high resolution numerical modeling of stress distribution in visco elasto plastic subducting slabs abstract we employ high resolution 2d thermomechanical modelling with complex elasto visco plastic rheology to analyse stresses in subducting and overriding plates the model contains a dynamic subduction channel defined as a few km thick zone of specific weak visco plastic rheology between the slab and the overriding plate our high resolution model corresponds to the zoom in of the large scale thermomechanical model of the central andes with the 4 fold resolution increase initial model configuration material temperature and stress distributions was interpolated from the parent model boundary conditions including slab pull and push velocities as well as velocity of the overriding plate were also re mapped from the large scale model we demonstrate that for shallow dipping overridden slabs the dominant stress is unbending stress at 50 100 km depth this stress reaches 1 1 5 gpa and strongly prevails over the slab bending stress near the trench which is limited by the frictional plastic yield unbending creates double 5 10 km distance between maximums compression zone in the upper part of the slab and a zone of extension some 20 30 km below pressure in these zones differs from the lithostatic pressure to as much as plus minus 0 8 gpa details of the distribution and magnitude of bending and unbending stresses in the slab are mainly controlled by the overriding velocity and direction of the slab pull force overpressure in the subduction channel in all models does not exceed 100 mpa large over and under pressures in the slab do not however lead to significant more than 10 km offsets of main dehydration reactions in crust and mantle of the slab

desert pavement coated surfaces in extreme deserts present the longest lived landforms on earth all exposed rocks on earth s surface experience erosion the fastest rates are documented in rapidly uplifted monsoonal mountain ranges and the slowest occur in extreme cold or warm deserts millennial submeterscale erosion may be approached only in the latter the oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings thus these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape while the surrounding landscape may erode faster and be younger we present erosion rate and exposure age data from the paran plains a typical environment in the near east where vast abandoned alluvial surfaces 10 2 10 4 km 2 are covered by well developed desert pavements these surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m y major factors that reduce erosion converge in these regions extreme hyperaridity tectonic stability fl at and horizontal surfaces i e no relief and effective surface armoring by a clast mosaic of highly resistant lithology the 10 be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern negev israel representing the sahara arabia deserts indicate simple exposure ages of 1 5 1 8 ma or correspond to maximum erosion rates of 0 25 0 3 m m y 1 the 36

assessment of erosion and deposition in steep mountain basins by differencing sequential digital terrain models abstract digital elevation models dems built from repeated topographic surveys permit producing dem of difference dod that enables assessment of elevation variations and estimation of volumetric changes through time in the framework of sediment transport studies dem differencing enables quantitative and spatially distributed representation of erosion and deposition within the analyzed time window at both the channel reach and the catchment scale in this study two high resolution digital terrain models dtms derived from airborne lidar data 2 m resolution acquired in 2005 and 2011 were used to characterize the topographic variations caused by sediment erosion transport and deposition in two adjacent mountain basins gadria and strimm vinschgau venosta valley eastern alps italy these catchments were chosen for their contrasting morphology and because they feature different types and intensity of sediment transfer processes a method based on fuzzy logic which takes into account spatially variable dtms uncertainty was used to derive the dod of the study area volumes of erosion and deposition calculated from the dod were then compared with post event field surveys to test the consistency of two independent estimates results show an overall agreement between the estimates with differences due to the intrinsic approximations of the two approaches the consistency of dod with post event estimates encourages the integration of these two methods whose combined application may permit to overcome the intrinsic limitations of the two estimations the comparison between 2005 and 2011 dtms allowed to investigate the relationships between topographic changes and geomorphometric parameters expressing the role of topography on sediment erosion and deposition i e slope and contributing area and describing the morphology influenced by debris flows and fluvial processes i e curvature erosion and deposition relations in the slope area space display substantial differences between the gadria and the strimm basins while in the former erosion and deposition clusters are reasonably well discriminated in the latter characterized by a complex stepped structure we observe substantial overlapping erosion mostly occurred in areas that show persistency of concavity or transformation from convex and flat to concave surfaces whereas deposition prevailingly took place on convex morphologies less expected correspondences between curvature and topographic changes can be explained by the variable sediment transport processes which are often characterized by alternation of erosion and deposition between different events and even during the same event

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

avulsions and the spatio temporal evolution of debris flow fans debris flows are water laden masses of sediment that move rapidly through channel networks and over alluvial fans where they can devastate people and property episodic shifts in the position of a debris flow channel termed avulsions are critical for debris flow fan evolution and for understanding flow hazards because avulsions distribute debris flow deposits through space and time however both the mechanisms of flow avulsion and their effects on the long term evolution of debris flow fans are poorly understood here we document and analyze the spatial and temporal patterns of debris flow activity obtained by repeat topographic analyses dendrogeomorphic and lichenometric reconstructions and cosmogenic radionuclide dating on 16 fans from japan usa switzerland france and kyrgyzstan where possible we analyze the observed spatio temporal patterns of debris flow activity in conjunction with high resolution topographic data to identify the main controls on avulsion we identify two main processes that control avulsions on debris flow fans operating over distinct time scales first during individual flows or flow surges deposition of sediment plugs locally blocks channels and forces subsequent flows to avulse into alternative flow paths plug deposition is a stochastic process but appears to depend in part on the sequence of flow volumes the geometry of the channel and the composition of the flows second over time scales of tens of events the average locus of debris flow deposition gradually shifts toward the topographically lower parts of a fan highlighting the importance of topographic compensation for fan evolution our documented debris flow avulsions often but not always follow a pattern of channel plugging backstepping of deposition toward the fan apex over one or more flows avulsion and establishment of a new active channel large flows can have contrasting impacts depending on whether or not they follow smaller flows that have deposited channel plugs these results suggest that avulsions and spatio temporal patterns of debris flow fan formation strongly depend on both the magnitude frequency distribution and the sequence of the flows feeding a fan while individual avulsions are generally abrupt and difficult to predict the presence of debris flow plugs and patterns of backstepping may be useful as indicators of impending avulsions over longer time scales the compensational tendency of flows to avulse into topographic depressions on the fan may also be used to identify sectors of the fan that are at risk of future inundation

comparison of methods to determine q in shallow marine sediments from vertical reflection seismograms four methods for the determination of q in marine sediments are compared two traditional methods i e the risetime and the spectral ratio method and two newly established ones the spectrum modeling and the wavelet modeling method in the latter one q and the reflection time t are determined simultaneously which gives a much better accuracy for t than reading it from the seismogram the risetime and the spectral ratio methods are used for obtaining q directly from the data the principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of q and the best fit between the observed and the calculated data leads to the optimum result numerical tests on synthetic data show that a precision of more than 25 for data containing noise or superposed arrivals can hardly be achieved in any case wavelet modeling is the superior method application to data from a vertical reflection profile in the baltic sea yields q in the range of 15 100 for different layers which is to be expected in the sedimentary environment of this area

discussion of an interpretation of the variscan structures of sw england discussion concernant un article publie dans le j geol soc london vol 139 4eme partie 1982 pp 533 541

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

tectonic overpressure and underpressure in lithospheric tectonics and metamorphism the lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes here various aspects of non lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature the main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones they are in particular responsible for some geological processes which are otherwise difficult to explain such as downward water suction into the interior of subducting slabs magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to 100 of the lithostatic pressure and up to a gpa level these magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process rheological heterogeneity of deforming rock units which is common in nature has a tendency to enhance overpressure and underpressures large overpressure can typically be expected in rheologically strong dry bending rock units in particular in the mantle lithosphere however rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks therefore the influence of significant tectonic overpressure and or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono metamorphic scenarios many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface some controversy exists concerning overpressure magnitudes for the ultrahigh pressure uhp rocks and several conflicting hypotheses are proposed which need to be thoroughly tested in the future in this respect the alpine region may offer a unique opportunity for the testing of geological scale overpressures in u hp rocks by combining structural geological and petrological data with realistic lithospheric scale numerical modelling

geomorphologic evidence for the late pliocene onset of hyperaridity in the atacama desert the atacama desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on earth but the duration of this aridity is poorly constrained we reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features alluvial fans hillslope soils soil chemistry river profiles in the southern portion of the present desert topographic reconstructions of a large watershed 11 000 km 2 show deep incision and sediment removal between the late miocene and the end of the pliocene and modest to negligible incision in post pliocene times these changes in incision suggest an 50 280 reduction in river discharge which should reflect corresponding changes in precipitation changes in the nature of hillslope soils in the atacama desert indicate that in the pliocene or earlier hillslopes were mantled with silicate derived soil this mantle was stripped off and locally deposited as alluvial fans late pliocene to early pleistocene that now block or otherwise cause a rearrangement of pliocene and earlier river channels finally the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late pliocene stripping suggesting a decline in annual precipitation of at least 125 mm yr 1 or more mean annual precipitation map is now 1 embedded in the long post pliocene era of salt accumulation there are a variety of features suggesting overland flow on hillslopes rills striped gravel deposits piping and water spouts and large infrequent storms that infiltrated gentle alluvial fans due to the depth of salt rich horizons despite evidence for episodes that punctuate the hyperaridity the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate chloride and nitrate the late pliocene cessation of many fluvial features is coincident with recent research on the tropical pacific which shows that the pacific was in a permanent el nino state until ca 2 2 ma at which time sea surface temperatures offshore of south america declined greatly relative to those of the western pacific in turn setting up the present el nino southern oscillation enso climate system these observations indicate that the latest period of aridity has been prolonged and largely continuous and it appears to have occurred in step with the onset of the enso climate system beginning 2 m y ago

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

rifting assisted by shear heating and formation of the lomonosov ridge abstract we suggest that thermo mechanical feedback between shear heating and temperature dependent viscosity in continental zones of oblique extension has the potential to significantly weaken the lithosphere and facilitate subsequent rapid breakup we perform thermo mechanical experiments of strike slip and coeval but longer lasting extension using the finite element method the style of rifting in our model depends mainly on the efficiency of shear heating which in turn depends on the magnitude of lithospheric stresses and strain rates we develop and apply a numerical model to explain the formation of the lomonosov ridge microcontinent our proposed model provides a physically consistent mechanism for the detachment of the lomonosov ridge from the barents kara sea margin at 68 56 ma the narrow rift mode predicted by the numerical model is supported by the crustal structure of conjugate passive margins of the eurasia basin

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

slab dip vs lithosphere age no direct function one paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones with the negative buoyancy proportional to the age of the oceanic lithosphere we measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth we show here that this relationship is far more irregular than previously suggested and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere younger oceanic lithosphere may show steeper dip than older segments of slabs e g central america vs south america in contrast with predictions of models considering only slab pull the combination of slab age and subduction rate better accounts for slab dip however the correlation is not satisfactory correlation coefficient equal to 0 450 these results suggest that supplemental forces or constraints have to be accounted for such as thickness and shape of the hangingwall plate absolute plate velocity presence of lateral density variations in the hosting upper mantle effects of accretion erosion subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate d 2005 elsevier b v all rights reserved

convection in the mantle wedge above the slab and tectonic processes in subduction zones some nonconductive heat transport in the mantle wedge above the subducting lithosphere is necessary to explain the volcanic and thermal activities in subduction zones the physical mechanisms that have been proposed to explain the observed thermal processes are classified into the following three categories 1 secondary convection mechanically driven by the motion of the slab 2 thermal convection due to a large horizontal temperature gradient associated with the presence of the cool slab and 3 flow induced by ascending mantle diapirs containing low density melt the relative importance of these three mechanisms is evaluated using a simple model in which the conduit of the convective flow is prescribed the first mechanism of mechanically driven flow dominates the other two when the effective viscosity of the mantle wedge exceeds 1020 pa s 1021 p known estimates of wedge viscosity suggest that either the second or third mechanism is more likely to be the one that actually governs the tectonic process it is proposed that both the second and third mechanisms are realized in different stages of a single tectonic sequence and that arc volcanism and back arc spreading occur corresponding to these different modes of convection

effect of hydrothermal circulation on slab dehydration for the subduction zone of costa rica and nicaragua abstract dehydration of subducting oceanic plates is associated with mantle wedge melting arc volcanism intraslab earthquakes through dehydration embrittlement and the flux of water into the mantle in this study we present two dimensional thermal models of the costa rica nicaragua subduction zone to investigate dehydration reactions within the subducting cocos plate seismic and geochemical observations indicate that the mantle wedge below nicaragua is more hydrated than that below costa rica these trends have been hypothesized to be due to a variation in either the thermal state or the hydration state of the subducting slab despite only small variations in plate age along strike heat flow measurements near the deformation front reveal significantly lower heat flow offshore nicaragua than offshore costa rica these measurements are interpreted to reflect an along strike change in the efficiency of hydrothermal circulation in the oceanic crust we parameterize thermal models in terms of efficient and inefficient hydrothermal circulation and explore their impact on slab temperature in the context of dehydration models relative to models without fluid flow efficient hydrothermal circulation reduces slab temperature by as much at 60 c to depths of 75 km and increases the predicted depth of eclogitization by 15 km inefficient hydrothermal circulation has a commensurately smaller influence on slab temperatures and the depth of eclogitization for both regions the change in eclogitization depth better fits the observed intraslab crustal seismicity but there is not a strong contrast in the slab thermal structure or location of the main dehydration reactions consistent with other studies these results suggest that observed along strike differences in mantle wedge hydration may be better explained by a northwestward increase in the hydration state of the cocos plate before it is subducted

coarse sediment connectivity in river channel systems a conceptual framework and methodology identification of coarse sediment connectivity down river channel systems is important for understanding the linkages between river reaches the influence of sediment sources on channel morphology and the mechanisms and liability to propagation of morphological change a conceptual framework is developed in which several types and degrees of connectivity are proposed it is hypothesised that sediment connectivity varies with sediment sources and with stream competence to transport the coarse fractions a method of identifying the status of reaches is developed based on field mapping of morphology and sedimentology the analysis is exemplified by two channels one an ephemeral channel in se spain the other a perennial channel in nw england questions arise over whether reaches that lack channel stores transfer coarse sediment three scenarios are suggested 1 lack of flux due to lack of competence of the reach 2 flushing through of sediment due to high competence and 3 potential transport of coarse sediment but exhaustion or lack of availability changes in connectivity will take place over different time scales depending on the nature of exchange and time scales of storage short term variations in connectivity can be identified by repeated mapping

the composition and structure of volcanic rifted continental margins in the north atlantic further insight from shear waves abstract imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non volcanic rifted and convergent margins massive volcanism occurred during break up at 70 of the passive margins bordering the atlantic ocean the causes and dynamics of which are still debated this paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four component ocean bottom seismometers along two north atlantic continental margin profiles this traveltime tomography was performed using two different approaches the first a flexible layer based parameterisation enables the quality control of traveltime picks and investigation of the crustal structure the second with a regularised grid based parameterisation requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via monte carlo analyses the velocity models indicate high lower crustal velocities and sharp transitions in both velocity and vp vs ratios across the continent ocean transition the velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins interpretation of the high quality seismic reflection profile on the faroes margin confirms that this mixing is through crustal intrusion converted shear wave data also provide constraints on the sub basalt lithology on the faroes margin which is interpreted as a pre break up mesozoic to paleocene sedimentary system intruded by sills

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

linking environmental regimes space and time interpretations of structural and functional connectivity connectivity as a concept has been increasingly part of discussions or explanations in hydrology geomorphology and ecology we address recent critiques of this approach by demonstrating how a refinement which distinguishes structural connectivity from functional connectivity can be used to explain patterns observed in very different environmental systems these systems are found in linkages between surface and subsurface flowpaths and the hyporheos in the river don a temperate river channel in yorkshire uk in surface and subsurface fluxes in agricultural land in the uk and in vegetation and surface conditions in a degrading environment at the sevilleta lter site in the semi arid southwest usa first we demonstrate long term geological and structural controls mediated by in channel processes second human organization of landscape elements is a significant control on runoff and erosion so that similar events can produce very different responses and vice versa third linkages between the removal of grass vegetation and runoff and erosion produce non linear and path dependent feedbacks which control the subsequent degradation of the landscape making the process difficult to reverse as a result of these studies we argue that even in cases where connectivity cannot be directly quantified at least at present this limitation does not prevent the concept from being a useful heuristic device for exploring responses of complex systems furthermore this result implies that an increasing need exists for disciplinary connectivity to investigate such systems

estimates of upper crustal heterogeneity in the baltic shield from seismic scattering and borehole logs abstract constraints on small scale heterogeneity in the upper crust of the baltic shield were obtained from stochastic modelling of a variety of seismic data from the babel and siljan ring surveys a non linear least squares inversion scheme was applied to wavefield fluctuations in seismic reflections to obtain statistical parameters of the medium through which the wave energy propagated the reflections are from dolerite sills intruded at depths of 6 12 km into the palaeoproterozoic upper crust of the central baltic shield analysis of the babel data set reveals that the upper crust is a weakly scattering stochastic medium with velocities characterised by an exponential autocorrelation function with a correlation length of 150 50 m and rms velocity perturbation of 1 5 0 5 further inversions show that most of the scattering occurs within the upper few kilometres of the crust analysis of the sijan seismic data set reveals heterogeneity with length scales of 210 50 m and a velocity perturbation of about 1 0 the autocovariance and power spectrum of the sonic log and studies of frequency dependent q in vsp data from the gravberg 1 borehole give consistent estimates of basement heterogeneity the upper crust in this region is characterised by a correlation distance of 35 5 m and rms velocity perturbation of 4 6 in the uppermost 1 2 km of crust associated with near surface fracturing and a correlation distance of 140 50 m and rms velocity perturbation of 1 5 0 5 in the uppermost 6 8 km associated with compositional heterogeneity

quantification of l band insar coherence over volcanic areas using lidar and in situ measurements abstract interferometric synthetic aperture radar insar is a powerful tool to monitor large scale ground deformation at active volcanoes however vegetation and pyroclastic deposits degrade the radar coherence and therefore the measurement of 3 d surface displacements in this article we explore the complementarity between alos palsar coherence images airborne lidar data and in situ measurements acquired over the piton de la fournaise volcano reunion island france to determine the sources of errors that may affect repeat pass insar measurements we investigate three types of surfaces terrains covered with vegetation lava flows a a pahoehoe or slabby pahoehoe lava flows and pyroclastic deposits lapilli to explain the loss of coherence observed over the dolomieu crater between 2008 and 2009 we first use laser altimetry data to map topographic variations the lidar intensity which depends on surface reflectance also provides ancillary information about the potential sources of coherence loss in addition surface roughness and rock dielectric properties of each terrain have been determined in situ to better understand how electromagnetic waves interact with such media rough and porous surfaces such as the a a lava flows produce a higher coherence loss than smoother surfaces such as the pahoehoe lava flows variations in dielectric properties suggest a higher penetration depth in pyroclasts than in lava flows at l band frequency decorrelation over the lapilli is hence mainly caused by volumetric effects finally a map of lai leaf area index produced using spot 5 imagery allows us to quantify the effect of vegetation density radar coherence is negatively correlated with lai and is unreliable for values higher than 7 5

crustal laminations in deep seismic profiles in france and neighbouring areas summary remarkable crustal features appear on the ecors profiles carried out in northern france and the bay of biscay as well as on the swat profiles shot in the western channel and the celtic sea the most striking one is the occurrence of flat laminations in the lower crust dipping events and laminations are also present in the upper and lower crust especially in the swat profiles they can readily be related to tectonic events variscan in age some of them identified in the field the flat laminations in the lower crust are at first interpreted as resulting from delamination shearing magmatism and metamorphism at the crust mantle transition during the variscan orogeny this interpretation raises some difficulty concerning the space and time correlation of the laminations with the variscan orogeny they seem to have been emplaced after the permian triassic infilling of the plymouth bay basin and before the early cretaceous opening of the bay of biscay an early to middle jurassic age is suggested a period when large cratonic basins were formed without noticeable extension heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations choosing between orogenic and non orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination

permian triassic high thermal regime in the alps result of late variscan collapse or continental rifting validation by numerical modeling 1 the permian triassic igneous activity associated with regional scale deformation developed under high temperature low pressure ht lp metamorphic conditions and widely recorded in the pre alpine crust of the european alps can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process in order to reduce this ambiguity we use a two dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean continent convergence followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the paleozoic mesozoic evolution of the alpine area the modeling predictions are compared with the pt climax conditions of variscan and permian triassic metamorphism affecting the continental crust of the helvetic to southalpine domains the good agreement between model predictions and natural data realized for the early to neovariscan evolution indicates that during paleozoic the pre alpine crust of the alps was part of an active ocean continent convergence margin and an intracontinental suture zone furthermore modeling results support the interpretation envisaging a permian triassic lithospheric extension as responsible for the ht metamorphism and related intense igneous activity this evolution was precursor of the mesozoic oceanization during which the tectonic units coupled and accreted during the variscan subduction and collision were separated to form the two passive european and adriatic continental margins

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

effective q determination using frequency methods on birps data abstract an investigation to determine q for compressional waves from marine reflection seismic records is presented the spectral ratio method is used to determine values for the lower crust results from spectral modelling from a known synthetic source wavelet are used to give minimum estimates for the whole crust the effective q for the reflective lower crust around britain is found to be 500 200 which is consistent with laboratory results for dry rocks and is probably too large to allow significant amounts of fluid to be present

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

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

the alps 2 controls on crustal subduction and ultra high pressure rock exhumation in alpine type orogens building on our previous results we use 2 d upper mantle scale thermomechanical numerical models to explore key controls on the evolution of alpine type orogens and the alps per se focusing on ultra high pressure u hp metamorphic rocks the models show that uhp rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit here we test the sensitivity of the models to surface erosion rate crustal heat production plate convergence divergence rates geometry of the subducting continental margin and strength of the retrocontinent surface erosion affects crustal exhumation but not early buoyant exhumation metamorphic temperatures increase with crustal radioactive heat production maximum burial depth prior to exhumation increases with plate convergence rates but exhumation rates are only weakly dependent on subduction rates onset of absolute plate divergence does not trigger exhumation in these models we conclude that contrasting peak pressures exhumation rates and volumes of u hp crust exhumed in the alps orogen primarily reflect along strike contrasts in the geometry thermal structure and or strength of the subducting microcontinent brianconnais and continental european margin crust the experiments also support the interpretation that the western alps u hp internal crystalline massifs exhumed as composite stacked plumes and that these plumes drove local crustal extension during orogen scale shortening for weak upper plate retrocrusts postexhumation retrothrusting forms a retrowedge overall these results are consistent with predictions using the exhumation number ratio of buoyancy to side traction forces in the conduit which expresses the combined parameter control of the depth volume of crustal subduction and the transition to buoyant exhumation

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

seismic attenuation in the vicinity of the geothermal anomaly at urach obtained from near vertical reflection profiles1 the attenuation in the vicinity of the geothermal anomaly at urach was determined by means of two near vertical reflection profiles the attenuation in the sediments and in the upper crust 3 4 km depth was estimated by interpretation of the first refracted arrivals for calculating the attenuation the amplitude decay with respect to distance was used corrections for the spread factor i e the geometric amplitude divergence was deduced from the traveltime curves below the anomaly higher attenuation values q 1 0 008 were observed compared with those in the undisturbed crust q 1 0 002 this effect is probably due to the cracks and fissures in the upper part of the crystalline basement the attenuation in the middle and lower crust was determined using near vertical reflections from this depth interval the use of the spectral ratio method leads to higher values of the effective attenuation q 1eff below the heat flow anomaly compared to those of the normal crust this zone of high q 1eff coincides with the low velocity body below the heat flow anomaly both effects the higher attenuation and the lower velocities could be caused by high temperatures cracks and fissures in the crust

geomorphology on geologic timescales evolution of the late cenozoic pacific paleosurface in northern chile and southern peru abstract the atacama desert on the western margin of the central andes is one of the driest and oldest deserts in the world it is defined by a distinct and ancient surface known as the pacific paleosurface pps or atacama paleosurface the age of this surface is determined as the time at which sediment deposition ceased and the surface was effectively abandoned early studies suggested that this abandonment took place between 14 and 10 ma and was related to both the uplift of the andes and the onset of hyperaridity in the region here we provide a regional re examination of the pps compiling existing work on the underlying geology sedimentology surface exposure dating and seismic profiling we also present new multispectral satellite maps of the pps and 45 new cosmogenic 3he and 21ne surface exposure ages in order to constrain the formation age and the preservation and incision history of the paleosurface we conclude that the pps is not a single paleosurface but instead is a mosaic of smaller surfaces that were formed by aggradational and degradational processes over 19 million years or more and should be termed collectively as the pacific paleosurfaces the time at which individual paleosurfaces formed is related to regional climate where the location of each is controlled by regional tectonic activity cosmogenic surface exposure ages suggest that the surfaces are a record of regional scale climate events

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

variability in erosion rates related to the state of landscape transience in the semi arid chilean andes we quantify erosion rates in the higher sectors of the huasco valley in the main cordillera of the semi arid andes of chile using elevation differences between three successive geomorphic markers pediments and paleo valleys and the present day valley available ar ar ages of neogene pediments are used to estimate mean erosion rates for the three periods 16 to 13 my 13 to 8 my and following 8 my the landscape of the huasco valley is in a transient state as indicated by well preserved pediment surfaces in interfluves valleys deeply incised by fluvial and glacial erosion and scarped head valleys that represent the current knickzones higher erosion rates 45 75 m my are calculated for the more recent period 8 my during which deep incision developed compared to previous periods 6 31 m my quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the main cordillera comparison with erosion rates calculated in other drainage basins of the chilean andes suggests that the variability of erosion rates depends on the landscape s transient erosive state the landscape s geomorphologic response to the uplift of the main cordillera results in the retreat of a knickzone for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity copyright 2011 john wiley sons ltd

reflectivity of the crystalline crust hypotheses and tests summary the nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms but most geological information still comes from speculative interpretations of seismic experiments run in different regimes pronounced lower crustal reflectivity detected worldwide is ascribed in various geological hypotheses to primary lithologic layering to ductile strain banding or to trapped fluids a birps deep crustal profile across the atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone study of worldwide crustal profiles shows that exposed high pressure terranes are not as reflective as in situ lower crust at high pressure suggesting either that these granulite terranes are not representative of the lower crust or that physical properties possibly the presence of fluids or thermally controlled ductile strain banding are more likely responsible for observed reflectivity than are simple lithologic boundaries the argument for the importance of physical properties in causing observed lower crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat flow 1 geophysical constraints the notion that geometric effects thin layer interference and focussing in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature because many reflections are observed on deep crustal profiles velocity and density cannot increase monotonically with depth while remaining within bounds set by refraction experiments implying the existence of a funtion of velocity with depth that is oscillatory while the concept thus developed of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity for normal incidence no structure embedded in a constant impedance medium gives a stronger reflection than a quarter wavelength layer which produces an amplitude twice that due to a simple interface complex structures can increase the total energy reflected but the energy is spread over longer times and the peak reflected amplitude is not increased observed reflection amplitudes from the lower crust vary by factors u2 so that the presence or absence of thin layer interference cannot be the only effect causing lower crustal

lower crustal reflectivity from waveform inversion summary deep seismic reflection profiles show that the lower part of the continental crust often contains many strong subhorizontal reflections based on forward modelling these reflections are interpreted as a sequence of alternating high and low velocity layers a waveform inversion provides an optimum model by minimizing the misfit between observed data and synthetic seismograms however it is very difficult to invert for the whole data set partly because of the large computation time needed in modelling and partly because the high amplitude reflected energy in the upper crust dominates the misfit function to overcome these difficulties we propose a three step strategy for inverting the deeper part of seismic reflection data over a narrow aperture in which the velocity structure can be assumed to be 1 d first both source wavelet and recorded wavefield are propagated in the plane wave domain downwards to a particular depth for a given 1 d medium because the source receiver distances are short for deep reflections it is assumed that these reflections are at normal incidence it is also assumed that the near surface velocity is known or has been estimated using other inversion schemes the downward propagation algorithm takes account of nearsurface effects such as multiple reflections and attenuation second starting from a known background velocity model a non linear iterative waveform inversion is applied to the propagated data set the method is based on minimizing the difference sample by sample between observed and calculated wavefields in a least squares sense the forward modelling which consists of calculating the data from the model uses a reflectivity method to compute the synthetic seismogram in the plane wave domain the model perturbation is calculated with the help of the conjugate gradients method the non linear inversion gives the short wavelengths of impedance contrast which is a function of density and velocity in the lower crust third an interpretational step is tried in order to recover the medium wavelengths of velocity the algorithm is tested on synthetic data and then applied to a part of the wam data set south west of england a good fit to the real data with the optimum model predicts that the velocity contrast of the layers in the lower crust varies from 0 2 km s to 0 5 km s and the thickness from at least 0 075 km to 1 km

some attributes of wavefields scattered from ivrea type lower crust abstract the ivrea zone in northern italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of alpine lithospheric shortening based on two 1 25 000 geological maps from the central ivrea zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained stochastic seismic models of ivrea type lower crust a stochastic rather than a deterministic approach is applied due to the limited availability of detailed geological maps and the high degree of small scale structural and petrophysical complexity of the ivrea zone the primary characteristic of the resulting model is its layered self affine or fractal structure and its bimodal velocity distribution while we are not suggesting that ivrea type structure is universal for extended lower continental crust synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data 1 1 ivrea type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust 2 2 observed lower crustal q factors can be largely explained by scattering losses and intrinsic q factors of reflective lower crust are likely to be larger than 1000 which is incompatible with the presence of free volatiles 3 3 in agreement with recent broad band studies of the lower crust ivrea type lower crust does not create any tuning effects and 4 4 for ivrea type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures which suggests that multiple scattering may be important in the reflective lower crust

tectonic setting of post orogenic granites within sw fennoscandia based on deep seismic and gravity data within the svecononvegian province of sw scandinavia granite intrusions are abundant prominent ones are the bohus iddefjord and the fla granites the so called bohus fla granite belt the age of these granites consistent at 900 ma coincides with the late extensional stage of the svecononvegian grenvillian orogen gravity observations and deep seismic profiling lines are presented that also cover the skagerrak sea the geophysical data suggest that the bohus granite continues seaward for at least 80 km and its thickness offshore is estimated to be 2 4 km where the seismic profiling lines intersect the inferred seaward extension of the bohus granite a distinct reflection pattern is observed at 1 6 s twt this coincides with a gravity modelled thickness of 4 km the seismic profiles also show a large moho offset beneath the modelled granite it is proposed that this offset is related to svecononvegian crustal underthrusting and that the granite melt could have formed by anatexis of mid crustal rocks downthrusted to greater depths in the vicinity of the seismically observed moho offset

an eulerian technique for thermomechanical modeling of lithospheric extension a two dimensional eulerian spline function finite element method for modeling the coupled thermomechanical processes which control lithospheric extension is presented the model domain comprises a cross section through the lithosphere and part of the asthenosphere material may enter or leave through the bottom and side boundaries on the side boundary a fixed velocity or a fixed force condition can be applied highly nonlinear and thermally activated creep laws are used for the brittle and ductile rheologies of the lithosphere compositional differences between crust and mantle are described by a continuous function which is advected and which affects density rheology and heat production an approximate topography is calculated from the normal stress at a free slip surface boundary a set of sample case studies is presented which highlights the capabilities and possible applications of the method it also serves to study the influence of numerical parameters and of conceptual assumptions in various nontrivial applications the examples comprise narrow rifting continuing into the stage of seafloor spreading formation of a wide basin formation of a metamorphic core complex and impact of a mantle plume onto the bottom of the lithosphere in the last case the entire upper mantle is modeled and the instability of a thermal bottom boundary layer gives rise to a diapiric mantle plume the main weakness of the eulerian approach as compared to the lagrangian technique was found to be the effective blurring of material discontinuities like the moho this is of little consequence in pure shear deformation but when strong simple shear occurs it impairs the resolution the main advantage of the eulerian technique is that very large deformation poses no problem as it does for the lagrangian method the new approach is especially suited to study processes where strong differential flow occurs in part of the modeled system a further possible advantage of the method is its efficiency in terms of computer time

modeling the dynamics of subducting slabs cold dense subducting lithosphere provides the primary force driving tectonic plates at earth s surface the force available to drive the plates depends on a balance between the buoyancy forces driving subduction and the mechanical and buoyancy forces resisting subduction because both the buoyancy and rheology of the slab and mantle depend on temperature composition grain size water content and melt fraction unraveling which of these processes exert a first order control on slab dynamics and under what circumstances other processes become first order effects can be challenging laboratory and numerical models of slab dynamics provide a powerful method for testing the combined effects of buoyancy and strength changes that accompany the slab evolution in the upper mantle transition zone and lower mantle recent studies have focused on understanding how rheologic variations newtonian versus non newtonian viscosity or water content geometry 2d versus 3d and plate motions trench roll back or advance influence the evolution of slabs in the upper mantle and how they sink into the lower mantle these models suggest that spatial and temporal variations in slab strength and the history of subduction determine whether slabs sink directly into the lower mantle or are trapped in the transition zone

fluid flow during unbending implications for slab hydration intermediate depth earthquakes and deep fluid subduction abstract we calculate the tectonic stress profile and associated direction of fluid flow during unbending and dehydration of oceanic plates for a range of critical parameters that affect their combined elasto plastic and viscous behaviour such as bending curvature age pore fluid pressure and viscous flow laws in all models negative pressure gradients are established at moho depths down to the base of the slab elastic core fluids released at these depths flow downward across the plate wetting or further hydrating the underlying dry levels and ultimately accumulate at the base of the elastic core increasing the pore fluid pressure and triggering deep seismicity the thickness of the wet layer increases for low bending curvatures low pore fluid pressure old slabs and dry viscous rheologies wetting of the upper 10 30 km of the slab has important implications for its rheological and anisotropic structure unbending favours the redistribution and trapping of significant amounts of fluids in subducting oceanic plates that will subsequently be released at the base of the upper mantle

numerical simulations of an ocean continent convergent system influence of subduction geometry and mantle wedge hydration on crustal recycling the effects of the hydration mechanism on continental crust recycling are analyzed through a 2 d finite element thermomechanical model oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method hydration is accomplished by lawsonite and serpentine breakdown topography is treated as a free surface subduction rates of 1 3 5 7 5 and 10 cm yr slab angles of 30 45 and 60 and a mantle rheology represented by dry dunite and dry olivine flow laws have been taken into account during successive numerical experiments model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material the larger the viscosity contrast between hydrated and dry mantle the larger the percentage of recycled material into the mantle wedge slab dip variation has a moderate impact on the recycling metamorphic evolution of recycled material is influenced by subduction style tpmax generally representative of eclogite facies conditions is sensitive to changes in slab dip a direct relationship between subduction rate and exhumation rate results for different slab dips this relationship does not depend on the used mantle flow law thermal regimes predicted by different numerical models are compared to pt paths followed by continental crustal slices involved in ancient and recent subduction zones making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

lg wave attenuation in britain summary the lg wave quality factor qlg in britain has been modelled using data from the uk seismic network operated by the british geological survey the data set consists of 631 vertical mostly short period recordings of lg waves from 64 earthquakes 2 7 4 7 ml and 93 stations we have inverted for both regional average qlg and tomographic images of qlg and simultaneously a source term for each event and a site term for each station for 22 frequencies in the band 0 9 10 0 hz the regional average model is 266 f 0 53 between 1 0 and 10 0 hz and indicates that attenuation in britain is slightly higher than in france and significantly higher than in eastern north america and scandinavia tomographic inversions at each frequency indicate that qlg varies spatially broadly speaking southeastern england the lake district and parts of the east irish sea basin and a small region between the highland boundary fault and the southern uplands fault are characterized by higher than average attenuation southwestern england eastern central england and northwestern scotland are regions of relatively low attenuation to some extent these regions correlate with what is known about the tectonics and structure of the crust in the uk

exhumation by debris flows in the 2013 colorado front range storm the relative importance of extreme events in the long term exhumation of landscapes is difficult to determine the degree to which modern records capture long term mean process rates is therefore commonly unknown in september 2013 a massive precipitation event in the front range of colorado usa triggered over 1100 landslides and debris flows in a historically sedate landscape here we employ topographic differencing utilizing repeat aerial lidar to quantify net sediment transport volumes in 120 of these mass movements within an area of 102 km 2 west of boulder colorado comparing our results against published long term erosion rates we find that these mass movements evacuated the equivalent of hundreds to thousands of years of hillslope weathering products we conclude that 1 rare debris flows perform the majority of sediment transport and channel erosion within steep channels along the eastern edge of the front range potentially explaining discrepancies between modern sediment yields and long term erosion rates in such settings and 2 the high spatial density of debris flows along the corridors bounding steep canyons suggests that the landscape switches to debris flow dominance as knickpoints pass headward along the canyons

major crustal reflectors beneath sw england strong wide angle reflection events were detected during a crustal seismic experiment carried out in sw britain in 1979 three crustal reflectors have been identified referred to as rl r2 and r3 rl lies at a depth of about 8 km and appears to be restricted to the interior of the granite batholith underlying sw england the nature of this reflector is unknown r2 is a mid crustal reflector lying at a depth of 10 15 km and interpreted as rising gradually northwards it coincides locally with the base of the granite as predicted by gravity modeling but extends well beyond the northern limit of the granite body it is suggested that r2 may represent a major thrust of late variscan age r3 is the deepest reflector identified and lies at a depth of 27 30 km r3 is interpreted as the base of the crust and its computed depth is in close agreement with the local depth to the moho derived from an earlier seismic experiment in sw britain

integration of natural data within a numerical model of ablative subduction a possible interpretation for the alpine dynamics of the austroalpine crust a numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the alpine belt the austroalpine domain the model predictions and the natural data for the austroalpine of the western alps agree very well in terms of p t peak conditions relative chronology of peak and exhumation events p t t paths thermal gradients and the tectonic evolution of the continental rocks these findings suggest that a pre collisional evolution of this domain with the burial of the continental rocks induced by ablative subduction of the overriding adria plate and their exhumation driven by an upwelling flow generated in a hydrated mantle wedge could be a valid mechanism that reproduces the actual tectono metamorphic configuration of this part of the alps there is less agreement between the model predictions and the natural data for the austroalpine of the central eastern alps based on the natural data available in the literature a critical discussion of the other proposed mechanisms is presented and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results these factors include variations in the continental and or oceanic thickness variation of the subduction rate and or slab dip the initial thermal state of the passive margin the occurrence of continental collision and an oblique convergence

rill network development and sediment budgets threshold conditions for rill incision are well known but few studies have examined the effect of network geometry on water and sediment fluxes within an evolving rill system this paper reports the first in a series of studies designed to identify the influence of soil properties on rill network and confluence geometry and on water and sediment fluxes in rill systems the object of this study was to prepare detailed rill network water and sediment budgets identifying important sources and sinks simulated rainfall experiments were carried out on a canadian silt loam soil in a 7 1 m 2 4 m flume on a 5 slope rill networks of varying complexity developed which were ultimately constrained by flume boundaries sediment and water fluxes and hydraulic conditions were measured within networks and at a terminal weir networks evolved by initial knickpoint incision in the lower flume as flow shear velocities reached critical levels of 4 5 5 cm s 1 followed by headward migration and tributary development microtopography determined tributary location but the timing and intensity of development were controlled by the incision and migration rate of the main channel which changed local thalwegs and base levels raising shear velocities on side slopes above critical levels sediment discharge at the weir broadly reflected rill incision intensity and transport limited conditions but as active incision moved headward the linkage became attenuated and identification of discrete erosional incidents e g local bank collapse in the signal of the weir record became very difficult detailed water and sediment budgets showed much more complex patterns of localized incision and deposition within networks strongly influenced by local changes in thalweg and in water discharge due to seepage or return flow and by the effect of confluences on hydraulic conditions results indicate the value of detailed sediment budgets in interpreting weir water and sediment flux records and the necessity of linking such measurements for erosion plots and hillslope segments to rill network characterization experimental results are consistent with a simple model of rill system evolution based on channel incision headward migration and the critical shear velocities for rill initiation copyright 2000 john wiley sons ltd

an integrated analysis of the march 2015 atacama floods in march 2015 unusual ocean and atmospheric conditions produced many years worth of rainfall in a 48 h period over northern chile s atacama desert one of earth s driest regions resulting in catastrophic flooding here we describe the hydrologic and geomorphic drivers of and responses to the 2015 atacama floods in the salado river we estimated a flood peak discharge of approximately 1000 m3 s which caused widespread damage and high sediment loads that were primarily derived from valley fill erosion hillslopes remained surprisingly intact despite their lack of vegetation in the coastal city of chanaral flooding of the salado river produced maximum water depths over 4 5 m meters thick mud deposition in buildings and along city streets and coastal erosion the atacama flooding has broad implications in the context of hazard reduction erosion of contaminated legacy mine tailings and the atacama s status as a terrestrial analog for mars

gravity signature of crustal subduction inferred from numerical modelling summary gravitational signatures of subduction are a major feature of the earth s gravity pattern being visible as lineaments parallel to the arc trench systems the novelty of our study resides in the simulation of this subduction fingerprint via a newly developed dynamic subduction model based on the finite element approach which includes the compositional stratification of the lithosphere into a light upper crust and a dense lithospheric mantle crustal material sinks in the mantle in proximity to the top of the slab due to the viscous dragging forces and off scraping of the overriding plate we show that the sinking of light crustal material from both the subducting and overriding plates to depths of the order of 200 km may be responsible for a trough of about 100 mgal extending 100 200 km in a direction perpendicular to the trench this anomaly is superimposed on a global positive gravity anomaly due to thermal contraction of the mantle lithosphere system the negative gravity anomaly due to compositional stratification of the crust has comparable magnitude and horizontal extent of that due to the negative dynamic topography of the trench the gravity patterns from compositional stratification of the crust and from dynamic trench topography are evaluated for different tectonic subduction styles related to active convergence with and without roll back in general we found that case without roll back produces a broader and smoother dynamic topography gravity anomaly than with roll back while the gravity signature from compositional stratification is less sensitive to the style of subduction

interrelation between rifting faulting sedimentation and mantle serpentinization during continental margin formation including examples from the norwegian sea the conditions permitting mantle serpentinization during continental rifting are explored within 2 d thermotectonostratigraphic basin models which track the rheological evolution of the continental crust account for sediment blanketing effects and allow for kinetically controlled mantle serpentinization processes the basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur the isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions a systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures sediment supply has therefore through the temperature dependence of the viscous flow laws strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high in a case study for the norwegian margin we test whether the inner lower crustal bodies lcb imaged beneath the more and voring margin could be serpentinized mantle multiple 2 d transects have been reconstructed through the 3 d data set by scheck wenderoth and maystrenko 2011 we find that serpentinization reactions are possible and likely during the jurassic rift phase predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on lcbs from seismic and gravity data we conclude that some of the inner lcbs beneath the norwegian margin may be partially serpentinized mantle

a systematic 2 d investigation into the mantle wedge s transient flow regime and thermal structure complexities arising from a hydrated rheology and thermal buoyancy 1 arc volcanism at subduction zones is likely regulated by the mantle wedge s flow regime and thermal structure and hence numerous studies have attempted to quantify the principal controls on mantle wedge conditions in this paper we build on these previous studies by undertaking a systematic 2 d numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge s flow regime and associated thermal structure we quantify the role of a range of plausible i water contents 0 5000 h 106si ii subduction velocities 2 10 cm yr and iii upper plate ages 50 120 myr finding that small scale convection ssc resulting from rayleigh taylor instabilities or drips off the base of the overriding lithosphere is a typical occurrence the morphology of ssc varies with viscosity and subduction parameters with drips at their most prominent when subduction velocities and wedge viscosities are low our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner flow regime and strong upper plate erosion below the arc region by contrast we find that back arc upper plate erosion by ssc is largely controlled by wedge viscosity occurring when i viscosities are 5 1018 pa s and ii the length of the upper plate available for destabilization exceeds the characteristic wavelength of instabilities thus if hydrous weakening of wedge rheology extends at least 100 150 km from the trench our 2 d models predict an unstable flow regime resulting in temperature fluctuations of 50 100 k which are sufficient to influence melting and the stability of hydrous minerals

3 d numerical modelling of the conductive heat flow of sw england summary the effect of 3 d geology on conductive heat flow was investigated using the finite element technique on two models from sw england one a regional model centred on the comubian batholith as a whole and the other a more detailed model centred on the cammenellis pluton only two rock types granite and country rock were used to represent the geology of the region and yet good agreement between modelled and observed data was obtained provided that the granite was assumed to be highly homogeneous with surface heat production values persisting throughout its depth and the base of the batholith was assumed to be flat at a depth of 15 km the results show that the surface heat flow pattern is dominated by the 3 d shape of the batholith which controls the relative importance of the two opposing effects that operate at its margins heat refraction due to conductivity contrasts enhancing heat flow in the granite and lateral flow of heat caused by heat production contrasts diminishing it the highest heat flows occur at the boundaries of outcrops facing the axis of the batholith where there is significant heat refraction but little lateral loss a high degree of correlation between mineralized zones and areas of very high heat flow in both granite and country rock is also noticeable the temperature field at any depth displays wide ranging lateral variations over relatively short distances the highest values occurring in areas of granite which are covered by insulating country rock and which lie towards the centre of the batholith regions of highest heat flow are not necessarily the regions where the highest sub surface temperatures are likely to be found and it is shown that the hot dry rock geothermal test project at rosemanowes is not optimally sited from temperature considerations when the constraints on the physical parameters of the model are sufficiently reliable as they are in sw england the heat flow and temperature predictions from 3 d numerical models can be of enormous value in the assessment of the geothermal potential of a region

effective timescales of coupling within fluvial systems this paper presents a review of the coupling concept in fluvial geomorphology based mainly on previously published work coupling mechanisms link the components of the fluvial system controlling sediment transport down the system and the propagation of the effects of base level change up the system they can be viewed at several scales at the local scale involving within hillslope coupling hillslope to channel coupling and within channels tributary junction and reach to reach coupling at larger scales coupling can be considered as zonal coupling between major zones of the system or as regional coupling relating to complete drainage basins these trends are illustrated particularly by the examples of hillslope to channel coupling in the howgill fells northwest england badland systems in southeast spain alluvial fans in spain usa and uae and base level induced dissection of neogene sedimentary basins in southeast spain as the spatial scales increase so do the timescales involved effective temporal scales relate to magnitude and frequency characteristics recovery time and propagation time the relative importance changing with the spatial scale for downsystem coupling at the local scale the first two are important with propagation time increasing in importance in larger systems especially in those involving upsystem coupling related to base level change the effective timescales range from the individual event with a return period of decades through decadal to century timescales for downsystem coupling to tens to hundreds of thousands of years for the basinwide response to base level change the effective timescales influence the relative importance of factors controlling landform development

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

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

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

oligocene miocene age of aridity in the atacama desert revealed by exposure dating of erosion sensitive landforms the age of onset of hyperaridity in the atacama desert chile which is needed to validate geological and climatological concepts has been heretofore uncertain measurement of cosmogenic 21 ne in clasts from erosion sensitive sediment surfaces in northern chile shows that these surfaces have been barely affected by erosion since 25 ma surface exposure ages of sediment clasts give replicate values at 25 20 and 14 ma and individual values at 37 and 9 ma predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on earth our findings are compatible with the hypothesis that the onset of aridity in the atacama desert could be the reason for rather than the consequence of uplift of the high andes

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

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust

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

small scale convection under the back arc occurring in the low viscosity wedge abstract water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly thus we may expect a low viscosity wedge lvw above the subducting slabs the lvw coupled with a large scale flow induced by the subducting slabs may allow the existence of roll like small scale convection whose axis is normal to the strike of the plate boundary such a roll structure may explain the origin of along arc variations of mantle temperature proposed recently in northeast japan we study this possibility using both 2d and 3d models with without pressure and temperature dependent viscosity 2d models without pressure and temperature dependence of viscosity show that with a reasonable geometry of the lvw and subduction speed small scale convection is likely to occur when the viscosity of the lvw is less than 10 19 pa s corresponding 3d model studies reveal that the wavelength of rolls depends on the depth of the lvw the inclusion of temperature dependent viscosity requires the existence of further low viscosity in the lvw since temperature dependence suppresses the instability of the cold thermal boundary layer pressure i e depth dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities the model which shows a relatively moderate viscosity decrease in the lvw most of the lvw viscosity is 10 18 10 19 pa s and a wavelength of roll 80 km has a rather small activation energy and volume 130 kj mol and 4 cm 3 mol of the viscosity this small activation energy and volume may be possible if we regard them as an effective viscosity of non linear rheology

shear heating in extensional detachments implications for the thermal history of the devonian basins of w norway article i nfo the supra detachment basins in western norway were formed during lower to middle devonian in the hangingwall of the extensional system of detachments known as the nordfjord sogn detachment zone nsdz the basins experienced elevated peak temperatures up to 345 c adjacent to the main detachment fault during the terminal stages of the extension in this study we show that the heat generated by the de formation of the rocks within the shear zone of the detachment also known as shear heating played a role in the thermal evolution of the entire region during the extension and could have induced the elevated peak temperatures of the supra detachment basins we numerically analyse the influence of the rheology and the deformation style within the shear zone the rate of exhumation of the footwall and the thickness of the sedimentary accumulation on the top of the system the model reproduces the elevated temperatures recorded in the supra detachment basins where locally 100 c and 25 of the total heat budget can be attrib uted to shear heating we show that this additional heat source may increase the peak temperatures of the hangingwall as far as 5 km away from the detachment