chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

laurentide cordilleran and montane glaciation in the western peace river grande prairie region alberta and british columbia canada abstract geological investigations over the last decade in the western peace river grande prairie district of northwestern alberta and northeastern british columbia have provided important chronostratigraphic information regarding laurentide cordilleran and montane quaternary glacial events numerous 14 c age determinations as well as stratigraphic and sedimentological analyses of numerous exposures indicate that this area was first glaciated by cordilleran ice sometime before the late wisconsinan during the late wisconsinan ca 22 000 14 000 bp laurentide glacier ice covered much of the area between approximately 15 000 and 10 000 bp montane ice affected only the western part of the peace region there is no evidence for synchroneity of the ice advances and therefore no evidence for ice coalescence in this region the resultant ice free corridor was most likely a laterally fluctuating zone which existed during a several thousand year period in the late pleistocene climatic conditions along the corridor during this period were probably unfavorable for human migration but adequate for some plants and small animals numerous isolated areas along the western foothills of the northern rocky mountains west of the laurentide glaciation limit were never glaciated during the late wisconsinan

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

microfracturing paleostress and the growth of faults abstract we have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip we have also recorded the orientation of microfractures as a function of distance from the associated fault surface in order to control as many parameters as possible shallow faults with similar lithologies were chosen they showed the same kind of deformation namely microfracturing without block rotation recrystallization extensive granulation or crystal plasticity the results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip we interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip the angular relationship between the shear plane and microfractures was commonly found to be between 5 and 20 significantly less than the 30 expected between the far field maximum principal stress and the shear surface this low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode ii fault tip other possible explanations for the small angle for tensional microfractures and the shear surface are 1 the effective confining pressure was very low or 2 shear strength at failure was much larger than expected

effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading a grain based universal distinct element code model was developed to generate a deformable polygonal grain like structure to simulate the microstructure of brittle rock it takes into account grain scale heterogeneity including microgeometric heterogeneity grain scale elastic heterogeneity and microcontact heterogeneity the microgeometric heterogeneity can be used to match the grain size distribution of the rock the discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy the modeling approach was evaluated using lac du bonnet granite and a aspo diorite the microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading the crack initiation stress was found to be controlled primarily by the microscale geometric heterogeneity while the microcontact heterogeneity controlled the strength characteristics the effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined

richardson mountains yukon northwest territories the northern portal of the postulated ice free corridor abstract the richardson mountains located to the east of the porcupine plateau and old crow basin constitute the first barrier that migrating organisms would have to traverse in order to follow any potential ice free corridor in western canada two gaps exist in the mountain range mcdougall pass in the north and peel river canyon in the south the existence of the ice free corridor in this region thus hinges upon the chronology and extent of laurentide glacial advances in these areas three glacial events have been recognized in the richardson mountains peel plateau region an initial event occurred during the middle quaternary depositing sediments subsequently reworked into the brown bear gravels of the central peel plateau a second glacial event occurred at some later time prior to the late wisconsinan this glaciation correlative to the buckland event of the yukon coastal plain reached mcdougall pass blocking the northern access through the richardson mountains the glaciation did not extend into the bonnet plume basin but the presence of glacial ice in the southern peel plateau impounded a large lake in the basin depositing lacustrine sediments and preventing use of the flooded peel river canyon route the most recent major glaciation of late wisconsinan age was less extensive than the second event in the central and northern peel plateau although glacial ice did not penetrate mcdougall pass at this time lacustrine impoundment in the pass and the bell basin precluded use of this route by any potential migrants during the late wisconsinan in the bonnet plume basin late wisconsinan ice advanced over the deformable previously deposited lacustrine sediment reaching its maximum position at hungry creek the southern passage through the richardson mountains was blocked by glacial ice during the late wisconsinan and thus no suitable gateways to the corridor route existed prior to the ultimate deglaciation of the region

the eureka river landslide and dam peace river lowlands alberta the eureka river landslide of june 1990 at 50 mm3 is one of the largest historical landslides on the interior plains of canada it is one of seven large translational landslides to have occurred in the peace river lowlands within the last 65 years each landslide occurred in quaternary sediments deposited within a preglacial valley each landslide formed a dam the rupture surface of the eureka river landslide in preglacial lacustrine sediment 125 m below the peace river lowlands plains extended beneath the river channel causing the channel to be elevated the resulting landslide dam was over 20 m high forming a lake exceeding 8 km in length the river cut a new channel around the toe of the landslide abandoning the prelandslide channel as the new channel is free of armour incision has been rapid after 10 years the dam now stands approximately 5 m high key words landslide landslide dam peace river alberta preglacial valley geomorphology

western frontal fault of the canyon range is it the breakaway zone of the sevier desert detachment geologic evidence developed from surface exposures demonstrates that the western frontal fault of the canyon range is a major structure representing the eastern breakaway zone of the sevier desert detachment the western frontal fault of the canyon range east of delta utah is a north trending west dipping low angle 18 24 normal fault that juxtaposes syntectonic conglomerates and large rock avalanches of the miocene oak city formation against footwall rocks that have been ductilely cataclastically and brittlely deformed the oak city strata dip variably to the east dips are shallow in eastern outcrops 5 15 steeper in central outcrops 20 60 and shallow in the western outcrops 10 20 in eastern exposures the shallow dips of the oak city formation 5 15 and the acute angle of 20 35 between the western frontal fault and oak city units limit past rotation of the fault to 15 the systematic variation in the dip of oak city units from east to west suggests that the western frontal fault probably flattens beneath present oak city outcrops linking the fault to the shallow reflector seen in the cocorp consortium for continental reflection profiling utah line 1 seismic line beneath the westernmost outcrops of the oak city formation upper plate movement is constrained by oak city clast compositions and rock avalanche lithologies fault geometry and lower plate reconstruction to a maximum of 6 7 km since oak city syntectonic deposition at 12 13 ma maximum upper plate displacement for the entire sevier desert detachment cannot be determined from these breakaway zone exposures

archaeological constraints on some geological interpretations of a late wisconsinan site at varsity estates calgary alberta abstract late glacial history in western alberta has been a subject of debate as this area lies in the zone of assumed contact of the cordilleran glaciers and laurentide ice shield the geographic limits and timing of the coalescence of the two ice masses however are not clear except for the west central part of the province with the documented merger in the athabasca valley recently arguments for a complete late wisconsinan coalescence in the prairies all along the rocky mountain foothills have been presented based on 14c dates 21 ka and cosmogenic 36cl dates

timing of terminal pleistocene deglaciation at high elevations in southern and central british columbia constrained by 10be exposure dating abstract the cordilleran ice sheet cis covered most of british columbia and southern yukon territory at the local last glacial maximum llgm during marine oxygen isotope stage 2 however its subsequent demise is not well understood particularly at high elevations east of its ocean terminating margin we present 10be exposure ages from two high elevation sites in southern and central british columbia that help constrain the time of initial deglaciation at these sites we sampled granodiorite erratics at elevations of 2126 2230 m a s l in the marble range and 1608 1785 m a s l in the telkwa range at the western margin of the interior plateau the erratics at both sites are near ice marginal meltwater channels that delineate the local ice surface slope and thus the configuration of the ice sheet during deglaciation the locations of the erratics and their relations to meltwater channels ensure that the resulting 10be ages date cis deglaciation and not the retreat of local montane glaciers our sample sites emerged above the surface of the cis as its divide migrated westward from the interior plateau to the axis of the coast mountains two of the four samples from the summit area of the marble range yielded apparent exposure ages of 14 0 0 7 and 15 2 0 8 ka these ages are 1 8 3 0 ka younger than the well established llgm age of ca 17 ka for the puget lobe of the cis in washington state they are 1 7 ka younger than the llgm age for the puget lobe if a snow shielding correction to their uncertainty weighted mean age is applied the other two samples yielded much older apparent exposure ages 20 6 1 4 and 33 0 1 5 ka indicating the presence of inherited isotopes four samples collected from the summit area of the telkwa range in the hazelton mountains yielded well clustered apparent exposure ages of 10 1 0 6 10 2 0 7 10 4 0 5 and 11 5 1 1 ka significant present day snow cover introduces a large uncertainty in the apparent exposure ages from this site a snow shielding correction based on present day snow cover data increases the uncertainty weighted mean exposure age of the telkwa range erratics to 12 4 0 7 ka consistent with deglacial 14c ages from areas near sea level to the west our exposure ages show a thinning of the southern portion of the cis shortly after the llgm and persistence of a remnant mountain ice cap in the central coast mountains into the younger dryas chronozone our data also show that the summit area of the marble range was ice covered during the llgm the presence of an ice body of considerable dimension in north central british columbia until or possibly even after the younger dryas highlights the need for geomorphological and geochronological studies of the ice dispersal centre over the skeena mountains in northwest british columbia and the need for better understanding of the response of the cis to lateglacial climate fluctuations

multiphase flow of the late wisconsinan cordilleran ice sheet in western canada in central british columbia ice flow during the late wisconsinan fraser glaciation ca 25 10 ka occurred in three phases the ice expansion phase occurred during an extended period when glaciers flowed westward to the pacific ocean and east southeastward onto the nechako plateau from ice centers in the skeena hazelton coast and omineca mountains initially glacier flow was confined by topography along major valleys but eventually piedmont and montane glaciers coalesced to form an integrated glacier system the cordilleran ice sheet in the maximum phase a cordilleran ice divide developed over the nechako plateau to 300 km inland from the pacific coast at this time the surface of the ice sheet extended well above 2500 m above sea level and flowed westward over the skeena hazelton and coast mountains onto the continental shelf and eastward across the rocky mountains into alberta in the late glacial phase a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys and restricted accumulation centers to high mountains discordant directions in ice flow are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness ice centers probably shifted in response to climate irregular growth in the ice sheet rapid calving ice streaming and drainage of proglacial and subglacial water bodies crosscutting ice flow indicators and preservation of early valley parallel flow features in areas exposed to later cross valley glacier erosion indicate that the ice expansion phase was the most erosive and protracted event

ams dated late pleistocene taiga vole rodentia microtus xanthognathus from northeast british columbia canada a cautionary lesson in chronology dissected colluvial sediments on a peace river terrace at bear flat northeast british columbia enclosed a late pleistocene micromammalian faunule the fossil remains including a few loosely articulated skulls and mandibles were dominated by taiga voles microtus xanthognathus the bear flat site constitutes the second fossil occurrence in the region of this elusive species which is unknown in british columbia in historic times the late pleistocene age determined by accelerator mass spectrometry directly on taiga vole bone collagen is consistent with the ages of widespread taiga vole records peripheral to the laurentide ice sheet in western mid western and eastern north america the presence of allo chronous remains within a comprehensively dated sedimentary sequence provides a cautionary note about straightforward acceptance of relative stratigraphic dating

seventeenth canadian geotechnical colloquium the effect of cohesion loss and stress path on brittle rock strength stress strain curves for brittle rocks show three characteristic stress levels crack initiation long term strength and peak strength damage controlled testing at low confining stresses has shown that the long term and peak strengths are sensitive to the amount of induced damage i e the greater the amount of damage the lower the long term and peak strengths these tests also showed that the brittle failure process is characterized by a loss of cohesion as friction is mobilized excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel the back calculated strength for the failed rock around the tunnel is approximately one half of that measured in laboratory tests crack induced damage of lac du bonnet granite both in the laboratory and in situ begins when the load exceeds approximately one third of the unconfined compressive strength however the stress level associated with failure is a function of loading path in the laboratory where the loading path monotonically increases the ultimate strength of an unconfined sample is 225 mpa numerical studies suggest that in situ the loading path around the tunnel is more complex involving stress increase and decrease and stress rotation for this loading path failure initiates at a stress between 100 and 120 mpa conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel the results from the damage controlled laboratory tests and the microseismic monitoring carried out during tunnel construction indicate a constant deviatoric stress criterion is a reliable indicator for predicting the onset of damage this criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel the fundamental assumption in the constant deviatoric stress criterion is that at low confining stresses such as those which occur around underground openings the brittle failure process is dominated by cohesion loss

nunataks and valley glaciers over the mountains and through the ice models of the first peopling of the americas characterize arrival routes either along the coast or through the ice free corridor following the last glacial maximum while the pendulum has currently swung somewhat towards the coastal route archaeological evidence for either entry is lacking in this paper we introduce a third option an icy corridors entry route we argue that the traditionally envisioned corridor is an unnecessary feature for the terrestrial arrival of clovis or clovis predecessors below the ice sheets the recent genomic data is fully compatible with a re envisioned peopling route

fractals in geology and geophysics the definition of a fractal distribution is that the number of objectsn with a characteristic size greater thanr scales with the relationn r d the frequency size distributions for islands earthquakes fragments ore deposits and oil fields often satisfy this relation fractals were originally introduced by mandelbrot to relate the length of a coastline to the length of the measuring stick this application illustrates a fundamental aspect of fractal distributions scale invariance the requirement of an object to define a scale in photographs of many geological features is one indication of the wide applicability of scale invariance to geological problems scale invariance can lead to fractal clustering geophysical spectra can also be related to fractals these are self affine fractals rather than self similar fractals examples include the earth s topography and geoid

late pleistocene chronology and environment of the ice free corridor of northwestern alberta abstract pollen and macrofossil analyses of two radiocarbon dated lake sediment cores in the upper peace river district were used to investigate the controversial late glacial geochronology of the ice free corridor the basal mineral rich sediments contain reworked radiogenically dead palynomorphs as well as intrusive modern carbon analyses of the basal sediments from boone lake show that two 14c ages greater than 12 000 yr b p are spuriously old due to contamination by organic matter of cretaceous age the data support occlusion or near occlusion of laurentide and cordilleran ice in the peace river area during the late wisconsinan period the sediment record began around 12 000 yr b p in the ice dammed and enlarged boone lake an initially open sedge dominated cover was invaded by sage willow grass and poplar by 11 700 yr b p suggesting that a habitable landscape has existed in the area for at least 12 millennia the data however do not support the ice free corridor arguments of b o k reeves 1973 arctic and alpine research 5 1 16 1983 in quaternary coastlines and marine archaeology towards the prehistory of land bridges and continental shelves p m masters and n c fleming eds pp 389 411 academic press new york who suggests that ice occlusion did not occur in the peace river valley during the last 55 000 yr

a large rockslide debris avalanche in cohesive soil at pink mountain northeastern british columbia canada abstract in early july 2002 a large rockslide debris avalanche occurred on the west slope of the pink mountain anticline in northeastern british columbia 57 04 n 122 52 w the slopes have been subjected to previous deformation and landsliding pre and post slide digital elevation models dems were created from aerial photography approximately 1 04 m m 3 of sandstone and shale travelled nearly 2 km from an elevation of 1460 m to 1010 m the landslide has a fahrboschung of 11 6 and covered 43 ha the rock debris entrained a certain amount of fine textured till and colluvium increasing its volume by 15 this entrainment caused partial liquefaction of the underlying soil thereby enhancing the mobility of the landslide we used a bingham rheological model to dynamically model the landslide with excellent correspondence between the analysis of the actual observed behaviour of the landslide

the role of lithological layering and pore pressure on fluid induced microseismicity the success of hydraulic fracturing treatments is often judged by the shape and size of the resulting microseismic cloud however it is challenging to predict the anticipated microseismic cloud prior to treatment we use geomechanical modeling to predict the distribution of the microseismicity prior to the hydraulic fracture treatment we analyze the likelihood of tensile and shear failure due to 1 d variations in local stresses and rock strengths induced by layering and pore pressure for two field cases the deviation in the local stresses from the regional stress field is induced by vertical variations in stiffness this promotes failure of the stronger layers instead of the weaker ones since the stronger layers can become essentially load bearing the simulations and field studies show that 1 microseismic events tend to locate preferentially where layers reach tensile failure due to fluid injection and the number of events tends to decrease in layers that do not reach tensile failure 2 shear initiation can occur in different layers from those failing in tension thereby creating additional fluid migration paths and 3 reactivation of preexisting fractures may occur due to fluid migration even if their orientations are unfavorable numerical modeling is a significant aid in understanding the interplay of regional and local stresses and associated in situ failure due to variations in rock strength pore pressure and stiffnesses in a wider perspective it gives fundamental insights into the understanding of earthquakes and fault localization the mechanisms of fracture development and the role of fractures on fluid circulation and on the in situ stress field

late wisconsin deglaciation and holocene river evolution near fort nelson northeastern british columbia glacial lakes were formed in the fort nelson area by eastward retreating laurentide glacier ice that dammed a number of streams these lakes drained through spillways at progressively lower elevations as the ice retreated eastward a terrace on the muskwa river containing sediments dated ca 9000 years bp indicates that the river had entrenched itself between 50 and 70 m below the surrounding upland by that time prior to final entrenchment by the muskwa river to its present grade a pause in downcutting ca 9000 years bp occurred resulting in the formation of the terrace

microseismic moment tensors a path to understanding frac growth microseismic monitoring is a valuable tool in understanding the efficacy of hydraulic fracture treatments determination of event locations and magnitudes leads to estimations of the geometry of the fracture zone and certain dynamics of the fracturing process with sufficient resolution the hypocenters may even reveal failure planes or other underlying structures controlling the distribution of events and of interest to petroleum engineers to test various hypotheses on fracture growth

laurentide and montane glaciation along the rocky mountain foothills of northeastern british columbia mapping the surficial geology of the trutch map area nts 94g provides new data on the timing of continental and montane glaciations along the foothills of northeastern british columbia striated surfaces on mountain crests were dated to the late wisconsinan substage by cosmogenic dating the striations were produced by eastward flowing ice emanating from the region of the continental divide this ice was thick enough to cross the main ranges and overtop the rocky mountain foothill summits at 2000 m above sea level asl it is argued here that such a flow unhindered by topography could only have been produced by the cordilleran ice sheet and not by local cirque glaciation during this time the cordilleran ice sheet dispersed limestone and schist erratics of western provenance onto the plains beyond the mountain front conversely the laurentide ice sheet did not reach its western limit in the foothills until after cordilleran ice retreated from the area during its maximum the laurentide ice penetra

ultrasonic velocities acoustic emission characteristics and crack damage of basalt and granite acoustic emissions ae compressional p shear s wave velocities and volumetric strain of etna basalt and aue granite were measured simultaneously during triaxial compression tests deformation induced ae activity and velocity changes were monitored using twelve p wave sensors and eight orthogonally polarized s wave piezoelectric sensors volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface p wave velocity in basalt is about 3 km s at atmospheric pressure but increases by 50 when the hydrostatic pressure is increased to 120 mpa in granite samples initial p wave velocity is 5 km s and increases with pressure by 20 the pressure induced changes of elastic wave speed indicate dominantly compliant low aspect ratio pores in both materials in addition etna basalt also contains high aspect ratio voids in triaxial loading stress induced anisotropy of p wave velocities was significantly higher for basalt than for granite with vertical velocity components being faster than horizontal velocities however with increasing axial load horizontal velocities show a small increase for basalt but a significant decrease for granite using first motion polarity we determined ae source types generated during triaxial loading of the samples with increasing differential stress ae activity in granite and basalt increased with a significant contribution of tensile events close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly a concomitant increase of double couple events indicating shear suggests shear cracks linking previously formed tensile cracks

late wisconsin subglacial megaflood paths in alberta evidence of late wisconsin subglacial megafloods includes fields of giant flutings drumlins tunnel channels and scoured bedrock tracts scoured tracts are marked by channeled scabland water eroded depressions s forms and postglacial development of solonetzic saline clay pan soils on cretaceous bedrock belts of hummocky terrain and small zones of moraine plateaus are remnants left by incomplete sheet flood erosion of initial subglacial sediment later modified by ice pressing among multiple subglacial discharges the largest was the livingstone lake event which transmitted laurentide water from the northwest territories and northern saskatchewan through alberta to montana and the mississippi drainage this flood was augmented by cordilleran water relayed under valley glacier piedmonts coalescent with south western laurentide ice the short duration megaflood probably occurred within the longer time interval of 18 15 ka

on the stress dependence of the earthquake b value laboratory experiments have shown that the b value in the size distribution of acoustic emission events decreases linearly with differential stress there have been a number of observations that indicate that this relation may also hold for earthquakes here using a simple frictional strength model for stresses in the continental lithosphere combined with earthquake b values measured as a function of depth in a wide variety of tectonic regions we verify and calibrate that relation finding b 1 23 0 06 0 0012 0 0003 1 3 where the stress difference 1 3 is in megapascal for subduction zones we find that b value correlates linearly with the slab pull force and with the net reduction of plate interface normal force both of which also indicate a negative linear relation between b value and differential stress

acoustic emissions document stress changes over many seismic cycles in stick slip experiments 1 the statistics of large earthquakes commonly involve large uncertainties due to the lack of long term robust earthquake recordings small scale seismic events are abundant and can be used to examine variations in fault structure and stress we report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes those generated in the laboratory we investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory created fault zones we show that b values mirror periodic stress changes that occur during series of stick slip events and are correlated with stress over many seismic cycles moreover the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop consequently b value variations can be used to approximate the stress state on a fault a possible tool for the advancement of time dependent seismic hazard assessment citation goebel t h w d schorlemmer t w becker g dresen and c g sammis 2013 acoustic emissions document stress changes over many seismic cycles in stick slip experiments geophys res lett 40 2049 2054 doi 10 1002 grl 50507

mapping quaternary paleovalleys and drift thickness using petrophysical logs northeast british columbia fontas map sheet nts 94ithis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers the relatively subdued topography of british columbia s northern interior plains does not reflect the irregular buried bedrock surface many areas have been deeply incised by preglacial rivers tha

moment tensors and micromechanical models abstract a numerical modelling approach that simulates cracking and failure in rock and the associated seismicity is presented and a technique is described for quantifying the seismic source mechanisms of the modelled events the modelling approach represents rock as an assemblage of circular particles bonded together at points of contact the connecting bonds can break under applied stress forming cracks and fractures in the modelled rock if numerical damping is set to reproduce realistic levels of attenuation then energy is released when the bonds break and seismic source information can be obtained as damage occurs a technique is described by which moment tensors and moment magnitudes can be calculated for these simulated seismic events the technique basically involves integrating around the source and summing the components of force change at the surrounding particle contacts to obtain the elements of the moment tensor matrix the moment magnitude is then calculated from the eigenvalues of the moment tensor the modelling approach is tested by simulating a well controlled experiment in which a tunnel is excavated in highly stressed granite while microseismicity is recorded the seismicity produced by the model is compared to the actual recorded seismicity underground the model reproduces the spatial and temporal distribution of seismicity observed around the tunnel and also the magnitudes of the events a direct comparison between the actual and simulated moment tensors is not performed due to the two dimensional nature of the model however qualitative comparisons are presented and it is shown that the model produces intuitively realistic source mechanisms the ability to obtain seismic source information from the models provides a unique means for model validation through comparison with actual recorded seismicity once it is established that the model is performing in a realistic manner it can then be used to examine the micromechanics of cracking failure and the associated seismicity and to help resolve the non uniqueness of the geophysical interpretation this is demonstrated by examining in detail the mechanics of one of the modelled seismic events by observation of the time dependence of the moment tensor and by direct examination of the particle motions at the simulated source

glacier extent and climate in the maritime alps during the younger dryas we would like to acknowledge prof rea and dr pellitero for fruitful discussions on various aspects of this work and in particular on the use of glacier reconstruction and ela gis tools and extraction of climatic variables at a glacier ela dr ortu for kindly providing easy access to pollen data and for discussions on the climate of the younger dryas across the alps prof edwards for discussions on pollen analyses and prof federici for inspiring glaciological research in this beautiful region the constructive and useful feedback provided by dr monegato and an anonymous reviewer is greatly appreciated l cignoni is thanked for reviewing the english ms acknowledges support from nerc ciaf 9092 1010

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

flow pattern evolution of the laurentide and cordilleran ice sheets across west central alberta canada implications for ice sheet growth retreat and dynamics during the last glacial cycle this paper presents a reconstruction of the geometry dynamics and flow pattern of the laurentide and cordilleran ice sheets along the albertan portion of the eastern canadian rocky mountains during the last glaciation sediment landform associations relating to the evolution of these ice sheets document their initial convergence and mutual deflection across west central alberta the continued advance of laurentide ice locally displaced cordilleran ice westward into the front ranges and deflected trunk ice emerging from the athabasca river valley south eastwards along the foothills these convergent flow patterns remained major features in the regional geometry of both ice sheets throughout the last glacial maximum the onset of deglaciation was characterized by the northward retreat of the laurentide ice sheet which progressively unblocked valleys along the mountain front the configuration of buttress removal initiated the lateglacial expansion and sequential drawdown of the eastern cordilleran ice sheet along fast flowing outlet glaciers subsequent regional retreat of laurentide and cordilleran ice was associated with the development of large proglacial lakes which amplified deglaciation by destabilizing the margins of both ice sheets triggering extensive readvances due to periodic changes in water level associated with the opening and closing of outlets by ice margin fluctuations

coalescence of late wisconsinan cordilleran and laurentide ice sheets east of the rocky mountain foothills in the dawson creek region northeast british columbia canada abstract geomorphic stratigraphic and geochronological evidence from northeast british columbia canada indicates that during the late wisconsinan approximately equivalent to marine oxygen isotope stage mis 2 a major lobe of western sourced ice coalesced with the northeastern sourced laurentide ice sheet lis high resolution digital elevation models reveal a continuous 75 km long field of streamlined landforms that indicate the ice flow direction of a major northeast flowing lobe of the cordilleran ice sheet cis or a montane glacier 200 km wide was deflected to a north northwest trajectory as it coalesced with the retreating lis the streamlined landforms are composed of till containing clasts of eastern provenance that imply that the lis reached its maximum extent before the western sourced ice flow crossed the area since the lis only reached this region in the late wisconsinan the cis montane ice responsible for the streamlined landforms must have occupied the area after the lis withdrew stratigraphy from the murray and pine river valleys supports a late wisconsinan age for the surface landforms and records two glacial events separated by a non glacial interval that was dated to be of middle wisconsinan mis 3 age

brittle to ductile transition due to large strains along the white rock thrust wind river mountains wyoming abstract the deformed foreland of the rocky mountain cordilleras is characterized by major reverse faults that bring precambrian basement rocks to the surface the white rock thrust in the wind river mountains of wyoming is one such fault that developed at fairly shallow levels in the crust detailed study of grain size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation a transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain size along the fault zone diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses this may be one way of obtaining large displacements along major faults at shallow levels in the crust

damage mechanisms in stressed rock from acoustic emission to better understand the phenomena leading to the failure of rock unconfined compression experiments on charcoal granite specimens were performed with the monitoring of acoustic emission ae localization in the form of spalling near the free surface was detected by the concentration of hypocenters the ae locations which look random in space and time before localization actually were clustered and have fractal structure in either space or time after localization the fractal dimensions reduced significantly in a certain range of distance and time the analysis of interaction between spatial and temporal clustering revealed the size of clusters in both space and time the cluster sizes may be related to the intrinsic properties of the rock the seismic moment tensor was evaluated through a deconvolution technique to obtain the source mechanism and orientation of each ae event the dominant mode of failure from ae was sliding on inclined planes although a significant number of source planes were parallel to the loading axis while the growth of cracks perpendicular to the loading axis was inhibited this preferential growth of microcracks is related to a tensorial measure of damage and is used to study stress induced anisotropy

landform assemblages produced by the laurentide ice sheet in northeastern british columbia and adjacent northwest territories constraints on glacial lakes and patterns of ice retreatthis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers the laurentide ice sheet reached the canadian cordillera during the last glacial maximum in northeastern british columbia and adjacent northwest territories and all regional drainage to unglaciated areas in the north was dammed by the ice converging ice flow patterns near the mountain front suggest that the laurentide ice sheet likely coalesced with the cordilleran ice sheet during the last glaciation with deglaciation the ice masses separated but earlier ice retreat in the south meant that meltwater pooled between the mountain front and the laurentide margin the level of the flooding was controlled by persistent ice cover on the southern franklin mountains glacial lake liard formed when the laurentide ice sheet retreated east of the southern liard range and at its maximum extent may have impounded water at least as far south as the fort nelson river deglaciation of the plains was marked by local variations in ice flow caused by a thin ice sheet becoming more affected by the topography and formin

micromechanical modeling of cracking and failure in brittle rocks dynamic micromechanical models are used to analyze crack nucleation and propagation in brittle rock models of rock are created by bonding together thousands of individual particles at points of contact the feasibility of using these bonded particle models to reproduce rock mechanical behavior is explored by comparing model behavior to results from actual laboratory tests on different rock types the behavior of two granite models are examined in detail to study cracking and failure patterns that occur during compressional loading because discontinuum models are being used the rock models are free to crack and break apart under stress such that the micromechanics of cracking can be examined stress waves are allowed to propagate outward from each crack and it is shown that these dynamic waves significantly affect the rock behavior as the peak stress in the modeled rock is approached and many of the bonds are close to breaking a passing wave from a nearby crack is sufficient to break more bonds this causes clusters of cracks to be created and then eventual macroscopic shear failure occurs as these clusters connect to bisect the sample the failure patterns observed in the granite models are similar to those observed in actual laboratory tests

quaternary stratigraphy of the prophet river northeastern british columbiathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers exposures in the prophet river valley in northeast british columbia provide a unique glimpse into the quaternary history of the northwest canadian boreal plains the region shows evidence of late wisconsinan laurentide glaciation in the form of widespread till containing abundant erratic clasts derived from the canadian shield vertical sections along the prophet river expose non glacial and advance glacial sediments below this till pre late wisconsinan non glacial or interglacial floodplain sediments are interbedded with fluvial gravels at many sites macrofossils within horizontally laminated organic rich black clay and silt indicate deposition on the floodplain of the paleo prophet river within an oxbow lake the climate during deposition is interpreted to be similar to present supporting a dominantly spruce forest wood obtained from eight sites provided non finite radiocarbon ages and one sample provided an age of 49 300 2000 bp which is also considered non finite glaciolacustrine clays and s

late quaternary stratigraphy sedimentology and history of the jasper townsite area alberta canada geomorphic stratigraphic and sedimentologic evidence obtained from quaternary sediments near jasper alberta is used to reconstruct depositional environments in the vicinity of two former confluent valley glaciers sedimentation occurred mainly in the valley centre in a pre existing river channel lateral ice marginal sedimentation along the inset valley walls was important during both the advance and the retreat stages prior to and during glacial advance horizontally stratified gravels sands and silts were deposited in a braided stream near the valley bottom diamictons containing trough shaped sand lenses were deposited by subsequent proglacial debris flows and intermittent fluvial activity at higher stratigraphic positions outwash deposits confined to the valley wide are unconformably overlain by massive diamicton interpreted as subglacial till deposited as ice began to override the area the diamicton grades towards the valley side into ice contact debris deposited at the margins of the ex

late pleistocene and holocene vertebrates and palaeoenvironments from charlie lake cave northeast british columbia excavations outside charlie lake cave peace river district british columbia revealed deposits dating from 10 700 bp to the present the earliest fauna 10 700 10 000 bp was deposited when the newly deglaciated landscape was largely unforested and included bison bison sp ground squirrel spermophilus sp a large hare lepus sp snowshoe hare lepus americanus and a variety of birds including the cliff swallow hirundo pyrrhonota by 10 000 bp snowshoe hare lepus americanus was the most numerous mammal indicating the development of forested conditions by 9000 bp the fauna resembled the modern peace river fauna prior to european settlement typical of a largely forested landscape with wetland areas indicated by aquatic avian species subsequent holocene climatic fluctuations are not evident in the faunal record

stick split mechanism for anthropogenic fluid induced tensile rock failure fluids play a critical role in natural and human induced rock failure it is unclear however if propagation of a tensile fracture is inherently an episodic or continuous process for example typical average propagation speeds of hydraulic fracture tips on the order of 1 10 m min suggest continuous crack growth possibly at subcritical stress intensities in contrast using field observations and numerical and mathematical analyses we show that fracture growth due to anthropogenic hydraulic fracturing is most likely to occur in an episodic fashion characterized by stick split behavior that is analogous to stick slip motion of earthquakes the stick split mechanism is regulated by cyclic variations in fluid pressure near the crack tip in which each successive failure produces a local pressure drop that temporarily halts or slows fracture propagation a pressure drop results in partial fracture closure producing noncontinuous fracture propagation through a process that is reminiscent of hand clapping rupture speeds for individual failure events are on the order of the shear wave velocity of the medium thus continuous crack growth is not a likely mechanism for anthropogenic hydraulic fracturing treatments despite slow average tip propagation speeds

late wisconsinan cordilleran and laurentide glaciation of the peace river valley east of the rocky mountains british columbia in the past researchers have disagreed over the maximum extent of the cordilleran ice sheet in the peace river valley during the late wisconsinan some workers argued that cordilleran ice reached beyond the rocky mountains and briefly coalesced with the laurentide ice sheet on the westernmost interior plains in contrast others asserted that cordilleran ice did not reach beyond the eastern front of the rocky mountains stratigraphic interpretation of three sections within a middle wisconsinan paleovalley and re examination of a previously published regional stratigraphic framework show that western sourced ice likely the cordilleran ice sheet extended east of the mountain front during the late wisconsinan prior to the incursion of the laurentide ice sheet into the area this conclusion has implications for cordilleran ice sheet reconstruction and modelling and provides insight into the interactions between the cordilleran and laurentide ice sheets during the last glaciation

temporal constraints for the late wisconsinan deglaciation of western canada using eolian dune luminescence chronologies from alberta abstract the laurentide ice sheet lis covered most of canada during the last glacial maximum sometime after 20 cal ka bp the lis began to recede from western canada and by 11 cal ka bp it had retreated from most of the province of alberta due to the scarcity of datable contemporaneous organic materials the precise timing of the retreat of the ice sheet from the region remains poorly constrained so that the chronology of the sequential positions of the ice sheet margin between 20 and 11 cal ka bp is largely tentative in this study we use luminescence dating of postglacial eolian deposits sourced primarily from glaciolacustrine and deltaic sediments in central and northern alberta to provide an updated chronology for the retreat of the lis from the region we examine 14 new and 13 previously published quartz optically stimulated luminescence osl ages as well as 19 previously published k feldspar infrared stimulated luminescence ages the chronologies are used to constrain successive regional ice sheet margin positions that are geomorphically recognizable across the region between 16 and 11 ka calendar years these data suggest that the lis may have retreated from central alberta earlier than portrayed in radiocarbon derived deglaciation models we propose that the osl chronology from postglacial dunes constrains the deglaciation of the region more accurately because luminescence dating does not rely on the development of vegetation to produce material for dating implications for the revised chronology include an improved timing of the emergence of an ice free corridor that may have been used for paleoindian migration into the americas as well as enhanced age constraints on when meltwater drainage routes to the arctic became open in relationship to the younger dryas the chronology could also provide improved constraints for geophysical models that reconstruct regional isostatic rebound which followed deglaciation

beryllium 10 dating of the foothills erratics train in alberta canada indicates detachment of the laurentide ice sheet from the rocky mountains at 15 ka the foothills erratics train consists of large quartzite blocks of rocky mountains origin deposited on the eastern slopes of the rocky mountain foothills in alberta between 53 5 n and 49 n the blocks were deposited in their present locations when the western margin of the laurentide ice sheet lis detached from the local ice masses of the rocky mountains which initiated the opening of the southern end of the ice free corridor between the cordilleran ice sheet and the lis we use 10 be exposure dating to constrain the beginning of this decoupling based on a group of 12 samples well clustered in time we date the detachment of the western lis margin from the rocky mountain front to 14 9 0 9 ka this is 1000 years later than previously assumed but a lack of a latitudinal trend in the ages over a distance of 500 km is consistent with the rapid opening of a long wedge of unglaciated terrain portrayed in existing ice retreat reconstructions a later separation of the western lis margin from the mountain front implies higher ice margin retreat rates in order to meet the younger dryas ice margin position near the boundary of the canadian shield 2000 years later

an assessment of the minimum timing of ice free conditions of the western laurentide ice sheet abstract this study presents an investigation of the minimum timing of retreat of the western laurentide ice sheet which covered parts of canada and the northern united states during the wisconsin glaciation the retreat of the ice sheet is poorly constrained due to low spatial resolution of chronological data that indicate ice free conditions a limitation of radiocarbon and luminescence dating methods is that it is only possible to determine the time when it is certain that ice is absent from a region which may differ substantially from the true date of ice margin retreat the minimum timing of ice free conditions is determined using a model of ice sheet retreat inferred from landforms that formed during the late stages of glaciation and extrapolating the calibrated probability distribution of chronological data in the opposite direction the analysis excludes radiocarbon samples from materials that have been shown to give artificially old ages such as bulk sediments the results of the analysis identify 55 dates that have the most impact on the model the retreat of the western laurentide ice sheet commenced by at least 15 000 cal yr bp while the minimum timing of ice free conditions between the laurentide ice sheet and the cordillera is after 11 000 cal yr bp a lack of data in the keewatin sector prevents the determination of a precise timing of ice free conditions in the last vestiges of the laurentide ice sheet the model provides a guide to where additional samples could improve chronological control on ice margin location

quaternary stratigraphy and glacial history of the peace river valley northeast british columbiathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers two cordilleran and three laurentide glacial advances are recorded in quaternary sediments and landforms in the peace river valley northeast british columbia the advances are inferred from fluvial gravels glaciolacustrine sedi ments and tills within nested paleovalleys excavated during three interglaciations and from the distribution of granitoid clasts derived from the canadian shield till of the last late wisconsinan laurentide glaciation occurs at the surface ex cept where it is overlain by postglacial sediments the advance that deposited this till was the most extensive in the study area and the only advance definitively recognized in western alberta south of the study area late wisconsinan cordil leran till has not been found in the study area but cordilleran and laurentide ice may have coalesced briefly during the last glaciation support for this supposition is provided by the inferred deflection of laurentide flutings to the southeast by cordilleran ice the earliest laurentide advance may have been the least extensive of the three laurentide events recog nized in the study area erratics attributed to this advance occur only east of the halfway river beatton river drainage divide

seismic moment tensors of acoustic emissions recorded during laboratory rock deformation experiments sensitivity to attenuation and anisotropy summary seismic moment tensors can provide information on the size and orientation of fractures producing acoustic emissions aes and on the stress conditions in the sample the moment tensor inversion of aes is however a demanding procedure requiring carefully calibrated sensors and accurate knowledge of the velocity model in field observations the velocity model is usually isotropic and time independent in laboratory experiments the velocity is often anisotropic and time dependent and attenuation might be significant due to opening or closure of microcracks in the sample during loading in this paper we study the sensitivity of the moment tensor inversion to anisotropy of p wave velocities and attenuation we show that retrieved moment tensors critically depend on anisotropy and attenuation and their neglect can lead to misinterpretations of the source mechanisms the accuracy of the inversion also depends on the fracturing mode of aes tensile events are more sensitive to p wave anisotropy and attenuation than shear events we show that geometry of faulting in anisotropic rocks should be studied using the source tensors since the p and t axes of the moment tensors are affected by velocity anisotropy and deviate from the true orientation of faulting the stronger the anisotropy is the larger the deviations are finally we prove that the moment tensor inversion applied to a large dataset of aes can be utilized to provide information on the attenuation parameters of the rock sample the method is capable of measuring anisotropic attenuation in the sample and allows for detection of dilatant cracking according to the stress regime

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

glacigenic deposits at the edmonton convention centre edmonton alberta four main stratigraphic units are present at the edmonton convention centre site in downtown edmonton the lowermost sediment exposed is cretaceous sandstone of the horseshoe canyon formation this is overlain by preglacial sands and gravels of uncertain age which contain recumbent drag folds developed by glacial overriding of previously existing periglacial involutions these sediments are surmounted by a glacigenic complex consisting of subglacially modified sand and four till facies formed through passive melt out the uppermost quaternary sediment present is a proglacial lacustrine unit dominated by silts and clays with interbedded diamictons a single glacial episode is recorded in the sediments stratigraphic relationships with other exposures throughout the edmonton region indicate that the glacigenic complex formed during the final ice advance variation in the orientation of glacially induced deformations in the preglacial sands and gravels and the basal glacigenic sand indicate that the directi

laurentide glaciation in west central alberta a single late wisconsinan event the grande prairie region in west central alberta shows evidence of late wisconsinan laurentide glaciation in the form of a widespread till containing abundant erratic clasts derived from the canadian shield two sections located on the smoky river at watino and on the simonette river 80 km to the southwest expose sediment lying stratigraphically below the till a similar sequence is exposed in both localities 5 10 m of coarse quartzite gravel overlain by a thick sequence of sand and silt the gravel contains no laurentide erratics and is thus preglacial the contact between the basal gravel and the overlying sand and silt is sharp and conformable 14c dates from the base of the simonette section and throughout the watino section suggest a middle wisconsinan age thus the sediment exposed is interpreted as a conformable middle wisconsinan sequence the sedimentary sequences were deposited in a moderate to low energy braided stream system flowing towards the north the general fining up trend in the

multiple meltwater discharge and ice rafting events recorded in the deglacial sediments along the beaufort margin arctic ocean abstract the first regional model for the deglacial history of the beaufort margin slope in the western arctic is presented the conceptual model was developed using new high resolution chirp seismic reflection multibeam bathymetry and sediment core data acquired along the margin this synthesis provides important constraints on sediment source and dispersal patterns for the last deglaciation the continental slope from barrow canyon to the mackenzie trough is characterized by thick holocene sediments mostly sourced from barrow canyon and continental discharge this acoustically transparent fine grained unit overlies acoustically laminated sediments sourced from the mackenzie river and ice rafting the deglacial history of the margin from mackenzie trough to the amundsen gulf is characterized by many ice rafting and meltwater discharge events ice rafted debris layers were deposited around 14 6 ka and 14 1 ka and likely document enhanced ice shedding events from the amundsen and m clure ice streams as they retreated there are three inferred meltwater discharge events all sourced from the mackenzie region the oldest discharge event had two phases one sometime between 14 5 ka and 14 2 ka and another between 14 0 ka and 13 0 ka this discharge deposited finely laminated sediments more than 7 m thick sourced from proglacial lakes in the area following this was a flood event starting at 12 94 ka which generated high amplitude reflectors deposited coarse debris and caused a lowering in the 18o record this is possibly a major outburst flood from glacial lake agassiz with an additional component of agassiz meltwater that was diverted from its southern drainage pathway down the mississippi to the gulf of mexico finally a third discharge event was initiated by 11 3 ka which deposited coarse laminated sediments focused in the mackenzie trough timing of these second and third events correlates with the onset of the younger dryas cold period and the preboreal oscillation event respectively suggesting that fresh water discharge may have reached a tipping point leading to those climate events

late quaternary paleoenvironments of northwestern north america implications for inland versus coastal migration routes abstract although the long held normative view of new world colonization relies on entrance through an interior ice free corridor by terrestrially adapted big game hunters a recently demonstrated human presence in southern chile dating prior to the existence of the corridor route implies different colonization scenarios which must be seriously considered a renewed focus of archaeological and paleoecological research on early post glacial landscapes in northwestern north america has revitalized the ice free vs coastal corridor controversy geological findings from glacial geology and paleo sea levels support the possibility of coastal migration from beringia to the pacific northwest between about 14 000 and 10 000 bp and preliminary paleoecological data suggest that the coastal landscape was in part vegetated and probably able to support a terrestrial fauna including humans the same cannot be said about the ice free corridor

geology of northeast british columbia and northwest alberta diamonds shallow gas gravel and glaciersthis article introduces a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers this special issue reports on some of the results of a multi disciplinary research program conducted in the boreal plains of northwest alberta and northeast british columbia several innovative approaches to conducting geoscientific research in this remote drift covered region are presented including geochemical analysis of bentonites for evaluating kimberlite potential the use of electromagnetic surveys for mapping buried aggregate deposits and paleo topographic mapping techniques to define buried channels results of the program include the discovery of several large aggregate deposits the first kimberlite indicator minerals in northeast british columbia a significant sphalerite dispersal train in northwest alberta the first documented report of kimberlite sourced bentonites and numerous previously unknown interglacial sites together these papers provide a greatly enhanced understanding of the glacial history quaternary stratigraphy and kimberlite geology of northwest alberta and northeast bri

seismic moment tensor and b value variations over successive seismic cycles in laboratory stick slip experiments the formation of fault damage due to slip under high normal stresses can rarely be monitored under in situ conditions to advance our understanding of microfracture processes we investigated stick slip events on westerly granite samples containing the following 1 a planar saw cut fault and 2 a fault developed from a fresh fracture surface we examined temporal changes of seismic moment tensors and b values of acoustic emission ae events during experiment on the saw cut surface small aes exhibiting non double couple components were observed continuously and strong aes displaying double couple components were visible only when approaching the slip onsets sliding on naturally fractured surfaces showed in addition to double couple components significant volumetric contributions especially during the interslip periods and immediately after stick slip events indicating substantial shear enhanced compaction within a relatively broad damage zone the obtained results shed light on how differences in fault structure control the kinematics of microseismicity during different periods of the seismic cycle

brittle ductile transition and associated seismicity experimental and numerical studies and relationship with the b value the acoustic emission ae and the mechanical behavior of granite samples during triaxial compression tests have been analyzed the size of ae events displays power law distributions conforming to the gutenberg richter law observed for earthquakes which is characterized by the b value as the confining pressure increases the macroscopic behavior becomes more ductile for all different stages of the rock mechanical behavior linear nonlinear prepeak nonlinear postpeak shearing there is a systematic decrease of the b value with increasing confining pressure a numerical model based on progressive elastic damage and the finite element method allows simulations of the main experimental observations on ae and of a wide range of macroscopic behaviors from brittleness to ductility the model reproduces a decrease in the b value that appears to be related to the type of macroscopic behavior brittle ductile rather than to the confining pressure both experimental and numerical results suggest a relationship between the b value and the brittle ductile transition moreover these results are consistent with recent earthquake observations and give new insight into the behavior of the earth s crust

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

ancient dna reveals northwest range extension of richardson s ground squirrel urocitellus richardsonii into northeastern british columbia canada during the late pleistocene ancient dna was extracted from 12 500 to 10 500 year old ground squirrel bones from tse k wa an archaeological site in the peace river region of northeastern british columbia canada analysis of

cordilleran ice sheet mass loss preceded climate reversals near the pleistocene termination the cordilleran ice sheet cis once covered an area comparable to that of greenland previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost canada as late as 12 5 thousand years ago ka new data indicate that substantial areas throughout westernmost canada were ice free prior to 12 5 ka and some as early as 14 0 ka with implications for climate dynamics and the timing of meltwater discharge to the pacific and arctic oceans early bolling allerod warmth halved the mass of the cis in as little as 500 years causing 2 5 to 3 0 meters of sea level rise dozens of cirque and valley glaciers along with the southern margin of the cis advanced into recently deglaciated regions during the bolling allerod and younger dryas

the pattern and style of deglaciation at the late wisconsinan laurentide and cordilleran ice sheet limits in northeastern british columbia this paper reports on the landform assemblages at the northern confluence of the late wisconsinan laurentide and cordilleran ice sheets with montane and piedmont glaciers in the northern rockies and southern mackenzie mountains recent observations in northeastern british columbia refine our knowledge of the pattern and style of ice sheet retreat glacial lake formation and meltwater drainage at the onset of deglaciation confluent laurentide and cordilleran terminal ice margins lay between 59 n 124 30 w and 60 n 125 15 w from this terminal limit ice sheets retreated into north central british columbia and yukon territory with remnant cordilleran ice and montane glaciers confined to mountain valleys and the liard plateau distinctive end moraines are not associated with the retreat of cordilleran ice in these areas laurentide ice retreated northeastward from uplands and the plateaus then separated into lobes occupying the fort nelson and petitot river valleys ice retreat landforms include recessional end moraines sometimes overridden and drumlinized hill hole pairs crevasse fill deposits de geer like ribbed till ridges hummocky moraines kames meltwater features and glacial lake deposits that fall within the elevation range of glacial lake liard and glacial lake fort nelson ca 840 380 m meltwater and sediment transport into glacial lakes fort nelson liard nahanni and mackenzie was sustained by remnant ice in the liard river and fort nelson river drainage basins until the end of glaciation optical dating of sand from stabilized parabolic dunes on the liard plateau indicates that proglacial conditions lake formation and drainage began before 13 0 0 5 ka calendar years the petitot fort nelson and liard rivers all occupy spillways incised into glacial deposits and bedrock by meltwater overflow from glacial lakes peace and hay

thrust timing growth of structural culminations and synorogenic sedimentation in the type sevier orogenic belt western united states most of the regional shortening in the type area of the sevier orogenic belt in central utah was accommodated by displacement on the canyon range neocomian aptian pavant aptian albian paxton cenomanian campanian and gunnison late campanian paleocene thrust systems inception of each thrust system generated synorogenic sediment associated with frontal thrust tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland the sevier culmination a large antiformal duplex cored by crystalline basement rocks developed during paxton and gunnison thrusting west of and structurally beneath the canyon range and pavant thrusts growth of the sevier culmination was coeval with reactivation of the canyon range and pavant thrust systems and produced a second culmination in proterozoic lower cambrian rocks in the canyon range these structural highs provided much of the sediment to the adjacent foreland basin from late cenomanian to late paleocene time and may have helped to maintain critical taper in the thrust belt