petrology of the archean pontiac and kewagama sediments and implications for the stratigraphy of the southern abitibi belt discussion lajoie and ludden presented data on the petrography traceelement chemistry and bedding characteristics of the pontiac and kewagama sedimentary units from which they proposed a paleogeographic environment of sedimentation and discussed the relative position of these units in the stratigraphy most of the sedimentological and chemical data concord with our paleogeographical model dirnroth et al 1982 and as such they are a valuable addition to our paper thus textural parameters sedimentary structures and bedding characteristics suggest that pontiac sediments are turbidites derived from the subaerial erosion of a northern high relief and tectonically unstable source area and that they accumulated in a relatively deep water environment below wave base lajoie and ludden p 1310 trace element compositions of the two sedimentary units indicate sources that are partly volcanicplutonic in nature and fractionated in lightlheavy rare earth elements ree relative to the adjacent blake river group lajoie and ludden p 13 11 h e s e chemical data are consiitent with the ree contents of three mica schists of the pontiac group published by ujike 1984 the petrography of the kewagama sediments also suggests a derivation from a nearby subaerial source of high relief and mostly volcanic composition lajoie and ludden p 131 1 unfortunately lajoie and ludden s interpretation of the paleogeographic model is weakened by a misrepresentation of the pontiac metasediments as being metaquartzite with framework consisting of more than 90 quartz this statement is based on an evaluation of the detrital mode of the framework composition of pontiac metasediments as presented in the qm p l triangle of dickinson and suczek 1982 lajoie and ludden fig 3 however it is inappropriate to evaluate the framework composition of strongly deformed and metamorphosed greywackes in the q p l triangle as lajoie and ludden have done ingersoll and suczec 1984 p 985 warn

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

centroid depth and mechanism of trench outer rise earthquakes trench outer rise earthquakes occur by reactivation or creation of normal faults caused as the oceanic lithosphere approaches a subduction zone and bends into the deep sea trench these faults may cut deep enough into the mantle to allow sea water to penetrate into the lithosphere causing serpentinization the amount of water carried into the mantle is linked to the maximum depth that the tensional faults cut into the lithosphere which in turn is directly linked to the maximum focal depths of outer rise normal faulting earthquakes we analysed teleseismic p and s waves of seven earthquakes from the trench outer rise offshore of central america using teleseismic waveform inversion of broad band data for the computation of green s functions for waveform inversion probabilistic earthquake locations were calculated to study the rupture process earthquake centroid depths and focal mechanisms for a sequence of subevents were calculated both hypocentral depths from the relocation process and the estimated centroid depths from the waveform inversion show that all events occur at shallow depths 30 km furthermore the locations of the subevents relative to each other suggest that fault planes for mw 6 are in the order of 50 km in length and only 5 10 km in width rupture generally propagates downdip and the focal mechanisms change for most events from normal faulting to strike slip or oblique thrusting with time the depth at which this mechanism change is observed may represent the depth of the nodal plane between tensional and compressional regions in the incoming plate

new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

zircon lu hf systematics and the evolution of the archean crust in the southern superior province canada a combined lu hf and u th pb isotopic study was made of 25 zircons and 2 whole rocks from the late archean crust 2 888 2 668 ma in the southern superior province canada the relative abundances of u th lu and hf in zircons from the low grade michipicoten and gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks zircons from the high grade lower crustal regions adjacent to these belts kapuskasing structural zone are distinguished from the low grade zircons by their strong depletions of lu and hf the low hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the zr hf ratio

rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

eocene and oligocene basins and ridges of the coral sea new caledonia region tectonic link between melanesia fiji and zealandia this paper presents 34 geochemical analyses 24 ar ar ages and two u pb ages of igneous rocks from the back arc basins and submarine ridges in the coral sea new caledonia region the d entrecasteaux ridge is a composite structural feature primitive arc tholeiites of eocene age 34 56 ma are present along a 200 km length of the ridge and arguably were part of the initial line of subduction inception between fiji and the marianas substantial eocene arc edifices are only evident at the eastern end where bougainville guyot andesite breccias are dated at 40 2 ma the south rennell trough is confidently identified as a 28 29 ma early oligocene fossil spreading ridge and hence the flanking santa cruz and d entrecasteaux basins belong in the group of sw pacific eocene early miocene back arc basins that include the solomon sea north loyalty and south fiji basins the rate and duration of spreading in the north loyalty basin is revised to 43 mm yr between 28 and 44 ma longer and faster than previously recognized the direction of its opening was to the southeast that is parallel to the continent ocean boundary and perpendicular to the direction of coeval new caledonia ophiolite emplacement medium and high k alkaline lavas of 23 25 ma late oligocene age on the northern norfolk ridge are an additional magmatic response to pacific trench rollback

seismicity and attenuation in the central vanuatu new hebrides islands a new interpretation of the effect of subduction of the d entrecasteaux fracture zone the spatial distribution of the intermediate depth earthquakes in the central vanuatu new hebrides islands includes a remark able gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km no well located earth quakes determined from teleseismic d ata during a 16 year period nor any microearthquakes located by a local seismograph network during a 5 year period have occurred in the gap a zone of attenuation of high frequency shear waves over laps the seismicity gap no contortion of the benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent however the gap is close to the extrapolated location of the subducted part of the d entrecasteaux fracture zone dfz the dfz is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island a rc a pogitive thermal anomaly associated with the dfz may cause the gap of intermediate depth seismic ity and produce anomalous absorption of high frequency shear waves alternatively a zone of strong scattering associated with the subducted part of the dfz may also account for the anoma lous attenuation

geochronology of orthogneiss adjacent to the archean lake of the woods greenstone belt northwestern ontario a possible basement complex deformed and recrystallized amphibolite facies trondhjemitic to granodioritic orthogneiss along the north margin of the archean lake of the woods greenstone belt has an rb sr isochron age of 2950 150 ma and an initial 87sr 86sr ratio of 0 7028 0 0014 preserved primary textures and complex internal intrusive relationships document the original plutonic nature of the orthogneiss based on isotopic age and degree of deformation and recrystallization the orthogneiss is interpreted to be basement to the adjacent metavolcanic sequence of the greenstone belt the contact between the orthogneiss and greenstone belt is the locus of deformation but may be an unconformity it is also the boundary between the english river subprovince on the north and the wabigoon subprovince on the south similar orthogneiss ranging in age from 2800 to 3800 ma is widespread in the canadian shield it is basement to the 2650 2750 ma archean volcanism at least in some areas but is not necessarily basement to the older 2800

late paleoproterozoic magmatism in delhi fold belt nw india and its implication evidence from epma chemical ages of zircons abstract zircons from six granitoid plutons occurring in the northern part of the delhi fold belt nw india have been chemically dated using an electron microprobe most of the zircons occurring in the granites are chronologically unzoned and gave ages around 1 78 1 71 ga but younger ages of 1 1 0 9 ga have also been obtained from the rim of some grains monazite and zircons from the metasediments of delhi supergroup adjacent to the granitoids were also analyzed the older age of 1 9 1 7 ga obtained for monazites could possibly indicate their provenance age detrital zircon grains mostly rounded in the delhi metasediments which gave older ages of 1 9 ga could possibly have been derived from the archaean basement rocks exposed in this aravalli delhi fold belt the geochemical signatures of these granitoids are consistent with an extensional tectonic regime it is quite likely that contrary to the prevailing assumption of the granitoids being intrusive our geochronologic data suggests them to be the basement for sedimentation the new geochronologic information on these granitoids in the northern part of the delhi fold belt and their comparison with other correlative events establishes a widespread thermal event during late paleoproterozoic

u pb zircon ages from the indian plate in northwest pakistan and their significance to himalayan and pre himalayan geologic history u pb zircon isotopic ages of seven rock samples are combined with field data to provide constraints on deposition intrusion and metamorphism in the northwestern part of the indian plate pakistani hinterland south of the indus suture zone in pakistan the results suggest deformation intrusion and regional metamorphism at ca 2174 ma an event that may correlate with granulite facies metamorphism in the nanga parbat area this was followed by erosion and deposition prior to a second proterozoic deformation at ca 1850 ma which was associated with widespread intrusion and possibly with low grade regional metamorphism metasedimentary and intrusive rocks of this age are present within the lesser himalaya of nepal but are apparently absent in the high himalayan crystalline slab north of the mct this intrusive deformational event was followed by erosion and late proterozoic deposition with development in the cambrian of an epicontinental shallow marine shelf intrusion in late cambrian middle ordovician resulted only in minor uplift erosion and development of a disconformity marine shelf conditions were reestablished in the middle paleozoic prior to a widespread intrusive event in the carboniferous permian and normal faulting erosion and syndeformational deposition and volcanism in the late permian marine shelf conditions were reestablished in the triassic prior to the himalayan orogeny zircons with a concordant age of 89 ma coupled with field and published isotopic age data suggest that himalayan deformation and metamorphism in the pakistani hinterland began between 90 and 75 ma due to subduction of the indian plate beneath indus ophiolitic melange and reached peak amphibolite facies conditions between 70 and 48 ma this metamorphism precedes eocene 54 50 ma collision of india with the kohistan arc complex field data suggest that the presently exposed pakistani hinterland from afghanistan to babusar was never significantly overthrust or buried by the kohistan arc rather than initiating the metamorphism the collision of kohistan with india resulted in uplift exhumation and cooling of the metamorphic pile

systematic changes in the incoming plate structure at the kuril trench 1 recent seismic structural studies in trench outer rise regions have shown that vp within the incoming oceanic plate systematically decreases toward the trench probably owing to bending and fracturing of the plate to understand the mechanisms acting to reduce vp vs is critical because the vp vs ratio is a sensitive indicator of lithology porosity and the presence of fluid in the outer rise region of the kuril trench we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in vp vs and vp vs our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending related fractures at the top of the oceanic crust consistent with the seawater percolation our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

tectonic implications of palaeoproterozoic anatexis and late miocene metamorphism in the lesser himalayan sequence sutlej valley nw india unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units and structural elements that bound them are properly identified and characterized in the sutlej valley western himalaya the munsiari and vaikrita thrusts have both been correlated with the main central thrust the sequence of amphibolite grade rocks the jutogh group bounded by these faults has been variously assigned to the lesser himalayan sequence based on provenance ages and to the greater himalayan sequence from its metamorphic grade trace element and geochronological data from leucogranites in the jutogh group 1 indicate crustal melting at c 1810 ma before the deposition of the greater himalayan sequence thus correlating the jutogh group with the lesser himalayan sequence and 2 record proterozoic metamorphism overprinted at 10 5 1 1 ma established from u pb analysis of uraninite during the himalayan orogeny pressure temperature time data indicate that the jutogh group and greater himalayan sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion this precludes extrusion along a single widening channel and requires a southward shift of the locus of movement during the late miocene coincident with present day precipitation patterns

evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the main central thrust zone garhwal himalaya india inferences from finite strain and ams analyses abstract the footwall of the main central thrust mct zone along the bhagirathi valley comprises a wide zone of mylonitic quartzite and deep level tectonites the systematic variation of finite strain parameters es k and v in the mylonites indicates heterogeneous deformation which is determined to vary between simple shear and non coaxial flattening type in such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement in order to supplement the finite strain studies anisotropy of magnetic susceptibility ams analyses were carried out on the mylonitic quartzites a systematic variation in degree of anisotropy p with distance from the mct is documented and is interpreted to be tectonic in origin based on these results it is concluded that p can be used as a strain intensity gauge at least on an outcrop scale where a systematic variation in p values from one part of the outcrop to the other can be established however the quantitative relation between principal axes of finite strain ellipsoid and ams axes magnitude of principal susceptibility difference k1 and k3 and finite strain magnitude e1 ln 1 e1 and e3 ln 1 e3 were related by a logarithmic relationship with a correlation coefficient of 0 844

geochemistry of precambrian mafic magmatic rocks of the western himalaya india petrogenetic and tectonic implications abstract precambrian sequences of the higher himalayan crystallines vaikrita group and the lesser himalaya chail jutogh and jaunsar groups in the garhwal and himachal regions of the western himalaya include abundant metamorphosed mafic lavas and dykes amphibolites a gabbroic body within the chail group has been dated at 1907 91 ma initial 87 sr 86 sr ratio of 0 7022 0 0008 by the whole rock rb sr method this age is consistent with several age data 1800 2000 ma whole rock rb sr method assigned to the granitoids and gneisses of this region these amphibolites and gabbros exhibit low ti tholeiite characteristics the lesser himalayan samples are enriched in light rare earth elements lree and large ion lithophile element lile with distinct negative nb sr p and ti anomalies conversely the vaikrita group samples are characterized by less enriched lree lile and absence of the above anomalies but have distinct positive sr anomalies the chemical characteristics of the lesser himalayan samples are remarkably similar to the basal aravalli volcanics of the nw indian shield inferred to reflect significant components from enriched mantle sources and crustal contamination samples of the vaikrita group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative nb p and ti anomalies these distinct geochemical characteristics are used to demarcate the main central thrust along the vaikrita thrust the chail and jutogh groups are affiliated with the lesser himalaya and the chail group gabbro body may represent portion of one among the numerous magma chambers that fed a large precambrian magmatic province in the south of the main central thrust the similar emplacement ages 1800 2000 ma for the mafic and felsic magmatic rocks suggest that this region experienced a major episode of crustal generation and evolution in a rift environment

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

dehydration induced weakening and fault slip in gypsum implications for the faulting process at intermediate depth in subduction zones 1 high pressure fluids generated by metamorphic dehydration reactions can have profound effects on both rock strength and the stability of faults a decrease in effective pressure so produced could induce earthquakes at intermediate depth in the subducting slab either by embrittlement of an intact rock mass or by the reactivation of preexisting faults the purpose of this study was to experimentally investigate both processes using gypsum as an analog material constant strain rate relaxation and friction tests were performed on both intact cylindrical samples and saw cut aggregates both room temperature and dehydration conditions were investigated for comparison the maximum confining pressure and temperature were 50 mpa and 150 c respectively and the strain rate was varied between 1 5 10 6 and 1 5 10 5 s 1 this investigation shows that dehydrating gypsum is an excellent analog material for studies on reaction induced weakening however the lack of important brittle features like shear fractures and stress drops implies that intact gypsum aggregates do not show dehydration embrittlement in contrast this study demonstrates that depending on both the level of fault normal stress and the strain history dehydration induced unstable slip on preexisting faults is possible although some of the observations might be particular to gypsum the results also indicate that strong mechanical locking of the fault may continue to promote ductile flow unless new more favorably oriented faults are formed

geochemistry of metasedimentary rocks in the late archean hemlo heron bay greenstone belt superior province ontario implications for provenance and tectonic setting the late archean hemlo heron bay greenstone belt of the superior province near the hemlo gold deposit ontario consists mainly of fine grained clastic metasedimentary rocks with subordinate amounts of metavolcanic rocks cordierite anthophyllite rocks are also locally present but restricted to a single metasedimentary sequence and most likely represent metamorphosed mg and fe rich sedimentary rocks derived from predominantly mafic ultramafic sources the clastic metasedimentary rocks are geochemically highly immature some of their geochemical characteristics such as low al2o3 na2o ratio low cia k2o na2o 1 0 uniform th u ratio 3 6 and strong correlations of transition metals and mgo with fe2o3 are similar to those of typical archean sedimentary rocks however their ree compositions high contents of lree and steep chondrite normalized patterns are different from those of most archean sedimentary rocks and similar to those of more mature proterozoic sedimentary rocks the clastic metasedimentary rocks from the hemlo heron bay greenstone belt were derived mainly from felsic or intermediate volcanic sources admixed with variable amounts of mafic ultramafic volcanic components and possibly some continental weathering products and deposited rapidly as turbidites the geochemical uniformity of all clastic metasedimentary rocks indicates that the hemlo heron bay greenstone belt represents a single tectonic environment possibly equivalent to a modern island arc system rather than a juxtaposition of suspect terranes

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

the significance of source versus process in the tectonic controls of magma genesis arculus r j 1987 the significance of source versus process in the tectonic controls of magma genesis in s d weaver and r w johnson editors tectonic controls on magma chemistry j volcanol geotherm res 32 1 12 despite the association of certain characteristic trace element signatures with particular tectonic environments of eruption there are accumulating data which would result in significant tectonic misassignments ambiguity of signals appears in active arc back arc systems of the southwestern pacific and particularly in some intracontinental plate suites given the selective preservation of continental as opposed to oceanic lithosphere inappropriate paleotectonic inferences are probable using trace element criteria alone strong relative fractionation of the alkalis and alkaline earth elements aee with respect to the rare earth elements ree in the majority of arc related magmas and a number of intraplate continental basalts is strongly suggestive of the involvement of hydrous fluids at some stages in the respective petrogenetic processes occurring in these two tectonic regimes in contrast fractionation of high field strength elements hfse such as nb and ta with respect to the ree in the same suites is most readily explained by the involvement at some stage in the magma formation process of high sio2 melts a number of widely applied tectonic discriminants makes use of aee hfse fractionation but the processes and sources involved in subduction zone petrogenesis may be duplicated during interaction of mantle derived basalt with the heterogeneous components of continental lithosphere both mantle and crust a significant role for both volatile dominated fluids and silicate melts is implicated in collision and some intracontinental plate magmatism

the d entrecasteaux zone southwest pacific a petrological and geochronological reappraisal abstract the d entrecasteaux zone southwest pacific is an arched submarine horst and graben structure which extends from the northern end of the new caledonia ridge to the western border of the new hebrides island arc a review of the bathymetry seismic reflection data paleomagnetism gravimetry seismology and dsdp data available for this area is combined with a study of basaltic samples dredged along the horsts of this regional fracture zone these basalts show strong petrographic and chemical affinities with morb their fissiontrack ages range between 56 ma paleocene eocene boundary and 36 ma eocene oligocene boundary it is suggested that the d entrecasteaux zone represents the northern arcuate extension of the northeast dipping eocene subduction obduction zone located along the new caledonia loyalty islands ridge while its present morphology appeared from post obduction extensional movements resulting in a progressive uplift of basaltic ocean floor since middle miocene times

sm nd age of the garhwal bhowali volcanics western himalayas vestiges of the late archaean rampur flood basalt province of the northern indian craton abstract whole rock sm nd isotope dating of the garhwal and bhowali mafic volcanics of the kumaun lesser himalayan sector yield an age of 2 51 0 08 ga and an initial nd 5 1 0 5 this age and initial nd are the same as that of the rampur mandi volcanics which occur to the west in the lesser himalayas in addition to confirming the widespread presence of the late archaean crustal elements in the himalayas these data constrain the age of the oldest rifting event and deposition of supracrustal sediments in the himalayas and set at rest the long standing controversy about the age and inter relationship of these mafic volcanics despite their present day disparate occurrence as a result of the post eruption deformation the area covered by these mafic volcanics is large enough to classify them as flood basalts the rampur flood basalt rfb province rift related volcanism of the same age is known from the contiguous aravalli region of the nw indian shield the orientation of the rifts in the two regions suggests the possibility of a mantle plume setting for the rfb

geometrical analysis of mesoscopic shear zones in the crystalline rocks of mct zone of garhwal higher himalaya abstract the crystalline rocks of the mct zone of garhwal higher himalaya exhibit well preserved mesoscopic shear zones majority of these shear zones are of ductile and brittle ductile type with both sinistral and dextral sense of movement detailed analysis of mesoscopic shear zones reveals that sinistral shear zones exhibit a strike variation from nne to ene and dextral shear zones exhibit variation from nnw to wnw directions thus forming a conjugate pair the bisectors of statistically preferred orientations of the two sets of the shears indicate that they generated due to nne ssw horizontal compression these dextral and sinistral shear zones exhibit strike slip geometry developed during progressive ductile shearing

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

age and geochemistry of the paleoproterozoic bhatwari gneiss of garhwal lesser himalaya nw india implications for the pre himalayan magmatic history of the lesser himalayan basement rocks

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

proterozoic crustal evolution in the nw himalaya india as recorded by circa 1 80 ga mafic and 1 84 ga granitic magmatism abstract single zircon dating of the rampur metabasalts of the larji kullu rampur window in the lesser himalayas yielded an evaporation age of 1800 13 ma the zircon age is considerably younger than the previously published whole rock sm nd age of 2510 90 ma suggesting that the sm nd age may be geologically meaningless and that the sm nd whole rock array may have resulted from mixing in the nw himalaya there is also evidence for extensive silicic melt generation in the paleoproterozoic zircons from a metarhyodacite in the larji kullu rampur window yielded an evaporation age of 1840 16 ma which we interpret as the minimum age of magmatism the main central thrust granitic mylonites are interpreted as the basement of the neoproterozoic haimanta group metasediments together with the granitic rocks from the lesser himalaya they were derived from pre existing continental crust prior to 1 84 ga the nd depleted mantle model ages are in the range of 2 6 2 4 ga suggesting a contribution of archean crust a recycled archean component is also documented by a 2 9 ga domain in one of the zircons

bimodal cyclical archean basalts and rhyolites from the michipicoten wawa greenstone belt ontario geochemical evidence for magma contributions from the asthenospheric mantle and ancient continental lithosphere near the southern margin of the superior province abstract the petrographic and geochemical variations in the middle to late archean volcanic rocks of the michipicoten wawa greenstone belt mgb ontario record bimodal basalt rhyolite magmatic activity at approximately 2 9 2 75 and 2 7 ga along what has previously been interpreted to be a southward dipping convergent plate margin which is underlain by continental crust cycle 1 komatiitic basalts have elevated mg number 0 51 0 83 compared to cycle 2 and 3 tholeiites mg number 0 44 0 56 and dominantly flat ree patterns with negative ta nb anomalies la ta 4 35 cycles 2 and 3 are more evolved than cycle 1 but all three cycles have broadly similar ratios of the incompatible trace elements compared to modern oceanic basalts the basaltic members of cycles 1 3 have a moderate ta nb anomaly la ta 4 46 and low la yb n 1 18 1 56 low la ta is a feature of oceanic lavas whereas high la ta is a feature of magmas which contain contributions from a crustal reservoir the mafic volcanic rocks are believed to be derived from asthenospheric mantle with assimilation of older continental crust the felsic volcanic rocks have elevated th nb 0 7 1 6 and ce yb n 10 2 16 4 and are enriched in light rare earth elements lree and large ion lithophile elements lile relative to rhyolites from other superior province greenstone belts we suggest that the rhyolites record the geochemical traits of their sources and these could be interpreted to be moderate to high degree melts of tonalitic upper crust rather than fractionated magmas or partial melts derived from the underlying mafic volcanics this contrasts with models for many other rhyolites in superior province greenstone belts such as the southern volcanic zone of the abitibi greenstone belt where variations require a more mafic crustal source to generate the fractionated low la sm gd yb and high zr th rhyolites believed to be locally associated with volcanogenic massive sulphide deposits the absence of significant volumes of compositionally intermediate volcanic rocks and the evidence for crustal contributions to mgb cyclical magmatism raises the possibility that these are archean analogues of the phanerozoic flood basalts which straddle the margins of continental shield areas such as the java ontong sequence and are not relics of either archean oceanic crust or subduction related volcanics

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

comparative sedimentology of the principal volcanic sequences of archean greenstone belts in south africa western australia and canada implications for crustal evolution abstract significantly different assemblages of sedimentary rocks are present in the volcanic sequences of archean greenstone belts in the barberton mountain land pilbara block and canadian shield thin sedimentary layers in the 3 55 3 3 ga old onverwacht and warrawoona groups in the barberton mountain land and eastern pilbara respectively are made up of silicified volcaniclastic sediments banded cherts carbonaceous cherts silicified evaporites and locally stromatolites deposited under shallow water anorogenic conditions the volcanic sequences in these belts essentially lack terrigenous debris derived by weathering and erosion algoma type iron formation volcanogenic massive sulfide deposits hyaloclastic breccias and evidence for the existence during their accumulation of older sialic basement sedimentary rocks in the volcanic sequences of the 2 8 2 6 ga old superior and slave provinces canada and perhaps in other late archean belts in the rhodesian craton and yilgarn block include generally unsilicified volcaniclastic units mafic hyaloclastic breccias terrigenous clastic layers abundant algoma type iron formation and massive sulfide deposits sedimentation took place under relatively deep water conditions local tectonic activity was widespread and older sialic basement was present these younger volcanic sequences contain few shallow water deposits such as carbonaceous cherts evaporites and stromatolites the volcanic sequences of both generations of greenstone belts seem to have formed platforms that stood topographically above the surrounding sea floor differences between older and younger greenstone belt volcanic sequences appear to reflect the increasing abundance and influence of sialic crust from early to late archean time and an accompanying increase in ocean water depth over the volcanic platforms evolution of the sialic crust can be traced through at least five archean and early proterozoic stages 1 1 an early period of ensimatic shallow water greenstone belt formation 3 55 3 3 ga ago 2 2 an early episode of intrusive and metamorphic activity from ca 3 3 2 9 ga ago during which some of the older greenstone belts were cratonized 3 3 a late period of greenstone belt formation 2 8 2 6 ga ago during which volcanism occurred under relatively deep water conditions upon and or adjacent to areas of older sialic basement 4 4 a late archean interval of continental crust formation during which the younger greenstone belts were cratonized 2 7 2 5 ga ago 5 5 a latest archean and early proterozoic period dominated by stable cratons and relatively minor greenstone belt activity

tectonic setting of late archean bimodal volcanism in the michipicoten wawa greenstone belt ontario the tectono stratigraphic relationships depositional environments rock associations and major and trace element compositions of the late archean 2744 2696 ma bimodal basalt rhyolite volcanic rocks of the michipicoten wawa greenstone belt ontario are compatible with an origin along a convergent plate margin that varied laterally from an immature island arc built on oceanic crust to a more mature arc underlain by continental crust this environment is similar to that of the cenozoic taupo kermadec tonga volcanic zone michipicoten basaltic rocks most of which are proximal deposits compositionally similar la yb n 0 63 1 18 to modern oceanic island arc tholeiites are interpreted as having formed along the largely submerged island arc voluminous michipicoten rhyolitic pyroclastic rocks la yb n 4 3 18 7 ybn 5 7 15 9 probably erupted subaerially from the continental arc with distal facies deposited subaqueously on the adjacent oceanic island arc and proximal facies deposited in subaeri

along strike variation in subducting plate seismicity and mantle wedge attenuation related to fluid release beneath the north island new zealand abstract we analyze seismicity in the subducted slab of the hikurangi subduction zone along a 600 km length beneath the north island new zealand the volcanic character changes along strike with extremely productive rhyolitic volcanism in the central taupo volcanic zone moderate andesitic volcanism in the northern and south central zones and subduction without volcanism in the southernmost hikurangi zone we have relocated slab earthquakes with 3 d velocity models the relocated seismicity shows more detail of the varied distribution and abundance of slab seismicity which below 50 km depth may be related to embrittlement from high fluid pressure the depth of the buoyant subducted hikurangi plateau ranges from 40 to 140 km depth at depths where the subducting slab interacts with the mantle wedge along arc variation in slab seismicity is a fundamental characteristic with patches of abundant seismicity separated by low seismicity zones the largest most numerous patch at 150 220 km depth underlies a pronounced low qp zone in the mantle wedge which is associated with the rhyolite dominant taupo caldera extensive melt in the region of low qp requires high h 2 o flux from the underlying slab the abundant taupo seismicity suggests a correlation between melt production and regions of earthquake fracture permeability following embrittlement which promote migration of dehydration fluid the hydration history of the incoming slab may be a key factor in producing variations in dehydration and intraslab fluid migration broader extensive outer rise yielding and hydration may have occurred near the approach of the re entrant hikurangi plateau forming the slab section that currently has high seismicity slab seismicity deepens from 240 to 330 km along arc as the subduction rate increases from 40 mm yr the southwestern slab seismicity is bounded by an unusually narrow zone with 110 km depth extent this is inferred to be a dehydration front related to heating at a slab edge that is located 70 km further southwest

himalayan architecture constrained by isotopic tracers from clastic sediments we report the first geochemical traverse to integrate u pb ages and hf data on single detrital zircons with bulk rock sm nd rb sr isotopic measurements across the breadth of the himalayan orogen in northwest india our dataset from key tectonostratigraphic units in the sutlej valley provides detailed insights into both the provenance of the sedimentary formations and their tectonic history whole rock nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt regardless of their current metamorphic grade u pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks while lu hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains complex whole rock sr systematics hint at the extent of pre himalayan thermal events our data confirm the geochemical distinction between the older lesser himalayan formations and those of the depositionally younger units including the high himalayan crystalline series that has been recorded along the strike of the himalaya from the western syntaxis to nepal palaeoproterozoic metasediments of the jutogh group and rampur formation yield end 500 values below 17 whereas neoproterozoic and younger metasediments yield end 500 above 13 the latter group encompasses the vaikrita group gneisses of the high himalayan crystallines and the overlying haimanta group but also includes a number of units in thrust sheets north of the main boundary thrust in the lesser himalayan zone designated the douter lesser himalayat to distinguish them from the palaeoproterozoic dinner lesser himalayat zircon populations from the inner lesser himalaya are characterised by palaeoproterozoic late archaean ages 2 6 1 8 ga whereas the depositionally younger units contain populations both of this age and a younger period meso to neo proterozoic 1 1 0 8 ga detrital zircon ages are younger than their respective hf isotope derived crustal formation ages by 0 7 2 1 ga indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism the

horizontal and vertical interseismic velocity fields in the vanuatu subduction zone from gps measurements evidence for a central vanuatu locked zone we analyzed interseismic velocity fields along the vanuatu arc subduction zone from more than 45 gps station measurements spanning 15 years convergence with respect to the australian plate ap is normal to the trench with amplitudes that vary greatly from a maximum of 170 mm a a1 at vanikoro to 27 mm a a1 facing the d entrecasteaux ridge der motions expressed relative to the pacific plate pp highlight arc segmentation both the northern and southern segments undergo back arc spreading in a roughly ne sw direction rotating anticlockwise and clockwise respectively while the central block moves eastward at a faster rate with respect to the pp than to the ap more convergence is therefore accommodated at the eastern bound of the arc than at the vanuatu trench a right lateral strike slip movement 26 1 mm a a1 along a zone normal to the trench direction n70 is necessary to accommodate the rate variation between the central and southern parts of the arc horizontal and vertical rates indicate partitioning between malekula southernmost santo and central northern santo blocks simple elastic modeling of the interseismic stage deformation gives locked zone characteristics 25 dip and 50 km width and a 54 mm a a1 long term convergence rate between the ap and the central part of the vanuatu arc from gps horizontal and vertical velocities estimated south of the der the modeling does not fit the observed velocities facing the der probably because it does not account for the inelastic deformation associated with buoyant ridge subduction and back arc thrusting

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

the kumaun and garwhal lesser himalaya india part 1 structure and stratigraphy our understanding of the geologic evolution of the himalaya remains incomplete particularly in regard to structural and geochronologic details of the proterozoic paleozoic lesser himalayan sequence we conducted an integrated field mapping geochronological study and geochemical analysis of the lesser himalayan sequence strata in the kumaun and garwhal himalaya of nw india 78 00 80 30 e structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the main central thrust footwall in the south and at lower structural levels the main central thrust footwall is characterized by parallel folding and sparse development of axial cleavage quartz microstructures indicate that deformation occurred at temperatures below 350 c in contrast in the north and at higher structural levels close to the main central thrust deformation is characterized by replacement of original bedding the development of penetrative cleavage and schistosity locally folds are tight in places isoclinal and overturned the corresponding quartz microstructures indicate that deformation occurred above 350 c dating of detrital zircons from lesser himalayan sequence metasedimentary units and igneous zircons from schistose gneisses throughout the lesser himalayan sequence allow refinements to the stratigraphic framework first unlike has been observed in western nepal immediately west of our study area u pb zircon dating suggests the absence of paleoproterozoic strata in the ramgarh thrust hanging wall this suggests that the thrust cuts upsection laterally along strike second u pb detrital zircon dating suggests that metasedimentary strata in the ramgarh thrust hanging wall are correlative with basal units of the outer lesser himalayan sequence removing the necessity that the ramgarh thrust hanging wall was allochthonous with respect to its footwall lesser himalayan sequence units schistose gneisses with u pb ages of ca 1850 ma are older than lesser himalayan sequence units in the area suggesting that they are thrust emplaced lesser himalayan sequence basement we conclude that the ramgarh thrust is an out of sequence thrust postdating a folding event in its footwall the earlier proposal that neoproterozoic cambrian lesser himalayan sequence strata are the southern extension of the tethyan himalayan sequence requires that the tons thrust which separates the distal and proximal facies of the lesser himalayan sequence be a major south directed structure with a slip magnitude of 50 100 km

precise u pb zircon ages from the gamitagama greenstone belt southern superior province volcanism in the gamitagama lake greenstone belt 50 km south of wawa ontario on the east shore of lake superior took place ago at the same time as similar volcanism in the much larger abitibi greenstone belt to the east two post orogenic intrusions in the metavolcanic metasedimentary terrain are coeval at and and could very well be comagmatic as suggested by ayres a granitic sill in a metasedimentary granitic migmatite terrain that borders the greenstone belt to the southwest has an age of 2615 10 ma and is much younger than the post orogenic plutons to the northeast this migmatite region represents either an uplifted deep zone that remained hot long after volcanism or a region that accumulated heat as a result of its sedimentary cover

paleogeographic and paleotectonic response to magmatic processes a case history from the archean sequence in the chibougamau area quebec the chibougamau area quebec is characteristic of the internal zone of the archean abitibi orogenic belt the paleogeographic paleotectonic and magmatic history of the archean sequence in the chibougamau area is subdivided into three stages in the first stage a submarine volcanic chain formed mainly by the effusion of submarine lava flows composed of primitive potash poor tholeiitic basalt the volcanic chain gradually grew to sea level in the second stage volcanic islands emerged and grew mainly pyroclastic eruptions of strongly differentiated calc alkaline andesite and dacite concentrated on the volcanic islands whereas effusion of basalt continued at first in the surrounding basin a felsic volcaniclastic apron was deposited around the volcanic islands in the third stage the volcanic islands were uplifted and were eroded to the level of their subvolcanic plutons the debris derived from this volcanic plutonic terrain was deposited in downfaulted marine and continental basins the contemporaneous volcanism was shoshonitic the first paleogeographic stage is interpreted as the growth of an immature island arc during the second stage the island arc became mature and its crust was thickened by accretion of plutonic material the third stage is a period of back arc extension

geochemistry and petrogenesis of garhwal volcanics implications for evolution of the north indian lithosphere abstract middle proterozoic magmatism in the garhwal himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets geochemical data reveal that the volcanic suite is tholeiitic with the dyke sheets showing considerably more fe enrichment than the lava flows all the rocks are distinctly enriched in incompatible elements and light ree relative to primordial mantle abundances but have an equally distinct continental signature reflected in marked negative nb sr p and ti anomalies in their mantle normalised spidergrams although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source other proterozoic volcanics in northern india have closely similar geochemical features as have other classic occurrences of younger continental flood basalt suites in adjacent areas of gondwanaland these geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but in the case of younger cfb suites this may have been modified to a greater or lesser extent by subsequent additions of plume mantle material

thermal petrological controls on the location of earthquakes within subducting plates we find that in young and warm subducting plates earthquakes occur just below the moho in older plates earthquakes occur throughout the subducting oceanic crust as well as the subducting mantle we document this behavior in several subduction zones where there are independent constraints on earthquake locations and slab structure specifically for northern and southern japan alaska and cascadia the differences in earthquake depth relative to subducting crust may reflect large differences in temperature and thus locations of major dehydration reactions in colder slabs the crust passes through blueschist facies dehydration reactions while in cascadia and nankai the major dehydration reactions in crust may be due to zoisite and amphibole breakdown or associated melting the cold paths allow more mineral bound h2o to be retained within the crust at shallow depths eventually released upon dehydration over shorter time intervals than warm paths the cold path dehydration reactions also result in net positive volume changes of solid fluid with solid volume decreasing less than the volume of h2o produced on hot paths the net volume changes are negative with solid volumes decreasing more than the volume of h2o produced the difference in behavior could drive a net increase in pore pressure upon dehydration for the cold but not the hot crustal paths the difference in rate of release in h2o and difference in sign of net system volume change may promote seismogenesis in cold subduction zones but inhibit it in the crust of warm slabs within the mantle of the downgoing plate earthquakes mostly occur where serpentine is stable or breaks down in both settings

seismicity and shallow slab geometry in the central vanuatu subduction zone the vanuatu arc in the southwest pacific ocean is one of the world s most seismically active regions with almost 39 magnitude 7 earthquakes in the past 43 years convergence rates are around 90 120 mm yr along most of the arc but drop to 25 43 mm yr in the central section probably due to the subduction of the d entrecasteaux ridge we characterize the slab geometry and tectonic state in this central section by analyzing data from a 10 month deployment of 30 seismometers over this section we located more than 30 000 events all less than magnitude 5 5 constructed an improved 1 d velocity model calculated focal mechanisms and cluster geometries and determined the 3 d geometry of the interplate seismogenic zone the seismogenic zone has a shallow bulge in front of the d entrecasteaux ridge which could be explained by the ridge s buoyancy contributing to the uplift of the fore arc islands the seismogenic zone extends to 45 km depth significantly below the 26 27 km depth of the fore arc moho indicating that the upper mantle wedge is not significantly serpentinized which is consistent with the relatively high thermal parameter of the subducting plate the maximum width of the seismogenic zone is 80 km indicating an upper earthquake magnitude limit of mw 7 85 0 4 assuming standard rupture zone aspect ratios the data also reveal a double seismic zone 20 to 30 km below the seismogenic zone which is presumably caused by flexure of the downgoing plate

petrography and provenance of silicified early archaean volcaniclastic sandstones eastern pilbara block western australia intense post depositional alteration has profoundly affected sandstones in the volcanic portions of early archaean 3 5 3 3 ga greenstone belts the mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism but grain textures are commonly well preserved consequently microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes silicified early archaean volcaniclastic sandstones assigned to the panorama formation and duffer formation warrawoona group eastern pilbara block western australia were originally composed of volcanic vrf and sedimentary srf rock fragments volcanic quartz feldspar traces of ferromagnesian minerals and pumice only volcanic megaquartz remained stable during alteration all other primary components were replaced by granular microcrystalline quartz gmc and sericite in most areas the sandstones were composed of dacitic to rhyolitic vrfs now totally replaced by sericite poor gmc and recognized by preserved microporphyritic textures in a few areas quartz poor dacitic to andesitic vrfs dominated the detrital assemblage minor srfs and mafic vrfs now replaced by gmc are recognized on the basis of colour internal structures and internal textures including skeletal possible spinifex textures detrital feldspar is represented by blocky sericite rich grain pseudomorphs a semi quantitative point count scheme developed for the analysis of heavily altered sandstones indicates the following primary detrital mode ranges for panorama arenites quartz 0 28 feldspar 0 28 vrfs 58 86 and srfs 0 25 in about half the point counted samples feldspar could not be distinguished from rock fragments in such cases both were counted as one grain type lv which makes up from 84 to 100 of the framework modes of these rocks these sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments but locally including minor mafic ultramafic and sedimentary rocks much but not all of the felsic volcaniclastic sand represents reworked pyroclastic debris there is no evidence for contributions from plutonic or metamorphic sources the panorama modal assemblage represents a provenance that is lithologically more restricted than that of archaean greywackes and other siliciclastic units common in the sedimentary portions of these same early archaean greenstone belts and younger greenstone belts 3 0 2 7 ga

link between plate fabric hydration and subduction zone seismicity in alaska subduction carries water into the earth where it can influence seismicity analysis of the structure of the alaskan subduction zone suggests fluid delivery is influenced by faults in the oceanic plate that formed at the mid ocean ridge

cooling and uplift histories of the crystalline thrust stack of the indian plate internal zones west of nanga parbat pakistan himalaya abstract amphibole and mica k ar ar ar and rb sr geochronology for the crystalline internal zones of the indian plate define both an extensive pre himalayan thermal history and a post himalayan metamorphism cooling history south of the main mantle thrust near besham hornblende ar ar ages from basement gneisses record an ca 1850 ma mid proterozoic thermal event hornblende muscovite and biotite cooling ages from cover sequences metamorphosed during the himalayan orogeny are 35 4 30 to 24 and 29 to 22 ma respectively the mica ages together with those derived from zircon and apatite fission track data zeitler 1985 demonstrate a rate of cooling of about 30 c ma during the late oligocene to early miocene that was greater than that either before or since this rapid cooling was initiated during the post metamorphic evolution of the indian plate south verging crustal scale thrust stack during which cover sequences metamorphosed during the himalayan orogeny were imbricated with basement rocks thermally unaffected during that event most of the cooling which happened during the stripping of some 10 2 km of overburden reflects exhumation due to a combination of erosion recorded in the miocene molasse sediments of the foreland basin and major crustal extension within the mmt zone both erosion and extension were the direct consequence of the evolution of the thrust stack

serpentinization in the trench outer rise region offshore of nicaragua constraints from seismic refraction and wide angle data summary recent seismic evidence suggested that most oceanic plate hydration is associated with trenchouter rise faulting prior to subduction hydration at trenches may have a significant impact on the subduction zone water cycle previous seismic experiments conducted to the northwest of nicoya peninsula northern costa rica have shown that the subducting cocos lithosphere is pervasively altered which was interpreted to be due to both hydration serpentinization and fracturing of the crustal and upper mantle rocks new seismic wide angle reflection and refraction data were collected along two profiles running parallel to the middle american trench axis offshore of central nicaragua revealing lateral changes of the seismic properties of the subducting lithosphere seismic structure along both profiles is characterized by low velocities both in the crust and upper mantle velocities in the uppermost mantle are found to be in the range 7 3 7 5 km s 1 thus are 8 10 per cent lower than velocities typical for unaltered peridotites and hence confirm the assumption that serpentinization is a common process at the trench outer rise area offshore of nicaragua in addition a prominent velocity anomaly occurred within the crust beneath two seamounts here velocity reduction may indicate increased porosity and perhaps permeability supporting the idea that seamounts serve as sites for water percolation and circulation

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

a thick dacitic debris flow sequence lake of the woods greenstone terrane central canada resedimented products of archean vulcanian plinian and dome building eruptions abstract a 4 km thick vertically dipping south facing homoclinal intermediate to felsic volcaniclastic sequence in central lake of the woods is underlain and overlain by subaqueous tholeiitic basalt flows volcaniclastic lithologies comprise a lower tuff breccia and lapilli tuff assemblage an upper turbiditic greywacke formation and three lenticular upward fining conglomerate formations interspersed within other units tuff breccia and lapilli tuff are characterized by very thick poorly defined beds pronounced heterogeneity mostly angular blocks and lapilli and a plagioclase quartz crystal component that in places is incompatible with phenocryst populations in fragments these appear to be subaqueous debris flow deposits that resulted from multistage mixing during downslope mass transport of fragments produced by subaerial phreatomagmatic vulcanian type eruptions anomalous crystal contents represent intermixing of subaerial subplinian or plinian ash conglomerate formations comprise very thick poorly defined beds composed almost entirely of well rounded fine to medium grained trondhjemite they are also subaqueous debris flow deposits but were apparently derived from wave erosion of large flank dome complexes that periodically blocked normal transport of volcanic debris from higher on the volcano the upper greywacke represents a shift in eruptive processes to largely plinian or subplinian eruptions

detachment of part of the downgoing slab and uplift of the new hebrides vanuatu islands several seisniological observations suggest that there is a gap within the downgoing slab of australian lithosphere plunging beneath the new hebrides and ages of elevated coral tenaces on the new hebrides islands suggest that the islands arc rising rapidly we suggest that the creation of the gap in the slab within the last 1 m y occurred by part of the slab detaching from the rest and sinking rapidly into the underlying asthenosphere this lowered the downward force on the slab at shallower depths and therefore on the overriding plate and the continued decrease of this force as the deeper slab sinks freely has allowed the islands above it to rise although this suggestion cannot account for all aspects of the uplift of the islands it provides a simple mechanism for explaining the fairly young uplift and its distribution along the new hebrides arc

bimodal volcanism in archean greenstone belts exemplified by greywacke composition lake superior park ontario in the archean gamitagama lake greenstone belt a metagreywacke metasiltstone formation at least 3 6 km thick largely had an easily eroded unconsolidated subaerial pyroclastic felsic volcanic provenance the remains of this provenance are now represented by a 0 600 m thick felsic formation sandwiched between mafic flow formations in an adjacent and laterally equivalent 4 5 km thick metavolcanic sequence the felsic formation includes flows plus various pyroclastic units including crystal rich ash flow tuff volcanism erosion transportation and sedimentation appear to have been essentially contemporaneous with subaerially erupted ash that was being transported rapidly by rivers to the sedimentary basin where it was deposited in a submarine fan angular to rounded framework sand grains form 39 of the greywacke in the proportion 49 plagioclase 36 quartz and 15 lithic clasts dominantly felsic volcanic despite the moderately high quartz content a felsic pyroclastic volcanic provenance can be sho

quaternary uplift of the torres islands northern new hebrides frontal arc comparison with santo and malekula islands central new hebrides frontal arc coral reef terraces on the torres islands have recorded the quaternary uplift of part of the northern new indian plate underthrusts the arc from the west 14c and 230thp34u ages for torres fossil corals indicate that for approximately the past 100 000 years the islands uplifted at a constant rate ranging geographically from 0 7 to 0 9 yr the occurrence of similar uplift rates and eastward tilting directions and the absence of major crosscutting structures or rupture zone boundaries suggest that the islands may be part of a single tectonic block or arc segment the frontal arc morphology in the torres area is typical of frontal arcs farther south the central new hebrides frontal arc has no physiographic trench at the plate boundary west of santo and malekula islands and the axes of the maximum uplift rate are at the seaward edge of the plate the d entrecasteaux ridge dr a massive e w trending bathymetric feature on the indian plate is underthrusting santo and malekula the eastern parts of santo and malekula are physiographically equivalent to the torres islands and seem to have been on a former axis of maximum uplift rate however in response to subduction of the dr the uplift axes of santo and malekula may have shifted westward in a few hundred thousand years the new uplift axes caused the western parts of these islands to achieve their present topographic dominance over the earlier uplift axes holocene uplift rates on southern santo and northern malekula are about twice the average rates of the previous 100 000 years the accelerated holocene uplift could be explained if a particularly prominent part of the dr has recently underthrust santo and malekula d hebrides frontal arc these isles lie about midway between the volcanic chain and trench axis where the

anomalous low velocity zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture zone 1 a detailed investigation of the hypocenter distribution beneath kanto japan reveals a nw se trending linear alignment of seismicity within the subducted pacific slab we estimate the 3d seismic velocity structure in the pacific slab to understand the factors controlling the genesis of such intraslab earthquakes a narrow low velocity zone is imaged within the subducted slab over a length of 150 km which partly penetrates into the mantle portion of the slab the low velocity zone correlates in space with the nw se trending earthquake cluster a reactivation of hydrated fracture zone formed prior to subduction is probably related to the low velocity anomaly dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone accompanied by intraslab earthquakes these observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes