nucleation and seismic tremor associated with the glacial earthquakes of whillans ice stream antarctica 1 the ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes here we usedata from a gps and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of whillans ice stream west antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream additionally we show that rupture can be independently tracked by increased levels of microseismic activity including harmonic tremor that are coincident with the onset of slip at any location thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

basal characteristics of the main sticky spot on the ice plain of whillans ice stream antarctica abstract understanding the processes that affect streaming ice flow and the mass balance of glaciers and ice sheets requires sound knowledge of their subglacial environments previous studies have shown that an extensive deformable subglacial sediment layer favors fast ice stream flow however areas of high basal drag termed sticky spots are of particular interest because they inhibit the fast flow of the overriding ice the stick slip behavior of whillans ice stream wis is perhaps the most conspicuous manifestation of a subglacial sticky spot we present new ice thickness and seismic reflection measurements collected over the main sticky spot in the ice plain of wis allowing us to elucidate its role in the behavior of the ice stream ice thickness and surface elevation data show that the sticky spot occupies a subglacial topographic high water flow in response to the hydrological potential gradient will be routed around the sticky spot if effective pressures are similar on the sticky spot and elsewhere the seismic experiment imaged a laterally continuous basal layer approximately 6 m thick having compressional wave velocities of greater than 1800 m s 1 and density greater than 1800 kg m 3 indicative of a till layer that is stiffer than corresponding till beneath well lubricated parts of the ice stream this layer likely continues to deform under the higher shear stress of the sticky spot and some water may be pumped up onto the sticky spot during motion events

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

effective stress profiles and seepage flows beneath glaciers and ice sheets the resistance to sliding and the extent of till deformation beneath soft bedded glaciers depend on the spatially averaged level of effective stress n which is controlled by the distribution of water pressure at the bed major subglacial conduits that facilitate large scale water transport are expected to be predominantly aligned with the direction of maximum hydraulic gradient which is normally parallel to the slope of the glacier surface when the basal heat flow promotes net melting or freezing seepage transport can enable water exchange between these conduits and the rest of the basal surface area for a simple glacier geometry with subglacial conduits that are aligned parallel to a uniform slope the seepage transport is driven primarily by gradients in effective stress balance equations determine how n varies with conduit spacing and the heat flow regime considerations of thermodynamic equilibrium require that ice penetrates the pore space at high effective stress even when the glacier base experiences net melting for a given heat flow regime there are limits on the conduit spacing that can be attained before a finite till layer becomes partially frozen throughout during net freezing the resistance to flow through partially frozen sediments limits the steady state conduit spacing the partially frozen zone can actually be restricted to smaller thicknesses when the freezing rate is greater

a theory for ice till interactions and sediment entrainment beneath glaciers and frozen onto or melted from the glacier base can achieve a steady state that is in balance with the rate that latent heat is transported to or from the basal interface at constant n when a gradual increase in heat flow from the glacier base causes the rate of melting to decrease h increases and continues to do so when the heat flow is great enough to produce freezing as freezing becomes more rapid and h increases further the rate of fluid supply to the glacier base reaches a maximum when the effective permeability is sufficiently reduced by the partial ice saturation in the fringe larger h can be achieved with slower freezing at the glacier base but steady states with larger h are unstable the maximum rate of fluid supply to the glacier base is greater at lower n higher temperature gradients and for sediments with higher permeabilities unsteady behavior can lead to large changes in h when there is a mismatch between the rate that latent heat can be extracted and the rate that fluid is supplied to the glacier base transient behavior driven by abrupt changes in n is characterized by rapid variations in freezing rate followed by slower adjustments to h that are limited by the timescale for the conduction of latent heat the resulting patterns of sediment deformation are expected to commonly be distributed over finite depth ranges even when shear is perfectly localized at any single instant in time

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

dynamics of stick slip motion whillans ice stream antarctica article i nfo the stick slip motion and associated seismic emissions of whillans ice stream wis west antarctica are two of the many recent observations of unexpected ice sheet behavior that are challenging traditional models of rapid glacier motion here we find that the wis slip events repeatedly nucleate from a sticky spot located in the middle of the ice stream acting similar to an asperity in traditional models of earthquake physics this region shows less motion than surrounding areas during the inter slip periods thus concentrating stress and producing a pulse of seismic energy at the onset of slip the propagating rupture breaks through an additional asperity in the northern part of the ice stream producing another pulse of seismic energy 6 12 min after initiation both asperities are regions of higher hydraulic potential than surrounding regions suggesting they may have greater bed friction due to reduced water lubrication tidal pacing of the stress accumulation combined with fault healing controls the applied stress at failure with higher stress giving faster propagation of the rupture front and higher slip velocities these differences are reflected in the timing of the teleseismic arrivals our results highlight both the great sensitivity of large ice streams to small changes in external forcing and the importance of limited regions of the subglacial bed in controlling their motion as well as providing insights to the mechanics of repeating earthquakes

conditions for thrust faulting in a glacier 1 dipping arcuate bands of debris rich ice outcropping near the margins of glaciers are often interpreted as thrust faults assumed to originate in zones of longitudinal compression identification of thrusts is typically based either on the geometry and sedimentology of the debris bands or on the crystal fabric of surrounding ice but the physical processes necessary to generate thrusts are rarely evaluated herein we combine a numerical model of compressive ice flow near a glacier margin with theoretical stress and strain rate criteria for ice fracture and stress criteria for frictional slip to determine the conditions necessary for thrust faulting in glaciers this model is applied to two different glaciological settings where longitudinal compression has been documented 1 the transition between warm based and cold based ice near the terminus of storglaciaren sweden and 2 the downglacier extent of the 1983 surge front of variegated glacier where surging ice encountered stagnant ice simulations representing the margin of storglaciaren indicate that peak compressive strain rates are six orders of magnitude too small to induce fracture whereas at variegated glacier strain rates were an order of magnitude too small for compressive fracture in both groups of simulations preexisting fractures governed by coulomb friction are susceptible to slip if they span the ice thickness are oriented close to the optimal fracture angle and in the case of storglaciaren are subject to water pressures that are a large fraction of ice overburden pressure variations about the optimal fracture orientation low or zero water pressure high sliding friction coefficient and limited vertical or lateral fracture extent each tend to suppress thrusting

debris entrainment and landform genesis during tidewater glacier surges the englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed the subsequent melt out of debris rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity we investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice englacial structures and ridge networks exposed at the margins of tunabreen a tidewater surge type glacier in svalbard the basal ice facies display clear evidence for brittle and ductile tectonic deformation resulting in overall thickening of the basal ice sequence the formation of debris poor dispersed facies ice is the result of strain induced metamorphism of meteoric ice near the bed debris rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults reoriented crevasse squeezes and preexisting fractures these structures are observed to melt out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff ridge networks are also located at the terrestrial margins of tunabreen neighboring von postbreen and in a submarine position within tempelfjorden analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris rich englacial structures this in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges

variable deceleration of whillans ice stream west antarctica the whillans ice stream ice plain wip has been slowing since at least 1963 prior constraints on this slowdown were consistent with a constant long term deceleration rate new observations of ice velocity from 11 continuous and 3 seasonal gps sites indicate the deceleration rate varies through time including on interannual time scales between 2009 and 2012 wip decelerated at a rate 6 1 to 10 9 2 m yr2 that was double the multidecadal average 3 0 to 5 6 2 m yr2 to identify the causes of slowdown we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher order inverse model all model results support the conclusion that the observed deceleration of wip is caused by an increase in basal resistance to motion at a rate of 10 to 40 pa yr subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on changes in subglacial hydrology or increases in the area of resistant basal substrate through differential erosion the observed variability in wip deceleration rate suggests that dynamics in subglacial hydrology plausibly driven by basal freeze on and or activity of subglacial lakes plays a key role in modulating basal resistance to ice motion in the region

till stratigraphy and late glacial events in the lake michigan lobe of eastern wisconsin four late wisconsinan reddish brown tills are present along the lake michigan shoreline in eastern wisconsin these tills are distinguished by differences in grain size distribution clay mineral content and stratigraphic position the ozaukee and haven tills underlie the two creeks forest bed the younger valders till although not present at a forest bed locality is also pre twocreekan age based on several arguments including correlation with tills of the adjoining green bay lobe one greatlakean age till the two rivers till has been recognized and this clearly overlies the two creeks forest bed as previously reported we conclude that the valders till is most likely of pre twocreekan age and that valderan is an inappropriate term as a substage name for post twocreekan time and that the use of the term greatlakean is justified by stratigraphic studies

seismic and geodetic evidence for grounding line control of whillans ice stream stick slip events the tidally modulated stick slip events of whillans ice stream in west antarctica produce seismic energy from three locations near the grounding line using ice velocity records obtained by combining time series from colocated broadband seismometers and gps receivers installed on the ice stream during the 2010 2011 and 2011 2012 austral summers along with far field seismic recordings of elastic waves we locate regions of high rupture velocity and stress drop these regions which are analogous to asperities in traditional seismic fault studies are areas of elevated friction at the base of the ice stream slip events consistently initiate at one of two locations near the center of the ice stream where events associated with the ross sea high tide originate or a grounding line spot where events associated with the ross sea low tide initiate as well as occasional high tide events following a skipped low tide event the grounding line site but not the central site produces rayleigh waves observable up to 1000 km away through fast expansion of the slip area grounding line initiation events also show strong directivity in the downstream direction indicating initial rupture propagation at 1 5 km s compared to an average of 0 150 km s for the entire slip event following slip initiation additional seismic energy is produced from two sources located near the grounding line first at the downstream end of subglacial lake engelhardt and second toward the farthest downstream extent of the ice stream this evidence suggests that the stronger higher friction material along the grounding line controls motion throughout the stick slip region

basal icequakes recorded beneath an alpine glacier glacier d argentiere mont blanc france evidence for stick slip motion while basal icequakes associated with glacier motion have been detected under antarctica for several decades there remains very little evidence of stick slip motion for alpine glaciers here we analyzed 2357 basal icequakes that were recorded at glacier d argenti ere mont blanc massif between february and november of 2012 and that are likely to be associated with basal sliding these events have been classified into 18 multiplets based on their waveforms the strong similarity of the waveforms within each multiplet suggests an isolated repeating source despite this similarity the peak amplitude within each multiplet varies gradually in time by up to a factor of 18 the distribution of these events in time is relatively complex for long time scales we observe progressive variations in the amplitudes of events within each multiplet for intermediate time scales hours the events occur regularly in time with typical return times of several minutes up to several hours for short time scales from 0 01 to 100 s the largest multiplet shows clus tering in time with a power law distribution of the interevent times the location of these events and their focal mechanisms are not well constrained because most of these events were detected by a single seismometer nevertheless the locations can be estimated with an accuracy of a few tens of meters using a polarization analysis the estimated average depth of the basal events is 179 m which is in good agreement with the estimated glacier thickness the relative changes in distance between the source and the sensor can be measured accurately by correlating separately the p wave and s wave parts of the seismograms of each event with the template waveforms which are obtained by averaging the signals within each multiplet we observed small variations in the times between the p wave and the s wave of up to 0 6 ms over 50 days these variations cannot be explained by displacement of the sensor with respect to the glacier but might be due to small changes in the seismic wave velocities with time finally we found using numerical simulations that the observed signals are better explained by a horizontal shear fault with slip parallel to the glacier flow than by a tensile fault these results suggest that the basal events are associated with stick slip motion of the glacier over rough bedrock the rupture length and the slip are difficult to estimate nonetheless the rupture length is likely to be of the order of meters and the total seismic slip accumulated over one day might be as large as the glacier motion during the most active bursts

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

earthquake cycle simulations with rate and state friction and power law viscoelasticity abstract we simulate earthquake cycles with rate and state fault friction and off fault power law viscoelasticity for the classic 2d antiplane shear problem of a vertical strike slip plate boundary fault we investigate the interaction between fault slip and bulk viscous flow with experimentally based flow laws for quartz diorite and olivine for the crust and mantle respectively simulations using three linear geotherms dt dz 20 25 and 30 k km produce different deformation styles at depth ranging from significant interseismic fault creep to purely bulk viscous flow however they have almost identical earthquake recurrence interval nucleation depth and down dip coseismic slip limit despite these similarities variations in the predicted surface deformation might permit discrimination of the deformation mechanism using geodetic observations additionally in the 25 and 30 k km simulations the crust drags the mantle the 20 k km simulation also predicts this except within 10 km of the fault where the reverse occurs however basal tractions play a minor role in the overall force balance of the lithosphere at least for the flow laws used in our study therefore the depth integrated stress on the fault is balanced primarily by shear stress on vertical fault parallel planes because strain rates are higher directly below the fault than far from it stresses are also higher thus the upper crust far from the fault bears a substantial part of the tectonic load resulting in unrealistically high stresses in the real earth this might lead to distributed plastic deformation or formation of subparallel faults alternatively fault pore pressures in excess of hydrostatic and or weakening mechanisms such as grain size reduction and thermo mechanical coupling could lower the strength of the ductile fault root in the lower crust and concomitantly off fault upper crustal stresses

experimental determination of a double valued drag relationship for glacier sliding the contribution of glaciers to sea level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice bed interface results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double valued drag relationship like that suggested by some sliding theories steady drag on a rigid sinusoidal bed increases peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations drag decreases with increased sliding speed if cavities extend beyond the inflection points of up glacier facing surfaces so that adverse bed slopes in contact with ice diminish with further cavity growth these results indicate that shear tractions on glacier beds can potentially decrease due to increases in sliding speed driven by weather or climate variability promoting even more rapid glacier motion by requiring greater strain rates to produce resistive stresses although a double valued drag relationship has not yet been demonstrated for the complicated geometries of real glacier beds both its potential major implications and the characteristically convex stoss surfaces of bumps on real glacier beds provide stimulus for exploring the effects of this relationship in ice sheet models

instruments and methods direct measurement of sliding at the glacier bed sliding at the base of trapridge glacier yukon territory canada was measured using a drag spool we describe this simple and inexpensive instrument as well as its installation and operation from 1990 to 1992 seven sites were instrumented with drag spools at six of the sites basal sliding during the period of observation accounted for 50 70 of the total flow observed at the glacier surface the contribution from ice creep is known to be small so most of the remaining surface motion must be attributed to subglacial sediment deformation for the seventh site the observed sliding rate was 90 of the total flow an indication that the sliding contribution varies spatially across the bed diurnal variations in the response of one of our instruments appear to be correlated to subglacial water pressure fluctuations and are interpreted in terms of changes in sliding velocity rather than the opening and closing of basal cavities

inversion of basal friction in antarctica using exact and incomplete adjoints of a higher order model basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics here we use a complete map of surface velocity of the antarctic ice sheet to infer the basal friction over the entire continent by combining these observations with a three dimensional thermomechanical higher order ice sheet numerical model from the ice sheet system model open source software we demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization at a spatial resolution as high as 3 km along the coastline we compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation the results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80 of the ice sheet the basal friction coefficient which relates basal friction to basal velocity is however significantly heterogeneous it is low on fast moving ice and high near topographic divides areas with low values extend far out into the interior along glacier and ice stream tributaries almost to the flanks of topographic divides suggesting that basal sliding is widespread beneath the antarctic ice sheet 2013 american geophysical union all rights reserved

slow slip events on the whillans ice plain antarctica described using rate and state friction as an ice stream sliding law the whillans ice plain wip antarctica experiences twice daily tidally modulated stick slip cycles slip events last about 30 minutes have sliding velocities as high as 0 5 mm s 15 km yr and have total slip 0 5 m slip events tend to occur during falling ocean tide just after high tide and just before low tide to reproduce these characteristics we use rate and state friction which is commonly used to simulate tectonic faulting as an ice stream sliding law this framework describes the evolving strength of the ice bed interface throughout stick slip cycles we present simulations that resolve the cross stream dimension using a depth integrated treatment of an elastic ice layer loaded by tides and steady ice inflow steady sliding with rate weakening friction is conditionally stable with steady sliding occurring for sufficiently narrow ice streams relative to a nucleation length stick slip cycles occur when the ice stream is wider than the nucleation length or equivalently when effective pressures exceed a critical value ice streams barely wider than the nucleation length experience slow slip events and our simulations suggest that the wip is in this slow slip regime slip events on the wip show a sense of propagation and we reproduce this behavior by introducing a rate strengthening region in the center of the otherwise rate weakening ice stream if pore pressures are raised above a critical value our simulations predict that the wip would exhibit quasi steady tidally modulated sliding as observed on other ice streams this study validates rate and state friction as a sliding law to describe ice stream sliding styles

mechanics of grain size reduction in fault zones 1 recent observations of nanometer scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive especially if significant shear heating is produced during an earthquake commercial crushing and grinding operations encounter a grind limit near 1 mm below which particles deform plastically rather than fracturing a fragmentation model and low temperature plasticity mechanics indicate that it is not possible to produce under compressive loading and short timescale significantly smaller particles at any strain rate however shock loading and subcritical crack growth can produce nanometer scale fragments in compression under tensile loading the fragment size is determined by a competition between the nucleation of cracks and stress relaxation in their neighborhoods therefore higher tensile strain rates produce smaller fragments the ultimate limit is determined by the availability of elastic strain energy which does not place a significant constraint on the minimum grain size grain growth kinetics suggests that survivability of grains is very temperature sensitive a 10 nm quartz fragment will double its size in 0 1 s at 1000c in 20 s at 800c in 14 h at 600c and in 10 years at 400c the observation of grains smaller than 10 nm places meaningful constraints on the dynamic fields and permeability of the fault zone during a large earthquake microstructural analysis of the grains and rock damage may be used to infer whether fragmentation occurred under macroscopic tension or compression

transient effective stress variations forced by changes in conduit pressure beneath glaciers and ice sheets the resistance to sliding beneath soft bedded glaciers depends on the effective stress n which is controlled by the subglacial hydrological system while large scale water transport is primarily through conduits the effective stress profile beneath most of the glacier base is expected to be controlled by seepage flows through the permeable sediments models of the response to forced changes of the near conduit effective stress demonstrate that perturbations in n decay over a characteristic lateral distance that is typically only a few times the sediment thickness this implies that only relatively small changes to the spatially averaged effective stress n can be produced if conduit spacing is comparatively large and the glacier and sediments remain in contact to produce larger changes to n it is likely that flotation must be achieved beneath a significant portion of the glacier base at higher values spatial variations in n can produce gradients in the thickness h of a fringe of ice infiltrated sediments immediately adjacent to the glacier base this has implications for the development of glacial landforms when sliding causes sediment transport

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

viscoelastic simulation of earthquake cycle using a simple spring dashpot mass system with a friction law we examine the viscoelastic effect on earthquake cycle using a simple spring dashpot mass system with a laboratory derived friction law the temporal change of the frictional stress is large at the early interseismic stage and gradually decreases in contrast to the linear change in an elastic medium this temporal change depends on the maxwell time the smaller relaxation time leads to the larger change of the stress accumulation rate such a rate change in stress accumulation may affect the seismicity at subduction zones moreover we find that the smaller relaxation time produces the smaller period of earthquake cycle and that if the system relaxation time becomes smaller than the period of earthquake cycle the bifurcation phenomenon where the period splits into two occurs it is also found that the smaller relaxation time causes the earlier occurrence of the precursory stress decrease preceding the rapid stress drop

stick slip motion of an antarctic ice stream the effects of viscoelasticity stick slip behavior is a distinguishing characteristic of the flow of whillans ice stream siple coast antarctica distinct from stick slip on northern hemisphere glaciers which is generally attributed to supraglacial melt the behavior is thought be controlled by basal processes and by tidally induced stress however the connection between stick slip behavior and flow of the ice stream on long time scales if any is not clear to address this question we develop a new ice flow model capable of reproducing stick slip cycles similar to ones observed on the whillans ice plain the model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice till interface the model results suggest the long term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick slip behavior such as velocity magnitude during the slip phase and factors that regulate it e g elastic modulus furthermore it is found that factors controlling purely viscous flow such as temperature influence stick slip contribution to long term flow in much the same way additionally we show that viscous ice deformation traditionally disregarded in analysis of stick slip behavior has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick slip observations

rate weakening drag during glacier sliding accurately specifying the relationship between basal drag on a hard rough glacier bed and sliding speed is a long standing and central challenge in glaciology drag on a rigid bed consisting of steps with linear treads inclined upglacier a good idealization for the bedrock morphology of some hard bedded glaciers has been considered in sliding theories but never studied empirically balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit equilibrium condition sometimes called iken s bound in this study we used a large ring shear device to slide ice at its pressure melting temperature across a stepped bed over a range of steady sliding speeds 29 348 m yr 1 and under a steady effective pressure 500 kpa contrary to expectation drag decreased 42 with increasing sliding speed and cavity size experimental deviations from theory cannot explain this decrease in drag with increasing sliding speed i e rate weakening we suggest that stress bridging in ice between ice bed contact zones and cavities causes stress gradients that require viscous deformation of ice to sustain stress equilibrium so that contact zones can be at shear stresses below limit equilibrium values a parameter linearly dependent on sliding speed that scales the extent of ice deformation to areas of ice bed contact allows the experimental drag relationship to be fitted with a simple sliding model rate weakening drag has now been observed for two contrasting bed morphologies stepped and sinusoidal highlighting the need to consider such behavior in glacier flow models

swarms of repeating stick slip icequakes triggered by snow loading at mount rainier volcano we have detected over 150 000 small m 3000 m on the glacier covered edifice and occur primarily in weeklong to monthlong swarms composed of simultaneous distinct families of events each family contains up to thousands of earthquakes repeating at regular intervals as often as every few minutes mixed polarity first motions a linear relationship between recurrence interval and event size and strong correlation between swarm activity and snowfall suggest the source is stick slip basal sliding of glaciers the sudden added weight of snow during winter storms triggers a temporary change from smooth aseismic sliding to seismic stick slip sliding in locations where basal conditions are favorable to frictional instability coda wave interferometry shows that source locations migrate over time at glacial speeds starting out fast and slowing down over time indicating a sudden increase in sliding velocity triggers the transition to stick slip sliding we propose a hypothesis that this increase is caused by the redistribution of basal fluids rather than direct loading because of a 1 2 day lag between snow loading and earthquake activity this behavior is specific to winter months because it requires the inefficient drainage of a distributed subglacial drainage system identification of the source of these frequent signals offers a view of basal glacier processes discriminates against alarming volcanic noises documents short term effects of weather on the cryosphere and has implications for repeating earthquakes in general

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

microearthquakes and subglacial conditions 1 ten passive seismic recording stations were deployed in a 9 km by 6 km array on rutford ice stream west antarctica to detect microearthquakes from the ice stream bed the array covered an area where a varying distribution of basal sedimentary conditions had been identified from seismic reflection surveys and interpreted as areas of both basal sliding and subglacial sediment deformation the passive seismic experiment was designed to give an independent confirmation of this interpretation as basal sliding is believed to be associated with the release of more acoustic energy than a deforming bed the results show that this is the case basal sliding producing six times more events this confirms that the spatial pattern of basal conditions does indeed reflect the distribution of different ice flow mechanisms it also shows that microearthquake monitoring can be a valuable new technique for mapping ice stream basal conditions over wide areas

laboratory study of the frictional rheology of sheared till 1 deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength experiments were conducted on till a mixed size granular material from matanuska glacier alaska and the scioto ohio lobe of the laurentide ice sheet caesar till we deformed till in double direct shear under fixed shear velocity or shear stress creep normal stress ranged from 50 kpa to 5 mpa at shearing rates ranging from 1 to 300 m s for 1 cm thick samples creep was induced via small step perturbations in stress fabric development within till layers was investigated by varying shear strain prior to creep tests in velocity controlled experiments till deforms as a nearly coulomb plastic material with slight velocity strengthening corresponding to a stress exponent n 60 creep experiments conducted well below the shear strength indicate lower n values increasing as shear stress increases with increasing initial strain and inferred fabric development the creep strain rate decreases while n increases experiments at a normal stress of 1 mpa and no initial strain show n 6 8 increasing to n 17 5 at higher shear strains however strain rate was still decreasing and thus these values represent maximum estimates our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less at low strains n likely depends on consolidation state permeability and dilation deformation is nearly rate insensitive coulomb plastic at shear stress near the shear strength or at high strain

glacier sliding seismicity and sediment entrainment the evolution of glaciers and ice sheets depends on processes in the subglacial environment shear seismicity along the ice bed interface provides a window into these processes such seismicity requires a rapid loss of strength that is typically ascribed to rate weakening friction i e decreasing friction with sliding or sliding rate many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds to our knowledge however these studies have all found rate strengthening friction here we investigate the possibility that rate weakening rock on rock friction between sediments frozen to the bottom of the glacier and the underlying water saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds we test this entrainment seismicity hypothesis using targeted laboratory experiments and simple models of glacier sliding seismicity and sediment entrainment these models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity we propose that stagnation at the whillans ice stream west antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins our results suggest that basal shear seismicity may indicate geomorphic activity

deformation of debris ice mixtures mixtures of rock debris and ice are common in high latitude and high altitude environments and are thought to be widespread elsewhere in our solar system in the form of permafrost soils glaciers and rock glaciers these debris ice mixtures are often not static but slide and creep generating many of the landforms and landscapes associated with the cryosphere in this review a broad range of field observations theory and experimental work relevant to the mechanical interactions between ice and rock debris are evaluated with emphasis on the temperature and stress regimes common in terrestrial surface and near surface environments the first order variables governing the deformation of debris ice mixtures in these environments are debris concentration particle size temperature solute concentration salinity and stress a key observation from prior studies consistent with expectations is that debris ice mixtures are usually more resistant to deformation at low temperatures than their pure end member components however at temperatures closer to melting the growth of unfrozen water films at ice particle interfaces begins to reduce the strengthening effect and can even lead to profound weakening using existing quantitative relationships from theoretical and experimental work in permafrost engineering ice mechanics and glaciology combined with theory adapted from metallurgy and materials science a simple constitutive framework is assembled that is capable of capturing most of the observed dynamics this framework highlights the competition between the role of debris in impeding ice creep and the mitigating effects of unfrozen water at debris ice interfaces

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

isostatic geoid anomalies on a sphere long wavelength geoid anomalies due to lateral variations in the density or thickness of a thin layer in local isostatic equilibrium on the surface of a sphere depend sensitively on the assumed state of stress within the layer a number of common intuitive definitions of local isostasy generally believed to be essentially identical are associated with quite different states of isostatic stress and the corresponding theoretical geoid anomalies can vary by more than a factor of 2 this sensitivity of the theoretical anomaly to the exact definition of local isostasy constitutes an obstacle to any proposed program of interpreting or exploiting observed global isostatic geoid anomalies such as those associated with the aging of the oceanic lithosphere

implications of basal micro earthquakes and tremor for ice stream mechanics stick slip basal sliding and till erosion abstract the whillans ice plain wip is unique among antarctic ice streams because it moves by stick slip the conditions allowing stick slip and its importance in controlling ice dynamics remain uncertain local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick slip and a window into current basal conditions but the spatial extent and importance of this basal seismicity are unknown we analyze data from a 2010 2011 ice plain wide seismic and gps network to show that basal micro seismicity correlates with large scale patterns in ice stream slip behavior basal seismicity is common where the ice moves the least between unstable slip events with small discrete basal micro earthquakes happening within 10s of km of the central stick slip nucleation area and emergent basal tremor occurring downstream of this area basal seismicity is largely absent in surrounding areas where inter slip creep rates are high the large seismically active area suggests that a frictional sliding law that can accommodate stick slip may be appropriate for ice stream beds on regional scales variability in seismic behavior over inter station distances of 1 10 km indicates heterogeneity in local bed conditions and frictional complexity wip unstable slips may nucleate when stick slip basal earthquake patches fail over a large area we present a conceptual model in which basal seismicity results from slip weakening frictional failure of over consolidated till as it is eroded and mobilized into deforming till