large scale pseudotachylytes and fluidized cataclasites from an ancient subduction thrust fault in the kodiak accretionary complex kodiak island alaska pseudotachylyte occurs in black locally vitreous ultrafine grained fault rock microscopic observations show that the pseudotachylytes are composed of glass with vesicles amygdules microlites and flow structures indicating a frictional melt the pseudotachylyte is gradational to cataclasite and shows outcrop scale injection and ductile deformation structures the cataclasite was ductily mobile i e fluidized simultaneous with the formation and emplacement of pseudotachylyte melt the pseudotachylytic rocks postdate the stratal disruption fabric of associated shear zone melanges and show similar direction of thrust transport and have undergone limited subsequent deformation we interpret the stratal disruption as resulting from underthrusting of the subducting plate and pseudotachylyte development as the final activity of this thrust surface the gradational contacts between pseudotachylyte and cataclasite demonstrate that the cataclasite also formed as a seismic product and may represent paleoseismic rupture zones possibly of very great earthquakes with or without accompanying pseudotachylytes the pseudotachylytes are voluminous and many are spatially disconnected from generation surfaces this style is distinct from pseudotachylytes described in other environments and this may explain the rarity of documented examples of subduction thrust pseudotachylyte

brittle deformation during alpine basal accretion and the origin of seismicity nests above the subduction interface geophysical observations on active subduction zones have evidenced high seismicity clusters at 20 40 km depth in the fore arc region whose origin remains controversial we report here field observations of pervasive pseudotachylyte networks interpreted as evidence for paleo seismicity in the now exhumed valpelline continental unit dent blanche complex nw alps italy a tectonic sliver accreted to the upper plate at c 30 km depth during the paleocene alpine subduction pre alpine granulite facies paragneiss from the core of the valpelline unit are crosscut by widespread mm to cm thick pseudotachylyte veins co seismic heating and subsequent cooling led to the formation of ti rich garnet rims ilmenite needles ca rich plagioclase biotite microliths and hercynite micro crystals 39ar 40ar dating yields a 51 54 ma age range for these veins thus suggesting that frictional melting events occurred near peak burial conditions while the valpelline unit was already inserted inside the duplex structure in contrast the base of the valpelline unit underwent synchronous ductile and brittle seismic deformation under water bearing conditions followed by a re equilibration at c 40 ma 39ar 40ar on retrograded pseudotachylyte veins during exhumation related deformation calculated rheological profiles suggest that pseudotachylyte veins from the dry core of the granulite unit record upper plate micro seismicity mw 2 3 formed under very high differential stresses 500 mpa while the sheared base of the unit underwent repeated brittle ductile deformation at much lower differential stresses 40 mpa in a fluid saturated environment these results demonstrate that some of the seismicity clusters nested along and above the plate interface may reflect the presence of stiff tectonic slivers rheologically analogous to the valpelline unit acting as repeatedly breaking asperities in the basal accretion region of active subduction zones

estimation of deformation temperatures flow stresses and strain rates from an intra continental shear zone the main boundary thrust nw himalaya uttarakhand india abstract this work generates thermal and tectonic data from the mylonitized rocks of the main boundary thrust mbt south and north to this major thrust the siwalik range and the lesser himalaya respectively have been receiving a renewed interest amongst the petroleum geoscientists understanding temperature flow stress and strain rates related to a major thrust is crucial in thermal mechanical modelling of the terrains adjacent to it strongly sheared proterozoic chandpur phyllites and abundant syntectonic quartz veins occur along the mbt zone one of the major foreland verging thrusts in the himalayan orogen near the sahenshahi temple at dehradun uttarakhand state india microstructure analyses show intense ductile top to s shear in terms of s c fabric and xrd analyses indicate presence of clays e g illite and clinochlore these clays not only bear the signatures of the intense fluid activity but also enhance the fault movement by reducing the mechanical strength of the fault zone rocks thermometry based on qualitative quartz grain boundary mobility 300 550 c and quantitative laser raman spectroscopy for carbonaceous materials 340 370 c reveal the maximum estimates of the deformation temperature and metamorphic temperature respectively the higher temperature microstructures can be found in lower temperature domains under the presence of fluids hence the quartz thermometry estimates a broader temperature range than the raman thermometry previous works report that the phyllites present at the base of a thrust sheet experiences deformation at

dynamics of vulcano island tyrrhenian sea italy investigated by long term 40 years geophysical data abstract vulcano island is a composite volcanic edifice located in the south central sector of the aeolian archipelago tyrrhenian sea italy historic activity has been characterized by frequent transitions from phreatomagmatic to minor magmatic activity the last eruption in 1888 90 saw powerful explosive pulses and this eruption defines what we call vulcanian also for other volcanoes since then volcanic activity has been in the form of fumarolic emanations of variable intensity and temperature mainly concentrated at la fossa crater we investigated vulcano dynamics through ca 40 years of ground deformation and seismicity data collected by the discrete and continuous ingv monitoring networks we considered levelling gps edm seismic and tilt data edm and levelling measurements began in the middle of the 1970s since the late 1990s the edm benchmarks have been measured using gps we performed a data inversion identifying for the 1999 2013 period the action of a deflating magmatic source located under vulcanello at 4 5 km a s l just at the top of a wide regional tectonic tabular source we analyzed data on different time space scales distinguishing the dynamics of different sectors of vulcano piano vulcanello la fossa cone and caldera and three periods 1974 1984 1984 1999 1999 2013 and the time evolution of the dynamics of the island we also show how the regional tectonic stress plays an important role in the transition of the volcanic system from a phase of stability to one of unrest inducing the heating and expansion of shallow hydrothermal fluids

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

mechanisms of formation of deep basins on continental crust in the verkhoyansk fold belt miogeosynclines and cratonic basins abstract the verkhoyansk fold belt is a wide folded area in northeastern asia a large sedimentary basin 10 15 km deep was formed there by a slow subsidence of continental crust during 1000 ma in the late proterozoic middle paleozoic there are no deformations in the sedimentary cover that could indicate significant lithospheric stretching or shortening during this slow subsidence a number of deep basins 5 15 km of the same type exist or existed in other areas they can be called cratonic basins a slow gabbro eclogite transformation in the lowermost crust that occurred in a thick cratonic lithosphere can be suggested as a mechanism for their formation several deep water basins were formed in the verkhoyansk fold belt by rapid subsidence of continental crust during 1 5 ma in the late paleozoic and early mesozoic the subsidence occurred without significant stretching or thrust loading the basins formed were strongly folded in the late mesozoic under the convergent plate motions there were many basins of the same type in the other fold belts among deep basins on continental crust only those produced by rapid subsidence were intensely folded they can be called miogeosynclines rapid gabbro eclogite transformation with the destruction of the basaltic layer may be suggested as the mechanism of their formation this probably occurred under upwelling of hydrous anomalous mantle to the base of the crust

immiscible sulfide droplets in pseudotachylyte evidence for high temperature 1200 c melts abstract in the north cascade mountains washington rocks that underwent friction melting commonly contained sulfide minerals mostly pyrrhotite and pyrite during pseudotachylyte melt formation sulfides melted to form immiscible sulfide droplets present in five distinct textural settings the largest droplets formed through melting of lithic clasts whereas micron scale sulfide droplets are common in many of the pseudotachylytes veins microprobe analysis indicates that nearly all droplets are pyrrhotite the disappearance of pyrite indicates that melt temperatures must have exceeded 750 c but other indications suggest that the melt temperature must have been much higher the extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted requiring a minimum melt temperature of 1200 c in some samples evidence for fluid rich bubbles and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt the presence of sulfide droplets appears to be common especially in relatively low oxygen fugacity melts that formed at shallow crustal levels this can provide a good textural marker of melting and therefore of pseudotachylyte formation especially where other indications of melting i e high temperature microlites vesicles etc are lacking and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events

stress release in exhumed intermediate and deep earthquakes determined from ultramafic pseudotachylyte stresses released by coseismic faults during subduction toward lawsonite eclogite facies conditions in the alpine subduction complex of corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured congruent peridotite melting at ambient conditions of 1 5 gpa and 470 c requires a temperature increase of 1280 c to 1750 c we assume that more than 95 of the work is converted to heat during faulting hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain minimum estimates of released stress are typically greater than 220 mpa and as high as 580 mpa the abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth

consequences of foreland basin development on thinned continental lithosphere application to the aquitaine basin sw france abstract we present an analysis of the consequences of foreland basin development on thinned continental lithosphere inherited from pre orogenic phases of extension bathymetry at the transition from pre orogenic extensional basin to foreland basin and compaction of pre orogenic sediments contribute to the accommodation space for foreland basin sediments and thrust loads in addition the extension induced transient thermal state of the lithosphere results in ongoing thermal subsidence and a flexural rigidity which changes through time quantitative modelling of the phase of extension and the foreland basin stage of the aquitaine basin southern france shows that the inherited transient thermal state of the lithosphere contributes significantly to 1 the total foreland basin depth and width 2 the post compressional subsidence history and 3 the cratonward onlap pattern accounting for the thermo mechanical effects of pre orogenic extension significantly reduces the estimates of both the flexural rigidity 30 43 for the aquitaine basin and the required topographic or thrust load 40 for the aquitaine basin at foreland basins emplacement of thrust loads below sea level as expected in a pre orogenic extensional basin setting further reduces the required topographic load this sheds light on the wide range of flexural rigidity values reported for continental lithosphere from foreland basin modelling studies and explains in many instances the inferred hidden load or subsurface load in flexural modelling studies at foreland basins the present study has shown that pre orogenic extension phases significantly affect the record of vertical motion and the stratigraphy of the aquitaine basin and is probably important for foreland basin evolution in general

accretion and exhumation structures formed by deeply subducted seamounts in the kamuikotan high pressure temperature zone hokkaido japan 1 the lower cretaceous iwashimizu complex in the southern kamuikotan zone japan was formed by accretion of seamounts which subducted to the lawsonite albite to blueschist facies the complex is a pile of four nappe units each unit comprises a duplex with horses of seamount volcanics sedimentary rocks and ramp shear zones suggesting a thin skinned peeling of subducted seamounts each unit bounded fault is regarded as the floor thrust of an overlying duplex and the roof thrust of an underlying duplex reflecting a downward progression of underplating accretion the structurally upper pirashuke unit was metamorphosed at higher pressure than the structurally lower units this inverted arrangement of metamorphic grade can be explained by shallowing of progressively younging subducted slab structurally upper units were exhumed synchronously with growth of the structurally lower units this synaccretionary exhumation can be explained by repetition of undercritical duplex forming compression and overcritical extensional unroofing during seamounts subduction

tomographic evidence for hydrated oceanic crust of the pacific slab beneath northeastern japan implications for water transportation in subduction zones 1 we estimate detailed seismic velocity structure around the pacific slab beneath northeastern japan by double difference tomography a remarkable low velocity zone with a thickness of 10 km which corresponds to much hydrated oceanic crust is imaged coherently along the arc at the uppermost part of the slab the zone gradually disappears at depths of 70 90 km suggesting the occurrence of intensive dehydration reactions there the concentration of intraslab earthquakes at these depths supports dehydration embrittlement hypothesis as a mechanism for generating intraslab earthquakes a low velocity zone imaged immediately above the slab at depths 70 km probably reflects a hydrous layer that absorbs water expelled from the slab and carries it to deeper depths along the slab our observations suggest that an along arc variation in arc volcanism might be related to that in the development of the hydrous layer above the slab

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

polyphase seismic faulting in the ivrea zone italian alps revealed by 40ar 39ar dating of pseudotachylytes terra nova 23 162 170 2011 abstract most fault related pseudotachylytes in the val sesia area ivrea zone nw italian alps occur in two areas near the canavese line in the kinzigite formation paragneisses and in an elongate belt in the mafic complex gabbros away from the canavese line 40ar 39ar dating indicates that pseudotachylytes from the kinzigite formation were formed in the late eocene likely during the early stages of the collision between the nw corner of the adriatic indenter and the european margin pseudotachylytes from the mafic complex were formed under a transpressive strain regime as suggested by kinematic indicators and yield early cretaceous albian 40ar 39ar ages their formation is possibly related to shortening during the inception of the tectonic inversion of the adriatic passive margin

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

from passive margins to orogens the link between ocean continent transition zones and ultra high pressure metamorphism a lithostratigraphic association consisting of serpentinized mantle rocks continent derived allochthons mid oceanic ridge gabbros of jurassic age and post rift sediments typical of an ocean continent transition is found in the eclogitic piemonte units in the western alps in situ u pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement in contact with serpentinites primary magmatic zircons of permian age were overgrown by a second generation of zircon at ca 166 150 ma likely related to melt infiltration associated with the intrusion of the underlying gabbroic body this indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the jurassic and they were probably part of hyper extended crust related to the opening of the tethys therefore the complex lithological association described here which is also characteristic of several ultra high pressure melange zones worldwide was acquired prior to the orogenic event during which it was only partly reworked ocean continent transitions are in positions favorable to reach ultra high pressure conditions following negatively buoyant oceanic lithosphere into subduction and then being accreted to the orogen in response to the arrival of more buoyant continental lithosphere resisting subduction the ocean continent transition is now found in the immediate footwall of a 500 m thick shear zone which accommodated multiple episodes of deformation during eocene oligocene time suggesting an important link between alpine deformation and rift related structures

the east carpathians indications of phase transitions lithospheric failure and decoupled evolution of thrust belt and its foreland abstract an analysis is presented of the tectonic development of the east carpathians since the late oligocene the geological and geomorphological data show that five epochs of convergence and nappe emplacement took place with the last one occurring 11 12 m y b p after these compressional events the nappe surface remained near sea level the crustal uplift which formed the present mountains began 3 m y b p and after 99 of the shortening of the carpathians had been completed the pre carpathian foreland basin up to 3 5 km deep formed on the margin of the east european platform in the middle miocene analysis of the evolution of the basin shows that it deepened away from the nappe stack and most of the subsidence occurred when there was no convergence this precludes lithospheric flexure due to convergence as a mechanism for basin formation at 11 12 m y b p the inner part of the basin 50 km wide was overridden by the carpathian nappe up to 12 14 km thick at that time a slight crustal uplift took place rather than subsidence in the shallow remnant outer basin part the present foredeep which emerged above sea level an additional 7 8 km of subsidence then occurred in the foredeep of the southeastern carpathians in the late miocene early pleistocene when very little convergence took place the evolution of the carpathian foredeep shows that lithospheric flexure of 2 5 km occurred in regions 20 40 km wide similar amounts of flexure and large basement faults exist under the nappe this style of deformation indicates flexural rigidity of the lithosphere that is several orders of magnitude smaller than in stable cratonic regions under a low flexural rigidity the crust should be close to local isostasy then the isostatic response to superposition of a 10 13 km thick nappe on the platform margin which was near sea level would form high mountains the crustal surface however remained at a low altitude we interpret this to be the result of contraction of gabbro in the lower crust with the formation of dense garnet granulites during nappe emplacement a layer of these dense rocks 20 km thick with p wave velocities 7 5 7 6 km s exists in the lower crust of the east carpathians according to the geophysical data there is no cool high velocity slab now subducting towards the intra carpathian area an increase in the mean height of the carpathians by only 1 km since the termination of convergence precludes a recent slab detachment rapid metamorphism of mafic rocks in the lower crust requires the infiltration of volatiles from the asthenosphere infiltration of volatiles into mantle lithosphere and the upper crust could strongly reduce their viscosity these phenomena might be the cause of an abrupt loss of lithospheric rigidity on the margin of the east european platform in the neogene lithospheric failure allowed the incorporation of this region into the carpathian fold belt after a long period of cratonic development

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

aftershock seismicity of the 27 february 2010 mw 8 8 maule earthquake rupture zone on 27 february 2010 the mw 8 8 maule earthquake in central chile ruptured a seismic gap where significant strain had accumulated since 1835 shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity here we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines by processing the seismic data between 15 march and 30 september 2010 from stations from iris ipgp gfz and university of liverpool we determined 20 205 hypocentres with magnitudes mw between 1 and 5 5 seismic activity occurs in six groups 1 normal faulting outer rise events 2 a shallow group of plate interface seismicity apparent at 25 35 km depth and 50 120 km distance to the trench with some variations between profiles along strike the aftershocks occur largely within the zone of coseismic slip but extend 50 km further north and with predominantly shallowly dipping thrust mechanisms along dip the events are either within the zone of coseismic slip or downdip from it depending on the coseismic slip model used 3 a third band of seismicity is observed further downdip at 40 50 km depth and further inland at 150 160 km trench perpendicular distance with mostly shallow dipping 28 thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture and presumably above the intersection of the continental moho with the plate interface 4 a deep group of intermediate depth events between 80 and 120 km depth is present north of 36 s within the maule segment a large portion of events during the inter seismic phase originated from this depth range 5 the magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the mw 8 8 maule 2010 earthquake 6 pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of pichilemu 34 5 s these crustal events occur in a 30 km wide region with sharp inclined boundaries and oriented oblique to the trench the best located events describe a plane dipping to the southwest consistent with one of the focal planes of the large normal faulting aftershock mw 6 9 on 11 march 2010

review of neogene and quaternary volcanism of the carpathian pannonian region abstract development of the pannonian basins as well as thrusting and folding of the carpathians was accompanied by widespread volcanic activity this volcanism can be divided into three main genetic types 1 1 early miocene mainly acidic volcanism tuffs and ignimbrites the recent spatial separation of these volcanics may be due to large right lateral displacement along the mid hungarian shear zone during the early miocene 2 2 middle miocene pliocene calc alkaline mainly intermediate stratovolcanic complexes in the inner western carpathians and in the east carpathians related to a subducted oceanic slab their geochemical compositions show a transitional character between active continental margin and island arc type magmatic rocks the thickness of the crust increased with time and from west to east beneath this volcanic arc 3 3 pliocene pleistocene alkali basaltic volcanism was related to an upwelling then cooling asthenospheric dome this is thought to have initiated the thermal regime from which these melts ascended the compositional difference in space and in time might reflect the existence of local individual small sized diapiric bodies as well as several processes e g fractional crystallization mixing which modified the original magma the eruption of these basaltic melts was temporally associated with the final phase of development of the pannonian basin this volcanism was associated with recent uplifts and tectonic rejuvenation of the area

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

high pressure metamorphism in the nappes of the valle dell orco traverse western alps collisional belt a detailed field and petrological study of rocks from nappes cut by the valle dell orco italian western alps in particular the sesia lanzo composite unit has revealed geological and metamorphic histories which started in pre alpine times and lasted up to the alpine subduction collisional processes during these processes the nappes sustained an early high p low t stage and a later low p greenschist facies stage but followed partly distinctive p t time trajectories this paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction underthrust to the later exhumation stage the metamorphic crystallization is often governed by incomplete and or local equilibrium and the pervasive syn metamorphic deformation and the composition of the syn metamorphic fluid phase if present have exerted an effective local control on reaction kinetics

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

mechanism of continental crust subsidence in fold belts the urals appalachians and scandinavian caledonides abstract formation of deep basins on continental crust is usually explained by lithospheric stretching stretching produces the large deformations in the upper brittle part of the crust that have been found in many rift valleys the sedimentary cover of deep basins on continental crust is exposed at the surface in fold belts in many places some undeformed basins have been covered by detailed seismic profiling and drilling the sedimentary cover structure in the urals appalachians and in the scandinavian caledonides has been analysed no deformations typical of stretching were found in the greater part of the basins on continental crust high magnitude subsidence without stretching can be produced by thrust loading or by a large density increase in the crust or mantle paleotectonic conditions exclude thrust loading as a cause of formation of the major basins in the above fold belts most of them were formed in cool cratonic areas by rapid subsidence in a few m y their initial depth was 1 3 km subsidence of this type cannot be produced by thermal relaxation without significant stretching gabbro eclogite transformation in the lower crust is proposed as a possible mechanism intense shortening of continental crust is observed only where rapid subsidence of a large magnitude took place before the folding not one cratonic block was strongly compressed in the fold belts considered

the problem of the moho in the alps abstract the tertiary alps are the product of continental collision and material balance estimates require the subduction of continental crust and the underlying moho the present moho in the alpine region consists of three parts that of the european plate me that of the adriatic african plate ma and a new moho m2 believed to have developed in the subducted continental crust by high pressure metamorphism at 40 60 km depth the fate of m2 however is uncertain some of it is apparently destroyed subsequently as some eclogite facies and even highest p coesite bearing rocks somehow find their way to the surface where they form exotic bodies from microscopic to nappe size on the other hand the continental moho exposed in the ivrea body tells a different story here continental granulite facies rocks are intruded by ultramafic and mafic masses this suggests an origin in an extensional environment possibly in subduction related gravitational collapse basins such successor basins succeed the basins of the original tethys that had been largely consumed by the late cretaceous they puncture the 1000 2000 km wide belt of africa europa collisional interaction e g the aegean the tyrrhenian and the pannonian basins although in some ways similar to the back arc basins of the pacific these mostly intracontinental basins differ in important aspects such as their small size in these basins asthenospheric upwelling replaces m2 by a new moho m3 and the eclogitic rocks underlying m2 by the ultramafic mafic intrusions of the ivrea type material balance considerations suggest that most of the eclogues remain part of the mantle for some time on the other hand recent deep reflection seismic surveys in several places indicate delamination of a part of the lower crust and the uppermost mantle with the formation of imbrications and duplexes some of which are wedged into the middle crust of the opposite plate they may be the first stage in the formation of granulite and eclogite nappes

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

evolution of quartz microstructure and c axis crystallographic preferred orientation within ductilely deformed granitoids arolla unit western alps abstract we have studied quartz microstructures and the c axis crystallographic preferred orientations cpos in four granitoid samples representative of increasing ductile shear deformation from a weakly deformed granitoid stage 1 to a mylonitic granitoid stage 4 the quartz c axis cpo measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area all the samples belong to the austroalpine arolla unit western alps and were deformed at greenschist facies conditions the quartz c axis cpo was analyzed using a u stage and the optical orientation imaging technique the magmatic plagioclase forming more than 50 of the volume of the granitoid is extensively replaced by a mica rich aggregate even in weakly deformed samples of stage 1 these aggregates flow to form an interconnected weak matrix with increasing deformation wrapping relatively less strained quartz grains that undergo dominantly coaxial strain recrystallization of quartz ranges from less than 1 in the weakly deformed granitoid to up to 85 in the mylonitic granitoid with average grain strain of 41 and 64 respectively with increasing strain and recrystallization quartz grains in the granitoids show a sequence of transient microstructures and cpos crystal plastic deformation is initially accomplished by dislocation glide with limited recovery and at 50 grain strain it results in a cpo consistent with dominantly basal a slip at 60 grain strain recrystallization is preferentially localized along shear bands which appear to develop along former intragranular cracks and the recrystallized grains develop a strong c axis cpo with maxima orthogonal to the shear band boundaries and independent of the host grain orientation within the granitoid mylonite at an average quartz grain strain of 64 recrystallization is extensive and the c axis cpo of new grains displays maxima overlapping the host c axis orientation and therefore unrelated to the bulk sense of shear the host controlled cpo is inferred to reflect pervasive recrystallization by progressive subgrain rotation the switch from shear band control to host control on c axis cpo occurred between 40 and 70 of recrystallization in the quartz mylonite the quartz c axis cpo develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions this study indicates that the cpo evolution of quartz may significantly differ in cases of polymineralic vs monomineralic rocks under the same deformation conditions if quartz in the polymineralic rock behaves as a strong phase

locking of the chile subduction zone controlled by fluid pressure before the 2010 earthquake large subduction zone earthquakes are thought to occur where the down going and overriding tectonic plates are strongly locked analysis of geodetic and seismic data collected in the decade before the 2010 chile earthquake shows that variations in pore fluid pressure correlate with the degree of plate interface locking and may therefore control earthquake rupture

alpine deformation and 40ar 39ar geochronology of synkinematic white mica in the siviez mischabel nappe western pennine alps switzerland we explore the timing of deformation and exhumation of the siviez mischabel nappe western swiss alps which has been considered a classic example of a midcrustal crystalline nappe since the studies of argand 1916 this study presents 40ar 39ar ages obtained on both synkinematic white mica from permo triassic cover sediments and more complex white mica populations from basement gneisses of the siviez mischabel and middle pennine nappes primary foliation developed in cover units by nucleation growth and rigid rotation of mica grains during noncoaxial alpine deformation although some samples show a crenulation of this primary foliation mica growth appears to have occurred only during the development of primary foliation the main phase of greenschist facies deformation related to imbrication of the siviez mischabel nappe and other middle pennine nappes good agreement exists between independent estimates of the timing of deformation and reported 40ar 39ar white mica ages from cover units of the central and southern siviez mischabel nappe in cover units from the central and southern siviez mischabel regions of the study area 40ar 39ar ages appear to date synkinematic white mica growth results suggest that the siviez mischabel nappe was emplaced and developed foliation during a 5 m y period from 41 to 36 ma in cover units from the eastern siviez mischabel however 40ar 39ar white mica ages appear to date postkinematic thermal events these thermal events may be related to oligocene magmatic activity in the lower pennine nappes or to miocene development of the simplon fault zone variations in the relation between alpine age and grain size for cover samples from the central eastern and southern siviez mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism when considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western swiss alps these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high pressure units in the suture zone of the western alps are intimately related and synchronous on the scale of a few million years

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

ductile brittle transition in pre alpine amphibolite facies mylonites during evolution from water present to water deficient conditions mont mary nappe italian western alps in the austroalpine mont mary nappe italian western alps discrete zones of mylonites ultramylonites developed from coarse grained upper amphibolite facies metapelites of pre alpine age the syn mylonitic mineral assemblage is quartz biotite muscovite plagioclase garnet sillimanite ilmenite graphite and formed via the model hydration reaction grt 1 kfs h 2 o bt 2 ilm 2 qtz ms sil grain size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite and by compositional changes of garnet and biotite deformation took place at temperatures of 510 580 c under low pressure conditions 0 25 0 45 gpa and corresponds to the latest stages of pre alpine metamorphic evolution the pre alpine mylonitization conditions were close to the brittle ductile transition as indicated by syn mylonitic generation of pseudotachylytes and high diverential stress inferred from quartz grain size piezometry the brittle ductile behaviour at a relatively high temperature and the absence of annealing textures in quartz aggregates are suggestive of water deficient conditions during mylonitization these were accomplished through progressive consump tion of water by syn kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization

pressure solution in siliciclastic hp lt metamorphic rocks constraints on the state of stress in deep levels of accretionary complexes abstract the magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured the only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high pressure low temperature metamorphic rocks recorded during their crustal evolution microfabrics in hp lt metamorphic t 400 50 c p 10 2 kbar phyllites and quartzites of the phyllite quartzite unit on the island of crete southern aegean reveal 1 that progressive deformation was by pressure solution dissolution precipitation creep 2 that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation 3 that the unilaterally rational 001 mica quartz phase boundaries were sites of strongly enhanced dissolution and 4 dislocation creep was restricted to a minor role in quartzites poor in mica consequently the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial to a depth of more than 30 km and subsequent exhumation currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 mpa at temperatures around 400 c at a depth of 30 km this implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

probing the transition between seismically coupled and decoupled segments along an ancient subduction interface the transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip as well as the focus site of episodic tremor and slip features exhumed remnants of the former alpine subduction zone found in the swiss alps allow analyzing fluid and deformation processes near the transition zone region 30 40 km paleodepth the dent blanche thrust dbt is a lower blueschist facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge we report field observations from the dbt region where multiple several tens of meters thick foliated cataclastic networks are interlayered within the basal dbt mylonites petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions 400 500 c 1 1 1 3 gpa during subduction of the tethyan seafloor in eocene times 42 48 ma some of these networks exhibit mutual crosscutting relationships between mylonites foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep whole rock chemical compositions in situ 40ar 39ar age data of recrystallized phengite and sr isotopic signatures reveal that dbt rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the dbt from the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates 10 12 s 1 accompanied by fluctuation of supralithostatic and quasi lithostatic fluid pressures 1 0 95 dbt brittle plastic deformation switches highlight the diversity of deformation processes and fluid rock interactions in the transition zone region of the subduction interface

fault rocks within the blueschist metabasalts of the diamante terranova unit southern italy potential fossil record of intermediate depth subduction earthquakess vitale et al fault rocks within blueschist metabasalts

ultrafine grained quartz mylonites from high grade shear zones evidence for strong dry middle to lower crust creep strength of the crust depends upon the rheology of the most common mineral usually quartz recrystallized quartz grains in many high grade shear zones from the middle to lower crust are typically large millimeter sized implying active grain boundary migration but equivalents from old polymetamorphic and water deficient basement sheared at similar crustal depths can be very small for the latter strong crystallographic preferred orientation cpo of quartz with c axes aligned close to y progressive misorientation of crystals subgrain and dislocation development and core mantle structures with recrystallized grains of 2 8 m size all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation recrystallized grain size piezometry of such quartz indicates high flow stress in fine grained shear zones while the synkinematic metamorphic mineral assemblage and the cpo are typical of amphibolite facies conditions this is evidence that middle to lower crust is not inevitably weak due to its high temperature water content also has an important influence

subcrustal density inhomogeneities of northern eurasia as derived from the gravity data and isostatic models of the lithosphere for the territory of northern eurasia 6 e 165 w 30 75 n the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model the method of model construction is based on the admittance technique the experimental admittance presents a relation between the part of the outer load uncompensated by the moho undulations and the residual gravity field and is used to select the best model the 1 1 averaged values of topography elevations basement and moho depths sedimentary cover density and gravity anomalies have been used as initial data according to the correlation equation relating the outer load and moho depths the mean density contrast between the lower crust and the subcrustal lithosphere is 0 43 g cm3 but the moho undulation can not provide complete isostatic equilibrium in some areas the part of the outer load uncompensated by moho undulations may be as large as 107 kg m2 and the residual gravity field is as intensive as 260 mgal assuming that for loads of wavelength 200 km local isostatic compensation is valid in accordance with the admittance analysis the anomalous masses compensating for the part of the outer load which is not compensated by moho undulations have to be located partly in the lower crust and in the subcrustal layer the regional trend of anomalous compensating masses is negative under western europe the mediterranean eastern asia and adjacent marginal seas and positive under the east european platform and western and central asia the local compensating masses correspond to particular tectonic units the isostatic gravity anomalies of northern eurasia have been determined and the long wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

seismicity near the slip maximum of the 1960 mw 9 5 valdivia earthquake chile plate interface lock and reactivation of the subducted valdivia fracture zone understanding the processes behind subduction related hazards is an important responsibility and major challenge for the earth sciences few areas demonstrate this as clearly as south central chile where some of the largest earthquakes in human history have occurred we present the first observation of local seismicity in the villarrica region 39 40 s based on a temporary local network of 55 stations installed from the chilean coast into the argentinian back arc for one year while consistent with the chilean national catalog ssn our results allow us to observe smaller magnitudes with a completeness of about 2 0 and image the geometry of the wadati benioff zone from the chile trench down to 200 km offshore a gap in interplate seismicity is observed in the region of the 1960 valdivia earthquake slip above the interface two offshore seismicity clusters possibly indicate ongoing stress relaxation in the subducting nazca plate we find a prominent seismicity cluster along the extrapolated trace of the oceanic valdivia fracture zone vfz the seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike slip events it indicates weakening and reactivation of the major vfz by dehydration of oceanic crust and mantle interpreting the subducted vfz section as a localized reservoir of potential fluid release offers an explanation for the villarrica volcanic complex that is located above the reactivated vfz and shows the highest volcanic activity in south america crustal seismicity is observed near puyehue volcano which recently started to erupt june 2011 key points we observe local seismicity near the 1960 valdivia maximum slip region the subducted valdivia fracture zone is reactivated by fluid release the seismogenic zone is free of seismicity events occur at faults and volcanoes

fault creep and microseismicity on the hayward fault california implications for asperity size 1 the hayward fault is documented to undergo significant creep with some patches accommodating 50 or more of the long term fault displacement in spite of this the fault has also experienced moderate to large earthquakes by comparing the patterns of microseismicity observed on the fault with models of fault zone creep we can investigate the long term displacement deformation history of the fault in terms of the relative roles of aseismic creep fault slip accommodated through microseismicity and strain accumulation slip deficit we find that microseismicity on the hayward fault produces a negligible percentage of the seismic moment dissipated on the fault combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault for small asperities associated with repeating earthquakes on the hayward fault the rupture areas of these asperities range from 20 to 60 m2

origin of pseudotachylites in slow creep experiments we conducted a series of experiments on granitoid cataclasites under mid crustal conditions pc 500 mpa t 300 c and slow displacement rates 10 8 ms 1 d 10 7 ms 1 i e within an order of magnitude of plate tectonic velocities the samples reach high peak strength 1250 1375 mpa then weaken slightly by 40 mpa and continue to deform without any abrupt stress drops up to a of 4 microstructural observations show that at peak strength nanocrystalline mean grain size 34 nm smallest crystalline particles 8 nm to partly amorphous material develops weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions during higher strain deformation deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop this material shares many similarities with naturally occurring pseudotachylites however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes 1 ms 1 we conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered

tectonic overpressure at shallow depth in the lithosphere the effects of boundary conditions abstract in dynamic geological settings the total pressure p comprises dynamic and litho static pl components which are indistinguishable when pressure is estimated thermodynamically the difference between the two components is called tectonic overpressure here defined as top p pl the magnitude of which has been debated for decades in geodynamics in this paper we report on a parametric study carried out to evaluate the magnitude of top in various geodynamic settings under variable boundary conditions the top in the brittle frictional field is limited by the frictional strength and the cohesion of rocks and by pore fluid pressure in the viscous field it is dependent on viscosity and flow configuration we analyse pressure in the viscous field using the navier stokes equation with gravity and conclude that there are some configurations and boundary conditions favourable for the development of high top 2 at relatively shallow lithospheric levels so facilitating the exhumation of high and ultrahigh pressure rocks in subduction channels this is the case for upward tapering subduction channels with rigid or deformable walls and no outlet pressure condition at the bottom natural subduction zones are characterized by non steady behaviour over time and space therefore if a subduction zone transiently goes through the favourable conditions mentioned above the top can be 2 at shallow lithospheric levels and high and ultrahigh pressure rocks can be rapidly exhumed to the surface by return flow close to the hanging wall

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

superheated friction induced melts in zoned pseudotachylytes within the adamello tonalites italian southern alps pseudotachylytes and cataclasites are present along a strike slip fault zone in the tonalites of the adamello intrusion italian southern alps ambient conditions during faulting were 0 25 0 3 gpa and 250 300 c pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain the thickness of the microlitic domains x increases linearly with the half total thickness a of the pseudotachylyte vein commonly 2 cm according to the relation x 0 29 0 12 a the spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts a numerical model explains zoning as the result of the different cooling rates and clast melt interactions at the center and periphery of thick veins zoning is compatible with the injection of a single pulse of superheated friction induced melt tmelt 1450 c melt temperatures estimated by the clast matrix ratio o hara 2001 are considerably lower 316 577 c it is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites pseudotachylytes

pre oceanic rifting in the alps abstract a review of mesozoic mainly earliest jurassic to mid jurassic rifting in the western and central alps normal faults with throws up to a few kilometers are moderately inclined and involve rotation of crustal blocks the amount of stretching and thinning of the crust by jurassic rifting is discussed the rifting 200 165 m y pre dates the opening of the alpine oceans by spreading and transform movements 165 110 m y

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths

fossil intermediate depth earthquakes in subducting slabs linked to differential stress release the cause of intermediate depth 50 300 km seismicity in subduction zones is uncertain it is typically attributed either to rock embrittlement associated with fluid pressurization or to thermal runaway instabilities here we document glassy pseudotachylyte fault rocks the products of frictional melting during coseismic faulting in the lanzo massif ophiolite in the italian western alps these pseudotachylytes formed at subduction zone depths of 60 70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite this rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present day subduction the pseudotachylytes locally preserve high pressure minerals that indicate an intermediate depth seismic environment these pseudotachylytes are important because they are hosted in a near anhydrous lithosphere free of coeval ductile deformation which excludes an origin by dehydration embrittlement or thermal runaway processes instead our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks seismic activity within subducted slabs could be caused by differential stress release according to analysis of fossilized remnants of earthquake slip in an exhumed slab these deep earthquakes were previously thought to mark either slab dehydration or thermal runaway processes