a self regulating model of bedrock river channel geometry 1 the evolution of many mountain landscapes is controlled by the incision of bedrock river channels while the rate of incision is set by channel shape through its mediation of flow the channel shape is itself set by the history of bedrock erosion this feedback between channel geometry and incision determines the speed of landscape response to tectonic or climatic forcing here a model for the dynamics of bedrock channel shape is derived from geometric arguments a normal flow approximation for channel flow and a threshold bed shear stress assumption for bedrock abrasion the model dynamics describe the competing effects of channel widening tilting bending and variable flow depth transient solutions suggest that channels may take 1 10 ky to adapt to changes in discharge implying that channel disequilibrium is commonplace if so landscape evolution models will need to include bedrock channel dynamics if they are to probe the effects of climate change

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

calculation of bed variation in alluvial channels in an effort to develop a simple numerical model to predict the variation of bed topography in real worked rivers a depth averaged numerical model was applied to evaluate the variation of the bed topography in alluvial streams the model was used to simulate the symmetric and asymmetric meander loops the simulated results showed good agreement with experimental results applications of the model to the development of meso scale bed configurations in straight channels were also performed the migration velocity of bars in meandering channels was investigated in alluvial rivers the interactions between hydraulic conditions and plan geometries result in different kinds of bed confugurations that are highly dependent on formation cessation and migration of bars the study shows that the 2 dimensional model can predict these phenomena for given hydraulic and geometric conditions which may be useful in practical engineering works

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

resetting monazite ages during fluid related alteration abstract fluid related coupled dissolution reprecipitation processes can modify the composition of monazite and other orthophosphate minerals at temperatures well below diffusional closure temperatures it is critical to evaluate the effects of alteration processes on th u pb dates especially because incomplete removal of pb during alteration could lead to spurious dates either older or younger than the actual age of the monazite formation experimental alteration of monazite at 450 mpa and 450 c has resulted in the production of altered domains in the vicinity of the grain rim as well as in the vicinity of inclusions and microfractures without any obvious structural control within grains the composition of the altered domains is relatively constant from domain to domain and from grain to grain thorium was reduced from ca 7 wt tho 2 to ca 2 wt uranium and pb were almost entirely removed from the altered domains while unaltered areas of the monazite grains as well as the original monazite grains before the experiment yield very consistent dates of ca 350 ma dates from the altered domains are consistently near zero the implication is that fluid aided alteration of monazite for the conditions of this experiment has resulted in a near complete resetting of the monazite age at temperatures well below the closure temperature because alteration textures in monazite are relatively common in nature careful analysis of such textures offers a means of recognizing and potentially dating fluid rock interactions

monazite chemical composition some implications for monazite geochronology an investigation of the chemical composition of monazite from a number of localities has been carried out samples used include monazites from metamorphic rocks granitic rocks and a hydrothermal ore deposit the ree distribution pattern of monazite varies greatly in accordance with its geological environment a remarkable feature of the monazites studied is that their chondrite normalised ree distribution patterns are mostly uniform between grains within the same sample but differ significantly from sample to sample this characteristic apparently indicates that there is an important effect on the ree distribution of monazite exerted by the host rock or source material from which monazite crystallised another important feature shown by the monazites studied is that monazites in rocks containing garnet as a major mineral show extreme depletion of hree whereas monazites in rocks without garnet or monazite that formed after the garnet breakdown contain significantly higher amounts of y and hree this suggests that the phase assemblage especially garnet plays an important role in the ree distribution of monazites in these rocks the value of ree distribution in monazite is exemplified with regard to the origin of monazite in the lewisian metamorphic rocks which is a fundamental issue in monazite geochronology

precipitation of rutile and ilmenite needles in garnet implications for extreme metamorphic conditions in the acadian orogen u s a we report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the merrimack synclinorium connecticut and present a precipitation model for their origin the rocks were strongly metamorphosed and deformed during the devonian acadian orogeny the needles are primarily elongated parallel to in garnet rutile has anomalous extinction angles as great as 35 cf griffin et al 1971 rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long other oxide inclusions that may be present include submicrometer to micrometer scale twinned rutile bicrystals as well as srilankite and a crichtonite group mineral some garnet cores have unusual box shaped quartz inclusions which coexist with ti fe oxide needles and commonly contain micrometer scale rods of f oh cl apatite negative garnet crystal pores are also widespread ti fe oxide needles are restricted to garnet core regions rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite spinel cordierite and k feldspar consequently the garnet core regions represent an earlier distinct period of growth relative to the rims garnet cores contain 25 35 pyrope and a host of minor and trace constituents including tio2 0 07 0 6 wt cr2o3 0 01 0 10 wt na2o 0 01 0 03 wt p2o5 0 01 0 09 wt and zro2 up to 150 ppm na2o and zro2 correlate positively with tio2 titanium zoning is preserved in some garnets zoning profiles and two dimensional chemical mapping show that ti and to a lesser degree cr are depleted around ti fe oxide inclusions therefore we conclude that the needles are precipitates that formed from ti bearing garnet during exhumation and cooling garnet contained sufficient ti to form precipitates no ti source external to garnet was necessary titanium bearing garnets that contain oriented ti fe oxide needles are known primarily from ultrahigh pressure metamorphic rocks mantle peridotites and pyroxenites and high pressure granulites thus the presence of needle bearing garnets in connecticut strongly suggests that a previously unrecognized domain of extreme pressure and or temperature metamorphism exists in the acadian orogen

preservation of old prograde metamorphic u th pb ages in unshielded monazite from the high pressure paragneisses of the variscan ulten zone italy abstract three samples of garnet kyanite paragneiss from the variscan ulten zone northern italy were studied in detail for u th pb monazite dating monazite in these gneisses is abundant shows highly variable grain size and occupies different textural positions within the matrix as inclusion in garnet and kyanite within apatite aggregates monazite shows different deformation features as a function of the textural position enclosed shielded monazite is generally more fractured than matrix unshielded monazite the integration of textural information with deformation features and in situ u th pb analyses by la icp ms indicates that there is no direct correlation between textural site and monazite ages old ages of 351 343 ma determined on portions of large matrix unshielded monazite and on rare domains of monazite shielded by garnet have been related to a prograde stage of the variscan metamorphic evolution of the ulten zone ages of 330 326 ma which are related to the thermal peak are recorded by small matrix monazite external domains of large matrix monazite and by domains of fractured monazite enclosed in garnet and kyanite large old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation re crystallisation partially occurred

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

yo yo subduction recorded by accessory minerals in the italian western alps high pressure metamorphic rocks form during subduction of earth s crust to mantle depths at convergent plate margins their exposure at the surface of earth today provides a record of the subduction zone process in general such metamorphic rocks record only a single cycle of subduction and exhumation yet tectonic models suggest that individual rock units should undergo multiple subduction exhumation cycles here we investigate the microstructure and chemical composition of metamorphic minerals in high pressure rocks exposed in the sesia zone in the italian western alps we find that the minerals white mica garnet allanite and zircon each exhibit multiple generations of mineral overgrowths in particular two generations of white mica with high silicon content indicative of formation at high pressure are separated by an overgrowth with low silicon content that formed during exhumation at low pressures furthermore the trace element signatures of distinct zones within zircon and allanite also reveal two episodes of high pressure metamorphism separated by a period of rapid exhumation we use uranium lead dating of zircon and allanite overgrowths to constrain the timing of this subduction exhumation subduction cycle to 79 65 myr ago we conclude that slices of the sesia zone crust experienced two cycles of burial to mantle depths in less than 20 million years the yo yo subduction occurred during oblique convergence between the african and european plates which involved a continental margin

ordovician early silurian intrusive rocks in the northwest part of the upper allochthon mid norway plutons of an iapetan volcanic arc complex

application of k feldspar jadeite quartz barometry to eclogite facies metagranites and metapelites in the sesia lanzo zone western alps italy the eclogite facies assemblage k feldspar jadeite quartz in metagranites and metapelites from the sesia lanzo zone western alps italy records the equilibration pressure by dilution of the reaction jadeite quartz albite the metapelites show partial transformation from a pre alpine assemblage of garnet alm 63 prp 26 grs 10 k feldspar plagioclase biotite sillimanite to the eo alpine high pressure assemblage garnet alm 50 prp 14 grs 35 jadeite jd 80 97 di 0 4 hd 0 8 acm 0 7 zoisite phengite plagioclase is replaced by jadeite zoisite kyanite k feldspar quartz and biotite is replaced by garnet phengite or omphacite kyanite phengite equilibrium was attained only in local domains in the metapelites and therefore the k feldspar jadeite quartz kjq barometer was applied only to the plagioclase pseudo morphs and k feldspar domains the albite content of k feldspar ranges from 4 to 11 mol in less equilibrated assemblages from val savenca and from 4 to 7 mol in the partially equilibrated samples from monte mucrone and the equilibrated samples from montestrutto and tavagnasco thermodynamic calculations on the stability of the assemblage k feldspar jadeite quartz using available mixing data for k feldspar and pyroxene indicate pressures of 15 21 kbar 1 6 1 9 kbar at 550 50 c this barometer yields direct pressure estimates in high pressure rocks where pressures are seldom otherwise fixed although it is sensitive to analytical precision and the choice of thermodynamic mixing model for k feldspar moreover the kjq barometer is independent of the ratio p h 2 o p t the inferred limiting a h 2 o for the assemblage jadeite kyanite in the metapelites from val savenca is low and varies from 0 2 to 0 6

the growth of fault bounded tilt blocks a series of uniformly tilted fault bounded blocks is a common feature in actively extending regions such as the basin and range province if the tilted blocks were produced by rigid domino style rotation one would predict large voids at either end of a series of these blocks using tilt data and a simple flexural calculation we suggest that much of the apparent rigid behavior could also be produced by internal block deformation in our model of normal fault growth isostatic elastic uplift of the footwall is coupled with hanging wall downdrop within the region between faults resulting in the appearance of a tilted rigid block we present tilt data sampled at varying distances from several block defining faults within the northeast basin and range province tilt measurements between a series of 30 km spaced block defining faults are found to be uniform while tilts between more widely spaced faults exhibit a pattern of tilt that diminishes to zero in less than 30 km using a simple flexural calculation for internal block deformation we show that for this region the patterns of tilt are consistent with a flexural length scale of 8 12 km and deflections of 2 4 km these estimates are compatible with both the lower limit to seismicity and basin depth determined from earthquake and seismic reflection studies

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

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

microdiamond on areskutan confirms regional uhp metamorphism in the seve nappe complex of the scandinavian caledonides metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust here we present a new occurrence of diamond within the seve nappe complex snc of the scandinavian caledonides on areskutan in jamtland county sweden microdiamond is found in situ as single and composite diamond carbonate inclusions within garnet in kyanite bearing paragneisses the rocks preserve the primary peak pressure assemblage of ca mg rich garnet phengite kyanite rutile with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite calculated p t conditions for this stage are 830 840 c and 4 1 4 2 gpa in the diamond stability field the ultrahigh pressure uhp assemblage has been variably overprinted under granulite facies conditions of 850 860 c and 1 0 1 1 gpa leading to formation of ca mg poor garnet biotite plagioclase k feldspar sillimanite ilmenite quartz this overprint was the result of nearly isothermal decompression which is corroborated by ti in quartz thermometry chemical th u pb dating of monazite yields ages between 445 and 435 ma which are interpreted to record post uhp exhumation of the diamond bearing rocks the new discovery of microdiamond on areskutan together with other evidence of ultrahigh pressure metamorphism uhpm within gneisses eclogites and peridotites elsewhere in the snc provide compelling arguments for regional at least 200 km along strike of the unit uhpm of substantial parts of this far travelled allochthon the occurrence of uhpm in both rheologically weak gneisses and strong lithologies eclogites peridotites speaks against the presence of large tectonic overpressure during metamorphism

intergranular coesite and coesite inclusions in dolomite from the dabie shan constraints on the preservation of coesite in uhp rocks intergranular coesite is extremely rare in and bears crucial information on the formation and preservation of ultrahigh pressure uhp rocks here we report the first occurrence of intergranular coesite in a metasedimentary rock which occurs in the ganjialing area in the dabie shan east central china and contains abundant coesite inclusions in both garnet and dolomite we investigated the content of structural water in these minerals with fourier transform infrared spectroscopy our new results undermine the ubiquity of the pressure vessel model and highlight the role of reaction kinetics in preserving coesite due to the availability of water in uhp rocks this article is protected by copyright all rights reserved

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

tracing two orogenic cycles in one eclogite sample by lu hf garnet chronometry the adula nappe central alps was thought to be a melange of rock fragments some of which were subducted and then exhumed from the mantle lu hf dating of two populations of garnets in one rock sample instead indicates that the nappe twice experienced subduction and exhumation as one coherent unit

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

multiple metamorphic stages within an eclogite facies terrane sesia zone western alps revealed by th u pb petrochronology convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel studies of high pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels but the details of such dynamics are controversial to understand the dynamics of a subduction channel we report the results of a petrochronological study from the central sesia zone a key element of the internalwestern alps this comprises two polymetamorphic basement complexes eclogitic micaschist complex and gneiss minuti complex and a thin dismembered cover sequence scalaro unit associated with pre alpine metagabbros and metasediments bonze unit structurally controlled samples from three of these units eclogitic micaschist complex and scalaro bonze units yield unequivocal petrological and geochronological evidence of two distinct high pressure stages ages u th pb of growth zones in accessory allanite and zircon combined with inclusion and textural relationships can be tied to the multi stage evolution of single samples two independent tectono metamorphic slices showing a coherent metamorphic evolution during a given time interval have been recognized the fondo slice which includes scalaro and bonze rocks and the druer slice belonging to the eclogitic micaschist complex the new data indicate separate stages of deformation at eclogite facies conditions for each recognized independent kilometer sized tectono metamorphic slice between 85 and 60 ma with evidence of intermittent decompression p 0 5 gpa within only the fondo slice the evolution path of the druer slice indicates a different p t time evolution with prolonged eclogite facies metamorphism between 85 and 75ma our approach combining structural petrological and geochronological techniques yields field based constraints on the duration and rates of dynamics within a subduction channel

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

calibration of the garnet biotite al2 sio5 quartz geobarometer for metapelites a garnet biotite al2sio5 quartz gbaq geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under p t conditions of 450 950 c and 1 17 kbar in the calibration activity models of garnet and biotite identical to those in the garnet biotite geothermometer of holdaway 2000 were used therefore the gbaq geobarometer and the garnet biotite geothermometer can be simultaneously applied to iteratively estimate metamorphic p t conditions successful application of the gbaq geobarometer to natural metapelites certify its validity most importantly when plagioclase is absent or cao in garnet and or plagioclase are deficient this geobarometer may prove useful for estimating metamorphic pressures the random error of the present gbaq geobarometer is inferred to be around 1 8 kbar an electronic spreadsheet is available as table s4 to apply the gbaq geobarometer in combination with the garnet biotite geothermometer this article is protected by copyright all rights reserved

modeling fluvial incision and transient landscape evolution influence of dynamic channel adjustment 1 channel geometry exerts a fundamental control on fluvial processes recent work has shown that bedrock channel width depends on a number of parameters including channel slope and is not solely a function of drainage area as is commonly assumed the present work represents the first attempt to investigate the consequences of dynamic gradient sensitive channel adjustment for drainage basin evolution we use the channel hillslope integrated landscape development child model to analyze the response of a catchment to a given tectonic perturbation using as a template the topography of a well documented catchment in the footwall of an active normal fault in the apennines italy that is known to be undergoing a transient response to tectonic forcing we show that the observed transient response can be reproduced to first order with a simple detachment limited fluvial incision law transient landscape is characterized by gentler gradients and a shorter response time when dynamic channel adjustment is allowed the differences in predicted channel geometry between the static case width dependent solely on upstream area and dynamic case width dependent on both drainage area and channel slope lead to contrasting landscape morphologies when integrated at the scale of a whole catchment particularly in presence of strong tilting and or pronounced slip rate acceleration our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution they stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution particularly when the relative uplift field is nonuniform

new constraints on sediment flux dependent river incision implications for extracting tectonic signals from river profiles we present new field data from rivers draining across active normal faults that incise across the same lithology at the fault have been subjected to similar climatic regimes and tectonic settings and were perturbed by a well documented increase in fault slip rate ca 1 ma in spite of these similarities the rivers exhibit markedly different long profiles and patterns of catchment incision we use channel slope and hydraulic geometry data for each river to calculate bed shear stresses b and show that there is no simple relationship between peak b and the relative uplift rates across the faults u which differ by a factor of four the long term average sediment supply to each channel q s estimated from time averaged catchment erosion rates can explain the b versus u data if bedload modulates bedrock incision rate e in a strongly nonlinear way together these field data allow us for the first time to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems

the role of extensional structures on experimental calderas and resurgence the structure and shape of collapses and resurgences is often controlled by pre existing discontinuities such as normal faults in rift zones in order to study the role of extensional structures on collapse and resurgence we used analogue models dry sand simulated the brittle crust silicone located at the base of the sand pack simulated magma in the experiments regional extension pre dated collapse or resurgence forming normal faults in a graben like structure the graben was filled with additional sand simulating post rift deposits a piston then moved the silicone downward or upward inducing collapse or resurgence within the previously deformed sand the collapses showed an ellipticity length of minor axis length of major axis between 0 8 and 0 9 with the major axis parallel to the extension direction the partial reactivation of the pre existing normal faults was observed during the development of the caldera reverse faults which conversely to what was expected from experiments without pre existing extension became partly inward dipping resurgence showed an elongation of the uplifted part with the main axis perpendicular to the extension direction at depth pre existing normal faults were partly reactivated by the reverse faults formed during resurgence these locally became outward dipping normal faults a total reactivation of pre existing faults was also observed during resurgence the experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre existing structures during differential uplift such a reactivation is mainly related to the loss in the coefficient of friction of the sand the results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers

an anticlockwise p t t path at high pressure high temperature conditions for a migmatitic gneiss from the island of fjortoft western gneiss region norway indicates two burial events during the caledonian orogeny

complexity in the behavior and recrystallization of monazite during high t metamorphism and fluid infiltration abstract a detailed study of monazite grains in silica undersaturated and quartz bearing mg al metapelite from the oygarden group of islands east antarctica reveals a complex history of growth and recrystallization during two separate events in the neoproterozoic and earliest cambrian monazite grains from garnet poor and garnet rich metapelite preserve core domains that have ages corresponding to growth and or recrystallization at granulite facies p 9 1 0 gpa t 850 900 c conditions during the rayner structural episode between 930 and 890 ma high th rims 22 wt tho 2 that are in textural equilibrium with sapphirine orthopyroxene symplectites formed after garnet during late rayner decompression occur on monazite grains in garnet rich assemblages and give electron microprobe ages 883 18 ma within error of core domains 903 14 ma these high th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth in garnet poor metapelite monazite grains that have grain boundaries in textural equilibrium with the granulite facies assemblage also show alteration to higher th compositions on rims and along vein like fractures this compositional shift is accompanied by partial to complete resetting of ages to 500 ma along with minor modification of core domain ages textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution reprecipitation reaction process but in contrast to garnet bearing assemblages this alteration occurred during a fluid influx at lower temperatures during regional pan african tectonism this study highlights the susceptibility of monazite to resetting with fluid present conditions dramatically increasing the potential for recrystallization

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

evolution of a gneiss in the seve nappe complex of central sweden hints at an early caledonian medium pressure metamorphism abstract ultrahigh pressure uhp metamorphism has recently been reported from various crustal rocks in the seve nappe complex snc in which microdiamonds were found we studied a gneiss from the areskutan nappe in central sweden this nappe of the snc was recently interpreted to have experienced uhp metamorphism the studied medium grained and well foliated gneiss contains quartz biotite garnet k feldspar plagioclase and potassic white mica with accessory kyanite sillimanite rutile ilmenite and monazite pressure p temperature t pseudosections were mainly used to decipher the metamorphic evolution of the gneiss according to polyphase inclusions such as biotite chlorite potassic white mica in the garnet core the prograde p t conditions were approximately 600 c and 8 8 5 kbar an isobaric heating to 700 c followed constrained by homogenous garnet with xca 0 03 and xmg 0 295 the highest si content of 3 15 per formula unit in potassic white mica and zr in rutile thermometry a late migmatitic stage occurred at about 725 c and 6 5 kbar u th pb dating of monazite included in garnet with the electron microprobe yielded two populations at 463 12 2 ma y2o3 2 0 wt and 458 7 2 ma y2o3

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

microdiamond discovered in the seve nappe scandinavian caledonides and its exhumation by the vacuum cleaner mechanism when a continent collides with an island arc or other continent continental crust of the subducted continent may be buried to depths exceeding 100 km and exposed to pressures that can cause formation of coesite and diamond this process leads to substantial density increase in sio2 rich rocks and in turn to a reduction of the buoyancy of the subducted material which should inhibit exhumation nevertheless coesite and diamond bearing continental crustal rocks are known from several occurrences worldwide we report on the discovery of microdiamond in kyanite garnet gneiss from allochthonous metasediments of the seve nappe complex in the scandinavian caledonides our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust we propose that the diamond bearing rocks were subducted in an arc continent collision setting and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block

rates of thermal equilibration at the onset of subduction deduced from diffusion modeling of eclogitic garnets yukon tanana terrane canada well preserved eclogitic rocks near faro yukon tanana terrane canada underwent a prograde evolution from 510 c and 1 1 gpa to 690 c and 1 5 gpa followed by nearisobaric cooling to 540 c a remarkable feature of the garnet porphyroblast cores is the presence of minute garnet inclusions of distinctly different composition preservation of a sharp compositional gradient at the interface between the host and the inclusion garnets and results of diffusion modeling indicate that the counterclockwise pressure time evolution took place on a very short time scale of about 0 2 m y minimum rates of heating 950 c m y and burial 7 cm yr calculated along the prograde part of the path are in good agreement with values extracted from thermal models of newly initiated subduction zones

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

monazite in a variscan mylonitic paragneiss from the munchberg metamorphic complex ne bavaria records cadomian protolith ages small oval shaped unshielded monazite grains found in a variscan garnet muscovite bearing mylonitic paragneiss from the liegendserie unit of the munchberg metamorphic complex mmc in the northwestern bohemian massif central europe yield only pre variscan ages these ages determined with the electron microprobe have maxima at about 545 520 and 495 ma and two side maxima at 455 and 575 ma and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the nw bohemian massif the pressure p temperature t history of this mylonitic paragneiss determined from contoured p t pseudosections involved an initial stage at 6 kbar 600 c reaching peak p t conditions of 12 5 kbar 670 c with partial melting followed by mylonitization and retrogression to 9 kbar 610 c the monazite representing detrital grains derived from igneous rocks of a cadomian provenance between 575 455 ma has survived these variscan metamorphic deformational events unchanged because this mineral has probably never been outside its p t stability field during metamorphism this article is protected by copyright all rights reserved

evolution of metamorphic volatiles during exhumation of microdiamond bearing granulites in the western gneiss region norway fluid inclusions in garnet kyanite and quartz from microdiamond bearing granulites in the western gneiss region norway document a conspicuous fluid evolution as the rocks were exhumed following caledonian high and ultrahigh pressure hp uhp metamorphism the most important of the various fluid mixtures and daughter minerals in these rocks are n2 co2 magnesian calcite n2 co2 ch4 graphite magnesian calcite n2 ch4 n2 ch4 h2o co2 and h2o nacl cacl2 nahcolite rutile also occurs in the n2 co2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions volatiles composed of n2 co2 magnesian calcite characterise the ambient metamorphic environment between hp uhp peak and early retrograde metamorphism during progressive decompression the mole fraction of n2 increased in the fluid mixtures as amphibolite facies conditions were reached ch4 and later h2o appeared in the fluids concomitant with the disappearance of co2 and magnesian calcite graphite is ubiquitous in the host lithologies and fluid inclusions thermodynamic modelling of the metamorphic volatiles in a graphite buffered c o h system demonstrates that the observed metamorphic volatile evolution was attainable only if the fo2 increased from c 3 5 0 3 to 0 8 0 3 log units relative to the fmq oxygen buffer external introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the fo2 the oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the caledonian orogeny

spatial and temporal variations in growth rates along active normal fault systems an example from the lazio abruzzo apennines central italy abstract the geometry kinematics and rates of active extension in lazio abruzzo italian apennines have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems we present fault map traces throws throw rates and slip directions for 17 parallel en echelon or end on active normal faults whose 20 40 km lengths combine to form a soft linked fault array ca 155 km in length and ca 55 km across strike throw rates derived from observations of faulted late glacial features and holocene soils show that both maximum throw rates and throw rate gradients are greater on centrally located faults along the strike of the array total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike when summed across strike throw rates are increasingly high towards the centre of the array relative to summed throws we interpret the above to suggest that throw rates have changed in the recent past ca 0 7 ma from spatially random fault growth rates initiating at 2 5 3 3 ma to growth rates that are greater on centrally located faults we interpret this as evidence for fault interaction producing throw rate variations that drive throw profile readjustment on these crustal scale soft linked faults the results are used to discuss seismic hazards in the region which are quantified in a second paper in this issue