ichnofabric characterization of a deep marine clastic system a subsurface study of the middle eocene ainsa system spanish pyrenees this paper documents a subsurface trace fossil and ichnofabric study of the proximal parts of a structurally confined and channelized sand rich lower slope and proximal basin floor deep marine system in the middle eocene ainsa basin spanish pyrenees five depositional environments are recognized based on sedimentary facies associations depositional architecture and stratigraphic context channel axis channel off axis channel margin levee overbank and interfan as well as a channel abandonment phase each environment is characterized by distinct and recurring ichnofabrics ichnological measurements and observations were recorded from six cores recovered from six wells drilled at a spacing of between 400 m and 500 m at outcrop and totalling 1213 m in length from channel axis to levee overbank environments there is a trend of increasing bioturbation intensity and ichnodiversity ichnofabrics in channel axis and channel off axis environments are characterized by low bioturbation intensity and low ichnodiversity thalassinoides dominated firmground ichnofabrics associated with erosive sediment gravity flows are common in these environments in contrast channel margin and levee overbank environments are characterized by ichnofabrics associated with high bioturbation intensity and ichnodiversity sediments of the interfan are characterized by the highest bioturbation intensity associated with burrow mottling and an absence of primary sedimentary structures this paper demonstrates that in core based studies ichnofabric analysis is an important and valuable tool in discriminating between different environments in channelized deep marine siliciclastic systems the results of this study should find wide applicability in reservoir characterization studies in the petroleum industry in field based analogue ichnofabric studies and other core based studies in deep water siliciclastic systems worldwide such as the integrated ocean drilling program

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

geochronologic studies in central new england ii post acadian hinged and differential uplift sup 40 ar sup 39 ar analyses of muscovite biotite and k feldspar from central new england reveal a remarkable pattern of mineral ages the ages are progressively younger from central to western new hampshire and rise sharply near the vermont border to ages more typical of post acadian cooling this distribution is attributed to differential uplift via isostatic rebound of an anomalously thickened crust at the bronson hill anticlinorium this explanation requires that between 6 and 8 km of normal fault motion has occurred on structures in western new hampshire not previously recognized to have accommodated this kind or magnitude of displacement this hinged differential uplift occurred from approximately 360 170 ma and is consistent with a time constant for rebound of approximately 80 m y

milankovitch forcing of bioturbation intensity in deep marine thin bedded siliciclastic turbidites abstract trace fossils have been used to infer a milankovitch type control on pelagic and hemipelagic sediments but there have been no comparable studies in deep marine siliciclastic turbidite successions here we present the results of a quantitative analysis of trace fossil abundance and intensity in an essentially continuous middle eocene 230 m long well a6 core comprising very thin and thin bedded siliciclastic turbidites in the deep marine ainsa basin spanish pyrenees after removing the sediment slides and debris flow deposits as these represent geologically instantaneous events approximately two orders of magnitude thicker than the typical laminated turbiditic sediments in the core spectral analysis was conducted on the bioturbation intensity data spectral analysis of bioturbation intensity in the a6 core suggests for the first time from a siliciclastic turbidite succession at a tectonically active plate margin thrust top basin in a foreland basin a cyclicity that is interpreted to reflect the 41 kyr and an 112 kyr possibly an average of the 95 and 125 kyr milankovitch frequencies we propose that this drove environmental changes in bottom water conditions in the ainsa basin most likely leading to critically varying levels of oxygenation stratification of bottom waters our age model for the ainsa basin 4 km of deep marine sediments in 10 myr yields an average sediment accumulation rate of 40 cm kyr 1 that is consistent with that inferred from the spectral analysis 30 cm kyr 1 for fine grained sedimentation an implication of these results is that global environmental change probably glacio eustasy acted as a driver on deep marine siliciclastic sedimentation patterns during the middle eocene

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

new radiometric ages on gneisses of the oliverian domes in new hampshire and massachusetts gneissic plutons of the oliverian domes mantled by ammonoosuc volcanics are located along the axis of the bronson hill anticlinorium from new hampshire to connecticut the contacts between the plutonic and volcanic rocks appear to be concordant on a regional scale but gneiss intrudes the volcanics in several domes available radiometric and fossil evidence suggests that the ammonoosuc volcanics have a middle ordovician age but are somewhat older than the oliverian gneisses new u pb zircon data from oliverian gneisses of six domes plot on a concordia diagram as an almost colinear array that yields an upper intercept age of about 444 m y the plotted data vary from nearly concordant to moderately discordant the degree of discordance correlating with sup 207 pb sup 206 pb ages that range from 459 to 415 m y the pattern of discordance does not relate to the uranium contents of the zircons nor to the geographic distribution of the domes if sup 207 pb sup 206 pb ages are considered individually without an assumed consanguinity of the units however they do not find particular support in geologic relationships thus they prefer the concordia intercept age of 444 8 m y for the suite as the best estimate for the time of crystallizationmore of the oliverian gneisses possibly the whitefield gneiss in the jefferson dome represents a 10 to 15 m y older unit although they are cautious about claiming such resolution with the present data less

trondhjemite and metamorphosed quartz keratophyre tuff of the ammonoosuc volcanics ordovician western new hampshire and adjacent vermont and massachusetts the ammonoosuc volcanics and equivalent rocks of ordovician age are exposed in the oliverian domes along the bronson hill anti clinorium bha between northern new hampshire and southern connecticut in western new hampshire and adjacent vermont and massachusetts the ammonoosuc lithology consists of a lower mainly mafic unit of homblende plagioclase amphibolite and an upper mainly felsic metamorphosed quartz keratophyre tuff these lithologies are locally interlayered and both are intruded by sills dikes and plugs of trondhjemite trondhjemite also constitutes the interior gneissic core of several small domes or plutons the trondhjemite is highly siliceous sio 2 73 81 low in a1 2 o 3 11 3 13 5 generally contains 2 o and thus resembles some trondhjemites in island arc or continental margin settings chemical trends of both trondhjemite and ammonoosuc volcanics felsic and mafic are essentially calc alkaline variations in both major and trace elements of trondhjemites in several of the domes suggest several somewhat different sources along the bha overall however the major and minor element chemistry of the trondhjemites is closely similar to that of the ammonoosuc quartz keratophyre tuff these rocks could have been produced either by partial melting or by fractional crystallization of basaltic source rocks the partial melting model is preferred because of the largely bimodal basalt quartz keratophyre ammonoosuc assemblage in which andesitic and other intermediate compositions are virtually lacking the relatively thin ammonoosuc section appears to preclude generation of trondhjemite at the presently exposed base of an island arc as has been postulated for very similar trondhjemite amphibolite assemblages twillingate trondhjemite little port complex in newfoundland instead generation of the felsic ammonoosuc rocks more likely occurred at deeper levels along a subduction zone dipping eastward under the bha as postulated in current plate tectonic models the close juxtaposition in space and time of sialic crust and ammonoosuc volcanics may explain the calc alkaline trends of the latter and suggests a paleotectonic environment of convergent oceanic continental plate margins possibly with significant crustal shortening across the arc

the ainsa fold and thrust oblique zone of the central pyrenees kinematics of a curved contractional system from paleomagnetic and structural data this work was funded by the project intectosal cgl2010 21968 c02 01 and by the grup de recerca de geodinamica i analisi de conques 2009sgr1198 secretaria d universitat i recerca del departament d economia i coneixement de la generalitat de catalunya josep poblet acknowledges financial support by research projects geociencias en iberia estudios integrados de topografia y evolucion 4d topo iberia csd2006 041 and desarrollo de fracturas y venas asociadas al plegamiento fraveple cgl2011 23628 funded by spanish ministries

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

uncertainties in differential thermodynamic gibbs method p t paths a combined analytical and numerical evaluation of the uncertainties in p t paths is made for three assemblages that propagates errors in the parameters initial pressure initial temperature initial composition change of composition of monitor parameters endmember entropy and endmember volume propagated errors along an isobaric heating path t 77 c for assemblage 1 grt bt pl qtz ms chl h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite and ignoring uncertainties in thermodynamic properties are approximately 320 bars 1 and 8 3 c 1 if rim compositional uncertainties of 5 in major cations are assumed or 2 5 c 1 and 50 bars 1 if electron microprobe analytical uncertainties are assumed for compositions the largest source of uncertainty is from the errors in the monitor parameters and p t path uncertainties can depend critically on which monitor parameters are used if the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase then propagated uncertainties are worse than 29 c 1 and 5800 bars 1 p t path uncertainties also depend on assemblage 1 precisions in assemblages 2 grt bt pl qtz ms sil h2o and 3 grt bt qtz kfs sil h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite are calculated to be 267 bars and 43 4 c and 372 bars and 8 2 c respectively estimates of the accuracies in p t paths that include potential errors in endmember entropy of 1 j mol k and in endmember volume of 1 cm3 mol are 324 bars and 8 5 c assemblage 1 341 bars and 48 6 c assemblage 2 and 388 bars and 9 7 c assemblage 3 use of different garnet plagioclase and muscovite activity models can change the length of a p t path by as much as 15 but does not typically change directions in p t space significantly models that incorporate changes of fluid composition shorten p t paths in assemblages 1 and 3 but do not change trajectories significantly assemblage 2 is virtually unaffected by fluid phase models for the mineral assemblages considered here and using appropriate monitor parameters propagated errors are small compared to the total path length suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism and hence for interpreting orogenic processes

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

metamorphic thermal and tectonic evolution of central new england mountain skitchewaug and big staurolite nappes reactivation of a new detailed tectonic model is presented for the acadian orogenic this fabric during thrusting is recorded in some rocks of the big belt of central new england vermont and new hampshire that staurolite nappe by rotated garnets that grew during near isothermal accounts for a wide range of petrological and structural observations loading only the sillimanite isograd crosses the fall mountain three belts are considered the eastern vermont merrimack and skitchewaug nappe boundary metamorphic breaks across the skitintervening bronson hill belts specific observations in eastern chewaug big staurolite nappe boundary at the base of the big vermont that are accounted for in the model include the following staurolite nappe and at the margin of the keene and alstead domes p t paths are clockwise with maximum pressures near the athens require post metamorphic thrusting when p t conditions were in chester and strafford domes of 8 11 kbar but with maximum the greenschist facies these observations can be explained by a pressures decreasing to 3 5 kbar at the boundary with the bronson relatively simple model involving in sequence thrusting from east to hill belt differential exhumation of the vermont domes relative to west commencing in central new hampshire at 400 410 ma the rocks in easternmost vermont is required by the recorded differences preservation of the low grade belt along the vermont new hampin maximum pressure 5 6 kbar 15 20 km and the present day shire border requires that crustal thickening in vermont was not geographical separation 7 10 km specific observations in new caused by emplacement of new hampshire nappes onto eastern hampshire that are explained include the following p t paths in vermont and that the nappes of western new hampshire had time the merrimack belt are counter clockwise with maximum pressures to cool before final juxtaposition against the low grade belt cooling of 4 5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early acadian plutons p t paths in the bronson hill carboniferous suggesting that the acadian was a prolonged event belt are intimately associated with structural position an early spanning as much as 100 myr contact metamorphism is evidenced in the skitchewaug and fall mountain nappes near contacts with the early acadian bethlehem gneiss 400 410 ma peak metamorphic temperature rises upwards in the nappe sequence an inverted metamorphic sequence

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

measurement and correlation of microstructures the case of foliation intersection axes recent studies have used the relative rotation axis of sigmoidal and spiral shaped inclusion trails known as foliation inflexion intersection axis fia to investigate geological processes such as fold mechanisms and porphyroblast growth the geological usefulness of this method depends upon the accurate measurement of fia orientations and correct correlation of temporally related fias this paper uses new data from the canton schist to assess the variation in fia orientations within and between samples and evaluates criteria for correlating fias for the first time an entire data set of fia measurements is published and data are presented in a way that reflects the variation in fia orientations within individual samples and provides an indication of the reliability of the data analysis of 61 fia trends determined from the canton schist indicate a minimum intrasample range in fia orientations of 30 three competing models are presented for correlation of these fias and each of the models employ different correlation criteria correlation of fias in model 1 is based on relative timing and textural criteria while model 2 uses relative timing orientation and patterns of changing fia orientations and model 3 uses relative timing and fia orientation as correlation criteria importantly the three models differ in the spread of fia orientations within individual sets and the number of sets distinguished in the data relative timing is the most reliable criterion for correlation followed by textural criteria and patterns of changing fia orientations from core to rim of porphyroblasts it is proposed that within a set of temporally related fias the typical spread of orientations involves clustering of data in a 60 range but outliers occur at other orientations including near normal to the peak distribution consequently in populations of fia data that contain a wide range of orientations correlation on the basis of orientation is unreliable in the absence of additional criteria the results of this study suggest that fias are best used as semiquantitative indicators of bulk trends rather than an exact measurement for the purpose of quantitative analyses

garnet rotation and the development of axial plane crenulation cleavage rotated helicitic garnets syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold the garnet rotation is inconsistent with simple minded shear models for the development of the crenulation cleavage and it is useful to consider this problem in order to understand the development of crenulation cleavage better various models are considered in an attempt to explain both garnet rotation and cleavage morphology earlier models were based on the assumption that garnet rotation is driven by shear parallel to s2 they were mostly unsatisfactory but one involving migration of the crenulation hinges is viable when combined with other models described here the new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface s1 the new foliation s2 is considered to be a passive structure at least in the early stages of its development these models satisfactorily explain garnet rotation and development of the cleavage including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets comparison of observed and predicted helicitic structures strongly supports the proposed models evidence is presented to show that where a garnet has an se that is a schistosity and an si that is a crenulation cleavage the schistosity can have developed by unfolding of the crenulations

high resolution correlation patterns in the turbidite systems of the hecho group south central pyrenees spain detailed bed by bed correlations in sheetlike lobes and basin plain elements of the hecho group were carried out through a hierarchical decomposition based on three orders of correlation this procedure provides the most detailed correlation framework with chronostratigraphic significance first order decomposition is based on physical correlation by mapping of major megaturbidites second order decomposition consists in matching a series of marker beds within first order units leading to the definition of units several metres thick finally third order decomposition relies on the matching of serial features within second order packages number vertical arrangement and facies features of beds and further testing the coherence of the downstream evolution of facies in single beds sheetlike lobes and basin plain are closely related and display a distinctive sheet like geometry basin plain beds are deposited by the same flows that build the sheetlike lobes with the existence of a transitional facies change from the lobe to the basin plain thickening of some beds towards the basin plain due to ponding of the fine grained silt and mud divisions a diagnostic feature of the basin plain controls the sheet like stacking pattern by balancing the wedging out of thinner lobe beds upcurrent from or within the basin plain as a consequence the whole area can be geomorphically classified as a basin plain which comprises both lobe and basin plain facies associations

pressure temperature reaction history of metapelitic rocks from the maryland piedmont on the basis of correlated garnet zoning and plagioclase inclusion composition regional structural interpretation thermobarometry and gibbs method calculations were combined to determine an approximate p t path for kyanite grade metapelitic rocks of the baltimore gneiss terrane garnet growth along the estimated p t path was modeled using the computer program diffgibbs models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite bearing sample from the area modifications in the p t path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model garnet profiles in the most successful model garnet growth began at 550 c and 6500 bars in the assemblage chlorite biotite garnet muscovite plagioclase quartz with increasing temperature and with decreasing and then constant pressure at 570 c and 5724 bars staurolite was added to the assemblage and some garnet was consumed in the model garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure the region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615 c at 6474 bars after kyanite was added to the assemblage grossular increased to the constant rim value only if temperature decreased and pressure increased cooling and decompression affected garnet zoning only very near the rim measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model this study demonstrates that complex garnet zoning can be modeled successfully even when the reaction history includes episodes of garnet consumption over a path that is consistent with geologic evidence details of garnet zoning provide surprisingly tight constraints on the p t path

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

deconvolving tectono climatic signals in deep marine siliciclastics eocene ainsa basin spanish pyrenees seesaw tectonics versus eustasy the eocene deep marine siliciclastic fill of the ainsa basin spanish pyrenees gives unprecedented temporal resolution of the causes and timing of coarse clastic sediment supply to a deep marine basin early eocene tectonic subsidence linked to pyrenean orogenesis created the ainsa basin with water depths of similar to 400 800 m above a foundered shallow marine mixed carbonate and clastic shelf the similar to 25 sandbodies or channelized submarine fans in the basin were controlled by the similar to 400 k y milankovitch frequency with modes at similar to 100 k y and similar to 41 k y possibly stacked similar to 23k y influencing bottom water conditions in the basin causing periodic stratification in the water column across a submarine sill at the western basin margin early boltana anticline intrabasinal tectonics defined and controlled the position of eight sandy systems and their constituent fans in a process of seesaw tectonics by 1 westward lateral offset stacking of sandy fans due to growth of the eastern side of the basin represented today by the mediano anticline and 2 eastward orogenward back stepping of the depositional axis of each sandy system due to phases of relative uplift of the boltana anticline during basin infill uplift of the boltana anticline led to increasing basin narrowing and depositional confinement unlike the earlier depositional systems the youngest deep marine system was fed from a more southern sediment source between the growth anticlines as was the overlying deltaic system all the older deep marine sandy systems were fed from southeast point sources from canyons and erosional lower slope channels eroding the growing mediano anticline the depositional style outlined in this paper might be common to other active margins where siliciclastic basins evolve between active thrusts

age and setting of the bronson hill magmatic arc a re evaluation based on u pb zircon ages in southern new england new u pb zircon ages of selected rocks from the bronson hill anticlinorium in southern new england are as follows microcline gneiss of the dry hill gneiss in the pelham dome believed to be metamorphosed alkali rhyolite is confirmed as late proterozoic yielding an age of 613 3 ma another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 ma plagioclase rich gneisses with associated amphibolites swanzey pauchaug monson and fourmile gneisses now believed to represent metamorphosed intrusive igneous rocks yield ages from 454 to 442 3 2 ma thus ranging from early late through latest ordovician consistent with other absolute ages of the oliverian plutonic series in the region quartz phyric rhyolite from the upper member of the ammonoosuc volcanics and rhyolite tuff from the partridge formation are dated at 453 2 ma and 449 3 2 ma respectively indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m y additional zircon ages are reported from a crosscutting but deformed tonalite dike in the swanzey gneiss 381 3 ma and a coarse grained gabbro from the prescott intrusive complex 407 3 2 ma intrusive into the partridge formation metamorphic overprinting of the region during both the acadian and alleghanian orogenies is indicated by u pb titanite ages of 357 5 ma and 292 5 ma from a tonalite gneiss on prescott peninsula and the dry hill gneiss respectively the above ages and other zircon ages as well as petrography and geochemistry of the fourmile monson swanzey pauchaug and other plagioclase rich gneisses of the bronson hill anticlinorium suggest that they represent the plutonic root of a calc alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in late ordovician time ca 454 442 ma the physically overlying cover sequence of chemically different tholeiitic arc volcanic and sedimentary rocks of the ammonoosuc volcanics and partridge formation previously thought to rest unconformably on the plagioclase rich plutonic gneisses is now shown to have had a magmatic history overlapping with them the zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the late ordovician caradocian emplacement of the giddings brook slice of the taconian allochthons and too young to have formed above an early and medial ordovician subduction zone

evolution of a turbiditic foreland basin and analogy with an accretionary prism example of the eocene south pyrenean basin the south pyrenean tertiary basin was an east west trending foreland trough which developed from early eocene to miocene times in a north south trending compressive regime a turbiditic series of lower and middle eocene age hecho group enables us to define the typical features of the tectono sedimentary evolution of the basin 1 slope and shelf derived carbonate turbidites and megaturbidites bear witness to the seismic activity of the basin borders 2 the southern basin border evidenced by the onlap of the turbidites onto slope and shelf deposits migrated progressively southwards in time and 3 the deformation pattern is characterized in the meso cenozoic sediments by a complex thrust fold system of thin skinned tectonics style its specific geometry and kinematics were influenced by the pre deformation geometry onlap this thrust fold system is related to south directed basement nappes and its front advanced progressively southwards in time from this data we infer an evolutionary model for this highly mobile basin it was a flexural basin linked to the northward bending of the lithosphere in the foreland of a south directed crustal accretionary prism as the nappes advanced the basin migrated progressively southwards in the foreland direction as for the actual basin evolution many analogies with accretionary prism trench systems of active margins exist

u pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites abstract u pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern connecticut are used to date metamorphism and deformation most of the major minerals have 206 pb 204 pb ratios less than 100 however u pb mineral isochron and mineral whole rock isochrons constructed using aliquots of the same mineral yield concordant ages with uncertainties as low as 5 ma staurolite kyanite and garnet are older than metamorphic monazites and zircons these data are summarized below 5 mineral 206 pb 204 pb system the straits schist trap falls fm monazite zircon 207 pb 206 pb 379 384 ma 380 2 ma staurolite 48 623 238 u 206 pb 394 400 ma 398 5 ma garnet 22 46 238 u 206 pb 383 ma 414 10 ma 235 u 207 pb 418 ma 207 pb 206 pb 623 ma kyanite 21 27 238 u 206 pb 399 ma chlorite anatase ilmenite 20 27 238 u 206 pb 350 370 ma apatite quartz kspar 18 19 transposed pegmatitic intrusions in the straits schist yield monazites with u pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists growth of monazite and zircon in the schists may be related to high t fluid influx associated with pegmatite intrusion age discordance in the garnets from the straits schist is most consistent with continuous loss of intermediate daughter products of the 238 u decay chain from about 420 ma to 380 ma garnets from the adjacent trap falls formation do not display this behavior

architecture of the tectonically influenced sobrarbe deltaic complex in the ainsa basin northern spain abstract the syn tectonic sobrarbe deltaic complex eocene of the ainsa basin spanish pyrenees is confined by lateral thrust ramps and influenced by intra basinal growth anticlines six facies associations have been distinguished within the deltaic complex i slope marls and turbidite sandstones ii distal delta front silty and bioturbated sandstones iii proximal delta front and delta plain deposits tidal inlet tidal flat mouth bar bay distributary channel and floodplain environments iv biogenic deposits at flooding surfaces v collapse zone sediments and vi nummulite dominated shallow marine carbonates the deltaic complex comprises four composite sequences which each contain a number of minor sequences of the genetic maximum flooding surface bounded type regressive unconformities and their correlative surfaces separate the composite sequences cs and each cs can be divided into lowstand transgressive and highstand components at the base the comaron cs is a wnw prograding deltaic complex containing six minor sequences characterized by basinward stepping sandstone wedges separated by transgressive mudstones double regressive unconformities are often associated with these sandstone wedges proximally in the basin growth of the intra basinal arcusa anticline caused gentle folding of the comaron cs and syn growth strata at the base of the overlying las gorgas cs contain a prominent forced regressive sandstone wedge the las gorgas sandbody and display thinning of bedsets onto the crest of the anticline moreover it can be demonstrated that relative sea level rise took place in the growth synclines at the same time as a fall in relative sea level occurred on the growth anticlines the tectonic deformation caused a shift in the direction of deltaic progradation towards the north northwest as demonstrated by the well developed clinoforms in the highstand sequence set of the las gorgas cs a second phase of tectonic activity this time triggering large scale collapse of the delta front occurred at the transition between the las gorgas and barranco el solano cs tilting of the basin floor led to the development of coeval transgressive and regressive strata separated by a fulcrum line of zero net vertical movement of the shoreline in the post tilt phase reduced sediment supply and accelerated subsidence led to widespread transgression and a change from clastic to carbonate dominated sedimentation this phase was terminated by a gradual increase in sediment supply causing progradation of a mixed clastic carbonate system dominated by large nummulite banks and bioturbated shoreface marls and sandstones finally a pronounced fall in relative sea level associated with the transition to the buil cs triggered slope instabilities and eventually established fluvial conditions all over the study area with the exception of the anticline crests the relatively high rate of tectonic subsidence in the study area suppressed relative sea level falls resulting in development of discontinuous lowstand strata and bounding surfaces the segmented nature of many regressive unconformities may be related to incremental growth of thrust related structures and each segment is separated from neighbouring segments by gaps of limited vertical and lateral displacement precursors of progressive unconformities are seen in the upper proximal part of the comaron cs by downdip fanning and updip convergence of minor sequences these differentially tilted stratal units provide reliable indicators of syn sedimentary tectonism

monazite geochronology in central new england evidence for a fundamental terrane boundary monazite crystallization ages have been measured in situ using sims and emp analysis of samples from the bronson hill anticlinorium in central new england in west central new hampshire each major tectonic unit nappe displays a distinctive p t path and metamorphic history that requires significant post metamorphic faulting to place them in their current juxtaposition and monazite ages were determined to constrain the timing of metamorphism and nappe assembly monazite ages from the low pressure high temperature fall mountain nappe range from c 455 to 355 ma and y zoning indicates that these ages comprise three to four distinct age domains similar to that found in the overlying chesham pond nappe the underlying skitchewaug nappe contains monazite ages that range from c 417 to 307 ma 40 ar 39 ar ages indicate rapid cooling of the chesham pond and fall mountain nappes after 350 ma which is believed to represent the time of emplacement of the high level chesham pond and fall mountain nappes onto rocks of the underlying skitchewaug nappe garnet zone rocks from western new hampshire contain monazite that display a range of ages c 430 340 ma both the metamorphic style and monazite ages suggest that the low grade belt in western new hampshire is continuous with the vermont sequence to the west rocks of the big staurolite nappe in western new hampshire contain monazite that crystallized between c 370 and 290 ma and the same unit along strike in northern new hampshire and central connecticut records ages of c 257 300 ma conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks these monazite ages confirm that the metamorphism observed in the big staurolite nappe occurred significantly later than that in the units structurally above and below these data support the hypothesis that the big staurolite nappe represents a major tectonic boundary along which rocks of the new hampshire metamorphic series were juxtaposed against rocks of the vermont series during the alleghanian

synchronous extension and contraction in frontal thrust sheets of the spanish pyrenees the ainsa marine foreland basin of the southern pyrenees contains eocene deltaic and turbidite sequences that were deformed synchronously by both thrusting and extensional faulting during sedimentation two basin scale extensional truncation surfaces developed within a prograding imbricate thrust wedge in the footwall of the cotiella thrust sheet detailed mapping and analysis have shown that these features are major extensional detachment faults that produced complex sedimentological and stratal relations

scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite grade phyllites se vermont detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south eastern vermont show that grain scale zoning is common and sympathetic zoning in adjacent minerals is nearly universal we interpret this to reflect a pressure solution mechanism for cleavage development where precipitation from a very small fluid reservoir fractionated that fluid multiple analyses along single muscovite biotite and chlorite grains 30 200 m in length show zoning patterns indicating tschermakitic substitutions in muscovite and both tschermakitic and di trioctahedral substitutions in biotite and chlorite using cross cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite grade chlorite grade overprinted by biotite grade and biotite grade zoning patterns are comparable in all three settings requiring a similar cleavage forming mechanism independent of metamorphic grade moreover the use of 40ar 39ar geochronology demonstrates this is true regardless of age furthermore samples with chlorite grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of al si mg and fe ions are greater than the temperature of the biotite isograd 400 c parallel and smoothly fanning tie lines produced by coexisting muscovite chlorite and muscovite biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization these results do not support theories suggesting cleavages form in fluid dominated systems if crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir then the compositions of individual grains defining these cleavages would be uniform on the contrary the fine scale chemical zoning observed probably reflects a grain scale process consistent with a pressure solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation thus the role of fluids was probably limited to one of catalysing pressure solution and fluids apparently did not drive cleavage development

filling the north european early middle eocene ypresian lutetian boundary gap insights from the pyrenean continental to deep marine record abstract the early middle eocene ypresian lutetian transition is represented by a hiatus in many north european sections including those in which the classic stratotypes were originally defined however the global stratotype section and point of the lutetian stage which is still pending definition should be placed at a globally correlatable event included within that unrepresented interval the pyrenean eocene outcrops display sedimentary successions that offer the rare opportunity to analyse the ypresian lutetian boundary interval in almost continuous sections and in very different settings seven reference stratigraphic sections were selected on the basis of their quality and correlated by means of biomagnetostratigraphic data this correlation framework casts light on the sequence of chronostratigraphic events that characterize the ypresian lutetian boundary interval which may prove useful in defining the main correlation criterion of the base of the lutetian all of the pyrenean sections show a similar sedimentary evolution despite being up to 350 km apart from each other containing deposits of different origins intrabasinal carbonate sediments siliciclastic sediments sourced from the iberian plate and terrigenous sediments sourced from the uplifting pyrenees and despite having been accumulated in different sedimentary environments from continental to deep marine and in different geodynamic settings piggy back basin foreland basin and cratonic margin this common evolution can be readily interpreted in terms of a sea level driven depositional sequence whose lowstand and transgressive systems tracts are included within the ypresian lutetian boundary interval the pyrenean ypresian lutetian depositional sequence can reasonably be correlated with depositional sequences from classic north european areas shedding light on the palaeoenvironmental history which in those areas has not been recorded furthermore these depositional sequences may possibly correlate with others from the antarctic ocean and from new jersey as well as with oceanic temperature variations suggesting that they might be the result of climatically driven glacioeustatic sea level changes should this hypothesis prove correct it would confirm previous suggestions that the onset of antarctic glaciations needs to be backshifted to the late ypresian at least

the distribution style and intensity of alleghanian metamorphism in south central new england petrologic evidence from the pelham and willimantic domes abstract alleghanian metamorphism in south central new england is characterized by late pennsylvanian documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm kyanite mathrm muscovite mathrm staurolite end document grade heterogeneous reconstitution of migmatitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepack

a new age model for the middle eocene deep marine ainsa basin spanish pyrenees abstract the middle eocene ainsa basin spanish pyrenees comprises 4 km of deep marine sedimentary rocks belonging to the hecho group despite extensive study of these exemplary deep marine clastic successions there has been no comprehensive chronostratigraphic framework that provides primary ages for the submarine fan and related deposits here we present a new composite basin stratigraphy based upon biostratigraphic analyses of the upper hecho group submarine fan and interfan deposits calcareous nannofossil data suggest that deposition of the gerbe through to guaso systems occurred during biozones np14 16 42 6 and 48 9 ma based on our age model additional biostratigraphic ages from the lower hecho group suggest that the entire hecho group no older than biozone np13 51 ma the improved chronostratigraphic control enables correlations between submarine canyons and submarine fans of the ainsa and jaca basins to be assessed our new age model provides a means of comparison of stratigraphic events with regional sections allowing a better understanding of the lateral and temporal evolution of these depositional systems from source to sink

microstructural evidence for n s shortening in the mount isa inlier nw queensland australia the preservation of early w e trending foliations in porphyroblasts revealed by independent 3d measurement techniques 3d microstructural analyses of porphyroblast inclusion trails using both the asymmetry switch method for determining foliation intersection axes preserved in porphyroblasts fias and the recently developed fitpitch method reveal w e and n s trending fia sets in the white blow formation of the mount isa inlier each method reveals two subsets of fias centered on each of these major trends these were distinguished based on the relative timing trend and orientation of inclusion trail patterns thirty six samples were analyzed using both techniques and produced very similar results pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin sections and trends in horizontal sections yield distinct near orthogonal modal orientations from all the analyzed samples this indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes w e and n s trending fias have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages garnet and staurolite growth during bulk north south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain this period of bulk shortening and associated metamorphism formed during a period of orogenesis called o1 w e shortening formed n s striking foliations that preserve a period of orogenesis o2 and another succession of metamorphism involving more phases of porphyroblast growth preserving n s trending fias overprinting of successive fia trends wsw ene wnw ese nnw sse and ssw nne suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from n s to w e overall with a major tectonic break or decompression between o1 and o2 the porphyroblast inclusion trail patterns preserving w e trending fias provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations

petrology of the regional sillimanite zone west central new hampshire u s a with implications for the development of inverted isograds abstracr sillimanite from the regional sillimanite zone in west central new hampshire is fibrolitic and overprints f2 folds nappe stage of earlier mica foliation regional sillimanite zone samples show no evidence for earlier staurolite parageneses despite the fact that staurolite is abundant at lower grade because sillimanite was produced directly from garnet chlorite by the prograde heating reaction garnet chlorite muscovite quartz sillimanite biotite h20 the pressure at which this reaction occurs is sensitive to the mno and cao contents of garnet and phase equilibrium arguments reveal that at the regional pressures of west central new hampshire 2 4 kbar staurolite parageneses are only possible in rocks with low mno cao the inferred p t path is counterclockwise with nearly isobaric initial heating at 2 kbar followed by loading t heating to a peak metamorphic temperature of 600 t 25 c at 4 kbar followed by nearly isobaric cooling garnets were compositionally homogenized near the metamorphic peak but subsequent cooling was rapid 100 c m y these data argue against the folded isotherm model of chamberlain 1986 instead the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high grade thrust sheets on lower grade rocks followed by depression of high grade rocks to lower structural levels

obtaining equilibrium oxygen isotope fractionations from rocks theory and examples a generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope closure temperatures examples using literature data and new analyses from 32 natural amphibolite facies schists demonstrate both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks by screening samples according to the theoretical models natural data are shown to have highly consistent mineral fractionations 2 reproducibilities of 0 16 to 0 54 that within uncertainty reproduce experimental determinations among the minerals quartz biotite muscovite and calcic amphibole this correspondence indicates that the proposed theoretically based selection criteria improve the likelihood of measuring equilibrium fractionations the new data further corroborate the expected progressive enrichment of 18o in the orthosilicates with increasing al si relative to fe mg ky grt 1 05 st grt 0 6 and st cld 0 3 at 525 575 c in contrast typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz biotite and amphibole that are strongly disequilibrium because of exchange during cooling theoretical screening of samples prior to isotope analysis allows robust independent assessment of theoretical and experimental determinations of equilibrium isotope fractionations

climate forcing of fine grained deep marine systems in an active tectonic setting middle eocene ainsa basin spanish pyrenees abstract a multi proxy approach to understand environmental change in deep time was undertaken on about 150 m of core from a 230 m long middle eocene core from the ainsa basin spanish pyrenees representing deep marine siliciclastic sediments using detailed sedimentary logging high resolution multi element xrf geochemistry total organic carbon and stable carbon isotopes the well a6 was drilled as part of an industry university consortium through siltstones fine very fine grained sandstone turbidites and hemipelagic structureless mudstones that were deposited as overbank and off axis deposits from a sandy submarine fan and interfan deposits for comparative mineralogy between the sandstone turbidites and siltstones scanning electron microscopy sem and x ray diffraction xrd were undertaken on selected samples the sandstone turbidites show enrichment of detrital elements such as si zr and ti that can be linked to greater quartz and heavy mineral content compared with adjacent siltstones structureless hemipelagic mudstones comprise mainly clay minerals and carbonate we interpret the sandstone turbidites as from hyperpycnal flows during high river sediment discharge whilst hemipelagic mudstones resulted from the suspension fall out of hypopycnal flows cyclostratigraphic analysis of the core reveals milankovitch cyclicity at frequencies of 0 03 cycles m short eccentricity 0 09 cycles m obliquity 0 15 cycles m precession couplet and 0 19 cycles m precession couplet orbital parameters appear to have controlled the cyclic delivery of coarser grained sediment by turbidity currents two equally plausible depositional models both as milankovitch driven can explain the cyclical changes in the deep marine sediments 1 climatic cycles with humid periods of enhanced chemical weathering increased storminess and greater riverine run off leading to high sediment flux to the deep basin as sandstone turbidites 2 climatic cycles with cooler conditions linked to high frequency small scale eustatic sea level fluctuations with lowstand shelf edge delta progradation resulting in greater volumes of coarse detrital sediment to the seafloor by hyperpycnal flows this study provides an insight into the likely depositional effects of orbitally induced climate change on the nature and delivery of terrigenous sediment into deep marine environments

paleomagnetically derived folding rates southern pyrenees spain well dated syntectonic unconformities with the spanish pyrenees allow characterization of sedimentation and deformation within an orogenic foreland the spanish pyrenees are composed of a south vergent thrust belt and foreland basin in which folds developed both parallel and perpendicular to tectonic transport unconformity geometries and magnetostratigraphic data from synorogenic strata provide a precise temporal framework for the quantitative analysis of the mediano anticline the largest of the transverse folds folding began in the early eocene and ended by 42 ma at which time an anticline 20 km long and 5 km in wavelength had been produced angular and progressive unconformities within syntectonic sedimentary units preserve the history of fold development fold growth resulted from west ward salt movement driven by prograding sediments and was characterized by 2 3 m y intervals of slow continuous limb tilting 2 2 4 2 m y preserved as progressive unconformities within syntectonic strata because the emplacement of the cotiella montsec thrust sheet was episodic intervals of slow fold growth were punctuated by intervals lasting

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

flattening the bhutan himalaya a detailed thermobarometric transect of 35 samples across the greater and tethyan himalayan sequences in central bhutan demonstrates a tectonostratigraphically intact section with uniform apparent thermal and baric field gradients of 2072 1c km and 0 5770 08 kbar km pressure temperature paths determined from chemically zoned garnets in 6 samples demonstrate that these p t conditions correspond with maximum pressures the super lithostatic baric gradient cannot be explained by pre to syn metamorphic tectonic processes or by extension within an inclined slab instead the data imply 50 post peakmetamorphic flattening of the himalayan metamorphic core accommodated by distributed top to thenorth shear consistent with microstructural analysis orogenic flattening best explains the development of the south tibetan detachment system as a strain incompatibility feature rather than a structure bounding the top of a tectonically inserted wedge and helps reconcile debate attributing first order himalayan structural features to either wedge failure critical taper or pipe flow channel flow

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

pressure temperature and metamorphic zonation studies of pelitic schists in the merrimack synclinorium south central new hampshire metamorphic isograds in pelitic rocks in nine 15 quadrangles in south central new hampshire are divisible into the upper 5 of tracy s 1975 high grade zones 2 the staurolite sillimanite zone 3 the sillimanite zone 4 the sillimanite alkali feldsparmuscovite zone 5 the sillimanite alkali feldspar zone 6 and the sillimanite alkali feldspar cordierite zone these zones replace the single alkali feldspar sillimanite zone originally mapped by thompson and norton 1968 biotite garnet geothermometry and plagioclase composition studies confirm the zonal pattern deduced from the mineralogical relations three stages of acadian metamorphism are recognized an early m 1 andalusite metamorphism is evidenced by sillimanite pseudomorphs after andalusite and flecky gneisses occurring near the contacts of the devonian kinsman series plutons this early metamorphism has been strongly overprinted bv m 2 metamorphism which locally attained cordierite alkali feldspar grade and produced the dominant isograd pattern m 2 metamorphism cross cuts devonian kinsman and spaulding series plutons in this region m 3 locally retrograded the m 2 terrane and produced a lower temperature decussate muscovite

metamorphic p t paths from calcic pelitic schists from the strafford dome vermont usa metamorphism of the gile mountain formation and waits river formation in the strafford dome and townshend brownington syncline in east central vermont records two nappe style events d1 and d2 followed by doming d1 formed a muscovite biotite ilmenite schistosity subparallel to compositional layering so and was followed by heating to garnet grade the temperature and pressure at the end of d1 are estimated to be c 450d c and 6 8 kbar d2 variably crenulated and folded s1 during a nearly isothermal pressure increase of 1 2 kbar calculated from compositions of garnet which have inclusions trails with progressive crenulation and rotation of the s1 fabric similar p t paths are computed for most of the area suggesting that the later schistosity developed during emplacement of a regional nappe 3 6 km thick there is a general lack of d3 dome stage microstructures near the strafford willoughby arch staurolite and kyanite overgrew s2 in pelites and plagioclase with increasing xan overgrew s2 in calcic pelites reflecting post d2 heating to a maximum of 550 600d c metamorphic pressures at the end of d2 are fairly constant on the west side of the dome indicating minor dome stage uplift in contrast pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome the observed pattern of isotherms and isobars is mainly the result of post metamorphic differential uplift and unroofing finally a minor retrograde metamorphism produced the assemblage albite epidote k feldspar muscovite chlorite with grade increasing east toward the connecticut river

the piermont allochthon revisited and redefined at its type locality regional lithostratigraphic and structural mapping of the piermont allochthon of northwestern new england at its type locality indicates that the foster hill fault the supposed sole thrust is a stratigraphic boundary rather the allochthon lies above the bean brook fault covers a much smaller area than suggested previously and is limited to the vicinity of piermont western new hampshire the allochthon which primarily consists of rocks correlated with the silurian age rangeley formation of maine has horizons of metamorphosed polymictic conglomerate at its base rocks directly below the allochthon are correlated with the metasedimentary ordovician aged albee formation and contain abundant metamorphosed dykes and sills that do not cross the sole detachment of the allochthon analysis of macro and mesoscale structures indicates local preservation of fold hinges and overprinting foliations oblique to and later than those previously described and has allowed the deformation history to be significantly extended by several events emplacement of the allochthon early in the deformation history at metamorphic conditions caused nw se elongation lineations of clasts in conglomerate at the base of the allochthon the relict foliations and fold hinges are commonly preserved below the allochthon suggesting that these rocks have had a more protracted deformation history relative to rocks in the allochthon the new interpretation of the piermont area provides a framework to build a more thorough tectonic history for this part of the new england appalachians

cyclicity and basin axis shift in a piggyback basin towards modelling of the eocene tremp ager basin south pyrenees spain

the south pyrenean eocene carbonate megabreccias revisited new interpretation based on evidence from the pamplona basin abstract the south pyrenean foreland basin contains numerous units of eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow marine carbonate platforms previous studies were limited mainly to the foreland eastern part known as the jaca basin the present study from the pamplona basin a western part of the foreland trough sheds new light on the origin and regional significance of these south pyrenean eocene carbonate megabreccias specms the number of the specm units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed the specm units appear to occur as time stratigraphic clusters which can be correlated with the relative sea level lowstands and linked with phases of tectonic activity the megabreccias were derived from a carbonate platform system hosted by the foreland basin s southern passive margin the episodic instability and mass wasting were triggered by phases of structural steepening forebulge uplift accompanied by high magnitude earthquakes with the former causing platform emergence increased load stresses and excess pore water pressure in the carbonate ramp the specm deposits were emplaced by cohesive debris flows evolving into high density turbidite currents an ideal specm unit consists of 1 an immature homogeneous debrite in the proximal part 2 a differentiated bipartite debrite and turbidite in the medial part and 3 an incomplete base missing debrite overlain by turbidite or a turbidite alone in the distal part the debrite component volumetrically predominates in the specm units and the original terms megaturbidite and seismoturbidite thus seem to be inappropriate for these deposits

the anso transect of the southern pyrenees basement and cover thrust geometries a balanced geological section through the western central pyrenees allows the alpine geometry of hercynian basement to be determined this shows marked differences with other transects of the southern part of the chain basement involved thrust sheets do not form an antiformal stack but are imbricated and define steps separated by frontal culmination limbs basement thrusts propagate upwards and have formed distinct thrust systems in the cover in this transect the north pyrenean zone is bounded to the south by gently dipping thrusts of large displacement and the axial zone massif at its western termination area is a flat topped culmination caused by a single thrust fault the gavarnie thrust this thrust emerges to the south in the interior of the syntectonic jaca basin the geometry of which is also controlled by underlying basement thrusts various generations of structures can be correlated with distinct episodes of syntectonic sedimentation a sequential restoration of the section illustrates how successive foreland sedimentary wedges were generated and progressively involved in deformation tectonic shortening is 48 km almost half of that estimated further to the east for the southern pyrenees the structure deduced at the ecors pyrenees seismic section cannot be translated to this transect

timing of acadian deformation in northeastern maine 40ar 39ar ages determined for 24 mineral separates from the pleasant lake hunt ridge and cochrane lake plutons bear on the timing of the main phase of the acadian orogeny in northern maine six hornblende samples and one clinopyroxene sample have only minor disturbances in their release spectra the increments with concordant ages form overlapping plateaus at 403 4 ma many of the biotite samples have discordant release spectra the average of their total gas ages is 400 6 ma calculated closure temperatures for the hornblendes and biotites are 550 c and approximately 350 c respectively the difference between closure temperatures and concordancy in ages demonstrates that the plutons cooled very rapidly upon emplacement therefore the 40ar 39ar ages date both the time of argon retention in the minerals and the time of emplacement of the plutons the best estimate for their time of emplacement is the average of the combined ages of the micas hornblendes and clinopyroxene 402 6 ma the plutons are undeformed and crosscut upright open acadian folds in the carys mills smyrna mills and madrid formations the age of the pleasant lake hunt ridge and cochrane lake plutons therefore places a lower limit of 402 6 ma on the time of acadian folding in northern maine

monsoon speeds up indian plate motion article i nfo short term plate motion variations on the order of a few myr are a powerful probe into the nature of plate boundary forces as mantle related buoyancies evolve on longer time scales new reconstructions of the ocean floor spreading record reveal an increasing number of such variations but the dynamic mechanisms producing them are still unclear here we show quantitatively that climate changes may impact the short term evolution of plate motion by linking explicitly the observed counter clockwise rotation of the indian plate since 10 ma to increased erosion and reduced elevation along the eastern himalayas due to temporal variations in monsoon intensity by assimilating observations into empirical relations for the competing contributions of erosion and mountain building we estimate the first order decrease in elevation along the eastern himalayas since initial strengthening of the monsoon furthermore we show with global geodynamic models of the coupled mantle lithosphere system that the inferred reduction in elevation is consistent with the indian plate motion record over the same period of time and that lowered gravitational potential energy in the eastern himalayas following stronger erosion is a key factor to foster plate convergence in this region our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the euler pole falls within its associated plate

geochronologic studies in central new england i evidence for pre acadian metamorphism in eastern vermont sup 40 ar sup 39 ar geochronology on hornblendes from central new england reveals a grenvillian cooling age 1026 plus minus 12 ma along the eastern margin of the green mountain massif acadian ages in east central vermont 350 397 ma and along the bronson hill anticlinorium in new hampshire and massachusetts 407 280 ma and late ordovician to silurian ages 440 451 ma along a belt in eastern vermont and central massachusetts these ages indicate a minimal acadian overprint in eastern vermont the highest degree of acadian overprint being located along the axis of the domes in the connecticut valley synclinorium these data suggest that the formation of the domes occurred in the acadian moreover the late ordovician to silurian cooling ages in eastern vermont suggest that much of the metamorphic mineralogy and fabric observed in the connecticut valley synclinorium may be pre silurian in age

stacking of nappes with different pressure temperature paths an example from the menderes nappes of western turkey pressure temperature p t paths have been estimated from metapelitic rocks in the lowermost cine nappe and the directly underlying bozdag nappe of the anatolide belt in western turkey peak metamorphic conditions in the lowermost cine nappe are 670 to 730 c 6 2 to 6 3 kb prograde garnet grt i growth occurred largely before and during the formation of the regional s p a foliation the suffix pa indicates a pre alpine age formation of a second garnet generation grt ii which discordantly overgrew grt i followed at 550 to 620 c 6 4 to 6 5 kb peak metamorphic conditions in the underlying bozdag nappe vary from 480 to 540 c 6 1 to 7 6 kb at the base to 610 to 660 c 8 5 to 10 8 kb at the top of the nappe and attest to an inverted metamorphic field gradient in the bozdag nappe differential thermodynamic modelling gibbs method yielded a prograde path for garnet growth during the formation of s p a because d p a structures formed during prograde metamorphism we relate them to crustal shortening which was associated with a top to the north northeast shear sense in present day coordinates this event caused deformation of the cine and bozdag nappes under different metamorphic conditions we propose that the cine nappe was heated by granitoid intrusions before being emplaced above a foreland unit we speculate that the bozdag nappe belonged to the lower parts of this foreland unit cross cutting relationships of d p a structures with dated granites indicates an age of 540 to 550 ma for this tectonometamorphic event the final juxtaposition of the cine nappe and bozdag nappes and also the inversion of the metamorphic field gradient in the bozdag nappe occurred during greenschist facies conditions during the alpine orogeny in the eocene

the stacking of thrust slices in collision zones and its thermal consequences the thermal consequences of stacking up thrust slices in collision zones are investigated using simple one dimensional thermal models the metamorphic evolution geotherms and p t time paths of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects cooling and pressure decrease associated with erosion cooling by the lowest units screen effect heating by the upper units cover effect it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone the metamorphic evolutions are very sensitive to the following tectonic parameters the number of thrusted units involved in thickening the time delay between each thrust and finally the mode of stacking of the different units over and understacking it appears that for a given depth of burial the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units the screen effect during understacking is more efficient for a short time delay 10 ma between each thrust overstacking leads to higher temperatures before uplift when compared to understacking it is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small 10 km it is shown that for an erosion controlled uplift the shape of the p t t path depends on the position of the rocks within a given unit and on the unit position within the pile finally the general metamorphic evolutions during crustal thickening hp lt metamorphism and subsequent overprint are discussed in terms of the previously mentioned parameters in the case of the western alps it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events

thrust sequences in the eastern spanish pyrenees detailed mapping of the south pyrenean zone in the eastern spanish pyrenees shows that a great number of alpine thrusts affect the basement and the cover the geometry of the thrust structures and their propagation sequence can be interpreted from balanced sections the folding dies out downwards from the higher thrust sheets to the lower ones and demonstrates a piggy back thrusting sequence these thrusts form a duplex the sole thrust of this duplex is the vallfogona thrust the roof thrust is rooted somewhere within the axial zone the thrusts climb southwards from the axial zone basement up into tertiary cover sediments these thrusts have been folded so that they now dip to the south and form the southern side of the axial zone antiformal stack locally smaller antiformal stacks developed and produced strong folds in the higher thrust sheets the south pyrenean zone studied is equivalent to the gavarnie nappe and the connection between them below the detached south pyrenean central unit may be the nogueres zone

metamorphic zones and field gradients in orogenic belts comment on tectonic controls of metamorphic field gradients by l j sonder and c p chamberlain

reaction softening by dissolution precipitation creep in a retrograde greenschist facies ductile shear zone new hampshire usa we describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host littleton schist near claremont new hampshire here na poor muscovite and chlorite progressively replace first staurolite then garnet and finally biotite porphyroblasts as the core of the fault zone in approached across the transect higher grade fabric forming na rich muscovite is also progressively replaced by fabric forming na poor muscovite the mineralogy of the new phyllonitic fault rock produced is dominated by na poor muscovite and chlorite together with late albite porphyroblasts the replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement in contrast element mapping shows that fabric forming na rich muscovite is selectively replaced at high strain microstructural sites indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite the replacement of strong high grade porphyroblasts by weaker na poor muscovite and chlorite constitutes reaction softening the crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated na rich muscovite defined foliation destroys not only the metastable high grade mineralogy but also its stronger geometry this process constitutes both reaction and microstructural softening the deformation mechanism here was thus one of dissolution precipitation creep activated at considerably lower stresses than might be predicted in quartzo feldspathic rocks at the same lower greenschist facies conditions this article is protected by copyright all rights reserved

ion microprobe dating of zircon from quartz graphite veins at the bristol new hampshire metamorphic hot spot detrital zircons entrained in hydrothermal quartz graphite rutile veins found near the bristol new hampshire metamorphic hot spot are overgrown by thin rims ion microprobe analyses of these rims date their growth at 408 plus minus 6 ma these measurements quantitatively confirm textural evidence that the graphite veins were emplaced during peak metamorphism associated with the acadian orogeny and they provide a direct positive test of the hypothesis based on petrological and stable isotope evidence that the hydrothermal systems responsible for the quartz graphite veins were also responsible for the hot spot metamorphism

oxygen isotope geochemistry of the amphiboles isotope effects of cation substitutions in minerals abstract the occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fractionations among the amphiboles to be recovered from natural samples oxygen isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition 18o in the order hornblende gedrite 0 9 previously published data for hornblende actinolite glaucophane and garnet show that act hbl 0 2 gln grt 1 and hbl grt 0 thus glaucophane strongly partitions 18o relative to the calcic amphiboles the fractionation between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers pyroxene endmembers and exchange components such as caal nasi 1 naal camg 1 camg 1 mgfe 1 femn 1 kna 1 kal si 1 and fe3 al 1 applications of the exchange component method reproduce measured amphibole fractionations to within 0 1 to 0 2 whereas other predictive methods cause misfit for typical metamorphic hornblende of 0 5 at 575 c although the isotope effects of cation exchanges may be small at high t they magnify dramatically for minerals formed in surficial diagenetic and low t metamorphic environments different composition clays are predicted to have equilibrium 18o differences of 2 9 if the isotope fractionation can be determined for one mineral endmember then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho ring chain and sheet silicates

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

architecture of the castissent fluvial sheet sandstones eocene south pyrenees spain the castissent formation represents a phase of strong fluvial progradation within the fluvio deltaic montanyana group probably enhanced by the late ypresian sea level fall the structural setting is characterized by the emplacement of the south pyrenean nappes the major architectural feature of the formation is the superposition of three multilateral and multistorey sheet sandstone complexes a b and c interbedded in finer floodplain deposits and dark grey mudstones of brackish transitional origin the structure of the sheet sandstone complexes is exemplified by the detailed analysis of the lower sheet sandstone complex a 25 m thick 6 km wide and exposed over 25 km of its length three types of channel fill sandstones serve as components 1 lenticular bedded bodies interpreted as braided stream deposits 2 lateral accretion bedded bodies intepreted as meandering stream deposits and 3 ribbons originating from laterally stable and short lived offshoot or crevasse channels by coalescence and or stacking of these channel fill types higher order architectural units a1 a2 a3 have been formed that together build the sheet sandstone complex on the basis of downstream variation in proportion and geometrical relation between the channel fill components three depositional zones have been differentiated within the fluvial system each zone is characterized by specific architectural processes 1 alternation of minor aggradational degradational cycles in the proximal valley 2 generation of new alluvial belts in the intermediate nodal avulsion zone and 3 successive random avulsion processes with development of semipermanent crevasse channels or minor distributaries in the distal distributive zone the sedimentation of the castissent formation was structurally controlled by an interplay of vertical basement movement due to thrust stacking in the hinterland and surficial thrust displacement to the foreland resulting in alternating southward and northward shift of the fluvial system

metamorphic history of the central himalaya annapurna region nepal and implications for tectonic models pressure temperature time p t t conditions of metamorphism have been determined in the annapurna region of central nepal that place new constraints on the structural and tectonic evolution of the himalayan orogenic wedge peak p t conditions increase structurally upward 525 c and 8 kbar in the lesser himalayan sequence 650 c and 12 kbar at the base of the greater himalayan sequence across the main central thrust 750 c and 12 kbar in the middle of the greater himalayan sequence and 775 c and 13 kbar near the top of the greater himalayan sequence metamorphic monazite ages in the greater himalayan sequence also increase structurally upward 16 21 ma for subsolidus growth at the base of the greater himalayan sequence to 25 ma for peak t metamorphism and anatexis near the top of the greater himalayan sequence these ages are several million years older than at equivalent structural levels at langtang 200 km to the east the p t t data recommend reinterpretation of the bhanuwa fault within the greater himalayan sequence as a thrust and the presence of a different thrust structurally above the bhanuwa thrust here named the sinuwa thrust the new data are consistent with progressive stacking of tectonic slices with calculated overthrust rates that are consistent with some but not all models that presume 2 cm yr convergence across the himalaya since 25 ma despite differences in absolute ages similarities among the chemical systematics of monazite peak p t conditions and overthrust rates calculated for annapurna when compared to langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers although the timing and duration of movement on discrete thrust surfaces may differ

the piermont allochthon revisited and redefined at its type locality discussion timms 2004 1 has made a significant contribution to understanding a controversial structural feature of central new england the piermont allochthon see hadley 1942 2 1950 3 rumble 1969 4 moench 1990 5 1992 6 1996 7 1999 8 billings 1992 9 rankin 1996 10