textures of peritectic crystals as guides to reactive minerals in magmatic systems new insights from melting experiments peritectic crystals in igneous rocks may be derived from either the source or country rocks or may have formed by reactive assimilation of source inherited solids primary magmatic minerals during self or magma mixing or country rock xenoliths or xenocrysts identifying such peritectic crystals is important for constraining the components and textures of igneous rocks and the underlying processes of magmatic evolution in this study we demonstrate that peritectic olivine formed in melting experiments crystallizes as clusters of euhedral to subhedral crystals olivine replacing orthopyroxene amphibole and phlogopite forms crystal clusters with distinct crystal to melt ratios 2d surface area grain boundary segmentation and inclusion relations in our experiments the textures of peritectic crystals are primarily controlled by the stability temperature and decomposition rate of reactive minerals high temperature minerals such as orthopyroxene slowly decompose to form high density clusters of large crystals with long grain boundary segments the sio2 rich peritectic melt produced favours formation of melt inclusions low temperature minerals such as amphibole and phlogopite rapidly decompose to form low density clusters of small crystals with short grain boundary segments the relatively sio2 poor peritectic melt produced results in the formation of fewer melt inclusions host melt composition has a minor effect on the textures of peritectic olivine formed in the melting experiments of this study and previous contamination experiments but affects the assemblages of the peritectic crystal clusters cluster density and 2d surface area of peritectic olivine tend to decrease whereas grain boundary segment length increases with increasing experimental temperature and h2o content using textural criteria that distinguish olivine formed after different minerals in our melting experiments we hypothesize that two olivine populations from a basaltic andesitic lava flow of the tatara san pedro volcanic complex chile may be peritectic crystals formed after amphibole and orthopyroxene both amphibole and orthopyroxene are common in xenoliths preserved in some tatara san pedro lava flows one notable difference between the experimental and natural olivine crystals is that the natural olivine crystals have 2d surface areas and 2d grain boundary segments up to 1000 and 100 times larger respectively than those produced in our experiments we propose that this size difference is primarily controlled by comparatively slow heating and decomposition of reactive crystals and textural coarsening of peritectic crystals during prolonged magma residence in the natural system

glacial valley cross profile morphology tian shan mountains china abstract the morphology of glacial valley cross sections can be described in terms of power law y ax b or quadratic equations y a bx cx 2 fitted to empirical data the quadratic solution provides a more robust way of describing the morphology of glacial valley cross sections whereas the power law has more potential in understanding the cross sectional shapes and their evolution these two functions are used to study the cross sectional morphology of glacial valleys in the middle and western tian shan mountains and to discuss the comparison with fluvial channels the major conclusions are 1 power law equation parameters a and b are sensitive to the origin selection with larger sensitivity in vertical and a a ln a values conversely c values of the quadratic equation are more stable regardless of different origins selected 2 hirano and aniya earth surf processes landforms 13 1988 707 716 suggested two characteristic patterns in the relationship between the power law exponent b and the valley form ratio fr however glacial valleys in these areas do not fit the rocky mountain model for b fr values described by hirano and aniya for alpine glacial valleys this indicates that this rocky mountain model cannot be applied to all alpine glacial areas 3 the c values of the quadratic equation represent a curvi linear trend with its corresponding fr s at the same time power law parameters a and b fit a closed linear relationship both from these areas and others in the published literature 4 the cross sectional shapes of glacial valleys show clear differences with fluvial channels by comparing a b values of glacial valleys with the hydraulic geometry of fluvial channels this implies that the a b relationship and the variation range of b values commonly with 1 5 2 5 may be helpful to differentiate valleys formed by different processes

some forms of mountain peaks in the canadian rockies controlled by their rock structure abstract different mountain shapes in sedimentary sequences in the canadian rockies were enhanced by glacial erosion and have been modified post glacially by gravity driven slope processes slope modification by both glacial erosion and post glaciation landslides is related to slope geometry and rock structure particularly bedding dip five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips 1 castellate and 2 matterhorn mountains occur in sub horizontal beds and their slopes are generally 35 65 and oblique to both bedding and joints 3 cuestas develop in gently to moderately dipping beds dip slopes and steeper normal escarpments form their cataclinal and anaclinal sides respectively with the dihedral angle between them about 90 4 hogbacks in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments the dihedral angle between the slopes is usually slope gradients on a sample of 34 peaks are closely related to landslides particularly rock slides and rock topples these landslide modes are controlled by bedding dips

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

postglacial topographic evolution of glaciated valleys a stochastic landscape evolution model the retreat of valley glaciers has a dramatic effect on the stability of glaciated valleys and exerts a prolonged influence on the subsequent fluvial sediment transport regime we have studied the evolution of an idealized glaciated valley during the period following retreat of ice using a numerical model the model incorporates a stochastic process to represent deepseated landsliding non linear diffusion to represent shallow landsliding and an approximation of the bagnold relation to represent fluvial sediment transport it was calibrated using field data from several recent surveys within british columbia canada we present ensemble model results and compare them with results from a deterministic linear diffusion model to show that explicit representation of large landslides is necessary to reproduce the morphology and channel network structure of a typical postglacial valley our model predicts a rapid rate of fluvial sediment transport following deglaciation with a subsequent gradual decline similar to that inferred for holocene time we also describe how changes in the model parameters affect the estimated magnitude and duration of the paraglacial sediment pulse

polynomial and power functions for glacial valley cross section morphology an empirical evaluation of glacial trough cross section shape is performed on seven vertical cross sections in three sierra nevada valleys glaciated during the late quaternary power and second order polynomial functions are fitted by statistical regression power functions are very sensitive to subtle valley bottom topographic features and require precise specification of the valley bottom centre location this dependency is problematic given under representation of valley bottoms by conventional contour sampling methods and the common alteration of valley bottom morphology by non glacial processes power function exponents vary greatly in response to these and other non genetic factors and are not found to be reliable indicators of overall valley morphology second order polynomials express overall valley shape in a single robust function they are applied to both bedrock and sediment floored glacial valleys with negligible statistical bias except where side slopes are stepped or convex upward or where valley form is asymmetrical they can describe alluviated or severely eroded valleys and can objectively identify indi vidual components of polymorphic valleys because valley bottom and centre locations need not be specified mathematical expressions of parameters useful for geomorphic measurements and glaciological modelling are analytically derived from the polynomials as functions of the three polynomial coefficients these parameter equations provide estimates of valley side slopes mean and maximum depth midpoint location width area boundary length form ratio and symmetry

structural tectonic and glaciological controls on the evolution of fjord landscapes abstract the fjord landscape of south america stretching 1500 km between golfo corcovado 43 s and tierra del fuego 56 s is the largest continuous fjord landscape on earth this paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models first order geological structures are represented by strike slip faults forming lineaments up to hundreds of kilometres long the strike slip faulting has been active since late cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to 10 km long a detailed analysis of these second order lineaments from digital image data was carried out in three sectors in sector 1 located northwest of the north patagonian icefield there are three distinct mean orientations characterized by a main nearly orogen parallel orientation az 145 and two orogen oblique secondary orientations az 20 and az 65 in sector 2 located west of the south patagonian icefield there are also three separate mean orientations with most of the lineaments concentrated between azimuths 0 and 80 mean at 36 and two other orogen oblique means at azimuth 122 and 163 in sector 3 around the cordillera darwin there is a single main orogen parallel mean at 100 115 in all three sectors mapped fjord orientations bear a striking similarity to the structural data with fjords orientated preferentially in the same direction as structural lineaments we infer that successive glaciations followed the same ice discharge routes widening and deepening pre existing geological structures at the expense of the surrounding terrain to create the fjord landscape this study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological

valley formation by fluvial and glacial erosion cross valley profiles from the west slope of the olympic peninsula washington are used to investigate the relative effects of fluvial and glacial erosion on valley formation unlike most ranges where glaciers and rivers sequentially occupied the same valleys neighboring valleys in the olympic mountains developed in similar lithologies but were subject to different degrees of glaciation allowing comparison of the net effect of glacial and fluvial processes integrated over many glacial cycles upslope drainage area was used to normalize comparisons of valley width ridge crest to valley bottom relief and valley cross sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley spanning transects valley width relief and cross sectional areas are similar for glaciated partly glaciated and unglaciated fluvial valleys with drainage areas of 10 km 2 but diverge downslope glaciated valleys draining 50 km 2 reach two to four times the cross sectional area and have to 500 m greater relief than comparable fluvial valleys partly glaciated valleys have intermediate dimensions at distances of 5 km from valley heads the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in fluvial valleys the finding of strong differences in the net result of valley excavation by fluvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to pleistocene glaciation

using parabolas to describe the cross sections of glaciated valleys it is widely accepted that the simple power curve y axb can be used to describe the cross section of glaciated valleys there are however good reasons to suppose that this particular form of equation is inappropriate for this purpose

spatial distribution of sediment storage types and quantification of valley fill deposits in an alpine basin reintal bavarian alps germany spatial patterns of sediment storage types and associated volumes using a novel approach for quantifying valley fill deposits are presented for a small alpine catchment 17 km 2 in the bavarian alps the different sediment storage types were analysed with respect to geomorphic coupling and sediment flux activity the dominant landforms in the valley in terms of surface area were found to be talus slopes sheets and cones followed by rockfall deposits and alluvial fans and plains more than two thirds of the talus slopes are relict landforms completely decoupled from the geomorphic system notable sediment transport is limited to avalanche tracks debris flows and along floodplains sediment volumes were calculated using a combination of polynomial functions of cross sections seismic refraction and gis modelling a total of 66 seismic refraction profiles were carried out throughout the valley for a more precise determination of sediment thicknesses and to check the bedrock data generated from geomorphometric analysis we calculated the overall sediment volume of the valley fill deposits to be 0 07 km 3 this corresponds to a mean sediment thickness of 23 3 m the seismic refraction data showed that large floodplains and sedimentation areas which have been developed through damming effects from large rockfalls are in general characterised by shallow sediment thicknesses 20 m by contrast the thickness of several talus slopes is more than twice as much for some locations e g narrow sections of valley the polynomial generated cross sections resulted in overestimations of up to one order of magnitude whereas in sections with a moderate valley shape the modelled cross sections are in good accordance with the obtained seismic data for the quantification of valley fill deposits a combined application of bedrock data derived from polynomials and geophysical prospecting is highly recommended d 2003 elsevier b v all rights reserved

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

investigating the spatial distribution of summit flats in the uinta mountains of northeastern utah usa isolated laterally extensive gently sloping surfaces known as summit flats are present at high elevations in many laramide ranges and are particularly well developed in the uinta mountains of northeastern utah to investigate the spatial distribution of these surfaces and to consider possible controls on this pattern a map of summit flats in the uintas was developed from digital elevation data summit flats were identified as unglaciated areas of the landscape above an elevation of 3400 m having a slope of less than 0 3 m m 1 and an area greater than 510 2 km 2 as defined summit flats comprise 43 of the unglaciated land area above 3400 m in the uintas with the largest individual flat covering nearly 34 km 2 to quantitatively evaluate the distribution of summit flats in the uintas the area of summit flats was normalized to the total unglaciated area above 3400 m in 10 km wide swaths oriented normal to the range axis values of percent summit flats obtained by this method decrease dramatically westward from a high of more than 60 at the eastern end of the uintas to 0 at the western end given that individual summit flats can be diminished through lateral erosion by surrounding valley glaciers and that the summit flats themselves were apparently never glaciated this result suggests that glacial erosion has been more effective in the western uintas over the course of the quaternary focused glacial erosion at the upwind end of the range is consistent with the hypothesis that the proximity of lake bonneville enhanced precipitation over the western uintas during the last glacial maximum munroe j s and mickelson d m 2002 last glacial maximum equilibrium line altitudes and paleoclimate northern uinta mountains utah u s a journal of glaciology 48 257 266

subsidence in magma chamber and the development of magmatic foliation in oman ophiolite gabbros abstract in the oman ophiolite the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified starting in the root zone of the sheeted dike complex rzsdc located above this horizon 18 sections down to the upper gabbros unit have been mapped in great detail in selected areas of the southern massifs of this ophiolite they are complemented by 133 sites located throughout the entire ophiolite where the transition from the rzsdc to the uppermost foliated gabbros is well exposed altogether half the sites and 11 cross sections display within a few tens of meters beneath the rzsdc a magmatic foliation which is parallel to the overlying sheeted dikes in the other sites and cross sections the gabbro foliation is either flat lying or steep but not parallel to the sheeted dikes compared to the rzsdc isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths in the topmost steeply foliated gabbros clinopyroxene is idiomorphic becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende moving down from these uppermost gabbros and over one hundred meters the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized granular gabbros of the gabbro unit these repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying main magma chamber the topmost gabbros beneath rzsdc which were expelled from the melt lens by drifting very soon after settling on the melt lens floor display in plagioclase a spectacular zoning pointing to a fast cooling moving downwards stronger foliation increased compaction and recovery recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude 50 to 500 yr over this vertical distance this field based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis

magma chambers at oceanic ridges how large seismic models of magma chambers at fast spreading oceanic ridges predict small perched magma chambers above a wide low velocity zone seismic attenuation in a low velocity zone allows for 10 melt in hot gabbros in order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the oman ophiolite we propose that the low velocity zone belongs to the magma chamber in such a dynamic magma chamber gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension this process results in a drastic reduction of the melt fraction that is compatible with magmatic flow as confirmed by preliminary experiments we propose here that dynamic magma chambers below ridges may contain as little as 10 melt

the seismic stratigraphy of okanagan lake british columbia a record of rapid deglaciation in a deep fiord lake basin this paper presents the first detailed data regarding the newly discovered deep infill of okanagan lake okanagan lake 50 00 n 119 30 w is 120 km long 3 5 km wide and occupies a glacially overdeepened bedrock basin in the southern interior of british columbia this basin and other elongate lakes of the region e g shuswap kootenay kalamalka canim and mahood lakes mark the site of westward flowing ice streams within successive cordilleran ice sheets an air gun seismic survey of okanagan lake shows that the bedrock floor is nearly 650 m below sea level more than 2000 m below the rim of the surrounding plateau the maximum thickness of pleistocene sediment in okanagan lake basin approaches 800 m forty six seismic reflection traverses and an axial profile show a relatively simple stratigraphy composed of three seismic sequences argued to be no older than the last glacial cycle 30 ka a discontinuous basal unit sequence i characterized by large scale diffractions and up to 460 m thick infills the narrow v shaped bedrock floor of the basin and is interpreted as a boulder gravel deposited by subglacial meltwaters overlying seismic sequence ii is composed of two sub sequences sub sequence iia is a chaotic to massive facies up to 736 m thick lakeshore exposures close to where this unit reaches lake level show deformed and chaotically bedded glaciolacustrine silts containing gravel lens and large ice rafted boulders the surface topography of this sub sequence is irregular and in general mimics the form of the underlying bedrock as a result of compaction this sequence passes laterally into stratified facies sub sequence iib at the northern end of the basin seismic sequence ii appears to record rapid ice proximal dumping of glaciolacustrine silt as the okanagan glacier backwasted upvalley in a deep lake a thin 60 m max laminated seismic sequence iii drapes the hummocky surface of sequence ii and represents postglacial sedimentation from fan deltas the extreme thickness of sequences i and ii in okanagan lake reflects the focussing of large volumes of meltwater and sediment into the basin during deglaciation pre existing sediments that pre date the last glacial cycle appear to have been completely eroded glaciological conditions during sedimentation may have been similar to marine based outlet glaciers calving in deep water in fiord basins in contrast to marine settings where ice bergs are free to disperse large volumes of dead ice were trapped within the basin structural evidence for sedimentation around dead ice blocks has been previously used to argue that the cordilleran ice sheet downwasted in situ we emphasize in contrast the trapping of dead ice left behind by rapidly calving lake based outlet glaciers

ice sheet erosion patterns in valley systems in northern sweden investigated using cosmogenic nuclides erosion patterns associated with glaciation of trunk and hanging valley systems in northern sweden were investigated using cosmogenic nuclide 10be apparent exposure ages and inferred nuclide inheritance sequences of samples taken across valleys known to have been covered repeatedly by the fennoscandian ice sheet revealed two primary patterns of erosion in vavlavagge the exposure age pattern is consistent with 2 m of glacial erosion during the last glacial cycle along the entire profile at ravtasvaggi dievssavaggi and alisvaggi exposure ages in the valley bottom contrast with apparent exposure ages two to four times older on the valley sides the older ages on the valley sides reflect cosmogenic nuclide inheritance due to limited 2 m bedrock erosion of the valley sides during the last glacial cycle the pattern and scale of erosion in these valleys indicates that glacial valley formation is a result of multiple glacial cycles rather than the result of topographic modification during a single glacial cycle initial data comparing hanging valley and trunk valley sites do not show distinct differences in apparent exposure ages slightly older ages for samples from hanging valley bottoms may suggest nuclide inheritance indicating lower erosion than in trunk valley bottoms as would be expected given the marked topographic step between hanging and trunk valleys although quantifying the amount of erosion depends on the assumed cosmogenic nuclide inheritance prior to the onset of erosion the pattern of erosion is independent of this hence the observed pattern of cosmogenic nuclide concentrations provides constraints on spatial patterns of erosion and helps to refine understanding of the timing and extent of landform modification by glaciation copyright 2005 john wiley sons ltd

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

anatectic migmatites from the roof of an ocean ridge magma chamber a well preserved contact aureole at the base of the sheeted dyke is likely to be centred at or very close to the sheeted complex in the troodos ophiolite cyprus records the partial melting dyke gabbro boundary this region of the crust is a of the roof of a mid ocean ridge magma chamber hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10 30 m of the sheeted dyke in rheology and thermal structure it is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust e g deand closest to the underlying gabbros 30 at water undersaturated conditions and temperatures if not all of the lower crust e g phipps morgan chen 875 c geological considerations indicate that the duration of 1993 quick denlinger 1993 boudier et al 1996 melting events was probably on the time scale of eruptive cycles i e and magmatic heat is rapidly transferred across a thin years to centuries this resulted in disequilibrium partial melting 100 m conductive boundary layer e g lowell and highly imperfect melt segregation within the contact aureole burnell 1991 and advectively transported out of the we hypothesize that partial melting is a common process within crust by hydrothermal fluids e g cann et al 1985 the roof zones of axial magma chambers at fast spreading ridges geophysical surveys along the east pacific rise epr and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties e g melt volumes proportion of melt and magma field relations suggest that where the highest degrees of crystals of amcs are not static and change on the time melting occurred a mixture of anhydrous hornfels and hydrous scale of eruptive cycles years to centuries wilcock leucocratic melt disaggregrated and was assimilated into the magma delaney 1996 singh et al 1998 chamber it is well known from ophiolite studies that the roofs of amcs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks particularly within the uppermost few hundred

a reassessment of the role of ice sheet glaciation in the long term evolution of the east greenland fjord region abstract the long term evolution of the east greenland fjord region is investigated using geomorphological and stratigraphical evidence to 1 assess the nature of landscape modification caused by late cenozoic ice sheet glaciation and 2 relate patterns of glacial landscape modification to first order i e continent margin scale topography and geology geomorphological and stratigraphical evidence demonstrates evolution of the first order topography and incision of at least part of the present first order fjord system by 55 ma this hypothesis is tested using apatite u th he ages for samples from two bedrock profiles near kong oscar fjord the thermochronology supports landscape evolution before 55 ma followed by relative tectonic stability because it indicates rapid denudation around the time of rifting that occurred prior to continental breakup i e between 75 and 55 ma the nature of landscape modification caused by late cenozoic glacial erosion appears to have been controlled by first order topography and geology with selective ice sheet erosion in areas of high elevation caledonian basement and apparently little glacial erosion of low elevation mesozoic sedimentary strata nevertheless fjord morphometry demonstrates systematic evolution of the first order fjord system from confined and overdeepened fjords in caledonian basement to wider and disproportionately larger fjords in mesozoic strata the latter indicates that changes in lithological strength enabled the development of more efficient fjord morphology under full glacial conditions that may have promoted fast ice flow

early east antarctic ice sheet growth recorded in the landscape of the gamburtsev subglacial mountains the gamburtsev subglacial mountains are regarded as a key nucleation site for the antarctic ice sheet and they retain a unique long term record of pre glacial and early glacial landscape evolution here we use a range of morphometric analyses to constrain the nature of early glaciation and subsequent ice sheet evolution in the interior of east antarctica using a new digital elevation model of the gamburtsev subglacial mountains derived from an extensive airborne radar survey we find that an inherited fluvial landscape confirms the existence of the gamburtsev subglacial mountains prior to the onset of glaciation at the eocene oligocene climate boundary ca 34 ma features characteristic of glaciation at a range of scales are evident across the mountains high elevation alpine valley heads akin to cirques identified throughout the mountains are interpreted as evidence for early phases of glaciation in east antarctica the equilibrium line altitudes associated with these features combined with information from fossil plant assemblages suggest that they formed at or prior to 34 ma it cannot be ruled out that they may have been eroded by ephemeral ice between the late cretaceous and the eocene 100 34 ma hanging valleys overdeepenings truncated spurs and steep sided linear valley networks are indicative of a more widespread alpine glaciation in this region these features represent ice growth at or before 33 7 ma and provide a minimum estimate for the scale of the east antarctic ice sheet between ca 34 and 14 ma when dynamic fluctuations in ice extent are recorded at the coast of antarctica the implications are that the early east antarctic ice sheet grew rapidly and developed a cold based core that preserved the alpine landscape the patterns of landscape evolution identified provide the earliest evidence for the development of the east antarctic ice sheet and can be used to test coupled ice climate evolution models

the roof of an axial magma chamber a hornfelsic heat exchanger the heat flux from an axial magma chamber into the convecting hydrothermal system at mid ocean ridges is governed by the conductive boundary layer separating them the nature of this critical interface has been modeled and seismically imaged but its characteristics have hitherto rarely been documented in ocean crust here hornfelsic rocks from the sheeted dike gabbro transitions at two tectonic exposures of fast spreading east pacific rise crust at the pito and hess deeps and in the oman and troodos ophiolites are described these rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700 1000 c these temperatures in conjunction with their geological relationships imply that the hornfels are preserved fragments of conductive boundary layers their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200 400 m the heat flux across the hornfelsic aureoles 11 44 mw km is comparable to hydrothermal fluxes released along fast spreading ridges confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers

assimilation of crustal xenoliths in a basaltic magma chamber sr and nd isotopic constraints from the hasvik layered intrusion norway contaminated layered intrusions known thousands of recrystallized strontium and neodymium isotopic data for mafic cumulates chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins and adjacent crustal rocks of the hasvik layered intrusion are thought to represent the remnants of the assimilated material north norwegian caledonides are reported together with new the xenoliths spalled off the roof during magma emplacement and mineralogical and whole rock analytical data to constrain the extent together with the elevated temperatures 400 600 c of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks led to a high degree of assimilation in the magma chamber initial sr sr 700 ma of 0 7045 and end hasvik magma chamber 700 ma of 3 03 for the chilled margin which has a tholeiitic composition akin to the chilled rocks of the skaergaard intrusion demonstrate that the parental magma was derived from a depleted mantle source the basal cumulates 0 335 m show an up section decrease in sr sr from 0 7045 to 0 7038 and a correlative

temporal constraints on glacial valley cross profile evolution two thumb range central southern alps new zealand abstract a key problem in understanding valley evolution in alpine landscapes has been the lack of an independent timescale against which to measure morphological change here we outline that by using space as a proxy for time the role of glaciation in producing classic u shaped valleys can be quantified the two thumb range in new zealand s central southern alps is undergoing rapid tectonic transport and uplift as it is advected towards the alpine fault repeated cycles of glacial erosion during the quaternary have fashioned an alpine landscape in the north of the range close to the main divide and alpine fault while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles using known rates of tectonic transport and uplift of the southern alps coupled with an offshore oxygen isotope 18o record from the chatham rise and glacio geological reconstructions time constraints can be applied to the erosional development of valleys it is possible to estimate the duration of glacial occupancy of valleys in the two thumb range and the pattern is one of very limited glaciation of the southern valleys with glacial occupancy increasing northwards the key trend identifiable is that valley cross profiles become u shaped with c 400 600 kyr of glacial occupancy other models have shown a flattening of valley long profiles with increased glaciation implying glaciers erode larger volumes of rock from mountains than rivers from a cross profile perspective the simple change in geometry from v to u shape concurs with these models indeed the transition from v to u shape with increasing relief and glaciation indicates glaciers may erode greater volumes of rock from mountain belts than rivers

dem analysis on longitudinal and transverse profiles of steep mountainous watersheds longitudinal and transverse characteristics of watersheds are fundamental to describe watershed topography only a limited number of studies however have analyzed the characteristics of transverse profiles in watersheds and almost no studies have examined longitudinal and transverse profiles at the same time how transverse profiles change within a watershed and how longitudinal and transverse characteristics of watersheds are related remain to be examined to address these issues this study employs dems gis and computer programming techniques to acquire a series of longitudinal and transverse profiles of watersheds basic morphometric parameters representing the shape of these profiles were analyzed methods to classify the profiles and to identify points where the morphometric characteristics change abruptly were also developed these methods were then applied to severely dissected steep watersheds in the japanese alps the results show that spatial change in transverse characteristics reflects two factors general topography of the watersheds and local differences in channel development the form of the transverse profiles is also affected by erosion processes that create mature v shaped valleys with a characteristic slope angle the longitudinal and transverse profiles play different roles in determining watershed topography and the overall longitudinal slope exerts a major influence on the mean watershed slope the discontinuity of the longitudinal and transverse profiles however tends to occur at the same location which may reflect the occurrence of knickzones and their influences on the development of valley side slopes

controls on spatial variations of glacial erosion in the qilian shan northeastern tibetan plateau abstract understanding interactions between topography and glacial erosion is a key to advancing knowledge of mountain landscape evolution and is often hampered by the lack of quantitative data for glacial erosion here we examine spatial variations in glacial erosion in the qilian shan using a power law model y axb fit to cross profiles of glacial valleys we further analyze the impacts of glacial dynamics topography and lithology on the degree of glacial erosion using qualitative and quantitative methods although the distribution of glacial erosion is controlled by the combined effects of glacial dynamics topography and lithology average glacier size or ice flux and mean slope above the equilibrium line altitude are dominant controls in addition the topographic data analyzed through a b fr diagram suggests two opposite types of glacial valley cross profile development temperate maritime glacier valleys are mainly dominated by overdeepening while continental glacier valleys are mainly dominated by a widening process although this varies with lithology

glacial geomorphology modeling processes and landforms abstract the primary goal of glacial geomorphology is to provide physically based explanations of the past present and future impacts of glaciers and ice sheets on landform and landscape development to achieve this requires the integration of studies of landform with studies of the processes responsible for form development over a wide range of spatial and temporal scales during the twentieth century significant improvements in approaches to recognizing and describing glacial landforms have been matched by impressive advances in understanding and modeling ice flow and glacial erosion and deposition processes at present process models are being tested explicitly in terms of predicting the development of known forms which also provides new insight into the controls on form development evaluations of the implications of deformable beds for process and form development are also being attempted finally we are reassessing long held beliefs about the significance of glacial action in landform development and sediment production as we head towards the twenty first century glacial geomorphology will advance through the use of three dimensional numerical models that include ice flow basal sliding with explicit consideration of deformable beds erosion and deposition processes and underlying material characteristics these models will be used to address form evolution and test process models and will include both the temporal and spatial aspects of form development space based landform recognition as well as the challenges presented by the concerns of global change will place heightened emphasis on large scale problems which will include reexamination of the basic significance of glacial versus nonglacial action in landform development and sediment production

geochemistry oxygenation by a changing crust serpentine minerals in earth s early upper continental crust suppressed atmospheric oxygen levels until the upper crust became granitic

climate controlled variations in scree production southern alps new zealand the interaction of fluvial glacial and hillslope processes controls the development of mountain belts and their response to tectonic and climatic forcing studies on the contribution of hillslope processes to mountain erosion have focused on bedrock landslides as they have a profound and readily observed impact on sediment yield and slope morphology despite the ubiquity of scree or talus mantled slopes in mountainous terrain the role of frequent low magnitude 100 m 3 rockfall events is seldom addressed in the context of landscape evolution here we quantify the contribution of rockfall erosion across an 80 by 40 km transect in the southern alps new zealand by analyzing the spatial extent of scree slopes mapped from aerial photographs and estimating long term 10 15 k y rockfall erosion rates from the accumulation of slope deposits below bedrock headwalls and in debris and alluvial fans along the rapidly uplifting high rainfall western margin where high rates of bedrock landsliding have been previously documented scree mantled slopes are sparse rainfall decreases exponentially east of the main divide and the proportion of slopes mantled by scree increases monotonically attaining a maximum value of 70 the systematic distribution of scree deposits cannot be attributed to lithologic variation seismicity or the legacy of glaciation instead climate may serve as a primary control on scree production as nearly 70 of the mapped scree deposits in our transect are confined to a narrow elevation range of 1200 1600 m above sea level masl our analysis of altitudinal controls on annual temperature variations indicates that scree production via frost cracking processes may be maximized between elevations of 1600 and 2300 masl as higher elevations are subject to persistent permafrost which obviates the frost cracking process rates of rockfall erosion near the rapidly uplifting main divide are low 0 1 mm yr whereas rates in the scree dominated eastern areas average 0 6 mm yr and may approximately balance rock uplift

a discussion of hirano and aniya s 1988 1989 explanation of glacial valley cross profile development in a rational explanation of cross profile morphology for glacial valleys and of glacial valley development hirano and aniya 1988 present an explanation for the apparent catenary form of glaciated valley cross profiles based on the assumption that glacial erosion works towards minimizing friction between a glacier and its channel however as there is as yet no convincing justification for the application of such extremum principles to this problem it is not clear how useful this type of analysis might be in more detail hirano and aniya s analysis includes assumptions about patterns of basal water pressures and the controls on friction inconsistent with other work in this area furthermore as hirano and aniya 1989 point out the catenary solution they derive is the maximum rather than the minimum friction solution the underlying reason for this surprising result is that the variational principle hirano and aniya use is inappropriately structured for the problem being addressed as it does not involve a cross section area i e ice discharge constraint hirano and aniya also discuss the progressive development of glacial valley cross profiles but their analysis is hindered by the fact that most of the data they present are purely morphological and thus cannot be used to assess form development as a function of either time or extent of glacial erosion the only possible exceptions are graf s 1970 data which give morphological parameters as a function of valley order but these do not unequivocally support hirano and aniya s conclusions and are used with implicit assumptions about the relationship between valley order and time of glacial occupation or magnitude of glacial erosion that are incorrect or uncorroborated respectively however if we accept the common assumption that glacial erosion results in a progression from initial v shaped to broadly u shaped parabolic or catenary forms hirano and aniya s argument that there are two distinct types of form development incision in alpine situations versus valley widening under ice sheets is justified and is an important observation that needs to be addressed in future models of glacial valley development

a note on cross profile morphology for glacial valleys we provide an improvement to the hirano aniya catenary model for the cross profile morphology of a glacial valley copyright 2005 john wiley sons ltd

longitudinal variations in cross section morphology along a glacial valley a case study from the tien shan china a new model the variable width depth ratio vwdr model is used to analyze longitudinal variations in cross section morphology along glacial valleys in the vwdr the cross sectional shape of a valley is expressed as a function of the width depth ratio at various heights above the valley floor two parameters m a measure of the breadth of the valley floor and n a measure of the steepness of the valley sides appear in the model we have used the vwdr model to study morphological variations of cross sections along glacial valleys in the middletien shan mountains china and find that 1 in valleys without tributaries m increases the valley floor becomes wider and n becomes more negative valley sides become steeper from the head of the valley to the equilibrium line and then m decreases and n becomes less negative to the end of the valley 2 in valleys with tributaries a similar pattern is observed with an oscillating maximum in m and minimum in n in those sections where a tributary enters the main valley these characteristics are believed to reflect a maximization of glacial erosion potential in the vicinity of the equilibrium line and in locations of confluence

structure and dynamics of ridge axial melt lenses in the oman ophiolite 1 the oman ophiolite is regarded as derived from a fast spreading oceanic ridge as in its present day marine analogs expectedly the gabbro unit crystallized in a large magma chamber underneath a small melt lens in ophiolites this melt lens is reduced to a horizon mapped in the field where the floor and the roof of the melt lens joined the magmatic activity of the former melt lens has been studied in gabbros located below the melt lens horizon beneath a stable melt lens gabbros subside and drift through the steep walls of the magma chamber as steeply foliated gabbros in contrast in unstable melt lenses more specifically considered here flat lying gabbros are exposed beneath the lens these gabbros were settling on the floor while the melt lens was retreating their strong magmatic foliations and lineations point to a dynamic deposition on the floor and lineation trends record the structure of convection rolls within the lens time constraints suggest a 100 year periodicity of melt recharge by short and powerful melt pulses followed by a shorter time of convection activity after recharge and a longer period of static magma settling as recently proposed for the east pacific rise at each new melt pulse the lens nearly doubles its volume by surface expansion within a short time lapse before slowly and progressively retreating to feed crustal accretion large horizontal surface inflation of the lens better explains the seawater hydration of the accreting crust than melt lens vertical motion through the lid

numerical models of magma chambers in the oman ophiolite long standing studies conducted in the oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the east pacific rise epr this is true in particular for the shape size and internal magmatic flow of magma chambers thus justifying numerical modeling our modeling founded on the equations of fluid mechanics is based on a two dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber boundary conditions deduced from the geophysical data of the epr and geological observations from the oman ophiolite the model predicts a magma chamber shape consistent with field studies whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills it has only a weak influence on the shape of the chamber the viscosity of the magma plays a crucial role for the setting of convection in the chamber according to geological observations which argue against the existence of convective flow patterns the viscosity of the gabbroic mush within the chamber must exceed 1014 1015 pa s depending on the oceanic plate velocity the flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive foliations oriented inward or active foliations oriented outward the active structural analysis in the oman model is supported by ophiolite data

the relative efficiency and influence of glacial and fluvial erosion on tibetan plateau landscapes abstract driven by climate change periods of dominating glacial and fluvial erosion have both sculpted mountain ranges during the quaternary to understand the evolution of mountain landforms there is considerable interest in knowing which erosion agent leads to more incision how glacial thermal regime impacts erosion and landscape development and whether glacial erosion can result in increased relief to investigate the relative efficiency of glacial and fluvial erosion and the influence of glacial thermal regime on topography we compare two different mountain ranges the qionglai shan monsoonal temperate glaciation and lenglongling subcontinental glaciation in western china we use a 1 d model to construct presumptive fluvial longitudinal profiles if glaciation had not occurred and compare these with the actual glacial profiles to obtain relative incision rates of fluvial and glacial erosion results demonstrate that glacial incision is smaller than fluvial incision in lenglongling and larger than fluvial incision in the qionglai shan which may be influenced by different tectonics and glacier types topographic analysis of glacial and fluvial valleys with similar drainage areas show that glacial erosion results in valley widening local valley relief increase induced by isostatic uplift and slope gradient increase in lenglongling however there is no local valley relief and slope increase observed in the qionglai shan which may reflect slope process responses to threshold hillslope limiting the height above valley bottoms

contaminating the lower crust in the oman ophiolite contamination of magmas via assimilation of country rock is widely recognized as an important process in the thermal and chemical evolution of magmas emplaced into continental crust its importance at mid ocean ridges has however received much less consideration lavas from the fast spreading east pacific rise show evidence for contamination through an enrichment in seawater derived chlorine that is not observed in basalts from slow spreading ridges in this study magmatic amphiboles in plutonic rocks from the oman ophiolite an intermediate to fast spreading ridge analogue have been analyzed to determine the extent of this contamination in the lower oceanic crust these data suggest an enrichment in chlorine throughout much of the lower ocean crust if this enrichment is representative of lower oceanic crust formed at typical intermediate to fast spreading ridges assimilation may be a more important process at mid ocean ridges than previously recognized

glacial valley cross profile development the influence of in situ rock stress and rock mass strength with examples from the southern alps new zealand abstract the evolution of the glacial valley cross profile form is commonly attributed primarily to the glaciological variables that control the erosion of the channel however studies in the new zealand southern alps suggest that the rock mass strength rms of the eroded rock mass is a major control on slope stability and hence on the final form of the trough rms of the slope rock will alter with time and erosional excavation and oversteepening of the valley slopes the in situ rock stress field induced by 1 the extreme topography in the axial ranges of the new zealand southern alps and 2 tectonically by collision of the australian and pacific crustal plates may play a role in valley slope development by controlling the location of rock failure and reducing rms this provides weakened rock that may provide sites for selective glacial erosion of the rock mass hence valley form evolution models should also take into account the rms and the in situ stress field acting in the eroded rock mass this study has implications for the development and modification of alpine glacial troughs in similar tectonic settings elsewhere

development of glacial valley cross sections under conditions of spatially variable resistance to erosion abstract a simulation model that examines the development of valley cross profiles as a result of glacial erosion has been constructed by placing a finite element model for ice flow within an iterative program that modifies channel cross section form on the basis of an erosion equation the simulation model has previously been used to examine how different flow laws erosion laws and ice discharge histories affect medium scale landform development this allows us to better understand the development of glacial landforms and also provides a critical test for erosion and flow laws within the erosion component of the simulation program it is possible to specify spatial variations in bedrock erodibility in order to examine the effect of spatially variable lithologies on form development as an initial test of form development under these conditions a central area of enhanced erodibility is placed in the valley center in this case active glacial channels develop that are narrower and deeper than those which occur with homogenous bedrock this change in form occurs as long as the weaker zone is small enough that the active channel does not eventually narrow to a stage such that it is entirely contained within the weaker material and as long as it is large enough that it continues to have a significant impact on overall discharge through the section form variation occurs because the weaker central zone enhances the lateral gradient of the erosion rate which is critical for form development a central area of more erodible bedrock changes not only the form of the active glacial channel but also the convexity of side slopes that are left above the glacier as it erodes downwards into the landscape the absence of a classic u shaped cross section should not be taken a priori to suggest either absence of glacial erosion or ineffective glacial erosion rather the combination of active glacial and hillslope processes in a landscape with spatially and temporally variable ice discharge and rock characteristics can create a diverse set of cross section forms

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

using a gis filtering approach to replicate patterns of glacial erosion jansson k n stroeven a p alm g dahlgren k i t glasser n f goodfellow b w 2011 using a gis filtering approach to replicate patterns of glacial erosion earth surface processes and landforms 36 3 408 418

assessing glacial modification of bedrock valleys using a novel approach abstract analysis of valley cross sections is an established technique for evaluating the effects of glacial erosion here we present a semiautomated approach for assessing glacial modification of bedrock valleys by quantifying morphological variability in valley cross sections extracted in bulk from digital elevation models dems this method can be tailored to derive cross sections of user specified width and spacing and can generate cross sectional data sets of multiple orders of magnitude we demonstrate the technique by extracting 27 000 cross sections from the sierra nevada california and computing measures of valley morphology including a form ratio a quadratic curve fit and a power law curve fit for each cross section in addition we introduce a novel metric the v index which is calculated by comparing valley cross sectional area to that of an ideal v shaped cross section and is thus an expression of the deviation from an ideal v shaped form statistical comparisons of the different measures suggest that the v index is a robust alternative to traditional morphological measures because it can accommodate irregular valley cross sections and discriminate between glacial and nonglacial forms we document the distribution of valley forms throughout the sierra nevada using the v index to identify range scale patterns in cross sectional morphology and to investigate the role of lithology on glacial modification of bedrock valleys

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

the magmatic hydrothermal transition in the lower oceanic crust clues from the ligurian ophiolites italy abstract the gabbroic bodies from the jurassic ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the mid atlantic ridge initial cooling of the ligurian gabbros is associated with local development of hornblende bearing felsic dykes and hornblende vein networks the hornblende veining is correlated with the widespread development of hornblende as coronas pseudomorphs after the clinopyroxene in the host gabbros in addition the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins the hornblendes from the felsic dykes and the hornblende rich rocks within the mantle sliver show a similar geochemical signature characterized by low mg cao and al2o3 negligible cl and high tio2 k2o ree y zr and nb concentrations the whole rock sm nd isotopic compositions of the felsic dykes and the hornblende rich rocks define a sm nd isochron corresponding to an age of 154 20 ma and an initial end of 9 2 0 5 the 18o of the hornblendes and coexisting zircons from these rocks about 4 5 and 5 8 respectively do not indicate the presence of a seawater component in these melts the formation of the felsic dykes and of the hornblende rich rocks within the mantle sliver involved sio2 rich silicate melts with negligible seawater component which presumably were derived from high degree fractional crystallization of mor type basalts the vein and the coronitic pseudomorphic hornblendes show high mg and cao significant cl 0 02 0 17 wt and low tio2 and k2o concentrations the coronitic pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and 18o values 1 0 to 0 7 close to seawater suggesting an origin by reaction between migrating seawater derived fluids and the host gabbros the vein hornblendes commonly show slight lree enrichment relatively high concentrations of nb up to 2 5 ppm and 18o ranging from 3 7 to 0 8 the crystallization of these hornblendes most likely involved both seawater and magmatic components

an extended perfect plasticity method for estimating ice thickness along the flow line of mountain glaciers 1 direct measurement of the thickness of mountain glaciers is difficult over large areas yet knowledge of the thickness is essential for calculating their volumes and future evolution we develop a new method for estimating the ice thickness along glacier flow lines using the perfect plasticity rheological assumption that relates the thickness and surface slope to a yield stress previous studies have used this assumption with the shallow ice approximation to estimate the ice thickness but the standard approach neglects the effect of side drag on glacier stress balance our method addresses this shortcoming and extends the standard method by accounting for the side drag via the glacier width besides the assumed yield stress the inputs for our method are the outline and surface topography of the glacier surface velocity and mass balance data are unnecessary we validated the extended method on five glaciers in northwest china where thickness data are available from radio echo soundings finding that it can reproduce measured thicknesses with a mean absolute error of 11 8 like the standard method moreover for long glacier tongues confined to flow between parallel valley sides this method is found to give more accurate thickness estimates than does the standard method with a mean absolute error of as low as 5 3 sensitivity analysis shows that the estimated ice thickness depends strongly on yield stress and surface slope and less strongly on glacier width because this method is physically more realistic than the standard method and its inputs are easily derivable from remote sensing observations it has the potential to be used for processing large glacier data sets

a rational explanation of cross profile morphology for glacial valleys and of glacial valley development the fact that the cross profile of the glacial valley could be well approximated by parabolas y axb b 2 0 is explained by the variation principle assuming that the glacier erosion works towards minimizing thefriction between ice and bedrock the variation principle proves that the ideal or fully developed morphology of the glacial valley should be a catenary the curve which a chain hanging from two fixed points forms maclaurin s series expansion of the catenary equation shows that a parabola is a very good approximation of the catenary hence the good approximation of the cross profile by parabolas different catenaries are generated by changing the form ratio depth rim width and are then approximated by y axb by the method of last squares the b values obtained become only fractionally larger than 2 0 with invreasing form ratios of up to 1 0 indicating that b values would range in practice between 1 0 and about 2 0 two types of trend in the relationship between b values and the form ratio were obtained from several glaciers for one type the b value becomes larger with increasing form ratios and for the other the opposite the first type is called the rocky mountain model after its source of data and represents overdeepening of the glacial valley development the second type is caalled the patagonia antarctica model representing a widening instead of a deepening process of development these differences are attributed to the nature of the glaciers which produced these valleys i e alpine glaciers and continental ice sheets

quantified time scale for glacial valley cross profile evolution in alpine mountains space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the two thumb range southern alps new zealand the range is undergoing rapid tectonic transport and uplift as it is advected toward the alpine fault repeated cycles of glacial erosion during the quaternary have fashioned u shaped valleys in the north of the range close to the main divide while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles tectonic transport and uplift rates coupled with an offshore oxygen isotope 18o record and glacial geological reconstructions allow time constraints to be applied to valley development along the range valley width and depth measurements and power law equations were used to quantify the shape of cross profile transects in 37 valleys valleys evolved into a characteristic u shaped cross profile formed over 400 k y of glacial occupancy this regardless of other models of glacial flattening of valley long profiles suggests that alpine glaciation excavates greater volumes of rock indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts

valley shape influence on deformation mechanisms of rock slopes stress distribution in mountainous areas is influenced by local morphology valley morphology and the relationship between main and tributary valleys strongly depend on geological characteristics and evolution they may control the evolution of slope instabilities especially when interacting with pervasive structural features we performed parametric three dimensional 3d numerical modelling of simplified slope geometries with variable slope angle from 218 to 358 length combining different orientations for different slope sectors and changing attitude of pervasive planes of anisotropy foliation schistosity bedding data used in the 3d models are the initial slope geometry rock mass properties and internal anisotropy we assumed mohr coulomb behaviour with the presence of ubiquitous joints and different piezometric levels the model results show that plastic deformation initiates near the highest ridge just after deglaciation commences a shear zone develops and propagates toward the toe of the slope and its shape is strongly controlled by slope geometry anisotropy and in situ stresses the thickness of the failing mass for model slope reliefs up to 3200 m increases from 50 mto some hundreds of metres during glacier retreat and it depends on geometry of slopes anisotropy and in situ stresses results are compared to examples of deep seated slope deformations from the alps which helps in the interpretation of such phenomena and in the understanding of their influence on valley evolution

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

the shapes of some mountain peaks in the canadian rockies different mountain shapes in sedimentary sequences in the canadian rockies were enhanced by glacial erosion and have been modified postglacially by gravity driven slope processes slope modification by both glacial erosion and postglaciation landslides is related to rock structure particularly bedding dip rock mass strength and slope geometry five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips castellate 1 and matterhorn 2 mountains occur in sub horizontal beds and their slopes on all sides follow combinations of bedding planes and joints the overall slopes are generally 37 to 65 and oblique to both bedding and joints slopes in sub horizontal beds may be controlled by their rock mass strength cuestas 3 develop in gently to moderately dipping beds dip slopes and steeper normal escarpments form their cataclinal and anaclinal sides respectively with the dihedral angle between them about 90 hogbacks 4 in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments the dihedral angle between the slopes is usually less than 90 dogtooth 5 mountains occur in steeply dipping to sub vertical beds and the dihedral angle can be as low as 60 slope gradients in inclined beds are closely related to landslides whose modes are controlled by bedding dips copyright 1999 john wiley sons ltd

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

the influence of rock mass strength on glacial valley cross profile morphometry a case study from the southern alps new zealand the erosional morphology in the vicinity of the main divide of the southern alps and fiordland new zealand appears to be a product of the interaction between alpine fault induced tectonic processes rock mass strength of the uplifted and eroded bedrock and the processes acting to denude the developing mountain landscape the magnitude of the effects of glacial erosion on the landscape is directly controlled by the size and physical properties of the glaciers whilst the form of the trough is a direct consequence of the rock mass strength rms properties of the slope rock realistic models of development of the cross profile shape of glacial valleys must take into consideration the rms properties of the eroded substrate

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

glacial and gully erosion on mars a terrestrial perspective the mid to high latitudes of mars host assemblages of landforms consistent with a receding glacial landscape on earth these landforms are postulated to have formed 5 ma under a different climate regime when mars orbital obliquity was on average 10 higher than today here we investigate the spatiotemporal relationship between gullies and glacial landforms both common in the mid latitudes gullies are kilometre scale landforms with a source alcove transportation channel and depositional apron the glacial landforms comprise 1 extant viscous flow features vff that extend from the base of crater walls into the interior of crater floors and are widely interpreted as debris covered glaciers containing extant ice and 2 landforms such as arcuate ridges at the base of crater walls that have been interpreted as relicts of more recent less extensive glacial advances focussed on crater walls we measure headwall retreat associated with glacial landforms and date their host craters to constrain minimum headwall retreat rates we record headwall retreat rates up to 102 m my 1 for the youngest suite of glacial landforms equivalent to erosion rates of wet based glaciers on earth and to headwall retreat rates associated with martian bedrock gully systems we find extensive evidence for a single erosional episode dating 5 10 ma which postdates emplacement of the majority of vff but seems to predate formation of the gullies we propose that the wet based glacial episode was associated with glaciation focussed on the crater walls rather than melting of the glacial ice deposits on the crater floors vff this is consistent with our observations of crater wall morphologies including the presence of arcuate ridges consistent with terrestrial glaciotectonic features that require liquid water to form textural alteration of the eroded bedrock surface consistent with ice segregation and frost shattering and the presence of downslope pasted on terrain tentatively interpreted here as glacial till deposits sourced from glacial erosion of the crater wall the pasted on terrain is usually interpreted as a thicker latitude dependant mantle located on sloping terrain formed from airfall of ice nucleated on dust but we suggest that it has been reworked by glaciation and is predominantly glacial in origin although our results cannot substantiate that gullies are produced by meltwater the discovery of this wet glacial event does provide evidence for widespread meltwater generation in mars recent history

comparison of late pleistocene and modern glacier extents in central nepal based on digital elevation data and satellite imagery abstract late pleistocene and modern ice extents in central nepal are compared to estimate equilibrium line altitude ela depressions new techniques are used for determining the former extent of glaciers based on quantitative objective geomorphic analyses of a 90 m resolution digital elevation model dem for every link of the drainage network valley form is classified as glacial or fluvial based on cross valley shape and slope statistics down valley transitions from glacial to fluvial form indicate the former limits of glaciation in each valley landsat multispectral scanner imagery for the same region is used to map current glacier extents for both full glacial and modern cases elas are computed from the glacier limits using the dem and a toe to headwall altitude ratio of 0 5 computed ela depressions range from 100 900 m with a modal value of 650 m and a mean of 500 m values consistent with previously published estimates for the central himalaya but markedly smaller than estimates for many other regions we suggest that this reflects reduced precipitation rather than a small temperature depression consistent with other evidence for a weaker monsoon under full glacial conditions

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

the glaciation of dartmoor the southernmost independent pleistocene ice cap in the british isles the granite uplands of dartmoor have traditionally been considered to be relict permafrost and periglacial landscapes that lay beyond the limits of quaternary glaciations but a variety of landform evidence indicates that a plateau icefield existed on the northern part of the moor constituting the southernmost independent ice cap in the british isles overdeepened or weakly u shaped valley segments fringing north dartmoor document an early extensive phase of glaciation but the most convincing landform evidence relates to more recent valley based glacier occupancy a moraine ridge on the slipper stones represents the most unequivocal palaeo glacier on north dartmoor with a palaeo ela of c 460 m above sea level asl although this relates to the youngest and most restricted phase of glaciation a longer term ela is likely to be represented by the corn ridge proto cirque at 370 410 m asl more extensive valley glaciers are recorded in each of the major drainage basins of north dartmoor by arcuate and linear bouldery ridges and hummocky valley floor drift which are interpreted as latero frontal moraines deposited by outlet lobes of a plateau icefield recession of these lobes is marked by inset sequences of such ridges and occasional meltwater channels plateau ice was predominantly thin and protective and snowblow and preferential accumulation in valley heads facilitated the modest glacial erosion and debris transport recorded in the landforms and sediments it is proposed that the highest plateaux have been occupied by ice for the longest cumulative period of time throughout the quaternary average glacial conditions explaining the distribution of different tor types on northern dartmoor this also explains the lack of tors on the most expansive of the highest plateau terrain ice dispersal centres as the product of a average glacial conditions preferentially removing tors or dampening their production rates b the survival of high relief type 1 tors during glaciation if they occupy summits too narrow to develop significant plateau icefields and or ridges that are bypassed by faster moving ice in adjacent deep valleys and c the survival of subdued type 2 tors in areas glaciated less regularly during the quaternary simple ice flow modelling indicates that a plateau icefield type glaciation is required for significant ice flow to occur and confirms thin ice cover in particular on narrow summits thereby supporting the explanation of tor class distribution the modelling allows us to spatially correlate the geomorphological evidence of margin positions into two major stages and further indicates a strong altitude mass balance feedback leading to an ice cap that is not in balance with its climate and with an extent that is limited by the length of the cold phases rather than their severity

on the mathematical description of glaciated valley cross sections morphometric analyses of glaciated valleys typically involve attempts to fit empirical data from valley cross profiles to quadratic parabolic or power law equations with the choice of equation depending on the goal of the analysis in assessing the relative merits of these two types of equations some confusion has arisen because of apparent variations in coordinate system datums between studies which have made use of the power law equation however we show that this confusion relates to a simple mathematical error rather than any true difference in methodology of more significance however is the fact that in fitting a power law equation to observed profiles significant bias is introduced by the use of a logarithmic data transformation because of this implicit bias power law equation parameters are influenced most strongly by data points close in elevation to the centre of the valley and so failure to account for the depositional elements which frequently occur in these areas can lead to significant errors if the aim of the analysis is to characterize the form of the erosional profile

high resolution imaging science experiment hirise observations of glacial and periglacial morphologies in the circum argyre planitia highlands mars 1 the landscape of the argyre planitia and adjoining charitum and nereidum montes in the southern hemisphere of mars has been heavily modified since formation of the argyre impact basin this study examines morphologies in the argyre region revealed in images acquired by the high resolution imaging science experiment hirise camera and discusses the implications for glacial and periglacial processes distinctive features such as large grooves semicircular embayments in high topography and streamlined hills are interpreted as glacially eroded grooves cirques and whalebacks or roche moutonnee respectively large boulders scattered across the floor of a valley may be ground moraine deposited by ice ablation glacial interpretations are supported by the association of these features with other landforms typical of glaciated landscapes such as broad valleys with parabolic cross sections and stepped longitudinal profiles lobate debris aprons interpreted as remnant debris covered glaciers or rock glaciers and possible hanging valleys aligned boulders observed on slopes may also indicate glacial processes such as fluting alternatively boulders aligned on slopes and organized in clumps and polygonal patterns on flatter surfaces may indicate periglacial processes perhaps postglaciation that form patterned ground at least portions of the argyre region appear to have been modified by processes of ice accumulation glacial flow erosion sediment deposition ice stagnation and ablation and perhaps subsequent periglacial processes the type of bedrock erosion apparent in images suggests that glaciers were at times wet based the number of superposed craters is consistent with geologically recent glacial activity but may be due to subsequent modification

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids