earthquake nests as natural laboratories for the study of intermediate depth earthquake mechanics article the physical mechanism of intermediate depth earthquakes is still under debate in contrast to conditions in the crust and shallow lithosphere at temperatures and pressures corresponding to depths 50 km rocks ought to yield by creep or flow rather than brittle failure some physical process has to enable brittle or brittle like failure for intermediate depth earthquakes the two leading candidates for that are dehydration embrittlement and ther mal shear runaway given their great depth intermediate depth earthquake processes can t be observed direct ly insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity in this paper we focus on three intermediate depth earthquake nests vrancea hindu kush and bucaramanga and what they reveal about the mechanics of intermediate depth earthquakes we review published studies of tectonic setting focal mechanisms precise earthquake locations and earthquake source physics at these locations with an emphasis on the bucaramanga nest all three nests are associated with subducting litho sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity in contrast the bucaramanga nest has a larger b value larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two we also report for the first time finding a signifi cant number of repeating earthquakes some with reverse polarity

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

source characteristics of large deep earthquakes constraint on the faulting mechanism at great depths 1 source parameters for eight large deep earthquakes are inferred from broadband data of the global seismograph network we completed an unprecedented data set composed of 14 largest deep earthquakes mw ranging from 7 0 to 8 3 from six different subduction zones studied with identical source parameter inversion methods the study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate events in cold slabs show high aftershock activity high rupture velocity and high seismic efficiency in contrast earthquakes in warm slabs generally have low aftershock productivity low rupture velocity and low seismic efficiency however the 23 june 1991 argentina event has source properties that are rather characteristic for earthquakes in cold slabs and that differ from those of other large south american events we suggest that two types of deep earthquakes occur in south america and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes in this model shear failure occurs through a thermal runaway mechanism within a shear zone at great depth and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production the 1991 argentina event located in a pocket of deep seismicity within the slab core may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones whereas the other known south american large deep events such as 1963 peru 1970 colombia and 1994 bolivia are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

role of pore fluid pressure on transient strength changes and fabric development during serpentine dehydration at mantle conditions implications for subduction zone seismicity abstract to further investigate the dehydration embrittlement hypothesis and its possible link to subduction zone seismicity we conducted deformation experiments on antigorite serpentinite in a griggs type apparatus at conditions below and above antigorite stability temperature ramps crossing the antigorite thermal stability were used in conjunction with a new experimental method that allows fluid produced during dehydration reactions to be drained partially drained or undrained during temperature ramps weakening coupled with transient slip initiated at 650 c coincident with the predicted phase transition of antigorite to olivine and talc at 1 gpa the weakening rate and steady state strength were dependent on drainage conditions undrained samples weakened over a few minutes and supported the lowest shear stress 50 mpa while drained samples weakened over a few hours and supported the highest shear stress 210 mpa the coefficient of friction shear stress over normal stress in drained samples decreased from 0 4 to 0 16 after the temperature ramp the strengths of samples that were first annealed at 700 c for 12 h then deformed were similar to those observed in the temperature ramp experiments strain localization along fractures occurred in all samples during temperature ramping regardless of the drainage conditions however microstructural observations indicate deformation by ductile mechanisms at higher strain under both undrained and drained conditions the rheology and microstructures suggest dehydrating serpentinite deforms via semibrittle flow with grain scale ductile deformation more active at high pore fluid pressures our results suggest that earthquakes in serpentinized mantle do not nucleate as a direct result of unstable frictional sliding along fractures generated at the onset of dehydration reactions

a note on the glacial geology and postglacial emergence of the lake harbour region baffin island n w t ice flow indicators in the lake harbour region of northern hudson strait define two flow directions affecting this area during the late wisconsinan glaciation a pronounced southward flow direction indicated by medium and large scale erosional and depositional features represents ice flow from an ice dome centered to the north perhaps foxe basin and or amadjuak lake carbonate rich till and striations represent eastward southeastward ice flow down the axis of hudson strait convergence of ice sheet flow with a rapidly moving ice stream has been observed and modelled for west antarctic ice streams and involves sharp bending of flow lines at the point of convergence a similar scenario is proposed for the lake harbour region to explain the two contrasting ice flow patterns impingement of an ice stream in hudson strait onto the southern coast of baffin island suggests the influence of northerly flowing ice perhaps from the ungava plateau radiocarbon dates on marine shells and archeological samples are

deep earthquake rupture histories determined by global stacking of broadband p waveforms 1 we conduct a systematic investigation of time histories of moment release in deep earthquakes whose physical mechanism remains unknown we have constructed source time functions stfs of 111 large deep earthquakes depth 100 km and mw 6 4 by stacking p waveforms from broadband seismograms recorded by the global seismographic network aligning and stacking waveforms cancels noise and allows us to identify the rupture initiation and termination times accurately each stf is scaled to a reference moment of 1019 n m using the duration moment relation m01 3 in order to investigate depth dependence of the stfs we bin them into three depth ranges 100 350 km 350 550 km and deeper than 550 km based on physical properties of the mantle and the distribution of the data we infer a change in character near 550 km earthquakes deeper than about 550 km are shorter have simpler time functions and are more uniform in their characteristics further we find longer than expected scaled durations and greater complexity in the 350 550 km depth range the scaled durations of the 350 550 km earthquakes do not decrease with depth as 1 vs instead possessing a slightly longer average scaled duration than those 100 350 km deep we further categorize the earthquakes by subduction zone and find a similar depth dependence within individual zones finally the events that have achieved highest moment rate in the first 1 2 s after initiation tend to be deeper and or larger events the latter suggesting that dynamic rupture propagation affects the final size of the earthquake the depth dependence of duration and complexity suggests more complicated rupture histories between 350 and 550 km and a possible change in the physical mechanism of rupture at about 550 km

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

late glacial flow patterns deglaciation and postglacial emergence of south central baffin island and the north central coast of hudson strait eastern canadian arctic new georaorphic sedimentologic and chronologic data are used to reconstruct late quaternary ice sheet flow patterns deglaciation and isostatic uplift along the largest marine trough connecting the laurentide ice sheet with the north atlantic ocean the lake harbour region was targeted for study given its potential to record flow from several ice dispersal centers striations and sediment provenance indicators define flow patterns thirty four radiocarbon dates constrain a chronology of events centuries or millennia before deglaciation a southeast flowing ice stream impinged on southernmost big island as recorded by a single striation site and delimited in extent by geomorphic evidence of cold based ice during the cockburn substagc 9000 8000 bp the region was scoured by southward to southwestward flow from an ice cap on meta incognita peninsula as recorded by 60 striation sites along 200 km of coastline carbonate erratics are uncommon in till above the marine limit where present they sugg

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

experimental investigation of eclogite rheology and its fabrics at high temperature and pressure eclogite plays an important role in mantle convection and geodynamics in subduction zones an improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology however the deformation processes and associated fabrics in eclogite are not well understood incompatible views of deformation mechanism have been proposed for both garnet and omphacite we present here deformation behaviour of eclogite at temperatures of 1027 1427 c confining pressures of 2 5 3 5 gpa and strain rates of 1 10 5 s 1 to 5 10 4 s 1 we obtained a power law creep for the high temperature and pressure deformation of a dry eclogite 50 vol garnet 40 omphacite and 10 quartz with a 103 3 1 0 n 3 5 0 4 e 403 30 kj mol 1 and v 27 2 cm3 mol 1 the two principal minerals of eclogite have greatly different strengths progressive increase of garnet results in a smooth increase in strength analysis by electron back scattered diffraction shows that 1 garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions 2 omphacite shows pronounced lattice preferred orientations lpos suggesting a dominant dislocation creep mechanism further investigation into the water effects on eclogite show 3 water content does not influence the style of omphacite fabric but increases slightly the fabric strength 4 grain boundary processes dominate the deformation of garnet under high water fugacity or high shear strain conditions yielding a random lpo similar to that of non deforming garnet despite the strong shape preferred orientation spo observed 110 001 slip may dominate the deformation of rutile quartz displays complicated and inconsistent lpos in eclogite these results are remarkably similar to observations from deformed eclogites in nature

two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth s crust abstract four silicate rocks granite gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature and were found to exhibit brittle fracture behaviour from the measurements of the compressive and frictional strengths and the activity of acoustic emission ae and the observations of the microstructure of fractured specimens the pattern of fracture was found to change when the compressive strength became equal to the frictional strength two types of fracture mechanism low pressure and high pressure types were inferred the low pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers the features of high pressure type fracture are similar to those observed between brittle fracture and ductile creep in high temperature triaxial experiments microcracks are not concentrated close to the main fault and the main fault is sharp and oriented at approximately 45 to the stress direction this suggests that the high pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature for granite and gabbro samples of a few centimeters size high pressure type fracturing begins at 0 8 gpa confining pressure while for dunite and eclogite it begins at 1 0 and 2 0 gpa respectively the size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens 1 m this estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 mpa the significance of this is that it may be necessary to model earthquakes using the high pressure type fracturing because in the absence of pore pressure 60 mpa confining pressure corresponds to a depth of 3 km faulting processes in the crust including both the initial faulting and the movement of existing faults were inferred using the experimental results and the estimated size effect on strength

detection and analysis of microseismicity in multi anvil experiments abstract we present a new technique to measure microseismicity at gigapascal pressures this new technique uses existing acoustic emission ae and multi anvil press technologies with minor modifications to make them compatible as a proof of concept we present results from acoustic emission studies on serpentine dehydration to 8 gpa and the olivine wadsleyite transformation at 16 gpa these case studies demonstrate the power of gigapascal acoustic emission monitoring and point to future developments which will further strengthen the technique

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

earthquakes in the deep continental crust insights from studies on exhumed high pressure rocks summary recent studies on two exhumed anorthositic and gabbroic high pressure complexes in the bergen arcs and western gneiss region of norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may resist metamorphic re equilibration for geologically significant periods of time this persistent metastability exercised a profound control on the mechanical properties of these rocks as evidenced by abundant pseudo tachylyte veins cataclasites and fractures documenting eclogite facies seismic failure at depths exceeding 60 km in the thickened caledonian crust our observations indicate that the brittle seismogenic state of some lower crustal rocks and their propensity to remain metamorphically metastable despite prevailing conditions of t 600 800 c and p 1 5 to 2 0 gpa is largely controlled by

transient discontinuities revisited pseudotachylyte plastic instability and the influence of low pore fluid pressure on deformation processes in the mid crust abstract occurrences of pseudotachylyte cyclically introduced as melts into the outer hebrides thrust are demonstrably synkinematic with crystal plastic mylonites classical interpretations of these rock types as respectively products of pressure dependent frictional melting and thermally activated intracrystalline deformation create a paradox in that these processes are to a large extent mutually exclusive detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy the ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 c and stresses of 140 210 mpa on which were imposed much higher transient stresses in response to heterogeneous non uniform flow the latter conditions are sufficient for the development of plastic instabilities in feldspar rich crust if pore fluid pressures are sufficiently low the latter is consistent with the absence of hydration during exhumation observed in the rocks under study low pore fluid pressure during thrust exhumation of deep crust enables activation of high strength ductile processes dominated by dislocation glide which may be a prerequisite to instability whereas other studies have demonstrated the possible occurrence of such melt generating instabilities it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure temperature conditions grain size reduction to produce ultramylonites dominated by grainsize sensitive flow is achieved by both deformation induced dynamic recrystallization and crystallization of instability generated melts

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

a new class of microstructures which lead to transformation induced faulting in magnesium germanate 1 transformation induced faulting has been studied extensively in natural olivine ice and a variety of other olivine analog systems over the last 15 years and remains a likely candidate mechanism for initiating deep focus earthquakes the mechanism involves the formation of microscopic packets of fine grained denser materials from a metastable host which then behave as mode i features that coalesce to allow bulk mode ii shear failure we present evidence for a new class of transformation microstructures that also allows shear failure at high pressures but which does not require the formation of widely distributed mode i features prior to shear failure we have observed thin planar zones of transformation in coarse grained 150 m mg2geo4 olivine that form along crystallographic planes in deformed grains at temperatures cooler than those that allow significant development of anticracks the development of these zones is rapid and only occurs after 25 bulk strain increasing strain leads to progressive development of these zones in turn leading to bulk shear failure once a continuous pathway of fine grained spinel is formed that traverses the specimen transmission electron microscopy analysis shows that slip bands in olivine along 011 and 010 formed from dislocation pileups on these slip planes transform preferentially to a nanocrystalline aggregate of the denser spinel phase aided by the high strain energy of these bands this new partially strain driven mechanism compliments shear failure by anticracks as described in previous studies and adds new possibilities for the triggering of bulk shear failure by rapid mineralogical transformations

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

was autotomy a pervasive adaptation of the crinoid stalk during the paleozoic the crinoids were the most diverse and numerically most common echinoderms in the paleozoic as well as being one of the most important groups of sessile invertebrates autotomy self mutilation in the stem that is the casting off of the more distal part is only known to occur in one extant group of crinoids the post paleozoic isocrinine sea lilies these crinoids have a long distal stalk with regularly spaced articulations i e cryptosymplexies adapted for autotomy they are connected together by short mutable collagenous tissues that undergo irreversible destabilization during autotomy the distal opening of the axial canal is then sealed by new growth of calcite paleozoic crinoids lacked cryptosymplectial articulations but have other adaptations that may have singly or collectively facilitated autotomy these include the linking of adjacent short lengths of stalk i e noditaxes by short ligaments the evolutionary trend to narrow the axial canal occlusion of the axial canal in some species and the ability to seal the axial canal other morphological features of relevance to autotomy include the common division of the stem into morphologically distinct regions and attachment by a distal recumbent radicular runner the ability to autotomize the distal stem in many paleozoic crinoid clades was at the very least latent if overt it would have facilitated escape from environmental disturbance it may in part explain the great abundance of disarticulated crinoid stem fragments in many ordovician to permian sedimentary deposits

comparison of uranium series radiocarbon and amino acid data from marine molluscs baffin island arctic canada uranium series and sup 14 c dates and the extent of amino acid racemization are reported for 24 marine shell samples from three areas of baffin island arctic canada when the radiometric dates are plotted against the ratio of d alloisoleucene l isoleucene in the shells five broad age groups are recognized the uranium series data indicate that sup 231 pa is incompletely retained in most fossil shells and that sup 230 th is lost from some of the samples therefore their apparent ages are minimum however a few dated samples in each group have yielded useful age results and the minimum ages of the five groups of samples are estimated as 7 000 to 11 000 greater than or equal to70 000 greater than or equal to136 000 greater than or equal to190 000 and 300 000 yr calculated integrated thermal histories based on the epimerization reaction in the mollusc hiatella arctica linne give paleotemperature estimates of around 5 sup o c compared to the present mean annual air temperature of about 11 sup o c

rapid fluctuations of the laurentide ice sheet at the mouth of hudson strait new evidence for ocean ice sheet interactions as a control on the younger dryas ice directional features and erratic lithologies of the last glaciation demonstrate that a major outlet glacier of the laurentide ice sheet flowed nne across outer hudson strait rather than se down the strait as previously hypothesized ice advanced more than 600 km from a labradorean source crossing hudson strait and outer frobisher bay and advancing onto hall peninsula the highest summits on loks land circa 400 m asl a large island at the se tip of hall peninsula were inundated by labradorean ice three distinct late wisconsin advances have been recognized the earliest most extensive advance crossed frobisher about 11 5 kyr b p and was maintained for circa 1 kyr differences in the extent of amino acid racemization in shells from interstadial and deglacial sites on eastern loks land limit the duration of the most extensive late wisconsin advance s to a brief interval of no more than 2 kyr radiocarbon dates on in situ shells and on shells incorporated in till indicate that frobisher bay was ice free from at least 10 5 to 10 2 kyr b p after which a second labradorean advance the gold cove readvance attained nearly the same extent as the earlier advance the readvance was short lived and the bay was deglaciated before 9 5 kyr b p a final readvance occurred during the cockburn substage 9 to 8 kyr b p but ice failed to cross frobisher bay the maximum extent of the labradorean advance coincides with the onset of younger dryas cooling based on a minimum cross sectional area and reasonable estimates of ice velocity the iceberg flux to the north atlantic ocean at this time was between 300 to 2400 km year 1 about the same as the freshwater influx from the diversion of lake agassiz drainage to the st lawrence the input of a significant volume of ice into the north atlantic via the labrador sea would have cooled sea surface and air temperatures increased sea surface albedo and diminished the effectiveness of wind mixing of surface waters thereby reducing the volume of water required to effectively cap the north atlantic we suggest that a combination of a massive iceberg flux and increased st lawrence discharge may have been required to initiate younger dryas cooling when the outflow of lake agassiz was again routed down the st lawrence at 9 5 kyr b p a second younger dryas event failed to occur because by that time labradorean ice had retreated within hudson strait and was unable to generate a substantial climate altering volume of icebergs the possibility of an iceberg flood in the north atlantic could be tested by measuring the concentration of ice rafted detritus ird traditionally defined as quartz feldspar grains 125 m in deep sea cores however sediment entrained by the labradorean ice is dominated by detrital carbonate and only a small fraction of the noncarbonate component is 125 m hence a revised definition of ird is required to evaluate the iceberg flood hypothesis

glacial geology of the torngat mountains labrador two late quaternary alloformations are identifed throughout the torngat mountains by degree of soil profile development in till with a granitic composition morphologic expression of deposits and their relative areal distribution the two alloformations are interpreted to represent two successively limited glaciations of the torngat mountains by the laurentide ice sheet deposits of the shoal cove alloformation mantle hillslopes distal to deposits of the younger saglek alloformation deposits are primarily soliflucted till found around coastal regions soils developed in till of the shoal cove alloformation are characterized by well expressed bw horizons 19 cm thick deposits of the younger saglek alloformation are characterized by prominent well preserved moraines outwash fan surfaces and ice marginal channels soils described in till are poorly developed with a horizons directly overlying cox horizons or separated from cox horizons by thin transitional ac ca horizons or poorly expressed bw

analysis of small scale erosional data and a sequence of late pleistocene flow reversal northern new england a sequence of latest pleistocene ice flow reversal principally based on analysis of small scale erosional features has been reconstructed for northernmost new england single and crossing striations together with small scale stoss and lee or other erosional forms provided more than 1 200 widely dispersed striation sets along with relative age data the striations were grouped into grids from which vector mean data were extracted and in turn five zones delineated each characterized by similarities in ice flow a sequence of six flow direction shifts between and within zones for example east south east flow gives way to north flow was determined using a transition matrix of 159 striation sets nondirectional striation data till fabric measurements and glacial dispersal studies further complement the striation information all these data were integrated to produce a sequence of ice flow that began with interval a east southeast flow of the laurentide ice sheet through maine interval b involved a reversal of flow direction from eastward to northwestward in northern maine with development of an ice divide during interval c this divide strengthened and migrated southeastward so that by interval d it occurred nearly 120 km southeast of the quebec maine border interval e encompassed a final northward flow from the northern flank of the boundary mountains and the start of rapid large scale stagnation of the last glacier of northern new england

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

confirmation of two types of fracture in granite deformed at temperatures to 300 c abstract shimada m 1992 confirmation of two types of fracture in granite deformed at temperatures to 300 c in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 259 268 dry granite samples were triaxially compressed at confining pressures of up to 2 gpa at temperatures to 300 c the existence of two types of fracture previously observed in room temperature experiments was confirmed the low pressure type occurs when the compressive strength is higher than the frictional strength and the high pressure type occurs when the compressive strength is equal to the frictional strength the rate of increase of strength with confining pressure in the high pressure type regime is lowered as temperature increases as if an ideal coulomb fracture criterion at a fracture angle of 45 might be applicable with increasing temperature near the boundary between the low and high pressure type regimes unusually high values of strength were measured the specimens after such runs gave the suggestion that both low and high pressure types of fracture coexist there are some discrepancies in the values of strength from room temperature and those performed previously which are considered to be due to the experimental variability however the present experimental results to 300 c confirmed that two types of brittle fracture exist and that the fracture type changes when the compressive strength becomes equal to the frictional strength this tends to confirm previous speculations that many crustal fractures are of the high pressure type if so the low pressure type of fracture in which precursory ae acoustic emission activity before final fracturing is observed would be anticipated only at very shallow depth at ordinary pore pressure conditions it would occur at the depth of seismogenic zones only in the case of very high pore pressure

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

limits on rock strength under high confinement abstract understanding of deep earthquake source mechanisms requires knowledge of failure processes active under high confinement under low confinement the compressive strength of rock is well known to be limited by frictional sliding along stress concentrating flaws under higher confinement strength is usually assumed limited by power law creep associated with the movement of dislocations in a review of existing experimental data we find that when the confinement is high enough to suppress frictional sliding rock strength increases as a power law function only up to a critical normalized strain rate within the regime where frictional sliding is suppressed and the normalized strain rate is below the critical rate both globally distributed ductile flow and localized brittle like failure are observed when frictional sliding is suppressed and the normalized strain rate is above the critical rate failure is always localized in a brittle like manner at a stress that is independent of the degree of confinement within the high confinement high strain rate regime the similarity in normalized failure strengths across a variety of rock types and minerals precludes both transformational faulting and dehydration embrittlement as strength limiting mechanisms the magnitude of the normalized failure strength corresponding to the transition to the high confinement high strain rate regime and the observed weak dependence of failure strength on strain rate within this regime are consistent with a localized peierls type strength limiting mechanism at the highest strain rates the normalized strengths approach the theoretical limit for crystalline materials near theoretical strengths have previously been observed only in nano and micro scale regions of materials that are effectively defect free results are summarized in a new deformation mechanism map revealing that when confinement and strain rate are sufficient strengths approaching the theoretical limit can be achieved in cm scale sized samples of rocks rich in defects thus non frictional failure processes must be considered when interpreting rock deformation data collected under high confinement and low temperature further even at higher temperatures the load bearing ability of crustal rocks under high confinement may not be limited by a frictional process under typical geologic strain rates

are intermediate depth earthquakes caused by plastic faulting abstract the increase in confining pressure and temperature with depth precludes frictional sliding beyond a few tens of kilometers yet earthquakes occur to depths greater than 600 km although rock failure is often conceptualized as either brittle under low pressure and temperature or ductile at higher pressure and temperature an intermediate brittle like failure mode sometimes called transitional or semi brittle faulting is well known experimentally in contrast to brittle and ductile failure models for semi brittle terminal failure strength are lacking we show that semi brittle failure is consistent with the dictates of adiabatic instabilities that result in failure via plastic faulting although adiabatic instabilities have been suggested as a possible cause of deep 500 km earthquakes extrapolation of laboratory semi brittle faulting data using the theory of plastic faulting with a rheology and geotherm typical of subducting oceanic crust indicates that adiabatic instabilities are more likely at intermediate depths to about 300 km

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

late glacial ice margins and deglacial chronology for southeastern baffin island and hudson strait eastern canadian arctic radiocarbon dates from marine piston cores and from onshore raised marine stratigraphic sections in the hudson strait region were used to reconstruct deglacial isochrons for 9900 9500 8800 8500 and 8000 bp at the culmination of the gold cove readvance 9900 bp labrador ungava ice flowed northeastward across hudson strait and outer frobisher bay and stood for the last time on the baffin island continental shelf subsequent retreat by calving was rapid and profound opening the entire hudson strait marine trough by 9500 bp at this time ice dispersal from foxe basin labrador ungava and local ice on meta incognita peninsula supported tidewater margins along much of the coastline with the exception of northernmost ungava peninsula where the ice margin stabilized onshore this onshore margin remained in place throughout the cockburn substage while a major northeastward readvance of ungava bay ice the noble inlet readvance from 8800 to 8500 bp crossed outer hudson strait grounding on the hudson st

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

evidence for a relict glacial landscape in quebec labrador abstract two major glacial landform systems occur in labrador these are the radial lineation and esker swarm reflecting late wisconsinan decay and the ungava bay lineation and esker swarm reflecting convergent northward flow in some sectors the two landform systems give conflicting evidence regarding deglaciation pattern we have interpreted the ice sheet dynamics in labrador from morphological data using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns in the george river area we have found that the ungava bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation a similar overprint also exists along the intersection line in west central labrador these relations show that the previously accepted relative ages of the two landform systems hughes 1964 boulton and clark 1990a b klassen and thompson 1993 have to be reversed we interpret the ungava bay lineation and esker swarm to represent a 0 25 106km2 pre late wisconsinan relict landscape formed during the deglaciation of an older ice sheet and later preserved in a dry based central zone of the labrador dome

a heinrich like event h 0 dc 0 source s for detrital carbonate in the north atlantic during the younger dryas chronozone in the north atlantic we define h 0 as a heinrich like event which occurred during the younger dryas chron on the se baffin shelf prior to 11 ka surface water productivity was reasonably high as measured by the numbers of diatom and planktic foraminifera per gram but an abrupt increase in detrital carbonate dc 0 event from approximately 15 up to 50 carbonate by weight occurred at 11 14c ka and continued to circa 10 ka these deposits 2 6 m thick are dominated by detrital calcite and silt and clay sized sediments during this event dc 0 h 0 ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over labrador ungava as a consequence the pattern of ice rafted debris and sediment provenance shown by h o in the north atlantic is different from that during h 1 14 5 ka or h 2 20 ka when the ice sheet extended along the axis of hudson strait and may have reached the shelf break for example there is no concrete evidence for dc o is cores on the floor of the labrador sea due east of hudson strait hu75 55 56 but h o has been noted in cores off newfoundland and west of ireland a coeval carbonate event to dc 0 but this one dominated by dolomite occurs in hu82 su5 on the west side of davis strait with a source either from northern baffin bay or cumberland sound although other sources for north atlantic detrital carbonate cannot be totally excluded our evidence suggests that h 0 represents the expression of glaciological instability of the laurentide ice sheet within the general region of hudson strait and probably to the north cumberland sound and northernmost baffin bay there is one younger dc event dated circa 8 4 ka present in sediments along the labrador margin and in hudson strait which represents the final collapse of the ice sheet within hudson strait and hudson bay

late deglaciation of the central labrador coast and its implications for the age of glacial lakes naskaupi and mclean and for prehistory the age of the marine limit and associated deglaciation has been estimated from relative sea level curves for the hopedale and nain areas of the central labrador coast as approximately 7600 200 and 8500 200 yr ago respectively these ages indicate that the ice margin remained on the coast for up to 3000 yr longer than previously estimated because the central coast is due east of glacial lakes naskaupi and mclean the earliest the lakes could have formed was 8500 200 yr ago with their largest phases being fully established only after 7600 200 yr ago this suggests that the age of the lakes and associated deglaciation of the central labrador ungava region is younger by at least 1500 yr than previously estimated a late glacial marine based ice mass in ungava bay that dammed the lakes collapsed ca 7000 yr ago within this time frame therefore the glacial lakes only existed for 500 yr the persistence of the laurentide ice sheet margin on the central labrador coast until 7600 yr ago probably restricted the northward movement of early prehistoric people into northern labrador

ice sheet dynamics and glacial history of southeasternmost baffin island and outermost hudson strait southeasternmost baffin island is mantled by hudson strait drift it contains abundant limestone erratics and 20 to 50 carbonate in the matrix to the northwest it is replaced by drift dominated by locally derived rock of the canadian shield the sense and orientation of ice erosional features demonstrate that hudson strait drift is associated with northeasterly ice flow that crossed the tip of meta incognita peninsula local drift associated with ice flow s10 w along the hudson strait coast was derived from a dispersal center on the peninsula erratic lithologies contained in the hudson strait drift indicate a labradorean provenance large scale bedrock molding and the distribution of cirques indicate ne flowing ice has been dominant throughout the middle and late quaternary radiocarbon dates of in situ shells confirm that deglaciation began more than 11 000 yr ago with the frobisher bay coast becoming ice free by 9300 yr ago five dates from hudson strait suggest that the strait was deglaciated before 9000 yr bp however ice from the labradorean sector recrossed hudson strait during the cockburn substage about 8500 yr ago damming drainage from the west final retreat of labradorean ice from baffin island was complete by 8000 yr ago at which time the sea was able to penetrate hudson bay

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

ductile to brittle transition in thermally stable antigorite gouge at mantle pressures general shear experiments on antigorite rich serpentinite show a transition from ductile distributed to brittle localized deformation with increasing temperature from 300 c to 500 c at confining pressures from 1 to 2 gpa the coefficient of friction associated with slip along fractures decreases from 0 23 to 0 07 with an increase in temperature from 300 c to 500 c velocity stepping experiments exhibit a positive rate dependence as parameterized by a b values that decrease modestly with increasing temperature from 0 015 at 300 c to 0 01 at 500 c fractures contain fine grained foliated antigorite and there is no evidence of dehydration all samples have a moderate foliation and show microstructural evidence for both plastic and brittle deformation mechanisms under certain conditions the transition to brittle deformation at high pressures and temperatures in antigorite might generate earthquakes which could explain the occurrence of some intermediate depth seismicity within subduction zones in serpentinized regions that are too cold to induce dehydration

on the initiation of shear faults during brittle compressive failure a new mechanism brittle materials loaded under compression generally fail by shear faulting this paper addresses the initiation of the fault it presents direct observational evidence from ice which is used as a model material for rock and shows that wing cracking and splay cracking are important processes in the localization of deformation both prior to and during fault initiation wing cracks develop at the tips of sliding intergranular cracks and tend to align with the maximum principal stress splay cracks emanate from one side of the sliding parent crack the theme of the paper is that the splay cracks play the dominant role in triggering the fault the central idea is that the slender columns between the splay cracks are more likely to buckle and fail than are the columns between adjacent wing cracks because they do not have two fixed ends instead the end stemming from the inclined parent crack is free a moment is then applied by frictional sliding of the parent inclined crack and this causes the fixed free columns to break at a much lower stress than the fixed fixed columns columns created near a free surface are more likely to fail than those created elsewhere and this explains the observation that shear localization tends to initiate near free surfaces a first order calculation shows that the failure stress of the splay created columns is of the same order of magnitude as the terminal failure stress

postglacial emergence of ungava peninsula and its relationship to glacial history a series of 178 radiocarbon dates of late glacial and postglacial age from raised marine terraces on the hudson strait hudson bay and ungava bay coasts permit a new synthesis of deglaciation h

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

deglacial reservoir age and implications foxe peninsula baffin island marine reservoir age is reported for foxe basin canada during deglaciation of the laurentide ice sheet radiocarbon 14c measurements were made on pairs of contemporaneous molluscs and salix willow macrofossils yielding a mean marine reservoir age of 985 10 14c a mean regional r is calculated to be 615 20 14c a relative to a mean global reservoir age of ca 400 14c a previous studies in the canadian arctic that have been conducted on modern pre bomb molluscs give r values of ca 100 300 14c a the difference between modern and deglacial reservoir ages indicates that r in foxe basin has changed over time which has implications for the reconstruction of past events based on marine 14c dates we recalculate the timing of deglaciation of hudson strait with this new reservoir age and relate this to the 8 2 cal ka event we recommend that local r be determined for the time period being examined whenever possible and that in cases where this is impossible a r value of 615 14c a be applied when calibrating marine samples from foxe basin and vicinity during deglaciation copyright 2010 john wiley sons ltd

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1