highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

sheath fold like structures in pseudotachylytes abstract sheath fold like structures in pseudotachylyte samples from a gold mine in the witwatersrand basin south africa are explained by injections of frictionally produced silicate melt within the shear zone due to the formation of high and low pressure zones if the shear zone geometry is known the orientation of the fold closures can be used to determine the direction and the movement sense however if it is not possible to establish the shear zone geometry only the azimuth of movement can be estimated from the orientation of the sheath fold like folds the flow banding in these fault rocks is indicative of laminar flow which suggests a low reynold s number

ultracataclasis sintering and frictional melting in pseudotachylytes from east greenland abstract large volumes of pseudotachylyte an intrusive fault related rock interpreted to form by a combination of cataclasis and melting occur in tertiary normal faults and accommodation zones along 400 km of the east greenland volcanic rifted margin analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures mineralogies and chemistries in the aphanitic pseudotachylyte matrix three distinct types of pseudotachylyte referred to as angular rounded and glassy are identified based on these characteristics angular pseudotachylyte found primarily in dike like reservoir zones is characterized by angular grains visible on all scales with micron scale fragments of mica and amphibole its matrix is enriched in fe2o3 mgo and tio2 relative to the host rock with minor increases in cao k2o and small decreases in na2o rounded pseudotachylyte is found in reservoir zones injection veins pseudotachylyte filled extension fractures and fault veins small faults with pseudotachylyte along their surfaces it is characterized by smooth surfaced compacted grains on microscopic scales and encloses rounded interpenetrative lithic clasts on outcrop scale its matrix is enriched in fe2o3 mgo tio2 and al2o3 relative to the host rock with minor depletion in na2o and k2o glassy pseudotachylyte is found primarily along fault surfaces its matrix is characterized by isotropic conchoidally fractured material containing microscopic strain free amphibole phenocrysts and is enriched in tio2 al2o3 k2o fe2o3 mgo cao and na2o relative to the host rock these observations suggest that angular pseudotachylyte was produced by cataclasis with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting producing rounded and glassy pseudotachylyte respectively compositional and textural observations constrain the temperatures reached during frictional heating 700 900 c which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip our results suggest that the east greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels consistent relative ages and widespread occurrence of pseudotachylyte bearing faults in east greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin

shrimp zircon and titanite u pb ages lu hf isotope signatures and geochemical constraints for 2 56 ga granitic magmatism in western dharwar craton southern india evidence for short lived neoarchean episodic crustal growth abstract shrimp u pb zircon and titanite age data for granites adjacent to the gadag greenstone belt ggb in the western dharwar craton wdc india establish the occurrence of voluminous 2 56 ga granitic magmatism and indicate the development and evolution of the ggb over a time span of 70 m y these new data show that the ggb has an evolutionary history distinct from the chitradurga greenstone belt cgb to the south which was previously considered to be linked to that of the ggb based on the continuity of lithological associations whole rock geochemical data indicate granites marginal to the ggb are potassic were derived from intracrustal melting of an arc crust without the involvement of mantle and demonstrates the prevalence of horizontal tectonics in the evolution of dharwar craton lu hf zircon studies indicate that these rocks were derived from a heterogeneous arc crust with distinct crustal histories on either side of the ggb the voluminous 2 56 ga granitic magmatism surrounding the ggb in the wdc in addition to the known 2 61 and 2 56 2 52 ga felsic magmatism supports a number of short lived episodic crustal growth events within the dharwar craton during the neoarchean over a period of 100 m y short lived episodic crustal growth during the archean typically within the time scale of few million years marks the distinction with long lived modern style subduction processes these variations are often attributed to hotter archean mantle and prevailing geodynamic regime

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

late quaternary changes in desert dust inputs to the red sea and gulf of aden from 87sr 86sr ratios in deep sea cores abstract strontium isotope ratios of the hcl insoluble residue isr and foraminifera of cores from the red sea and gulf of aden are used to monitor effects of hydrothermal fluvial and desert dust transport to these regions during the past 0 5 ma while the gulf of aden was open ocean during low glacial sea levels the red sea was a semi isolated basin allowing the possibility to study the effects regional versus global inputs during glacial interglacial cycles the isr from the gulf of aden and the red sea display different ranges of 87 sr 86 sr ratios of 0 7085 0 7107 and 0 7062 0 7085 respectively these reflect mixtures between three components granitic hydrothermal and loess strontium with representative 87 sr 86 sr of 0 711 0 706 and 0 7085 respectively gulf of aden isr represent mixtures of the loess and granitic sources while red sea isr are mixtures of the loess and sea floor hydrothermal sources in the gulf of aden loess sources dominate during glacials indicating intensification of the ne moonsonal wind regime and granitic sources dominate during interglacials reflecting wetter conditions related to an enhanced regional sw monsoon red sea isr show no clear glacial interglacial distinction but display a general temporal increase in 87 sr 86 sr ratios over the past 380 ka toward loess like values indicating increasing loess contributions toward the present day the ranges of isr 87 sr 86 sr ratios in the red sea and the gulf of aden were distinct prior to the last glacial period 87 sr 86 sr ratios following major climate transitions at 10 80 130 190 240 and 330 ka bp that coincide with sapropel episodes in the eastern mediterranean which originated from the african monsoonal system and indicate enhanced wetness in the desert belt 87 sr 86 sr ratios of foraminifera show a very narrow range from 0 70912 to 0 70917 over 530 ka and in most samples are consistent with the contemporaneous global ocean in the red sea foraminifera and pteropods show slightly more variability than the gulf of aden a few red sea samples fall slightly above the seawater trend in marine isotope stages 5 and 9 and below during the last deglaciation suggesting local effects that occurred when the flow of surface ocean water from the gulf of aden to the red sea was limited and the red sea behaved like an amplifier basin

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

subduction zone metamorphism calc alkaline magmatism and convergent margin crustal evolution abstract at the dawn of the age of plate tectonics akiho miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages described contrasting geologic occurrences and inferred their characteristic ranges of physical conditions he advanced the paired metamorphic belt concept involving an oceanward narrow high p low t blueschist zone intimately intermixed with ophiolites and a landward broad low p high t realm associated with arc volcanic plutonic rocks more recent studies have illuminated the following 1 two main types of convergent plate junction can be distinguished pacific underflow of thousands of km of oceanic lithosphere and alpine closure of an intervening oceanic basin leading to continental collision 2 plate descent carries mafic felsic lithologies to depths of 35 120 km or more at relatively low temperatures producing high pressure hp pacific type and ultrahigh pressure uhp alpine type metamorphic terranes respectively 3 exhumed hp uhp complexes display low aggregate bulk densities reflecting chiefly buoyancy driven ascent of quartzofeldspathic allochthons 4 at a depth of 35 km in a warm lithosphere subduction zone sialic crust should melt if an aqueous fluid were abundant it does not hence the activity of h2o must be low at such depths 5 global observations indicate that volcanic plutonic arcs are sited 100 20 km above subducting oceanic plates devolatilization of sinking hydrated oceanic crust mantle promotes the generation of calc alkaline arc melts in this magmagenic zone 6 pacific type underflow of basaltic crust capped plates produces new and recycled continental crust whereas alpine type convergence reshuffles collided terranes but does not generate juvenile sialic crust

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

immiscible sulfide droplets in pseudotachylyte evidence for high temperature 1200 c melts abstract in the north cascade mountains washington rocks that underwent friction melting commonly contained sulfide minerals mostly pyrrhotite and pyrite during pseudotachylyte melt formation sulfides melted to form immiscible sulfide droplets present in five distinct textural settings the largest droplets formed through melting of lithic clasts whereas micron scale sulfide droplets are common in many of the pseudotachylytes veins microprobe analysis indicates that nearly all droplets are pyrrhotite the disappearance of pyrite indicates that melt temperatures must have exceeded 750 c but other indications suggest that the melt temperature must have been much higher the extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted requiring a minimum melt temperature of 1200 c in some samples evidence for fluid rich bubbles and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt the presence of sulfide droplets appears to be common especially in relatively low oxygen fugacity melts that formed at shallow crustal levels this can provide a good textural marker of melting and therefore of pseudotachylyte formation especially where other indications of melting i e high temperature microlites vesicles etc are lacking and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events

the importance of late and post orogenic crustal growth in the early proterozoic evidence from smnd isotopic studies of igneous rocks in the makkovik province canada abstract sm nd isotopic studies suggest that large tracts of 1900 1700 ma old crust in laurentia baltica is of juvenile origin this crust has generally been ascribed to arc magmatism sustained over long periods and most conceptual models for crustal growth emphasize this process the late to post orogenic granitoid rocks that areally dominate many ancient orogenic belts are commonly viewed as anatectic derivatives of the earlier arc type crust however in regions of short crustal residence the time resolution of isotopic tracers such as the sm nd system does not permit discrimination between this model and continued growth of the crust during post orogenic magmatism the relative contributions of recycled crust and new mantle derived material in late to post orogenic magmas can only be assessed where they also transect much older crustal blocks the nd isotopic signatures of 1800 1720 ma igneous suites in the makkovik province define such a boundary between an archean craton and a juvenile proterozoic domain in the juvenile domain the nd signatures of most igneous suites are equivocal initial nd 0 to 2 and they could be anatectic derivatives of slightly older orthogneisses however addition of new mantle derived material is documented by a type granites with initial nd up to 4 in the cratonic domain temporally and compositionally equivalent igneous suites mostly have initial nd of 7 to 3 significantly above local archean basement which has nd of 15 at 1800 ma conservative calculations suggest that most of these suites contain more than 50 new mantle derived material these results suggest significant crustal growth via late stage magmatism and direct interaction of mantle derived magmas and lower crustal rocks in contrast 1650 ma igneous suites lack clear systematic variation in nd and are interpreted as representing crustal growth via later distal arc type magmatism with crust mantle mixing mainly facilitated by sediment subduction crustal growth in this area thus involved significant addition of new material through late to post orogenic magmatism this process may partially explain the unusually high growth rates calculated for some segments of proterozoic crust which approach or exceed modern global arc addition rates this stage 3 crustal growth may have been proportionally more important in early earth history due to higher ambient mantle temperatures

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

initiation of leaking earth an ultimate trigger of the cambrian explosion abstract for life to have dramatically evolved and diversified during the so called cambrian explosion there must have been significant changes in the environmental conditions of earth a rapid increase in atmospheric oxygen which has been discussed as the key factor in the evolution of life cannot by itself explain such an explosion since life requires more than oxygen to flourish let alone survive the supply of nutrients must have played a more critical role in the explosion including an increase in phosphorus p and potassium k which are key elements for metabolisms to function so what happened at the onset of the cambrian to bring about changes in environmental conditions and nutrient supply and ultimately evolution of life an ultimate trigger for the cambrian explosion is proposed here the geotherm along subduction zones of a cooling earth finally became cool enough around 600 ma to allow slabs to be hydrated the subduction of these hydrated slabs transferred voluminous water from the ocean to the mantle resulting in a lowering of the sea level and an associated exceptional exposure of nutrient enriched continental crust along with an increase in atmospheric oxygen this loss of water at the surface of the earth and an associated increase in exposed landmass is referred to here as leaking earth vast amounts of nutrients began to be carried through weathering erosion and transport of the landmass rock fragments of the landmass would break down into ions during transport to the ocean through river providing life forms prokaryote sufficient nutrients to live and evolve also plume driven dome up beneath the continental crusts broadened the surface area providing a supply of nutrients an order magnitude greater than that produced through uplift of mountains by continental collision simultaneously atmospheric oxygen began to increase rapidly due to the burial of dead organic matter by enhanced sedimentation from the emergence of a greater landmass which ultimately inhibited oxidation of organic matter hence oxygen began to accumulate in the atmosphere which when coupled with a continuous supply of nutrients resulted in an explosion of life including an increase in the size an enhanced oxygen supply in the atmosphere resulted in the formation of an ozone layer providing life a shield from the uv radiation of the sun this enabled life to invade the land in addition to a change in the supply of nutrients related to a leaking earth the evolution of life was accelerated through mass extinction events such as observed during snowball earth possibly related to a starburst in our galaxy as well as mutation in the genome due to radiogenic elements sourced from carbonatite magma atomic bomb magma in rift valley there are two requirements to find a habitable planet 1 the initial mass of an ocean and 2 the size of a planet these two conditions determine the history of a planet including planetary tectonics and the birth of life this newfound perspective which includes the importance of a leaking planet provides a dawn of new planetary science and astrobiology

asteroids and early precambrian crustal evolution abstract major enigmas arising from the archaean crustal record are capable of resolution in terms of the combined role of episodic mega impacts and of long term mantle processes the terrestrial impact record suggests that for very large impact structures the degree of preservation of proximal deformation effects decreases with the increase in the magnitude of the associated thermal effects a consequence of extensive recrystallization and melting this focuses attention on thermal igneous and metamorphic peaks as possible clues of impact events isotopic age studies define principal thermal events about 3 5 3 0 and 2 7 0 1 gyr and less well defined episodes succeeded by protracted thermal fluctuations in infracrustal high grade metamorphic zones the bulk of the igneous activity associated with these episodes took place over relatively short time spans of c 20 30 106 years interpretations of these episodes in terms of internal dynamics of the earth are inconsistent with the thermal behavior of silicate rheologies in a continuously convecting mantle regime a triggering of these episodes by the mantle rebound response to intermittent extra terrestrial asteroid impacts is considered on the basis of 1 identification of major impacts from distal ejecta horizons marked by iridium anomalies and impact earthquake triggered diamictite breccia and olistostromes 2 the relationships between microtektite horizons and volcanic activity 3 geochemical and experimental evidence for mantle upwelling from the transition zone and catastrophic adiabatic melting required to generate peridotitic komatiites episodic differentiation accretion growth of sial consequent on these events is capable of resolving the volume problem which arises from comparisons between modern continental crust and sial produced by continuous two stage mantle melting processes taking the temporal decline in heat flow into account impact shock effects on the archaean crust have been largely obscured by 1 outpouring of voluminous basic ultrabasic lavas inundating shock deformed crust and extending beyond the perimeters of impact excavated basins 2 gravity subsidence and downfaulting of terrestrial mare accounting for the anatexis of sub greenstones basement 3 extensive shearing and recrystallization of impact structures breccias and mineral deformation features beneath impact excavated basins relics of which may be retained in structural windows in high grade metamorphic terrains isostatic subsidence and anatexis of thick mare type piles and underlying impacted crust resulted in intracrustal comagmatic plutonic and volcanic suites within periods in the order of15 30 106 years as controlled by post impact mantle convection cooling subsequent isostatic updoming of remobilized basement gneiss terrains and the rise of their anatectic products along major normal shears bearing an analogy to metamorphic core complexes resulted in the multiple gneiss granite complexes amidst greenstone dominated terrains recurrent post tectonic thermal fluctuations and magmatic activity reflect a long term existence of anomalous mantle regions beneath original mega impact sites and thermal perturbations related to distal impacts a broad age zonation in some archaean shields suggests lateral migration and terrane accretion in a convection driven plate tectonic regime

frictional melting of sedimentary rock during high speed diamond drilling an analytical sem and tem investigation abstract high speed drilling at 900 rpm with a downhole motor driven natural diamond drag bit has resulted in the extreme alteration of sedimentary rock in an offshore canadian wildcat well these effects as revealed in the rock cuttings retrieved from the drilling fluid at surface include discolouration darkening cataclasis slickensiding and partial fusion calcareous siltstones are the most susceptible to alteration especially melting whereas sandstone and limestone cuttings mainly exhibit fracturing and surface glazing analytical scanning electron microscopy reveals that the calcareous siltstones have undergone decalcification via decarbonation and the loss of cao to the drilling fluid decarbonation combined with dehydration resulted in the vesiculation and explosive fragmentation of certain cuttings analytical transmission electron microscopy shows that glass is present in the calcareous siltstone cuttings most of the modifications exhibited by the sedimentary rocks can be attributed to the effects of high strain rate deformation and frictional heating at the bit rock interface that melting took place is supported by thermal calculations which indicate that for a coefficient of friction of 0 5 temperatures of approximately 1200 c were realized at diamond rock contacts such high temperatures can accelerate the wear of diamond and its substrate and hence shorten bit life as well as obliterate potentially important geological information in the rock cuttings

crustal volumes of the continents and of oceanic and continental submarine plateaus global topographic data and the assumption of airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus the calculated crustal volumes are 7182 x lo6 km3 for the continents 242 x lo6 km3 for continental submarine plateaus and 369 x lo6 km3 for oceanic plateaus the falkland plateau and the lord howe rise are the two largest continental submarine plateaus with volumes of 48 x lo6 km3 and 47 x lo6 km3 respectively total continental crustal volume is 7581 x lo6 km3 including the volume of continental sediments on the ocean floor 160 x lo6 km3 in good agreement with previous estimates continental submarine plateaus on the seafloor comprise 3 2 of the total continental crustal volume the largest oceanic plateaus in order of decreasing size are the ontong java plateau the kerguelen plateau the caribbean the chagos laccadive ridge the ninetyeast ridge and the mid pacific mountains together they comprise 54 of the total anomalous crustal volume in oceanic plateaus an upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3 7 km3 yr a value that assumes accretion of all oceanic plateaus with a total volume of 4 9 of the continental crustal volume on a 100 myr time scale even if a substantial fraction of the crustal volume in oceanic plateaus is subducted accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate subduction of continental submarine plateaus with the oceanic lithosphere on a 100 myr time scale gives an upper bound to the continental crustal subtraction rate of 2 4 km3 yr much larger than recent estimates of crustal subtraction by subduction of seafloor sediments effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus a potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents entrapment of the continental fragments in the seafloor and subduction of the fragments with the oceanic lithosphere this process may be occurring in the mediterranean for corsica and sardinia

multi stage modification of paleoarchean crust beneath the anabar tectonic province siberian craton abstract according to present views the crustal terranes of the anabar province of the siberian craton were initially independent blocks separated from the convecting mantle at 3 1 daldyn terrane 2 9 magan terrane and 2 5 ga markha terrane rosen 2003 2004 rosen et al 1994 2005 2009 previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the markha terrane concluded that the evolution of the crust of the markha terrane is very similar to that of the daldyn terrane to test this conclusion we present results of u pb and hf isotope studies on zircons in crustal xenoliths from the zapolyarnaya kimberlite pipe upper muna kimberlite field located within the daldyn terrane and the botuobinskaya pipe nakyn kimberlite field in the center of the markha terrane the data on xenoliths from the botuobinskaya kimberlite pipe record tectonothermal events at 2 94 2 8 2 7 and 2 ga the event at 2 ga caused pb loss in zircons from a mafic granulite u pb dating of zircons from the zapolyarnaya pipe gives an age of 2 7 ga all zircons from the studied crustal xenoliths have archean hf model ages ranging from 3 65 to 3 11 ga this relatively narrow range suggests that reworking of the ancient crust beneath the nakyn and upper muna kimberlite fields was minor compared with the daldyn and alakit markha fields shatsky et al 2016 this study when combined with dating of detrital zircons implies that tectonic thermal events at 2 9 2 85 2 75 2 7 and 2 0 1 95 ga occurred everywhere on the anabar tectonic province and could reflect the upwelling of superplumes at 2 9 2 7 and 2 ga the presence of the same tectonic thermal events in the daldyn and markha terranes rosen et al 2006a b supports the conclusion that the identification of the markha terrane as a separate unit is not valid

the history of a continent from upb ages of zircons from orinoco river sand and smnd isotopes in orinoco basin river sediments abstract we report shrimp u pb ages of 49 zircons from a sand sample from the lower orinoco river venezuela and nd model ages of the fine sediment load from the main river and tributaries the u pb ages reflect individual magmatic or metamorphic events the sm nd model ages reflect average crustal residence ages of the sediment sources together they allow delineation of the crust formation history of the basement precursors of the sediments the u pb ages range from 2 83 to 0 15 ga and most are concordant or nearly so discrete age groupings occur at 2 8 2 1 and 1 1 ga the oldest group contains only three samples but is isolated from its closest neighbors by a 600 ma age gap larger age groupings at 2 1 and 1 1 ga make up about a third and a quarter of the total number of analyses respectively the remaining analyses scatter along concordia and most are younger than 1 6 ga the 2 8 and 2 1 ga ages correspond to periods of crust formation of the imataca and trans amazonian provinces of the guyana shield respectively and record intervals of short but intensive continental growth these ages coincide with 2 9 and 2 1 ga nd model ages of sediments from tributaries draining the archean and proterozoic provinces of the guyana shield respectively indicating that the u pb ages record the geological history of the crystalline basement of the orinoco basin zircons with ages corresponding to the major orogenies of the north atlantic continents the superior at 2 7 ga and hudsonian at 1 7 1 9 ga were not found in the orinoco sample the age distribution may indicate that south and north america were separated throughout their history nd model ages of sediments from the lower orinoco river and andean tributaries are 1 9 ga broadly within the range displayed by major rivers and dusts this age does not coincide with known thermal events in the region and reflects mixing of sources with different crust formation ages the igneous and metamorphic history of these sources as recorded by the detrital zircons is that of the orinoco basin basement this implies that despite evidence of fast sedimentary recycling global similarities in nd crustal residence ages and the probability of cross continent mixing through continental drift the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin the global semblance in nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large scale sampling of the continents which are heterogeneous in age on smaller regional scales a large portion of the continental crust in the orinoco basin formed during the trans amazonian orogeny at 2 0 2 1 ga and smaller portions formed both earlier at 2 8 ga and later after 1 6 ga these observations which are consistent with the relative sizes of crustal age provinces in the orinoco basin indicate that sediments from the lower orinoco and andean tributaries contain 25 35 of material added to the crust since trans amazonian times nd model ages of these sediments underestimate the average crust formation age of the basement of the orinoco basin by only about 10 if this relationship holds in other river basins then nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is 1 9 2 ga

injection veins of crushing originated pseudotachylyte and fault gouge formed during seismic faulting this paper describes the injection veins of crushing originated pseudotachylyte and fault gouge formed along the iida matsukawa fault in granitic cataclasite southern nagano prefecture central japan the pseudotachylyte which shows a dense and aphanitic appearances and fault gouge occur as simple veins fault vein along the main fault plane and as complex network veins injection vein in the neighboring cataclasite locally the injection veins of pseudotachylyte and fault gouge can be traced back to the fault veins powder x ray diffraction patterns and petrological analysis indicate that both of the pseudotachylyte and fault gouge consist entirely of fine grained angular clasts and that the pseudotachylytes and fault gouges have a similar x ray diffraction patterns with that of the host granite the similarity of chemical compositions and distribution patterns of grain size also show that the injection veins of pseudotachylyte and fault gouge have the same source material as that of fault veins field occurrences and petrological characteristics strongly suggest that the injection veins of pseudotachylyte and fault gouge formed during seismic faulting by a rapid intrusion and fluidization of crushed fine grained materials generated in the shear zone in a gas solid fluid system

a rift drift delamination model of continental evolution palaeozoic tectonic development of eastern australia the adelaide lachlan and new england fold belts of eastern australia record the continental growth of eastern gondwanaland during the palaeozoic the new england fold belt nefb represents a tectonic collage formed by subduction accretion during the late paleozoic but contains remnants of subduction related rocks that date from the cambrian the lachlan fold belt lfb developed inboard from the nefb initially as an amalgam of proterozoic continental and cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern gondwanaland convergent tectonism at 500 ma welded the fragments to the craton and formed the adelaide fold belt oceanward dispersal of detritus across the lfb produced an overlap assemblage of quartzose ordovician turbidites and a new subduction zone developed at the eastern margin the tectonic setting was similar to the modern philippines plate of the western pacific behind the arc silurian devonian convergent tectonics converted the 1700 2000 km wide continental margin of the proto lfb into a 750 km wide thin skinned fold magmatic belt within 60 ma the driving force was delamination which produced a ubiquitous basaltic underplate that generated 1 regional low p metamorphism 2 local anticlockwise p t t paths and crustal scale isobaric cooling 3 voluminous syn to late tectonic granitoids emplaced at rates approximately twice that of modern arcs and 4 normal thickness crust despite an average of 60 shortening delamination occurred in two separate areas originally 1000 km apart first in the eastern then in the western part of the lfb the lfb and its northern counterpart the thomson fold belt represent approximately one fifth of the australian continent and evolved from dispersed fragments into stable continental crust in 300 ma such rapid growth rates are typical of the 2 7 2 5 ga and 1 9 1 7 ga periods when voluminous widespread granitoids and large turbidite dominated low p metamorphic belts were produced in settings that are not obviously subduction related therefore a similar process of rift drift delamination riddel tectonics may have periodically operated throughout earth history

rapid growth of some major segments of continental crust some major segments of continental crust display a narrow range of sm nd crustal formation ages the sizes of the canadian shield the svecokarelian province of northern europe the west central united states and the arabian nubian shield suggest rapid crustal growth island arc accretion models rank among the most favored tectonic models for the formation of these areas a quantitative comparison of the growth rates of these crustal segments to mesozoic cenozoic arc addition rates shows however that island arc accretion alone seems insufficient to account for the amount of crust that was produced in each of these terrains other additional mechanisms such as hot spot volcanism and underplating may have been active in addition to arc accretion alternatively large amounts of preexisting basement have gone so far undetected

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

late carboniferous high end t ehf t granitoids enclaves and dikes in western junggar nw china ridge subduction related magmatism and crustal growth abstract we report results of petrologic geochronological and geochemical investigation of the late carboniferous diorites granodiorites amphibole am bearing granites and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the keramay area of the western junggar region of central asian orogenic belt caob zircon u pb dating suggests that they were generated in the late carboniferous 316 304 ma the diorite and granodiorite compositions extend over a wide range of sio2 53 70 wt sr 240 602 ppm and mg 41 58 values and are characterized by moderately fractionated rare earth element ree patterns nb ta depletion and relatively low y and yb contents the mafic dikes consist of dolerites diorite porphyries and minor granodiorite porphyries and have variable sio2 51 59 wt and high mg cr and ni values with the exception of two samples with relatively high heavy ree hree contents the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites the am bearing granites and a granite porphyry dike sample have high levels of sio2 73 77 wt hrees e g yb 3 46 15 7 ppm and low mg cr and ni contents along with clearly negative eu ba and sr anomalies similar to typical a type granites all granitoids enclaves and dikes in this region have high positive end t 7 13 to 9 74 and zircon ehf t 10 to 16 values and moderate initial 87sr 87sr ratios 0 7004 0 7049 mineral composition data suggest that the parental magmas for mafic dikes are similar to cenozoic sanukitoids in the setouchi arc area japan and were possibly generated under water rich and high oxygen fugacity nno 1 5 to nno 2 7 conditions they most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust derived melts and minor fluids and subsequently underwent fractional crystallization the diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust derived magmas coupled with minor crystal fractionation the am bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels taking into account widespread contemporaneous magmatism including morb type basalts and slab derived adakites in western junggar we suggest that the keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window which played an important role in the crustal growth of the caob

the bothnian coupled oroclines of the svecofennian orogen a palaeoproterozoic terrane wreck the accretion of magmatic arcs gives rise to elongate linear orogens and is a key process in forming new continental crust many precambrian continents are however presently equidimensional or have large areas without any clear linearity such as the central part of the palaeoproterozoic svecofennian orogen 1 92 1 77 ga one way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines here we reinterpret existing data and demonstrate the occurrence of coupled bothnian oroclines in the svecofennian orogen palinspastic restoration of the southern and northern bothnian oroclines brings a 1000 km long segment of the svecofennian orogen into an originally linear nw striking geometry that restores the lithological belts metamorphic zones and structural vergences to a common direction and which indicates that the orogen consists of a sw facing arc which has been shortened along ne verging folds and thrust faults

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

the oldest cambrian skeletal fossils of spain cadenas ibericas aragon diverse skeletal assemblage has been discovered in the ediacaran cambrian strata of the codos locality in ne spain aragon cadenas ibericas this assemblage includes at least seven genera only three of which can be ascribed to known taxa all the fossils are preserved in phosphate but their original microstructures are traceable by both elementary composition and microstructural features that are indicative for primary aragonite and high magnesium calcite biomineralogies already these early skeletal fossils show sophisticated microstructures represented by heterogeneous multilayered composites to satisfy the requirements for better protection against both chemical dissolution and mechanical predator deterioration the most common and best preserved fossil which is tubicolous codositubulus grioensis gen et sp nov is described here the composition of the fossil assemblage and its stratigraphic position are indicative for the lowermost terreneuvian lower cambrian or even pre terreneuvian age of this fauna

the nature and significance of pseudotachylite from the nason terrane north cascade mountains washington abstract pseudotachylite has been recently discovered at numerous localities in the nason terrane of the north cascade mountains washington the pseudotachylite is widespread not associated with any single major fault or shear zone and tends to occur in narrow foliation parallel zones single pseudotachylite veins range in thickness from less than 0 1 to 10 cm veins and networks of veins occur in six distinct types and these form the basis of a suggested classification scheme multiple generations of pseudotachylite are present in some samples host lithologies include quartz diorite semipelitic schist tonalite amphibolite and especially graphitic pelitic schist glassy veins are rare most pseudotachylite is apparently holocrystalline spherulites vesicles and dendritic microphenocrysts are present and indicate a frictional melt origin the ubiquitous occurrence of cataclasite xenoliths within the pseudotachylite along with other textural observations indicates that in the nason terrane the development of cataclasite zones was the precursor to pseudotachylite generation it is inferred that cataclastic deformation of the schist provided the strain softening which resulted in faulting and pseudotachylite generation the water rich and ultra fine grained matrix of the cataclasite was an ideal starting material for pseudotachylite generation based upon bulk chemistry of host lithologies and microprobe analyses of the pseudotachylite it appears that the pseudotachylite formed as a result of a virtually complete melting of the pelitic i e non quartz vein component of the schist where pseudotachylite occurs within pelitic schist alone the composition of the pseudotachylite is very similar to the pelitic component of the schist while the composition of veins cutting tonalite gneiss in addition to pelitic schist appears to define a mixing line between these two lithologies this suggests either participation of the tonalite during initial melting or subsequent assimilation of gneiss into the melt

crustal structure beneath the montserrat region of the lesser antilles island arc 1 we use receiver function analysis to estimate the first order subsurface seismic structure in the vicinity of the island of montserrat located in the northern lesser antilles arc and home of the active soufriere hills volcano near surface complexity in the island structure inhibits our ability to resolve lateral variations in seismic structure so we average the signals from eight stations in two directions to produce a first order regional seismic velocity model although we are unable to resolve a precise sharpness of the crust mantle transition we can limit its thickness to less than 4 km lateral variations in shallow structure complicate a receiver function based estimate of poisson s ratio but the data indicate that in both directions sampled by the receiver functions the lower crustal poisson s ratio is greater than 0 27 and more likely in the range 0 29 0 30 we estimate a mean crustal thickness of 30 km and the observations suggest that the crust may be slightly thinner northwest of the island 26 30 km than it is to the south 30 34 km the high p wave speed and poisson s ratio indicate a generally mafic lower crust with rocks of intermediate composition not precluded in the upper part of the lower crust we did not find evidence for a thick high speed lower crust 7 4 km s as has been inferred in some other arcs

ductile deformation of biotite in foliated cataclasite iida matsukawa fault central japan abstract the textures of foliated granitic cataclasites found along the iida matsukawa fault nagano prefecture central japan are examined at mesoscopic and microscopic scale the foliated cataclasites are mainly characterized by preferred orientation of biotite clasts and grain aggregations of quartz and feldspar porphyroclasts unlike cataclastically fractured quartz and feldspar in the foliated cataclasite biotite shows some fabric characters of ductile deformation like those observed in mylonites such as cleavage step mica fish elongated grain shapes and kink bands these fabric variations between the biotite quartz and feldspar suggest that there are marked differences in their relative deformational behavior that the former mainly ductiley deformed and the latter two mainly by brittle processes it is estimated that the foliated granitic cataclasite formed at temperatures between 150c and 250 c corresponding to depths of 5 8 km

petrogenesis of early syn tectonic monzonite granodiorite complexes crustal reprocessing versus crustal growth abstract the 563 7 6 1 ma old early syntectonic mon repos complex is a predominantly metaluminous magnesian calc alkalic granodioritic to granitic intrusion major and trace element variations imply that the rocks evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite rocks less evolved than granodiorite show evidence of hornblende accumulation initial sr 87sr 86sr 0 7090 0 7111 and nd end 5 3 to 12 3 isotopic compositions of the granodiorites and granites are highly heterogeneous and vary with sio2 contents indicating that assimilation of older crustal components occurred initial 206pb 204pb 16 65 17 65 and 207pb 204pb ratios 15 52 15 60 are similar to other mafic intermediate complexes from the damara belt one monzodiorite and two quartz monzonites are k2o lile and hfse enriched and have similar sio2 contents as the granodiorites these samples are similar to post collisional mafic magmas from elsewhere in the world their e nd values 3 8 to 4 3 87sr 86sr ratios 0 7051 0 7073 and trace element characteristics imply that their unexposed parental melts are derived from a lithospheric mantle source that was contaminated and metasomatized by crustal material during ancient subduction processes the data are explained by assuming a flat subduction model where melting predominantly involves ancient continental mantle lithosphere and crust with limited if any melting of the underlying asthenospheric mantle during flat subduction a sliver consisting of buoyant oceanic lithosphere was amalgamated with the base of the ancient continental lithosphere of the overriding plate the oceanic mantle lithosphere and the oceanic crust dehydrated but did not melt and these fluids lowered the solidus of the overlying continental mantle lithosphere and crust this scenario can explain the occurrence of rare k2o and lile enriched monzodiorites and quartz monzonites with crustal like isotopic compositions observed in this study as well as some more alkaline rocks in the belt because the metasomatized continental lithospheric mantle has lower solidus temperatures than peridotitic mantle it is very likely that such metasomatized rocks may melt early during flat subduction thus k2o enriched monzodiorites to monzogranites are not restricted to late stages of the evolution of an orogen but may already form at the onset of an orogeny

particle size and energetics of gouge from earthquake rupture zones surprisingly considering the forces that we see at the surface not all the energy involved in an earthquake is transformed into ground motion much of it is consumed by frictional heating and rock pulverization an analysis of pulverized rock or gouge collected from the san andreas fault in california and from a rupture zone at 2 km depth in a south african gold mine contradicts the commonly held view that gouge surface energy is a negligible component of earthquake energy balance the analysis reveals unexpected diminutive particles in gouge samples indicating that pulverization not frictional heating is the major consumer of earthquake energy at depth this might explain the heat flow anomaly seen in the san andreas fault system and perhaps some other long standing puzzles in earthquake energetics grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales1 2 3 4 and most slip along mature faults is observed to have been localized within gouge zones5 6 this fine grain gouge is thought to control earthquake instability3 6 7 8 and thus understanding its properties is central to an understanding of the earthquake process7 9 here we show that gouge from the san andreas fault california with 160 km slip and the rupture zone of a recent earthquake in a south african mine with only 0 4 m slip display similar characteristics in that ultrafine grains approach the nanometre scale gouge surface areas approach 80 m2 g 1 and grain size distribution is non fractal these observations challenge the common perception that gouge texture is fractal10 11 and that gouge surface energy is a negligible contributor to the earthquake energy budget3 9 12 we propose that the observed fine grain gouge is not related to quasi static cumulative slip but is instead formed by dynamic rock pulverization during the propagation of a single earthquake

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

amorphous material formed by the mechanochemical effect in natural pseudotachylyte of crushing origin a case study of the iida matsukawa fault nagano prefecture central japan abstract pseudotachylytes from the iida matsukawa fault central japan iida pseudotachylytes are described down to the nanometer scale using high resolution electron microscopy scanning electron microscopy analyses of chemical compositions and mercury intrusion porosimetry the pseudotachylyte matrix chiefly consists of nanoscale particles and elongated submicron fragments with biotite composition amorphous material of several tens of nanometers in size is also observed despite the absence of melt textures lattice fringe images reveal that the amorphous phase coexists with lattice distortion in deformed biotite fragments that are several hundreds of nanometers in size the presence of these submicrostructures indicates that the amorphous material formed by mechanical stress during the comminution process that is via the mechanochemical effect we conclude that amorphous material does not always provide evidence of the rapid cooling of melt in pseudotachylyte the amorphous materials present in the iida pseudotachylyte resulted from the mechanochemical effect due to both shear stress and normal stress during the comminution process that accompanied fault motion

roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting abstract we examine the roundness rd of clasts derived from fault rocks and discuss its possible application as an indicator for fracturing and chipping or frictional melting the roundness of quartz and feldspathic clasts derived from four frictional melting originated pseudotachylytes and one crushing without melting originated pseudotachylyte as well as three cataclastic rocks was measured between 230 and 270 clasts 10 m in size were measured for each sample the analyses show that rd is lower than 0 4 in all the clasts included in the cataclastic rocks and crushing originated pseudotachylyte whereas it varies from 0 1 to 1 0 and 35 90 of clasts have a roundness 0 4 in the melting originated pseudotachylyte it is suggested that the clasts having a roundness 0 4 were rounded by frictional melting rather than by fracturing or chipping in pseudotachylytes and that the roundness of clasts may be used as a special index of disintegration attributed to attrition and melting

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

ridge collision at convergent margins implications for archean and post archean crustal growth abstract growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion lateral and vertical related to 1 1 steady state subduction 2 2 non steady state episodic processes ridge forearc collision is one episodic process that can cause significant highly diachronous thermal and tectonic effects when a buoyant bathymetrically high and thermally active oceanic spreading ridge approaches collides with and potentially subducts below a continental margin near field effects within 100 km of collision site in the forearc may include anomalous heat anomalous magmatism ophiolite obduction forearc deformation and tectonic erosion of the base of the forearc far field effects generally postdate collision and may include changes in the thermal structure of the continent changes in the composition of arc magmas a reduction or hiatus in arc magmatism uplift and or subsidence of the continental margin marginal basin formation underplating of subducted slabs and intraplate deformation effects of ridge collision can vary depending on the triple junction geometry the chile margin triple junction cmtj where the actively spreading chile ridge is colliding with the south american forearc is the best modern example of ridge collision near field effects of pliocene recent ridge collisions near the cmtj include 1 1 emplacement of the pliocene taitao ophiolite less than 15 km from the trench 2 2 intrusion of pliocene granitic plutons and 3 3 forearc deformation formation of a pull apart basin on the continental shelf and late tertiary recent movement on a major fault along the arc forearc boundary far field effects include reduction in subduction related seismicity and volcanism chemical changes in arc magmatism and possibly uplift and or subsidence ridge collison features in southern chile are similar to features of archean greenstone belts suggesting that some greenstone sequences may have formed in a ridge collision setting and that ridge collision may have been an important process in archean crustal growth these features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin mafic ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc alkaline and morb character intercalated with volcaniclastic sedimentary rocks differences between archean and phanerozoic tectonic processes mainly related to higher heat production in the past probably have affected the nature and importance of ridge collision in archean crustal growth during the archean greater ridge length and smaller plates probably resulted in more ridge collisions and related processes contributing to a higher continental growth rate in particular underplating of subducted slabs and direct slab melting may have been important processes in archean continental growth but are of relatively minor importance today

glassy pseudotachylyte veins from the fuyun fault zone northwest china glassy pseudotachylytes occur in granitic mylonites from the fuyun fault zone northwest china powder x ray diffraction data indicate that the fine grained matrices of the fuyun pseudotachylytes vary from almost crystalline to complete glass a few to 90 wt glass powder x ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains microlites displaying quenched or rapid cooling textures vesicles and amygdules flow structures and rounded fragments are also common in the fuyun pseudotachylytes the average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks but the glass matrix has a low sio2 component which is 5 15 wt lower than that of the host rocks the pseudotachylytes contain 2 3 wt water h2o using experimental data which relate the solubility of water to pressure approximately 400 bars lithostatic pressure corresponding to approximately 1 5 km depth of pseudotachylyte formation is estimated a minimum temperature estimate of 1450 c can be estimated due to the presence of the pure sio2 glass the x ray diffraction and chemical data indicate that the fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces

singularity analysis of global zircon u pb age series and implication of continental crust evolution abstract recently compiled global databases of igneous and detrital zircon u pb ages have been integrated with other types of isotope data e g neodymium hafnium and oxygen to characterize the episodic growth of the continental crust and the development of supercontinents however to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long standing question instead of analyzing amplitudes and periodicity of the age series here the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time series records the results indicate that zircon age distributions around peaks follow power law distribution regardless of the bin size used to measure the age distribution based on the commonly accepted mechanisms phase transition self organized criticality and multiplicative cascade processes for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to avalanches superplumes slab breakoff etc occurred during episodic convection of the mantle the exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity the values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature with a mean duration of approximately 600 800 myr both intensity of the zircon episodes and their duration for the interval from 3 ga to 0 5 ga depict a descending trend which may signify mantle cooling

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

the quasi rigid premise in precambrian tectonics abstract the proposition that movements between multiple continental fragments driven by plate tectonics have taken place during the earlier eons of geological time can be simply tested a random distribution will be derived when the collective palaeomagnetic poles are rotated into any quasi rigid reconstruction this elementary test surmounts the most serious limitation of precambrian palaeomagnetic data namely that the ages of magnetisation are mostly poorly known in the event the late archaean and proterozoic poles 2850 590 ma produce a non random distribution with a single peak which is only compatible with a quasi rigid continental configuration occupying a preferential position on the globe this solution is not favoured because uncertainties in definition of the poles contribute to their scatter and bias the solution towards the random one hence confirmation of the quasi rigid premise is derived in a debased form but is not suppressed by the deficiencies of the database this palaeomagnetic constraint explains the distinctive isotopic geochemical and lithofacies signatures of the proterozoic eon and is independently supported by the trend and long temporal continuity of crustal lineaments it does not preclude the opening of ocean basins between the shields but it does predict that development of these basins was limited by the influence of controlling mantle systems this constrained the continental crust into a form which was essentially rigid on the gross scale resolvable by palaeomagnetism

mid crustal dynamics and island arc accretion in the arabian shield insight from the earth s natural laboratory abstract the late proterozoic terrane of the arabian shield uniquely exposes the records of magmatic processes and deformation patterns that shaped the earth over half a billion years ago this precambrian to cambrian terrane is composed of supracrustal rocks underlain and intruded by igneous rocks the plutonic shedding and accretion of the crust has been studied in detail satellite images tm landsat and spot were processed to enhance both lithologic content and structural patterns using spectral transformation and spatial filtering techniques results obtained by integrating the image interpretation with detailed ground studies are discussed the supracrustal rocks display complex fold interference patterns associated with the emplacement of the basic plutons jarshah and mifsah and mantled gneiss domes qardath and miktaa which took place coeval with crustal shortening by regional deformation the intrusion of a younger pluton arafadi caused the collision of two pre existing plutons jarshah and mifsah involved extensive sideway shedding of supracrustal rocks and formed a major shear zone qurayn finally a subvolcanic complex mahala was emplaced which includes a 2 5 km radius ringdike of several hundred meters thickness the geologic features of our study area can be explained by a simple model of island arc evolution in six stages i ii pre tectonic and pre plutonic paleogeographic situation 0 9 to 0 66 ga iii syntectonic basic plutonism 0 65 ga iv syntectonic gneiss domes 0 62 ga v post tectonic acidic plutonism 0 59 to 0 54 ga and vi post orogenic volcanism 0 54 to 0 5 ga in our scenario the relative ages of events inferred on the basis of field relationships are compatible with the geochronologic ages of rock units previously dated by radiometric methods only the accretion rate of the arabian shield region is here estimated at 0 1 km3 yr 1 or 10 of the total global accretion rate at the present day the average arabian arc addition rate is 33 km3 km 1 ma 1 for the late proterozoic which is rather close to estimates of modern island arc accretion rates of 30 km3 km 1 ma 1 we conclude that the process and rate of island arc accretion onto active continental margins of arabia in the late precambrian eon evolved in a fashion similar to that of island arc accretion in the phanerozoic eon this conclusion is important because it counters the view that based on unrealistic assumptions accretion rates for the late proterozoic crust of the arabian craton were as much as 10 times faster than the modern crustal accretion rates

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

evolution of continental crust in southern africa abstract nd isotopic data from the zimbabwe and kaapvaal cratons and the limpopo kalahari namaqualand and damara mobile belts imply that over 50 of present day continental crust in this region had separated from the mantle by the end of the archaean and that

episodic earth evolution abstract u pb ages of zircons from precambrian granitoids and major rivers are grouped into a series of major peaks at about 2 7 2 5 2 1 1 9 and 1 1 ga recently these peaks have been interpreted as times of enhanced preservation of the continental crust associated with the assembly of supercontinents an older interpretation which we support is that they correspond instead to periods of accelerated crustal growth related to episodic convection of the mantle in this paper we use fluid mechanics experiments to develop a new model of mantle convection and crustal growth a dense layer at the base of the mantle persists until 2 7 ga when it destabilizes and generates large domes that rise into the upper mantle there they cause a large increase in the rate of subduction which leads to enhanced granite magmatism at convergent margins and thus to a pulse of crustal growth the domes heat the upper mantle which partially melts at mid ocean ridges to produce thick oceanic crust that resists subduction the subsequent period of subdued plate motion is broken by the next generation of mantle domes before and after the archean proterozoic period of episodic crustal growth plate tectonics operated quasi continuously

crustal reworking at convergent margins traced by fe isotopes in i type intrusions from the gangdese arc tibetan plateau abstract voluminous magmatism at active continental margins and collision zones is marked by i type intrusions paleocene eocene intrusions exposed in the lhasa terrane of the tibetan plateau are part of the gangdese arc that operated during neo tethyan ocean subduction and subsequent india asia collision arc like geochemical trace element signatures and radiogenic isotope systematics are indicative of juvenile i type magmatism with variable silica contents consequent to igneous differentiation here we present the first fe isotope data for this fossil arc terrane which have highly variable 57fe values relative to irmm 014 of 0 05 to 0 57 the data show no obvious correlations with major and trace element systematics except for highly evolved rocks with 70 wt sio2 based on trace element systematics deuteric fluid exsolution is excluded as a cause for the isotope variations in less evolved rocks furthermore there is no apparent relation between fe isotope values and the established tectonic evolution from syn to post collisional magmatism comparison with i type granites of the australian lachlan fold belt and cordilleran snake river plain reveals an on average lighter 57fe in the gangdese suite in primitive lavas yet distinctively heavier than intra oceanic arcs the heavier 57fe values for primitive lachlan fold belt and snake river plain rocks are interpreted here as crustal contribution in the form of s type magmas derived from crustal anatexis which are absent in the gangdese arc gangdese belt data yield an average 57fe of 0 13 0 02 2 which is proposed as the best estimate for juvenile crust at active continental margins rhyolite melts modelling suggests that the gangdese belt data can be reproduced through fractional crystallization along a liquid line of descent at oxygen fugacity between fmq 0 to 2 typical for arc related melts crucially the majority of data is coherent with partial melts of existing mafic crust as the major contributor to the juvenile component in the intrusions with minor mantle derived melts evidenced through heavier primitive fe isotopes this indicates that for the gangdese belt samples the primitive parental melts are derived from lower crustal successions through predominantly crustal reworking with only subordinate crustal growth hence the fe isotope data indicates a two step evolution with melting of underplated mafic material sometime in the geologic past although juvenile in nature these successions are part of the existing crust supporting scenarios in which crustal reworking in convergent margin intrusions is an important process in evolved magma generation

gouge formation by dynamic pulverization during earthquake rupture it is proposed here that fault gouge forms by rock pulverization within the tip region of a fast propagating earthquake due to the intensity of stress and rates of strain in this zone to examine this mechanism we calculate the deformation conditions of a dynamic shear fracture propagating close to the limiting rayleigh wave velocity the mechanical conditions close to the fracture tip are extreme tensile stresses approach 5 gpa volumetric strain rates exceed 105 s 1 and volumetric expansion alternates with volumetric contraction it is expected that a localized zone a few millimeters wide will be pulverized under these conditions this mechanism of dynamic gouge formation provides new insight to recent observations of the texture of gouge from the san andreas fault zone and from rupture zones in south african mines further as such extreme conditions develop only during fast propagating earthquakes presence or absence of pulverized gouge may serve as an indicator of earthquake propagation velocity

fracturing mode analysis and relative age dating of faults by surface textures of quartz grains from fault gouges abstract surface textures of quartz grains taken from about 250 samples of fault gouges in faults ranging in length from several metres to several tens of metres with the faults extending several tens to several hundreds of kilometres in length are examined by means of the scanning electron microscope and are categorized into four groups from i to iv based on the smoothness of the surface degree of undulation and development of cavities as mentioned in a previous paper based on surface features of quartz grains from faults the geological age of movement of which is known the groups i to iv are tentatively related to the age of formation of quartz grains and the period of formation of surface textures this correlation makes it clear that quartz grain surface textures are an important clue to dating a relative age of faulting river patterns striations granular fracture surfaces and dimple like textures are observed to appear on less corroded surfaces of quartz grains from fault gouges fractographic analysis of these surfaces of quartz grains should be a clue to revealing the mode of fracture of the fault movement which released the quartz grains into the fault gouge

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration