bedforms of the keewatin ice sheet canada abstract by compiling glacial bedforms on a map that covers most of one sector of the laurentide ice sheet it is possible to make some suggestions about their genesis based largely on spatial relationships it can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar dynamic conditions for either landform to exist however there must have been enough sediment available in the base of the glacier to leave or form a feature large enough to be recognizable the presence or absence of sufficient load is related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice water system likewise given sufficient load it is evident that whether drumlins formed or whether ribbed moraine formed in a certain area is a function of the physical nature of the load which is again related to geology of the source outcrops whether the physical characteristics come into play after the sediment has been released from the ice and is being reshaped by basal drag streamlining etc or whether the nature of the load while entrained changes the behaviour of the basal part of the ice is unclear physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of entrained debris and ice to form ribbed moraine on melting whereas drumlins may reflect moulding of plastic subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock the observation that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial drainage are separated in time and circumstance the general occurrence of drumlins and eskers throughout the sediment rich portions of the keewatin ice sheet from zone 1 to its edge is difficult to reconcile with the restriction and intimate association of these forms with ribbed moraine almost exclusively in zone 2 because such a zonal relationship exists to some extent around other ice divides at least in labrador nouveau quebec and newfoundland it seems that some condition changed or existed in this zone throughout or at some specific time during the existence of the keewatin ice sheet possibly around the last ice divides reactivation of the ice sheet by alimentation associated with climatic deterioration may have promoted flow of thin brittle ice this may have shattered the glacier forming stacked thrust plates of ice and debris with subsequent stagnation the thrust plates may have melted out without substantial deformation

southern laurentide ice lobes were created by ice streams des moines lobe in minnesota usa abstract regional mapping in southern minnesota has illuminated a suite of landforms developed by the des moines lobe that delimit the position of the lobe at its maximum and at lesser readvances the ice lobe repeatedly advanced discharged its subglacial water and subsequently stagnated recent glaciological research on antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine based areas of the laurentide ice sheet i postulate that such ice streams may develop in land based areas of an ice sheet as well and that the des moines lobe 200 km wide and 900 km long was an outlet glacier of an ice stream it appears to have been able to advance beyond the laurentide ice sheet as long as adequate water pressure was maintained however the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys stagnation of the lobe is not equivalent to stoppage of the ice stream because ice repeatedly advanced into and onto the stagnant margins stacking ice and debris similar landforms are also seen in other lobes of the upper midwestern united states

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

ice stream surface texture sticky spots waves and breathers the coupled flow of ice till and water this paper addresses the coupling of flows of ice till and water and the issue of whether such coupling provides mechanisms for meso scale kilometres to tens of kilometres variability in ice sheet flow and texture the question of whether effective pressures at the ice bed interface are statically or hydraulically controlled is examined in this paper the answer is scale dependent and has a significant effect on the relationship between ice surface and basal topography the consequences of these considerations on till flow coupled ice till flow and coupled ice till and water flow are examined an analysis of till flow kinematics and shock formation is carried out the linear stability of coupled long wavelength ice till flow is analysed and regions in parameter space where this flow is unstable with rather small rate constants are found upstream moving ice surface waves are predicted the linear stablity of coupled ice till water flow is examined where water flow is modelled using a basal flow system with effective pressure dependent properties again regions in parameter space where the system is linearly unstable are found this time with relatively rapid rate constants the water pressure exhibits breather modes these analyses assume that there is a substantial basal traction a problem with models of ice streams wholly restrained at the side is identified they seem to predict erosion rates which are unfeasibly large there appears to be sufficient variability in the ice till water system to potentially explain texture in ice stream surfaces variations in ice stream thickness of tens of metres not directly relatable to topography and waves moving upstream or downstream most importantly the ice stream bed system is shown to exhibit meso scale variability simply by coupling ice flow according to the shallow ice approximation till flow according to the hydrostatic thin till approximation and water flow according to an effective pressure dependent hydraulics

the morphological characteristics of ribbed moraine abstract ribbed rogen moraines are large subglacially formed transverse ridges that cover extensive areas of the former laurentide scandinavian and irish ice sheets given their ubiquitous and conspicuous nature it is surprising that their characteristics are poorly understood to date most ribbed moraine studies have been spatially restricted and rely on small sample sizes thus published accounts of their characteristics are most likely unrepresentative of ribbed moraines generally this study addresses this deficit by producing the first representative data set on ribbed moraine size shape pattern and distribution and we describe their spatial characteristics various remote sensing and gis techniques were used to map ribbed moraines over a combined area of 81 000 km 2 in canada ireland and sweden producing a database of just over 33 000 individual landforms in all 25 082 ridges were mapped in two areas in central and northern quebec the lac naococane and river kaniapiskau regions 5637 in the lake rogen area of central sweden and 2500 in north central ireland in comparison against the published accounts of ribbed moraine we demonstrate that some widely held assertions are inaccurate or untrue and we show that ribbed moraine morphological characteristics are more complex than has been hitherto reported our data reveals they exist over a larger scale range than was previously stated this study concludes by presenting a list of ribbed moraine characteristics and argues that formational hypotheses must account for these if they are to remain valid explanations of ribbed moraine genesis

glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

bed ribbing instability explanation testing a numerical model of ribbed moraine formation arising from coupled flow of ice and subglacial sediment 1 ribbed moraines are large up to 16 km long ridges of sediment produced transverse to ice flow direction that formed widely beneath palaeo ice sheets since ice sheet stability is sensitive to conditions operating at the bed an understanding of ribbed moraine genesis will provide critical information on ice sheet dynamics currently there is no consensus on ribbed moraine formation and various competing hypotheses have been presented to account for their genesis only one of these theories has been developed into a physically based numerical model that quantitatively describes ribbed moraine formation this theory known as the bed ribbing instability explanation brie argues that ribbed moraines are produced by a naturally arising instability in the coupled flow of ice and till brie demonstrates that transverse subglacial ridges i e ribbed moraine spontaneously grow under certain parameter combinations and it predicts their wavelength spacing between ridges the model represents a significant advance because it is the first time a theory of subglacial bedform generation has been developed to make quantitative predictions which can be formally tested this paper discusses the types of tests that are currently possible and reports the results from the first testing of brie this analysis centers on the ability of brie to predict the primary characteristics of ribbed moraine which are patterning and wavelength results show that brie successfully predicts the correct ribbed moraine pattern and appropriate wavelengths the tests fail to falsify the model and it is concluded that brie remains a viable explanation of ribbed moraine formation

three dimensional seismic velocity structure of the san francisco bay area seismic travel times from the northern california earthquake catalogue and from the 1991 bay area seismic imaging experiment basix refraction survey were used to obtain a three dimensional model of the seismic velocity structure of the san francisco bay area nonlinear tomography was used to simultaneously invert for both velocity and hypocenters the new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm numerous inversions were performed with different parameters to test the reliability of the resulting velocity model most hypocenters were relocated 12 km under the sacramento river delta 6 km beneath livermore valley 5 km beneath the santa clara valley and 4 km beneath eastern san pablo bay the great valley sequence east of san francisco bay is 4 6 km thick a relatively high velocity body exists in the upper 10 km beneath the sonoma volcanic field but no evidence for a large intrusion or magma chamber exists in the crust under the geysers or the clear lake volcanic center lateral velocity contrasts indicate that the major strike slip faults extend sub vertically beneath their surface locations through most of the crust strong lateral velocity contrasts of 0 3 0 6 km s are observed across the san andreas fault in the middle crust and across the hayward rogers creek calaveras and greenville faults at shallow depth weaker velocity contrasts 0 1 0 3 km s exist across the san andreas hayward and rogers creek faults at all other depths low spatial resolution evidence in the lower crust suggests that the top of high velocity mafic rocks gets deeper from west to east and may be offset under the major faults the data suggest that the major strike slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike slip motion the extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity strong ground motion and regional stress

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

rogen moraine an example of two step formation of glacial landscapes abstract a number of sections in rogen moraine ridges are described they show very different types of structures and sedimentology ranging from compact basal till to meltwater sediments in many instances with glaciotectonic features in others totally undisturbed this illustrates the statement that the morphology and the sedimentology including internal structures are the result of different processes the formation took place in two steps which may be the effect of one and the same glacial phase or of two or more different glaciations

fresh arguments against the shaw megaflood hypothesis a reply to comments by david sharpe on paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

pressure dependent viscosity and interfacial instability in coupled ice sediment flow we study an interfacial instability in the coupled flow of ice and subglacial sediment both modelled as viscous media unlike other interfacial instabilities in coupled viscous flows at zero reynolds number the mechanism considered here does not rely on buoyancy or the effect of an upper free surface but on the pressure dependence of the sediment viscosity specifically the instability relies on sediment rheology being such that when sediment flows in simple shear sediment flux increases with compressive normal stress at the ice sediment interface when the velocity of the interface is kept constant when ice moves over a shallow bump in the interface it generates a higher compressive stress on the bump s upstream side than in its lee if in addition the effective sediment viscosity is low compared with that of ice interfacial velocity remains approximately constant and this then implies that more sediment flows into the bump than out of it causing it to grow modelling ice as a newtonian material we show that this mechanism works for a wide range of sediment rheologies including the highly nonlinear shear thinning ones typically thought most appropriate for the description of nearly plastic sediment the instabilities predicted are essentially two dimensional with infinite transverse wavelength and a nonlinear model shows that growth is unbounded until cavitation occurs in the lee of evolving bumps on the interface the instability mechanism does not seem to predict the formation of common glacial landforms but may explain the formation of water filled cavities on deformable glacier beds

glacial sedimentary evidence supporting stick slip basal ice flow last glacial late devensian subglacial sediments in the southern sperrin mountains north central ireland comprise tabular bedrock rafts which are interbedded with diamicton till and brecciated bedrock and separated by glaciotectonic shears bedrock rafts and diamicton beds alternate laterally and vertically in the profile suggesting the ice bed interface was chequered with both high strength rock rafts and low strength diamicton patches during accumulation of the sediment pile the overall facies arrangement is interpreted qualitatively as reflecting stick slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice bed interface it is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower pressure regions infilling shallow cavities on the surrounding diamicton surface slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces causing ice bed uncoupling the presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the late devensian mid latitude ice sheets

positive feedbacks associated with erosion of glacial cirques and overdeepenings the principal points of water input to a glacier are the bergschrund in cirques and crevasse fields lower on the glacier crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds the amplitude of subglacial water pressure fluctuations is large just down glacier from these points of water input erosion by quarrying is likely in such areas erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings in the case of overdeepenings this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface resulting in erosional forces that accentuate the perturbation when subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases in such situations subglacial conduits are maintained by high water pressures rather than by melting of conduit walls in the limit water pressures apparently become so high that water is forced out along the ice bed interface and the conduits collapse the products of erosion are then no longer flushed out and a protective till layer accumulates by limiting erosion on such adverse bed slopes this till layer controls the geometry of these overdeepened basins

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

tectonic controls on magmatism in the geysers clear lake region evidence from new geophysical models in order to study magmatism and geothermal systems in the geysers clear lake region we developed a detailed three dimensional tomographic velocity model based on local earthquakes this high resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km the most significant velocity contrasts in the geysers clear lake region occur in the steam production area where high velocities are associated with a quaternary granitic pluton and in the mount hannah region where low velocities occur in a 5 km thick section of mesozoic argillites in addition a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern california this regional model sampled the whole crust but at a lower resolution than the local model the regional model outlines low velocities at depths of 8 12 km in the geysers clear lake area which extend eastward to the coast range thrust these low velocities are inferred to be related to unmetamorphosed mesozoic sedimentary rocks in addition the regional velocity model indicates high velocities in the lower crust beneath the clear lake volcanic field which we interpret to be associated with mafic underplating no large silicic magma chamber is noted in either the local or regional tomographic models a three dimensional gravity model also has been developed in the area of the tomographic imaging our gravity model demonstrates that all density contrasts can be accounted for in the upper 5 7 km of the crust two dimensional magnetotelluric models of data from a regional east west profile indicate high resistivities associated with the granitic pluton in the geysers production area and low resistivities in the low velocity section of mesozoic argillites near mount hannah no indication of midcrustal magma bodies is present in the magnetotelluric data on the basis of heat flow and geologic evidence holocene intrusive activity is thought to have occurred near the northwest geysers mount hannah sulphur bank mine and perhaps other areas the geophysical data provide no conclusive evidence for such activity but the detailed velocity model is suggestive of intrusive activity near mount hannah similar to that in the felsite of the geysers production area the geophysical models seismicity patterns distribution of volcanic vents heat flow and other data indicate that small young intrusive bodies that were injected along a northeast trend from the geysers to clear lake probably control the thermal regime

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

morphometry and core type of streamlined bedforms in southern sweden from high resolution lidar abstract this paper generates a new data set of streamlined glacial bedforms in southern sweden which is used both to test conclusions from previous work on streamlined bedform morphometrics and to advance our knowledge of streamlined bedforms on the scandinavian shield the data set consists of streamlined glacial bedforms in southeast sweden mapped from the new lidar derived swedish national height model which has a pixel resolution of 2 0 m with a vertical resolution of 1 cm we have mapped 10 311 features of the mapped features 135 are known to have an unconsolidated sediment core 2120 a bedrock component and 8055 whose core composition is unknown the extracted morphological variables are then subjected to a univariate and bivariate analysis we find that the extracted characteristics broadly fit into the lower end of the modal and median value spectrum of similar bedforms from around the world the distribution of the variables is found to be log normal to a first order approximation the covariant relationships between height and length width and area are examined after the variables have been log transformed and are found to be significant if not particularly strong rock cored features are found to have a longer modal length than soft cored features which suggests that reconstructions of past flow velocities from streamlined landforms need to closely consider core type additionally we find no support for a derived scaling law for streamlined features by plotting length width and elongation ratio against one another

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

quaternary geology and geomorphology of the dinosaur provincial park area and surrounding plains alberta canada the identification of former glacial lobes drainage diversions and meltwater flood tracks abstract the quaternary geology and geomorphology of the lower red deer river drainage basin centred on the dinosaur provincial park badlands provides information on pre late wisconsinan drainage patterns and the dynamics of former lobes of the laurentide ice sheet in western canada patterns of deglaciation proglacial lake evolution and spillway incision are also reconstructed based upon the distribution of surface materials and glacial glaciofluvial landforms the empress group fluvial and glaciolacustrine sediments which could be as young as 27 ka bp infill the precursor red deer river and its tributaries and document the initial advance of glacier ice into southern alberta glaciotectonic disturbance of older sediments and bedrock the production of deformation tills and the construction of a megafluting complex and cupola hills record the advances of a glacier lobe centred over the study area stratified inter and intra till beds record pulses of subglacial meltwater between phases of subsole deformation the thickening of tills towards the margin of the lobe represents a till wedge an expected product of sediment advection by glaciers moving over deformable beds the eastern margin of the glacier lobe is demarcated by the interlobate suffield hummocky moraine belt which contains overprinted thrust ridges which record diachronous oscillations of neighbouring lobes within the ice sheet proglacial and glaciofluvial sediments were deposited in the area in association with proglacial lake bassano patricia which drained eastwards when the suffield moraine was dissected by spillways changes in the size of glacial lake bassano patricia are clearly documented by a sequence of spillway incisions which culminated in the erosion of scabland topography and the initiation of a new course for the red deer river a 15 km southward diversion of the main channel in distinct contrast to the documented incision histories of other small rivers in alberta one tree creek and little sandhill creek did not start major incisions of the quaternary sediments over buried valley positions until the late holocene when environmental conditions were characterized by higher precipitation

thermodynamics of basal freeze on predicting basal and subglacial signatures of stopped ice streams and interstream ridges we have constructed a numerical model that simulates the response of subglacial sediments to basal freeze on the model is set up to emulate the basal zone of drilling sites in the ross sea sector of thewest antarctic ice sheet we treatbasal freeze on atanice sedimentinterfaceasathermodynamicprocessthatcouplestheflowofwater heat and solutes inunfrozensubglacial sedimentsunderlyinga freezingicebase thecouplingof these flows occurs through the clapeyron equation which specifies the dependence of the basal freezing melting temperature on ice pressure water pressure solute concentration and surface tension effects thermally driven water flow is induced when an ice base becomes supercooled below the pressure melting point because ice water surface tension inhibits ice growth in small pore spaces of fine grained subglacial sediments our model results show that basal freeze on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges these changes are associated with specific signatures that compare with borehole observations and geophysical surveys water pres sure levels are reduced and thick layers of debris laden basal ice develop these basal ice layers andunderlying sedimentscontain a distinct isotopic signal the predicted stable iso tope ratios reflect rayleigh type isotopic fractionation whose significance increases with increasing freezing rates supercooling of the ice base induces also measurable changes in the ice temperature profile of the glacier till porosity represents another quantity whose evolution is influenced strongly by basal freeze on in particular measurements of vertical porositydistributionbeneathstoppedicestreamscouldbeusedtoback calculatethetiming of the onsetof basal freezing our modelresults show thatthebasal zoneof ice streams and interstream ridges responds sensitively to changes inbasal melting freezing rates this sen sitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples our model resultsalso indicate thatmeltwater from fast flowing ice streams may be driven towardsthe freezing icebase of interstream ridges

glacial landsystems of the southwest laurentide ice sheet modern icelandic analogues landform assemblages and associated stratigraphy sedimentology and structure are used in the reconstruction of palaeo ice sheet dynamics in alberta western canada interpretations are based upon the modern analogues from four outlet glaciers at the margins of vatnajokull and myrdalsjokull iceland in the area between lloydminster and lac la biche central alberta an extensive landform assemblage of megaflutings crevasse squeeze ridges and thrust block moraine arcs document the former surging of part of the margin of the laurentide ice sheet during later stages of recession this landform assemblage including numerous exposures of glacitectonised bedrock and quaternary sediments is comparable to the landsystem of the surging glaciers eyjabakkajokull and bruarjokull in iceland near high river southern alberta the former existence of an ice lobe characterised by active recession is recorded by closely spaced low amplitude recessional push moraines that drape tunnel valleys these are comparable in form and pattern to annual push moraines and fluted till surfaces produced by breidamerkurjokull and sandfellsjokull iceland and also include rimmed depressions produced by the escape of artesian water during ice marginal pushing this study provides interpretations of the regional glacial geomorphology of alberta based upon comparisons of form and stratigraphy with modern glacial analogues and provides an alternative to recent models which invoke large floods of subglacial meltwater to explain many of these same features implications for ice dynamics and regional till stratigraphies are discussed copyright 1999 john wiley sons ltd

thermal constraints on earthquake depths in california the high quality databases for california seismicity from the southern and northern california earthquake centers and an extensive compilation of thermal measurements in california are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone the base of this zone below which only a small percentage of crustal earthquakes occur is termed the cutout depth and it is at or near the brittle ductile transition in the crust based on laboratory deformation studies this transition should be temperature strain rate lithology and stress state dependent in this study qualitative comparisons between the heat flow in california and earthquake hypocentral distributions show first that as expected earthquake cutout depths are inversely related to heat flow second the epicentral distributions tend to parallel thermal transitions this correlation is probably related to stress concentrations in these locations an ancillary observation is that the cooler western mojave block in southern california appears to be behaving similarly to the tibetan indenter block in asia as faults tend to go around it into areas of higher heat flow where the seismogenic zone is thinner and the crust may be weaker and it is pushed toward and under the southern cold sierra nevada block third to quantitatively compare the data sets temperatures for the seismicity cutout depth in california were calculated using the steady state 1d heat conduction equation with the variables based on published values for heat flow heat generation and thermal conductivity the analysis was restricted to profiles along which the heat flow and earthquake databases were constrained which allowed the error in the temperature calculation to be determined using monte carlo simulations the results show that two distinct ranges of temperatures dependent upon location describe the cutout depth d 99 of seismicity for california 450 50 c and 260 40 c the 450 50 c cutout depth temperature is most widespread geographically occurs within many provinces and is higher than the frequently referenced temperature of 300 50 c for the brittle ductile transition the lower temperature 260 c seems to be restricted to provinces where the heat flow is near or below 50 mw m 2 such as the mojave block and sierra nevada the differences in these cutout temperatures suggest that additional factors such as strain rate stress regime and or lithology may contribute to the seismicity cutout depth fourth along a profile with significant seismic activity and a 450 c cutout temperature the envelope for the maximum energy released by earthquakes falls at or below the 300 c isotherm detailed characterization of the heat flow and earthquake synergy in this manner furthers the understanding of the earthquake process and can aid in the estimation of the maximum depth of rupture for great earthquakes particularly in areas of low seismicity thus reducing uncertainties in hazard calculations manuscript received 26 february 2003

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

the subglacial geomorphology of southeast alberta evidence for subglacial meltwater erosion a coherent pattern of landforms in southeast alberta forms a subglacial landform continuum scoured bedrock tracts flutes transverse bed forms and tunnel channels in this continuum are inferred to be products of erosion by turbulent subglacial meltwater flows beneath the laurentide ice sheet bedrock and glacigenic sediment are truncated by an erosional surface flutes and extensive boulder lags across the truncation surface strongly suggest erosion by high velocity turbulent flows tunnel channels dissect the erosion surface and record channelization of earlier sheet flows convex longitudinal profiles of channels indicate subglacial meltwater flow northeast southwest trending transverse bed forms are superimposed on the preglacial divide and may be either glaciotectonic ridges or fluvial bed forms landforms reflect the dominance of erosion in the subglacial environment likely by catastrophic meltwater flows of the livingstone lake megaflood event the subglacial meltwater hypothesis accounts for e

age intercalibration of 40ar 39ar sanidine and chemically distinct u pb zircon populations from the alder creek rhyolite quaternary geochronology standard abstract we report results from a 40 ar 39 ar sanidine and ca tims 238 u 206 pb zircon dating study of eruption and crystal residence timescales of the alder creek rhyolite acr california extruded during the cobb mountain normal polarity subchron c1r 2n a 40 ar 39 ar acr sanidine date of 1 1850 0 0016 ma 2 external uncertainty determined relative to the astronomically dated a1 tephra sanidine is interpreted as the acr eruption age this age is supported by ca tims u pb zircon dating guided by la icpms trace element analyses titanium in zircon tizr thermometry and cathodoluminescence cl imaging using these data two compositionally distinct zircon populations were revealed pre acr group b zircon exhibit oscillatory zoning large positive ce and negative eu anomalies high incompatible trace element contents tizr temperatures of 650 c 750 c and th u disequilibrium corrected 238 u 206 pb dates of 1 38 1 24 ma these crystals are interpreted as antecrysts inherited from earlier intrusives in the geysers cobb mountain magma source region acr related group a zircon present as discrete grains and overgrowths on group b zircon display less intense cl with diffuse zoning and have less pronounced positive ce and negative eu anomalies lower incompatible trace element contents higher tizr temperatures that range up to 840 c and significantly younger dates the youngest group a dates yield a weighted mean of 1 1978 0 0046 ma 2 including systematic uncertainties that is interpreted as the mean age of zircon crystallization prior to eruption the 13 5 ka offset between the 40 ar 39 ar and 238 u 206 pb dates can be attributed to zircon magma residence time recognition of a young population of acr zircon is consistent with the 40 ar 39 ar eruption age which coincides with the astronomical age estimate for the cobb mountain subchron determined by correlating the oxygen isotope record of the giant piston core md972143 to the la93 1 1 orbital solution on the basis of independent radio isotopic and orbital forcing results we propose the refined age of 1 1850 0 0016 ma 2 external uncertainty for the quaternary acr 40 ar 39 ar sanidine standard

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

regional three dimensional seismic velocity model of the crust and uppermost mantle of northern california 1 we present a three dimensional 3d tomographic model of the p wave velocity vp structure of northern california we employed a regional scale double difference tomography algorithm that incorporates a finite difference travel time calculator and spatial smoothing constraints arrival times from earthquakes and travel times from controlled source explosions recorded at network and or temporary stations were inverted for vp on a 3d grid with horizontal node spacing of 10 to 20 km and vertical node spacing of 3 to 8 km our model provides an unprecedented comprehensive view of the regional scale structure of northern california putting many previously identified features into a broader regional context and improving the resolution of a number of them and revealing a number of new features especially in the middle and lower crust that have never before been reported examples of the former include the complex subducting gorda slab a steep deeply penetrating fault beneath the sacramento river delta crustal low velocity zones beneath geysers clear lake and long valley and the high velocity ophiolite body underlying the great valley examples of the latter include mid crustal low velocity zones beneath mount shasta and north of lake tahoe

age and thermal history of the geysers plutonic complex felsite unit geysers geothermal field california a 40ar 39ar and u pb study abstract sixty nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the geysers geothermal field yield 207pb 206pb vs 238u 206pb concordia ages ranging from 1 13 0 04 ma to 1 25 0 04 1 ma the weighted mean of the u pb model ages is 1 18 0 03 ma the u pb ages coincide closely with 40ar 39ar age spectrum plateau and terminal ages from coexisting k feldspars and with the eruption ages of overlying volcanic rocks the data indicate that the granite crystallized at 1 18 ma and had cooled below 350 c by 0 9 1 0 ma interpretation of the feldspar 40ar 39ar age data using multi diffusion domain theory indicates that post emplacement rapid cooling was succeeded either by slower cooling from 350 to 300 c between 1 0 and 0 4 ma or transitory reheating to 300 350 c at about 0 4 0 6 ma subsequent rapid cooling to below 260 c between 0 4 and 0 2 ma is in agreement with previous proposals that vapor dominated conditions were initiated within the hydrothermal system at this time heat flow calculations constrained with k feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known geysers plutonic complex is required to maintain the geothermal system this requirement is satisfied by either a large underlying convecting magma chamber now solidified emplaced at 1 2 ma or episodic intrusion of smaller bodies from 1 2 to 0 6 ma

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

glacial curvilineations new glacial landforms produced by longitudinal vortices in subglacial meltwater flows abstract the glacial landscape of the dobrzyn plateau in central poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans the term glacial curvilineations is proposed to describe these enigmatic bedforms that occur as fields of parallel sinuous ridges separated by troughs the regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau we propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel forming flows parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau understanding curvilineation genesis may shed light on glacier hydrologic processes tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater such as drainage of subglacial and or supraglacial lakes the extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet like flows as wide as a few kilometres

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

monitoring deformation at the geysers geothermal field california using c band and x band interferometric synthetic aperture radar monitoring deformation at the geysers geothermal field california using c band and x band interferometric synthetic aperture radar d w vasco jonny rutqvist alessandro ferretti alessio rucci fernando bellotti patrick dobson curtis oldenburg julio garcia mark walters and craig hartline d w vasco jonny rutqvist patrick dobson curtis oldenburg earth sciences di vision building 74 berkeley laboratory 1 cyclotron road berkeley ca 94720 e mail dwvasco lbl gov jrutqvist lbl gov pfdobson lbl gov cmoldenburg lbl gov alessandro ferretti alessio rucci and fernando bellotti tele rilevamento europa via vittoria colonna 7 20149 milano italy e mail alessandro ferretti treuropa com alessio rucci treuropa com fernando bellotti treuropa com julio garcia mark walters and craig hartline calpine corporation 10350 socrates mine road middletown ca 95461 e mail julio garcia calpine com mark walters calpine com craig hartline calpine com

numerical investigations of the slow down of whillans ice stream west antarctica is it shutting down like ice stream c we investigate whether the recent slow down of whillans ice stream wis west antarctica may lead to its complete stoppage in the near future using a numerical model basal resistance to ice stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing we implement a basal drainage system which acts to hinder ice stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing the ice module is a standard flowline model with parameterized key out of plane effects raymond 1996 the most important result of our modeling effort is that we never obtained a slow down that would not be followed by a complete stoppage within 5100years wis slow down canbe avoided in the model if large basal water input rates are assumed to satisfy basal freezing in com parison thetestedperturbationsinwiswidthandmassbalancehada relativelysmalleffect onthetendencyof the simulated ice streamto slowdown these resultsunderscorethe need for morequantitativeconstraints ontheefficiencyof sub ice streamwaterdrainage we con jecture that the present day slow down of wis will evolve to shut down in the next few decades unless anadditionof basalwaterprevents freeze on drivenbed strengthening

superimposition of ribbed moraines on a palaeo ice stream bed implications for ice stream dynamics and shutdown the sediments and landforms preserved on palaeo ice stream beds can provide important information about their subglacial conditions and flow mechanisms and the processes accompanying their shutdown in this paper detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the dubawnt lake ice stream bed north west canadian shield are presented including their morphometry internal structure from ground penetrating radar gpr surveys and from glaciogeological analysis and sedimentological characteristics from sediment architecture and lithofacies analysis the observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development this hypothesis is based on evidence from a deformed lodgement till which subsequently underwent brittle deformation and developed prominent thrust shear structures and tension fractures tension fractures are observed in a sediment exposure and thrust structures are observed in gpr surveys where they occur most prominently in the ribbed moraine ridge crests the presence of the fractures and their association with a population of clasts in the till that are orientated with their a axes transverse to the inferred ice flow direction suggests a compressional flow regime it is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity but that at some point patches of till under the ice stream stiffened through dewatering the subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines we therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams i e sticky spots and or at the transition between slow and fast ice flow along parts of an ice stream the general absence of ribbed moraines on most other palaeo ice stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow sticky spots led to their shutdown e g ice depletion

similarity of organized patterns in driving and basal stresses of antarctic and greenland ice sheets beneath extensive areas of basal sliding the rate of ice transport from the interior of ice sheets to their margins and hence the rate with which it contributes to sea level is determined by the balance of driving stress basal resistance and ice internal deformation using recent high resolution observations of the antarctic and greenland ice sheets we compute driving stress and ice deformation velocities inferring basal traction by inverse techniques the results reveal broad scale organization in 5 20 km band like patterns in both the driving and basal shear stresses located in zones with substantial basal sliding both ice sheets experience basal sliding over areas substantially larger than previously recognized the likely cause of the spatial patterns is the development of a band like structure in the basal shear stress distribution that is the results of pattern forming instabilities related to subglacial water the similarity of patterns on the greenland and antarctic ice sheets suggests that the flow of ice sheets is controlled by the same fundamental processes operating at their base which control ice sheet sliding and are highly variable on relatively short spatial and temporal scales with poor predictability this has far reaching implications for understanding of the current and projection of the future ice sheets evolution

ribbed moraine formation ribbed rogen moraines are conspicuous landforms found in interior parts of formerly glaciated areas two major theories for ribbed moraine formation have been suggested in recent years i the shear and stack theory which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow followed by basal melt out of transverse moraine ridges and ii the fracturing theory according to which ribbed moraines form by fracturing of frozen pre existing till sheets at the transition from cold to warm based conditions under deglaciating ice sheets in this paper we present new data on the distribution of ribbed moraines and their close association with areas of frozen bed conditions under ice sheets in addition we show examples of ribbed moraine ridges that fit together like a jig saw puzzle these observations indicate that fracturing and extension of a pre existing till sheet may be a predominant process in ribbed moraine formation in summary we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines one implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen bed to thawed bed conditions under former ice sheets

reflections on soft subglacial beds as a mosaic of deforming and stable spots abstract much discussion regarding subglacial processes is centred around the question of how soft beds respond to ice overriding in particular how widespread and penetrative is their deformation how does it affect the ice stability and what are the consequences for large scale sediment redistribution patterns contrasting views have been proposed which are difficult to verify and reconcile in the light of scarcity of agreed criteria to identify deforming beds here we suggest that soft beds under warm based glaciers can be envisaged as a mosaic of deforming and stable spots transient in time and space according to changes in the control parameters at the ice bed interface the subglacial mosaic model is consistent with field data showing heavily deformed beds interspersed with largely undisturbed beds it is suggested that deformation found in tills is a cumulative effect of strain within a relatively thin ephemeral deforming spots under the ice sole affecting the till as it accretes during ice overriding in accord with the coulomb plastic rheology the role of clast ploughing in bed deformation is emphasised

ribbed bedforms on palaeo ice stream beds resemble regular patterns of basal shear stress traction ribs inferred from modern ice streams rapidly flowing ice streams are an important mechanism through which ice sheets lose mass and much work has been focussed on elucidating the processes that increase or decrease their velocity recent work using standard inverse methods has inferred previously unrecognised regular patterns of high basal shear stress sticky spots 200 kpa beneath a number of ice streams in antarctica and greenland termed traction ribs they appear at a scale intermediate between smaller ribbed moraines and much larger mega ribs observed on palaeo ice sheet beds but it is unclear whether they have a topographic expression at the bed here we report observations of rib like bedforms from dems along palaeo ice stream beds in western canada that resemble both the pattern and dimensions of traction ribs their identification suggests that traction ribs may have a topographic expression that lies between and partly overlaps with ribbed moraines and much larger mega ribs these intermediate sized bedforms support the notion of a ribbed bedform continuum their formation remains conjectural but our observations from palaeo ice streams coupled with those from modern ice masses suggest they are related to wave like instabilities occurring in the coupled flow of ice and till and modulated by subglacial meltwater drainage their form and pattern may also involve glaciotectonism of subglacial sediments

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

glacial geomorphology of terrestrial terminating fast flow lobes ice stream margins in the southwest laurentide ice sheet glacial geomorphological mapping of southern alberta canada reveals landform assemblages that are diagnostic of terrestrial terminating ice streams fast flowing outlet glaciers with lobate snouts spatial variability in features that comprise the landform assemblages reflects changes in a palaeo ice stream activity switch on off and b snout basal thermal regimes associated with climate sensitive steady state flow palaeo ice stream tracks reveal distinct inset sequences of fan shaped flowsets indicative of receding lobate ice stream margins former ice lobe margins are demarcated by a major often glacially overridden transverse moraine ridges commonly comprising glacitectonically thrust bedrock and b minor closely spaced recessional push moraines and hummocky moraine arcs details of these landform types are well exhibited around the former southern margins of the central alberta ice stream where larger scale more intensive mapping identifies a complex glacial geomorphology comprising minor transverse ridges mtr types 1 3 hummocky terrain ht types 1 3 flutings and meltwater channels spillways the mtr type 1 constitute the summit corrugation patterns of glacitectonic thrust moraines or major transverse ridges and have been glacially overrun and moderately streamlined the mtr type 2 sequences are recessional push moraines similar to those developing at modern active temperate glacier snouts the mtr type 3 document moraine construction by incremental stagnation because they occur in association with hummocky terrain the close association of hummocky terrain with push moraine assemblages indicates that they are the products of supraglacial controlled deposition on a polythermal ice sheet margin where the ht type 3 hummocks represent former ice walled lake plains the ice sheet marginal thermal regime switches indicated by the spatially variable landform assemblages in southern alberta are consistent with palaeoglaciological reconstructions proposed for other ice stream fast flow lobes of the southern laurentide ice sheet where alternate cold polythermal and temperate marginal conditions associated with climate sensitive steady state flow sequentially gave way to more dynamic streaming and surging activity

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

the stress state of the northwest geysers california geothermal field and implications for fault controlled fluid flow a dataset comprised of well constrained focal mechanisms for 6147 earthquakes recorded in the northwest geysers geothermal field during the period of 2005 2012 was utilized to conduct a detailed stress study within and below the geothermal reservoir the high quality focal mechanisms were organized into grid blocks of varying size using a 3d octree gridding algorithm in which discretization was governed by data density this method allows for separate inversions of contiguous blocks of seismicity at a relatively fine scale we obtained the three principal stress orientations for every grid block containing at least 25 events by inverting for the best fit stress tensor within each grid block the principal stress orientations were used to determine which of the two nodal planes for each focal mechanism had the highest ratio of resolved shear to normal stresses and was thus more likely to be the fault plane we found a normal strike slip faulting regime s hmax s v s hmin both within and below the reservoir consistent with the extensional and strike slip tectonics in the region surrounding the geysers in addition an average s hmax orientation of n26 e was obtained for the studied crustal volume these observations suggest that injection and production activities over the past 50 years do not appear to have significantly affected the local stress field the presumed fault planes are steeply dipping with northeast southwest to east west strike directions suggesting that these are the principal flow directions both within the low matrix permeability graywacke reservoir and in the wholly concealed granitic pluton locally referred to as the felsite basement below

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

ribbed moraines in sweden distribution pattern and palaeoglaciological implications abstract a detailed map of ribbed moraines in central and northern sweden is presented they are frequent features over much of the mapped area and the distribution has well defined limits the most abundant ribbed moraines are situated in close connection to areas that are inferred to have been cold based during the retreat of the late weichselian ice sheet while outer fully warm based areas lack ribbed moraine observations have also been made on a more local scale where cold based patches grade into ribbed moraines via a zone of a broken up till cover a model of origin is presented which includes contracting of a frozen core area of a retreating ice sheet at the transition from non sliding to sliding conditions there will be high stresses resulting in local extensional flow at the base of the ice sheet a rising phase change surface frozen thawed material would cause detachment and boudinage like breaking up of a pre existing till cover thawed at the bedrock contact but still frozen to the ice sole the orientation of ice flow during the formation of the ribbed moraines closely matches the ice flow directions of the deglaciating late weichselian ice sheet thus ribbed moraines are considered to be deglaciation landforms typical of shrinking cold based zones of a retreating ice sheet

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

geophysical characterization of the northwest geysers geothermal field california abstract the clear lake volcanic field in northern california is the youngest and northern most part of a long lived volcanic system that has produced recent 10 ka eruptions adjacent to the clear lake volcanic field is the world s largest energy producing geothermal field the geysers the hottest part of the geyser s geothermal field is in the northwest where temperatures reach 400 c at 3 km depth low permeability high thermal gradients and low steam saturation prescribed development of an enhanced geothermal system egs in the northwest geysers to increase energy producing capacity though the northwest geysers is known to be the hottest part of the field geophysical methods have failed to adequately image any inferred heat source this project aims to image the heat source of the northwest geysers using newly collected gravity and magnetotelluric mt measurements gravity data were jointly modeled with existing magnetic data along a two dimensional profile aligned with an existing geologic cross section the key feature of the potential field model is a low density low susceptibility body at 5 km depth bmsl under the egs mt data were modeled in three dimensions to characterize subsurface resistivity structure where the upper 3 km of the resistivity model agrees well with existing data lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data below 3 km depth bmsl the resistivity model images a possible young intrusion under the egs a possible zone of partial melt

subglacial landforms and deposits in central and northern quebec canada with emphasis on rogen moraines abstract the development of the anatomy and the morphology of a relatively simple till sheet deposited over parts of the canadian shield is discussed this is characterized by the common occurrence of well developed streamlined landforms and rogen moraines the sedimentation of the till sheet was an alternating succession of depositional and erosional or landforming events the former leading to the sedimentation of various till lithofacies the latter leading to the development of various morainic terrains this orderly sequence of subglacial events occurred during the deglaciation of central and northern quebec with the controlling factors being bed rock topography and geology ice thickness and basal ice temperatures till lithofacies include in ascending stratigraphic order lodgement till subglacial melt out till immature lodgement till and varieties of supra glacial ablation deposits morainic terrains include drumlinized and fluted ground moraine rogen moraines as well as fluted and unfluted hummocky moraine rogen moraines are formed subglacially a few kilometers behind the glacier margin as a consequence of localized compressive stresses which result in shearing and stacking of slices of near base englacial debris later deposited as a basal melt out till the moraines are believed to be a characteristic landform associated with shield type rock surfaces namely an irregular topography of asymmetric rock basins of all sizes resulting from the areal scouring of hard rocks as well the moraines are believed to be as well a characteristic landform resulting from the flattening profile of the margin of the waning ice sheet the formation of the moraines led to enhanced flow in marginal areas generating late events of streamlining and boulder plucking

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

surface form of the southern laurentide ice sheet and its implications to ice sheet dynamics reconstructions of the ice surface morphology of several lobes of the southern laurentide ice sheet reinforce previous arguments that this sector of the ice sheet was thin and low sloping driving stresses estimated from the geometry of the reconstructed ice surfaces are 0 7 4 3 kpa for the 14 ka des moines lobe 0 9 1 2 kpa for the 14 ka james lobe 0 9 1 7 kpa for the 18 20 ka lake michigan lobe 1 8 2 9 kpa for the 15 18 ka chippewa sublobe and 17 22 kpa for the 15 18 ka green bay lobe previous estimates of rates of ice margin advance 450 2 000 m yr indicate moderate to fast ice velocities for the ice lobes reconstructed driving stresses and velocity estimates of the des moines james and lake michigan lobes are analogous to the distal ends ice plains of low sloping 0 4 x 10 3 but fast moving 500 m yr west antarctic ice streams whose dynamics have been attributed to sliding and or subglacial sediment deformation by pervasive shear these reconstructions support recent models of the laurentide ice sheet which include movement by sliding or by subglacial sediment deformation along its southern western and northwestern sectors evidence for either mechanism should be represented in the sedimentologic and geomorphic records thin ice in these regions indicates that the laurentide ice sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles

glacial landforms of extreme size in the keewatin sector of the laurentide ice sheet assemblages of glacial landforms of a mega scale are here identified in the keewatin sector of the laurentide ice sheet large till belts or ridges apparent only on satellite imagery and lyi

the geysers cobb mountain magma system california part 2 timescales of pluton emplacement and implications for its thermal history abstract over 400 ion microprobe u pb isotopic ages measured for zircons extracted from 24 geothermal wells that penetrate the geysers plutonic complex gpc allow us to conclude that the entire known extent of the gpc crystallized during the early pleistocene nine samples of the microgranite porphyry that forms the shallow cupola 100 1 500 m below sea level mbsl of the gpc yield the oldest model u pb age 1 75 0 01 ma after correction for initial u series disequilibrium errors 1 twelve samples from the main intrusive phase orthopyroxene biotite granite present at depths 1 250 mbsl define a crystallization age of 1 27 0 01 ma this coincides with the age determined for a structurally and compositionally distinct body of granodiorite 1 25 0 01 ma n 5 samples that is intruded over a similar depth range two petrographically distinct varieties of orthopyroxene biotite granite yield ages of 1 46 0 03 gpc21 6000 and 1 16 0 02 ma ca5636 74f 21 three samples u pb zircon ages for dikes intruded in metagraywacke country rocks overlap with those obtained from the main body of the gpc and include the youngest material identified dike sample negu2 st1 7700 1 11 0 03 ma overall the u pb results demonstrate that the main body of the gpc 300 km3 was emplaced and crystallized within the upper crust over a short time interval 0 2 ma that overlaps with zircon crystallization ages of overlying silicic volcanic units

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock