tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

ionospheric current source modeling and global geomagnetic induction using ground geomagnetic observatory data long period global scale electromagnetic induction studies of deep earth conductivity are based almost exclusively on magnetovariational methods and require accurate models of external source spatial structure we describe approaches to inverting for both the external sources and three dimensional 3 d conductivity variations and apply these methods to long period t 1 2 days geomagnetic observatory data our scheme involves three steps 1 observatory data from 60 years only partly overlapping and with many large gaps are reduced and merged into dominant spatial modes using a scheme based on frequency domain principal components 2 resulting modes are inverted for corresponding external source spatial structure using a simplified conductivity model with radial variations overlain by a two dimensional thin sheet the source inversion is regularized using a physically based source covariance generated through superposition of correlated tilted zonal quasi dipole current loops representing ionospheric source complexity smoothed by earth rotation free parameters in the source covariance model are tuned by a leave one out cross validation scheme 3 the estimated data modes are inverted for 3 d earth conductivity assuming the source excitation estimated in step 2 together these developments constitute key components in a practical scheme for simultaneous inversion of the catalogue of historical and modern observatory data for external source spatial structure and 3 d earth conductivity

the role of strain heating in the evolution of megathrusts on the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws a model is developed for the interplay of thermal and mechanical processes along megathrust systems the model is based on the observation that thrust systems involving large portions of the lithosphere have a two part history during the early history of movement dry lower crustal upper mantle rocks are deformed in a shear zone where because of the nonlinear effect of temperature dependent diffusion strain heating rates are directly controlled by the initial thermal state that is heating is insignificant if initial temperatures are high but where temperatures are low large temperature increases are possible as a result strain heating acts as a thermal buffer such that upon arrival of upper crustal rocks at depths of 20 km or more the temperatures along the thrust would be controlled largely by the rheology of hanging wall rocks rather than the initial thermal state moreover the relatively high melting temperatures for dry lower crustal upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high as thrusting progresses this early history is successively eliminated as wet upper crustal rocks are emplaced beneath the preheated hanging wall the granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall either by hydrous fluxing of the hot hanging wall or by melting in the highest grade footwall rocks rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by tectonic pileup beneath the thrust thorough testing of the model will require thorough assessment of the time dependent problem however one dimensional steady state models together with energy considerations indicate that the general model is allowable the concept of thermal buffering by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology hence they represent real world systems where a thermal and mechanical system are intimately linked this rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

development of inverted metamorphic isograds in the western metamorphic belt juneau alaska an inverted metamorphic gradient is preserved in the western metamorphic belt near juneau alaska the western metamorphic belt is part of the coast plutonic metamorphic complex of western canada and southeastern alaska that developed as a result of tectonic overlap and or compressional thickening of crustal rocks during collision of the alexander and stikine terranes detailed mapping of pelitic single mineral isograds systematic changes in mineral assemblages and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km peak temperatures of metamorphism increase progressively from about 530 c for the garnet zone to about 705 c for the upper kyanite biotite zone silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9 11 kbar the metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt although truncation of the isograds north of juneau indicates that the sill intrusion continued after the isograds were established our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma garnet rim compositions and widespread growth of chlorite suggest partial re equilibration of the schists under pressures of 5 6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill the combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high t high p metamorphism associated with the collision of the alexander and stikine terranes was a long lived event extending from about 98 ma to about 67 ma

structural evolution of the alaska juneau lode gold deposit southeastern alaska the alaska juneau lode gold deposit is hosted by a series of polydeformed permian to late triassic volcanic pelitic volcaniclastic and mafic intrusive rocks rocks in the mine area have been sheared and metamorphosed to greenschist grade interpretation of rock fabrics indicates several generations of ductile and brittle deformation prior to mineralization reverse shear occurred along northwest striking and northeast dipping ductile shear zones mineralization consists of eocene auriferous quartz carbonate veins which cut the regional metamorphic fabrics mineralization was followed by reverse right lateral shear along northwest trending ductile brittle shear zones two northwest striking and steeply dipping vein sets host the bulk of the ore orientation of carbonate fibers within the quartz veins were used to determine the deformation regime that existed during mineralization plunge of the fibers indicate that down to the northeast extension occurred synchronous with mineralization structural da

a complex 660 km discontinuity beneath northeast china abstract we present a detailed seismic study of the 660 km discontinuity beneath northeast china using the receiver function technique we use seismic data collected from 24 broadband stations in northeast china analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128 0 e to 130 5 e and latitude 40 0 n to 44 0 n and then it splits into multiple discontinuities in the surrounding regions the complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab we speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area

receiver function migration image of the deep structure in the bohai bay basin eastern china 1 we apply a receiver function poststack migration method to image the crustal and upper mantle structures beneath the bohai bay basin eastern china the thick sediments of the basin appear to have a substantial influence on the migrated image of the deep structure by using the surface reflected ppps multiples instead of the commonly used ps converted phases a curved moho is coherently imaged showing a thinned crust of 30 km in the basin area compared with the 40 km thick crust in the west mountain range by incorporating the sedimentary velocity model into migration both the 410 km and the 660 km discontinuities present a simple structural feature with relatively flat topography within the basin s interior although possible local structural complexities may exist at the base of the upper mantle around the boundary areas of the basin the mantle transition zone is on average 10 15 km thicker than the global average reflecting a cold lower upper mantle environment that is distinct from the warm shallow upper mantle of the region

determining moho depth beneath sedimentary basins using regional pn multiples the study of the moho beneath thick sedimentary basins involving natural earthquakes is challenging as low velocity materials often cause strong reverberations that mask moho signals here we develop a method to determine the depth of the moho by taking advantage of the presence of the sediments the method utilizes the first pn crustal multiple from regional earthquakes pnpn and its differential travel time with respect to pn pnpn is usually weak in amplitude thus it is difficult to identify in regions without a sedimentary cover however pnpn is significantly amplified in the presence of low velocity sediments because of an increase in the near surface p to p reflection coefficient the arrival time amplitude and wave shape of pnpn if normalized by the reference pn are insensitive to earthquake source parameters such as focal mechanism and focal depth we demonstrate the potential of this method using both 1d and 2d waveform simulations synthetic waveforms suggest that pmppn and pnpmp one pn leg merges to pmp near the source or the receiver largely contribute to the pnpn amplitudes which depend on the near surface structure at their free surface p to p reflection points we further validate the method with two field examples in the imperial valley one is near the united states mexico border and the other is in oklahoma in the central united states both examples suggest that the method can be used to study the moho either near the source or the receiver

empirical garnet biotite plagioclase quartz gbpq geobarometry in medium to high grade metapelites on the basis of the net transfer reactions among garnet biotite plagioclase and quartz for both mg and fe end member models the garnet biotite plagioclase quartz gbpq geobarometer was empirically calibrated under physical conditions of p 1 0 11 4 kbar and t 515 878 c based on the input garnet biotite temperatures and garnet aluminosilicate plagioclase quartz gasp pressures of 224 natural aluminosilicate bearing metapelitic samples collated from the literature the calibrations are internally consistent with the asymmetric quaternary solid solution model of garnet the symmetric quaternary solid solution model of biotite and the al avoidance ternary solid solution model of plagioclase in calibrating the garnet biotite geothermometer and the gasp geobarometer the resulting two gbpq barometer formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent metapelites the two gbpq barometry formulae yielded identical pressures whether the sample was included or not included in calibrating the gbpq barometry the random error of the gbpq barometry may be expected as 1 2 kbar the dp dt slopes of these two gbpq formulae are close to that of the gasp barometer in p t space applications of the gbpq barometry of aluminosilicate absent metapelites to the rocks within a thermal contact aureole or rocks within a limited geographical area without post metamorphic structural discontinuity show no obvious pressure change it may be concluded that the two gbpq barometry formulae derived in this study may be used as practical tools for metamorphic pelites under the conditions of 515 878 c and 1 0 11 4 kbar in the composition range of xgros 3 in garnet xan 17 in plagioclase and x bio 3 in biotite application of the gbpq barometer beyond the calibration range should be undertaken with caution

possible thermal buffering by crustal anatexis in collisional orogens thermobarometric evidence from the nepalese himalaya one dimensional thermal models suggest that large thrust faults may exert first order control on the thermal structure of collisional orogenic belts and an increasing body of petrologic data may be interpreted as consistent with these models we present new thermobarometric data from the central nepalese himalaya which imply that large sections of the himalayan orogen may have been at roughly uniform temperatures during active thrusting a phenomenon that is not predicted by existing models we attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis if this process is common in the intermediate and lower crust of active orogens and if it is localized spatially one dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis

isostatic state and crustal structure of north china craton derived from goce gravity data abstract the crustal structure between different subregions of the north china craton ncc plays an important role in improving our understanding on the craton evolution here we present a flexural moho map and isostatic residual gravity field of the ncc derived from the go cons gcf 2 tim r3 model we first investigate the influence of additional sources on the forward flexure calculation especially the distinct thinning of the lithospheric thickness from west to east after removing the gravity effects of the lab undulations the flexural moho and isostatic residual gravity field of the ncc are calculated using the vening meinesz regional isostasy models there are high to medium positive residual anomalies beneath the eastern block and trans north china orogen while negative residual anomalies delineate the yinshan block ordos plateau and khondalite belt this obvious disparity is consistent well with the decreasing depth of the flexural moho from west to the east a profile across the eastern block trans north china orogen and the western block was employed to construct a density structure of the crust and uppermost mantle by comparing the surface topography bouguer gravity isostatic moho and seismological moho undulations we find that the flexural moho is much shallower than the seismic moho beneath the northern margin of the ordos block possibly indicating the over compensated isostatic state caused by the dominant nw se oriented compressional deformation in the lithospheric processes density modeling results further predicted that a high density body may exist in the lower crust beneath the northern margin of the ordos block which could be a westward dipping upper middle crust remnant stagnating in the lower crust beneath the western block medium to high pressure metamorphism of granulite facies or eclogite facies resulting from subsequent continental thickening during the continental collision may be responsible for modifying the remaining subduction remnant

metamorphic evolution of the central metamorphic belt klamath province california an inverted metamorphic gradient beneath the trinity peridotite metabasalts and metasedimentary rocks of the devonian central metamorphic belt comprise the lower plate of the east dipping trinity thrust system in the klamath province an inverted metamorphic gradient is preserved in the central metamorphic belt metamorphic conditions decrease from amphibolite facies adjacent to the trinity thrust through albite epidote amphibolite facies to upper greenschist facies at the base of the central metamorphic belt mineral chemistry mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 50 c at the trinity thrust to 500 50 c at the base of the central metamorphic belt under pressures of 5 3 kbar synmetamorphic ab qtz veins up to 1 m thick increase in abundance towards the trinity thrust infiltration of h2o co2 fluids derived from prograde devolatilization reactions in the central metamorphic belt caused extensive hydration and metasomatism of the trinity peridotite the hanging wall block of the trinity thrust zone geological relationships and the preserved inverted metamorphic gradient suggest that the central metamorphic belt formed in an east dipping devonian subduction zone in an oceanic environment the central metamorphic belt appears to represent a discrete slice of accreted oceanic crust several km thick rather than progressively accreted material metamorphic pressures recorded by the central metamorphic belt are intermediate between the 2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7 10 kbar preserved in subduction related inverted metamorphic gradients the lack of blueschist facies mineral assemblages in the central metamorphic belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth

u pb zircon geochronology from the alexander terrane southeast alaska implications for the greens creek massive sulphide deposit this paper presents results of a laser ablation inductively coupled plasma quadrapole mass spectrometer la icp qms u pb dating study of small in situ zircon grains from samples collected in the vicinity of the greens creek massive sulphide deposit on northern admiralty island southeast alaska the greens creek mine is a volcanogenic massive sulphide deposit in the central portion of the alexander triassic metallogenic belt atmb and is one of the top global silver producers despite having a dominantly mafic metavolcanic stratigraphic footwall the stratigraphic footwall is a mississippian mafic metavolcanic sequence with a protolith age of approximately 340 330 ma the first u pb zircon constrained chronostratigraphy for the area places the deposit near or at the base of the host late triassic stratigraphy just above an approximately 100 million year old unconformity and probably 10 15 million years older than mineralization at the palmer and windy craggy deposits in the northern portion of the

distribution and characteristics of metamorphic belts in the south eastern alaska part of the north american cordillera the cordilleran orogen in south eastern alaska includes 14 distinct metamorphic belts that make up three major metamorphic complexes from east to west the coast plutonic metamorphic complex in the coast mountains the glacier bay chichagof plutonic metamorphic complex in the central part of the alexander archipelago and the chugach plutonic metamorphic complex in the northern outer islands each of these complexes is related to a major subduction event the metamorphic history of the coast plutonic metamorphic complex is lengthy and is related to the late cretaceous collision of the alexander and wrangellia terranes and the gravina overlap assemblage to the west against the stikine terrane to the east the metamorphic history of the glacier bay chichagof plutonic metamorphic complex is relatively simple and is related to the roots of a late jurassic to late early cretaceous island arc the metamorphic history of the chugach plutonic metamorphic complex is complicated and developed during and after the late cretaceous collision of the chugach terrane with the wrangellia and alexander terranes the coast plutonic metamorphic complex records both dynamothermal and regional contact metamorphic events related to widespread plutonism within several juxtaposed terranes widespread moderate p t dynamothermal metamorphism affected most of this complex during the early late cretaceous and local high p t metamorphism affected some parts during the middle late cretaceous these events were contemporaneous with low to moderate p high t metamorphism elsewhere in the complex finally widespread high p t conditions affected most of the western part of the complex in a culminating late late cretaceous event the eastern part of the complex contains an older pre late triassic metamorphic belt that has been locally overprinted by a widespread middle tertiary thermal event the glacier bay chichagof plutonic metamorphic complex records dominantly regional contact metamorphic events that affected rocks of the alexander and wrangellia terranes widespread low p high t assemblages occur adjacent to regionally extensive foliated granitic dioritic and gabbroic rocks two closely related plutonic events are recognized one of late jurassic age and another of late early and early late cretaceous age the associated metamorphic events are indistinguishable a small late devonian or early mississippian dynamothermal belt occurs just north east of the complex two older low grade regional metamorphic belts on strike with the complex to the south are related to a cambrian to ordovician orogeny and to a widespread middle silurian to early devonian orogeny the chugach plutonic metamorphic complex records a widespread late late cretaceous low to medium high p moderate t metamorphic event and a local transitional or superposed early tertiary low p high t regional metamorphic event associated with mesozonal granitic intrusions that affected regionally deformed and metamorphosed rocks of the chugach terrane the chugach complex also includes a post late triassic to pre late jurassic belt with uncertain relations to the younger belts

seismic structure of the bohai bay basin northern china implications for basin evolution as part of an extensive seismic research program 33 portable broadband seismic stations were deployed along a line crossing the bohai bay basin northern china three teleseismic events were selected to constrain the seismic structure along a 280 km profile across the western edge of the basin we determined the basin structure that described the observed shear horizontal sh wave field the synthetic sh wave was calculated using a finite difference fd method with its computational domain localized in the basin area and input motions at the base of the model extrapolated from the displacement recorded at a nearby hard rock station synthetic seismographs calculated for the models match the observations well in both waveform and travel time numerical tests indicate that the structural features of the preferred models are well resolved the analysis of relations between structures and stratigraphic units along the cross sections allows multiple deformational events in the basin to be inferred in conjunction with a profile across the southern edge that has been presented previously by zhao et al 14 l zhao t y zheng w w xu modeling the jiyang depression northern china using a wave field extrapolation fd method and waveform inversion bull seismol soc am 94 2004 988 1001 the results reveal basin wide extension with local inversion features in the bohai bay basin d 2005 elsevier b v all rights reserved

crustal and uppermost mantle structure across the lower yangtze region and its implications for the late mesozoic magmatism and metallogenesis eastern south china abstract we present here high resolution seismic sections of crustal and uppermost mantle structure along two dense linear arrays across the lower yangtze region in the eastern south china block scb our seismic images reveal two low velocity anomalies lvas in the uppermost mantle which we interpreted as relic still cooling mantle magma chambers ponding and underplating beneath the base of the crust in the areas of the dexing porphyry copper deposits and the cretaceous south anhui granites the moho and lab discontinuities imaged also exhibit deformation directly above the two mantle lvas which probably indicate local mantle lithosphere destruction by asthenospheric upwelling in the mesozoic this local asthenospheric upwelling may have triggered melting of the mantle lithosphere and generated the observed mantle magma chamber system through interactions with lower crustal materials low vs bodies revealed in the middle upper crust at depth of 10 km may indicate the corresponding solidified intrusive granitoids these observations provide unambiguous seismological evidence connecting deep magmatism with surface geological features combined with previous petrologic and geochemical studies the findings presented here outline a mantle magma chamber based magmatic system that was underplated beneath the base of the crust and kept supplying heat and materials in forming the voluminous yanshanian igneous rocks and associated metallogenesis in the lower yangtze region

the p t evolution of the middle koli nappe complex scandinavian caledonides 68 n and its tectonic implications near 68 n the scandinavian caledonides are composed of 3 tectonic domains each of which has a different tectonostratigraphy the lower 2 domains can be related stratigraphically to scandinavia prior to caledonian deformation whereas the highest domain the middle koli nappe complex mknc represents a fore arc accretionary complex that was accreted to scandinavia during caledonian deformation subsequent to accretion the flyschoid sediments that dominate the mknc were metamorphosed to the amphibolite facies in the area covered by this study the mknc is composed of two nappes a lower langvatn nappe and an upper marko nappe each of which has a unique early metamorphic history pelitic mineral assemblages in the marko nappe constrain the peak p t to be 625 7 0 kbars whereas ultramafic mineral assemblages in the lower langvatn nappe constrain its peak temperature to be 580 c p t estimates from garnet biotite and garnet plagioclase geothermobarometry for both nappes overlap ranging from 528 c and 6 6 kbars to 620 c and 8 8 kbars with an average of 567 32 c and 8 0 0 9 kbar

recalibration of the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer for metapelitic assemblages the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer have been simultaneously calibrated under conditions of t 1 4 450 760 c and p 1 4 0 8 11 1 kbar using a large number of metapelitic samples in the compositional ranges x grt fe 1 4 0 53 0 81 x grt mg 1 4 0 05 0 24 x grt ca 1 4 0 03 0 23 in garnet x pl ca 1 4 0 17 0 74 in plagioclase and fe 1 4 0 04 0 16 mg 1 4 0 04 0 13 al 1 4 1 74 1 96 in muscovite on the basis of 11 oxygens the resulting gm thermometer yielded similar temperature estimates mostly within 50 c to that of the garnet biotite thermometer and successfully discerned the expected systematic temperature change of prograde sequences thermal contact zones and an inverted metamorphic zone the resulting gmpq barometer yielded similar pressure estimates mostly within 1 0 kbar to the garnet aluminum silicate plagioclase quartz gasp barometer and placed the aluminosilicate bearing samples in the appropriate aluminosilicate stability fields application of the gmpq barometer to thermal contact aureoles or rocks within limited geographical areas confirmed the expected constant pressures that should exist in these settings the random errors of the gm thermometer and the gmpq barometer are estimated to be 16 c and 1 5 kbar respectively when biotite or aluminosilicate is absent in metapelites metamorphic p t conditions may be determined by simultaneously applying the gm thermometer and the gmpq barometer

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

empirical garnet muscovite geothermometry in metapelites abstract two empirical garnet muscovite geothermometers assuming no ferric iron model a and 50 ferric iron model b in muscovite respectively were calibrated under the physical conditions of p 3 0 14 0 kbar and t 530 700 c the input temperatures and pressures were determined by simultaneously applying the garnet biotite thermometer am mineral 85 2000 881 and the gasp geobarometer am mineral 86 2001 1117 to natural metapelites to confirm internal thermodynamic consistency holdaway s am mineral 85 2000 881 garnet mixing properties were adopted muscovite was treated as a symmetric fe mg al vi ternary solid solution and its margules parameters were derived in this work the resulting two formulae reproduced the input garnet biotite temperatures well within 50 c and gave identical results for a great body of natural samples moreover they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence inverted metamorphic zone and thermal contact aureole pressure estimation has almost no effect on the two formalisms of the garnet muscovite geothermometer assuming analytical error of 5 for the relevant components of both garnet and muscovite the total random uncertainty of the two formulations will generally be within 5 c the two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 c low to high pressure in the composition ranges xalm 0 51 0 82 xpyr 0 04 0 22 and xgros 0 03 0 24 in garnet and fe tot 0 03 0 17 and mg 0 04 0 14 atoms p f u in muscovite

electromagnetic induction in a spherical earth with non uniform oceans and continents in electric contact with the underlying medium ii bimodal global geomagnetic sounding of the lithosphere summary the vertical electric currents flowing between the inhomogeneous surface layer and the conductive layers of the earth s mantle are substantially dependent upon the transverse resistivity of the earth s crust and upper mantle although these currents do not contribute straightforwardly to a surface magnetic field they change the vortex sub surface currents which contribute to it thus there exists an indirect influence of leakage currents upon the magnetic field this makes it possible to evaluate resistance of the high ohmic layers which could not be evaluated if the earth was laterally homogeneous and only the induction mode of the field was induced this paper presents the first results of the bimodal sounding based on s variation data an estimate of the lithosphere s transverse resistance t 3 x lo9 51 m2 has been obtained

a wave equation migration method for receiver function imaging 1 theory 1 a wave equation based poststack depth migration method is proposed to image the earth s internal structure using teleseismic receiver functions by utilizing a frequency wave number domain one way phase screen propagator for wave field extrapolation in the migration scheme common conversion point ccp stacked receiver functions are backward propagated to construct a subsurface structural image the phase screen propagator migration method takes into account the effects of diffraction scattering and travel time alternation caused by lateral heterogeneities and therefore it is particularly useful for imaging complex structures and deep discontinuities overlain by strong shallow anomalies synthetic experiments demonstrate the validity of the migration method for a variety of laterally heterogeneous models the migrated images show considerable improvement over the ccp images in recovering the model features influences of several factors on the image quality of the poststack migration are further investigated including interstation spacing noise level of the data velocity model used in migration and earthquake distribution incident direction of source fields on the basis of the sampling theorem and previous statistic results we discuss the relation of spatial resolution and signal to noise ratio of the migrated image with the frequency of the data surface station spacing and number of receiver functions used in stacking we show that both ccp stacking and poststack migration of receiver functions need to be designed in a target oriented way for reliable and efficient imaging our results also suggest that careful consideration of earthquake source distribution is necessary in designing seismic experiments aimed at imaging steeply dipping structures

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

mantle transition zone structure beneath southeastern china and its implications for stagnant slab and water transportation in the mantle we determined depth variation of the 410 and 660 km discontinuities beneath southeastern china by common converted point stacking of rm p wave receiver functions of 121 permanent chinese seismic stations we then combined the results with seismic velocity variation to estimate temperature and water content variations in the mantle transition zone of the region previous tomographic studies have shown a stagnant slab in the mantle transition zone in eastern asia that is connected to subduction of the western pacific temperature variations obtained clearly outline the shape of the stagnant slab with its western edge at 113 5 circ e and the southern edge at 28 5 circ n the correlation between the location of the stagnant slab and surface tectonics suggests that the cenozoic extension in eastern china is closely associated with the subduction of the western pacific and its eastward migration the water content of the stagnant slab is lower than in surrounding slabs suggesting that the water has already been released from the subducting slab into the upper mantle

the tatshenshini shear zone new in southwestern yukon canada comparison with the coast shear zone in british columbia and southeastern alaska and implications regarding the shakwak suture the tatshenshini shear zone extends from klukshu lake in southwestern yukon southward to the yukon british columbia border deformation in the shear zone ranges from hundreds of meters to a few kilometers in width and is characterized by protomylonitic and mylonitic turbidites belonging to the dezadeash formation jurassic cretaceous that are overlain by protomylonitic granodiorite of the ruby range batholith late cretaceous tertiary the dezadeash formation ruby range batholith contact bedding and deformation and magmatic fabrics within the tatshenshini shear zone are all coplanar striking northwest southeast and dipping moderately to the northeast kinematic indicators from the dezadeash formation and ruby range batholith record a top to the southwest sense of shear collision of the insular superterrane with the intermontane superterrane by the mid cretaceous resulted in the eastward directed underplating of yukon tanana terrane equivalent rocks by the dezadeash formation and the formation of the tatshenshini shear zone the tatshenshini shear zone preceded and controlled emplacement of the ruby range batholith but magmatism ending 58 56 myr ago outlasted deformation the results of this work support the correlation of the ruby range batholith with the adjacent great tonalite sill in southeastern alaska it also suggests a linkage between the tatshenshini shear zone and the coast shear zone in southeastern alaska and along which the great tonalite sill was emplaced the tatshenshini shear zone and the coast shear zone represent the coda of long lived tectonism along the shakwak suture the accretionary boundary between the insular and intermontane superterranes

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

small scale hot upwelling near the north yellow sea of eastern china 1 in order to image the structure of the upper mantle discontinuity beneath eastern china we have applied a common conversion point ccp stacking method of receiver function rf both the 410 km and the 660 km discontinuities hereafter called the 410 and the 660 clearly show continuous positive phases along the selected profile the 410 shows depression whereas the 660 shows uplift in the eastern section of the study profile the transition zone tz to the west of longitude 122 is thicker than the global average though the tz is thinner to the east of that longitude the thinnest part of the tz with 10 15 km of thinning an increase in temperature of up to 100c is located at the north yellow sea we suggest that either the small scale convection associated with the deep penetration of the sinking slab into the lower mantle or a small plume from lower mantle has generated hot upwelling in this region citation ai y t zheng w xu and q li 2008 small scale hot upwelling near the north yellow sea of eastern china geophys res lett 35 l20305 doi 10 1029 2008gl035269

formation of metamorphic core complexes in non over thickened continental crust a case study of liaodong peninsula east asia pre thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes mccs however the discovery of mccs in the liaodong peninsula where the crust has a normal thickness 35 km challenges the universality of this scenario therefore we implement a series of 2 d numerical thermo mechanical modeling experiments in which we investigate the conditions of mcc formation in normal crusts as well as the relationships between the underlying mechanisms and the syn rift basin evolution in these experiments we explore the impact of the lithostratigraphic and thermo rheological structure of the crust we also examine the lithosphere thickness strain softening extension rate and surface erosion sedimentation processes the experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome which is geometrically incompatible with the observations of the mccs in the liaodong peninsula according to our further findings the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift while synchronous surface erosion controls the polarity of the syn rift basin the synthetic model data are compatible with the geological observations and cooling history based on the thermo chronology for the eastern part of the east asia during the late mesozoic to the early cenozoic the model predicted p t t paths are essentially different from those inferred for the other known mccs confirming the exceptional character of the mcc formation in the wide rift system of the east asia

global 3 d imaging of mantle conductivity based on inversion of observatory c responses ii data analysis and results summary the global 3 d electrical conductivity distribution in the mantle in the depth range between 400 and 1600 km is imaged by inverting c responses estimated on a global net of geomagnetic observatories very long time series up to 51 years 1957 2007 of hourly means of three components of the geomagnetic field from 281 geomagnetic observatories are collected and analysed special attention is given to data processing in order to obtain unbiased c responses with trustworthy estimates of experimental errors in the period range from 2 9 to 104 2 d after careful inspection of the obtained c responses the data from 119 observatories are chosen for the further analysis squared coherency is used as a main quality indicator to detect and then to exclude from consideration observatories with a large noise to signal ratio during this analysis we found that along with the c responses from high latitude observatories geomagnetic latitudes higher than 58 the c responses from all low latitude observatories geomagnetic latitudes below 11 also have very low squared coherencies and thus cannot be used for global induction studies we found that the c responses from the selected 119 mid latitude observatories show a huge variability both in real and imaginary parts and we investigated to what extent the ocean effect can explain such a scatter by performing the systematic model calculations we conclude that 1 the variability due to the ocean effect is substantial especially at shorter periods and it is seen for periods up to 40 d or so 2 the imaginary part of the c responses is to a larger extent influenced by the oceans 3 two types of anomalous c response behaviour associated with the ocean effect can be distinguished 4 to accurately reproduce the ocean effect a lateral resolution of 1 1 of the conductance distribution is needed and 5 the ocean effect alone does not explain the whole variability of the observed c responses we also detected that part of the variability in the real part of the c responses is due to the auroral effect in addition we discovered that the auroral effect in the c responses reveals strong longitudinal variability at least in the northern hemisphere europe appears to be the region with smallest degree of distortion compared with north america and northern asia we found that the imaginary part of the c responses is weakly affected by the auroral source thus confirming the fact that in the considered period range the electromagnetic em induction from the auroral electrojet is small assuming weak dependence of the auroral signals on the earth s conductivity at considered periods and longitudinal variability of the auroral effect we developed a scheme to correct the experimental c responses for this effect with these developments and findings in mind we performed a number of regularized 3 d inversions of our experimental data in order to detect robust features in the recovered 3 d conductivity images although differing in details all our 3 d inversions reveal a substantial level of lateral heterogeneity in the mantle at the depths between 410 and 1600 km conductivity values vary laterally by more than one order of magnitude between resistive and conductive regions the maximum lateral variations of the conductivity have been detected in the layer at depths between 670 and 900 km by comparing our global 3 d results with the results of independent global and semi global 3 d conductivity studies we conclude that 3 d conductivity mantle models produced so far are preliminary as different groups obtain disparate results thus complicating quantitative comparison with seismic tomography or and geodynamic models in spite of this our 3 d em study and most other 3 d em studies reveal at least two robust features reduced conductivity beneath southern europe and northern africa and enhanced conductivity in northeastern china

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

silicate and sulphide thermobarometry of low to medium grade metamorphic rocks from holkham bay south east alaska the western metamorphic belt of the coast plutonic complex south east alaska and adjacent british columbia contains strongly deformed rocks and a prominent topographic low the coast range megalineament near holkham bay south east alaska the lineament separates the western metamorphic belt into a western low grade greenschist facies terrane and an eastern medium grade amphibolite facies terrane sphalerite compositions of grains in direct contact with pyrite and pyrrhotite in chlorite muscovite zone rocks in the low grade terrane give pressures of about 8 kbar compatible with pressures of 8 10 kbar at 500 c calculated from plagioclase biotite garnet muscovite assemblages adjacent to the windham bay pluton about 15 km away a pressure of 4 8 0 7 kbar was calculated from sphalerite compositions in staurolite zone rocks east of the coast range megalineament this is indistinguishable from pressures of 4 8 1 kbar at 585 c and 5 1 1 kbar at 680 c plagioclase garnet aluminum silicate quartz equilibria and 4 1 1 kbar at 585 c plagioclase biotite garnet muscovite equilibrium determined for the medium grade terrane an identical pressure of 4 8 0 7 kbar was calculated from sphalerite compositions in biotite zone rocks adjacent to the lineament this is considerably higher than a pressure of 3 1 1 kbar at 525 c obtained using plagioclase biotite garnet muscovite geobarometry from shear zones within the lineament the discrepancy may be explained by later equilibration of mineral phases within the shear zones the geothermobarometry suggests relatively low temperatures and high pressures for the low grade terrane 6 10 kbar and intermediate temperatures and pressures for the medium grade terrane to the east 4 6 kbar comparison of the barometers indicate that sphalerite can be used to estimate metamorphic pressures similar to those estimated from silicate mineral chemistry when pyrrhotite sphalerite pyrite assemblages are used

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

plagioclase relations in pelites central menderes massif turkey ii perturbation of garnet plagioclase geobarometers activity composition relations in oligoclase near the peristerite gap are investigated in pelites from the central menderes massif the pressure of metamorphism is estimated independently from garnet rutile ilmenite kyanite quartz as being in the range 4 7 kbar in the temperature range 450 600 c approximately both the newton haselton calibration of the garnet plagioclase kyanite quartz geobarometer and a related simple treatment of garnet plagioclase muscovite biotite give a wide range of apparent pressures correlated with plagioclase composition and ranging up to 11 12 kbar where the plagioclase is most sodic this effect is attributed to failure of the activity model for plagioclase used in the newton haselton treatment it is inferred that in the present area plagan decreases with increasing xplagan in the range an15 an25 the data can be interpreted in terms of high plagan in the high albite structure at these temperatures modified to lower values by e ordering in the more calcic oligoclases the ordering appears to be independent of the peristerite gap and the data do not support the interpretation of the gap as a solvus garnet plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately an20 and t 700 c and should instead be used to investigate the x behaviour of the plagioclase where independent geobarometry can be used as a constraint

inverted isograds at sulitjelma norway the result of shear zone deformation at sulitjelma norway there is a major inversion of metamorphic isograds beneath an inverted but undisrupted ophiolite the flysch like furulund schist in which the inverted isograds occur is also inverted and the early folds in it are downward facing the isograds cut across the axial surfaces of early folds and across the schistosity these relationships are explained as the consequence of metamorphism during the progressive development of a large overfold the inverted limb of the overfold is regarded as a major thick gently dipping shear zone separating the lower grade lower part of the caledonian allochthon below from the higher grade upper part of the allochthon above the association between stratigraphical inversion downward facing of syn schistosity folds and metamorphic inversion is explained by the progressive development of the shear zone it is suggested that the presence of such shear zones is a common feature of orogenic belts formed by continental collision

a wave equation migration method for receiver function imaging 2 application to the japan subduction zone 1 the newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the japan subduction zone using the fundamental research on earthquakes and earth s interior anomalies freesia broadband data three profiles are chosen in the subsurface imaging two in northeast ne japan to study the subducting pacific plate and one in southwest sw japan to study the philippine sea plate the descending pacific plate in ne japan is well imaged within a depth range of 50 150 km the slab image exhibits a little more steeply dipping angle 32 in the south than in the north 27 although the general characteristics between the two profiles in ne japan are similar the imaged philippine sea plate in eastern sw japan in contrast exhibits a much shallower subduction angle 19 and is only identifiable at the uppermost depths of no more than 60 km synthetic tests indicate that the top 150 km of the migrated images of the pacific plate is well resolved by our seismic data but the resolution of deep part of the slab images becomes poor due to the limited data coverage synthetic tests also suggest that the breakdown of the philippine sea plate at shallow depths reflects the real structural features of the subduction zone rather than caused by insufficient coverage of data comparative studies on both synthetics and real data images show the possibility of retrieval of fine scale structures from high frequency contributions if high frequency noise can be effectively suppressed and a small bin size can be used in future studies the derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure the observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates moreover the deep extent of the pacific plate image and the shallow breakdown of the philippine sea plate image are observed to correlate well with the depth extent of the seismicity beneath ne and sw japan such a correlation supports the inference that the specific appearance of slabs and intermediate depth earthquakes are a consequence of temperature dependent dehydration induced metamorphism occurring in the hydrated descending oceanic crust

titanium aluminum and interlayer cation substitutions in biotite from high grade gneisses west greenland detailed electron microprobe study pelitic to mafic metamorphic rocks from two localities higher ti k k na and possible k na similar al iv and lower al vi in granulite vs amphibolite grade samples modified journal abstract

development of inverted metamorphic gradient in the internal domain of the taconian belt gaspe peninsula the mont logan nappe is part of the taconian internal domain of the quebec appalachians and is entirely made up of synrift to passive margin elastics and volcanics of the shickshock group rocks o

crustal structure and deformation beneath eastern and northeastern tibet revealed by p wave receiver functions abstract the present day tibetan crust records the shallow response of the cenozoic continental collision between the indian and eurasian plates an analysis of the deep crustal structure beneath eastern and northeastern tibet is of vital significance for studying the geodynamic processes of crustal thickening and expansion of the tibetan plateau we herein provide detailed images of the crustal structure of eastern and northeastern tibet and the adjacent sichuan basin using teleseismic p wave receiver function p rf data from a nw se trending linear seismic array our p rf imaging result reveals distinct structural features of the study region including marked lateral variations in the depth to basement beneath the songpan ganzi block and the sichuan basin a seismically slow mid lower crust beneath the songpan ganzi block and a low velocity anomaly just above the moho around the easternmost kunlun fault area and obvious moho offsets near the boundaries of tectonics blocks these structural features may reflect various crustal responses within the continental interior to the india eurasia collision at the plate margin the rigid crust of the sichuan basin might have wedged into the tibetan crust in the longmenshan area which probably facilitated crustal thickening and enabled channelized mid lower crustal flow in the songpan ganzi block to the west being a pre existing tectonic boundary the kunlun fault could have acted as a focus of heating and hot mantle upwelling associated with the deep processes of the indian plate underthrusting and subduction possibly resulting in localized weakening and modification of the lower crust around this fault area the observed significant differences in the crustal structure of eastern and northeastern tibet suggest that crustal shortening in this region may have been absorbed by not only vertical thickening in the interiors of the tectonic blocks but also complex local deformation along the boundary zones

empirical garnet muscovite plagioclase quartz geobarometry in medium to high grade metapelites abstract based on the net transfer reactions among garnet muscovite plagioclase and quartz for both mg and fe end member models the garnet muscovite plagioclase quartz gmpq geobarometry was empirically calibrated under the physical conditions of p 1 0 11 4 kbar and t 505 745 c for 128 natural metapelitic rock samples collected from the literature the input temperatures and pressures were simultaneously determined by the garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gmpq calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same al avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet biotite geothermometer and the gasp geobarometer a symmetric fe mg al vi ternary solid solution model of muscovite was adopted and the margules parameters of muscovite were obtained through regression the mg and fe model reactions along with the assumption of whether the ferric iron content in muscovite is 0 or 50 resulted in four gmpq barometry formulae the gmpq barometry formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent samples the gmpq barometry formulae yield identical pressures for every sample whether the sample was included or not in calibrating the barometers for each of the mg or fe model reaction the formulae gave identical pressures within 40 bars the random error of the gmpq barometry may be expected as 1 4 kbar the d p d t slopes of these gmpq formulae are close to that of the gasp barometer in the p t space applications of the gmpq barometry to aluminosilicate absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference it may be concluded that the gmpq barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505 745 c and 1 11 4 kbar in the composition range of x gros 3 in garnet and x an 17 in plagioclase

polymetamorphism in medium to high grade pelitic metamorphic rocks west central maine a regional petrologic study of pelitic metamorphic rocks has been undertaken with emphasis on the area bounded by the cities of augusta lewiston norway rangeley kingfield and madison this report involves a regional isograd map interpretation of the nature of the various reactions and discussion of petrologic problems of polymetamorphism isograd distribution and widespread occurrence of muscovite in the k feldspar sillimanite zone a future contribution will deal more specifically with mineral chemistry geotherrnometry and geobarometry of much of the area after a low grade metamorphism m 1 the region suffered a higher grade early metamorphism m 2 which in many places produced andalusite and cordierite as well as staurolite this was followed by a somewhat higher pressure metamorphism m 3 involving staurolite chlorite and sillimanite in some areas m 3 has produced extensive retrograde effects with increasing grade in ms the major mineral reactions are 1 formation of staurolite from reaction of garnet chlorite and muscovite 2 formation of sillimanite from staurolite chlorite and muscovite 3 reaction of staurolite to form sillimanite biotite and in many rocks garnet and 4 formation of k feldspar from the partial reaction of muscovite quartz and plagioclase a possible fourth metamorphism m 4 produced narrow aureoles with andalusite and staurolite around the three latest plutons analysis of the isograd pattern demonstrates that the pressure was rather uniform throughout the area at a given time but increased about 0 5 kbar or more between m 2 and m 3 metamorphism the more regional distribution of isograds to the south is consistent with higher initial temperatures and more closely spaced magmatic heat sources but not necessarily with increasing pressure to the south the pressure increase between m 2 and m 3 might have been produced by intrusion of magma above the present level and or extrusion of rhyolitic volcanics

thermal evolution of the greater himalaya garhwal india the hanging wall of the main central thrust mct in garhwal india roughly 79 n 80 e 30 n 31 n exhibits an inverted metamorphic gradient sillimanite potassium feldspar assemblages near the top of the hanging wall or greater himalayan sequence are underlain by kyanite grade rocks near the fault textural relationships in pelitic samples from the alaknanda and dhauli river valleys indicate that the inversion is the product of two distinct metamorphic events an early harrovian event m1 which affected the entire greater himalayan sequence and a later buchan event m2 the effects of which are most obvious in the upper part of the sequence rim thermobarometry garnet inclusion thermobarometry and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak m1 conditions of 900 k and 960 mpa roughly 36 km depth before following an erosion controlled uplift path e g england and richardson 1977 m2 metamorphic temperatures in the upper part of the sequence also exceeded 900 k but maximum pressures 317 523 mpa indicate paleodepths of only 12 19 km calculated pressure temperature paths indicate that m2 was characterized by temperature increases of 80 k and roughly 5 km of tectonic burial we attribute m1 to tectonic burial of the greater himalayan sequence during the early stages of india eurasia collision we believe that the uplift and cooling path of the sequence was interrupted in late oligocene miocene time by a second burial and heating event m2 related to thrust imbrications in southern tibet this burial was coincident with the generation of leucogranites which are abundant near the top of the greater himalayan sequence but are virtually absent near the mct field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the greater himalayan sequence and were injected at their present structural level or were melted in situ if the granites were injected then they may have provided some of the heat necessary for m2 metamorphism although our data suggest no direct relationship between the main central thrust as mapped in garhwal and metamorphism in the greater himalaya anatectic melting of an unexposed portion of the greater himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped mct in central nepal cf le fort 1981 if the granites were produced by in situ m2 melting then we must appeal to a heat source within the upper part of the greater himalayan sequence such as locally high concentrations of heat producing elements cf pinet and jaupart 1987

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

rapid dewatering of the crust deduced from ages of mesothermal gold deposits the large scale migration of fluids through the continental crust has been well documented but there is no consensus regarding the timing of fluid migration relative to orogenic episodes or rates of crustal dewatering1 here we present40ar 39ar dates for muscovites from quartz veins along a major shear zone in southeast alaska which show that the veins were emplaced in the early eocene during the late stages of orogenic deformation hydrother mal activity took place for only about 1 myr and along a distance of at least 200 km the fluids were generated by metamorphic reactions in subducted crust along the north american plate margin and were apparently trapped in the crust by the low permeabilities accompanying a convergent tectonic regime until 56 myr ago the rapid dewatering event coincided with a change in plate motion at 56 55 myr which caused a shift from convergent to partly transcurrent tectonics we suggest that this change in tectonic regime led to increased crustal permeabilities and hence the possibility of large scale fluid migration

relationships between plate motions and late cretaceous to paleogene magmatism in southwestern alaska four major late cretaceous to paleogene magmatic belts have been recognized in southwestern alaska and tentative boundaries have been drawn for each the belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together though there has been significant postmagmatic tectonism the kuskokwim mountains belt 73 60 m y b p the alaska range belt 74 55 m y b p and the gulf of alaska belt 62 57 m y b p all may have been related to a period of rapid north northeastward motion of the kuia plate from about 74 to 56 m y b p the angle of convergence was low with respect to the present orientation of the belts suggesting the possibility of postmagmatic rotation of the belts a higher angle of convergence may have existed with respect to the alaska range belt a probable arc if postmagmatic rotation resulted from 300 400 km of right lateral displacement on the denali fault the gulf of alaska belt is in an anomalous forearc position with respect to the alaska range belt but probably formed by a convergence related mechanism the kuskokwim mountains belt is in an apparent back arc position but the mechanism of its origin is uncertain it may have been part of an unusually broad alaska range arc or less likely it may have been a separate arc magmatic quiescence occurred throughout south western alaska during a period of plate reorganization from about 56 to 43 m y b p a period which began with extremely rapid northward motion of the kuia plate magmatism resumed when the present period of rather slow northwestward pacific plate motion began at about 43 m y b p the aleutian arc probably formed at this time and has been episodically active in about the same location ever since

large scale ground deformation of etna observed by gps between 1994 and 2001 we have processed thirty global positioning system gps campaigns carried out at etna from 1994 to early 2001 between the last two main flank eruptions of the mt etna sicily italy this rest period allowed us to investigate the deep magma plumbing system of the mt etna the temporal dynamics of twenty three points observed three times or more were analyzed all the time series show a first order linear trend during the five years period it suggests that the volcano was continuously deformed by the action of a deep source while a discrete activity of the volcano was observed at the summit we have interpreted the residual deformation field as the result of an major eastward motion of the eastern flank of the volcano

calibration of a ti in muscovite geothermometer for ilmenite and al2sio5 bearing metapelites abstract the ti in muscovite geothermometer was empirically calibrated as ln t oc 7 258 0 289 ln ti 0 158 mg fe mg 0 031 ln p kbar using ilmenite and al2sio5 bearing assemblages in metapelites under p t conditions of 450 800 c and 0 1 1 4 gpa the calibration was conducted for muscovites containing ti 0 01 0 07 fe 0 03 0 16 mg 0 01 0 32 and mg fe mg 0 05 0 73 respectively on the basis of 11 oxygen per formula unit such compositional range covers more than 90 natural muscovites and the random error of this thermometer is estimated to be of 65 c the geothermometer was validated against a set of independently determined temperature conditions between different degrees in samples from different prograde inverted and contact metamorphic terranes application of this thermometer beyond the calibration conditions is not encouraged

deformational history of an outlier of metasedimentary rocks coast plutonic complex british columbia canada the khutzeymateen assemblage records a portion of the polyphase deformation experienced by rocks within the core of the coast plutonic complex this series of deformational events probably took place during late cretaceous to early eocene regional orogenic activity the khutzeymateen assemblage is dominated by metamorphosed graywackes and volcaniclastic material the earliest recognizable deformation involves thrust faulting that juxtaposed rocks of the khutzeymateen assemblage and central gneiss complex the next deformational event produced isoclinal folds f1 a penetrative foliation s1 and a strong mineral lineation l1 both f1 and l1 have a 340 15 orientation peak metamorphism p 450 50 mpa t 650 50 c was synchronous with this isoclinal folding event f1 folding was followed by a brittle chevron folding event f2 with a 335 20 orientation there is a strong lithologic control on the development of f2 minor folds which are developed predominantly within regularly layered qua

the upper mantle discontinuity structure beneath eastern china 1 we apply a common conversion point stacking technique of receiver functions from the north china interior structure project ncisp to image the upper mantle discontinuity structure beneath eastern china the stacking results of dense receiver functions reveal a prominent discontinuity structure the appearance of the 410 km discontinuity is sharp and consistent with little influence by western pacific subduction slabs the 520 km discontinuity is relatively weak and consists of splitting phases at depths ranging from 500 km to 550 km as for the 660 km discontinuity double discontinuities and a narrow depression zone are detected near a depth of 660 km indicating a more complex structure there phase transformations in non olivine components such as from garnet to perovskite seem to be a more plausible interpretation for a complex 660 km discontinuity in this region

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

the new approach to global deep sounding the new approach to global geomagnetic sounding is developed to overcome difficulties of spherical harmonical analysis and subsequent transfer function determination the approach is based on minimizing the discrepancy between experimental and simulated magnetic fields the discrepancy is considered as a function of the medium model parameters and the coefficients of external fields the method can be used for laterally inhomogeneous as well as homogeneous earth models an example of its application to a radially symmetric model is demonstrated

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

seismic evidence for the north china plate underthrusting beneath northeastern tibet and its implications for plateau growth abstract lithospheric deformation of the tibetan plateau is caused by subduction of the indian northward and asian southward plates the effects of this interaction on inner reaches of the asian continent between the tibetan plateau and the north china craton ncc for example remain uncertain due to poor geophysical data coverage in northeastern tibet ne tibet we provide here detailed knowledge of the lithospheric structure beneath ne tibet as determined from a dense broadband seismic profile traversing ne tibet to the westernmost ncc receiver function imaging reveals several significant features including a north dipping intracrustal converter nc moho offset overlap beneath major fault zones and a low velocity layer lvl in the middle lower crust the lithosphere asthenosphere boundary lab is clearly defined and appears as a south dipping interface that runs continuously from the alxa interior to the qilian orogen interpretation of these observations combined with other seismic evidence implies that the ncc lithospheric mantle has been persistently underthrust beneath the qilian orogen this process forms the thick skinned crustal accretionary wedges which develop above a middle lower intracrustal decollement our results provide further deep geophysical constraints on the cenozoic post collisional evolution of the convergent boundary between ne tibet and the ncc and help clarify the mechanism of plateau growth in this boundary area

seismic evidence for water deep in earth s upper mantle water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone observations of converted seismic waves provide evidence of a 20 to 35 kilometer thick discontinuity near a depth of 410 kilometers most likely explained by as much as 700 parts per million of water by weight

retrogressive dissolution of garnet effect on garnet biotite geothermometry cation exchange geothermometry is predicated on the basis of chemical equilibration between phases involved in ion exchange reactions if chemical equilibrium is attained between the phases at any temperature and subsequently locked in temperature determinations should be consistent throughout a given sample the attainment of chemical equilibrium is commonly inferred where phases used for thermobarometry are in mutual contact or where sharp straight grain boundaries exist between grains thermometric determinations on garnet biotite pairs in a metapelite from the cheyenne belt wyoming illustrate the problem of using these criteria to infer chemical equilibrium in one example garnet biotite pairs from different parts of a single zoned garnet grain yield apparent temperatures that range over 150 c and are anomalously higher than documented metamorphic conditions almandine garnets from low to intermediate grade pelitic rocks commonly show strong compositional zoning retrograde modification coupled with strain induced dissolution of garnet may result in meaningless temperatures because of nonequilibration of phases used for geothermometry apparent textural equilibrium or mutual contact of grains may not be indicative of chemical equilibrium dissolution of garnet under conditions of biotite stability can produce garnet biotite pairs with sharp straight grain boundaries suggestive of textural equilibrium in the absence of visible truncation of compositional zones compositional maps or microprobe traverses both parallel and normal to schistosity are necessary to evaluate the possibility of dissolution failure to do so may result in the incorrect interpretation of t p determinations

crust mantle structure difference across the gravity gradient zone in north china craton seismic image of the thinned continental crust the archean north china craton ncc has been tectonically modified and lost its thick lithospheric keel during the mesozoic and cenozoic the processes and mechanisms of the lithospheric modification and its appearance in and the relation between different subregions of the ncc are still poorly understood seismic data from 45 stations along a 470 km long profile cross the bohai bay basin bbb and the taihangshan mountain range tmr in the ncc were employed to construct a coherent structural image of the crust and uppermost mantle an integrated receiver function imaging technique combining the common conversion point stacking approach with waveform inversion and forward modeling was proposed to extract the structural information beneath the study region modeling of bouguer gravity anomalies was also applied to constrain the density distribution the imaging result reveals distinct structural features between the mountain range and the basin area and presents a picture of uneven crust thinning within the study region in the east bbb the crust is significantly thinned due mainly to the reduction in the thickness of the lower crust including the crust mantle transition zone by up to 12 km the west tmr in contrast is characterized by a relatively thick lower crust of 20 km the teleseismic waveform data and the gravity observation suggest a thicker crust and a buoyant mantle lithosphere beneath the tmr compared with the bbb the contrasting crustal structural features appear coupled with the lithospheric processes and possibly reflect that different tectonic mechanisms and deformation regimes dominated the evolution of the two regions the north south gravity lineament lying between the tmr and bbb might represent a deep intra continental boundary separating the ncc into topographically and tectonically different regions

distinct velocity variations around the base of the upper mantle beneath northeast asia abstract both the global and regional p wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions in particular low velocity anomalies have been observed beneath the descending high velocity slabs in a number of subduction zones the limited resolution at large depths and possible trade off between the high and low velocities however make it difficult to substantiate this feature and evaluate the vertical extent of the low velocity structure from broadband waveform modeling of triplicated phases near the 660 km discontinuity for three deep events we constrained both the p and sh wave velocity structures around the base of the upper mantle in northeast asia for the two events beneath the southern kurile the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending pacific slab our preferred models consistently suggest normal to lower p and significantly low sh velocities above and below the 660 km discontinuity extending to about 760 km depth compared with the global iasp91 model corroborating previous observations for a slow structure underneath the slab in contrast both high p and sh velocity anomalies are shown in our preferred models for the japan subduction zone region likely reflecting the structural feature of a slab stagnant above the 660 km discontinuity the velocity jumps across the 660 km discontinuity were found to be on average 4 5 and 7 for p and s waves under the south kurile and 3 and 6 under the japan subduction zone the respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under slab areas based on mineral physics data the depth averaged 1 5 p and 2 5 sh velocity differences in the depth range of 560 760 km between the two regions could be primarily explained by a 350 450 k temperature variation although the presence of about 0 5 1 wt water might also contribute to the subtle velocity variations near the base of the transition zone in the southern kurile from our modeling results we speculate that the slow structure in the southern kurile may be correlated to the low velocity zone observed previously around the 410 km discontinuity under northern honshu if this is the case both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast asia

moho depth variations from receiver function imaging in the northeastern north china craton and its tectonic implications a detailed knowledge of the crustal thickness in the northeastern north china craton ncc is important for understanding the unusual phanerozoic destruction of the craton we achieve this goal by employing a 2 d wave equation based migration method to p receiver functions from 198 broadband seismic stations using ps conversions and surface reflected multiples by combining receiver function images along 19 profiles we constructed a high resolution moho depth model for the northeastern ncc the results present dominant e w moho depth variations similar to previous observations and new regional n s variations beneath both sides of the north south gravity lineament to the west while a deeper moho approximate to 42km appears in the interior of the trans north china orogen a relatively shallow moho approximate to 38km exists in the northern margin of the trans north china orogen to western ncc to the east the crust beneath the yan mountains in the marginal area is thicker approximate to 32 40km than that approximate to 26 32km beneath the bohai bay basin in the craton interior and the moho further shallows from ne approximate to 32km to sw approximate to 26km within the basin along with other observations we conclude that the dominant e w difference may have been associated with the paleo pacific plate subduction under eastern asia since the mesozoic the newly observed complex n s variations may have reflected the structural heterogeneity of the cratonic lithosphere inherited since the formation of the ncc in the paleoproterozoic or spatially uneven effects on the cratonic lithosphere of subsequent thermotectonic events during the long term evolution of the craton or both

alpine metamorphism of pelitic schists in the nufenen pass area lepontine alps the effects of tertiary alpine metamorphism on pelitic mesozoic cover rocks have been studied along a cross section in the central lepontine alps in the nufenen pass area switzerland greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid garnet staurolite and kyanite in pelitic rocks regional metamorphism reached maximum conditions during the interkinematic period between a main alpine penetrative d2 and a late alpine d3 crenulation type deformation phase or synchronous with the late alpine deformation based on afm phase relationships four different metamorphic zones can be distinguished 1 chloritoid zone 2 staurolite chlorite zone 3 staurolite biotite zone and 4 kyanite zone the isograds that separate these zones can be modelled by univariant reactions in the kfmash system the conditions of metamorphism calculated from geological ther mobarometers for the maximum post d2 por phyroblast stage are from north to south 500 c at 5 6 kbar and 600 c at 7 8 kbar detailed thermobarometry of garnet por phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure early garnet growth occurred along a prograde p t path post d2 rims grew with increasing temperature but decreasing pressure and finally post d3 garnet formed along a retrograde p t path it may be concluded from the calculated pressure and temperature difference over a short distance 3 km across the mapped area that the isogradic surfaces of the post d2 metamorphism are steeply oriented the data also suggest that isobaric and isothermal surfaces are parallel much of the observed metamorphic pattern can be explained as the result of a significant post d2 differential uplift of the hot pennine area relative to the helvetic area along a tectonic contact zone the closely spaced isograds isotherms in the north may then be interpreted as a thermal effect owing to the emplacement of the hot pennine rocks against the got thard massif with its cover whereas in the pennine metasediments post d2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed it is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure the interpretation given here for the nufenen pass area may also apply to the luk manier pass area where similar metamorphic patterns have been reported by fox 1975 the formation of the northern steep belt as denned by milnes 1974b and the associated late alpine fold zones may therefore have significantly modified the metamorphic pattern of the helvetic penninic contact zone

structure of the mantle transition zone beneath the north china craton abstract detailed information on deep structural features can improve current understanding of lithospheric thinning and destruction of the north china craton in this study we analyzed receiver functions rfs from teleseismic p waveforms recorded by 246 seismic stations beneath the north china craton ncc to determine the topography of the mantle transition zone mtz the results reveal specific features in the 410 and 660 km discontinuities and outlined mtz thickness beneath the study area the mtz structure shows two anomalously thickened regions beneath the ncc up to 30 40 km thicker than the global average which are mainly concentrated in the eastern and western sections of the eastern ncc encc the thickened regions are discontinuous and separated by a distance of about 400 km double 660 km discontinuities also appear in the southeastern encc combining these results with previous findings we propose a model in which mtz thickening is caused by discrete remnants of the subducted western pacific slab discontinuities resulting from the stagnant slabs may retain evidence of changes in the rate of pacific plate subduction as well as the phase transition of the subducted slabs within the mtz

exhumation and tilting of the western metamorphic belt of the coast orogen in southern southeastern alaska the uplift history of the western metamorphic belt in southern southeastern alaska can be traced using emplacement depths and cooling ages of mid cretaceous plutons which intruded the metamorphic belt during the period 101 ma to 89 ma during the period 101 ma to 94 ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km pressure data indicate that by 89 ma the western portion of the metamorphic belt was at a shallower crustal level 23 km within the study area while the eastern side remained at a depth of approximately 30 km cooling curves constructed from minerals in mid cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side an observation consistent with the inferred uplift history southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high angle faults may have provided one mechanism for uplift during mid to late cretaceous time the eastern portion of the western metamorphic belt cooled rapidly at 55 ma probably in response to a period of rapid uplift the coast shear zone an early tertiary high angle fault with west side up displacement bounds the east side of the metamorphic belt we suggest tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault

computation of seismic profiles from mineral physics the importance of the non olivine components for explaining the 660 km depth discontinuity the recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals the updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles these profiles are chosen to stress out the influence of non olivine minerals on seismic parameters and to represent cold and horizontally averaged temperature profiles of the earth s mantle in a first part starting compositions of pyrolite and piclogite and a single layer convection are assumed the results clearly point out the importance of the non olivine part of the mineralogy two scenarios are found to explain the 660 km depth discontinuity

a detailed receiver function image of the sedimentary structure in the bohai bay basin abstract if the site at which receiver functions are constructed is filled by sediments then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct p arrival based on this observation waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the bohai bay basin a major continental petroliferous basin in eastern china an adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations the derived s velocity structure provides for the first time a basin scale seismic image of detailed sedimentary stratification the sedimentary cover of the basin is about 2 12 km thick consisting of cenozoic mesozoic and paleozoic strata from top to bottom the structural features presented in the s velocity image coincide quite well with the depression uplift type of tectonic system in the bohai bay basin the reconstructed morphology of the sedimentary layers provides seismological evidence for the two stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late mesozoic immediately following the lithospheric reforming of the eastern china continent

insight into the geodynamics of cratonic reactivation from seismic analysis of the crust mantle boundary 1 the reactivation of craton is a crucial issue for the understanding of continent lithosphere although the partial loss of the thick lithospheric root of the north china craton ncc was documented its mechanism remains a contentious topic in the study of cratonic evolution in this paper we imaged the crust mantle structures by using the data from 36 permanent stations with fine spatial coverage in the yanshan tectonic belt ystb and the taihangshan region tsr in the northern ncc two tectonic units with distinct crust mantle structures were recognized and their junction zone was identified the results reveal distinct features of the crust mantle boundaries of the ystb and the tsr which are characterized by a sharp interface and a thick transition zone respectively based on the analysis of the seismic structures of the crust mantle boundary and the upper mantle we propose that the ncc reactivation was possibly derived by a slow secular ascension of hydrated asthenospheric material atop the stagnant pacific slab which influenced the formation of crust mantle transition zone in the eastern ncc on the other hand regional delamination of the deep crust after orogenic thickened and or magmatic underplating beneath the ystb caused the spatial variation of the crust mantle boundary both of the mantle processes possibly changed the nature of the lithospheric mantle

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

comment and reply on tectonic accretion and the origin of the two major metamorphic and plutonic welts in the canadian cordillera comment