geobarometry of low temperature eclogites applications of isothermal pressure activity calculations estimation of metamorphic pressures in low temperature eclogite type c is difficult because of the high variance mineral assemblages and problems in geothermometry solution properties of low temperature omphacite and the thermodynamic properties of clinozoisite we have considered equilibria in the cao feo mgo tio2 al2o3 sio2 h2o cfmtash system involving the phase components quartz rutile kyanite ilmenite almandine pyrope grossular clinozoisite sphene diopside and h2o fluid there are four linearly independent equilibria involving the phase components in this system because kyanite can crystallize as a nearly pure phase the lack of kyanite in a rock indicates that a al2sio5 is 1 0 if we can estimate temperature independently we can solve for a al2sio5 and pressure by using two of the equilibria in isothermal pressure activity diagrams we have applied this approach to eclogites from new caledonia and from southwestern oregon for the new caledonia eclogites calculated pressures range from 11 2 to 13 6 kbar at 500 c and are consistent with the minimum pressures based upon the presence of jadeitic pyroxene quartz and the lack of stable albite oregon eclogites come from different tectonic blocks and calculated minimum pressures of 11 12 kbar are based upon the presence of jadeitic pyroxene rutile garnet and lack of stable albite and ilmenite at reduced values of a sio2 0 7 0 9

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

development of p t prograde and p retrograde t prograde isogradic surfaces during blueschist to eclogite regional deformation metamorphism in new caledonia as indicated by progressively developed porphyroblast microstructures the mid tertiary blueschists eclogites and eclogitic gneisses of northern new caledonia are the products of four phases of regional meta morphism and deformation d d4 omphacite lawsonite and mn rich garnet isogradic surfaces were developed during the second deformation d2 under prograde pressure and temperature conditions subsequent deformations d3 d folded these d isogradic surfaces however within the p retrograde t prograde metamor phic environment of the d phase omphacite altered to albite and chlorite as a result a late stage sub horizontal isogradic surface developed for omphacite out where this mineral is preserved as relics within syn d albite porphyroblasts other minerals that crystallized for the first time epidote or had rim additions almandine phengite during d also form nearly horizontal isogradic surfaces porphyroblastic garnet and albite contain inclusion trails which allow their microstructural development and crystallization of the matrix to be traced from d to d late syn d the temperature increased mark edly in association with an extensive exothermic decarbonation even though the rocks were in a state of pressure retrogression this caused considerable neocrystallization recrystallization and growth of matrix and porphyroblasts such that although s foliation crenulated by d and d is readily observable almost all signs of stored strain due to d and d have been removed and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization post dating deformation

new caledonia obducted peridotite nappe offshore extent and implications for obduction and postobduction processes one of the largest ophiolitic peridotite masses in the world covers a quarter of the island of grande terre new caledonia the peridotite nappe was obducted during the eocene is weakly deformed and corresponds to the highest of a structurally simple pile of thrust nappes we present new marine seismic data that allows us to track the offshore continuation of the peridotite nappe along strike for a distance of more than 500 km south of new caledonia and to image its pre syn and post obduction sedimentary records offshore the peridotite nappe underlies a c 150 km wide and 2 km deep basin flat topped horsts of peridotite are clearly bounded by major normal faults in contrast faults are obscure onland to the east the peridotite nappe roots along the eastern margin of the felicite ridge new name a c 300 x 25 km dome shaped ridge which we interpret as being the southern extension of the hp lt metamorphic core complex observed in new caledonia two alternative tectonic models address the relative timing and relationships between peridotite nappe emplacement uplift of a metamorphic core complex and extensional tectonics these models provide new ideas for the understanding the formation of the eastern margin of the zealandia continent our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins

the early diagenesis of iron in pelagic sediments a multidisciplinary approach abstract biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial atlantic are investigated by means of pore water chemistry chemical leaching experiments and total elemental determinations color reflectance spectroscopy rock magnetic measurements and tem observations of the magnetic fraction results indicate that in the presence of nitrate ascorbate a weak reducing agent leachable iron decreased with depth from the sediment surface within this upper sediment region iron assimilation by bacteria is indicated as magnetosomes were found by tem observations throughout the whole core extractions with a strong reducing agent dithionite representing iron bound to iron oxides correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone the abrupt decrease of red color reflection intensity relative to the amount of iron oxides below the iron redox boundary is the result of a decrease in the specific surface area of iron oxyhydroxide oxides magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary arising from an increase in biogenic magnetite formed by magnetotactic assimilatory bacteria dissolution of very fine grained magnetite and the gradual decrease of coarser grained terrigenous titano magnetite with depth the early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content

high pressure veins in eclogite from new caledonia and their significance for fluid migration in subduction zones abstract this paper describes the petrology and geochemistry of high pressure veins and associated metasomatic selvages that are hosted by eclogite in the pouebo eclogite melange of northern new caledonia isotopic and geochemical evidence indicates that the mafic host rock represents seafloor altered fractionated morb that underwent eclogite facies metamorphism in a subduction zone within the host rock are cm thick garnet quartz phengite veins that are enveloped by garnet poor omphacite rich selvages or bleach zones petrography thermometry oxygen isotope characteristics and mass balance calculations are used to show that the veins largely formed by fluid mediated mass transfer from the bleach zones during prograde metamorphism minerals in the veins are free of inclusions but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets vein garnets have mn and hree zoning patterns that are indicative of progressive garnet growth during prograde metamorphism whereas mg and ca contents reveal prominent sector zoning and fine scale intergrowth features we propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock fluid circulation may have been driven by episodic microcracking sealing around garnet porphyroblasts which led to significant mass transfer and progressive vein growth mass balance calculations and phengite trace element compositions also require the additional of pelite derived components to the veins these components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux this model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases the delay of fluid migration after hydrous mineral breakdown may provide an important source of water for arc magmas as free fluid is transported to deeper levels in subduction zones

seismic evidence for the migration of fluids within the accretionary complex of western canada multichannel deep seismic reflection data from the subduction zone of western canada delineate the wedge of accereted sediments and the principal terranes crescent pacific rim and wrangellia that form the convergent margin the top of the igneous oceanic crust is defined by subhorizontal reflections extending at least 100 km landward of the deformation front upon incorporation into the accretionary wedge the clearly defined stratigraphy of the incoming oceanic sedimentary section is destroyed over a distance of about 10 km initially an unreflective zone which correlates well with maximum fluid expulsion is formed farther landward a predominantly landward dipping reflectivity exists a number of reflections are thrust faults which appear to merge at depth with the subhorizontal reflections but most have another origin these reflections may be related to the movement of fluids generated by the compaction of sediments or possibly by the dehydration of the subducting plate they are strongest i

microstructures and petro fabrics of lawsonite blueschist in the north qilian suture zone nw china implications for seismic anisotropy of subducting oceanic crust abstract we conducted a detailed study on the microstructures and petro fabrics of massive and foliated lawsonite blueschist lbs in north qilian suture zone nw china the lattice preferred orientation lpo of glaucophane and lawsonite in foliated lawsonite blueschist lbs is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid body rotation respectively the lpo of glaucophane is mainly characterized by the 001 axis aligning parallel to lineation and the 100 axis and 110 pole plunging perpendicular to foliation in contrast the lpo of lawsonite features the maximum 010 axis concentrated close to lineation and the 001 axis strongly clustered normal to foliation the preferred orientation of 010 axis of lawsonite parallel to lineation is supported by a two dimensional numerical modeling using the finite volume method fvm the mineral lpos are much stronger in foliated lbs than in massive lbs in addition a kinematic vorticity analysis suggests that both pure shear dominant wm 0 18 0 26 and simple shear dominant wm 0 86 0 93 deformation regimes are present in foliated lbs the 001 axis and 010 pole of glaucophane and the 100 and 010 axes of lawsonite tend to distribute in a foliation parallel girdle in the pure shear dominant samples but simple shear dominant samples display more lineation parallel concentrations of a 001 axis of glaucophane and a 010 axis of lawsonite because the whole rock seismic anisotropies in foliated lbs are significantly higher than those in massive lbs and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite the delay time of fast s wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle 45 60 is expected to range from 0 03 to 0 09 s lower bound for massive lbs and from 0 1 to 0 3 s upper bound for foliated epidote blueschist

chloritoid bearing rocks associated with blueschists and eclogites northern new caledonia chloritoid bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the tertiary high pressure metamorphic belt of northern new caledonia the typical assemblage of chloritoid bearing rocks in the epidote zone is quartzchlorite muscovite garnet chloritoid in the omphacite zone epidote is an additional member of the chloritoid bearing assemblage paragonite is rare plagioclase was not detected and rutile and ilmenite are the fe ti oxide phases chloritoid glaucophane is not a common assemblage chloritoid bearing rocks have relatively low ca k na al ratios and the chloritoids are relatively mg rich with mg mg fe up to about 0 4 a comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560 c in the omphacite zone and a minimum temperature limit near 450 c at 10 kbar an empirical garnet chlorite fe mg exchange thermometer does not yield consistent results for the higher grade rocks suggesting ts ranging from 390 to 535 c in the omphacite zone and 420 465 c in the epidote zone the distribution coefficient kd fe mg ctd fe mg chl for chloritoid and chlorite ranges from 3 9 to 6 4 values which are lower than those 10 from lower greenschist facies rocks but are near those of upper greenschist facies and albite epidote amphibolite facies

the electrical conductivity distribution beneath vancouver island a region of active plate subduction magnetotelluric mt and geomagnetic depth sounding gds data were recorded at 25 locations across vancouver island canada over the subducting juan de fuca plate the gds data respond to a number of local conductive features at frequencies above 10 hz while at longer periods the in phase and quadrature induction arrows are colinear and are aligned approximately normal to the continental shelf with a maximum amplitude response near the southwest coast interpretation of the mt data show the presence of a conducting zone at depths greater than 20 km beneath vancouver island and correlating with a strong seismic reflector extensive testing with two dimensional models shows the mt and gds data are consistent with a proposed geoelectrical structure containing a conducting layer extending from the pacific ocean dipping beneath vancouver island and in electrical contact with the mainland conductive region the layer has a horizontal conductance of 200 s that may decrease to 100 s beneath the northeast side of vancouver island related geophysical studies suggest the conductive zone is above the actual thrust surface of the juan de fuca plate the conducting layer is most likely the result of cracks and pores filled with saline fluids which are supplied bymore water subducted with the oceanic crust and by dehydration reactions the presence of fluids has significant implications for thrust earthquakes and for metamorphic reactions that occur in subduction zones the conductance of the upper mantle between 90 and 390 km is best modeled at approximately 1 500 s less

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

eclogites and blueschists of the pam peninsula ne new caledonia a reappraisal high p rocks of the pam peninsula ne new caledonia are divided into three zones 1 an uppermost ferroglaucophane lawsonite zone of cretaceous to eocene metasediments and metavolcanics of the diahot terrane that experienced peak conditions involving p 7 9 kbar and t 400 58 c 2 albite epidote omphacite zone diahot terrane rocks that experienced blueschist facies conditions of p 12 6 1 2 kbar and t 570 36 c 3 lowermost metabasic eclogites of uncertain age that form the pouebo terrane which experienced high p conditions of p 23 9 3 0 kbar and t 600 c eclogite occurs as metre to kilometre scale pods in coarse grained hydrous mineral rich glaucophanite formed during hydration and decompression of the pouebo terrane metamorphism and deformation were consequent to 44 51 ma eocene convergence when sedimentary and ophiolitic nappes were thrust over the eclogites in a sw direction white mica ages constrain metamorphism to have ended by 37 1 ma large steps in metamorphic grade are coincident with sw dipping and ne dipping faults that separate the three zones and were formed during two stages 1 comparatively slow uplift and hydration of the pouebo terrane before it was juxtaposed with the albite epidote omphacite zone at p 14 kbar 2 comparatively rapid uplift of both the pouebo terrane and the albite epidote omphacite zone to form a domal core of eclogite flanked by significantly lower grade rocks to the sw and ne

the emplacement of the peridotite nappe of new caledonia and its bearing on the tectonics of obduction the peridotite nappe of new caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction here we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern new caledonia the klippes are flat lying and involve s sw vergent reverse slip shear zones which are true compressional structures in origin further northeast the nappe is folded in association with the development of a steep schistosity in low grade metasediments this difference in structural style indicates that the peridotite nappe experienced compression at greater depths toward its root zone suggesting a push from the rear mechanism of emplacement this supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence we establish a crustal scale cross section at the end of the obduction event before neogene extension this involves a large fold nappe of high pressure rocks bounded from below by a major thrust furthermore we show that obduction in new caledonia occurred through dextral oblique convergence oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it this obliquity can solve the paradox of the peridotite nappe seemingly being emplaced at the same time the high pressure rocks were exhumed oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen

40ar 39ar dating of white micas from the epidote to the omphacite zones northern new caledonia tectonic implications an 40ar 39ar study of white micas from high pressure metamorphic rocks of northern new caledonia yielded cooling ages of 37 1 ma for both epidote and omphacite zone samples whole rock samples from the lawsonite zone yielded ages in the range 44 51 ma with complicated age spectra probably reflecting both detrital and newly grown micas the areal extent of the mica samples over 300 km2 suggests that the epidote and omphacite zone rocks cooled through the muscovite closure temperature about 350 c as a coherent cooling unit simple thermal modeling suggests that these rocks could have closed at similar times if the unroofing rate were greater than 2 10 mm a 1 lawsonite zone rocks occur structurally within about 0 5 km of garnet omphacite rocks suggesting the possibility of major postmetamorphic tectonic displacement

open ocean to trench turbidity current flow in the nankai trough flow collapse and reflection the ocean drilling program leg 131 which drilled in the nankai trough in the western pacific ocean has shown for the first time that turbidity currents are deflected and reflected against the trench slopes such that in the trench outer slope there is a predominant pattern of currents carrying terrigenous sediments apparently coming from the open ocean shikoku basin directly toward the trench landward slope the purpose of this paper is to summarize the evidence for flow rebound at odp leg 131 site 808 and to show that these results have important implications for the interpretation of paleocurrents in ancient linear turbidite systems

comparison of the high pressure schist belts of new caledonia and sanbagawa japan the high pressure schist terranes of new caledonia and sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism the parent rocks were chemically similar as volcanic sedimentary trough or trench sequences and metamorphic temperatures in both belts were 250 to 600 c from phase equilibria curves total pressures were higher for new caledonia 6 15 kb than for sanbagawa 5 11 kb and the estimated thermal gradients were 7 10 c km and 15 c km respectively pt paths identify the higher pressure in new caledonia p differences 2 kb at 300 c and 4 kb at 550 c with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas lawsonite epidote omphacite new caledonia and chlorite garnet biotite sanbagawa in new caledonia the na amphibole is dominantly glaucophane and na pyroxenes associated with quartz are jadeite jd95 100 and omphacite in sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite jd50 for both areas garnet rims show increase in pyrope content with advancing grade but sanbagawa garnets are richer in almandine progressive assemblages within the two belts can be equated by such reactions as new caledonia sanbagawa glaucophane paragonite h2o albite chlorite quartz glaucophane epidote h2o albite chlorite actinolite and the lower pressure japanese associations appear as retrogressive phases in the new caledonia epidote and omphacite zones

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

prograde sulfide metamorphism in blueschist and eclogite new caledonia in new caledonia blueschist and eclogite preserve as inclusions in porphyroblastic minerals a record of sulfide present during prograde subduction processes sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids or grew later during retrogression cu fe sulfide mineral inclusions have been found across metamorphic grade within silicate defined metamorphic mineral zones spanning a crustal profile of 30 km bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew in general single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases high cu contents in sulfide inclusions are interpreted to be a consequence of fe lost from sulfide to growing garnet rather than the result of intrinsically high cu in the bulk rock the distribution of sulfide inclusion compositions across metamorphic grade considered together with the available thermodynamic data suggests that covellite nukundamite bearing inclusions in lawsonite high in both cu and s disappear at higher grades as these sulfide minerals are no longer stable similarly clustering near the ratio fe cu 1 1 may cease with increasing grade owing to the replacement of chalcopyrite intermediate solid solution iss by the denser assemblage pyrite pyrrhotite bornite digenite

mg fe carpholites from aluminous schists in the diahot region and implications for preservation of high pressure low temperature schists northern new caledonia abstract mg fe carpholite is widespread in the diahot region of new caledonia in highly aluminous schists and as veins in what was originally a clay rich hydrothermal alteration envelope about massive suphide deposits these carpholites have fe fe mg ratios of 0 03 0 65 and no significant mn component mg carpholite quartz occur in assemblages with chlorite or pyrophyllite pyrophyllite kaolinite and pyrophyllite diaspore temperatures of 230 320 c and minimum pressures of 7 kbar are indicated for the mg fe carpholite bearing rocks the regional distribution of aragonite and mg fe carpholite parallel to a major zone of dominantly transcurrent movement and oblique to the trend of the subduction complex indicates the high p low t schists owe their rapid uplift and preservation to the vertical component of the transcurrent faulting

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

coexisting amphiboles in an eclogite from the western alps new constraints on the miscibility gap between sodic and calcic amphiboles crystal chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the aosta valley western alps textural analysis gives evidence of three successive high pressure parageneses 1 pre kinematic high grade blueschist assemblages preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite stage a 2 synkinematic eclogite assemblages composed of garnet omphacite glaucophane actinolite white mica clinozoisite quartz rutile stage b 3 post kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite stage c p t conditions of the eclogitic metamorphism have been estimated at around 500 550 c 16 kbar glaucophane and actinolite coexist as discrete grains in stage a and b assemblages this texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles from nam4 0 80 in actinolite to nam4 1 70 in glaucophane at t 500 550 c a comparison with published analyses allows a new solvus along the glaucophane actinolite join to be drawn the later barroisitic amphibole stage c exhibits strong chemical zonation indicating disequilibrium growth this amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end members

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain