lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

high temperature overprint in u hpm rocks exhumed from subduction zones a product of isothermal decompression or a consequence of slab break off slab rollback abstract this paper presents and discusses petrological observations from high to ultrahigh pressure u hp metamorphic terrains in relation to existing geophysical and numerical models for subduction and exhumation processes in orogenic belts the interpretations are mostly based on observations from gneiss terrains bearing abundant bodies mafic meta eclogite and ultramafic garnet peridotite and pyroxenite exposed in collisional orogens the inclusions and compositional zoning of minerals are considered to be first order information that is needed to constrain pt paths of hp uhp rocks and reconstruct the related geodynamic models for subduction and exhumation of crustal and mantle rocks the bohemian massif of the european variscides is used as the basis for a model example to explain these processes but u hp rocks from various other terrains are taken into consideration to discuss available pt paths in relation to proposed subduction and exhumation rates of u hp rocks based on geophysical and geochronological data primarily information used in this respect include textural relations and preserved prograde zoning in minerals from many u hp rocks which reveal that a relatively cool geothermal gradient typical of subduction zones tended to prevail during the prograde and peak pressure segments of pt paths prior to initiation of exhumation and may have continued even with cooling if exhumation rates were rapid the commonly applied interpretation of isothermal decompression during exhumation is critically appraised considering whether a simple thermal relaxation and radiogenic heating during exhumation is responsible for formation of post peak pressure retrograde mineral assemblages and textures observed in u hp rocks we go on to consider whether this can satisfactorily explain the often pervasive medium pressure high temperature metamorphic re equilibration of u hp rocks or whether an additional external source of heat is a better explanation we conclude that the commonly observed high temperature metamorphic overprint exhibited by u hp rocks occurs mostly after rocks have been exhumed from the subduction channel and have reached normal crustal positions when mantle upwelling resulting from slab breakoff delamination or slab rollback takes place at the onset of continent continent collision we also explore contrasting pt trajectories for mantle rocks that have been entrained into crustal material during their subduction or exhumation pt paths of mantle and subducted crustal rocks tend to converge as mantle rocks impinge upon the cooler subduction zone and once entrained share a common evolution that depends on the exhumation mechanism and rate considering all of the data presented in this work we conclude that the diverse polyphase metamorphic evolution exhibited by u hp terrains embodied in the pt paths of hp and uhp rocks has important consequences for reconstructing their changing thermal regimes and provides important constraints for geodynamic models involving subduction and the transition to collision

evolution of amphibolite facies structural features and boundary conditions for deformation during exhumation of high and ultrahigh pressure rocks nordoyane western gneiss region norway 1 geologic mapping and structural analysis of baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high pressure rocks where strain partitioning has preserved evidence for interpreting the evolution of late structural features the earliest of these were extensional detachments juxtaposing eclogite facies rocks against overlying amphibolite facies rocks that show no evidence for eclogite facies metamorphism these early detachments are strongly overprinted and complexly folded and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels younger more localized mylonite zones formed synchronously with tubular sheath isoclinal tight and open folding that shows a progression from wnw to ene trends the earliest mylonite zones interpreted as originally subhorizontal range in strike through a 20 angle from 110 to 90 later steeply dipping mylonite zones formed under lower amphibolite facies conditions strike 75 and locally truncate earlier structures the youngest mylonite zones formed under lowest amphibolite conditions strike 50 and truncate all earlier structures folds developed during this progression show the range in orientation from wnw to ene reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing these changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation l s fabrics absence of axial planar foliation and chaotic orientations of axial surfaces of granulite to amphibolite facies folds indicate formation in a constrictional strain field this is also supported by estimates of the finite strain accumulated at 780 c and 45 km and similar observations by previous workers assuming a simple monoclinic deformation for transtension strain estimates and structural measurements indicate apparent transtensional angles of 9 11 that increased to greater than 20 and then decreased to less than 20 these changes in transtensional angle agree with changes in the orientation of the x y plane of strain during exhumation from 45 km to less than 20 km depth and appear to reflect changes in the boundary conditions of deformation this upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of hp and uhp metamorphic rocks

zircon geochronology and ca 400 ma exhumation of norwegian ultrahigh pressure rocks an ion microprobe and chemical abrasion study understanding the formation and exhumation of the remarkable ultrahigh pressure uhp rocks of the western gneiss region norway hinges on precise determination of the time of eclogite recrystallization we conducted detailed thermal ionization mass spectrometry chemical abrasion analysis and sensitive high resolution ion microprobe analysis of zircons from four ultrahigh and high pressure hp rocks ion microprobe analyses from the flatraket eclogite yielded a broad range of apparently concordant caledonian ages suggesting long term growth in contrast higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi step chemical abrasion yielded moderate pb loss from the first lowest temperature abrasion step possible minor pb loss or minor growth at 400 ma from the second step and a 407 404 ma cluster of slightly discordant 206pb 238u ages most likely free from pb loss from the remaining abrasion steps we interpret the latter to reflect zircon crystallization at 405 400 ma with minor discordance from inherited cores zircon crystallization occurred at eclogite facies possibly post peak conditions based on compositions of garnet inclusions in zircon as well as nearly flat hree profiles and lack of eu anomalies in zircon fractions subjected to chemical abrasion these ages are significantly younger than the 425 ma age often cited for western norway eclogite recrystallization implying faster rates of exhumation 2 5 8 5 km myr and coeval formation of eclogites across the uhp portion of the western gneiss region

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

importance of fracturing during retro metamorphism of eclogites presented textural and petrological data show that the deep to intermediate continental crust may fracture and that microfractures are the locus of fluid and mass transfer necessary for retrograde metamorphism kyanite eclogites from ulsteinvik norway underwent partial retrogression to granulite and amphibolite facies assemblages during near isothermal exhumation from depths equivalent to more than 2 0 gpa at temperatures of 700 800 c plagioclase bearing assemblages rich in hydrous phases formed along margins of eclogite lenses and along mesoscopic fracture systems hydrated zones are from 1 50 cm thick with adjacent wall rock eclogite replaced by symplectites at a low degree of reaction the secondary minerals in the wall rock are found along intra and intergranular microfractures typically 50 100 m wide minerals filling the microfractures include orthopyroxene plagioclase spinel in garnet plagioclase sapphirine plagioclase corundum and plagioclase spinel in kyanite and diopside plagioclase in omphacite the microfractures are often arranged en echelon and are connected through microfaults releasing bends filled with amphibole and spinel form along microfaults in garnet the faulting and fracturing caused localized chemical change in garnet the damage zones close to faults are enriched in feo and mno with steep compositional gradients 8 wt feo over 20 m these feo and mno enriched zones form wedge like structures around the tip of the faults horsetail structures and rose or flame like structures at sticking points along faults they may represent examples of stress induced chemical transport during fracture propagation the change from dry to amphibole bearing assemblages at the tip of the fracture and fractures ending in splays of fluid inclusions trails reflect the involvement of a fluid phase during fracture propagation this suggests that the dry granulite facies retrogression was also driven by fluid infiltration and that metamorphism at depth in collision zones may not be controlled by pressure and temperature alone

development and maintenance mechanism for the semiannual oscillation of the north pacific upper level circulation abstractthe north south semiannual oscillation sao of the north pacific jet stream part of the atmospheric sao in the northern hemisphere can be well depicted by the semiannual component of the monthly mean eddy streamfunction expressed by the semiannual eddy streamfunction budget the dynamic processes develop and maintain the sao including the adjustment between vorticity advection and convergence of vorticity flux of the monthly mean mode and the convergence of transient vorticity flux an empirical orthogonal function analysis of these dynamic processes shows an east west elongated cyclonic anticyclonic cell of the semiannual eddy streamfunction anomaly which appears in january and july october and april south of the siberia alaska landmass the maximum minimum adjustment processes by the monthly mean mode and the maximum minimum feedback impact of transient activity on the sao occur in december and june september and march a month ahead of the maximum minimum north south sao of th

monazite geochronology of uhp and hp metamorphism deformation and exhumation nordoyane western gneiss region norway u th pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of uhp and hp plates 820 c 39 kbar 780 c 18 kbar metamorphosed during the late silurian early devonian collision between baltica and laurentia the upper plate contains a microdiamond bearing kyanite garnet graphite gneiss and associated kyanite eclogites independently indicating probable diamond forming conditions in situ dating of monazite in the microdiamond sample using the shrimp ii at the geological survey of canada yielded ages of 415 6 8 ma for those included in garnet and 398 6 ma for those in the matrix these ages compare to 408 0 5 6 and 397 5 4 4 ma determined using the electron microprobe at the university of massachusetts both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100 950 ma and scattered grains with ages of 900 500 ma but no ages of 1680 1650 ma equivalent to the local baltica basement were found agreement between the two techniques allowed evaluation of monazite age domains 198 analyses from the microdiamond rock and a kyanite garnet sillimanite mylonite produced from it using a combination of high resolution element imaging and trace element analysis of u th pb and y this comparison yielded three mean ages of 407 0 2 1 ma 394 8 2 3 ma and 374 6 2 7 ma combining this geochronology with previous p t estimates we propose that the uhp unit reached its maximum depth of 125 km at a maximum age of 407 ma when monazite was included in garnet and experienced 65 km of exhumation at an average rate of 10 9 mm year during top southeast thrusting that brought it into contact with the hp unit following these events both units were exhumed together at an average rate of 3 8 mm year until reaching a depth of 37 km at 395 ma where these rocks experienced extensive re equilibration and top west and left lateral shearing after 395 ma these units continued to be exhumed at an average rate of 0 8 to 1 4 mm year until 375 ma the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite the exhumation histories of these units record a change in mechanism from syncollisional exhumation through late to post orogenic collapse that was a consequence of plate reorganization

exhumation of the high pressure tsakkok lens swedish caledonides insights from the structural and white mica 40ar 39ar geochronological record

geochronology of fluid induced eclogite and amphibolite facies metamorphic reactions in a subduction collision system bergen arcs norway rb sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes granulite facies rocks of the lindas nappe bergen arcs norway were subducted and exhumed during the caledonian orogeny the rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation along fractures and shear zones the first occurred at eclogite facies 15 21 kbar 650 750 c and a later one at amphibolite facies conditions 8 10 kbar 600 c the presence of fluids enabled local metamorphic equilibration only near fluid pathways in fluid absent domains preexisting assemblages were metastably preserved this resulted in a heterogeneity of metamorphic signatures on meter to m scales well preserved granulite facies rocks preserve their proterozoic rb sr mineral ages as does the u pb system of zircon in most lithologies six rb sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid induced eclogitization at 429 9 3 5 ma 2 weighted average mswd 0 39 an eclogite facies vein has yielded metamorphic zircon with concordant u pb ages of 429 3 ma identical to the u pb age of 427 4 0 9 ma for zircon xenocrysts in an amphibolite facies vein seven rb sr mineral isochron ages date amphibolite facies fluid infiltration at 414 2 2 8 ma mswd 1 5 an age value testifying to residence of the rocks in the deep orogenic crust at temperatures 600 c for nearly 15 ma the new data show that rb sr mineral isochron ages effectively date fluid induced re crystallization events rather than stages of cooling the direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the p t reaction time space which is essential for understanding exhumation histories and the internal dynamics of orogens in general

evolution of caledonian deformation fabrics under eclogite and amphibolite facies at vardalsneset western gneiss region norway the vardalsneset eclogite situated in the western gneiss region sw norway is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation the eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite actinolite clinozoisite kyanite quartz paragonite phengite and rutile the rocks record a five stage evolution connected to caledonian burial and subsequent exhumation 1 a prograde evolution through amphibolite facies t 490 63 c is inferred from garnet cores with amphibole inclusions and bell shaped mn profile 2 formation of l s tectonite eclogite t 680 20 c p 16 2 kbar related to the subduction of conti nental crust during the caledonian orogeny lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the l s fabric was coaxial 3 formation of sub horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90 disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3 locally occurring mylonitic eclogites t 690 20 c p 15 1 5 kbar with top w kinematics may indicate however that non coaxial deformation was also active at eclogite facies conditions 4 development of a widespread regional amphibolite facies foliation t 564 44 c p 10 3 8 1 kbar quartz veins and development of conju gate shear zones indicate that coaxial vertical shortening and sub horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions 5 amphibolite facies mylonites mainly formed under non coaxial top w movement are related to large scale movement on the extensional detachments active during the late orogenic extension of the caledonides the structural and metamorphic evolution of the vardalsneset eclogite and related areas support the exhumation model including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

quartz veins record earthquake related brittle failure and short term ductile flow in the deep crust abstract discordant quartz veins in a retrograded eclogite amphibolite western gneiss region norway formed at temperatures of about 500 to 550 c the parental fractures crosscut the foliation and show branching indicating rapid crack propagation at high differential stress the shape of the quartz veins indicates intense inhomogeneous ductile deformation of the host rock during opening and sealing controlled by the presence of the cohesionless fluid filled fractures the vein quartz microfabrics indicate crystallization in an open cavity implying a stage of very high permeability and fluid flow the event of brittle failure at 500 to 550 c followed by an episode of inhomogeneous ductile deformation is attributed to stress changes related to the earthquake cycle i e coseismic loading and postseismic creep the record suggests seismic activity at the nearby low angle normal fault at 15 to 25 km depth and provides insight into transient crustal properties and processes near active faults at great depth

characteristics of summer stationary waves in the northern hemisphere abstract summer stationary waves in the northern hemisphere are separated by a midlatitude transition zone into the subtropical monsoon regime with a vertical phase reversal and the subarctic regime with a vertically uniform structure the dynamics and maintenance mechanism of the subtropical stationary waves have been investigated in the context of monsoon circulation depicted in terms of streamfunction with 40 yr ecmwf re analysis era 40 the dynamic characteristics of stationary waves in the transition zone and the subarctic region are thus the focus of this study the dynamics and maintenance mechanism of these waves were explored with the streamfunction budget and the velocity potential maintenance equations stationary waves across the transition region consist of anticyclonic shear zones over the north pacific and north atlantic and a cyclonic shear zone in east eurasia these transition elements are linked to subtropical oceanic anticyclones and continental thermal lows at high latitudes a th

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

coseismic crustal deformation from microseismicity in the patras area western greece summary the patras area lies in the western part of central greece it is an area characterized by high seismicity and complex neotectonics several devastating earthquakes have occurred in the region since 600 bc contemporary crustal deformation is examined in this area using microearthquake data recorded over a lengthy period during 1983 84 by the patras seismic network principally and to a lesser extent by the volos seismic network the microseismicity 1 8 3 9 ml defines a zone deepening to the ne which justifies a possible extension of the gulf of corinth major graben towards the trikhonis lake to the nw spectra of 108 well located microearthquakes are estimated using p waves obtained by selective windowing designed to include only the p phase seismic moments in the range 0 3 45 7 1012 nm are obtained accompanied by estimates of seismotectonic source parameters including source radii average stress drop and average coseismic slip poor correlation is found between seismic moment and magnitude and the likely reason is the complex nature of the neotectonic regime existing in the area two zones differing in crustal deformation characteristics are observed the corinth trikhonis zone reveals two sets of characteristic faults the first set is represented by microearthquakes showing distinctive and relatively higher seismic moments in conjunction with lower stress drops and seismic slips this set of faults shows greater source radius than the second set and therefore the faults are longer the second set is characterized by an almost constant source radius within the range of uncertainty and a wide range of seismic moments stress drops and seismic slips the rio zone is characterized by low seismic slip stress drop and fault radii with the exception of the locality south of the city of patras where relatively higher seismic slip stress drop and fault radius are observed

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

origin of the intraseasonal variability over the north pacific in boreal summer abstractthe spatial structure and temporal evolution of the intraseasonal oscillation iso in boreal summer over the midlatitude north pacific ocean are investigated through the diagnosis of ncep reanalysis data it is found that the midlatitude iso has an equivalent barotropic structure with maximum amplitude at 250 hpa initiated near 120 w the iso perturbation propagates westward at a phase speed of about 2 4 m s 1 and reaches a maximum amplitude at 150 w a diagnosis of barotropic energy conversion shows that the iso gains energy from the summer mean flow in the iso activity region a center followed column averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the iso perturbation there is a two way interaction between iso flows and synoptic eddies while a cyclonic anticyclonic iso flow causes synoptic scale eddies to tilt toward the northwest southeast northeast southwest direction the tilted synoptic eddies then

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

statistical characterization of geosat altimetry noise dependence on environmental parameters 1 sea surface height ssh measurements from the geosat satellite altimetry mission are analyzed to statistically characterize their noise process which is defined here as the small scale 50 km time varying component of the altimetry data this analysis is one component of a larger effort to extract meaningful information about seafloor roughness from the small scale fabric of the altimetric gravity field both small scale seafloor topography e g abyssal hills and ssh noise significantly influence the measured gravity fabric hence a global quantitative understanding of ssh noise is required to infer what portion of the gravity fabric is associated with the true geoid it is also anticipated that the ssh noise analysis will prove useful in designing improvements to altimetry processing algorithms the ssh data are first processed by differencing nearest neighbor tracks to minimize the static component of variability and then by high pass filtering to remove variability associated with mesoscale oceanic circulation autocovariance analysis reveals that ssh noise can be decomposed into uncorrelated white noise and correlated semiperiodic components three parameters are measured which characterize the variance of both components and the correlation length of the correlated component the global distribution of uncorrelated noise variance is closely correlated to significant wave heights directly linking the uncertainty of the altimeter return time to the roughness of the sea surface the variance and correlation length of the correlated component of noise can be associated with three separate environmental parameters sea ice precipitation rates and upper atmospheric variability associated with the subtropical jet stream as widely differing as these environmental processes are it is hypothesized that features like sea ice rain storms and atmospheric turbulence can cause sudden perturbations in the return time of the altimetry signal creating decaying oscillations associated with the onboard tracking algorithms

temporal and tectonic evolution of the granulite eclogite association from the bergen arcs western norway the u pb and sm nd dating of deep crustal rocks from the bergen arcs system helps resolve enigmatic aspects of the tectonic evolution of the caledonian orogen in western norway and yields insights into the arrested stages of eclogite development within the granulites of the area the u pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 5 ma all errors two standard deviations 2 which is similar to other zircon ages from the granulite facies protolith the age is interpreted to represent the time of late proterozoic sveconorwegian granulite metamorphism the u pb ages of sphene and epidote show that the eclogites formed early in the evolution of the caledonian orogen pre scandian phase at about 460 ma an eclogite facies quartz vein yields a sm nd whole rock garnet isochron of 440 12 ma that may reflect the onset of cooling immediately after peak eclogite facies conditions although the sm nd systematics reveal some isotopic disequilibrium within the sample in tandem with previous 40ar39ar age determinations from an adjacent eclogite of 450 ma for hornblende and 430 ma for muscovite these data indicate that 30 ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid crustal levels this corresponds to minimum cooling rates of 14 c m y the timing relations suggest that the formation and exhumation of these eclogites from the overlying caledonian nappe wedge in western norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent continent collision the short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite isolated occurrences of eclogite within the granulite the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the dry granulite all point to the importance of fluids in the transformation and re equilibration of the granulite to eclogite together field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions

applications of inclusion behaviour models to a major shear zone system the nordfjord sogn detachment zone in western norway rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones we used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity strain nature of rigid inclusion matrix interface and confinement in the large scale ductile nordfjord sogn detachment zone nsdz of the caledonides of western norway our study shows that 1 the observed shape preferred orientation spo at higher structural levels of the nsdz at site 1 gjervika can be explained by simple shear pure shear simple shear ratio sr 0 associated with a slipping inclusion matrix interface 2 the observed spo at deeper structural levels of the nsdz at site 2 near sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone sr z 1 acting upon rigid inclusions in slipping contact with the enclosing matrix 3 observed back rotated boudins deeper in the nsdz at site 3 biskjelneset can form in confined flow associated with a considerable amount of shortening across the shear zone sr 0 4 4 the observed tails of porphyroclasts indicate a minimum at least local strain of ca g z 20 5 the clasts in the studied shear zones strongly depart from jeffery s model jeffery g b 1922 the motion of ellipsoidal particles immersed in a viscous fluid proceedings of the royal society of london a102 161e179 the large scale extensional nsdz under investigation shows evidence of strain partitioning rocks vary from protomylonites to ultramylonites and the simple shear and pure shear components are heterogeneously distributed therefore we conclude that flow in the nsdz was very heterogeneous both at the kilometre and the metre scale however the present study suggests that the amount of shortening across the shear plane throughout the nsdz increases with depth and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall 2007 elsevier ltd all rights reserved

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

thermochronology of the mont laurier terrane southern canadian grenville province and its bearing on defining orogenic architecture abstract convergence during pre grenvillian accretion led to crustal thickening and juxtaposition of shallowly southeast dipping imbricated terranes of the central metasedimentary belt in ontario and its equivalent to the north in quebec known as the mont laurier terrane mlt this investigation focuses on the otter lake region of the southern mlt which is composed of a mesoproterozoic back arc sequence metamorphosed to upper amphibolite granulite facies at conditions of 6 1 kbar and 650 700 c notable peak assemblages include fo bearing marbles and sil kfs migmatites with little to no retrogression the terrane is divided into the marble west and quartzite east domains separated by the east dipping heney shear zone hsz la icpms u pb geochronology on select migmatites from both domains yielded bimodal u pb zircon and monazite age populations at ca 1230 ma and 1180 1140 ma with mineral ages from the leucosomal portions of the migmatites populating the younger age bracket and implying that the metamorphism and anatexis took place during the accretionary shawinigan orogeny directly east of the hsz 40ar 39ar hornblende and phlogopite ages are ca 1145 ma and ca 1150 ma respectively whereas to the west in the marble domain hornblende ages are markedly younger ca 980 950 ma rapid cooling of the quartzite domain at the end of the shawinigan orogeny together with the lack of evidence for any metamorphic overprinting in the southern mlt during the long lived grenvillian orogeny 1095 980 ma is consistent with this region being part of an orogenic lid or suprastructure post grenvillian cooling of marble domain rocks through 500 c nearly 200 m y after cooling of the quartzite domain suggests a protracted middle crustal residence of the marble domain under thermally elevated conditions implying that portions of the orogenic lid remained warm above 500 c during the grenville orogeny moreover the disparate cooling ages on either side of the hsz provide evidence for important late displacement across the hsz comparable to that proposed for terrane bounding shear zones such as the robertson lake and bancroft shear zones these structures may be an expression of vertical orogenic collapse or lateral stretching of the orogenic lid biotite cooling ages are uniformly

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

heterogeneous extrusion and exhumation of deep crustal variscan assembly geochronology of the western tatra mountains northern slovakia abstract the nature and style of mid crustal assembly and exhumation during continental collision has been investigated in the tatra mountains of the western carpathians the pre alpine basement of the western carpathians represents the easternmost exposure of the variscan orogen in europe which marks the collision of laurasia with gondwanian affiliated terranes during the palaeozoic the tatric crystalline unit of the western tatra in northern slovakia displays an inverted metamorphic sequence where a high grade unit comprising migmatites with relicts of eclogite has been thrust over a lower grade mica schist unit new geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the western tatra the upper unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1 6 gpa to 1 0 1 2 gpa at 750 800 c which lead to intensive re equilibration at high pressure granulite facies conditions comparable to the peak metamorphic conditions of the host migmatite both eclogite and migmatite shared a retrograde p t path following the insertion of the eclogite assemblage into the migmatites the metamorphic evolution of the lower unit mica schist is constrained to peak p t conditions of 0 6 0 8 gpa and 640 and 660 c followed by retrogression this suggests that different rock types of the western tatra metamorphic core shared only their exhumation path from mid crustal levels id tims sm nd dating of garnet from eclogite yields a whole rock garnet isochron age of 337 10 ma with an initial e nd isotopic composition of 8 3 in situ u pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c 380 ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 11 ma age which is indistinguishable from sm nd garnet age and u pb age of zircons in the anatectic leucosome of the migmatite 347 7 ma a younger monazite age population from the migmatite of 300 16 ma is consistent with 40 ar 39 ar mica ages of c 310 ma this argues for a contemporaneous and likely shared exhumation path of the assemblage pair in situ monazite total pb analyses from the lower unit mica schists yields xenocrystic and c 370 ma ages but no geochronologic evidence for peak variscan tectonism exhumation of the deep crustal root occurred most probably in a two stage process the timing of the high pressure eclogite facies metamorphism before the onset of exhumation into the mid crust was likely between c 380 ma and 360 ma subsequent exhumation into the middle crust was coeval with migmatite generation at c 340 ma and garnet diffusion modeling suggest 30 c ma cooling rates the exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid crustal levels at a time coeval with anatexis and subsequently extruded with mid crustal material to the upper crust

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

the time space structure of the asian pacific summer monsoon a fast annual cycle view abstract despite the seemingly intricate and multifold time space structure of the mean asian pacific summer monsoon apsm its complexity can be greatly reduced once the significance of fast annual cycles has been recognized and put into perspective the apsm climatology is characterized by a slowly evolving seasonal transition slow annual cycle superposed by pronounced singularities in the intraseasonal timescale termed the fast annual cycle in this study the fast annual cycles show nonrepetitive features from one episode to another which are often divided by abrupt change events the apsm fast annual cycles are composed mainly of two monsoon outbreaks each marking a distinctive dry wet cycle the first cycle spans from the middle of may to early july and the second cycle from late july to early september when the first cycle reaches its peak in mid june a slingshot like convection zone described as the grand onset pattern rules an area from the arabian sea to the indochina peninsula then b

structural petrological and chemical analysis of syn kinematic migmatites insights from the western gneiss region norway evidence of melting is presented from the western gneiss region wgr in the core of the caledonian orogen western norway and the dynamic significance of melting for the evolution of orogens is evaluated multiphase inclusions in garnets that comprise plagioclase potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies the multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting such as segregations in mafic rocks leucosomes and pegmatites hosted in mafic rocks and in gneisses based on field studies these lithologies are found in three structural positions 1 as zoned segregations found in high pressure hp ultra mafic bodies 2 as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss and 3 as undeformed pegmatites cutting the main caledonian structures segregations post date the eclogite facies foliation and predate the amphibolite facies deformation whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post kinematic and were formed at the end of the deformation history geochemistry of the segregations leucosomes and pegmatites in the wgr defines two trends which correlate with the mafic or felsic nature of the host rocks the first trend with ca poor compositions represents leucosome and pegmatite hosted in felsic gneiss whereas the second group with k poor compositions corresponds to segregation hosted in ultra mafic rocks these trends suggest partial melting of two separate sources the felsic gneisses and also the included mafic eclogites the ree patterns of the samples allow distinction between melt compositions fractionated liquids and cumulates melting began at high pressure and affected most lithologies in the wgr before or during their retrogression in the amphibolite facies during this stage the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward buoyant crust partial melting of both felsic and mafic sources at temperatures below 800 c implies the presence of an h2o rich fluid phase at great depth to facilitate h2o present partial melting

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

interaction between fluid flow fracturing and mineral growth during eclogitization an example from the sunnfjord area western gneiss region norway in sunnfjord western gneiss region of south norway a proterozoic layered gabbro complex displays various degrees of transformation to eclogite in the unreacted parts layering in the gabbro is defined by modal variations of plagioclase olivine pyroxenes and minor fe ti oxide and spinel coronitic and foliated eclogite formed from the gabbro by hydration reactions at t 510 620 c causing a volume decrease of c 13 in the coronitic eclogite the mafic magmatic phases are replaced by aggregates of omphacite barroisite tremolite talc and rutile whereas the plagioclase domains are pseudomorphed by omphacite barroisite clinozoisite kyanite paragonite and garnet the felsic and mafic domains are separated by a garnet rim up to 5 mm thick garnet was also formed along dilational veins connecting and radiating out of coronas including the same eclogite facies minerals in addition microfractures filled by amphibole and omphacite cut through the corona and vein garnet oriented perpendicular to the garnet layering the transformation of dry gabbro to eclogite with hydrous minerals requires supply of water the timing of metamorphic reactions is therefore dependent on the timing of fluid introduction the inclusion pattern of garnet in the holt tyssedalsvatnet metagabbro complex indicates that transformation started under eclogite facies conditions brittle deformation in form of fractures allowing infiltration of fluids and mobilisation of elements is shown to be the most important process initiating transformation brittle deformation is thereby active in deep crustal levels corresponding to eclogite facies conditions fracturing is interpreted as having been caused by a combination of high fluid pressure volume changes during mineral transformations and external stresses ductile deformation started after the initial metamorphic transformation garnet chemistry and zoning pattern are controlled by the chemistry of the growth place the fluid influx and element supply abrupt chemical variations across corona garnet may have been formed by an interaction between rapid growth and transport along microfractures between plagioclase and mafic domain the metamorphic transition proceeded rapidly the transformation of the deep crust to eclogite can be viewed as a dynamic process where the volume changes result in fracturing and these fractures funnelling fluid and element enhancing mineral reaction the densification causes further fracturing indicating that the process is self driven provided that fluid is available

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

maintenance of summer monsoon circulations a planetary scale perspective abstract the monsoon circulation which is generally considered to be driven by the landmass ocean thermal contrast like a gigantic land sea breeze circulation exhibits a phase reversal in its vertical structure a monsoon high aloft over a continental thermal low is juxtaposed with a midoceanic trough underlaid by an oceanic anticyclone this classic monsoon circulation model is well matched by the monsoon circulation depicted with the observational data prior to the first global atmospheric research program garp global experiment fgge however synthesizing findings of the global circulation portrayed with the post fgge data it was found that some basic features of major monsoon circulations in asia north america south america and australia differ from those of the classic monsoon circulation model therefore a revision of the classic monsoon theory is suggested with four different wave regimes selected to fit the horizontal dimensions of these monsoon circulations basic features common to

electromagnetic precursors to earthquakes in the ulf band a review of observations and mechanisms despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes the scientific community is no closer to understanding why precursors are observed only in some cases laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed however field experiments have had much less success many purported observations of low frequency electrical precursors are much larger than expectations based on laboratory results in some cases no precursors occurred prior to earthquakes or precursory signals were reported with no corresponding coseismic signals nonetheless the field experiments are in approximate agreement with laboratory measurements maximum resistivity changes of a few percent have been observed prior to some earthquakes in china but the mechanism causing those changes is still unknown anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed finally piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields

rapid exhumation of lower crust during continent continent collision and late extension evidence from 40ar 39ar incremental heating of hornblendes and muscovites caledonian orogen western norway resolving enigmatic aspects of the age relations of deep crustal rocks in the bergen arcs area provides key insights into the controversial tectonic evolution of the caledonian orogen in western norway in the bergen arcs area eclogites of the lower crustal lindas nappe upper plate structurally overlie eclogite bearing basement gneiss of the western gneiss region lower plate across the bergen arcs shear zone laser 40 ar 39 ar incremental heating data for hornblendes and muscovites on mineral separates in tandem with detailed thermobarometry and structural observations allow evaluation of the tectonic cooling and exhumation histories of the deep crustal rocks as well as offset and movement history across tectonic boundaries in the lindas nappe the eclogites formed at 700 c and pressures 17 kbar hornblende and three separate muscovite grains from a single eclogite sample reveal well defined 40 ar 39 ar isotope plateaus at 447 7 4 2 432 6 0 5 429 2 0 9 and 429 5 0 6 ma 2 errors given here and below respectively indicating relatively rapid cooling of the high pressure rocks 10 15 c m y apparently older plateau ages from muscovites from other eclogites at 451 3 1 1 and 462 6 0 8 ma may be related to indiscernible excess argon high grade amphibolites 690 c 8 12 kbar of the lindas nappe yield ages of 439 3 3 5 and 455 4 1 5 ma in contrast hornblende from gneiss in the adjacent western gneiss region 670 c 8 kbar to the east across the bergen arcs shear zone reveal plateau ages of 409 3 2 7 and 394 8 2 2 ma similar to other hornblende and muscovite 40 ar 39 ar dates in the western gneiss region immediately to the north to the west of the lindas nappe allochthonous basement gneiss of the oygarden gneiss complex 670 c 8 kbar yields a plateau age of 408 1 3 4 ma the temperature estimates from all the samples analyzed are above the closure temperatures for hornblende and muscovite and thus indicate that the argon ages reflect cooling after peak metamorphic conditions the argon ages for the high pressure eclogites of the lindas nappe imply extreme crustal thickening 50 km prior to ca 450 ma together the data indicate that subduction of lowermost continental crust possibly related to continent continent collision occurred early ca 450 ma in the tectonic evolution of the orogen leading to the development and exhumation of the lower crustal eclogites and high grade amphibolites within the lindas nappe between 450 and 430 ma in contrast the high pressure eclogites of the western gneiss region which may have formed significantly later ca 425 ma were exhumed and cooled along with the basement gneiss late in the history of the orogen ca 400 ma the 40 m y offset across the bergen arcs shear zone which separates the lindas nappe upper plate and the western gneiss region lower plate is consistent with extensional movement along the fault resulting in the rapid exhumation and cooling of the region late in the tectonic evolution of the orogen ca 400 ma thus the deep crustal rocks in the overlying caledonian nappes in western norway appear to have been cooled and exhumed rapidly related to continent continent collision during a predominantly contractional phase early in the tectonic evolution of the orogen whereas lower crustal rocks of the underlying basement appear to have been exhumed extremely rapidly coincident with late orogenic to postorogenic extension temporally and possibly tectonically distinct mechanisms are illustrated for the exhumation of deep crust within the tectonic evolution of a continent continent collisional orogen

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

symplectite formation in the presence of a reactive fluid insights from hydrothermal experiments this study describes the microstructural and chemical development of symplectites obtained in fluid mediated mineral replacement experiments during the experiments polymineralic feldspar rich samples were exposed to aqueous na sio2 solution at 600 c and 2 kbar confining pressures for durations of 12 hr to 20 days the resulting reaction rims display high mineralogical and structural complexity and contain two varieties of symplectites represented by nanometre scale intergrowths of gehlenite zeolite and grossular zeolite grains the experimental fluid was enriched in 18o isotope in order to trace oxygen redistribution during the reaction the elevated 18o concentration in the reaction products and the heterogeneity in its distribution suggest that symplectite formation was controlled by dissolution precipitation mechanisms rather than volume diffusion processes microstructural and chemical observations suggest that symplectite formation occurred in multiple stages in response to spatially heterogeneous and temporarily evolving fluid composition at the reaction interfaces hence our results shed light on the fundamental processes involved in symplectite formation improving our ability to interpret symplectite microstructures this article is protected by copyright all rights reserved

deciphering high pressure metamorphism in collisional context using microprobe mapping methods application to the stak eclogitic massif northwest himalaya the stak massif northern pakistan is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the indian continent in the northwest himalaya although this unit was extensively retrogressed during the himalayan collision records of the high pressure hp event as well as a continuous pressure temperature p t path were assessed from a single thin section using a new multiequilibrium method this method uses microprobe x ray compositional maps of garnet and omphacitic pyroxene followed by calculations of 200 000 p t estimates using appropriate thermobarometers the stak eclogite underwent prograde metamorphism increasing from 650 c and 2 4 gpa to the peak conditions of 750 c and 2 5 gpa then retrogressed to 700 650 c and 1 6 0 9 gpa under amphibolite facies conditions the estimated peak metamorphic conditions and p t path are similar to those of the kaghan and tso morari high to ultrahigh pressure hp uhp massifs we propose that these three massifs define a large hp to uhp province in the northwest himalaya comparable to the dabie sulu province in china and the western gneiss region in norway

trace element composition and cathodoluminescence of kyanite and its petrogenetic implications kyanite crystals from fourteen localities worldwide were analysed for their abundances of the trace elements na mg k ca ti v cr mn and fe and cathodoluminescence cl properties based on protolith type metamorphic setting and distinctive trace element fingerprints a genetic classification of kyanite bearing rocks is suggested a al rich metasediments which commonly contain coarse grained quartz kyanite segregations b metamorphosed granitic rocks specifically granulites c metamorphosed argillic alteration zones hosted originally in felsic igneous rocks d metamorphosed argillic alteration zones hosted originally in mafic igneous rocks and e metamorphosed mafic to ultramafic rocks specifically eclogites vanadium and cr concentrations reflect both protolith and host rock compositions and therefore may provide a geochemical fingerprint for the nature of the protolith the incorporation of fe into kyanite is largely controlled by oxygen fugacity during kyanite formation and therefore in most cases its concentration cannot be related to that of the protolith from our results ti concentration appears to be related to metamorphic grade particularly formation temperature if proven by further studies ti in kyanite may provide a useful geothermometer correlation of trace element abundances with cl spectra confirms that common red cl which is composed of the spectral bands centred at 1 69 ev 734 nm 1 75 ev 708 nm and 1 80 ev 689 nm is related to cr3 defects cl spectra of most kyanites show in addition a low intensity blue emission centred at 2 56 ev 485 nm correlation of the intensity of the blue emission with ti suggests that it is related to or sensitized by ti4 or ti3 defects kyanites with 3200 gg 1 fe show generally no detectable cl due to the cl quenching effect of fe2 our findings provide new criteria in the exploration for and quality assessment of new kyanite deposits the ti content one of the critical contaminants of kyanite products besides fe ca and mg appears predictable from the observed correlation of ti with formation temperature iron will be hard to predict because its incorporation is mainly controlled by the oxidizing conditions during kyanite formation and the estimation of these conditions requires advanced analytical methods magnesium and ca are consistently low in all investigated samples from a regional exploration viewpoint group c and d kyanites have the lowest ti and relative low fe and therefore will be most refractory due to their attractive blue colour kyanite bearing rocks of group c have potential as ornamental or dimension stone

a thermomechanical model of exhumation of high pressure hp and ultra high pressure uhp metamorphic rocks in alpine type collision belts using a fully coupled numerical thermomechanical model handling strain localization surface processes and ultra high viscosity contrasts 11 orders of magnitude we test a number of possible mechanisms of high pressure hp low temperature lt high temperature ht exhumation in continental collision zones the model considers two end member cases low or high buoyancy of the downgoing crust the first model case predicts three levels of exhumation in the same collisional context the classical corner flow lp lt low pressure low temperature exhumation in the accretionary prism deeper 70 km hp ht exhumation for the thickened subducting crustal sedimentary wedge and ultra hp ht exhumation from the lower crustal chamber forming at the depth of 100 120 km and separated from the upper one by a narrow crustal channel the width of this channel can oscillate in the process of shortening thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber although both zones of crustal accumulation and the narrow channel between them resemble a vortex shaped nozzle this nozzle appears to be too soft to produce any significant overpressures from the upper crustal wedge the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material from the lower crustal chamber the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere in the second modelled case of high buoyancy the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface accelerated or slowed subduction in case of full or partial crustal decoupling respectively and upper plate extension d 2001 elsevier science b v all rights reserved

interaction between intraseasonal oscillations of the midlatitude flow and tropical convection during 1979 northern summer the pacific ocean abstract it has been inferred and illustrated by many studies in terms of outgoing longwave radiation olr and velocity potential that energy is input into the intraseasonal oscillation through tropical cumulus convection especially over the monsoon region the existence of this low frequency oscillation of regional extratropical oscillation has been observed by other studies previous findings lead us to speculate upon a hemispheric interaction between tropical cumulus convection and extratropical circulation in terms of an intraseasonal time scale this study demonstrates that during the 1979 summer a coherent intraseasonal oscillation existed between cumulus convection and divergent circulation over the equatorial western pacific and mid pacific troughs moreover the intraseasonal oscillations of streamfunction associated with these troughs the tropical olr and the tropical field exhibit coherent eastward propagations based upon these two coherent relations it is hypothesized that a tro

latest caledonian to present tectonomorphological development of southern norway abstract the regional resultant stress field of the northeastern north atlantic has shifted significantly throughout the phanerozoic in fennoscandian parts of the caledonian orogen mountain building which was characterized by nw se contraction reference to present north was followed by a collapse with transport both parallel and transverse to the mountain chain the late palaeozoic mesozoic saw several stages of e w to nw se extension varying in time and position local episodes of inversion are traceable in some cases particularly in connection with deep seated and long lived zones of weakness the cenozoic has to a larger degree been affected by compression including folding and basin inversion again some of the more pronounced effects of local inversion are related to pre existing fault systems neogene uplift of the western mountainous area in scandinavia can be unravelled by potential field study aft data and reflection seismic sections assuming that the region is close to isostatic equilibrium the uplifted areas must be supported at depth by substantial volumes of low density material within the crust or the mantle close to the crust mantle interface or close to the lithosphere asthenosphere interfaces a series of nw se oriented cross sections representing successive stages of development from the late caledonian to present were prepared and used to analyse the tectonomorphological development as related to the shifting stress configurations three stages are inferred stage 1 devonian late permian denudation of the caledonian mountain chain during the latest stage of closing of the iapetus ocean in early to mid devonian times the caledonian mountain chain reached its maximum elevation of perhaps as much as 8 9 km including the foreland basins to the east the average topographic gradient of the eastern flank may have been in the order of 1 3 the denudation of the caledonian mountains continued into the permian and by the end of the permian the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0 1 stage 2 triassic late cretaceous tectono thermal uplift of central south norway uplift of central south norway was rejuvenated in triassic times and continued throughout the jurassic and early cretaceous the gradient of the eastern flank increased to a maximum of approximately 0 15 and master structural elements like the laerdal gjende olestol fault complex again played important roles in the development stage 3 tertiary present renewed tectono thermal activity and post glacial rebound a second phase of post caledonian tectono thermal uplift of south central norway started in oligocene time again increasing the gradient of the eastern flank of south norway around 0 2

the structure and maintenance of stationary waves in the winter northern hemisphere abstract previous studies of extratropical stationary waves in the winter northern hemisphere nh often focused on effects of orography and land ocean thermal contrast on the formation structure and maintenance of these waves in contrast research attention to tropical stationary waves was attracted by the summer monsoon circulations and the enso related climate variability consequently the structure and basic dynamics of tropical stationary waves and the relationship of these waves with those in mid high latitudes have long been neglected thus the following several distinct features of observed winter nh stationary waves have not been explained 1 an abrupt change in the longitudinal phase across 30 n 2 a transition from the vertical phase reversal of tropical stationary waves to the vertically westward tilt of extratropical stationary waves and 3 a longitudinally quarter phase relationship between stationary waves and east west circulations and a reversal of this relationship across 30 n

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

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

high pressure metamorphism and deep crustal seismicity evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas the metamorphic complex of the western gneiss region wgr norway constitutes the root of the caledonian mountain belt and experienced temperatures of 700 800 c and pressures in excess of 20 kbar during peak metamorphism mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures the krakeneset gabbro located on the island of vagsoy in the mixed hp uhp zone of the western wgr display evidence for extensive metastability through the entire prograde and retrograde p t histories eclogite constitutes less than a few percent of the total volume of the body and high pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration the gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones 50 cm wide transecting the gabbro body the pseudotachylytes contain m to mm scale amoeboid and dendrite like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene omphacite amphibole and dolomite suggesting rapid disequilibrium growth of minerals during high pressure conditions textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid state alteration of a microcrystalline pseudotachylyte matrix the pseudotachylyte lined fault zones are in close spatial association with numerous amphibole carbonate filled hydrofractures with conspicuous fracture parallel alteration zones defined by hydrous eclogite facies assemblages these eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high grade metamorphic conditions geothermobarometric estimates ca t 650 700 c p 20 kbar and petrological data imply that hydrofracturing pseudotachylyte crystallization and the subsequent pseudotachylyte alteration process must have occurred during high pressure metamorphism our observations are suggestive of a deep crustal earthquake scenario where a high pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition the formation of weak deep crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the krakeneset gabbro during its residence deep in the former caledonian collision zone

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

genesis and development of spring rainstorms in northern southeast asia southwest china northern indochina and the northern south china sea abstractduring may and june the monsoon rainfall in northern southeast asia is primarily produced by rainstorms at the mature stage these storms coupled with a midtropospheric subsynoptic scale trough produce rainfall 50 mm 6 h 1 and exhibit a cyclonic surface vortex with a scale o 102 km rainstorms during the period of 1979 2016 are identified with station and satellite observations along with assimilation data several dynamic processes of rainstorm geneses are disclosed by an extensive analysis 1 maximum occurrence of rainstorm geneses is located in the midtroposphere of two regions northern vietnam southwestern china and the northern south china sea but eventually penetrates downward to the surface 2 the environment favorable for rainstorm genesis is a southwest northeast oriented narrow trough formed by the confluence of the midtropospheric northeasterly around the eastern tibetan plateau and the lower tropospheric monsoon southwesterlies because the criterion for charney stern i

northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation abstract using climatological pentad mean outgoing longwave radiation olr and european centre for medium range weather forecasts analysis winds the authors show that the northern hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations cisos the extreme phases of ciso characterize monsoon singularities monsoon events that occur on a fixed pentad with usual regularity whereas the transitional phases of ciso represent the largest year to year monsoon variations the ciso results from a phase locking of transient intraseasonal oscillation to annual cycle it exhibits a dynamically coherent structure between enhanced convection and low level convergent upper level divergent cyclonic anticyclonic circulation its phase propagates primarily northward from the equator to the northern philippines during early summer may july and westward along 15 n from 170 e to the bay of bengal during august and september the propagation of ciso links monsoon singular

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

distribution of hydrous components in jadeite of the dabie mountains two kinds of jadeite occur in the jadeite quartzite from the dabie mountains eastern china associated either with weak or strong deformation the former shows a uniform composition while the latter shows both uniform and zoning composition these jadeites were examined with infrared ir spectroscopy all jadeite displays hydroxyl oh stretching bands implying that hydrous components commonly exist in jadeite the concentration of the hydrous components in the jadeite with weak deformation is homogenous whereas the concentration of the hydrous components in strongly deformed samples is variable the correlation between the intensity of oh band absorbance and compositional zoning of jadeite with strong deformation indicates that na rich jadeite can accommodate more oh than na poor jadeite its interpretation is that there is more na associated with greater oh availability in the initial crystallization environment our data of the comparatively high oh concentrations recorded in the core of the jadeite which contains relative high vacancies in m2 site imply that both oh content and the vacancies in m2 site could decrease during exhumation of the jadeite quartzite the decrease of oh solubility in jadeite would result in the formation of h2o fluid during the early exhumation of uhp metamorphic rocks the h2o fluid transformed from oh during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep

the clinopyroxene plagioclase symplectite in retrograde eclogites a potential geothermobarometer the retrograde p t t path of intermediate and high temperature eclogites t 500 50 c is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae often this symplectite shows a two stage microstructural evolution i e a coarse lamellar width symplectite s1 followed by a thinner lamellar width symplectite s2 each symplectite may also evolve independently into a final globular microstructure sg

eclogite facies polyphase deformation of the drosdal eclogite western gneiss complex norway and implications for exhumation abstract exhumation of the deep crust during orogenic extension is accepted as a geological phenomenon but structures formed during burial and the earliest stages of exhumation are often overprinted by deformation events occurring at shallower crustal levels the western gneiss complex wgc of norway comprises variably retrogressed high pressure hp and ultrahigh pressure uhp eclogite bodies enclosed in predominantly felsic amphibolite facies rocks the drosdal body located in the sunnfjord area is one of the largest and best preserved eclogites in the wgc and comprises over ca 3 km2 of exposed eclogite facies rocks it is an excellent example of an eclogite tectonite and displays a wealth of structures formed during deformation at a minimum depth of 50 km and peak temperatures of ca 800 c large volumes of mylonites with the eclogite facies assemblage garnet clinopyroxene quartz amphibole clinozoisite phengite kyanite rutile are preserved structures associated with the eclogite facies metamorphism include e w trending isoclinal folds boudinage hinge parallel lineations and meter scale kyanite dominated veins the drosdal body was tightly folded on the kilometer scale under eclogite facies conditions subsequently the shapes of the drosdal body and other mafic bodies in the sunnfjord area were modified during eclogite to amphibolite facies e w directed stretching to give boudin like lenses the timing of formation of a pervasive eclogite facies lineation eclogite facies folds and kyanite bearing veins overlapped substantially and this portion of the deformation history was dominated by a constrictional strain field partitioning of deformation occurred after a substantial amount of eclogite facies deformation had already taken place and resulted in the relative rotation of linear features in specific zones on the scale of 400 m the eclogite facies lineations and fold hinges within the drosdal body are subparallel to amphibolite and greenschist facies structures throughout the wgc although circumstantial this suggests that structures belonging to each of these three metamorphic facies formed during one progressive deformation event corresponding to extensional exhumation our observations are consistent with models involving inhomogeneous deformation of the lower crust during early orogenic extension we envisage that transtensional noncoaxial deformation dominated the extensional deformation of the wgc but that coaxial deformation must have occurred locally

dicing with earthquakes the van group have been publicizing claims to be predicting earthquakes since 1981 their predictions of two basic parameters of earthquakes magnitude and location are very loosely constrained most van magnitude predictions are below 5 7 ms with a tolerance of 0 7 ms the uncertainty range spanned by a predicted magnitude is so enveloping that such forecasts cannot be viewed as plainly successful in terms of van s claims to spatial resolution these if successful might constrain the linear dimension of the source region of a magnitude 8 0 ms earthquake but do not adequately constrain a magnitude as large as 7 5 ms not only do van need to refine the tolerances in their magnitude and location predictions before careful judgements can be made but it is also necessary to provide an unambiguous statement of the prediction hypothesis used in relation to a pre published and communally archived dataset of complete seismic electric signal occurrences similar to seismological station bulletins

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin

the timing of stabilisation and the exhumation rate for ultra high pressure rocks in the western gneiss region of norway new petrographic evidence and a review of the latest radiometric age data are taken to indicate that formation of the ultra high pressure uhp eclogites within the western gneiss region of norway probably occurred within the 400 410 ma time frame thus this event took place significantly later than the previous widely accepted age of c 425 ma for the timing of the high pressure metamorphism in this part of the scandinavian caledonides garnet growth under uhp coesite stable conditions is recognised as a discrete younger event following on from earlier garnet formed under firstly amphibolite facies then quartz stable eclogite facies conditions currently the best constrained and most precise age specifically for uhp mineral growth is the 402 2 ma u pb age for metamorphic zircon some of which retain coesite inclusions from the hareidland eclogite exhumation must have followed shortly thereafter and based on synoptic pressure temperature and depth time curves must have been very fast our data and those of others indicate an initial fast exhumation to about 35 km depth by about 395 ma at a mean rate of about 10 mm a 1 this rapid exhumation rate may have been driven by the appreciable residual buoyancy of the deeply subducted continental crustal slab due to incomplete eclogitization of the dominant proterozoic orthogneisses during the short lived uhp event subsequent exhumation to 8 10 km depth by about 375 ma occurred at a much slower mean rate of about 1 3 mm a 1 with the late stage extensional collapse of the caledonian orogen playing an increasingly important role especially in the final unroofing of the western gneiss region with some remarkably preserved uhp rocks