paleocene plutonism and its tectonic implications north cascades washington u pb zircon geochronology has identified a northwest trending belt of paleocene 68 59 ma plutons within the crystalline core of the north cascades most of these plutons show amphibolite facies assemblages and solid state foiliation they document extensive paleogene dynamothermal metamorphism in the core and combined with other recent studies demonstrate ongoing metamorphism from mid cretaceous to mid eocene time the paleocene plutons were emplaced during an interval that marked a magmatic lull throughout much of the northern cordillera the distribution of latest cretaceous and paleocene plutons suggests that magmatism in the north cascades core records the same eastward retreat that occurred farther north in the cordillera

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

stabilizing feedbacks in glacier bed erosion glaciers often erode transport and deposit sediment much more rapidly than nonglacial environments1 with implications for the evolution of glaciated mountain belts and their associated sedimentary basins but modelling such glacial processes is difficult partly because stabilizing feedbacks similar to those operating in rivers2 3 have not been identified for glacial landscapes here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks we find that the long profiles of beds of highly erosive glaciers tend towards steady state angles opposed to and slightly more than 50 per cent steeper than the overlying ice air surface slopes and that additional subglacial deepening must be enabled by non glacial processes climatic or glaciological perturbations of the ice air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate

determination of the bedload transport rate in a small proglacial high arctic stream using direct semi continuous measurement abstract the article presents the results of a 34 day series of direct semi continuous measurements of the bedload transport performed during the melt season 2013 in the scott river catchment nw part of the wedel jarlsberg land spitsbergen the daily variability of the bedload transport rates and its relation to the dissolved and suspended load were analysed in two cross sections located in the lower course of a proglacial gravel bed river the bedload flux was measured by means of two 4 module sets of river bedload traps rbt in the research period a total of 34 day long samples of bedload and 34 water samples for the determination of the dissolved and suspended load were collected in each of the two cross sections 200 m apart the measurements showed longitudinal and temporal variability of bedload flux and the remaining components solutions suspensions corresponding with changes in water discharge the mean daily bedload flux qb in consecutive measurement cross sections xs amounted to 124 kg d 1 xs i and 59 kg d 1 xs ii respectively and showed high variability and an evident relationship with temperature the unequal bedload flux of the two measuring stations xs i and xs ii suggests aggradation within the alluvial fan that separates the two reaches the volume and rate of bedload flux were determined by the frequency of the occurrence of flood flows during which approximately from 59 xs i to 77 xs ii of total bedload was discharged the highest recorded daily values constituted 12 of total bedload flux through the cross section below the alluvial fan the river discharged approximately 6 million cubic meters m m3 of water 3 t of bedload lb 2238 t of suspended load ls and 613 t of dissolved load ld the relationships between the three components of the load suggest a 500 fold excess of suspended load and a 100 fold excess of dissolved load in relation to bedload flux high temporal and latitudinal variability of the bedload transport rate resulted in changes in the channel morphology and during floods also the floodplain

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

a multi century estimate of suspended sediment yield from lillooet lake southern coast mountains canada we use annually laminated lake sediments to estimate suspended sediment yield for a 3850 km2 montane catchment in the british columbia coast mountains sediment yield over the past 369 years averages 213 38 mg km 2 a 1 sediment yield increases to 285 50 mg km 2 a 1 during the first half of the twentieth century and declines thereafter the frequency of high yield events during the 369 year period is irregular 11 of the 34 events occur in the early part of the twentieth century a time when glaciers in the watershed underwent major retreat we fitted a generalized extreme value gev model to estimate quantiles of the sediment yield distribution and we used epoch analysis to examine persistence in sediment yield following 34 of the largest events persistence is greatest for the most extreme events it is more variable for events that recur on average every 10 25 years our results indicate that sediment yield is linked to long term changes in sediment supply to the lake the results of this study

diapiric spoke patterns abstract not all finger like diapirs evolve from or into linear walls as previously supposed that would imply that all contemporaneous finger diapirs lie in rows and many do not physical models demonstrate that diapiric linear walls or rows of fingers have to be forced by lateral forces lateral boundaries or internal edge effects by contrast the kinematics of the simplest isothermal gravity overturns responsible for finger like diapirs are geometrically kinematically and dynamically similar to the spoke patterns of vigorous thermal convection this is because rayleigh taylor rt diapirism and rayleigh benard rb thermal convection are end members of a continuous dynamic spectrum due to any combination of compositional or thermally induced density inversions all simple rt overturns are dynamically similar but the vigour of thermal convection varies with the heat flow the kinematics of spoke patterns are difficult to study in rapid multiple overturns of thermal convection in low viscosity fluids however spoke patterns in slow isothermal diapiric analogues can be analysed at any stage during their single overturn such studies indicate that two sets of spokes conduct fluids of different densities horizontally along the top and bottom boundaries to finger like diapiric stems or plumes where they rise or sink to spread in polygonal bulbs at the opposite boundary spokes converge to hubs at the bottom boundary beneath stems feeding diapiric bulbs at the top boundary and vice versa if the movement cells are considered centred on rising plumes the cell corners are mapped by sinking plumes the differences between spoke patterns and previously understood diapiric patterns are illustrated by geological examples with emphasis on relevance to the search for oil and gas physical arguments suggest that spoke patterns are probably the most common geometries of gravity driven overturns in layered rocks of both the crust and the mantle

magmatic history and geophysical signature of a post collisional intrusive center emplaced near a crustal scale shear zone the plechý granite pluton moldanubian batholith bohemian massif the plech pluton southwestern bohemian massif represents a late variscan complexly zoned intrusive center emplaced near the crustal scale pfahl shear zone the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches magmatic fabric acquisition and the late variscan stress field associated with strike slip shearing the magmatic history of the pluton started with the emplacement of the porphyritic plech and haidmuhler granites based on gravity and structural data we interpret that the plech and haidmuhler granites were emplaced as a deeply rooted ne sw elongated body its gross shape and internal fabric steep ne sw magmatic foliation may have been controlled by the late variscan stress field the steep magmatic foliation changes into flat lying foliation particularly recorded by ams presumably as a result of divergent flow magnetic lineations correspond to a sub horizontal ne sw finite stretch associated with the divergent flow subsequently the t istolicnik granite characterized by steep margin parallel magmatic foliation was emplaced as a crescent shaped body in the central part of the pluton the otherwise inward younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet bearing granite the marginal granite along the southeastern margin of the pluton

proglacial lakes character behaviour and geological importance proglacial lakes are ubiquitous within the quaternary record and can provide exceptional breadth and depth of palaeoenvironmental information present deglaciation is increasing the number and size of proglacial lakes around the world this study provides a synthesis of knowledge on proglacial lake character and behaviour and critically evaluates the importance of proglacial lakes from a geological perspective we show how ice marginal or ice contact lakes and other distal proglacial lakes can be distinguished from each other by geomorphological sedimentological chemical and biological characteristics the key controls on proglacial lake geomorphology and sedimentology are outlined and discussed proglacial lakes can exacerbate mountain glacier and ice sheet margin ablation via mechanical and thermal stresses but very large lakes can moderate summer air temperatures and relatively retard summer ice ablation proglacial lakes interrupt meltwater flux and are very efficient sediment traps hydrological routing and consequent geomorphological activity can be radically modified by sudden drainage of proglacial lakes and resultant glacial lake outburst floods exceptionally large proglacial lake drainages affected global ocean circulation and global climate during the quaternary overall analyses of proglacial lakes can provide a valuable insight into i patterns character and behaviour of mountain glaciers ice sheets and glaciations and ii the impacts of past present and future deglaciation

the record of glacial lake champagne in kusawa lake southwestern yukon territory late pleistocene glaciers dammed glacial lake champagne in the valley of dezadeash river between a westward flowing glacier in the takhini valley and eastward flowing glaciers from the st elias complex modern kusawa lake lies in the southern extension of lake champagne geophysical and geomorphic evidence documents the deglaciation of the lake the presence of lake champagne and the postglacial sedimentary environment of the basin during the main phase of lake champagne the water level stood at 772 m in the northern part of kusawa lake and 756 m in northern dezadeash valley both probably controlled by a spillway floored at 756 m to the north into the nordenskiold river this indicates differential isostatic rebound of 0 2 m km from south to north at that time a trunk glacier occupied the southern portion of kusawa lake depositing a thick sequence of sediment in the basin a glacier in the primrose valley and the takhini trunk glacier built large deltas into lake champagne subsequently the level

pressure temperature time paths from two dimensional thermal models prograde retrograde and inverted metamorphism two dimensional thermal models of time transitive crustal thickening and subsequent unroofing in large scale overthrust terrains generate pressure temperature time ptt paths that generally resemble those produced by earlier one dimensional instantaneous models but that differ in detail these differences in ptt path morphology are most pronounced proximal to major fault zones where postthrusting geotherms are characterized by large temperature inversions in one dimensional models but generally lack inversions in two dimensional models tests using the two dimensional fault model developed here indicate that 1 burial rate proportional to dp dt not thrust fault geometry dip angle controls the topology of synthrusting pt paths but plays only a minor role in determining the maximum temperature that rocks attain during the later unroofing stage in their thermal histories 2 normal ranges of thrusting and erosion rates and fault parameters result in pt paths with the usual sense clockwise on conventional pt diagrams counterclockwise in our diagrams 3 the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing dp dt during this period 4 fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing 5 the initial thermal state of the lithosphere prior to thrusting has a profound effect on pt path morphologies and on the peak metamorphic conditions attained by samples 6 for excess heat distributed across the entire lithosphere e g due to increased background thermal gradients a plot of peak temperatures experienced by metamorphic rocks versus structural depth tmax plot closely represents the initial geotherm 7 excess heat in the crust only e g increased radioactive heating in a layer yields a different result with the tmax plot corresponding to the initial geotherm only near the top of the hanging wall tmax plots for both 6 and 7 show no temperature inversion which exceeds the nominal uncertainties 50 k for geothermometric data 8 shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient is 0 6 or greater and 9 simultaneous thrusting and erosion produce pt loops significantly narrower than those resulting from sequential thrusting and erosion suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic ptt paths to various thermal mechanical and geometric factors related to tectonism but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data analytical inversion techniques that use a postthrusting thermal regime consistent with two dimensional forward models and that integrate values of dp dt dt dt and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains

sediment evacuation and glacial erosion rates at a small alpine glacier 1 glacial erosion is known to be driven by sliding which occurs in brief episodes in the melt season what is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production we document sediment evacuation rates for three seasons at bench glacier a small temperate alpine glacier in the chugach range of alaska two lines of evidence suggest that subglacial sediment storage is small strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up glacier inclined stoss slopes of bedrock bumps sediment evacuation rates at bench glacier should therefore closely track glacial erosion rates over annual timescales much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year suggesting that the conduits and well connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment this small glacier efficiently modifies the landscape lowering its bed by 1 2 mm yr while sliding only 1 2 m yr our sparse measurements of instantaneous bed load discharge average 41 of concurrent suspended sediment discharge assuming that half of the suspended load derives from abrasion of subglacially quarried clasts roughly two thirds of the sediment evacuated can be attributed to quarrying

short term geomorphological evolution of proglacial systems proglacial systems are amongst the most rapidly changing landscapes on earth as glacier mass loss permafrost degradation and more episodes of intense rainfall progress with climate change this review addresses the urgent need to quantitatively define proglacial systems not only in terms of spatial extent but also in terms of functional processes it firstly provides a critical appraisal of prevailing conceptual models of proglacial systems and uses this to justify compiling data on rates of landform change in terms of planform horizontal motion elevation changes and sediment budgets these data permit us to produce novel summary conceptual diagrams that consider proglacial landscape evolution in terms of a balance of longitudinal and lateral water and sediment fluxes throughout we give examples of newly emerging datasets and data processing methods because these have the potential to assist with the issues of i a lack of knowledge of proglacial systems within high mountain arctic and polar regions ii considerable inter and intra catchment variability in the geomorphology and functioning of proglacial systems iii problems with the magnitude of short term geomorphological changes being at the threshold of detection iv separating short term variability from longer term trends and v of the representativeness of plot scale field measurements for regionalisation and for upscaling we consider that understanding of future climate change effects on proglacial systems requires holistic process based modelling to explicitly consider feedbacks and linkages especially between hillslope and valley floor components such modelling must be informed by a new generation of repeated distributed topographic surveys to detect and quantify short term geomorphological changes

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

extreme sediment delivery events recorded in the contemporary sediment record of a montane lake southern coast mountains british columbia the extreme sediment delivery regime of a montane catchment was examined through the analysis of recent lacustrine varve deposits recovered from a high density sampling program years of anomalously high sediment yield from the green lake watershed were identified over a 70 year period 1930 2000 based on whole lake varve deposit volumes intra annual sediment delivery events were categorized and described for those anomalous years using historical hydroclimatic data and the stratigraphic record observed within inflow proximal sediment cores extreme sediment delivery coincided with high discharge conditions and elevated sediment availability resulting from 1 rapid glacier recession of the early twentieth century 2 late summer and autumn rainstorm generated floods and 3 freshet floods caused by unusual snowmelt conditions the thickness and physical characteristics of varves vary among years that experienced different types of moderate and extreme sediment delivery events in relation to the defin

lake core record of grinnell glacier dynamics during the latest pleistocene deglaciation and the younger dryas glacier national park montana usa abstract few records in the alpine landscape of western north america document the geomorphic and glaciologic response to climate change during the pleistocene holocene transition while moraines can provide snapshots of glacier extent high resolution records of environmental response to the end of the last glacial maximum younger dryas cooling and subsequent warming into the stable holocene are rare we describe the transition from the late pleistocene to the holocene using a 17 000 yr sediment record from swiftcurrent lake in eastern glacier national park mt with a focus on the period from 17 to 11 ka total organic and inorganic carbon grain size and carbon nitrogen data provide evidence for glacial retreat from the late pleistocene into the holocene with the exception of a well constrained advance during the younger dryas from 12 75 to 11 5 ka increased detrital carbonate concentration in swiftcurrent lake sediment reflects enhanced glacial erosion and sediment transport likely a result of a more proximal ice terminus position and a reduction in the number of alpine lakes acting as sediment sinks in the valley

numerical modeling of cosmogenic deglaciation records front range and san juan mountains colorado 1 we use cosmogenic radionuclide crn exposure ages from polished striated bedrock to constrain numerical simulations of deglaciation in the middle boulder creek valley colorado front range and the animas river valley san juan mountains colorado in both valleys the cosmogenic ages suggest initiation of deglaciation 20 ka and ongoing retreat until 12 13 ka while the first order trend in crn concentrations in each valley suggests a monotonic glacial retreat we evaluate other retreat scenarios with different implications for post last glacial maximum regional climate we use a 2 d numerical glacier simulation with a crn layer to investigate how crn based deglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance the crn layer simulates both production during periods of exposure and reduction by glacial erosion during readvances we simulate glacial occupation of the valleys as they respond to equilibrium line altitude ela histories characterized by stepwise change gradual rise or a rise punctuated by short periods of lowering each scenario generates a distinct spatial pattern of concentrations in the crn layer these results and the spatial pattern of measured concentrations in bedrock constrain the range of ela histories that reproduce the crn pattern in each valley in the animas river valley the exposure ages are well explained by a linear ela rise from full glacial to deglacial conditions ages in middle boulder creek valley are best explained by a deglaciation history including a stillstand or partial readvance between 16 and 14 ka followed by rapid retreat

environmental reconstruction from a varve network in the southern coast mountains british columbia canada cores of annually laminated sediments varves from five lakes in the southern coast mountains of british columbia canada document clastic sediment response to climate and geomorphic change over the past 120 years interannual varve thickness correlates with annual flood magnitude interdecadal trends in varve thickness are influenced by other environmental factors such as glacier recession despite major differences in the lakes and their contributing watersheds substantial concordance is observed among the records a pronounced change in the nature of lake sedimentation accompanied by higher interannual variability occurred in 1980 the change coincides with an increase in the magnitude of autumn flooding and major re organization of the north pacific climate system these results highlight new directions for palaeoenvironmental research using varved sediment records specifically to study the magnitude and spatial extent of past hydro climatic events

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

diapiric ascent of magmas through power law crust and mantle there has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust we argue here that this is mainly because the country rocks have previously been assumed to be newtonian and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten the ascent velocity of diapirs is calculated through an equation with the form of the hadamard rybczynski equation for the rise of spheres through newtonian ambient fluids this well known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid these correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of newtonian drops through power law fluids the equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks the velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth the depth of solidification of the diapir is systematically studied as a function of the geothermal gradient buoyancy of the body solidus temperature of the magma and rheological parameters of the wall rock the results show that when the wall rock behaves as a power law fluid the diapir s ascent rate increases without a similar increase in the rate of heat loss in this way diapirs rising at 10 to 102 m yr can ascend into the middle or upper crust before solidification strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates the thermal energy of the diapir is used to soften the country rock only at late stages of ascent the transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures

century scale variability in late summer rainfall events recorded over seven centuries in subannually laminated lacustrine sediments white pass british columbia formation of annually laminated sediments in summit lake white pass british columbia is controlled by runoff generated by snowpack and glacier melt and major rainfall events the 700 yr varve record is divided into two subannual series early and late based on sedimentological criteria and sedimentary structures within each varve a comparison of recent subannual laminae with nearby meteorological records supports the interpretation they are formed by river discharge events generated by major snow and glacier melt events and large late summer rainfall events a significant correlation exists between the late subannual thickness series and the size of the largest rainfall events in late summer the long record indicates there was an abrupt increase in the thickness and frequency of major rainfall induced sedimentary events at the end of the seventeenth century in addition the frequency of laminae generated by early runoff events also increased however early subannual varve thickness component remains statistically the same as the thickness prior to the end of the seventeenth century this suggests the change in varve thickness at this time is due to increases in major late summer rainfall frequency rather than increased sediment availability caused by regional little ice age glacier advances

post little ice age record of coarse and fine clastic sedimentation in an alaskan proglacial lake many assessments of glacial sediment yield rely solely on mea surements of fine grained suspended sediment we show that sus pended sediment contributes 20 of the total clastic sediment discharge into iceberg lake an alaskan proglacial lake drainage of this lake in 1999 exposed outcrops of varved lacustrine sediment that record suspended sediment deposition since a little ice age highstand two centuries ago lake level has dropped in four abrupt spillway controlled events that are recorded stratigraphically as transient increases in the basinwide deposition of unusually fine grained sediments varve counting constrains ages of these events and hence of each abandoned shoreline and associated delta vol umes of deltaic and lake floor sediments constrain specific yields from four multidecadal intervals between a d 1825 and 1999 to tal yield ranges from 3 8 to 4 9 3 10 3 t km 22 yr 21 and most of the sediment 81 86 is sand and gravel now in the deltas a range of evidence suggests that sediment yield faithfully records up stream sediment production in this small 66 km 2 and heavily gla cierized basin implying an effective subglacial erosion rate of 1 6 mm yr these data show that estimates of total yield based upon suspended sediment alone are subject to large errors and suggest that quarrying accounts for a large fraction of the glacial erosion in this system

testing a glacial erosion rule using hang heights of hanging valleys jasper national park alberta canada 1 in most models of glacial erosion glacier sliding velocity is hypothesized to control rates of bedrock erosion if this hypothesis is correct then the elevation difference between hanging and trunk valley floors the hang height should be dictated by the relative sliding velocities of the glaciers that occupied these valleys by considering sliding velocity to be proportional to balance velocity and using mass continuity hang height is expressed in terms of glacier catchment areas slopes and widths which can be estimated for past glaciers from the morphology of glacial valleys these parameters were estimated for 46 hanging valleys and their trunk valleys in three adjacent regions of jasper national park the variability in valley morphology can account for 55 85 of the hang height variability if erosion rate scales with balance velocity raised to a power of 1 3 this correspondence is in spite of spatial variations in glaciation duration snow accumulation rates and other variables that likely affected hang heights but cannot be readily estimated and so are not included in our formulation thus it appears that balance velocity and by extension sliding velocity if the two are proportional may be a reasonable control variable for assessing erosion rate

varve and radiocarbon dating support the rapid advance of jakobshavn isbrae during the little ice age abstract large outlet glaciers draining the greenland ice sheet significantly influence overall ice sheet mass balance considerable short term years to decades retreat and fluctuations in velocity of jakobshavn isbrae western greenland illustrate the complex nature by which large outlet glaciers respond to climate change making predictions of future ice sheet change challenging to provide a longer term view centuries we investigate the geological record of jakobshavn isbrae change we use continuous sediment records from lakes that were influenced by the recent advance of jakobshavn isbrae which took place during the little ice age in particular we explore the use of annually laminated lake sediments varves to precisely constrain the advance of the ice margin as it approached its late holocene maximum extent we find that the ice margin advanced recently at least after 1650 to 1700 ad and more likely 1800 ad we suggest that during this period jakobshavn isbrae advanced at a rate that was similar to its historically documented average retreat since 1850 ad our results indicate that jakobshavn isbrae and presumably other large marine calving glaciers have the ability to advance quickly in response to climate forcing

interdecadal patterns of total sediment yield from a montane catchment southern coast mountains british columbia canada article i nfo we reconstruct sediment yield for a mountain watershed of western canada since the mid twentieth century from studies of annually laminated lake sediments delta progradation and solute transfer total yield averaged 320 40 mg km 2 a 1 and comprised 35 suspended load 50 bedload and 15 dissolved load ratios between the individual yield components varied approximately threefold at interannual timescales because of significant variability in the suspended and bedload fractions asynchronous flux in suspended and bedload fractions through time arise from differences in sediment availability and transitory sediment storage in the channel periods of elevated yield coincide with rapid glacier recession an extreme rainstorm and a landslide our results indicate that in montane environments extrapolation from even decade long monitoring programs may lead to biased projections of long term yield and delivery mode proportions if variations in sediment supply and catchment response to hydroclimatic and geomorphic controls are not considered

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

the sediment budget of an alpine cirque cirques form and evolve as glaciers attack the bed and subaerial processes dismantle the surrounding walls collectively these processes which can make a cirque longer or deeper or both profoundly influence near divide regions of glaciated mountains and yet are rarely studied in a systematic way toward this end we developed a theoretical framework for the sediment budget of a cirque that includes sediment sources transport pathways and storage elements we quantified each component of the sediment budget using field measurements and remote sensing data of a glaciated alpine cirque in british columbia canada the cirque has a plan view area of 1 64 km 2 and relief of 780 m our budget values which correspond to a period of substantial glacier retreat are based on measurements reflecting time intervals ranging from 1 yr to 80 yr we report errors as a range enclosed in parentheses analogous to 95 confidence bounds on average 1640 250 7950 metric tons of rock are released by the headwall each year nearly 90 of this debris leaves the wall as small rockfalls or in snow avalanches our field observations indicated that snow avalanches originating as cornice failures are currently the most important transport process on the headwall we estimated the mass of debris transported annually by the glacier to the foreland using 1 the volume and age of the foreland ground moraine and 2 the product of rock mass per unit volume of ice and glacier velocity over the past several decades the glacier delivered 6440 1180 14930 tons yr to the foreland via forward ice motion and margin retreat mostly in subglacial till or sediment rich basal layers less than 3 of the glacierborne sediment flux traveled as supraglacial debris 170 50 320 tons yr at present sediment evacuation from the cirque occurs in a single meltwater stream we monitored water discharge and suspended sediment concentration in this stream between 29 june and 28 august 2007 by season s end 650 80 1860 tons of sediment had passed our gauging station equivalent to an erosion rate of 0 2 0 03 0 70 mm yr when averaged over the glacier bed approximately one third of the total annual streamborne sediment transport occurred over a 2 d period during the first major melt event of the year using our budget relations and flux magnitudes we estimate the glacier is removing between 1240 and 2470 tons of rock from its bed per year a rate equivalent to 0 5 0 9 mm yr of erosion glacierwide the headwall by comparison is being worn away horizontally at 1 2 0 2 5 9 mm yr thus our results suggest that the headwall is retreating at rates roughly equivalent to vertical incision by the glacier our sediment budget results demonstrate that the wide variety of sediment sources and transport processes active in cirques necessitates a holistic view of cirque formation one that most morphometric range scale and glacial erosion analyses ignore

four magmatic fabrics in the tuolumne batholith central sierra nevada california usa implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust for the first time four distinct magmatic fabrics are documented in a composite plutonic body the tuolumne batholith central sierra nevada california usa one type of fabric was formed by strain caused by highly localized magma flow type 1 whereas the other three chamber wide fabrics recorded strain increments during boundary processes along batholith margins type 2 superimposed by increments of heterogeneous regional tectonic strain types 3 and 4 our present work indicates that in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static actively deforming and rheologically complex crystal mushes at lower melt percentages we demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem placement regional tectonic strain however it may be commonly very problematic to infer the exact nature of flow or even fabric forming process from the preserved rock record our study also exemplifies how examination of magmatic fabric patterns in plutons complemented with geochronology may provide crucial constraints on the interplay among successive magma emplacement fabric preservation temporal evolution of strain fields in a crystallizing magma chamber and the development of crystal mush zones

role of transient water pressure in quarrying a subglacial experiment using acoustic emissions 1 probably the most important mechanism of glacial erosion is quarrying the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments although evidence indicates that erosion rates depend on sliding speed rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice bed separation to study quarrying in real time a granite step 12 cm high with a crack in its stoss surface was installed at the bed of engabreen norway acoustic emission sensors monitored crack growth events in the step as ice slid over it vertical stresses water pressure and cavity height in the lee of the step were also measured water was pumped to the lee of the step several times over 8 days pumping initially caused opening of a leeward cavity which then closed after pumping was stopped and water pressure decreased during cavity closure acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically with repeated pump tests this crack grew with time until the step s lee surface was quarried our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded natural basal water pressure fluctuations should also concentrate stresses on rock steps increasing rates of crack growth stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size which may help explain the effect of sliding speed on erosion rates

a 1500 year record of temperature and glacial response inferred from varved iceberg lake southcentral alaska we present a varve thickness chronology from glacier dammed iceberg lake in the southern alaska icefields radiogenic evidence confirms that laminations are annual and record continuous sediment deposition from a d 442 to a d 1998 varve thickness is positively correlated with northern hemisphere temperature trends and more strongly with a local 600 yr long tree ring width chronology varve thickness increases in warm summers because of higher melt runoff and sediment transport as expected but also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake varve thickness provides a sensitive record of relative changes in warm season temperatures relative to the entire record temperatures implied by this chronology were lowest around a d 600 warm between a d 1000 and a d 1300 cooler between a d 1500 and a d 1850 and have increased dramatically since then combined with stratigraphic evidence that contemporary jokulhlaups which began in 1999 are unprecedented since at least a d 442 this record suggests that 20th century warming is more intense and accompanied by more extensive glacier retreat than the medieval warm period or any other time in the last 1500 yr

the new north american varve chronology a precise record of southeastern laurentide ice sheet deglaciation and climate 18 2 12 5 kyr bp and correlations with greenland ice core records new glacial varve records from long cores combined with records from key surface exposures and new radiocarbon ages have allowed the correction consolidation expansion and calibration of ernst antevs9 original new england varve chronology nevc in the connecticut valley of new england u s a the varve records have been reformulated with corrections and a new numbering system as the new north american varve chronology navc which is a continuous 5659 yr varve sequence that spans most of the last deglaciation 18 200 12 500 yr bp in the northeastern united states rates of ice recession for separate intervals terminated by abrupt glacial stillstands and readvances have been determined for western new england ice recession history is coupled to varve thickness changes that depict changes in meltwater production in the connecticut valley and show the relationship of changes in ablation rate summer climate variation to glacial readvances and periods of halted and rapid ice recession up to 300 m yr comparison of varve thickness records to greenland ice core climate records show that after 15 000 yr bp climate changes of sub century and longer scales recorded in both records appear identical and synchronous after 15 000 yr bp therefore there was a link between north atlantic climate and marginal processes of the southeastern sector of the laurentide ice sheet lis prior to 15 000 yr bp when the lis was closer to an equilibrium condition retreat rates were generally lower and changes in varve thickness and ablation were more subtle but can still be linked to ice sheet activity only weak relationships between varve thickness changes and greenland climate are evident suggesting that changes in the southeastern lis during this time may have been significantly influenced by climate patterns unique to the north american continent or ice dynamics

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

linking bedrock discontinuities to glacial quarrying quarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock the morphologies of glacier hard beds depend on the relative effectiveness of these two processes as abrasion tends to smooth bedrock surfaces and quarrying tends to roughen them here we analyze concentrations of bedrock discontinuities in the tsanfleuron forefield switzerland to help determine the geologic conditions that favor glacial quarrying over abrasion aerial discontinuity concentrations are measured from scaled drone based photos where fractures and bedding planes in the bedrock are manually mapped a tukey honest significant difference test indicates that aerial concentration of bed normal bedrock discontinuities is not significantly different between quarried and non quarried areas of the forefield thus an alternative explanation is needed to account for the spatial variability of quarried areas to investigate the role that bed parallel discontinuities might play in quarrying we use a finite element model to simulate bed normal fracture propagation within a stepped bed with different step heights results indicate that higher steps larger spacing of bed parallel discontinuities propagate bed normal fractures more readily than smaller steps thus the spacing of bed parallel discontinuities could exert strong control on quarrying by determining the rate that blocks can be loosened from the host rock

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

unsteady late pleistocene incision of streams bounding the colorado front range from measurements of meteoric and in situ 10be 1 dating of gravel capped strath terraces in basins adjacent to western u s laramide ranges is one approach to document the history of late cenozoic fluvial exhumation we use in situ 10be measurements to date the broad surfaces adjacent to the eastern edge of the rocky mountains in colorado and compare these calculated ages with results from meteoric 10be measurements we analyze three sites near boulder colorado gunbarrel hill table mountain and pioneer that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the plio pleistocene transition our in situ 10be results reveal abandonment ages of 95 129 ka at table mountain 175 27 ka at pioneer and ages of 251 10 ka and 307 15 ka at gunbarrel hill all are far younger than previously thought inventories of meteoric 10be support this interpretation yielding ages that are comparable to table mountain and 20 lower than pioneer in situ ages we argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate and that vertical incision rates of several mm yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions in contrast to the traditional age chronology in the area our ages suggest that the deep exhumation of the western edge the high plains occurred relatively recently and at an unsteady pace

apatite u th he signal of large magnitude accelerated glacial erosion southwest british columbia alpine glaciers are efficient agents of erosion and capable of significantly modifying topography despite recent advances in theoretical and field studies that quantify glacial erosion processes few studies have documented glacial erosion rates over long 10 6 yr or large more than tens of kilometers scales we use apatite u th he ahe and apatite fission track aft cooling ages to address the late miocene to holocene erosion history across two 60 km long transects of the heavily glaciated southern coast mountains british columbia observed ahe cooling ages from equal elevation samples range between 1 5 and 8 ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1 5 4 0 m y we evaluated temporal and spatial variations in erosion rates using a three dimensional thermal kinematic model that predicted ahe and aft ages at the surface for different erosion histories comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300 over the past 1 5 7 m y coincident with the onset of glaciation of this range

nested temporal suspended sediment yields green lake basin british columbia canada suspended sediment yield was determined for a mountain watershed by calculating the volume of sediment stored in the watershed s outlet lake basin and monitoring sediment transport for the lake s primary inflow lake based yields for the 1931 2001 period 0 21 0 03 mg km 2 day 1 are 10 higher than estimates for the last 3000 years 0 19 0 05 mg km 2 day 1 fluvial 0 24 0 08 mg km 2 day 1 and lake based yields ca 0 21 0 07 mg km 2 day 1 are comparable over the period of overlap 1999 2001 persistence in the long term yield record is dominated by low frequency century to decadal periodicity this periodicity is shown to inflate the standard error of the sediment yield estimate for the basin elevated transport of suspended sediments following years of extreme yield is uncommon in the green lake record and indicates a sediment transport regime that is supply limited for this watershed approximately 50 years of suspended sediment monitoring would be required to reduce the uncertainty of the yield estimate to 37 of its average value a summer rainstorm in 1991 delivered more suspended sediment to the green lake basin than any other event in the last 3000 years

extensional pluton emplacement models do they work for large plutonic complexes shallow crustal to mid crustal magma emplacement models commonly rely on local or regional extension to solve the room problem we argue that existing extensional models explain the emplacement of sheeted dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons 1 rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall rock extension to be balanced by magma influx 2 rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma and 3 because of their sheetlike shapes and small thicknesses these small magma injections will cool rapidly and form sheeted plutons or dike swarms extension may however facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

magnetic fabric of the řicany granite bohemian massif a record of helical magma flow abstract a highly unusual fabric pattern and inferred flow mechanism was discovered in the shallow level icany granite pluton bohemian massif using an integrated structural and anisotropy of magnetic susceptibility ams study the pluton consists of an outer strongly porphyritic and an inner weakly porphyritic biotite granite separated by a wide gradational contact both varieties share a steep margin parallel magmatic foliation defined by k feldspar phenocrysts and biotite and magnetic ams foliation carried by biotite the steep foliation bears a shallowly plunging magnetic lineation arranged parallel to the circumference of the pluton margin and a steep magnetic lineation in the weakly porphyritic pluton center we propose a new mechanism of magma flow in which the bulk ascent along a steep sided cylindrical conduit was partitioned into a high viscosity phenocryst rich outer margin flowing helically and a low viscosity phenocryst poor center flowing vertically this interpretation is supported quantitatively by a simple model of magma flow within a cylindrical pipe in which the linear decrease in phenocryst content from the pluton margin inwards causes a power law decrease in the effective viscosity of the magma according to the poiseuille equation such a power law viscosity distribution across the conduit produces a central zone of high magma ascent velocity the magnetic ams fabric pattern in the icany pluton may thus preserve a record of helical magma ascent driven by viscosity partitioning in a steep sided conduit which presumably linked an underlying magma chamber with a volcanic feeder at the surface except for one deep seated granite migmatite complex no such helical fabric pattern has been documented in a granite pluton as of yet we propose that the viscosity partitioned helical flow may be a more common magma ascent mechanism in shallow level volcano plutonic systems than previously envisaged

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

melt bearing shear zones analogue experiments and comparison with examples from southern madagascar abstract analogue model experiments were conducted to investigate the influence of irregularly distributed weak sites in localising strain as an aid to understanding shear zone development in partially molten rocks the very weak inclusions consisted of vaseline in a homogeneous matrix of paraffin wax which has a power law viscous rheology boundary conditions were those of pure shear at constant natural strain rate and confining stress 3 the inclusions were initially perfect cylinders with axes parallel to the intermediate bulk strain axis y conjugate shear zones nucleate on the inclusions and link up to form an anastomosing pattern of high strain zones of concentrated shear surrounding much more weakly deformed pods of near coaxial strain the zones initiate at angles near 45 to the bulk shortening axis z but stretch and rotate towards the x axis with increasing bulk strain all inclusions nucleate shear zones so that with increasing development of the anastomosing pattern weak material occurs only within the high strain zones the restriction of migmatite leucosomes to shear zones in natural examples could also reflect a corresponding control of melt on the sites of shear zone nucleation rather than implying accumulation from the surrounding wall rock the model geometry is very similar to that observed in small scale shear zones in migmatites of southern madagascar elongate zones rich in weak inclusions originally either perpendicular or at 45 to the z axis were also modelled for direct comparison with the regional scale geometry of the pan african high grade shear zones on madagascar

exhumation rates in the st elias mountains alaska abstract reprocessing and interpretation of seismic profiles from the gulf of alaska in the vicinity of the st elias mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch approximately 10 000 years ago this estimate of the sediment volume does not include material moved out of the region by long shore currents transportation from the shelf to deeper waters or material trapped in fiords and basins on and near land thus an estimated sedimentation rate offshore of 7 9 mm year and the average erosion rate of 5 1 mm year from the st elias mountains derived from the estimated sedimentation rate represent minimum estimates we infer that these estimated rates are not typical of the approximately 5 m y history of mountain building in the area but rather they represent the result of efficient erosion by glaciers during this interglacial period

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

subglacial extensional fracture development and implications for alpine valley evolution 1 bedrock stresses induced through exhumation and tectonic processes play a key role in the topographic evolution of alpine valleys using a finite difference model combining the effects of tectonics erosion and long term bedrock strength we assess the development of near surface in situ stresses and predict bedrock behavior in response to glacial erosion in an alpine valley the matter valley southern switzerland initial stresses are derived from the regional tectonic history which is characterized by ongoing transtensional or extensional strain throughout exhumation of the brittle crust we find that bedrock stresses beneath glacial ice in an initial v shaped topography are sufficient to induce localized extensional fracturing in a zone extending laterally 600 m from the valley axis the limit of this zone is reflected in the landscape today by a valley shoulder separating linear upper mountain slopes from the deep u shaped inner valley we propose that this extensional fracture development enhanced glacial quarrying between the valley shoulder and axis and identify a positive feedback where enhanced quarrying promoted valley incision which in turn increased in situ stress concentrations near the valley floor assisting erosion and further driving rapid u shaped valley development during interglacial periods these stresses were relieved through brittle strain or topographic modification and without significant erosion to reach more highly stressed bedrock subsequent glaciation caused a reduction in differential stress and suppressed extensional fracturing a combination of stress relief during interglacial periods and increased ice accumulation rates in highly incised valleys will reduce the likelihood of repeat enhanced erosion events

thrust related diapiric and extensional doming in a frontal orogenic wedge example of the montagne noire southern french hercynian belt abstract the montagne noire which is situated at the toe of the orogenic wedge of the french massif central south european variscides appears to be a well suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins the data reported in the present paper show that the montagne noire dome is a particular type of basement involved frontal culmination in an orogenic wedge and foreland basin system this frontal culmination is characterized by a syn contractional ht decompression recorded by clockwise ptt paths and widespread strata overturning in thrust and fold structures which controlled the sedimentation in the adjacent foreland basin these unusual characteristics are interpreted to be a result of the succession of thrusting diapirism and extensional collapse antiformal stacking of syn metamorphic thrust sheets controlled the first stages of the foreland basin development diapirism was essentially responsible for the ht decompression and widespread strata overturning extensional doming was a result of late to post metamorphic collapse acting on the pre existing high amplitude dome diapirism and associated isothermal decompression metamorphism which constitute the essential difference between the montagne noire and ordinary frontal ridges in orogenic wedges were probably enhanced by a local partial melting of the upper to middle crust it is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline

reconstruction of climate and glacial history based on a comparison of varve and tree ring records from mirror lake northwest territories canada abstract this study compared the hydroclimatic signals recorded in annually laminated varved sediments and tree rings within a small study area in the selwyn mountains of the southwestern northwest territories southeastern yukon territory border region of canada the records were located immediately adjacent to each other and within 6 km of instrumental meteorological and hydrometric records which permitted a detailed analysis of climate in the area from ad 1704 to 1996 this study explored the challenges of annually resolved multi proxy hydroclimate analyses and examined how best to interpret the climate record given the differences in proxy formation and the respective signals the high frequency annual variability from both proxies showed a good match for some parts of the record e g mid 1700s and much of the 1900s but revealed different sensitivities at other times e g 1800s however both records contained a common low frequency decadal summer climate signal glacier dynamics influenced varve formation through altered sediment availability during the inferred little ice age lia maximum ice stand in the catchment from ad 1778 to 1892 and subsequent recession after this maximum as a result of reduced lia sedimentation july mean temperatures reconstructed from varve thickness during the 1800s were underestimated compared to the tree ring estimates other instances of low frequency divergence were apparent particularly during parts of the twentieth century despite the overall similarities the extended varve record also provided a glacial history for the southern selwyn mountains indicating probable glacier advances during ad 1300 1450 and ad 1600 1670 maximum ice stand during the 1800s followed by twentieth century recession

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

bedload to suspended load ratio and rapid bedrock incision from himalayan landslide dam lake record about 5400 cal yr bp a large landslide formed a n400 m tall dam in the upper marsyandi river central nepal the resulting lacustrine and deltaic deposits stretched n7 km upstream reaching a thickness of 120 m 14 c dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly 500 yr reconstructed volumes of lacustrine 0 16 km 3 and deltaic 0 09 km 3 deposits indicate a bedload to suspended load ratio of 1 2 considerably higher than the 1 10 that is commonly assumed at the downstream end of the landslide dam the river incised a new channel through 70 m of greater himalayan gneiss requiring a minimum bedrock incision rate of 13 mm yr over last 5400 yr the majority of incision presumably occurred over a fraction of this time suggesting much higher rates the high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments 2007 university of washington all rights reserved

sediments of ice dammed self draining ape lake british columbia the glacier damming ape lake has withdrawn from its neoglacial terminal moraines in the lake since early in this century at an average rate of 15 m a as a result the first known drainings of ape lake occurred in october 1984 and august 1986 in each event about 54 of the volume of the lake was lost through a tunnel in the damming glacier most of the remaining water was held in the deep basin of the lake behind partially breached neoglacial terminal moraines as the glaciers have withdrawn the character of the sediments has changed sediments in the deep basin of the lake are varved but the grain size especially of the summer deposits has decreased and rates of sedimentation have decreased from about 4 mm a to less than 2 mm a in shallow water deposition of varved sediments has given way to the deposition of massive sediments at rates of less than 1 mm a ice rafted debris is rare in deep water despite the presence of calving bergs during and following the first draining significant subaerial

modeling sediment transport in ice walled subglacial channels and its implications for esker formation and proglacial sediment yields sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization geometry and sedimentary structures ultimately eskers are a record of past glacio fluvial sediment transport but there is currently no physical model for this process we develop a 1 d model of morphodynamics in semicircular bedrock floored subglacial channels we adapt a sediment conservation law developed for mixed alluvial bedrock conditions to subglacial channels channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice to which we add the closure or enlargement due to sediment deposition or removal respectively we apply the model to an idealized land terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus or the formation of an incipient esker is inherent to the dynamics of the channelized water flow the alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation we show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply or transport limited system as has been hypothesized for proglacial sediment yields we also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply but perhaps more importantly ice surface slope and the temporal pattern of water delivery to the bed

a theory of glacial quarrying for landscape evolution models numerical models are used commonly to study the topographic evolution of glacially eroded landscapes these models are grounded on a power law rule that relates bedrock erosion rate to either the sliding speed or discharge of glaciers this rule however is poorly linked to the principal process of glacial erosion quarrying in which bedrock blocks are dislodged from the bed by sliding ice a new model of quarrying allows this erosion rule to be evaluated as in past quarrying models ice bed separation during sliding controls deviatoric stresses in the rock that cause crack growth unlike past models bedrock strength heterogeneity resulting from preglacial fractures is included using a weibull statistical distribution of rock strength this strength distribution is predicated on the observation that larger rock bodies have lower strengths because they have a greater probability of containing a large fracture results can indeed be closely fitted with a power law erosion rule but its nonlinearity the range of sliding speed over which it applies and erosion rates depend sensitively on bedrock strength heterogeneity and effective pressure this theory anchors large scale models of glacial erosion to the primary small scale process that these models hope to simulate and reinforces recent emphasis on the role of bedrock fractures in accelerating geomorphic processes moreover by linking basal water pressure to erosion rate the theory can improve efforts with numerical models to study feedbacks between subglacial hydrology and landscape evolution

bedload component of glacially discharged sediment insights from the matanuska glacier alaska the flux of glacially derived bedload and the proportions of the suspended and bedload components carried by proglacial streams are highly debated published data indicate a large range from 30 to 75 in the bedload percentage of the total load two vents where supercooled subglacial meltwater and sediment are discharged were sam pled over the course of an entire melt season in order to quantify the flux of glacially delivered bedload at the matanuska glacier alaska the bedload component contributed by these vents for the one melt season monitored is negligible furthermore the bedload fluxes appear to be strongly supply limited as shown by the poorly correlated discharge bedload flux magnitude and grain size caliber thus in this case any attempt to employ a predictive quantitative expression for coarse sediment production based on discharge alone would be inaccurate a nonglaciated basin proximal to the matanuska glacier ter minus yielded higher bedload sediment fluxes and larger clast sizes than delivered by the two monitored vents such nonglaciated basins should not be overlooked as potentially major sources of coarse bedload that is reworked and incorporated into valley outwash usually subequal although bedload may range from 30 to 75 of the total clastic load establishing portable relationships between suspended load bedload and discharge would enhance our understanding of how and when glaciers erode for the melt season studied we show that bedload is 1 of the total clastic load and does not have any obvious correla tion to discharge at two of the main vents at the terminus of the matanuska glacier alas ka these observations coupled with high measured bedload component of adjacent non glaciated valleys inspire several working hy potheses for the observed origin of coarse bedload in the matanuska river downstream of the ice margin

late holocene glacier fluctuations in the wrangell mountains alaska four intervals of late holocene glacier advance are recognized from study of nine valley glaciers in the wrangell and westernmost st elias mountains of alaska the oldest glacial advance is recognized at the nabesna and barnard glaciers where five radiocarbon ages suggest advance as early as 2700 cal calibrated yr b p two additional radiocarbon dated advances are centered on cal yr a d 300 and the beginning of the little ice age about a d 1200 the best documented little ice age advances occurred during the mid 1600s through the 1800s and are recognized at all nine glaciers these latter advances are dated by tree rings of trees overrun by glaciers in five glacier valley by 17 radiocarbon dates and by tree ring and lichen ages from 20 moraines that were deposited during the culmination of these advances the glacial chronology is broadly similar to chronologies from adjoining alaskan mountain ranges at both coastal and interior sites for the past 3000 yr there are however differences in timing of advances during the first millennium a d the glacial history for the past 2000 yr is also consistent with temperature sensitive proxy records from interior alaska and yukon territory

west directed flow of the west antarctic ice sheet across eastern basin ross sea during the quaternary abstract detailed seismic stratigraphic correlations and contour mapping time structure and time thickness of near surface units in eastern basin ross sea outer continental shelf suggest that strata outcropping at ross bank were deposited as a terminal moraine in association with west directed advance of the west antarctic ice sheet grounding zone from marie byrd land during the grounding event there was a major shift from west directed ice sheet flow to north directed ice stream flow seismic correlation to age control at dsdp leg 28 sites 271 and 272 suggests a pre last glacial maximum quaternary age for this major grounding event