma this indicates that the neoproterozoic magmatic rocks were derived from reworking of ancient paleoproterozoic to archean crust the results support the contention that the haizhou group is similar to the wulian group at the northwestern edge of the sulu orogen both having a scb affinity but that the penglai group does not belong to the scb because of the absence of neoproterozoic ages this lends support to the conclusion that the triassic suture between the north china and south china blocks is located along the baichihe yantai fault which lies north of the wulian complex and south of the jiaobei terrane thus the wulian yantai fault is not the suture zone as is traditionally proposed

zircon u pb geochronology and hf isotopic composition of the hongqiyingzi complex northern hebei province new evidence for paleoproterozoic and late paleozoic evolution of the northern margin of the north china craton abstract the hongqiyingzi complex located at the northern margin of the north china craton was previously defined as a paleoproterozoic unit it consists of volcanic and sedimentary rocks which underwent greenschist to amphibolite facies metamorphism this study presents zircon u pb ages and hf isotopic composition and whole rock nd isotopic composition of metamorphosed igneous and sedimentary rocks from the hongqiyingzi complex the age dating results show that different gneisses of this complex contain mainly late paleoproterozoic 1800 ma and late paleozoic 275 to 300 ma zircons and minor late archean zircons late paleozoic zircons either occur as magmatic grains or as metamorphic overgrowth and both types of zircon can be distinguished by their th u ratios and internal structures most zircon grains of a paleozoic magmatic rock sample hq14 have high e hf t values of 1 5 to 10 8 suggesting significant contribution of mantle or juvenile crustal material to the magma source s the results imply that the hongqiyingzi complex contains precambrian supracrustal sequences and paleozoic rocks of magmatic origin that underwent strong metamorphism and deformation during the late paleozoic related to the closure of the paleo asian ocean between siberia and the north china craton

u pb and hf isotope analyses of detrital zircons from late paleozoic sediments insights into interactions of the north china craton with surrounding plates abstract u pb dating and hf isotope analyses were performed on detrital zircons from the upper permian sandstones in the ordos and jiyuan basins with aims of defining the sediment provenance and by inference to determine the influence of the surrounding plates on the evolution of the north china craton ncc detrital zircons from these two basins fall into three major groups based on their u pb ages phanerozoic paleoproterozoic and neoarchean the youngest grain from every sample is only slightly younger than the real depositional age of strata the phanerozoic zircons from the jiyuan basin 244 390 ma show ehf t ranging from 30 3 to 1 3 suggesting a provenance from the inner mongolia paleo uplift 245 376 ma ehf t 18 9 to 1 7 a bi modal distribution of ages and ehf t is noted for detrital zircons from the ordos basin one resembles that of the jiyuan basin the other is characterized by early paleozoic ages and relatively high ehf t values 10 9 to 7 7 a hybrid source is thus inferred for the ordos sediments the northern qinling orogen 428 478 ma ehf t 10 9 7 7 must have served as a source in addition to the inner mongolia paleo uplift source this interpretation is consistent with the fact that the northern qinling orogen was an active margin during the early paleozoic and suggests that the western part of the northern qinling orogen was elevated relative to the center of the craton during the late paleozoic it is likely that the early paleozoic arc trench in the northern part of the dabie orogen was under thrusted below the ncc due to the subduction of the yangtze plate during the triassic the inner mongolia paleo uplift was strongly uplifted by subduction of the paleo asian ocean plate underneath the northern ncc and functioned as a sediment source for the inner craton during the late paleozoic these results reveal a heterogeneous pattern of destruction of the north china craton with its northern margin starting to be activated no later than late paleozoic

an early mesozoic transcontinental palaeoriver in south china evidence from detrital zircon u pb geochronology and hf isotopes detrital zircon geochronology reveals that late triassic early jurassic fluvial sandstones from the major basins of the south china craton have similar age patterns and define four populations at 2 6 2 4 ga 2 0 1 7 ga 850 700 ma and 480 210 ma the late palaeoproterozoic group is predominant in all of the five samples and yielded remarkable age peaks at c 1 85 ga these zircons have hf t values between 22 5 and 3 6 suggesting derivation from reworked archaean crust and minor juvenile crustal additions in the late palaeoproterozoic these characteristics differ from those of the yangtze block but correlate well with those of samples from the eastern cathaysia block palaeocurrent analysis of the early mesozoic sandstones shows predominant west and nw directed palaeoflows supporting derivation of the sediments from the cathaysia block the remarkable similarities in provenance signatures and spatial changes of lithofacies of the triassic jurassic around the south china craton delineate an east west trending sedimentary zone extending from korea to west china accumulation of these sediments was probably related to the development of an active continental margin produced by westward subduction of the palaeo pacific plate a c 2000 km long westerly draining transcontinental palaeoriver probably had existed in the early mesozoic and fed the basins in korea south china and west china supplementary material sensitive high resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon u th pb and hf isotope data are available at www geolsoc org uk sup18514

rupture dimensions and rupture processes of fluid induced microcracks in salt rock abstract we developed and applied a simple empirical green function method to study induced microcracks observed during hydraulic fracturing experiments in salt rock either unidirectional ruptures on rectangular faults or allround ruptures on elliptical faults are tested to explain the observed directivity effects in body wave amplitude spectra mostly the rectangular rupture model and horizontal fault planes are favored the average rupture lengths are between 15 and 27 mm the average rupture durations between 14 and 26 s small average rupture velocities of 30 of the s wave velocity of the rock are indicated the dispersive low frequency coda waves present in the data look similar to coda waves observed during other hydraulic fracturing experiments and to long period events from some volcanoes which have been explained by the resonance of a fluid filled crack the radiation pattern of first motion amplitudes of most events is dominated by a dip slip double couple indicating slip on horizontal or vertical planes we cannot distinguish whether the observed low frequency coda waves are influenced by a source effect or by a possible sensor borehole coupling however a simple method using gauss filter at different center frequencies and relocation is tested to analyze the low frequency coda waves in terms of source models

zircon u pb and pb isotope fractionation during stepwise hf acid leaching and geochronological implications abstract two homogeneous zircons zircon 91500 kuehl lake ontario and phalaborwa zircon south africa were analysed to study u and pb behaviour during stepwise hf acid leaching from the phalaborwa zircon 11 of radiogenic pb and 7 of u were extracted into solution at the first leaching step and a reverse discordance was obtained after extraction of 27 0 of the total radiogenic pb budget and 24 5 of the u the zircon residue gave concordant ages it contained 75 8 of total radiogenic 206pb budget 76 4 of total radiogenic 207pb budget and 68 0 of total radiogenic 208pb budget indicating different mobility of pb isotopes the 208pb 206pb ratios of the solutions and the residue decrease systematically from 1 10 to 0 60 this pattern probably demonstrates that domains with different th u ratios were successively sampled during acid attack in contrast to phalaborwa zircon only 1 of the total radiogenic pb budget was extracted in leaching steps from each of three zircon 91500 fragments highly reverse discordant ages are observed in the first leaching step of the phalaborwa zircon this phenomenon is consistent with the recent studies of mattinson partial dissolution analysis pdf of zircon past present future 11th annual v m goldschmidt conference abstract volume 3368 pdf 2001 and davis and krogh chem geol 172 2000 41 due to the fractionations between u and pb and between radiogenic pb isotopes observed in the first leaching step discordant domains or zonings of zircons have to be completely removed in the leaching steps to avoid any artificial high 207pb 206pb ratio in the residues evaluation of data from leaching studies shows that the extractability of pb is positively correlated with the total radiogenic pb budgets in zircons and subsequently with u contents and ages of zircons indicating an important role of radiation damage this correlation is consistent with the fact that old zircons with high u contents are much more susceptible to pb loss than younger ones with low u contents

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

structural and geochronological evidence for early cretaceous orogen parallel extension of the ductile lithosphere in the northern dabie orogenic belt east china abstract the wnw trending north dabie dome ndd in the northern dabie orogenic belt is bounded by the xiaotian mozitan ductile shear zone xmsz in the north and the wuhe shuihou shear zone wssz in the south fabric geometries and kinematics of both the xmsz and the wssz are similar they both show a top to nw or wnw sense of shear and display similar microstructures which indicate that the deformation temperature was about 600 650 c 40 ar 39 ar dating of hornblende and biotite from the mylonites of the xmsz suggests that the shear zone was developed prior to 142 ma u pb zircon la icpms dating of undeformed granite dikes that cut the xmsz suggests that the ductile shearing occurred before 130 ma the wssz was also developed in the early cretaceous the interior of the ndd is dominated by top to nw or wnw sense of shear in the migmatitic gneiss and deformed plutons of the earliest early cretaceous ages we suggest that the xmsz the wssz and the interior of the ndd were involved in the same early cretaceous deformation the xmsz and the wssz were originally a single connected more flat lying zone which we call the east dabie detachment zone eddz it developed at a crustal level of over 18 km in depth kinematics of the eddz suggest that the ductile crust and possibly part of the lithospheric mantle in the eastern dabie orogen underwent pervasive orogen parallel and ese ward extension at the beginning of the early cretaceous large scale magmatic intrusions following this deformation and the resultant increase in buoyancy led to the exhumation of the ndd and the warping of the eddz erosion separated the eddz into two zones the xmsz and the wssz as observed today many metamorphic core complexes of early cretaceous were developed in the eastern north china craton most showing top to nw or wnw sense of shear we suggest that such a widespread uniform shear sense reflects a uniform flow direction of the ductile lithosphere in the east china that has also been documented in the liaonan yiwulushan and yumenshan metamorphic core complexes in the northeastern part of the north china craton

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

zircon ages and nd hf isotopic composition of the zhaertai group inner mongolia evidence for early proterozoic evolution of the northern north china craton the mesoproterozoic rifting and accompanying magmatism and sedimentation in the north china craton are interpreted to be related to the breakup of the paleoproterozoic supercontinent columbia this study presents zircon ages and nd hf isotopic data of the low grade mesoproterozoic zhaertai group exposed along the northern margin of the north china craton sandstones of the zhaertai group contain detrital zircon populations of about 2500 ma which originated from the late archean basement rocks underlying this group a close match between the u pb ages and the tdm hf ages with high initial ehf values at 2500 ma for the detrital zircon grains imply a juvenile origin for protoliths of the basement rocks sedimentation of the zhaertai group likely began around 1750 ma tentatively constrained by the formation time of an intercalated volcanic layer the volcanic rocks have low initial end values of 8 0 to 8 5 at 1750 ma similar to those of contemporaneous mafic dyke swarms within the north china craton which have been interpreted to be related either to the breakup of the supercontinent columbia or to the post collisional collapse of the trans north china orogen a progressive increase of end values of whole rock samples upwards in the stratigraphic sequence likely indicates an increase of juvenile material with time probably related to the contribution of mesoproterozoic mantle derived rocks 2007 elsevier ltd all rights reserved

detrital zircons reveal no jurassic plateau in the eastern north china craton abstract u pb hf isotopic analyses have been carried out on detrital zircons from late permian jurassic strata in the western shandong bohai bay wsbb basins with aims of constraining the paleogeography and evolution of the eastern north china craton ncc four major groups of u pb ages namely 3 0 2 3 ga 2 2 1 4 ga 1250 430 ma and 410 150 ma are recognized for zircons from the fine grained sandstones the post paleoproterozoic zircons of the late permian sample show an age cluster of 344 255 ma with e hf t values of 18 to 8 being similar to those extracted from late paleozoic igneous rocks from the yinshan yanshan orogenic belt yyob on the northern margin of the ncc however the devonian jurassic zircons 390 150 ma from the jurassic sediments have e hf t values from 45 to 14 very similar to the mixture of phanerozoic zircons from the xing meng orogenic belt xmob and the yyob but significantly different from those of the liaodong peninsula jiaodong peninsula and the western shandong uplift in addition cambrian silurian zircons 535 430 ma of the jurassic samples have e hf t values from 27 to 7 resembling the early paleozoic zircons extracted from the northern qinling orogen nqo the u pb age spectra and hf isotope of devonian jurassic zircons as well as bulk nd isotopes suggest that the central eastern ncc received some juvenile materials from the xmob the yyob and the nqo during the jurassic this indicates widespread prototype basins and a low elevation during the jurassic arguing against the presence of a paleo plateau in the eastern ncc at that time the lack of evidence predicted by lithospheric delamination e g large scale elevation or doming after late triassic makes the delamination unlikely to be a viable mechanism of lithospheric thinning of the ncc the subsidence rather than elevation of the eastern ncc after late triassic might be related to thermo chemical erosion of the lithosphere by underlying convective mantle in a back arc setting

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

mesozoic evolution of the hefei basin in eastern china sedimentary response to deformations in the adjacent dabieshan and along the tanlu fault this paper presents a study of jurassic early cretaceous sedimentary evolution of the hefei basin in eastern china and explores the relationship between clastic sedimentation and coeval deformation of the dabieshan to the south and the tanlu fault to the east the hefei basin experienced a two stage evolution the basin was initiated in the early jurassic and expanded in the middle and late jurassic the synsedimentary jinzhai normal fault is considered to be a border fault of the basin because it controlled middle to upper jurassic proximal alluvial deposits persistent n to ne directed paleo flows in the southern hefei basin indicate that sediments came from the dabieshan and the presence of triassic coesite bearing detrital zircon in lower jurassic sediments documents exhumation of ultrahigh pressure rocks of the dabieshan to the surface as early as the early jurassic occurrences of eclogite clasts in an upper jurassic unit indicate continued denudation of the dabieshan at that time thickening of jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the jinzhai normal fault on the south and the ne striking left lateral transtensional tanlu fault on the east jurassic extensional subsidence of the hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the dabieshan which combines to indicate contemporaneous extension and contraction on the north and south sides of the dabieshan respectively vigorous volcanism and uplift of the southern hefei basin characterized the second stage of development of the hefei basin in the early cretaceous and this led to a synchronous shift of its main depocenter to the north this younger depocenter is characterized by lacustrine and fluvial deltaic sedimentation where alluvial and fan deltaic coarse grained deposition mainly occurred along the eastern edge of the basin early cretaceous subsidence is attributed to e w extension across the middle segment of the tanlu fault and the zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin a model is accordingly advanced to account for how the hefei basin developed in response to the tectonic exhumation of the dabieshan and the deformation of the tanlu fault in the mesozoic it illustrates that the hefei basin initiated and evolved during jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra high pressure rocks of the dabieshan early cretaceous development of the basin was controlled by magmatism related uplift of the dabieshan on the south and orthogonal normal faulting of the middle segment of the tanlu fault on the east this study provides an independent constraint upon the exhumation processes of ultrahigh pressure rocks of the dabieshan

kinematic and thermochronological constraints on the xincheng huangpi fault and mesozoic two phase extrusion of the tongbai dabie orogen belt abstract the tongbai dabie orogenic belt dob is composed of several fault bounded terranes with different metamorphic and deformation intensities these tectonically stacked terranes were extruded along their boundary faults from depths of 120 km during the mesozoic systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen in this paper we focus on the xincheng huangpi fault xhf which is located along the southern boundary of the central dob ultrahigh pressure complex unit uc comprehensive macro and micro structural u pb zircon and 40 ar 39 ar geochronological data are presented here to constraint the structural chronology of the xhf the xhf represents the lower detachment surface formed during extrusion of the uhp slab and experienced two episodes of ductile shearing the xhf experienced early southwestward thrusting around 234 ma and was exhumed to middle crustal levels during 234 195 ma with an average exhumation rates of ca 1 5 mm yr the uhp rocks then exhumed to the surface by erosion later nw se trending dextral strike slip shearing of the xhf occurred between 145 and 140 ma and was generally contemporaraneous with sinistral oblique slip of the xiaotian mozitan fault along the northern margin of the uc coeval dextral and sinistral oblique shearing along the southern and northern margins of the uc would have caused southeastern lateral extrusion this lateral extrusion has no relationship with the uhp exhumation

late proterozoic magmatism and metamorphism recorded in gneisses from the dabie high pressure metamorphic zone eastern china evidence from zircon u pb geochronology this study presents zircon ages and geochemical and nd isotopic data for metagranites and quartzites from the susong high pressure metamorphic zone in the dabie sulu ultrahigh pressure collisional belt this belt belongs to the eastern part of the qinling dabie orogenic belt that formed during early mesozoic collision of the north and south china blocks two metagranites give zircon u pb model ages of 785 7 and 205 12 ma likely representing a late proterozoic magmatic event and an early mesozoic overprint they have low initial end values 12 4 and 11 1 at 780 ma favouring a crustal origin flat heavy rare earth elements ree patterns gdn ybn 1 2 probably reflect that melting took place at a shallow crustal section where heavy ree bearing mineral phases are unstable all zircons of three quartzites yield young discordant u pb ages and define a discordia with u pb model ages of 784 6 and 213 3 ma identical to those of the metagranites we assume that all detrital zircons had lost radiogenic pb prior to the early mesozoic overprint probably facilitated by fluid participation during a metamorphic event contemporaneous with the intrusion of the metagranites this simultaneous metamorphic and magmatic event was probably related to a rift setting along the periphery of the yangtze south china block during late proterozoic

waveform inversion of small to moderate earthquakes located offshore southwest iberia summary in this paper we apply the kinematic waveform inversion kiwi tools to the study of smallto moderate earthquakes in southwest sw iberia the earthquakes have magnitudes in the range ml 3 5 4 9 with the exception of one earthquake with magnitude ml 6 0 most events are located offshore are recorded with a large azimuthal gap and generate waves that travel throughastronglyheterogeneous crustalstructure weobtainnewestimatesofcentroid depth seismic moment strike dip and rake for 12 of the 29 studied events the earthquakes whose waveforms we cannot successfully model are 1 located too far from the stations 2 have small magnitude hence low signal to noise ratio or 3 are located within the cadiz basin which is a major sedimentary basin that affects wave propagation notably our results indicate that onshore earthquakes occur at shallow depths 15 km whereas offshore earthquakes occur deeper down to 46 km focal mechanisms indicate transpressive faulting

late paleozoic subduction accretion along the southern margin of the north qinling terrane central china evidence from zircon u pb dating and geochemistry of the wuguan complex abstract the qinling orogen separating the north china plate from the yangtze plate is a key area for understanding the timing and process of aggregation between the two plates two competing and highly contrasting tectonic models currently exist to explain the timing and nature of collision one advocates a devonian continental collision while the other favors a triassic collision the wuguan complex between the early paleozoic north qinling and the mesozoic south qinling terranes can provide important constraints on the late paleozoic evolutionary processes of the qinling orogen metamorphosed sedimentary rock of the wuguan complex have a detrital zircon age spectrum with two major peaks at 453 ma and 800 ma several minor age populations of 350 430 ma and 1000 2868 ma and a youngest weighted mean age of 358 3 ma indicating a mixed source from the north qinling terrane the recrystallized zircons yield a weighted mean age of 333 2 ma representing the metamorphic age geochemical analyses imply that the sedimentary rocks were originally deposited in an active continental margin dominated by an acidic arc source with a subordinate mafic ultramafic source the youngest population of detrital zircons 358 ma suggests that the wuguan complex developed as forearc basin along the southern accreted margin of the north qinling terrane during the early carboniferous whereas the ca 520 460 ma mafic rocks with e morb n morb oib or island arc basalt signatures probably derived from the danfeng group in combination with regional data we suggest that the depositional age of the wuguan complex is ca 389 330 ma but it was subsequently incorporated into tectonic melange by the northward subduction of the paleo qinling ocean a long lived southward facing subduction accretionary system in front of the north qinling terrane probably lasted until at least the early carboniferous

the ruhr coal basin germany structural evolution of an autochthonous foreland basin abstract the ruhr coal basin is part of the external fold and thrust belt of the variscan orogen in central europe information from extensive coal mining outcrops in the south of the ruhr district reflection seismic surveys and about 800 exploration boreholes in the north support the interpretation of a mostly molasse type sequence more than 6000 m thick of namurian and westphalian age both the southwest northeast trending sedimentary basin structures and the fold structures of the ruhr carboniferous were caused by the compressive regime of the variscan folding in its hinterland but there is no direct relationship between sedimentary basin structures and the later folding structures coal formation started in the namurian c reached its maximum during the westphalian a and b and ended during the westphalian d in total about 250 coal seams were formed but only 50 of them are of economic importance at present strata thicknesses and coal content are generally greater in the southeast of the ruhr coalfield than in the northwest an important exception can be observed in the lower part of the westphalian a where in contrast strata thicknesses are greatest in the northwest in the munsterland region although the coal content remains the greatest in the southeast detailed isopach maps covering 100 200 m thick stratigraphic intervals reveal the existence of a southwest northeast trending zone of reduced subsidence in the ruhr coalfield that moved from southwest northeast during the westphalian this structure can be interpreted as a peripheral bulge coal seems are purer and thicker in the area of this structure which therefore must have been a paleogeographic element within the ruhr basin the general effect of a general decrease in the coal content of the upper carboniferous towards the northwest is superimposed on the migration of the coal content maxima of individual formations towards the northwest during the namurian c and westphalian a the coal content maxima were situated in the area of the river ruhr and during the westphalian b and c in the area of the river lippe the deformation of the ruhr coal basin is of post westphalian age as demonstrated by the concordant folding of the devonian and carboniferous strata the tectonic structure is mainly characterised by the following elements stockwerk tectonics axial elevations and a succession of compressional and extensional tectonics due to the general dip of the ruhr coal basin towards the north different structural levels stockwerks can be observed the southern area displays the lowermost stockwerk with many minor folds of about constant wavelength and low amplitudes thrusts are mainly small and some of them show increasing displacement upwards the central part of the mining area displays the intermediate stockwerk with large tight anticlinoria with minor folds separated by open synclines these are accompanied by folded northwest and southeast vergent thrusts in the northern ruhr district high anticlines and broad trough shaped synclinoria with only few thrusts represent the uppermost stockwerk large fold controlled thrusts die out at this level axial culminations and depressions have strongly influenced the structural style of the folding as well according to this model of stockwerk tectonics excess volume created by disharmonic folding is redistributed by thrusts thrusts dying out downwards at different stratigraphic and structural levels give evidence that there is no regional basal detachment below the ruhr coal basin this interpretation fits very well to new results achieved by the deep seismic reflection profile dekorp 2 n the section clearly shows thick skinned tectonics in the rhenish massif with a shortening of the whole thickness of crust the ruhr coal basin can therefore be interpreted in terms of an autochthonous foreland basin in front of a buried thrust front to the south investigations on the post carboniferous strata of the ruhr basin indicate different periods of active faulting cross and diagonal faults were formed partly at the end of the variscan folding and partly before and during deposition of the zechstein strata a further important period of tectonic movements occurred during the early kimmerian phase in the late triassic furthermore earlier extensional faults in the ruhr basin have been affected by late cretaceous transpression