multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

a high precision u pb age of metamorphic rutile in coesite bearing eclogite from the dabie mountains in central china a new constraint on the cooling history this paper first reports a high precision u pb age of 218 1 2 ma for rutile in coesite bearing eclogite from jinheqiao in the dabie mounteins east central china this work shows that the u pb mineral rutile omphacite isochron age of 218 2 5 ma and conventional rutile u pb concordia age of 218 1 2 ma obtained by common pb correction based on the pb isotopic composition of omphacite in the same eclogite sample are consistent proving that the omphacite with low u pb ratio 2 8 can be used for common pb correction in u pb dating of rutile oxygen isotope analysis of rutile aliquots gave the consistent 18o values of 6 1 0 1 demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix oxygen isotope thermometry yields temperatures of 695 35 and 460 15 c for quartz garnet and quartz rutile pairs respectively these oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet according to field based thermochronological studies of rutile an estimate of the tc of about 460 c for u pb system in rutile under rapid cooling conditions 20 c ma was advised based on this u pb age as well as the reported chronological data with their corresponding metamorphic and or closure temperature an improved t t path has been constructed the t t path confirms that the uhpm rocks in south dabie experienced a rapid cooling following the peak metamorphism before 220 ma and a long isothermal stage from 213 to 180 ma around 425 c

neotectonic faulting and forearc sliver motion along the atirro rio sucio fault system costa rica central america the atirro rio sucio fault system forms a major northwest trending strike slip fault zone in east central costa rica we examined the kinematics and temporal evolution of this fault system through geomorphic structural and seismologic analysis this 150 km long strike slip fault zone traverses the northern flank of the paleovolcanic cordillera de talamanca and extends northwestward into the active cordillera volcanica central historical seismicity includes frequent minor swarms and occasional moderate magnitude m 5 0 6 5 damaging earthquakes field geomorphic evidence fault kinematic data and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest striking faults continuity with other transcurrent faults in northwest costa rica indicates that the atirro rio sucio fault system may form the southeastern end of a regional network of northwest trending dextral faults that accommodate margin parallel displacement of the central american forearc sliver the atirro rio sucio fault system originates within the central costa rica deformed belt inboard of the indenting cocos ridge we infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral central costa rica deformed belt faults including the major structures of the atirro rio sucio fault system this zone of arc parallel extrusion thus represents the root of the central american forearc sliver consistent with recent geodynamic models we propose that northwestward sliver escape along the atirro rio sucio faults is driven by rigid indentation of the aseismic cocos ridge into southern costa rica

formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

variation in peak p t conditions across the upper contact of the uhp terrane dabieshan china gradational or abrupt the southern dabieshan terrane sdt has previously been divided into high pressure hp and ultrahigh pressure uhp terranes and its regional extent and the tectonic nature of its boundaries are hotly debated topics in this study an eclogite bearing area of 100 km2 near taihu is mapped in detail and divided into northern middle and southern zones on the basis of lithological characteristics the northern zone consists of epidote biotite gneiss and eclogite blocks the middle zone includes granitic gneiss biotite gneiss eclogites and amphibolite and the southern zone is composed mainly of garnet bearing mica schist the eclogites occur mainly as lens or blocks in the northern and middle zones the peak p t conditions for 61 eclogite samples across the area are estimated using the grt cpx fe2 mg thermometers and the grt cpx phe barometers the results indicate three different p t regions 2 82 4 09 gpa 759 942 c in the northern zone and 2 00 3 54 gpa 641 839 c in the granitic gneiss and 1 38 2 36 gpa 535 768 c in the biotite gneiss from the middle zone combined with the spatial distribution of eclogites across the area the p t values for eclogites increase continuously from the south to the north defining a reference geotherm of 5 c km 1 however some unreasonable apparent gradients can be established along two south north profiles across the area and display a p t difference between the northern and middle zones on the basis of the average p t data for eclogites across the area a gap of at least 0 3 gpa 20 c exists between the northern and middle zones by contrast the p t values of eclogites from the middle zone show a coherent pattern with transitional characteristics from hp in the south to uhp in the north we suggest that the sdt was a coherent slab during subduction and was broken up by a major fault during exhumation which was formed under uhp metamorphic conditions

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

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

anatomy of extremely thin marine sequences landward of a passive margin hinge zone neogene calvert cliffs succession maryland u s a abstract detailed examination of neogene strata in cliffs 25 35 m high along the western shore of chesapeake bay maryland reveals the complexity of the surviving record of siliciclastic sequences 150 km inland of the structural hinge zone of the atlantic passive margin previous study of the lower to middle miocene calvert plum point member and choptank formations documented a series of third order sequences 7 10 m thick in which lowstand deposits are entirely lacking transgressive tracts comprise a mosaic of condensed bioclastic facies and regressive highstand tracts are present but partially truncated by the next sequence boundary smaller scale fourth order cyclic units could not be resolved together these sequences constitute t e transgressive and early highstand tracts of a larger second order miocene composite sequence the present paper documents stratigraphic relations higher in the calvert cliffs succession including the upper miocene st marys formation which represents late highstand marine deposits of the miocene second order sequence and younger neogene fluvial and tidal inlet deposits representing incised valley deposits of the succeeding second order cycle the st marys formation consists of a series of tabular units 2 5 m thick each with an exclusively transgressive array of facies and bounded by stranding surfaces of abrupt shallowing these units which are opposite to the flooding surface bounded regressive facies arrays of model parasequences are best characterized as shaved sequences in which only the transgressive tract survives and are stacked into larger transgressive highstand and forced regression sets biostratigraphic analyses by others indicate that this onshore record contains the same number of third order 1 my duration units as present offshore and so thinning landward of the hinge zone was accomplished not by omission or erosion of entire cycles of deposition but instead by omission of some subsidiary elements e g lowstand tracts by erosional shaving of sequence tops removing the entire regressive tract in some sequences by a reduced number of component high order cycles surviving per larger set and by qualitative changes in the anatomy or composition of elements e g condensed transgressive tracts shaved sequences rather than parasequences all of these differences can be attributed to limited accommodation ut preservation of an onshore record of each baselevel cycle was probably also favored by the large amplitude and rapidity of eustatic fluctuations during the miocene

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

a re evaluation of the stratigraphy and evolution of the kohistan arc sequence pakistan himalaya implications for magmatic and tectonic arc building processes new field mapping and structural data combined with published geochemical data from the kohistan arc in the nw himalaya enable a re evaluation of the arc stratigraphy key lithological units and their relationships are more clearly defined permitting the construction of a revised magmatic tectonic history for the arc the oldest units are transitional oceanic type basalts which form the basement to the subduction related sequence arc type gabbroic sheets and plutons intrude the oceanic basalts together these form the kamila amphibolite belt metasediments and basaltic lavas were deposited within an extensional basin onto the kamila amphibolite belt basement this sequence exposed across the arc forms a distinct stratigraphic unit which is formally defined here as the jaglot group sediment charged turbidity currents transported material into the basin whilst submarine eruptions contributed the basaltic component this period of extension culminated in the eruption of high mg boninites of the chalt volcanic group which overlie the rocks of the jaglot group the earliest granitoids of the kohistan batholith predate suturing and intrude the jaglot and chalt sequences at c 100 ma kohistan sutured to asia suturing being accompanied by thickening of the arc with the development of major intra arc shear zones and a penetrative regionally developed steep cleavage at c 85 ma intra arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the chilas complex which clearly intrudes the kamila amphibolite belt to the south and the jaglot group to the north the chilas complex has been regarded as part of the pre suturing juvenile arc sequence field evidence summarized here show this to be not so heat advection associated with emplacement of the complex caused amphibolite facies regional metamorphism melting of the lower arc and plutonism some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 ma before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 ma at least three phases of extension and rifting each separated by short lived phases of compression charac terized arc evolution much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

chronology and geochemistry of mesozoic granitoids in the bengbu area central china constraints on the tectonic evolution of the eastern north china craton abstract we performed zircon u pb dating and analyses of major and trace elements and sr nd pb isotopes for granitoids in the bengbu area central china with the aim of constraining the magma sources and tectonic evolution of the eastern north china craton ncc the analyzed zircons show typical fine scale oscillatory zoning indicating a magmatic origin zircon u pb dating reveals granitoids of two ages late jurassic and early cretaceous 206pb 238u ages of 160 ma and 130 110 ma respectively the late jurassic rocks jingshan intrusion consist of biotite syenogranite whereas the early cretaceous rocks huaiguang xilushan nushan and caoshan intrusions are granodiorite syenogranite and monzogranite the late jurassic biotite syenogranites and early cretaceous granitoids have the following common geochemical characteristics sio2 70 35 74 56 wt k2o na2o 0 66 1 27 mainly the occurrence of neoproterozoic magmatic zircons 850 ma and inherited early mesozoic 208 228 ma metamorphic zircons within the late jurassic biotite syenogranites together with the occurrence of neoproterozoic magmatic zircons 657 and 759 ma and inherited early mesozoic 206 231 ma metamorphic zircons within the early cretaceous nushan and xilushan granitoids suggests that the primary magmas were derived from partial melting of the yangtze craton yc basement in contrast the occurrence of paleoproterozoic and paleoarchean inherited zircons within the huaiguang granitoids indicates that their primary magmas mainly originated from partial melting of the ncc basement the occurrence of yc basement within the lower continental crust of the eastern ncc indicates that the yc was subducted to the northwest beneath the ncc along the tan lu fault zone during the early mesozoic

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

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

sm nd and rb sr dating of pyroxene garnetite from north dabie in east central china problem of isotope disequilibrium due to retrograde metamorphism abstract oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the sm nd and or rb sr systems in eclogites that experience retrograde metamorphism this study demonstrates that sm nd rb sr and o isotopes for minerals from two portions of a clinopyroxene garnetite from north dabie in east central china record both equilibrium peak metamorphic conditions and disequilibrium conditions resulting from anhydrous retrograde metamorphism the minerals are homogeneous in terms of major elements and show no variation across the sample but clinopyroxene shows variable degrees of alteration pure primary mineral separates that are in o isotope equilibrium yield triassic sm nd isochron ages of 219 11 and 229 13 ma that match previously published zircon u pb ages for peak metamorphism thus the sm nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism in contrast altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the sm nd isochron defined by the pure minerals implying that like oxygen the sm nd systems in these clinopyroxene were disturbed during retrogression no meaningful isochrons are found from the rb sr dating even in the pure primary minerals implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of sr is faster than those of sm nd and o in mafic minerals our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of sm nd mineral isochrons in dating peak metamorphic events in addition variations in the sm nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids

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

neoproterozoic tectonic evolution of south qinling china evidence from zircon ages and geochemistry of the yaolinghe volcanic rocks abstract the south qinling terrain is located at the northern margin of the yangtze block neoproterozoic volcanic sedimentary sequences of the yaolinghe group play the most significant role in studying the tectono magmatic evolution of the south qinling based on temporal and spatial geochemical variations the yaolinghe group can be divided into three volcanic cycles basaltic lavas of cycle 1 crystallized at 847 8 ma and exhibit typical island arc basalt characteristics with enrichment of liles th u and lrees depletion of hfses nb ta zr hf ti and homogeneous initial 143nd 144nd ratios ranging from 0 51162 to 0 51172 corresponding to initial nd values of 1 6 to 3 5 these features suggest that they were derived from a depleted mantle source during a subduction period volcanic rocks of cycle 2 show a complete lithological evolution from basalt through andesite to rhyolite without a significant gap of sio2 content they formed between 731 11 ma and 768 7 ma and display trace element signatures similar to arc volcanics with positive lile and negative hfse anomalies these basalts have lower initial 87sr 86sr ratios from 0 7030 to 0 7052 and higher initial 143nd 144nd ratios from 0 51172 to 0 51194 with positive initial nd values ranging from 0 9 to 5 3 whereas intercalated felsic rocks show lower initial 143nd 144nd ratios from 0 51150 to 0 51177 with initial nd values varying from 4 5 to 1 9 suggestive of metasomatic alteration of lithospheric mantle by slab derived fluids melts cycle 3 lavas show undepleted hfses nb ta zr hf and slightly enriched liles th u and ti concentrations they were generated in a rift related setting between 680 ma and 650 ma geochronological and geochemical evidence from lavas of the yaolinghe group suggest that the panxi hannan arc system extended to northern south qinling and the magmatism in the northern margin of the yangtze block evolved from arc to rift magmatism during the neoproterozoic

overriding plate deformation and variability of fore arc deformation during subduction insight from geodynamic models and application to the calabria subduction zone in nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates such large plates can impose far field boundary conditions that influence the style of subduction and overriding plate deformation here we present dynamic laboratory models of progressive subduction in three dimensional space in which the far field boundary conditions at the trailing edges of the subducting plate sp and overriding plate op are varied four configurations are presented free both plates free sp fixed op fixed and sp op fixed we investigate their impact on the kinematics and dynamics of subduction particularly focusing on overriding plate deformation the results indicate that the variation in far field boundary conditions has an influence on the slab geometry subduction partitioning and trench migration partitioning our models also indicate that in natural narrow subduction zones assuming a homogeneous overriding plate the formation of back arc basins e g tyrrhenian sea aegean sea and scotia sea is generally expected to occur at a comparable location 250 700 km from the trench irrespective of the boundary condition in addition our models indicate that the style of fore arc deformation shortening or extension is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface trench suction our geodynamic model that uses the sp op fixed setup is comparable to the calabria subduction zone with respect to subduction kinematics slab geometry trench curvature and accretionary configuration furthermore the model can explain back arc and fore arc extension at the calabria subduction zone since the latest middle miocene as a consequence of subduction of the narrow calabrian slab and the immobility of the subducting african plate and overriding eurasian plate this setting induced strong trench suction driving fore arc extension and forced subduction to be accommodated almost entirely by slab rollback not trenchward subducting plate motion while trench retreat was accommodated almost entirely by back arc and fore arc extension not trenchward overriding plate motion comparable to our sp op fixed model

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

radiogenic isotopes in fluid inclusions radiogenic isotopes studied in fluid inclusions are still a limited field with great potential for expansion as analytical techniques improve the main limitation for sr ar and he isotope work is the very small number of radiogenic atoms produced in a typical fluid inclusion the requirements to analysts are correspondingly high examples show that isotopic tracing on fluid inclusion fluids can be a decisive tool in solving geological problems

migmatization by metamorphic segregation at subsolidus conditions implications for nd pb isotope exchange abstract we have carried out major and trace element analyses as well as nd and pb isotope measurements on a stromatic migmatite in the phikwe complex botswana in order to assess the effect of high grade metamorphic event leading to migmatization on apparent nd model ages the phikwe complex considered as an entity of the central zone of the limpopo belt underwent a high grade metamorphic event at 2 0 ga leading to migmatite formation migmatites in this area present a very good layering of migmatitic components leucocratic layers were segregated from mafic layers dominated either by hornblende or biotite on a centimeter to millimeter scale accessory minerals such as monazite apatite and zircon were not identified in hornblende rich and quartzo feldspathic layers the combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and or hornblende dehydration reactions or by metamorphic segregation at subsolidus conditions ree modeling applying batch fractional and disequilibrium melting equations allows to discriminate between these two processes the modeled ree results of the quartzo feldspathic layers match those observed via metamorphic segregation at subsolidus conditions the end values at 2 0 ga range from 0 3 to 8 9 with lower values in the ree rich migmatitic layers thus the distribution of major and trace elements as well as the scatter nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components the positive end value of hornblende rich layers suggests hornblende crystallization from a mantle derived source and sm nd fractionation at protolith formation age apparent nd model ages ranging from 2 3 to 3 1 ga do not reflect the crustal evolution in southern africa in archean to proterozoic time based on the geochemical features and the pb pb wr isochron at 2 0 ga mass conservation during migmatization can be assumed therefore chemical and nd isotopic compositions of the protolith prior to migmatization can be calculated the results indicate a si and k poor diorite as protolith moreover calculated protolith compositions yield a two point sm nd isochron indicating an age of 2 7 ga and very similar nd model ages of 2 73 2 75 ga those ages constrain the timing of an archean crust formation episode in the phikwe complex

mesozoic extensional structures of the fangshan tectonic dome and their subsequent reworking during collisional accretion of the north china block the fangshan area sw of beijing lies at the junction of the nne trending taihang mountain range and the southeastern portion of the yanshan intraplate orogenic belt and has undergone at least five stages of deformation mid to late triassic extensional deformation d 1 is represented by the formation of the fangshan tectonic dome during se directed extensional tectonics this deformation was later modified by nnw directed thrusting in the late triassic d 2 and wnw directed thrusting in the late jurassic d 3 these d 1 d 3 structures were deformed by the arcuate beiling syncline d 4 which formed during the intrusion of the 133 128 ma fangshan pluton d 5 deformation is represented by cretaceous to quaternary nne striking high angle normal faults the ages of these events demonstrate that the principal ene trending tectonic framework of the yanshan intraplate orogenic belt was established mainly in the mid to late triassic the indosinian event the jurassic to cretaceous yanshanian deformation with thick skinned thrusting was associated with intrusion of numerous plutons and strongly modified the older structures the tectonic evolution of the fangshan tectonic dome was linked genetically with the collision of the north china block and the siberian craton and the later collision of the north and south china blocks

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

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

flat subduction dynamics and deformation of the south american plate insights from analog modeling received 14 june 2007 revised 13 january 2008 accepted 12 march 2008 published 21 june 2008 1 we present lithospheric scale analog models investigating how the absolute plates motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction possibly resulting in the appearance of flat slab segments and how it changes the overriding plate tectonic regime experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab slab flattening is a long term process 10 ma which requires the subduction of hundreds of kilometers of buoyant plateau the overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate the increase of the overriding plate topography close to the trench results from 1 the buoyancy of the plate subducting at trench and 2 the overriding plate shortening experiments are compared to the south american active margin where two major horizontal slab segments had formed since the pliocene along the south american subduction zone flat slab segments below peru and central chile nw argentina appeared at 7 ma following the beginning of buoyant slab segments subduction in northern ecuador and northern chile the process of slab flattening resulting from the carnegie and iquique ridges subductions respectively seems to be active but not completed the formation of flat slab segments below south america from the pliocene may explain the deceleration of the nazca plate trenchward velocity citation espurt n f funiciello j martinod b guillaume v regard c faccenna and s brusset 2008 flat subduction dynamics and deformation of the south american plate insights

high pressure ultrahigh pressure eclogites from the hong an block east central china geochemical characterization isotope disequilibrium and geochronological controversy the hong an block western dabieshan exposes a series of hp uhp metamorphic rocks with a s to n distribution from blueschist greenschist kyanite free to kyanite and coesite bearing eclogites the available age data are inconclusive that hinder our understanding of the tectonic evolution of the block the metamorphic temperatures in the hong an block tmeta 700 to 500 c are lower by 50 150 c than that of the dabie and sulu terranes in this work we undertook new trace element and sr nd o isotopic analyses on minerals in order to gain more insight into the geochronological problems the results are as follows 1 trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium and the presence of high temperature lree rich mineral inclusions e g epidote in garnet and omphacite has contributed to isotope disequilibrium 2 sm nd isotope analyses yielded no isochron ages for the hong an eclogites 3 rb sr isotope analyses gave mixed results in some cases coexisting minerals are completely out of isotope equilibrium and in others isochron relationship is established yielding ages from 210 ma to 225 ma the pattern of rb sr isotope disequilibrium appears to be independent of the petrological and o isotope temperatures 4 in contrast to the unequilibrated sm nd isotopic systems oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near equilibrium oxygen isotope temperatures are comparable with petrological temperatures however this is an apparent feature due to mass balance constraints 5 whole rock 18o values show a large variation from 10 to 8 suggesting that their protoliths have undergone very different processes of water rock interaction in view of the overall geochronological information we conclude that the hp uhp metamorphism in the hong an block took place in the triassic at about 220 230 ma as observed in the dabie and sulu terranes the significance of published paleozoic dates 450 300 ma for the xiongdian eclogite is not clear however any hypotheses advocating two periods of uhp metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data

regional ultrahigh pressure coesite bearing eclogitic terrane in central china evidence from country rocks gneiss marble and metapelite the dabie mountains represent a fault bounded coesite bearing eclogite gneissic terrane within the collisional belt between the sino korean and yangtze cratons the following geologic evidence indicates that this terrane has been subjected to regional ultrahigh pressure metamorphism 1 the country rocks have a metamorphic pressure temperature evolution compatible with that of the eclogites on the basis of the occurrence of quartz pseudomorphs after coesite calcite pseudomorphs after aragonite relict high pressure assemblages in different rock types of the country rocks and consistent comparable compositional zonations of garnets in both country rocks and eclogites 2 peak metamorphic pressure temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the dabie mountains and clockwise pressure temperature paths are common for eclogites over the region 3 layered eclogites interbedded with gneisses and amphibolites contain omphacite kyanite and garnet indicating a minimum pressure of 20 kbar at 600 c for formation of these rocks

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

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

crustal evolution of the north qinling terrain of the qinling orogen china evidence from detrital zircon u pb ages and hf isotopic composition abstract the north qinling terrain reveals a different geological history from the north china block to the north and the south qinling micro continental block to the south the timing of events and geochemical features of the north qinling terrain are essential for understanding the evolution of the qinling orogenic belt that joins the north and south china blocks this study presents detrital zircon ages and hf isotopic compositions from the guandaokou group a cover sequence of the north china block and from the kuanping group an upper basement sequence of the north qinling terrain the youngest detrital zircon u pb ages constrain the initiation of sedimentation in the guandaokou group to 1 85 ga and the maximum depositional age of the kuanping group to 640 ma zircon u pb ages and hf data testify to a major phase of magmatism in the north qinling terrain at 1000 900 ma the magmatic rocks were largely produced from reworking of early paleoproterozoic 2 5 2 0 ga crust with minor contribution of late paleoproterozoic 1 7 ga juvenile material this age record differs in time from the two important thermal events of 1 85 ga and 2 5 ga widely manifested in the north china block the north qinling terrain can thus be regarded as a separate micro continent with a pre neoproterozoic evolution different from that of the north china block the final assembly of the north qinling terrain to the north china block took place after about 640 ma

extensional compressional tectonics associated with the aegean arc comparison with the andean cordillera of south peru north bolivia extensional compressional tectonics are analysed along the convergent boundaries of the aegean arc and of the andean cordillera of south peru and north bolivia this paper deals only with deformations of miocene pliocene and pleistocene ages the prevailing regime in the aegean basin is extensional yet two compressional tectonic phases of u miocene l pliocene and of l pleistocene age have been recorded they are also known in many places all around the mediterranean thus forces that induce compressional tectonics in the aegean seem to have their origin outside the region they seem to be linked with changes in the conditions of convergence change in rate between the arabo african and eurasian plates beneath the andean cordillera of south peru north bolivia the subduction zone plunges at an angle of 25 30 as it does under the aegean arc there as in the aegean sea tectonics are predominantly extensional and several short lived compressional phases break up this extensional regime the latter may also be induced by changes of the conditions of convergence here between the nazca and south american plates beneath central peru north of the nazca ridge the subduction zone plunges at a small angle about 10 there neotectonic deformations are different in the recent they are essentially compressional or expressed by strike slip faulting geological data from the aegean and andes seem to demonstrate that the compressional deformation in the crust above subduction zones is strongly controlled by the stress conditions along convergent boundaries

causes of compression and extension behind trenches abstract three factors have hampered attempts to find the causes of compression and extension within overriding plates of subduction zones 1 1 adjacent subduction zones are often similar in many subduction parameters 2 2 proposed causal variables are themselves correlated and 3 3 physical models are limited by uncertainties in physical quantities and complexity of the subduction process nevertheless modern subduction zones provide a rich source of observations that help to distinguish among models we consider ten variables or combinations of variables which may control strain regime behind trenches convergence rate vc slab age as shallow slab dip ds absolute motion voa asthenospheric flow slab pull force as vc as voa vc ds voa and vc ds as ten observations are used to test these models a key prerequisite to these tests is the semiquantitative classification of the strain regime of each modern subduction zone into one of seven strain classes from active back arc spreading class 1 through active folding and foreland thrusting class 7 statistical analysis indicates that the modern variance in strain classes is not adequately accounted for by any single or dual variable model but the combination of vc ds and either asorvoa can account for 77 of this variance r 0 88 rapid changes in strain class both with time and along a subduction zone are similarly more consistent with the 3 variable models the role of vc is also supported by maximum compression orientations parallel to vc the role of ds is also supported by the tendency for oceanic overriding plates to be more extensional than continental overriding plates two factors suggest that the vc ds as model is more likely than vc ds voa 1 1 the dominance of one or more of the variables vc as andds in other subduction relations slab length earthquake moment trench relative depth deep dip and arc trench gap and 2 2 the correlation between strain class and maximum earthquake moment which is affected by vc and as however no confident discrimination between the two 3 variable models is yet possible

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

differential subduction and exhumation of crustal slices in the sulu hp uhp metamorphic terrane insights from mineral inclusions trace elements u pb and lu hf isotope analyses of zircon in orthogneiss based on new evidence the sulu orogen is divided from south east to north west into high pressure hp crustal slice i and ultrahigh pressure uhp crustal slices ii and iii a combined set of mineral inclusions cathodoluminescence images u pb shrimp dating and in situ trace element and lu hf isotope analyses was obtained on zircon from orthogneisses of the different slices zircon grains typically have three distinct domains that formed during crystallization of the magmatic protolith hp or uhp metamorphism and late amphibolite facies retrogression respectively i oscillatory zoned cores with low pressure lp mineral inclusions and th u 0 38 ii high luminescent mantles th u 0 10 with hp mineral inclusions of qtz grt arg phe ap for slice i zircon and coe grt phe kfs ap for both slices ii and iii zircon iii low luminescent rims with lp mineral inclusions and th u 0 08 zircon u pb shrimp analyses of inherited cores point to protolith ages of 785 770 ma in all seven orthogneisses the ages recorded for uhp metamorphism and subsequent retrogression in slice ii zircon c 228 and c 215 ma respectively are significantly older than those of slice iii zircon c 218 and c 202 ma respectively while slice i zircon recorded even older ages for hp metamorphism and subsequent retrogression c 245 and c 231 ma respectively moreover ar ar biotite ages from six paragneisses interpreted as dating amphibolite facies retrogression gradually decrease from hp slice i c 232 ma to uhp slice ii c 215 ma and uhp slice iii c 203 ma the combined data set suggests decreasing ages for hp or uhp metamorphism and late retrogression in the sulu orogen from south east to north west thus the hp uhp units are interpreted to represent three crustal slices which underwent different subduction and exhumation histories slice i was detached from the continental lithosphere at 55 65 km depth and subsequently exhumed while subduction of the underlying slice ii continued to 100 120 km depth uhp before detachment and exhumation slice iii experienced a similar geodynamic evolution as slice ii however both uhp metamorphism and subsequent exhumation took place c 10 myr later magmatic zircon cores from two types of orthogneiss in uhp slices ii and iii show similar mid neoproterozoic crystallization ages but have contrasting hf isotope compositions ehf 785 2 7 to 2 2 and 17 3 to 11 1 respectively suggesting their formation from distinct crustal units mesoproterozoic and paleoproterozoic to archean respectively during the breakup of rodinia the uhp and the retrograde zircon domains are characterized by lower th u and 176 lu 177 hf but higher 176 hf 177 hf t than the neoproterozoic igneous cores the similarity between uhp and retrograde domains indicates that late retrogression did not significantly modify chemical and isotopic composition of the uhp metamorphic system

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

satellite laser ranging and geological constraints on plate motion we have analyzed satellite laser ranging slr observed baseline rates of change and compared them with rates determined from sea floor spreading rates and directions and earthquake solutions with the number of years of observation now over six for many of the baselines the inaccuracy of determining baseline rates of change has diminished so that in some cases it is less than a few mm per year thus a direct comparison between baseline rates of change and rates of change established using geophysical information which we call geological rates is now feasible in most cases there is good agreement between the rates determined from slr and geological rates but in some cases there appear to be discrepancies these discrepancies involve many of the data for which one end of the baseline is either quincy california huahine french polynesia or simosato japan we have devised a method for looking at the discrepancies for these slr observatories which allows us to calculate the motion not modelled by the geologic information the results are discussed in terms of what is known about plate margins and other information

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe 40 ar 39 ar technique ultra high pressure eclogite amphibolite grade metamorphism of the dora maira massif in the western alps is a well established and intensively studied event however the age of peak metamorphism and early cooling remains controversial the 40ar 39 ar step heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 ma which have been interpreted as the time of early cooling after the high pressure event recent u pb and sm nd results challenge this assertion indicating a much younger age for the event around 45 ma and hence a radically different timing for the tectonic evolution of the western alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39 ar dating the new technique allowed selective in situ analysis at a spatial resolution of 50 urn of quartz phengite biotite and k feldspar the results demonstrate the frequent occurrence of excess argon with high 40ar 39 ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios step heating analysis of minerals with this mixture would produce linear arrays on a40ar 36ar versus 40ar 36 ar correlation diagram leading to geologically meaningless plateau ages older than the true closure age in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 36 ar ages in many high pressure terrains

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

zircon u pb and k feldspar megacryst rb sr isotopic ages and sr hf isotopic composition of the mesozoic heyu pluton eastern qingling orogen china abstract mesozoic granitoids are ubiquitous in china and have attracted the interests of many geologists not only because of their mineralization potential but also because they carry information on source material emplacement mechanisms and crustal architecture most of the plutons were created by multi stage magma emplacement and offer particular challenge for unraveling the emplacement and cooling history a multi isotope approach combined with single grain dating analyses provides a means to understand magma geneses even in complex magmatic settings this study presents rb sr isotope data for k feldspar megacrysts and u pb ages and hf isotopic composition for zircons from the composite heyu pluton exposed in eastern qinling orogen central china zircon u pb concordia ages of 150 and 140 ma and single k feldspar rb sr isochron ages between 133 and 121 ma were obtained from early and late magmatic stages respectively consistent with slow cooling or isotopic re equilibration of the rb sr k feldspar system the distinct k feldspar morphology of early and late intrusive phases is controlled largely by the emplacement depth likely to be associated with a gravity driven sinking model corresponding zircon e hf t values are 21 8 to 18 0 and 17 2 to 10 3 for early and late intrusive phases respectively indicating different magma sources

thermochronological constraints on two stage extrusion of hp uhp terranes in the dabie sulu orogen east central china along the qinling dabie sulu orogenic belt in china crops out the world s largest terrane composed of ultrahigh pressure uhp metamorphic rocks differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high pressure hp and uhp slices in different parts of the belt the western part of the dabie orogen western dabie terrane holds a key to understanding of the transition from oceanic to continental subduction this paper reports geochronological results to test a two stage tectonic model for the exhumation of hp uhp rocks in western dabie this model involves two different stages and types of extrusion for exhumation of the hp uhp rocks in east central china mica ar ar ages ranging from 241 to 231 ma indicate a general middle triassic cooling probably driven by early upward extrusion during the collision between the north and south china blocks late triassic early jurassic cooling was associated with later eastward extrusion ranging from 200 to 184 ma the second event is recorded also in mica in the region that was not affected by later deformation and magmatism the lateral movement along lithosphere scale faults resulted in the eastward extrusion of the hp uhp metamorphic terrane which was followed in the late triassic early jurassic time by a major compressive event these two extrusion events are correlative with the two stages of triassic exhumation of the western dabie hp uhp rocks respectively wintin the framework of the qinling dabie sulu orogenic belt it is suggested for western dabie that the subduction exhumation of blueschist facies unit is related to the mianlue suture whereas the subduction exhumation of hp uhp eclogite facies units is related to the shangdan suture

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

zircon u pb age and o isotope evidence for neoproterozoic low 18o magmatism during supercontinental rifting in south china implications for the snowball earth event identification of neoproterozoic low 18 o igneous rocks is a key to demonstrate that high t water rock interaction and low 18 o magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the earth s interior to exterior at the onset of the snowball earth event low 18 o values of 10 9 to 4 4 permil characteristic of meteoric origin were observed on an areal extent over 30 000 km2 for mid neoproterozoic metaigneous zircons from the ultrahigh pressure eclogite facies metamorphic belt of triassic ages along the dabie sulu orogenic belt china although the zircon o isotope system could be reset to acquire its low 18 o values by exchange with matrix minerals during wet eclogite facies metamorphism the abnormal 18 o depletion is confirmed by occurrence of neoproterozoic low 18 o zircons in low grade metaigneous rocks in the same belt cathodoluminscence imaging of zircons from low grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin hornblende from two granites of low 18 o zircons gave ar ar plateau ages of 747 6 and 776 12 ma ruling out that phanerozoic metamorphism effected mineral o isotope values two groups of u pb ages were measured at 756 5 ma and 782 5 ma respectively for the low 18 o zircons this indicates that low 18 o zircons crystallized during two episodes of low 18 o magmatism in response to tectonic evolution from supercontinental rifting at 780 ma to breakup at 750 ma the second episode of low 18 o magmatism is contemporaneous with the continental kaigas iceage a plate rift model is advanced to account for the tectonic connection between the low 18 o magmatism the continental glaciation and the supercontinental rifting during the cryogenian period high t meteoric water rock interaction occurred prior to low 18 o magmatism in rift tectonic zones and caldera collapse was responsible for melting of hydrothermally altered low 18 o rocks during rifting of the south china block from the rodinia supercontinent

seismic structure in southern peru evidence for a smooth contortion between flat and normal subduction of the nazca plate rapid changes in slab geometry are typically associated with fragmentation of the subducted plate however continuous curvature of the slab is also possible the transition from flat to normal subduction in southern peru is one such geometrical change the morphology of the subducted nazca plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry the presence of a thin ultra slow velocity layer usl atop the horizontal nazca slab is identified and located the lateral extent of this usl is coincident with the margin of the projected linear continuation of the subducting nazca ridge implying a causal relationship wherein increased hydration of the ridge results in the formation of the usl downdip waveform modelling of the 2 d structure in southern peru using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab the seismicity and structural evidence suggests smooth contortion of the nazca plate along the transition from flat to normal subduction the slab is estimated to have experienced 10 per cent strain in the along strike direction across this transition

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

neoproterozoic granitoid did not record ultrahigh pressure metamorphism from the southern dabieshan of china abstract it has been often debated whether all granitic gneisses associated with coesite bearing eclogites in southern dabieshan china have also been subjected to ultrahigh pressure uhp metamorphism we show here that a metagranitoid adjacent to the bixiling eclogite ultramafic complex has preserved primary granitic textures and an igneous mineral assemblage of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm biotite mathrm plagioclase mathrm microcline mathrm quartz mathrm allanite pm mathrm amphi

protolith control on fluid availability for zircon growth during continental subduction zone metamorphism in the dabie orogen abstract different episodes of zircon growth are recognized by a combined study of cl images mineral inclusions u pb ages trace elements and lu hf isotopes for ultrahigh pressure uhp eclogite facies metamorphic rocks in the dabie orogen the results provide insights into the effect of protolith property on fluid liberation during continental collision fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth zircon u pb dating for uhp metabasalt and metasediment eclogite and its host paragneiss yields two groups of ages at 244 3 and 225 2 ma respectively mineral inclusion trace element and lu hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation respectively in contrast zircon u pb dating for uhp metaintrusive rocks granitic orthogneiss and its hosted eclogite gave only one group of age at 222 2 ma mineral inclusion trace element and lu hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation the difference between the two groups of zircon u pb dates is attributed to the difference in their protolith origin volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water pore fluid so that considerable amounts of aqueous fluid can be released from them during subduction this episode of fluid action was recorded by the growth of anatectic zircon at 244 ma in contrast intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals so that little fluid can be released from them during subduction nevertheless large amounts of retrograde fluid were released from uhp metamorphic rocks regardless of their protolith origin during decompression exhumation this episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220 225 ma therefore the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction which has great bearing on partial melting element transport and mineral growth during continental subduction zone metamorphism

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

petrology geochemistry and isotope data on a ultrahigh pressure jadeite quartzite from shuanghe dabie mountains east central china abstract in the dabie ultrahigh pressure terrane of east central china coesite bearing jadeite quartzites occur locally as intercalated layers with marble and mafic eclogite this rock assemblage is in turn enclosed within quartzofeldspathic gneisses metamorphic parageneses and kelyphitic textures reveal a multistage metamorphic evolution and complex exhumation history the primary peak metamorphic assemblage consists of jadeite garnet coesite rutile apatite minor coesite and coesite pseudomorphs occur as inclusions in jadeite and garnet three stages of retrograde assemblages are observed in the jadeite quartzites stage a is represented by the polymorphic transformation of coesite to quartz aggregates stage b is characterized by formation of coronas around jadeite porphyroblasts consisting of an inner layer of oligoclase amphibole and an outer layer of albite aegirine augite the last stage stage c involved total replacement of jadeite and most garnets by taramitic amphibole albite aegirine augite peak metamorphic p t conditions were 26 kbar at 660 c and are consistent with the estimates from the adjacent coesite bearing eclogites the jadeite quartzites display clockwise p t path that matches those of the adjacent eclogites major and trace element data suggest that the protolith of the jadeite quartzite could have been an albitized siltstone enriched in na and depleted in k and ca the highly negative present day end value 24 7 indicates a very old age for the protolith its late archean model age tdm of 2 58 ga is among the oldest so far identified for rocks from the dabie uhpm terrane concordant field relations and petrogenetic considerations suggest that all mafic politic carbonate and gneissic rocks have experienced in situ uhp metamorphism during triassic continental collision between the sino korean and yangtze cratons

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

fine scale isotopic heterogeneities and fluids in the deep crust a40ar 39ar laser ablation and tem study of muscovites from a granulite eclogite transition zone spectacular exposures on holsnoy island western norway reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids stepwise and laser40ar 39ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy tem analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling muscovites with 0 10 0 17na na k yield40ar 39ar plateau and integrated ages in the range 450 465 ma 20 35 myr older than40ar 39ar muscovite plateau ages from adjacent eclogite samples laser40ar 39ar analyses on 75 m spots across two single muscovite grains from these samples are approximately uniform at 444 2 1 9 21 spots and 443 3 1 7 ma 26 spots however incremental heating of high sodic 0 18 0 32 na na k muscovite failed to yield a plateau laser40ar 39ar analyses of 100 areas from two single grains from this sample reveal complex two dimensional patterns with variable apparent ages in the range of 445 625 ma electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to 001 all the tem bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase yet several of the selected area electron diffraction saed analyses document locally intergrown muscovite and sodic muscovite possibly in an exsolution relationship on a unit cell scale analytical electron microscopy aem analyses obtained over a 5 100 nm scale on the same sample reveal great range and heterogeneity inna k including pure muscovite although no separate paragonite domains were found in contrast muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inna k at the microprobe level these data provide evidence of variable fluid mediated incorporation of excess40ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions the data indicate that fine scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface despite the influx of free fluids in the rocks at high pressure the heterogeneities are best reconciled with local buffering and limited mobility of the fluids at least during the final stages of equilibration

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

origin ascent and oblique emplacement of magmas in a thickened crust an example from the cretaceous fangshan adakitic pluton beijing abstract the fangshan pluton in the western hills of beijing records a history of emplacement from primarily vertical ascent to oblique injection along a detachment fault thus the pluton provides a classic example answering the room problem this pluton and the nanjiao apophyse were produced by cretaceous magmatism that formed the regionally extrusive east china mesozoic igneous province the fangshan pluton includes two intrusive phases a 136 ma monzodiorite phase intruded by a 131 ma quartz monzodiorite phase rocks from both phases have chondrite normalized ree patterns enriched in lree and depleted in hree they have primitive mantle normalized trace element patterns with distinct negative nb and ti anomalies and high sr 1067 1348 ppm and low y 8 17 ppm with high sr y ratios 71 137 these features are characteristic of adakites and suggest that the magmas were derived by partial melting of the lower part of a thickened continental crust the pluton has magmatic foliations and lineation indicative of the intrusion of the earlier magmas slightly obliquely from se to nw the magmatic fabrics have enclave ratios rfmag rp of 3 0 3 5 asymmetric solid state deformational fabrics and strains rs for the northwestern 3 5 4 0 and southeastern parts 1 5 2 0 again suggest a later oblique injection of magmas from se to nw both the fangshan pluton and nanjiao apophyse suggest that magmas migrated most probably along the pre existing detachment fault such an emplacement is also supported by the aeromagnetic polarization data and strong deformation of the beiling syncline thus magma emplacement in the fangshan pluton probably involved creation of space by both faulting and wallrock strain

isotopic disequilibrium in ultrahigh pressure and retrograde metamorphism of eclogite and gneiss from the chinese continental scientific drilling in the sulu orogen china evidence from mineral nd sr o isotopic composition diffusion rates of sr and o in minerals are often comparable while nd has a lower diffusion rate during thermal overprint s thus the o isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of rb sr and sm nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism this study presents a combination of investigation on sm nd rb sr and o isotopic compositions of minerals separated from ultrahigh pressure eclogite and gneiss that were collected from the main hole of the chinese continental scientific drilling project located in the sulu orogen eastern china oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620 740 and 460 590 c representing diffusion cessation of isotopic exchange during the eclogite facies recrystallization and later amphibolite facies retrograde overprint rb sr mineral regressions of two eclogite samples give consistent triassic ages of 244 ma corresponding to eclogite facies metamorphism while the same minerals do not yield meaningful sm nd isochron ages this phenomenon likely suggests that rb sr isotopic equilibrium was achieved during eclogite facies metamorphism and preserved during late amphibolite facies retrogression in contrast sm nd isotopic equilibrium between the minerals of eclogite was not achieved under uhp metamorphic conditions regressions of epidote and biotite of one gneiss sample give a triassic sm nd age of 243 34 ma corresponding to the time of the eclogite facies metamorphism and a jurassic mineral rb sr age of 187 5 1 8 ma these results imply that fluids have played an important role to achievement of the sm nd isotopic equilibrium during eclogite facies metamorphism and re equilibration of the rb sr isotopic system during later retrograde overprint

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

stress fields of the overriding plate at convergent margins and beneath active volcanic arcs tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales volcanic arcs are a common element of overriding plates stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc a process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region

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

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene