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

late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

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

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

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

insights into the dynamics of an orogenic wedge from lubrication theory implications for the himalayan tectonics abstract using lubrication theory we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate the model suggests that the subducting plate motion produces a dynamic pressure in the wedge which supports the gravity load of the overriding plate lying above it stable condition a drop in the dynamic pressure results in a rotational collapse of the overriding plate unstable condition we present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges the lubrication model is also implemented in computational fluid dynamics cfd simulations to perform real scale numerical experiments considering mainly three variables 1 plate convergence angle 2 subduction rate us and 3 viscosity ratio r between the overriding plate and wedge the overriding plates attain mechanical stability when us exceeds a threshold value us otherwise they become unstable as us

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

cathodoluminescence guided zircon hf isotope depth profiling mobilization of the lu hf system during u hp rock exhumation in the woodlark rift papua new guinea abstract cathodoluminescence image guided hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the lu hf system s behavior during rapid exhumation of u hp rocks in the woodlark rift papua new guinea investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve 8 m thick domains in which ehf present differs by as little as 4 units in a metasedimentary gneiss 2 89 0 29 ma zircon overgrowths on cretaceous aged inherited cores have radiogenic ehf present indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization the garnet effect of zheng et al 2005 in a separate gneiss sample that originated as an exhumation related anatectic melt 3 66 0 13 ma zircons lacking inheritance contain sub domains that differ from each other by 15 ehf present some of these sub domains are radiogenic and can be explained by the garnet effect whereas others also contain highly elevated lu and yb in addition to their radiogenic hf compositions thus necessitating a medium derived from the complete breakdown of garnet zircons in this sample also contain non radiogenic sub domains that grew in the presence of hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the australian continental margin the results confirm that rapid exhumation of u hp rocks can result in the following 1 transmission of radiogenic hf and sometimes lu and the other hree from garnet bearing mafic lithologies into the quartzofeldspathic gneisses and 2 mobilization and transport of unradiogenic hf present within the quartzofeldspathic remnants of subducted continental crust

zr and hf diffusion in rutile abstract chemical diffusion of zr and hf under anhydrous conditions has been measured in synthetic and natural rutile the sources of diffusant used were hafnon or zircon powders or a hafnon rutile mixture experiments were run in crimped pt capsules in air or in sealed silica glass capsules with solid buffers to buffer at nno or qfm rutherford backscattering spectrometry rbs was used to measure zr and hf diffusion profiles from these measurements the following arrhenius relations were obtained for zr diffusion parallel to c over the temperature range 750 1100 c d zr c 9 8 10 15 exp 170 30 kj mol 1 rt m 2 s 1 for hf diffusion parallel to c over the temperature range 800 1000 c d hf c 9 1 10 15 exp 169 36 kj mol 1 rt m 2 s 1 for hf diffusion normal to c over the temperature range 750 1050 c d hf c 2 5 10 12 exp 227 62 kj mol 1 rt m 2 s 1 diffusivities for experiments buffered at qfm and nno are similar to those run in air diffusivities in synthetic and natural rutile are likewise similar indicating that these findings can be applied directly in determining zr diffusivities in rutile in natural systems these data indicate that rutile should be moderately retentive of zr chemical signatures with zr diffusivities within an order of magnitude of those for pb in rutile over most geologic conditions when applied in evaluation of the relative robustness of the recently developed zr in rutile geothermometer t zack r moraes a kronz temperature dependence of zr in rutile empirical calibration of a rutile thermometer contributions to mineralogy and petrology 148 2004 471 488 e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 these findings suggest that zr concentrations in rutile will be somewhat more likely to be affected by later thermal disturbance than the geothermometer based on zr concentrations in titanite l hayden e b watson d a wark a thermobarometer for sphene abstract 16th v m goldschmidt conference 2006 and much less resistant to diffusional alteration subsequent to crystallization than the ti in zircon geothermometer e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 e b watson t m harrison zircon thermometer reveals minimum melting conditions on earliest earth science 308 2005 841 844

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

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

from magmatic generation to uhp metamorphic overprint and subsequent exhumation a rapid cycle of plate movement recorded by the supra subduction zone ophiolite from the north qaidam orogen abstract the rapid subduction and exhumation of high pressure hp and ultrahigh pressure uhp rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments herein ophiolitic fragments eclogites and amphibolites from a subduction channel exposed in the dulan terrane north qaidam orogen were investigated garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation new uranium lead u pb zircon ages of the current study combined with previous rutile u pb ages from eclogites indicate a rapid cycle of events starting with the generation of the magmatic protolith at ca 446 ma its subsequent subduction reaching a depth of ca 100 110 km at ca 435 ma and its exhumation into shallow portions of the crust which occurred at ca 416 ma thus a rapid 30 million years myr protolith generation subduction exhumation cycle could be recorded according to an ophiolitic rock association with island arc basalt iab like and mid ocean ridge basalt morb to ocean island basalt oib like trace element compositions the slab derived components determined by sr nd hf isotopes and the short time span of only about 11 myr between photolith generation and uhp metamorphism in eclogites the protolith of eclogite is best explained as a supra subduction zone ophiolite this finding likely records the closure of the south qilian ocean and the initiation of a continent arc collision at ca 446 ma furthermore starting with subduction erosion during continent arc collision in the early palaeozoic ophiolitic and arc related materials become subducted in the north qaidam it was inferred that such a rapid succession of events from protolith formation to uhp overprint and final exhumation could be explained by movements of rock material in a subduction channel thus this study provides a new view in understanding the mechanics of different episodes of subduction systems in north qaidam uhp metamorphic belt

deep crustal source of gneiss dome revealed by eclogite in migmatite montagne noire french massif central in orogens worldwide and throughout geologic time large volumes of deep continental crust have been exhumed in domal structures extension driven ascent of bodies of deep hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents the dominant rock type in exhumed domes is quartzofeldspathic gneiss typically migmatitic that does not record its former high pressure hp conditions in its equilibrium mineral assemblage rather it records the conditions of emplacement and cooling in the mid shallow crust mafic rocks included in gneiss may however contain a fragmentary record of a hp history and are evidence that their host rocks were also deeply sourced an excellent example of exhumed deep crust that retains a partial hp record is in the montagne noire dome french massif central which contains well preserved eclogite garnet omphacite rutile quartz in migmatite in two locations one in the dome core and the other at the dome margin both eclogites record p 1 5 0 2 gpa at t 700 20 c but differ from each other in whole rock and mineral composition deformation features shape and crystallographic preferred orientation cpo extent of record of prograde metamorphism in garnet and zircon and degree of preservation of inherited zircon rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c 310 ma interpreted based on zircon rare earth element abundance in eclogite zircon as the age of hp metamorphism dome margin eclogite zircon retains a widespread record of protolith age c 470 450 ma the same as host gneiss protolith age whereas dome core eclogite zircon has more scarce preservation of inherited zircon possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and or the duration of metamorphic heating and extent of interaction with aqueous fluid affecting zircon crystallization differences in hp deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development regardless of differences both eclogites experienced hp metamorphism and deformation in the deep crust at c 310 ma and were exhumed by lithospheric extension with their host migmatite near the end of the variscan orogeny the deep crust in this region was rapidly exhumed from 50 to 10 km where it equilibrated under low p high t conditions leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome

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

paleomagnetic evidence for vertical axis rotations of crustal blocks in the woodlark rift se papua new guinea miocene to present day kinematics in one of the world s most rapidly extending plate boundary zones the continental woodlark rift in se papua new guinea lies west of a propagating oceanic spreading center in the woodlark basin and is currently one of few places on earth where active continental breakup is thought to be occurring here north south extension is localized on a few major normal faults we determined characteristic remanent magnetization chrm components from demagnetization profiles of 300 individual specimens from these 157 components contribute to paleomagnetic directions for six formations we compare early miocene 20 ma to late pliocene 3 0 0 5 chrm mean directions at four localities with contemporaneous expected field directions corresponding to the australian plate time varying finite rotations from cape vogel peninsula 28 12 suggest anticlockwise rotation had begun by 15 ma this rotation may have been accompanied by rifting 7 ma earlier than previously inferred furthermore that early extension may have occurred south of the present rift and that deformation later migrated north of the peninsula pliocene vertical axis rotations are consistent with gps determined plate motions suggesting that contemporary rift kinematics were established by 3 ma finite anticlockwise rotation 10 1 7 6 in the amphlett islands is accordant with seafloor spreading in the woodlark basin suggesting this locality has seen the full woodlark plate motion since 3 ma clockwise rotation of the goodenough bay block 6 5 11 2 since the late miocene has accomplished transfer of deformation between major extensional corridors and an especially rapid local rotation 16 3 9 5 in nw normanby island may suggest an incipient dextral transfer fault

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

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

compressional metamorphic core complexes low angle normal faults and extensional fabrics in compressional tectonic settingscompressional core complexesm searle t lamont metamorphic core complexes mccs are interpreted as domal structures exposing ductile deformed high grade metamorphic rocks in the core underlying a ductile to brittle high strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension extension is supposedly the driving force that has governed exhumation however numerous core complexes notably himalayan karakoram and pamir domes occur in wholly compressional environments and are not related to lithospheric extension we suggest that many mccs previously thought to form during extension are instead related to compressional tectonics pressures of kyanite and sillimanite grade rocks in the cores of many of these domes are c 10 14 kbar approximating to exhumation from depths of c 35 45 km too great to be accounted for solely by isostatic uplift the evolution of high grade metamorphic rocks is driven by crustal thickening shortening regional barrovian metamorphism isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel channel flow following peak metamorphism beneath a passive roof stretching fault in naxos low angle normal faults associated with regional aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening the fact that low angle normal faults exist in both extensional and compressional tectonic settings and can actively slip at low angles c 60 is needed

zircon dissolution and growth during metamorphism kohn et al 2015 this volume present models of zircon dissolution and growth in hydrous morb and metapelite bulk compositions under metamorphic conditions they evaluate the propensity metamorphic minerals in equilibrium with zircon to assess what minerals besides zircon affect whole rock zr mass balance results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon with zr preferentially incorporated into melt rutile garnet and hornblende with increasing temperatures and decreasing pressures

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

geometry and kinematics of shear zones formed during continental extension in eastern papua new guinea abstract the d entrecasteaux islands eastern papua new guinea lie in an area of continental extension that has been active since the mid miocene during the last 4 ma the metamorphic basement rocks composing most of the islands have been uplifted and tectonically exhumed from depths of approximately 35 km tectonic exhumation appears to have been controlled by deformation in broad kilometre scale mylonitic shear zones a progressive evolution is documented in the shear zones characterized by increasing localized deformation a change from dominantly ductile to dominantly brittle processes and decreasing metamorphic grade i e retrograde metamorphism the evolution is believed to be the result of uplift and cooling of the basement accompanying shear zone movement the kinematic history of the shear zones is complex a number of sinistrally offset extensional detachment zones separate a multiply deformed high grade metamorphic basement from a cover of largely undeformed ultramafic and mafic cover rocks the detachment zones are connected by transverse shear zones these transverse zones appear to act as sinistral strike slip zones which also accommodate transfer motion between the detachment zones

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

a tale of two eras pliocene pleistocene unroofing of cenozoic and late archean zircons from active metamorphic core complexes solomon sea papua new guinea u pb ion microprobe analyses of zircons from gneisses and granodiorites exposed in the d9entrecasteaux islands and from conglomerate sections of the goodenough no 1 well in the adjacent trobriand basin provide constraints on the age of magmatism peak metamorphism and nature of rocks unroofed during initial stages of metamorphic core complex formation in the solomon sea the youngest populations of zircons from felsic gneisses and granodiorites indicate late pliocene 206 pb 238 u ages no inherited zircons were identified in the granodiorites and the 206 pb 238 u ages 1 65 0 18 ma 1 98 0 08 ma 2 are interpreted as crystallization ages these synkinematically emplaced granodiorites intruded into actively extending continental crust are some of the youngest known granitoids currently exposed at the earth9 surface zircon ages from felsic gneisses 2 63 0 16 ma 2 72 0 28 ma 2 are interpreted to date zircon growth subsequent to eclogite facies metamorphism felsic gneiss samples also contained zircon xenocrysts from cretaceous miocene protoliths in striking contrast zircons from igneous and metamorphic clasts from the goodenough no 1 well indicate a single population with a 207 pb 206 pb age of 2781 9 ma 2 we speculate that they are derived from basement rocks unroofed during initial stages of development of the d entrecasteaux metamorphic core complexes these results provide the first direct evidence for the existence of archean protoliths in the basement rocks of southeastern papua new guinea

gps and seismological constraints on active tectonics and arc continent collision in papua new guinea implications for mechanics of microplate rotations in a plate boundary zone 1 the island of new guinea is located within the deforming zone between the pacific and australian plates that converge obliquely at 110 mm yr new guinea has been fragmented into a complex array of microplates some of which rotate rapidly about nearby vertical axes we present velocities from a network of 38 global positioning system gps sites spanning much of the nation of papua new guinea png the gps derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in png we simultaneously invert gps velocities earthquake slip vectors on faults and transform orientations in the woodlark basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region the data are best explained by six distinct tectonic blocks the australian pacific south bismarck north bismarck and woodlark plates and a previously unrecognized new guinea highlands block significant portions of the ramu markham fault appear to be locked which has implications for seismic hazard determination in the markham valley region we also propose that rapid clockwise rotation of the south bismarck plate is controlled by edge forces initiated by the collision between the finisterre arc and the new guinea highlands

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

along or across front survey strategy an operational example at an unstable front we present results of the optimization of near real time on board sampling strategy in the iceland faroes oceanic frontal area based on the outputs of a mesoscale 3d operational data assimilation forecasting experiment by minimizing a root mean square error cost function we show that in this example an along front sampling strategy i e with transects parallel to the front produces smaller errors in temperature salinity nitrate phytoplankton and zooplankton fields as a result of a combination of the direction of the sampling of the front and errors associated with the asynopticy of observations doppler effect this is contrary to the classic across front sampling strategies that are used in most field experiments reported in the literature i e where transects are perpendicular to the front a control model shows that at these spatio temporal scales the along front sampling strategy is optimal when the frontal instability has sufficiently developed

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

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

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

an experimentally calibrated thermobarometric solubility model for titanium in coesite titanic experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite powdered amorphous silica titania anatase zirconia and water were added to silver capsules and run in the coesite stability field at 32 35 and 40 kbar from 700 to 1050 c using a piston cylinder apparatus crystallization of coesite rutile and zircon from silica titania and zircon saturated aqueous fluids was confirmed by raman spectroscopy cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous the ti concentrations of coesite crystals are significantly higher than concentrations predicted using the ti in quartz calibration wark and watson in contrib mineral petrol 152 743 754 2006 https doi org 10 1007 s00410 006 0132 1 thomas et al in contrib mineral petrol 160 743 759 2010 https doi org 10 1007 s00410 010 0505 3 titanium k edge x ray absorption near edge structure xanes measurements demonstrate that ti4 substitutes for si4 on fourfold tetrahedral sites in coesite at all conditions studied a model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least squares method to fit ti concentrations in coesite to the simple expression rt ln x text tio 2 text coesite 55 068 0 00195 times t text k 1 234 times p text kbar rt ln a text tio 2 text rutile where r is the gas constant 8 3145 10 3 kj k p is pressure in kbar t is temperature in kelvin x text tio 2 text coesite is the mole fraction of tio2 in coesite and a text tio 2 text rutile is the activity of tio2 in the system referenced to rutile ti in coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure such as another thermobarometer in a cogenetic mineral e g rutile or other phase equilibria e g graphite diamond applications of the ti in coesite thermobarometer to samples from the western alps and papua new guinea are presented

divergent plate motion drives rapid exhumation of ultra high pressure rocks abstract exhumation of ultra high pressure u hp rocks by upper plate divergent motion above an unbroken slab first proposed in the western alps has never been tested by numerical methods we present 2d thermo mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate followed by upper plate divergent motion away from the lower plate results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of u hp rocks directly to the earth s surface without the need for significant overburden removal by erosion model exhumation paths are fully consistent with natural examples for a wide range of upper plate divergence rates exhumation rates are systematically higher than the divergent rate imposed to the upper plate and the modeled size of exhumed u hp domes is invariant for different rates of upper plate divergence major variations are instead predicted at depth for differing model scenarios as larger amounts of divergent motion may allow mantle wedge exhumation to shallow depth under the exhuming domes the transient temperature increase due to ascent of mantle wedge material in the subduction channel has a limited effect on exhumed continental u hp rocks already at the surface we test two examples the cenozoic u hp terranes of the western alps continental upper plate and eastern papua new guinea oceanic upper plate the good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre cenozoic u hp terranes where the geologic record of exhumation is only partly preserved

diapir versus along channel ascent of crustal material during plate convergence constrained by the thermal structure of subduction zones abstract subduction channel processes are crucial for understanding the material and energy exchange between the earth s crust and mantle crustal rocks can be subducted to mantle depths interact with the mantle wedge and then exhume to the crustal depth again which is generally considered as the mechanism for the formation of ultrahigh pressure metamorphic rocks in nature in addition the crustal rocks generally undergo dehydration and melting at subarc depths giving rise to fluids that metasomatize and weaken the overlying mantle wedge there are generally two ways for the material ascent from subarc depths one is along subduction channels the other is through the mantle wedge by diapir in order to study the conditions and dynamics of these contrasting material ascent modes systematic petrological thermo mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates ages of the subducting oceanic plate as well as the plate convergence rates the model results suggest that the thermal structures of subduction zones control the thermal condition and fluid melt activity at the slab mantle interface in subcontinental subduction channels which further strongly affect the material transportation and ascent mode the thick overriding continental plate and the low angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge where the along channel exhumation occurs exclusively to result in the exhumation of hp uhp metamorphic rocks in contrast the thin overriding lithosphere and the steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels which lead to significant hydration and metasomatism melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge derived melts by diapir through the mantle wedge this may correspond to the origination of continental arc volcanism from mafic to ultramafic metasomatites in the bottom of the mantle wedge in addition the plate convergence rate can also affect the material ascent mode e g diapiric extrusion versus along channel exhumation by changing the amount of supracrustal rocks carried into the subduction channels which further regulate the fluid melt activity and thermo rheological properties

teleseismic imaging of subducting lithosphere and moho offsets beneath western tibet teleseismic images suggest that the tarim plate plunges 45 s down to 300 km depth beneath nw tibet the 410 km discontinuity shallows by 10 km under the plateau implying 100 c cooler upper mantle the deepest moho on record 90 km lies under w qiangtang it rises abruptly by 20 and 10 km beneath the altyn tagh fault and bangong suture respectively vp vs ratios are normal except in the yecheng flexural basin and deep under the south karakax volcanics 1 92 w kunlun s neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward extruding thrust wedge of the tarim crust

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

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

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

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

tectonic and metamorphic architecture of the hp belt of new caledonia abstract new caledonia includes a well exposed example of high pressure orogenic belt formed as a result of cenozoic plate tectonic reorganizations at the australia pacific plate boundary the metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features the architecture of this belt however still lacks general consensus the aim of this paper is i to review the nature and origin of the main tectonic units of the belts and ii to revisit the available structural models describing its architecture and geodynamic evolution for that we integrate the available field petrological geochemical and geochronological data together with new results from large scale field survey and focused ar ar geochronological data all together these data allow proposing a new model for the tectonic and metamorphic architecture of the belt also comprising a newly established tectonostratigraphic unit this architecture resembles that characterizing alpine type metamorphic belts formed in non pacific settings and clearly differs from cordilleran type belts found in eastern pacific regions moreover the collected data also allow refining our understanding of the oceanic and transitional oceanic lithosphere involved in the eocene subduction and plausible analogies with hyper extended margins known in present day ocean continent transition zones

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

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

testing the precision of bioevents deciding which of two bioevents is the less diachronous is a common problem in biostratigraphy the most accurate correlation uses the finest timescale available chemostratigraphy or cyclostratigraphy offer a potential precision of about 10 ka graphic correlation can then be used to test the precision of bioevents and to quantify any mismatch it can also be used to determine in which section any event occurs earlier application of these ideas to correlation of the cenomanian turonian and coniacian santonian boundaries demonstrates that some bioevents are as precise as chemo and cyclostratigraphy but that most are not two problems occur with bioevents first they may not be recognizable in all sections second where they are recognizable they may be diachronous in the former case calculating confidence intervals on known ranges in sections where the relevant fossil has been recorded is an alternative test large confidence intervals suggest that both first and last occurrences of a fossil may be diachronous bioevents at the cenomanian turonian boundary the following bioevents in stratigraphic order appear to be reliable time planes for international correlation the last occurrences of 1 corolithion kennedyi 2 rotalipora greenhornensis 3 axopodorhabdus albianus 4 rotalipora cushmani 5 lithraphidites acutus 6 microstaurus chiastius and 7 the first occurrence of quadrum gartneri at the coniacian santonian boundary only the first and last occurrences of platyceramus undulatoplicatus and the first occurrences of platyceramus cycloides and lucianorhabdus cayeuxii have been identified as potentially reliable bioevents

mantle compensation of active metamorphic core complexes at woodlark rift in papua new guinea in many highly extended rifts on the earth tectonic removal of the upper crust exhumes mid crustal rocks producing metamorphic core complexes these structures allow the upper continental crust to accommodate tens of kilometres of extension but it is not clear how the lower crust and underlying mantle respond also despite removal of the upper crust such core complexes remain both topographically high and in isostatic equilibrium because many core complexes in the western united states are underlain by a flat moho discontinuity it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating these processes should decouple upper crust extension from that in the mantle in contrast here we present seismic observations of metamorphic core complexes of the western woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension these core complexes are actively being exhumed at a rate of 5 10 km myr 1 and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density as indicated by low seismic velocities we conclude that at least in this case the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon but must involve mantle extension

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

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

pb diffusion in rutile diffusion of pb was measured in natural and synthetic rutile under dry 1 atmosphere conditions using mixtures of pb titanate or pb sulfide and tio2 as the sources of diffusant pb depth profiles were then measured with rutherford backscattering spectrometry rbs over the temperature range 700 1100 c the following arrhenius relation was obtained for the synthetic rutile d 3 9 10 10exp 250 12 kj mol 1 rt m2s 1 results for diffusion in natural and synthetic rutile were quite similar despite significant differences in trace element compositions mean closure temperatures calculated from the diffusion parameters are around 600 c for rutile grains of 100 m size this is about 100 c higher than rutile closure temperature determinations from past field based studies suggesting that rutile is more resistant to pb loss through volume diffusion than previously thought

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

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure