multi stage exhumation of young uhp hp rocks timescales of melt crystallization in the d entrecasteaux islands southeastern papua new guinea abstract on the d entrecasteaux islands southeastern papua new guinea png a series of domes expose the youngest known ultrahigh pressure uhp to high pressure eclogites and associated gneisses abundant evidence of partial melting is preserved throughout the d entrecasteaux domes in the form of variably deformed leucosomes dikes and plutonic bodies in order to understand 1 the timing of melt emplacement relative to the uhp metamorphism and 2 the complete exhumation and deformational history of the eclogites and surrounding gneisses a series of leucosomes dikes and eclogites from the mailolo dome in western fergusson island were analyzed by u pb id tims zircon geochronology zircons extracted from the eclogite at the uhp locality yield 206pb 238u tims dates from 5 60 0 22 to 4 61 0 18 ma zircons from three foliation parallel leucosomes are interpreted to record melt crystallization from 3 49 0 01 ma to 2 98 0 02 ma in comparison weakly deformed granitic granodioritic dikes that cut the leucosomes yield zircon 206pb 238u dates of 2 48 0 01 ma 2 420 0 002 ma based on these new and previously published results the eclogites may have undergone uhp metamorphism for several million years from 7 1 0 7 ma to 4 61 0 18 ma based on tims dates alone a more restricted interval of 5 6 4 6 ma is proposed following uhp metamorphism the eclogite and gneisses were exhumed to lower crustal depths intruded by foliation parallel leucosomes by ca 3 5 3 0 ma and by ca 2 45 ma late discordant dikes multiple generations of leucosomes and dikes allow a timeline for the exhumation of the png uhp terrane to be constructed mantle 100 km depths to the base of the crust in 1 my storage in the lower crust for 1 my and final emplacement into the shallow crust 1 my later

40ar 39ar geochronological constraints on the age and evolution of the permo triassic emeishan volcanic province southwest china abstract the biostratigraphically constrained permo triassic emeishan volcanic province evp extending over wide areas in southwest china has been recently considered as a large igneous province contemporaneous with the siberian traps and the siliceous tuffs at the p t boundary in south china we report the first 40ar 39ar ages on this igneous province minimum ages have been obtained on phenocrystic plagioclase of the emeishan basalt which has undergone a pervasive metamorphism most likely during subsequent tectonization as a consequence of terrane amalgamation comparison between the ar release spectra obtained on clear vs cloudy plagioclase indicates a 40 30 ma sericite resetting time a minimum apparent age of 246 4 ma for plagioclase from a plagiogranite a late differentiate of the panzhihua layered complex and an age of 254 5 ma for phlogopite from a pyroxenite near lake erhai provide the first absolute age constraint on this igneous province additional ar ar age measurements on post emeishan alkaline and mafic magmatism yielded 104 2 and 100 2 ma for an alkaline complex near panzhihua and 42 1 ma for a gabbro sill emplaced near the ertan dam further study is still needed to determine the age of the emeishan volcanic emission accurately and to test the validity of the assumed short duration of the eruption

variation of ti in quartz in gneiss domes exposing the world s youngest ultrahigh pressure rocks d entrecasteaux islands papua new guinea 1 the d entrecasteaux island gneiss domes papua new guinea expose the world s youngest ultrahigh pressure rocks 5 8 ma which were exhumed nearly isothermally to the mid crust at rates of cm yr 1 because quartz grains are internally distorted by deformation bands and have strongly interlobate grain boundary microstructures indicative of rapid grain boundary migration we infer that ti content in the quartz was frozen in by dynamic recrystallization that accompanied the continued deformation of the gneisses as they were exhumed through the crust toward the surface we infer that average ti content of an assemblage of quartz grains in a sample depends on the ratio of cooling rate to dynamic recrystallization rate an increase in the former preserves and captures a larger fraction of relatively high ti high temperature grains whereas an increase in the latter removes such relict grains and replaces them with a newer set of lower ti low temperature recrystallized grains based on this concept we use ti in quartz as a tool to infer domains of differing relative cooling rate across four d entrecasteaux island domes ti concentrations in quartz grains from 89 samples of quartzofeldspathic gneiss and eclogite increase inwardly away from the dome margins 2 5 20 ppm toward the center of the domes 20 100 ppm from this we infer that the most rapidly cooled rocks were exhumed near the center of the domes our data reinforces structurally based arguments that the symmetric domes were emplaced vertically into the crust as diapirs rather than laterally exhumed as a result of large magnitude slip on dome bounding detachment faults

origin of eclogite bearing domed layered metamorphic complexes core complexes in the d entrecasteaux islands papua new guinea compositionally layered metamorphic rocks of the d entrecasteaux islands papua new guinea are folded into domes and antiforms bounded by faults parallel to metamorphic layering and foliation the structures are broadly similar to the metamorphic core complexes of western north america lenses of ultramafic rock lie on the bounding faults and the same faults have served as loci for quaternary andesitic volcanic activity metamorphic grade in the northern islands goodenough and fergusson is amphibolite facies with pockets of eclogite fergusson island only and granulite and is greenschist facies in the southern island normanby in all three islands there is a characteristic tectonostratigraphic sequence fmu sequence from felsic metamorphic rocks at base or internally through mafic metamorphic rocks to ultramafic rocks at top or externally the association of metamorphic and ultramafic rocks apparently developed in a north dipping paleogene subduction system and was exhumed to upper crustal level in the oligocene early miocene possibly by reversal of movement on faults in the former subduction system vigorous uplift and development of domes and antiforms in the pliocene was triggered by westward propagation of the woodlark basin spreading ridge and was accompanied by rifting rift related magmatism rapid erosion and deposition of coarse sediment in the adjacent trobriand basin

late permian triassic magmatic evolution in the jinshajiang orogenic belt sw china and implications for orogenic processes following closure of the paleo tethys growth of continental crust involves the complex interplay of subduction zone magmatism to generate the crust followed by stabilization through crustal thickening magmatism and ultimately isolation from the active plate margin the jinshajiang orogenic belt sw china provides an exceptional record of continental development as the result of closure of the paleo tethys seaway and ensuing collision a compilation of u pb age and geochemical data for the plutonic and volcanic rocks within the southern part of the jinshajiang orogenic belt including new high precision ages for the ludian granitoid batholith of 231 to 220 ma enables us to explore the interaction between magmatism and orogeny in the context of the paleo tethys closure and continental amalgamation these age and geochemical constraints in conjunction with other geologic evidence suggest that subduction of the paleo tethys ocean dominated local tectonics prior to the triassic creating a volcano plutonic arc along the eastern margin of the qamdo simao terrane following consumption of the ocean collision zone magmatism dated at 247 to 237 ma was manifested by eruption of voluminous volcanic rocks in a suture parallel zone crustal anatexis was contemporaneous with the earliest phases of collision producing high silica rhyolites of early triassic age ca 247 246 ma between 245 and 237 ma the local tectonic regime switched from compression to extension probably due to strain partitioning caused by oblique convergence which led to the development of rift basins and extensive syn tectonic bimodal volcanism associated with deep water sediments from 234 ma to 214 ma the emplacement of high k calc alkaline granodiorites monzogranites occurred prior to or during isostatic uplift and extension probably caused by breakoff of the subducted slab the resultant exhumation brought deep seated granitoid batholiths to the surface and was contemporaneous with intrusion of ultramafic mafic melts ophiolitic melange ca 362 294 ma and collision related magmatic suites 247 214 ma are unconformably overlain by a late triassic 229 217 ma conglomerate rich sequence that represents an overlap assemblage across the qamdo simao terrane indochina and yangtze block of south china

eduction extension and exhumation of ultrahigh pressure rocks in metamorphic core complexes due to subduction initiation the controversy over the exhumation of ultrahigh pressure uhp rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes eduction i e reverse subduction we present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together crust subducted deep enough is heated and weakened causing limited diapiric rise while crust subducted to shallower depths retains strength and is exhumed only by eduction the model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence this occurs spontaneously when subduction of buoyant crust causes a subduction zone to lock up in one place causing a new subduction zone to form in another the model is consistent with many features of the youngest region of uhp rock exhumation on earth the d entrecasteaux islands uhp exhumation and the amount of regional extension as well as the seismic structure around the islands can be explained by eduction ductile flow fabrics seen on the islands would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting reversal of motion on the north dipping continental subduction zone required by this model was likely triggered by initiation of the new britain trench as suggested previously our model implies that the crust of goodenough basin south of the islands was exhumed by eduction in the last 5 ma and this hypothesis can be tested by drilling

exhumation and doming of the thasos metamorphic core complex s rhodope greece structural and geochronological constraints abstract the thasos island metamorphic core complex s rhodope n aegean sea was generated between 26 and 13 ma progressive extensional tectonics was associated with upwarping of the deeper parts of this core complex to a narrow gneiss dome and with formation of a steep topographic high we present a combined structural microstructural and geochronological study that constrains the deformation time path of progressive extension and exhumation thasos island consists of three superimposed more or less flat lying metamorphic units with downward decreasing geochronologic ages which are separated by two systems of low angle normal detachment surfaces trending nw se the upper unit stayed in supracrustal level since 50 ma representing the upper plate during an early phase of extension not later than 26 23 ma upper levels of the core complex intermediate unit underwent pervasive mylonitization with top to the sw shear from about 21 ma onward the intermediate unit was rigid and top to the sw slip continued at least until 13 8 ma on the discrete detachment surface that separates the core complex from the upper plate yet in the 21 15 ma period deeper levels lower unit still deformed plastically and deformation successively concentrated in zones of high strain and in discrete detachment surfaces that caused decoupling of the lower from the intermediate unit these indicate downdip movements in opposite sense associated with doming of the lower unit at that stage simultaneously the intermediate unit was fragmented on high angle normal faults that sole into the dome bounding detachment surface and roof into the core complex bounding detachment surface these faults confine tilt blocks that are arranged around the lower unit gneiss dome doming of the core complex occurred simultaneously with subsidence of basins located on the upper plate

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

strontium isotopes in cenozoic volcanic rocks from southeastern papua new guinea abstract strontium isotope data are presented from 28 rock samples representing four distinct episodes of late cenozoic volcanism in southeastern papua eocene tholeiitic basalts have initial 87sr 86sr ratios 0 7037 which are higher than those in chemically comparable basalts and are thought to have been enhanced by rock sea water interaction late cenozoic are trench type volcanoes in the papuan islands have initial 87sr 86sr ratios which show little variation 0 7041 2 in contrast to those from the contiguous papua new guinea mainland 0 7036 0 7054 this isotopic discontinuity does not appear to be due to contamination by immediately underlying sialic metamorphics high k trachytes in the lusancay islands north of the late cenozoic are have comparable initial 87sr 86sr ratios wide variation 0 703 0 710 in the initial ratios measured in a group of apparently closely related rocks ranging in composition from transitional basalt to peralkaline rhyolite cannot be explained by differences in age or by late magmatic fractionation and continue to pose an enigma

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

zircon u pb geochronology and elemental and sr nd isotope geochemistry of permian mafic rocks in the funing area sw china mafic layered intrusions and sills and spatially associated andesitic basalts are well preserved in the funing area sw china the 258 3 ma layered intrusions are composed of fine grained gabbro gabbro and diorite the 260 3 ma sills consist of undifferentiated diabases both the layered intrusions and volcanic rocks belong to a low ti group whereas the diabases belong to a high ti group rocks of the high ti group have feo tio2 and p2o5 higher but mgo and th nb ratios lower than those of the low ti group they have initial 87sr 86sr ratios 0 706 0 707 lower and nd 1 5 to 0 6 higher than the low ti equivalents 0 710 0 715 and 9 6 to 4 0 respectively the high ti group was formed from relatively primitive high ti magmas generated by low degrees 7 3 9 5 of partial melting of an enriched oib type asthenospheric mantle source the low ti group may have formed from melts derived from an em2 like lithospheric mantle source the mafic rocks at funing are part of the emeishan large igneous province formed by a mantle plume at 260 ma

trace element and sr nd isotopic geochemistry of the pge bearing xinjie layered intrusion in sw china abstract the xinjie layered intrusion is one of a number of major ultramafic mafic bodies hosting fe ti v deposits and cu ni pge sulfide in the pan xi area of the sichuan province sw china the intrusion is a 7 5 km long 1 to 1 5 km wide and 1200 m thick ultramafic mafic sill which is associated with permian plume related emeishan flood basalts the intrusion is characterized by three lithological cycles each representing a sequence from ultramafic to basic felsic composition the basal part of the intrusion is composed of three lithological units namely the marginal peridotite and pyroxenite units two major pge mineralized layers occur within the marginal and pyroxenite units the marginal unit mu is characterized by marked nb ta sr and ti enrichments and obvious th depletion while the peridotite unit peu shows significant sr slight zr depletion and pronounced positive ti anomalies the pyroxenite unit pyu is depleted in nb and zr but significantly enriched in sr and ba with lower ratios of nb la and zr sm compared to those of the underlying two units the pge enriched layer in the marginal unit has elevated and highly variable rb la and ba th values and the lower part of the pyroxenite unit which includes a pge enriched layer exhibits high and decreasing rb la and ba th ratios upwards sr nd isotopic compositions of samples from the three units ca 90 m thick were systematically analyzed and fall in a range of 0 7056 to 0 7074 for 87sr 86sr i and of 5 26 to 2 79 for end i within the marginal and peridotite units high 87sr 86sr i and low end i values are demonstrated by contrast an enriched sr isotopic 87sr 86sr i 0 7074 and slightly depleted nd isotopic end i 2 5 signature occurs in the basal pyroxenite unit 8 m below the pge enriched layer in this unit the marginal unit is interpreted as the product of contamination of a plume component e g end i 5 and 87sr 86sr i 0 704 by continental crust the peridotite unit has undergone decreasing degrees of crustal contamination at the bottom of the pyroxenite unit a new magma input contaminated by upper crust is proposed where its slight nd sr decoupling may be attributed either to the much higher sr content than the nd content of the contaminant or the selective contamination of sr rather than nd the pge mineralization in the pyroxenite unit was caused by magma mixing between the new magma and the resident magma whereas the pge enriched layer in the marginal unit resulted from wallrock contamination however the displacement of the isotopic pulse from the pge enriched layer in the pyroxenite unit is due to later sulfide saturation being achieved in both end members both of the pge enriched layers are linked directly to the segregations of the fe ti oxides and sulfides

longevity of the permian emeishan mantle plume sw china 1 ma 8 ma or 18 ma after the formation of the 260 ma emeishan large igneous province there were two volumetrically minor magmatic pulses at 252 ma and 242 ma respectively alkaline mafic dykes intruding both 260 ma and 252 ma felsic plutons in the panxi region southwestern china have compositions similar to the emeishan flood basalts one dyke is dated using the shrimp zircon u pb technique at 242 2 ma 18 ma younger than the start of emeishan magmatism the dykes have enriched light rare earth element patterns la ybn 4 4 18 8 and trace element patterns similar to the emeishan flood basalts and average ocean island basalts some trace element ratios of the dykes zr nb 3 8 8 2 la nb 0 4 1 7 ba la 7 5 25 6 are somewhat similar to em1 source material however there are differences their end values end 2 6 and 2 7 and i sr i sr 0 704542 and 0 704554 ratios are indicative of a mantle source thus emeishan magmatism may have lasted for almost 20 ma after the initial eruption however geological evidence precludes the possibility that the post 260 ma magmatic events were directly related to emeishan magmatism which began at and ended shortly after 260 ma the 252 ma plutons and 242 ma dykes represent volumetrically minor melting of the fossil emeishan plume head beneath the yangtze crust the 252 ma magmatic event was likely caused by post flood basalt extension of the yangtze crust whereas the 242 ma event was caused by decompressional melting associated with the collision between the south china and north china blocks during the middle triassic

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

emplacement of ophiolite in papua new guinea summary the major ophiolite complexes of papua new guinea lie on the northeastern margin of the australian craton and are flanked externally by palaeogene volcanic arcs the ophiolites are segments of oceanic lithosphere which occupied the forearc zone prior to arc continent collision in the case of the easternmost ophiolite the papuan ultramafic belt there is evidence that emplacement was preceded by development of a secondary rupture in the down going plate at some distance from the subduction zone and was followed by extension buoyant emergence and reversed movement of the ophiolite the westernmost ophiolite april ultramafics has more complex structure and probably developed as a series of thrust sheets in a subduction system the thrust sheets were subjected to renewed deformation and thrusting by continued convergence after arc continent collision

quartz in garnet and ti in quartz thermobarometry methodology and first application to a quartzofeldspathic gneiss from eastern papua new guinea

features and effects of basement faults on deposition in the tarim basin the tarim basin as the largest inland oil producing basin in china is a large scale composite basin which formed from a palaeozoic craton basin that was overprinted by a mesozoic cenozoic foreland basin based on comprehensive analyses of gravitational data aeromagnetic data and seismic data from the basin 38 basement faults can be identified demonstrating that the tarim basin has undergone several tectonic cycles according to their distributional locations in the basin basement faults can be divided into marginal and internal basin basement faults moreover on the basis of the distribution characteristics in the plane and controlling effects on the basin the basement faults can be divided into 20 primary basement faults and 18 secondary basement faults combined with the interpretations of seismic profiles we can determine the relationship between the basement faults and sedimentary cover faults considering the section balance of seismic profiles we find that some basement faults are syn depositional with continuous long term activity during the basin s evolution and control the deposition in the basin by counting fault growth indexes of the main sections it is obvious that different parts of the same fault have different growth indexes but show similar activity within the same geological period the faults stress states remain consistent with their geological history although the growth indexes of different faults are inconsistent

tectonic uplift mechanism of the goodenough and fergusson island gneiss domes eastern papua new guinea constraints from seismic reflection and well data 1 the d entrecasteaux island dei gneiss domes are fault bounded domes with 2 5 km of relief exposing ultrahigh pressure uhp and high pressure hp metamorphic gneisses and migmatites exhumed in an oligocene miocene arc continent collision and subduction zone subject to late miocene to recent continental extension to study the style of continental extension accompanying exhumation of the dei gneiss domes a grid of 1518 km of 2 d multichannel seismic mcs reflection data and well data is interpreted from the offshore areas surrounding the dei including the trobriand basin and the goodenough basin the offshore study is combined with onshore geologic information to constrain the oligocene to recent tectonic evolution of the basins mcs and well data are consistent with the trobriand basin forming as a forearc basin caused by southward miocene subduction at the trobriand trench at 8 ma the margin transitioned to an extensional tectonic environment since then the trobriand basin has subsided 1 2 5 km with few normal faults deforming the basin fill south of the dei the goodenough rift basin developed after extension began 8 ma with the hanging wall of the north dipping owen stanley normal fault bounding the southern margin of the basin the lack of evidence of upper crustal extension accompanying subsidence in the trobriand and goodenough basins suggests depth dependent lithospheric extension from 8 to 0 ma has accompanied uplift of the dei gneiss domes and supports schematic model of uplift of the dei domes involving vertical exhumation of buoyant postorogenic lower crust far field extension from slab rollback and an inverted two layer crustal density structure

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

sedimentation and lithofacies paleogeography in southwestern china before and after the emeishan flood volcanism new insights into surface response to mantle plume activity abstract investigations into permian sedimentation and reconstruction of paleogeography in sw china are aimed at characterizing sedimentary responses to the emeishan mantle plume in addition to erosional features on the sediments underlying the uplifted emeishan basalts unusual depositions of permian age are also present in the emeishan large igneous province lip specifically carbonate gravity flows and submarine incised canyon fillings were developed in the western margin of the postulated uplifted area and rifting trenches were developed along the eastern margin alluvial fan deposits occur at the boundary between the inner and intermediate zones these depositions all rest on the maokou formation and are in turn covered by the emeishan basalts implying synchronism between crustal uplift and depositional events these deposits and the associated extension and normal faulting along the margin of and within the lip represent sedimentary features resulting from dynamic behavior of mantle plume com

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

platinum group element geochemistry of the hongge fe v ti deposit in the pan xi area southwestern china mafic and ultramafic intrusions in the pan xi area along the western margin of the yangtze block southwestern china are spatially associated with the late permian emeishan flood basalts of the emeishan large igneous province the hongge layered intrusion is one of the plutonic bodies of this province and hosts a giant fe v ti deposit this intrusion has three zones a lower olivine clinopyroxenite zone a middle clinopyroxenite zone and an upper gabbro zone each of these zones consists of one or two compositional cycles which have distinct mg values tio2 concentrations and total ree contents the middle clinopyroxenite zone and upper gabbro zone contain thick 14 84 m magnetite mineralized layers samples from the lower olivine clinopyroxenite zone and middle clinopyroxenite zone of the hongge intrusion are enriched in platinum and palladium relative to iridium and ruthenium rocks of the lower olivine clinopyroxenite zone have pd ir ratios 1 8 22 3 lower than those of the middle clinopyroxenite zone 6 2 to 83 in its lower part and 3 6 to 49 in its upper part the pd ir ratios increase progressively upwards in each cyclic unit chromite is the major phase controlling concentrations of iridium and ruthenium the cu pd ratios at the bottom of each cyclic unit are close to that of the mantle and increase upwards this variation suggests that each cyclic unit represents a new batch of magma and sulfide mineral segregation removes pge platinum group elements ni and cu each cyclic unit in the hongge layered intrusion might have resulted from crystal fractionation and mixing between a primary and evolved magma the high cu pd ti pd ni pd and cu ir ratios of the intrusion suggest that the sulfide mineral segregation may have played an important role in pge differentiation if this interpretation is correct then there is a potential to find economic pge rich horizon in the hongge type intrusions in the region

geophysical and morpho tectonic study of the transition between seafloor spreading and continental rifting western woodlark basin papua new guinea abstract two major morpho tectonic domains separated by a major transfer zone are described at the transition between seafloor spreading and continental rifting in the western woodlark basin off shore eastern papua new guinea the oceanic domain comprises new oceanic crust formed during the bruhnes epoch older transitional crust and the rifted continental margins two rift branches are recognized within the continental domain the southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crustal accretion based on magnetic data seafloor spreading in the western part of the woodlark basin commenced between 3 5 and 2 5 ma along the northern margin of the basin and maintained its position adjacent to this margin up to 0 8 ma jaramillo event just prior to the bruhnes epoch the seafloor spreading axis jumped southward to its current position frequent jumps of seafloor spreading centers and relocation of paleoaxes indicate the instability of the woodlark extensional system the transition between the two domains is characterized by differential localization of extensional strain variations in the localization can result from progressive change of extensional mode when the initial rifting evolves into seafloor spreading and or from differences in lithospheric rheology as the rift propagates into the continental margin a consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced across the transfer zone by broadly distributed deformation within continental lithosphere

imaging continental breakup using teleseismic body waves the woodlark rift papua new guinea this study images the upper mantle beneath the d entrecasteax islands papua new guinea providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers differential travel times from p and s wave teleseisms recorded during the 2010 2011 cdpapua passive seismic experiment are used to invert for separate vp and vs velocity models of the continental rift a low velocity structure marks the e w axis of the rift correlating with the thinnest crust high heat flow and a linear trend of volcanoes this slow region extends 250 km along strike from the oceanic spreading centers demonstrating significant mantle extension ahead of seafloor breakup the rift remains narrow to depth indicating localization of extension perhaps as a result of mantle hydration a high vp structure at depths of 90 120 km beneath the north of the array is more than 6 5 faster than the rift axis and contains well located intermediate depth earthquakes these independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction although the polarity of subduction cannot be resolved this geometry is gravitationally unstable downwelling or small scale convection could have facilitated rifting and rapid lithospheric removal although this may require a wet mantle to be realistic on the required time scales the high v structure agrees with the maximum p t conditions recorded by young ultra high pressure rocks exposed on the rift axis and may be implicated in their genesis

possible seismogenic shallow dipping normal faults in the woodlark d entrecasteaux extensional province papua new guinea active normal faults dipping less than 30 have not been documented in the seismic record but they are commonly observed in geologic structures one interpretation is that seismogenic low angle faulting is physically impossible and that the geologic structures do not form at shallow dips to better examine this problem teleseismic body waves were inverted to determine source parameters for five large earthquakes in the woodlark d9entrecasteaux region a rare example of active continental extension associated with pliocene quaternary metamorphic core complexes the inversions show that at least two and possibly four events are consistent with normal dip slip on shallow dipping faults the largest earthquake m w 6 8 is located near the mapped core complexes the shallow dipping plane for this event determined to dip between 10 and 25 is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low angle structures although slip on the conjugate high angle plane cannot be ruled out observations of quaternary uplift and the similarity to the adjacent core complex bounding structures favor motion on the low angle plane similar earthquake mechanisms have not yet been documented in most other extensional provinces perhaps because instances of active core complex exhumation are rare

geochemistry of the siberian trap of the noril sk area ussr with implications for the relative contributions of crust and mantle to flood basalt magmatism the sequence investigated of the siberian trap at noril sk ussr consists of at least 45 flows that have been divided into six lava suites the lower three suites consist of alkalic to subalkalic basalts the ivakinsky suite overlain by nonporphyritic basalts the syverminsky suite and porphyritic and picritic basalts the gudchikhinsky suite the upper three suites are tholeiitic the uppermost 750 m of dominantly non porphyritic basalt belong to the mokulaevsky suite and are characterized by a nearly constant mg number 0 54 0 56 sio2 48 2 49 1 wt ce 12 18 ppm and ce yb 5 8 the underlying 1100 m of dominantly porphyritic basalt belong to the morongovsky and nadezhdinsky suites there is a continuous increase in sio2 48 1 55 2 wt ce 12 41 ppm and ce yb 5 18 from the top of the mokulaevsky to the base of the nadezhdinsky with little change in the mg number 0 53 0 59 mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid ocean ridge basalts the change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by mokulaevsky magma the nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the noril sk talnakh mining camp the mokulaevsky and morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the noril sk region these magmas may have risen up out of a different conduit system

seismites in the kathmandu basin and seismic hazard in central himalaya soft sediment deformation structures have been analyzed at six sites of the kathmandu valley microgranulometric study reveals that silty levels 60 to 80 silt favor the development of soft sediment deformation structures while sandy levels 60 to 80 sand are passively deformed nonetheless well sorted sand levels more than 80 sand generate over fluid pressure during compaction if located beneath a silty cap leading to fluidization and dike development 3 d geometry of seismites indicates a very strong horizontal shearing during their development using a physical approach based on soil liquefaction during horizontal acceleration we show that the fluidization zone progressively grows down section during the shaking but does not exactly begin at the surface the comparison of bed thickness and strength depth evolution indicates three cases i no soft sediment deformation occurs for thin few centimeters silty beds ii the thickness of soft sediment deformation above sandy beds is controlled by the lithological contrast iii the thickness of soft sediment deformation depends on the shaking intensity for very thick silty beds these 3 cases are evidenced in the kathmandu basin we use the 30 cm thick soft sediment deformation level formed during the 1833 earthquake as a reference the 1833 earthquake rupture zone extended very close to kathmandu inducing there mmi ix x damages a 90 cm thick sediment deformation has therefore to be induced by an event greater than mmi x from a compilation of paleo and historic seismology studies it is found that the great m 8 1 historical earthquakes are not characteristic of the greatest earthquakes of himalaya hence earthquakes greater than m 8 6 occurred kathmandu is located above one of the asperities that laterally limits the extent of mega earthquake ruptures and two successive catastrophic events already affected kathmandu in 1255 located to the west of this asperity and in 1100 to the east

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

were the deccan flood basalts derived in part from ancient oceanic crust within the indian continental lithosphere abstract deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust and continental flood basalts cfbs are now thought by some to be derived from such eclogite bearing peridotite plumes eclogite peridotite mixtures have much lower solidi and produce much greater melt fractions for a given temperature than peridotite fe rich eclogite or pyroxenite bearing sources have been inferred for many cfbs however plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite besides cfbs are always located along pre existing lithospheric structures suture zones edges of thick cratons and commonly associated with lithospheric rifting and continental breakup india s major late mesozoic cfb the deccan traps erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent many deccan basalts are too fe rich to have been in equilibrium with a peridotite mantle source and have commonly been considered to be significantly fractionated derivatives of picritic liquids however it is possible to view them as relatively less evolved liquids derived from a source with extra fertility i e an fe rich source a new non plume plate tectonic model for icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized iapetus oceanic crust formerly trapped along the caledonian suture the model explains the geochemical petrological characteristics of icelandic basalts and is consistent with passive upper mantle upwelling under iceland inferred from recent seismic tomography based on the petrological and geochemical features of the deccan flood basalts of the type section in the western ghats i propose that old eclogitized oceanic crust trapped in the ancient indian suture zones could have produced voluminous basaltic melts during the deccan event

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

late paleozoic underplating in north xinjiang evidence from shoshonites and adakites abstract shoshonitic series volcanic rocks ssvr and adakites are widely distributed in the permian terrestrial volcanic strata of the yishijilike awulale range of west tianshan north xinjiang china isotopic dating yields permian ages of 280 250 ma the ssvr include absarokite shoshonite and banakite which are characterized by enrichment of alkalis particularly in k combined with lower ti higher al a nkc 0 70 0 99 metaluminous and fe2o3 feo the ssvr that are rich in lile with high ree contents and eu eu range from 0 59 to 1 30 they are rich in lree la yb n 2 15 11 97 and depleted in nb ta and ti tnt negative anomalies the adakites are metaluminous to weakly peraluminous a nkc 0 85 1 16 and belong to the high sio2 type of adakite hsa sio2 62 71 they are characterized by lower ree with strong lree enrichment la yb n 13 35 pronounced positive eu anomalies eu eu 1 02 1 27 very low yb contents and distinct tnt negative anomalies are evident the ssvr have end t 1 28 to 4 92 and 87sr 86sr i 0 7041 0 7057 that are similar to adakites in the regions which are characterized by end t 0 95 to 5 69 and 87sr 86sr i 0 7050 0 7053 trace element ree and sr nd isotopic compositions suggest that both ssvr and adakites possess similar source regions associated with underplated mantle derived basaltic materials lithosphere extension driven by magmatic underplating was responsible for the generation of both the ssvr and adakites this magmatism serves as a petrological indicator of underplating during the permian obviously thickened crust 62 52 km a complex moho discontinuity high heat flow 100 mw m 2 widespread contemporary alkali rich granites basic dike swarms k ar ages of 187 271 ma ar ar ages of 174 270 ma and rb sr ages of 255 28 ma end t 1 84 to 10 1 87sr 86sr i 0 7035 and 0 7065 and basic granulites shrimp u pb age of 268 279 5 6 ma provide additional evidences for the underplating event in this area during permian

break away of south china from gondwana insights from the silurian high nb basalts and associated magmatic rocks in the diancangshan ailaoshan fold belt sw china abstract the diancangshan ailaoshan fold belt yunnan sw china represents the remnants of the once vast ailaoshan ocean a branch or back arc basin of the eastern paleotethys however when and how this ocean back arc basin opened and rifted south china away from gondwana are still unresolved in this study ordovician silurian high nb basalts hnbs 420 ma mg rich andesite 435 ma high mg basalt 446 ma and i type granite 423 ma were newly identified in the diancangshan ailaoshan fold belt the hnbs are alkaline with high tio2 1 8 1 9 wt and nb 16 9 23 1 ppm contents high nb u 36 8 39 1 and nb th pm 1 17 1 21 ratios positive nb ta anomalies primitive mantle normalized and positive end t values 3 2 the hnbs were likely derived from an oib arc mixed mantle source in the garnet stability field the mg rich andesites are medium k calc alkaline andesite with negative end t values 10 1 and negative nb ta anomalies the magma was likely derived from a mixture of depleted mantle peridotites and subducted sediments under low pressure and high h2o conditions the high mg basalts are high k calc alkaline and characterized by high mgo contents 9 3 9 4 wt mg 70 high cr 369 379 ppm and low al2o3 13 6 13 9 wt contents they were likely derived from an enriched mantle source metasomatized by marine sediment derived fluids the i type granites show medium sio2 content 63 6 wt low a cnk ratio 0 9 and negative end t value 9 8 and may have formed by the partial melting of meta basalts these hnb mg rich andesite high mg basalt and i type granite were likely formed in an intracontinental back arc rift setting integrated with our published detrital zircon u pb age and lu hf isotope evidence we propose that south china may have commenced its break up from gondwana in response to the late ordovician silurian 446 420 ma proto tethyan subduction which opened a back arc basin along the ailaoshan fold belt

deep seated flow as a mechanism for the uplift of broad forearc ridges and its role in the exposure of high p t metamorphic terranes forearc systems evolve through the interplay of numerous processes but aside from lateral movements much of the observed variability in morphology can be related to evolution of ductility in the deeper levels of the subduction complex in the eastern aleutian lesser antilles makran and cascade arc trench systems a 50 to 200 km wide forearc ridge is developed along the inboard edge of the subduction complex these forearc ridges did not form until subduction accretion had expanded the arc trench gap to 275 300 km and the near vertical rise of the ridge occurred 50 km or more from the trench slope break the forearc high apparently maintained by accretion at the trench thus these broad forearc ridges are not emergent trench slope breaks the process responsible for uplift is occurring within the subduction complex and the size of the accreted mass is apparently a major factor rheological models assuming frictional sliding laws at shallow levels and power law rheologies at depth predict different behaviors for various parts of a typical large subduction complex the older forearc basement beneath which the inboard part of the subduction complex is emplaced represents a rigid brittle massif that extends 100 150 km from the volcanic arc a prediction supported by seismicity studies in modern convergent margins because of the wet quartz rich lithologies of the subduction complexes however the brittle to ductile transition apparently occurs at depths of 10 20 km within the accreted mass thus in large subduction complexes where the downgoing slab is 40 50 km beneath the inboard edge a significant region at the deeper levels of the subduction complex is apparently capable of flow the predicted zone of ductile deformation is directly beneath the area in which a forearc ridge is generally developed hence we conclude that the broad structural highs of large subduction complexes are dynamically maintained by deep seated flow the character of this flow is uncertain yet it is likely that the stripping of low viscosity partially subducted sediments plays a role and the volume of sediments added by underplating may be the controlling factor in the dynamic uplift of broad forearc ridges kinematically the flow could be a pure shear deformation induced by subhorizontal loading but we prefer the familiar corner flow mechanism because it provides the simplest explanation for the observations the interaction between offscraping at the trench and underplating affects the distribution of mass on the downgoing slab and probably controls uplift rates in broad forearc ridges because of the complexity of the interaction and the observation that modern forearc ridges appear to be rising slowly we conclude that high pressure rocks are only exposed in unusual forearc systems e g systems with either a prolonged period of uplift or large influxes of deep underplated sediment the evolution of ductility plays a major role in the development of forearc morphologies when the subduction complex is small brittle deformation and processes related to accretion at the trench dominate and the classic growth models are applicable however as continued growth of the subduction complex depresses the downgoing lithosphere and increases the arc trench gap the subduction complex becomes ductile at depth and ultimately deep seated flow leads to abrupt uplift along the inboard edge of the subduction complex

kinematics of a twisted core complex oblique axis rotation in an extended terrane betic cordillera southern spain 1 the sierra de las estancias is a metamorphic core complex that developed during the late oligocene to early miocene extensional collapse of the betic cordillera structural and paleomagnetic analysis shows that the northern half of the complex has been strongly modified by rotation about an inclined axis requiring reassessment of the kinematics of the extensional phase of deformation the extensional tectonic event produced two major detachment faults associated mylonitic foliation and lineation and pervasive sets of shear bands and brittle normal faults with a top to ne shear sense an older foliation was largely transposed and reoriented during this event the regional foliation now defines a broad antiform with a steep north dipping limb and the trend of shear related lineations swings from nne to ene from the southern gently dipping limb into the north dipping limb the remanent magnetization in a suite of near vertical early miocene mafic dikes intruded into the north dipping limb has a declination of 243 and an inclination of 27 assuming that the dikes were originally vertical a rotation of approximately 50 about an inclined ssw plunging axis preserves the near vertical orientation of the dikes and brings the remanence into agreement with the expected orientation for an early miocene age correction for this rotation brings the foliation and lineation in the surrounding schists into orientations subparallel to those on the present southern limb of the antiform and the dikes into a vertical e w orientation the corrected orientation of the shear related lineation nne normal faults and dikes suggests that the original direction of extension during the late orogenic extensional event in the internal betic cordillera may therefore have been around nne ssw rather than e w as currently accepted the observed net tectonic rotation can be decomposed into a series of geologically plausible rotations involving domino style rotation due to slip on the late brittle normal faults tilting about the regional fold axis related to thrust ramping onto the iberian margin during miocene collision and a 30 clockwise rotation about a vertical axis related to oblique convergence with the iberian margin

dependence of active normal fault dips on lower crustal flow regimes two key questions are raised by the presence in the basin and range province of subhorizontal normal faults which were active in and around the miocene first how did they develop when their existence seems to be precluded by elementary rock mechanics arguments second if they did slip in the recent geological past why do normal faults now active in the brittle upper crust have steeper dips both in the western usa and elsewhere one may now answer both questions by considering the stress field in regions where the isostatic response during extension involves horizontal flow in the plastic lower crust this stress tensor will have inclined principal axes making it possible for normal faults to form with either very steep or subhorizontal dips depending on the sense of flow this can be controlled by environmental conditions which influence rates of hanging wall sedimentation and footwall erosion and thus determine lateral variations in lithostatic pressure in the lower crust regional scale lateral variations in the geothermal gradient caused by subduction can also influence lower crustal flow regimes in adjacent zones of continental extension enabling extension to occur on low angle normal faults dipping away from the active continental margin

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

illawarra reversal the fingerprint of a superplume that triggered pangean breakup and the end guadalupian permian mass extinction abstract the permian magnetostratigraphic record demonstrates that a remarkable change in geomagnetism occurred in the late guadalupian middle permian ca 265 ma from the long term stable kiaman reverse superchron throughout the late carboniferous and early middle permian to the permian triassic mixed superchron with frequent polarity changes in the late permian and triassic this unique episode called the illawarra reversal probably reflects a significant change in the geodynamo in the outer core of the planet after a 50 million years of stable geomagnetism the illawarra reversal was likely led by the appearance of a thermal instability at the 2900 km deep core mantle boundary in connection with mantle superplume activity the illawarra reversal and the guadalupian lopingian boundary event record the significant transition processes from the paleozoic to mesozoic modern world one of the major global environmental changes in the phanerozoic occurred almost simultaneously in the latest guadalupian as recorded in 1 mass extinction 2 ocean redox change 3 sharp isotopic excursions c and sr 4 sea level drop and 5 plume related volcanism in addition to the claimed possible links between the above listed environmental changes and mantle superplume activity i propose here an extra explanation that a change in the core s geodynamo may have played an important role in determining the course of the earth s surface climate and biotic extinction evolution when a superplume is launched from the core mantle boundary the resultant thermal instability makes the geodynamo s dipole of the outer core unstable and lowers the geomagnetic intensity being modulated by the geo and heliomagnetism the galactic cosmic ray flux into the earth s atmosphere changes with time the more cosmic rays penetrate through the atmosphere the more clouds develop to increase the albedo thus enhancing cooling of the earth s surface the illawarra reversal the kamura cooling event and other unique geologic phenomena in the late guadalupian are all concordantly explained as consequences of the superplume activity that initially triggered the breakup of pangea the secular change in cosmic radiation may explain not only the extinction related global climatic changes in the end guadalupian but also the long term global warming cooling trend in earth s history in terms of cloud coverage over the planet

giant radiating mafic dyke swarm of the emeishan large igneous province identifying the mantle plume centre in many continental large igneous provinces giant radiating dyke swarms are typically interpreted to result from the arrival of a mantle plume at the base of the lithosphere mafic dyke swarms in the emeishan large igneous province elip have not received much attention prior to this study we show that the geochemical characteristics and geochronological data of the mafic dykes are broadly similar to those of the spatially associated lavas suggesting they were derived from a common parental magma based on the regional geological data and our field observations we mapped the spatial distribution patterns of mafic dyke swarms in the elip and recognized six dyke sub swarms forming an overall radiating dyke swarm and converging in the yongren area yunnan province this location coincides with the maximum pre eruptive domal uplift and is close to the locations of high temperature picrites our study suggests that the yongren area may represent the mantle plume centre during the peak of emeishan magmatism

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

tectonics of the new guinea region the new guinea region evolved within the obliquely and rapidly converging australian and pacific plate boundary zone it is arguably one of the most tectonically complex regions of the world and its geodynamic evolution involved microplate formation and rotation lithospheric rupture to form ocean basins arc continent collision subduction polarity reversal collisional orogenesis ophiolite obduction and exhumation of ultra high pressure metamorphic rocks we describe the major onshore and offshore tectonic and geologic components including plate boundaries seismicity faults and magmatism and we integrate these with emerging ideas about mantle dynamics to evaluate the cenozoic tectonic evolution of new guinea future research opportunities to resolve the mantle structure beneath new guinea will enable mantle dynamics to be linked to lithospheric and surface processes virtually all plate tectonic and mantle processes have been active in the new guinea region throughout the cenozoic and as such

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

geochemistry and petrogenesis of the permian mafic dykes in the panxi region sw china abstract numerous intrusive bodies of ultramafic mafic to felsic compositions are exposed in association with volcanic rocks in the emeishan large igneous province lip southwestern china in this paper we present new elemental and isotopic data for the permian mafic dykes from the panxi region which is located in the western part of the emeishan lip the characteristics of major and trace elements and sr nd isotopes in combination with isotopic ages suggest that these mafic dykes were originated from a similar mantle source related to an upwelling mantle plume and their formation was involved in a process of variable degrees of fractional crystallisation coupled with assimilation of enriched crustal materials the mafic dyke samples from the panxi region show some systematic variations in geochemical compositions the gabbros in yanyuan contain the lowest ree e g la 5 12 ppm the highest mgo 8 38 21 33 and compatible elements e g cr 104 2610 ppm ni 135 496 ppm and display depleted sr nd isotopic ratios initial 87sr 86sr 0 703752 0 703844 end t 4 87 to 5 13 these gabbros likely represent an undifferentiated composition similar to primary melts derived from a depleted mantle source in contrast the gabbro samples from panzhihua and huili have more enriched lree abundances and incompatible elements and display depleted to slightly enriched sr and nd isotopic signatures initial 87sr 86sr 0 704354 0 705436 end t 0 29 to 4 58 though their trace element spidergram patterns are similar to those of the yanyuan samples these gabbros were formed probably through the removal of a few percent of olivine clinopyroxene plagioclase and apatite from the melts that formed the yanyuan gabbros all the mafic intrusives in the region exhibit elemental and sr nd isotopic features generally comparable with those of the nearby volcanic rocks in addition their emplacement immediately preceded or was synchronous with the main pulse of volcanism that formed lavas over a large area along the western margin of the yangtze craton these lines of evidence suggest that there is a genetic link between the mafic dykes and flood basalts in the emeishan lip and the both share a common mantle source related to the emeishan plume

the late triassic rift related volcanic rocks from eastern qiangtang northern tibet china age and tectonic implications abstract the late triassic nadi kangri volcanic rocks with nearly ew trending outcrops within the qiangtang basin northern xizang tibet china are composed mainly of acid tuff dacite rhyolite and minor basic volcanic rocks there exists a significant depositional hiatus between the nadi kangri volcanic rocks and underlying strata therefore the nadi kangri volcanic rocks represent a new evolution history of the mesozoic qiangtang basin the magma emplacement age of the nadi kangri volcanic rocks in the geladaindong area is 220 4 2 3 ma representing the onset of the mesozoic qiangtang basin the nadi kangri basalts have high nb zr 0 049 0 058 ta hf 0 12 0 15 and zr y 4 95 6 01 ratios in the tectonic discrimination diagrams such as zr vs zr y and th hf vs ta hf the nadi kangri basaltic rocks mostly plot in the within plate setting field the geological background and the geochemical characteristics suggest that the nadi kangri volcanic rocks were formed in a continental rift setting

southeast papuan crustal tectonics imaging extension and buoyancy of an active rift southeast papua hosts the world s youngest ultra high pressure uhp metamorphic rocks these rocks are found in an extensional setting in metamorphic core complexes competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation to test these theories we analyze the cdpapua temporary array of 31 land and 8 seafloor broadband seismographs seismicity shows that deformation is being actively accommodated on the core complex bounding faults offset by transfer structures in a manner consistent with overall north south extension rather than radial deformation rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure they show crustal thinning beneath the core complexes of 30 50 and very low shear velocities at all depths beneath the core complexes on the rift flanks velocities resemble those of normal continents and increase steadily with depth there is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs also low density melt seems minor within the crust together with the extension patterns apparent in seismicity these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism although deeper mantle diapirs may be undetected a small number of intermediate depth earthquakes up to 120 km deep are identified for the first time just northeast of the d entrecasteaux islands they occur at depths similar to those recorded by uhp rocks and similar temperatures indicating that the modern seismicity occurs at the setting that generates uhp metamorphism

emeishan basalt ar ar overprint ages define several tectonic events that affected the western yangtze platform in the mesozoic and cenozoic abstract ar ar whole rock dating was carried out as part of a detailed stratigraphical investigation of the emeishan basalt large igneous province lip in the stratotype area sichuan china thirteen from twenty one specimens from three sections yielded reliable reversed isochron and plateau ages maximum 1 errors of 2 1 and 1 6 m y respectively and form two clusters centered on the late jurassic early cretaceous and early late cretaceous with a tail spanning 82 40 ma however all are appreciably younger than the magnetobiostratigraphically constrained late middle permian 255 260 ma age of the basalts reflecting varying degrees of thermal resetting during the mesozoic and cenozoic recently middle jurassic early cretaceous ages reported from elsewhere in the lip have been used to infer a single 40 m y long tectonic episode that affected the western yangtze platform developing this idea the new information have been combined with data we hold for other parts of the terrain and results from three emeishan lip ar ar dating studies published during 2002 to give 32 reliable age dates three 10 12 m y events appear to be recorded in different parts of the province middle jurassic late jurassic early cretaceous and early late cretaceous with a fourth shorter middle eocene episode the 40 m y mesozoic episode appears to be a sampling artifact following a review of the regional deformation tectonic features it is argued that activity related to various phases of deformation in the longmen shan thrust belt is the most likely cause of resetting the final suturing of the north and south china blocks may have also been responsible for the middle jurassic event however the oligocene present indentation of india into asia appears not to have had an impact possibly due to the large scale related strike slip faults that have effectively shielded the lip sichuan basin

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

apatite fission track thermochronology of the precambrian aksu blueschist nw china implications for thermo tectonic evolution of the north tarim basement the precambrian aksu blueschist is located in the northwestern margin of the tarim block nw china in recent decades many studies were carried out with focus on the metamorphic age however a complete understanding of the evolution of the tarim block requires the cooling history of the precambrian metamorphic rocks and the time temperature paths as determined by low temperature thermochronometry therefore apatite fission track aft technique was applied on the precambrian aksu blueschist to reveal the thermo tectonic evolution of the north tarim basement all of the six blueschist samples analysed in this study yielded aft ages spanning 107 5 62 5 ma much younger than the blueschist facies metamorphic age of neoproterozoic and confined track lengths are between 10 46 and 12 12 m based on regional stratigraphic sequences the aft thermal history modeling as well as previous chronological results the thermo tectonic evolution of the aksu blueschist can be roughly reconstructed with four stages 1 the precambrain aksu blueschist exhumed to the surface soon after its formation erosion during the early sinian is indicated by the lack of sedimentation until the late sinian 2 the late sinian strata are continuous while the middle upper silurian and the lower middle carboniferous strata are absent the total thickness of the late sinian and paleozoic strata probably reached 10 000 m and resulted in the total annealing and thermal resetting of aft ages 3 the aft ages in the cretaceous are related with the widespread uplift in tian shan and its adjacent regions that restarted the aft clock during the late mesozoic these reflect a distant effect of the collision of the lhasa terrane with eurasia in the late jurassic early cretaceous and 4 sediments of cenozoic are documented in the aksu area the aksu blueschist was heated to partial annealing zone with the overlying cenozoic sediments during miocene time the aksu blueschist was re exhumed which was probably a distant response to the ongoing india eurasia convergence

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

adakites from collision modified lithosphere 1 adakitic melts from papua new guinea png show adakitic geochemical characteristics yet their geodynamic context is unclear modern adakites are associated with hot slab melting and or remelting of orogenic mafic underplate at convergent margins rift propagation over collision modified lithosphere may explain the png adakite enigma as png was influenced by rapid creation and subduction of oceanic microplates since mesozoic times in a new rift tectonic regime decompressional rift melts encountered and melted remnant mafic eclogite and or garnet amphibolite slab fragments in arc collisional modified mantle and partially equilibrated with metasomatized mantle alternatively hot slab melting in a proposed newborn subduction zone along the trobriand trough could generate adakitic melts but recent seismic p wave tomographic models lack evidence for subducting oceanic lithosphere in the adakite melt region however they do show deep subduction zone remnants as a number of high p wave anomalies at lithospheric depths which supports our proposed scenario

geology and geochemistry of intrusions and flood basalts of the noril sk region ussr with implications for the origin of the ni cu ores geochemical studies indicate that five principal magma types characterize the 3 8 km vertical thickness of flood basalt of the noril9sk region from base to top these are 1 a fractionated type with alkalic affiliations ivakinsky and syverminsky 2 an ni rich suite including picritic basalt gudchichinsky 3 a primitive but ni depleted suite with fiat rare earth element ree profiles and also including picritic basalts tuklonsky 4 a light ree enriched crustally contaminated suite lower nadezhdinsky nd 1 above 4 there is a transitional suite upper nadezhdinsky nd 2 and morongovsky mr 1 and mr 2 which is overlain by the fifth type a second primitive suite without picrites mokulaevsky similar but not identical in its chemistry to the tuklonsky type this is followed by tuffs and basalts of the kharayelakhsky kumginsky and samoyedsky suites which are largely composed of similar uncontaminated tholeiitic magma the latter three suites have not been studied by us intrusions of the region fall into five main groups 1 alkalic intrusions 2 ti rich dolerite dikes 3 dolerite sills and dikes 4 differentiated bodies not related to ore junctions and 5 differentiated bodies related to ore junctions the ni cu ores are associated with ni and cr rich type 5 olivine rich intrusions with flat ree profiles that are geochemicaly similar to the mokulaevsky magma light ree enriched ni and cr poor olivine rich type 5 intrusions with uneconomic mineralization occur closely associated with the strongly mineralized variants and resemble geochemically the basalts transitional between the lower nadezhdinsky and upper morongovsky they are cut by the strongly mineralized bodies indicating that the latter are of late morongovsky to mokulaevsky age the mineralized intrusions are associated with about 1 5 to 1 10 of their mass in sulfides this is a factor of 25 to 50 more than could have been carried in solution and deposited by in situ liquation from the magma of these intrusions the platinum group element pge concentrations in the ores require that the sulfides giving rise to them equilibrated with 15 to 200 times more magma than is represented by the intrusions much of the ore is very massive and crosscuts the host intrusions the sequence of igneous activity and mineralization in the immediate vicinity of noril9sk and talnakh from the base of the nadezhdinsky upward is thus interpreted to be 1 extrusion of highly crustally contaminated ni and cu poor basalt 2 extrusion of a series of basalt flows each progressively less contaminated and less ni and cu depleted than the preceding one accompanied by intrusion of contaminated ni depleted magma with weak mineralization 3 extrusion of uncontaminated ni and cu rich basalt accompanied by intrusion of uncontaminated ni rich magma with strong mineralization followed very closely by introduction of associated massive ore and 4 continued extrusion of ni and cu rich basalt it is proposed that crustal contamination of the original ni and cu rich magma occurred at the top of a vertically extensive fault controlled magma chamber and caused segregation of immiscible sulfides with a low r factor these depleted this magma in chalcophile elements and settled deeper in the chamber as they settled they reacted with less contaminated magma scavenging additional chalcophile elements they finally came to rest near the base of the chamber to form a zone of sulfide and thus chalcophile element enriched magma eruption and in some instances intrusion of progressively stratigraphically lower less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions the answer to the strong enrichment in ni cu and pge associated with the ore bearing intrusions is that these metals came from the magma that is now represented by the depleted basalts

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

tectonics of rift propagation into a continental margin western woodlark basin papua new guinea oblique convergence along the irregular boundary between the pacific and indo australian plates in the sw pacific has resulted in a transpressional regime the drag of the overriding pacific plate has produced a breakup of the solomon microplate from the indo australian plate and has induced its rotation within the mega shear zone between the two major plates the microplate is being separated from the indo australian plate by passive rifting and seafloor spreading in the woodlark basin which is propagating westward along the continental margin of papua new guinea the present day pole of the microplate rotation with respect to indo australian plate is estimated to be near the owen stanley fault zone which represents the suture after paleocene eocene arc continent collision the average rate of the seafloor spreading propagation is estimated to be 150 mm yr 1 over the last 3 4 ma the transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault the style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks

feedback between rifting and diapirism can exhume ultrahigh pressure rocks article i nfo the processes by which crustal rocks are buried to tremendous depths and subsequently exhumed to earth s surface remain controversial rapid exhumation of earth s youngest ultra high pressure uhp rocks in the woodlark basin papua new guinea is occurring within an active rift in contrast to more common scenarios of uhp exhumation during plate convergence we use 2d and 3d thermo mechanical models to demonstrate that uhp exhumation can result from feedback between rifting and the diapiric rise of a previously subducted continental fragment through the lithosphere we infer that this feedback is responsible for the exhumation of the uhp rocks in gneiss domes in the woodlark basin our models successfully reproduce uhp exhumation paths and rates and geological structures within the gneiss domes we show that uhp exhumation by diapirism is mechanically consistent in post collisional rifts our models highlight the complex feedback between diapiric ascent and extension

mesozoic cenozoic tectonothermal evolution of the eastern part of the tibetan plateau songpan garze longmen shan area insights from thermochronological data and simple thermal modelling we present a synthesis of the tectonic and thermochronological evolution of the eastern tibet since the triassic the long term cooling histories obtained on magmatic and metamorphic rocks of the south songpan garze kunlun and yidun blocks are similar showing a very slow and regular cooling during late jurassic and cretaceous confirming the suspected lack of major tectonic events between c 150 and 30 ma the exhumation linked to the tertiary growth of the tibetan plateau initiated around 30 ma and concentrates at the vicinity of the major tectonic structures exhumation rates increased again from about 7 ma in the longmen shan to interpret this very slow cooling rate between late jurassic and early cenozoic from granites of this area we use a simple 1d thermal model that takes into account the thermal properties of both sediments and crust the results suggest that 1 high temperature 500 8c can be kept over a long period of time 2 during cretaceous cooling is mostly controlled by the thermal properties of sediments of continental origin and 3 the initial late triassic rapid cooling rate was caused by the large thermal contrast between the granite body and the sedimentary rocks rather than by a high exhumation rate

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

exhumation of high pressure metamorphic rocks during crustal extension in the d entrecasteaux region papua new guinea the d entrecasteaux islands of eastern papua new guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea floor spreading in the west woodlark basin the complexes are defined by mountainous domes 2500 m high of fault bounded high grade metamorphic rocks including eclogite facies intruded by 2 4 ma granodiorite plutons garnet clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730 900 c the jadeite component of omphacite indicates minimum pressure of 21 kbar suggesting depths of 70 km the metamorphic rocks have undergone widespread retrogression to amphibolite facies retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes p t conditions in the early stages of shear zone activity determined using the garnet biotite exchange thermometer and the gasp and grips barometers were 570 730 c and 7 11 kbar a second phase of re equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons one re equilibrated gneiss indicated maximum temperature of 730 c at estimated pressures of approximately 4 kbar this late high temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes both closely coincide spatially with high temperature metamorphic rocks the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement these observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the d entrecasteaux islands metamorphic core complexes

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

emeishan basalts sw china and the end guadalupian crisis magnetobiostratigraphic constraints abstract a magnetostratigraphic investigation of the permian emeishan lip large igneous province was carried out on a composite section in ebian county close to the type area in sichuan sw china the main succession of twelve flows 175 m thick carries a normal polarity whilst the one reliable site from the overlying 30 m thick volcanic waning sequence is marked by a reverse polarity the data enable a correlation to be proposed with an emeishan basalt sequence in western guizhou c 400 km to the se from our knowledge of the geomagnetic field9s reversal behaviour during the permian it suggests that the main lava pile along the eastern half of the emeishan basalt outcrop belt was erupted within a half to one million years using magnetobiostratigraphic data from the adjacent south china platform the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the maokou limestones constrained by conodont data the main emei basalts appear to be at least two biozones older 1 1 5 ma than the mid late permian boundary it is possible however that the emei basalt waning zone sequence which represents an explosive volcanic phase might be coeval with the end guadalupian biotic crisis thus arguments implicating emei basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative age constraints

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

anisotropy beneath a highly extended continental rift we have employed shear wave splitting techniques to image anisotropy beneath the d entrecasteaux islands in southeastern papua new guinea our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid ocean ridge tip we image the transition from continental to oceanic extension a dense shear wave splitting data set from a 2010 to 2011 passive source seismic deployment is analyzed using single and multichannel methods splitting delay times of 1 1 5 s are observed and fast axes of anisotropy trending n s parallel to rifting direction predominate the results this trend is linked to lattice preferred orientation of olivine primarily in the shallow convecting mantle driven by up to 200 km of n s continental extension ahead of the westward propagating woodlark rift this pattern differs from several other continental rifts that evince rift strike parallel fast axes and is evident despite the complex recent tectonic history we contend that across most of this rift the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid ocean ridge like mantle fabric stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region

crustal structure from three dimensional gravity modeling of a metamorphic core complex a model for uplift santa catalina rincon mountains arizona gravity modeling shows that the observed fluctuations in gravity over the catalina rincon metamorphic core complex of southeastern arizona are caused primarily by shallow crustal density contrasts the wilderness suite granites the probable source of a 20 mgal low over the central part of the core complex may extend to depths of 7 12 km below the surface seismic refraction information is consistent with the existence of a deep crustal root under the catalina rincon core complex we hypothesize that the voluminous wilderness suite granites are related to a crustal thickening event that built the root in early tertiary time modeling of flexural isostatic uplift resulting from an excess crustal root predicts two episodes of uplift for the catalina rincon metamorphic core complex 1 low relief flexural uplift in the middle tertiary and 2 complete uplift precipitated by high angle basin and range faulting at 10 15 ma the uplift and exposure of core complexes therefore can be an isostatically driven process that occurs in regions of locally thickened crust

plate motions and continental extension at the rifting to spreading transition in woodlark basin papua new guinea can oceanic plate kinematics be extended into continental rifts abstract woodlark basin provides a unique opportunity to compare brittle extension subsidence and extension predicted from long term plate motions at the rifting to spreading transition of a non volcanic margin detailed analysis of reflection seismic data near the current rifting to spreading transition yields 111 km 23 km of brittle extension subsidence along the same transect predicts 115 km 47 km of extension long term plate motions derived from seafloor spreading and transform azimuths in the eastern portion of woodlark basin predict 220 km of extension in this area these results imply no large discrepancy between brittle extension and extension predicted from subsidence contrary to observations at ancient rifted margins however there remains a pronounced two fold discrepancy between post breakup plate motions and extension during rifting this implies that continental extension rates do not match seafloor spreading rates and requires that the extension within the rift can only be modeled using euler pole kinematics after the initiation of seafloor spreading this is hypothesized to result from a decoupling of the lithosphere in which the upper crust moves more slowly than the mantle lithosphere pre breakup plate motions are accommodated primarily by extension of the mantle lithosphere as a result of rollback of the solomon sea plate after the initiation of seafloor spreading spreading rates are controlled by the motion of the mantle lithosphere and the upper crust moves at the same rate as the rest of the lithosphere

crust and upper mantle structure associated with extension in the woodlark rift papua new guinea from rayleigh wave tomography the woodlark seafloor spreading center is propagating westward into the australian plate near the d entrecasteaux islands di papua new guinea generating an active transition zone from continental rifting to seafloor spreading from march 2010 to july 2011 we deployed 31 on shore and 8 offshore broadband seismic stations around the di region to explore the dynamic processes of the lithosphere extension and the exhumation of the high pressure terranes exposed on those islands we measure the multiband 10 60 s rayleigh wave phase velocities from both ambient noise and earthquake signals these measurements are then inverted for a three dimensional shear velocity model for the crust and upper mantle the results indicate that the lithosphere extension is localized near the rift axis beneath the di with a shear velocity structure in the upper mantle that is similar to mid ocean ridges beneath the kiribisi basin west of di an ultraslow shear velocity anomaly 4 0 km s is observed at shallow mantle depth 30 60 km which can be interpreted either by the presence of excess partial melt due to slow melt extraction or by the existence of felsic crustal material buried to mantle depth and not yet exhumed

gently dipping normal faults identified with space shuttle radar topography data in central sulawesi indonesia and some implications for fault mechanics abstract space shuttle radar topography data from central sulawesi indonesia reveal two corrugated domal landforms covering hundreds to thousands of square kilometers that are bounded to the north by an abrupt transition to typical hilly to mountainous topography these domal landforms are readily interpreted as metamorphic core complexes an interpretation consistent with a single previous field study and the abrupt northward transition in topographic style is interpreted as marking the trace of two extensional detachment faults that are active or were recently active fault dip as determined by the slope of exhumed fault footwalls ranges from 4 to 18 application of critical taper theory to fault dip and hanging wall surface slope and to similar data from several other active or recently active core complexes suggests a theoretical limit of three degrees for detachment fault dip this result appears to conflict with the dearth of seismological evidence for slip on faults dipping less than 30 the convex upward form of the gently dipping fault footwalls however allows for greater fault dip at depths of earthquake initiation and dominant energy release thus there may be no conflict between seismological and mapping studies for this class of faults

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

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

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

volcanic evolution in eastern papua abstract late mesozoic and cenozoic volcanic rocks in eastern papua record a complex series of volcanotectonic events which reflect interaction between the indo australian and solomon sea plates basement formations of upper cretaceous and eocene submarine basalt are comparable to those characteristic of sea floor spreading centers and are thought to have originated during volcanic activity associated with spreading in the coral sea basin arc trench type andesitic volcanism was prominent during the late cenozoic but shows no clear relationship to a subduction event an alternative explanation links the development of thickened crust and consequent crust mantle interaction with the generation of andesitic magmas the tectonic environment of eastern papua during the late cenozoic was one of block faulting and uplift associated with crustal tension the presence of quaternary peralkaline rhyolites suggests that this environment is now being replaced by active rifting

geochemistry and ar ar muscovite ages of the daraban leucogranite mawat ophiolite northeastern iraq implications for arabia eurasia continental collision abstract daraban leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the mawat ophiolite kurdistan region ne iraq these coarse grained muscovite tourmaline leucogranites are the first leucogranite dykes identified within the mawat ophiolite they are mainly composed of quartz k feldspar plagioclase tourmaline muscovite and secondary phologopite while zircon xenotime corundum mangano ilemnite and cassiterite occur as accessories the a cnk value of the granite dyke samples varies from 1 10 to 1 22 indicating a strongly peraluminous composition cao na2o ranges from 0 11 to 0 15 and al2o3 tio2 from 264 to 463 similar to the strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas ar ar muscovite step heating dating yields 37 57 0 25 and 38 02 0 53 ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision related metamorphism mineral chemistry shows evidence of both primary and secondary types of muscovite with cores favouring the magmatic interpretation and slight effects of a late syn serpentinization fluid seen at the rims geochemical features of daraban leucogranite dykes favour a syn collisional tectonic setting they probably formed in response to the continental collision between eurasia and arabia during the initial stage of the opening of the gulf of aden at 37 ma the muscovite ages and geochemical features of daraban leucogranite are strong evidence for the timing of the continental collision between northeastern arabia and eurasia in kurdistan region of iraq