pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

seismic slip in the lower crust inferred from granulite related pseudotachylyte in the woodroffe thrust central australia pseudotachylytes generated in granulite facies rocks are found in the woodroffe thrust central australia petrologic evidence and structural and textural features show that these pseudotachylytes contain typical melting origin features including injection occurrence rounded and embayed clasts and microlites within the fine grained matrix which formed contemporaneously with the wall granulite facies rocks under conditions of 8 kbar and 650 700 c these granulite related pseudotachylyte g pt veins are overprinted in subsequent deformation stages by three other types of pseudotachylytes including mylonite related m pt and ultramylonite related um pt pseudotachylytes that are then transcut by cataclasite related pseudotachylytes c pt the overprinting occurred following uplift of the lower crust where the g pt veins formed through the brittle plastic transition zone where the m pt and um pt veins occurred within the brittle dominated regime of the c pt generation zone in the upper crust the coexistence of multiple generations of large voluminous c pt m pt um pt and g pt veins indicates that numerous large earthquakes accompanying distinct seismic slip which produced the pseudotachylytes occurred over an extended period of seismicity and various depths of the crust within the woodroffe thrust zone the spatial distribution and structural features of the multiple stage pseudotachylytes suggest that coseismic slipping caused by large earthquakes which nucleated in the brittle dominated seismogenic zone propagated downward through the brittle plastic transition zone into the plastic dominated granulite facies depth from the upper to lower crust

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

propagation of seismic slip from brittle to ductile crust evidence from pseudotachylyte of the woodroffe thrust central australia abstract the woodroffe thrust central australia is a 1 5 km wide mylonitized shear zone marked by large volumes of mm to cm scale pseudotachylyte veins the pseudotachylytes display typical melt origin features including rounded and embayed clasts spherulitic and dentritic microlites and flow structures within a fine grained matrix three types of pseudotachylyte are identified on the basis of deformation texture vein morphology and host rock lithology cataclasite related c pt mylonite related m pt and ultramylonite related um pt the m pt and um pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization these pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine grained vein matrix and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite these observations constrain the timing and environment of m pt and um pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust the m pt and um pt veins as well as the host mylonite are overprinted by cataclasis and multiple generations of late stage c pt veins that were generated in the brittle dominated regime of the upper crust during uplift and exhumation of the shear zone the coexistence of multiple generations of voluminous c pt m pt and um pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the woodroffe thrust the timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle dominated seismogenic zone and propagated down through the brittle ductile transition into the ductile dominated regime of the crust

reconstruction of former glacier surface topography from archive oblique aerial images abstract archive oblique aerial imagery offers the potential to reconstruct the former geometry of valley glaciers and other landscape surfaces whilst the use of structure from motion sfm photogrammetry with multiview stereopsis mvs to process small format imagery is now well established in the geosciences the potential of the technique for extracting topographic data from archive oblique aerial imagery is unclear here sfm mvs is used to reconstruct the former topography of two high arctic glaciers midtre and austre lovenbreen svalbard norway using three archive oblique aerial images obtained by the norwegian polar institute in 1936 the 1936 point cloud was produced using seven lidar derived ground control points located on stable surfaces in proximity to the former piedmont glacier termini to assess accuracy the 1936 data set was compared to a lidar data set using the m3c2 algorithm to calculate cloud to cloud differences for stable areas such as nonglacial surfaces vertical differences were detected between the two point clouds rms m3c2 vertical difference of 8 5 m with the outwash zones adjacent to the assessed glacier termini showing less extensive vertical discrepancies 94 of m3c2 vertical differences between 5 m this research highlights that historical glacier surface topography can be extracted from archive oblique aerial imagery but accuracy is limited by issues including the lack of camera calibration the quality and resolution of the archive imagery and by the identification of suitable ground control to demonstrate the value of historical glacier surfaces produced using oblique archive imagery the reconstructed glacier surface topography is used to investigate evidence of a potential former surge front at the high arctic valley glacier austre lovenbreen a glacier identified to have potentially exhibited surge type behaviour during the neoglacial a surface bulge of 15 20 m is resolved on the 1936 model however when compared with the now deglaciated former subglacial topography a surge origin for the surface feature becomes unclear the processed 1936 oblique imagery was also used to produce orthorectified nadir aerial imagery from which structural mapping was undertaken this adds to the existing 1948 1995 structural map series for these glaciers this research demonstrates the potential of sfm mvs for reconstructing historical glacier surfaces which is important for aiding our understanding of former glacier dynamics and enabling the rapid assessment of glacier change over time

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

ice cored moraine degradation mapped and quantified using an unmanned aerial vehicle a case study from a polythermal glacier in svalbard ice cored lateral frontal moraines are common at the margins of receding high arctic valley glaciers but the preservation potential of these features within the landform record is unclear recent climatic amelioration provides an opportunity to study the morphological evolution of these landforms as they de ice this is important because high arctic glacial landsystems have been used as analogues for formerly glaciated areas in the mid latitudes this study uses sfm structure from motion photogrammetry and a combination of archive aerial and uav unmanned aerial vehicle derived imagery to investigate the degradation of an ice cored lateral frontal moraine at austre lovenbreen svalbard across the study area as a whole over an 11 year period the average depth of surface lowering was 1 75 0 89 m the frontal sections of the moraine showed low or undetectable rates of change spatially variable rates of surface lowering are associated with differences in the quantity of buried ice within the structure of the moraine morphological change was dominated by surface lowering with limited field evidence of degradation via back wastage this permits the moraine a greater degree of stability than previously observed at other sites in svalbard it is unclear whether the end point will be a fully stabilised ice cored moraine in equilibrium with its environment or an ice free lateral frontal moraine complex controls on geomorphological change e g topography and climate and the preservation potential of the lateral frontal moraine are discussed the methods used by this research also demonstrate the potential value of sfm photogrammetry and unmanned aerial vehicles for monitoring environmental change and are likely to have wider applications in other geoscientific sub disciplines

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

multi model photogrammetric analysis of the 1990s surge of sortebrae east greenland sortebrae is a surge type tidewater glacier complex draining southeastward from the geikie plateau east greenland sortebrae s main flow unit surged around 1950 and again between 1992 and 1995 the 1990s surge affected the lower 50 km of sorte brae over an area of approximately 335 km2 over a period of 5 km surge velocities in the order of kilometres per annum are about 100 fold the quiescent velocities multi model photogrammetric analysis shows a thinning of the reservoir zone of up to 219 m and thickening of the receiving zone of up to 74 m the surge transported approximately 18 6 km 3 of ice down glacier the total calving volume as a result of the surge amounted to 11 7 km 3 equivalent to a calving flux of 3 9 7 3 km a 1 the surge characteristics and environmental setting suggest that the surge mechanism involves a switch in the subglacial drainage this surge of sortebrae is more similar to the fast short alaskan type surges than to the sluggish long svalbard type surges

fault plane processes and mesoscopic structure of a strong type seismogenic fault in tonalites adamello batholith southern alps abstract the gole larghe fault is an exhumed paleoseismic fault crosscutting the adamello tonalites italian southern alps ambient conditions of faulting were 9 11 km in depth and 250 300 c in the study area the fault accommodates 1100 m of dextral strike slip over a fault thickness of 550 m displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons faults1 2 3 fault displacement is in the range of few centimeters faults3 to a maximum of a few tens of meters in major faults1 faults1 2 nucleated on pre existing joints whereas faults3 are newly generated fractures produced during slip along faults1 2 each fault within the gole larghe fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production pseudotachylytes are usually generated at the host rock cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins pseudotachylytes and cataclasites have a similar chemical composition which is enriched in loss on ignition k rb ba u and fe3 compared to host rock we envision two models for the evolution of the gole larghe fault in both models synkinematic fluid rock interaction along a fault causes fault hardening by precipitation of abundant k feldspar epidote and minor chlorite in the cataclasite matrix conducive to final production of pseudotachylyte in the first model induration occurs progressively by differential precipitation related to fabric evolution in cataclasites in the second model induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir whatever the model the gole larghe fault represents a strong fault where hardening processes resulted in a low displacement fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement fault thickness ratios

controls on glacier surging in svalbard the geographical distribution of surge type glaciers worldwide displays a remarkably non random pattern surge type glaciers tend to be concentrated in certain glacierized areas and to be completely absent in others this observation suggests that special conditions are required for surges to occur however the factors controlling the spatial occurrence of surge type behaviour are not known to investigate this problem we performed probability statistical analysis on a sample population of 615 glaciers in svalbard the probability that a glacier in the sample population is surge type is 36 4 within the sampled area there is a spatial variation in the concentration of surge type glaciers several geometric and environmental factors associated with glaciers in the sample population were measured and tested to determine if they are related to the probability of surging of the geometric factors tested length slope elevation orientation and presence or absence of tributaries only glacier length is related to surging with surge probability increasing with increasing length elevated probabilities of surging were also found for glaciers associated with sedimentary subglacial rocks and sub polar thermal regimes the distributions of related factors were used to predict the spatial distribution of surge type glaciers however in each case the individual factors were unable to reproduce the observed pattern of surge type glacier distribution

transient creep of a composite lower crust 2 a polymineralic basis for rapidly evolving postseismic deformation modes postseismic horizontal strain and displacement following the june 28 1992 landers california earthquake mw 7 3 is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace both the observed strain at pinon flat observatory pfo and observed global positioning system receiver velocities evolve rapidly after the landers big bear earthquake sequence the observed exponential decay of these motions with timescales of 4 34 days may reflect a soft creep rheology in the lower crust and brittle ductile transition zone or even within the seismogenic crust itself here a simple model of a two dimensional screw dislocation in a layered maxwell viscoelastic earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes 4 34 days are consistent with the behavior of a biviscous lower crust the lowest viscosity of this system is derivable from laboratory experimental data on the long term creep of natural quarztites and the highest viscosity is consistent with isostasy related lower crustal flow in a continental extensional tectonic environment the model predicts significant stress relaxation at the base of the seismogenic crust near the base of the seismogenic zone and about 4 km away from the mainshock the rate of predicted relaxation is of the order of 0 01 mpa d 1 during the first 20 days of postseismic flow oblate spheroidal inclusions at 5 concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3 4 1015 pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at pfo after the landers mainshock it is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are in part the cause of such rapid postseismic evolution in southeastern california unmylonitized quartz rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes

controls on the distribution of surge type glaciers in svalbard we analyzed the possible controls on the distribution of surge type glaciers in svalbard using multivariate logit models including 504 glaciers and a large number of glacial and geological attributes specifically we examined the potential effect of geological boundaries mass balance conditions and thermal regime on surging it was found that long glaciers with relatively steep slopes overlying young fine grained sedimentary lithologies with orientations in a broad arc clockwise from northwest to southeast are most likely to be of surge type no relation between lithological boundaries and surge potential could be established possible explanations for length being conducive to surging are transport distance related substrate properties distance related attenuation of longitudinal stresses and the possible relation between thermal regime and glacier size analysis of glaciers with recorded radioecho sounding reveals that a polythermal regime accumulation area ratios close to balance and a large elevation span increase the surge potential the logit models also enabled us to detect 19 new surge type glaciers to reclassify six glaciers as normal and to identify unusual surge type glaciers our model results suggest that a polythermal regime and fine grained potentially deformable beds are conducive to the surge potential of svalbard glaciers

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

distribution and controls of platinum group element and base metal mineralization in the darwendale subchamber of the great dyke zimbabwe three major sulfide zones containing platinum group elements pge have been identified in the upper part of the ultramafic sequence of the great dyke the pge are at the bases of the sulfide zones the lower two have sparse sulfide mineralization over a thick interval the upper zone contains higher grade in a thinner stratigraphic layer none of the current models for the formation of stratabound sulfide deposits in layered intrusions adequately accounts for the mineralization the two lower sulfide zones are interpreted as resulting from progressive fractional segregation of sulfide from the silicate magma whereas the upper zone may have resulted from mixing of a relatively primitive magma at the base of the chamber with an evolved liquid derived from near the roof

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

formation of alpine mylonites and pseudotachylytes at the base of the silvretta nappe eastern alps optical microscopy and transmission electron microscopy tem have been used to investigate deformation induced microstructures in alpine pseudotachylytes mylonites and ultramylonites from the upper austro alpine silvretta nappe at the southwestern margin of the lower engadine window in the eastern alps of switzerland and austria the earliest alpine stage of deformation d1 produced mylonitic gneisses with dislocation glide being the dominant deformation mechanism in quartz in the second stage d2a mylonites were formed as a result of strain softening in quartz manifest by dynamic recrystallization and a lattice preferred orientation tem observations are used to characterize the initial stages of migration and rotation recrystallization pseudotachylytes were generated during d2b when strain hardening due to dislocation tangling was not outweighed by strain softening several generations of mylonites and pseudotachylytes are observed a model for the propagation of seismic fractures initiated in comparatively rigid amphibolites into an otherwise ductile environment is proposed for the pseudotachylytes as such they are formed as transient discontinuities in the mylonites d1 and d2a b are related to the eo alpine detachment of the silvretta nappe subsequent eo alpine metamorphism in the investigated area reached the stilpnomelane zone and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during d3 and peak metamorphism the increased temperature combined with fluid infiltration from the overridden penninic strata favoured grain boundary sliding superplasticity as the dominant deformation mechanism during d3

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

mass balance change as a control on the frequency and occurrence of glacier surges in svalbard norwegian high arctic the end of the little ice age lia in svalbard 76 81 n a climate sensitive region at the northern extreme of strong poleward heat transfer was marked by an abrupt increase in mean annual air temperature of up to 5 c around 1920 glacier mass balance has been consistently negative since this time and large cumulative net losses of mass have occurred at most glaciers energy balance modelling confirms the sensitivity of svalbard glaciers to climate change predicting a negative shift in net mass balance of up to 0 8 m a 1 water equivalent per degree temperature rise this climate related shift in glacier mass balance has reduced the intensity of glacier surge activity in svalbard one glacier known to have surged since the end of the lia has since failed to accumulate the mass required to re initiate the surge cycle and is also now cold at its base and incapable of rapid flow by basal sliding three overviews of the total number of actively surging glaciers in svalbard between 1936 90 show a decrease from 18 to 5 this is significant compared with the expected numbers of surges based on lia conditions post lia climate change in svalbard has therefore affected not only glacier extent but also ice dynamics this is trend will probably continue given co2 induced climate warming

kick and cook experiments on peridotite simulating coseismic deformation and post seismic creep terra nova 24 62 69 2012 abstract experiments comprising deformation at 600 c and annealing at 700 1000 c were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle the found olivine microstructures indicate that deformation occurs by low temperature plasticity at low and high annealing temperatures zones of high strain are replaced by subgrains and recrystallized grains respectively the microstructures after annealing at 1000 c resemble core and mantle structures from shear zone peridotites often interpreted as imprint of steady state dislocation creep our study shows that such structures can form by a sequence of low temperature plasticity at high stress and subsequent recrystallization at low stress as corresponding to coseismic deformation and post seismic creep in this case palaeopiezometers are not applicable and a local cpo in aggregates of new grains does not necessarily indicate the activated glide system

the landform and sediment assemblage produced by a tidewater glacier surge in kongsfjorden svalbard abstract this paper describes the landform and sediment assemblage produced by a surge in 1948 of the kongsvegen kronebreen tidewater glacier complex in northwest spitsbergen the main geomorphological products of this advance are two large thrustmoraine complexes on opposite sides of the fjord and a system of geometrical ridges revealed on glacier decay the thrust moraines are composed largely of diamicton sandy and muddy gravel gravelly sand sand and mud with minor laminites all of these appear to be derived from the fjord floor and represent both fine fjord basin sediments and coarse grounding line fan deposits thrusting was the principal mode of emplacement of the sediment onto the adjacent land areas during the 1948 advance however the geomorphology of the thrust moraine complexes on either side of the fjord is quite different reflecting a transpressive regime on the southwest side mainly long ridges and a normal compressive regime on the northeast side short ridges and pinnacles of a hummocky nature the advance which produced the moraine complex has previously been attributed to a surge of kongsvegen but the glaciological and geomorphological evidence suggests that the advance involved both kongsvegen and kronebreen comparison of the landform assemblage produced by this event with that produced by other tidewater glacier surges demonstrates the diverse range of landform assemblages associated with glacier surges or other episodes of rapid flow within glaciomarine environments

amphibolite facies pseudotachylytes in premosello metagabbro and felsic mylonites ivrea zone italy abstract pseudotachylytes solidified friction induced melts produced during seismic slip that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle ductile elasto frictional viscous plastic transition in the crust thin sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite facies premosello metagabbro and inside country felsic mylonites of the ivrea zone italian southern alps petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies 500 650 c metamorphic conditions the occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites the association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients related to downward propagation of seismic ruptures from the upper brittle crust followed by post seismic stress relaxation by ductile flow

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process