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

convective interactions between oceanic lithosphere and asthenosphere influence of a transform fault abstract current estimates of the asthenospheric viscosity favor the occurrence of small scale convection ssc beneath oceanic lithosphere of sufficient age since transform faults tfs juxtapose lithospheres of distinct ages and thicknesses they are likely to influence their dynamics in this paper we describe 3d numerical models of oceanic spreading for two ridge segments offset by a tf in domains that are sufficiently large for ssc to fully develop a framework based on thermal convection with temperature dependent viscosity and both internal and basal heating is adopted and parameters such as surface velocity age offset at the tf activation energy mantle viscosity and internal heating rate are explored in our numerical experiments the onset time for ssc is identical on each side of the fracture zone fz it is not modified by the presence of a tf moreover scaling relationships for the onset time obtained in 2d experiments remain valid earlier studies focused on the influence of a tf based on 2d experiments corresponding to slices parallel to the ridge axis they noted the rapid development of a sinking thermal instability located at the step in the lithospheric relief our results indicate that the wake of the tf is characterized by the development of a lateral edge driven buoyant flow from the younger thinner side to the older thicker side of the fz however due to the global shearing context this instability does not sink before the onset of ssc characterized by small scale instabilities on each side of the fz as observed in earlier studies the essentially 3d velocity field combining this lateral flow and the prescribed surface spreading velocity results in an angle formed by the tf trail with the spreading direction we describe the parameters controlling this angle in our experiments and propose a scaling relationship in oceanic settings the specific geometry of the ridge axis segment length orientation of the tfs may affect this angle in addition a large scale mantle flow of other geodynamical origin could interact with the spreading flow and also modify the tf trail angle this departure at depth of the boundary between the thick and thin lithospheres from the surface fz may explain the presence of oceanic volcanoes on the older side of the fz usually considered as thicker

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

constraints on upper mantle viscosity from the flow induced pressure gradient across the australian continental keel 1 the thickness of continental lithosphere varies considerably from tectonically active to cratonic regions where it can be as thick as 250 300 km embedded in the upper mantle like a ship when driven to move by a velocity imposed at the surface a continental keel is expected to induce a pressure gradient in the mantle we hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography we perform three dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity with the australian region as a case study when the upper mantle is strong but still weaker than the lower mantle positive dynamic topography is created around the leading edge and negative dynamic topography is created around the trailing edge of the keel which is measurable by positive and negative geoid anomalies respectively for a weak upper mantle the effect is much reduced we analyze geoidal and gravity anomalies in the australian region by spatiospectral localization using slepian functions the method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions regional geoid anomalies thus filtered are on the order of 10 m across the australian continent with a spatial pattern similar to that predicted by the models the comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure models with a two layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle the addition of a third weak upper mantle layer an asthenosphere amplifies the effects of keels our three layer models with lower mantle viscosity of 3 1022 pa s suggest that the upper mantle asthenosphere is 300 times weaker than the lower mantle while the transition zone 400 670 km depths has a viscosity varying between 1021 and 1022 pa s

fault reactivation and pseudotachylite generation in the semi brittle and brittle regimes examples from the gavilgarh tan shear zone central india in the sheared and foliated granitoids of the proterozoic gavilgarh tan shear zone gtsz in central india two types of pseudotachylite pt m and pt c are recognized pt m layers are interbanded with mylonite and ultramylonite show strong internal plastic deformation and buckle folding concurrent with the host rocks and appear to have formed within the greenschist facies 300 400 c in the brittle plastic transitional semi brittle regime pt c layers show sharp contacts with the host rock exhibit abundant coeval cataclasis preserve no evidence of subsequent plastic deformation and formed at shallower depths at temperature

internal tide scattering at seamounts ridges and islands 1 the scattering of mode 1 internal tides from idealized gaussian topography in a nonrotating ocean with constant and realistic stratifications is examined with a primitive equation numerical model incident mode 1 energy fluxes of 20 and 2000 w m 1 are used to examine the linear regime and a more realistic situation simulations using two dimensional or infinite ridges compare well with ray tracing methods and illustrate how the size and shape of the topography influence wave scattering the height affects energy transmission and reflection while the slope and width determine the conversion of low mode internal tides into beams or higher modes three dimensional topographic scattering is considered for seamounts finite width ridges and islands scattering from finite ridges focuses wave energy directly downstream while scattering from seamounts produces azimuthal energy dispersion scattering to higher wave modes occurs in the lee of near critical and supercritical seamounts and ridges nonlinear interactions transfer energy into the mode 1 m4 internal tide the mellor yamada level 2 5 submodel parameterizes turbulent mixing for the near critical and supercritical ridges with realistic stratification elevated mixing is found over the leading edge of the topography and along a tidal beam up to the first surface bounce a transition from a beam structure near the topography to a low mode structure farther away occurs due to an increased contribution from the mode 1 internal tide as it refracts around the topography and not due to turbulent dissipation internal tide scattering at topography leads to a loss of energy to mixing and to a redistribution of energy flux in space frequency and mode number

new evidence for dislocation creep from 3 d geodynamic modeling of the pacific upper mantle structure laboratory studies on deformation of olivine in response to applied stress suggest two distinct deformation mechanisms in the earth s upper mantle diffusion creep through diffusion of atoms along grain boundaries and dislocation creep by slipping along crystallographic glide planes each mechanism has very different and important consequences on the dynamical evolution of the mantle and the development of mantle fabric due to the lack of in situ observations it is unclear which deformation mechanism dominates in the upper mantle although observed seismic anisotropy in the upper mantle suggests the presence of dislocation creep we examined the thermo mechanical erosion of the lithosphere by thermal boundary layer instabilities in 3 d dynamical models this study demonstrates that the seismically derived thermal structure of the pacific lithosphere and upper mantle imposes an important constraint on the upper mantle deformation mechanism the predominant deformation mechanism in the upper mantle is dislocation creep consistent with observed seismic anisotropy the acceptable activation energy range of 360 540 kj mol is consistent with although at the lower end of those determined from laboratory studies

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

frictional melt pulses during a 1 1 ma earthquake along the alpine fault new zealand our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine grain size of frictionally fused rock and the lack of detailed high resolution microscopy studies that describe features at the crystal lattice scale in such a study we here report the complex nature of melting and crystallization in a symmetrically layered pseudotachylyte vein from the alpine fault new zealand two melt pulses are recognized attributed to successive but rapid injections of frictionally generated material the initial injection preserved at the vein margins was proximally derived and contains a high concentration of clasts and a si rich glass this was quickly followed by a second generation of a more distally derived melt dominated fraction which was injected into the weak vein center whereas fragments of wall rock biotite are preserved in the marginal zones neocrystallized microlitic biotite characterizes both margins and center the vein biotite is different in composition microstructure and polytypism from the metamorphic biotite of the wall rock in all melt layers newly formed biotite shows notable signs of syn flow crystallization strain features and erosion at crystal glass contacts with breakdown of neocrystallized microlites along both crystal edges and faces these characteristics imply cyclic pulses of heating melting and crystallization occurred during a single large earthquake episode and probably reflects the stick slip propagation properties of coseismic faulting 40 ar 39 ar total gas ages from the vein center give a 1 11 0 04 ma date for cyclic melting which based on current exhumation rates occurred at a crustal of depth v11 km 2003 elsevier science b v all rights reserved

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

non hotspot volcano chains originating from small scale sublithospheric convection 1 some oceanic volcano chains violate the predictions of the hotspot hypothesis for geographic age progressions one mechanism invoked to explain these observations is small scale sublithospheric convection ssc in this study we explore this concept in thermo chemical 3d numerical models melting due to ssc is shown to emerge in elongated features 750 km parallel to plate motion and not just at a fixed spot therefore volcanism occurs in chains but not with hotspot like linear age progressions the seafloor age at which volcanism first occurs is sensitive to mantle temperature as higher temperatures increase the onset age of ssc because of the stabilizing influence of thicker residue from previous mid ocean ridge melting mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism calculations predict many of the key observations of the pukapuka ridges and the volcano groups associated with the line cook austral and marshall islands

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

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

recrystallization of quartz after low temperature plasticity the record of stress relaxation below the seismogenic zone abstract quartz microfabrics in rocks from the silvretta basal thrust and the defereggen antholz vals dav shear zone in the eastern alps are analysed by polarized light and electron microscopy the microfabrics from both shear zones record a switch from low temperature plasticity at transient high stress to recrystallization at relaxing stresses at greenschist facies conditions the development of new grains is dominantly by subgrain rotation and subsequent strain induced grain boundary migration in areas of localized high strain developed during initial low temperature plasticity the findings suggest that new grains develop at almost random crystallographic orientations at fast rates of stress relaxation i e at low stress as indicated by recrystallized quartz zones in the silvretta fault rocks in contrast at slow rates of stress relaxation new grains develop at moderately high stresses with crystallographic preferred orientation characterized by high schmid factor for basal glide as indicated by vein quartz samples from the dav shear zone both recorded histories with transient peak stresses and different rates of stress relaxation are interpreted to be related to seismic activity of the fault systems this study demonstrates that characteristic microfabrics provide important information about the deformation history of natural shear zones developed in different tectonic regimes

stress and flow beneath island arcs summary the flow pattern stress distribution topography and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the aleutian arc the results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench in models with simple distributions of rheological parameters the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab models with the resistance to flow thus reduced had outer rises deep trenches horizontal tension seaward of the trench horizontal compression under the trench and downdip tension in the slab free air gravity anomalies which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow were not fit excellently by any of the models in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc an alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench this is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

three dimensional simulations of plume lithosphere interaction at the hawaiian swell rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date efficient thinning depends on small scale instabilities that convectively remove lithospheric material these instabilities are favored by hotter plumes or stronger temperature dependence of viscosity and a simple scaling independent of rheology controls their onset this scaling allows extrapolation of the results of numerical experiments to the earth s mantle mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small scale convective rolls aligned with the plate motion the unusual variation in heat flow across the hawaiian swell may be due to such instabilities it was found that the spreading of the plume creates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plume origin this isolation has important consequences for the geochemical heterogeneity of the lithosphere and upper mantle

microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta anorthosite the very fine grained 1 m polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the harris meta anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested the intrusive nature of the veins is shown by their geometry and internal structures clasts which are almost always only plagioclase are generally concentrated towards the centres of veins flow of tens of millimetres can account for this concentration in millimetre thick veins as a result of the bagnold effect the veins are generally thin 5 m to 5 mm or more are frequently zoned and always contain transparent granules of high relief probably al rich pyroxene and opaque granules of magnetite up to a few micrometres in size the granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures in some veins spherulitic or bow tie textures occur the coarsest textures are found in the centres of the thickest veins the groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins this fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0 2 1 5 m which show little sign of deformation the local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins the variation being within the compositional space defined by the minerals of the host rock this shows that homogenization of the pseudotachylite has occurred in cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock this was produced by segregation of the granules pyroxene being absent from the host rock the plagioclase in the pseudotachylite is more disordered than that in the host rock all the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine grained and intensely strained rock powder the physical state on and after intrusion was that of a melt and injection was followed by crystallization the melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation the fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys it thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow tie textures

scattered wave imaging of the lithosphere asthenosphere boundary several recent studies have imaged discontinuities interpreted as the lithosphere asthenosphere boundary using receiver function methods these are exciting results since this boundary is fundamental and yet relatively elusive its location is not well imaged nor are its properties well understood the receiver function method provides high depth resolution and the multitude of studies using this method have increased lateral resolution a better global understanding of the location and nature of the boundary is developing which ultimately will have implications for mantle dynamics and planetary evolution however these new results do not all form a clear and consistent picture and in some cases introduce new complexities and questions some apparent contradictions may represent unmodeled earth structure overall interpretation and synthesis requires careful consideration of the approaches and sensitivities of individual studies

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

a sharp cratonic lithosphere asthenosphere boundary beneath the american midwest and its relation to mantle flow abstract beneath the american midwest s to p sp converted wave imaging and multi mode surface wave tomography identify a north trending transition in seismic structure at 150 250 km depth to the east of this american midwest transition amt the lithosphere asthenosphere boundary lab is imaged as a 1 2 sp sv amplitude arrival at 200 240 km depth consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography to the west of the amt sp conversions are much shallower at 150 190 km depth and are much weaker 0 7 or absent azimuthal anisotropy constrained by surface wave tomography also changes across the amt with stronger anisotropy to the east of the transition beneath the thicker lithospheric root we suggest that the seismic changes across the amt can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root the mantle flow is dominantly driven by the drift of the north america plate locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root this mantle underflow could create a sharper seismic lab east of the amt via two effects first the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere and hence the isotropic velocity contrast second the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy our results suggest that seismically detectable lab sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection a secondary result is the image of a mid lithospheric discontinuity arrival at 80 110 km depth that is found primarily to the east of the amt this arrival is interpreted as produced by a layer of low velocity metasomatic minerals that have accumulated since the 1 8 ga creation of the lithosphere

small scale convection at the edge of the colorado plateau implications for topography magmatism and evolution of proterozoic lithosphere the colorado plateau of the southwestern united states is characterized by a bowl shaped high elevation late neogene quaternary magmatism at its edge large gradients in seismic wave velocity across its margins and relatively low lithospheric seismic wave velocities we explain these observations by edge driven convection following rehydration of colorado plateau lithosphere a rapidly emplaced cenozoic step in lithosphere thickness between the colorado plateau and adjacent extended rio grande rift and basin and range province causes small scale convection in the asthenosphere a lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized resulting in magmatism edgedriven convection also drives margin uplift giving the plateau its characteristic bowl shape the edge driven convection model shows good consistency with features resolved by seismic tomography

crystal plasticity and superplasticity in quartzite a natural example plastically deformed quartzites from the betic movement zone betic cordilleras spain exhibit microstructures indicative of crystal plasticity on a mineral grain scale quartzites with dynamically recrystallized grain sizes larger than 10 m have strong crystallographic preferred orientations narrow grain boundaries little creep damage and an inverse proportionality of dislocation density and grain size mylonites with grain sizes smaller than 10 m have low crystallographic preferred orientations wide grain boundaries up to 1000 a abundant creep damage and decreasing dislocation density with diminishing grain size this is thought to reflect a clear cut shift in deformational regimes from dislocation creep to superplastic flow at 10 m grain size superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 m during an initial dislocation creep stage dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain boundary sliding in the mylonites it seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens in the mylonites dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms and an estimate for late stage stresses is provided by unbound dislocation densities in both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization

edge modulated stagnant lid convection and volcanic passive margins the initial oceanic crust along volcanic passive margins is a factor of 3 greater than that of typical oceanic crust 20 versus 6 7 km convection driven by the edge of the continental lithosphere may cause mantle material to circulate through the shallow zone of significant melting beneath the nascent ocean basin and cause the volume of melted mantle to exceed that required to replace the diverging lithospheric plates this hypothesis is a well known alternative to mantle plumes i obtain dimensional scaling relations and develop numerical models which indicate that the melted volume is unlikely to be enhanced by a factor of 3 this conclusion agrees with the results of numerical calculations by nielsen and hopper 2002 2004 dimensional scaling provides the relationship which shows that the vigor of convection in terms of laterally averaged heat flow or flow velocity depends on the square of local thickness of the rheological boundary layer for linear viscosity the derivation does not involve undulations in lithospheric thickness and thus provides the inference that the vigor of stagnant lid convection depends only weakly on those features numerical models support this inference lithospheric edges do nucleate instabilities but these instabilities are similar in magnitude to those in models started with a tiny perturbation from laterally homogeneous temperature subsequent convection is also similar for models started with tiny perturbations and with large lateral variations in lithospheric thickness in terms of the scaling relationship the asthenosphere beneath the thin lithosphere within rifts is an unfavorable site for circulation and extensive melting the rheological boundary layer is in general thin below thin lithosphere which causes stagnant lid convection there to be weak care needs to be taken in numerical modeling of convection beneath passive margins starting conditions that drive large instabilities of features that could not have persisted in the earth need to be avoided models should start rifting only after mature stagnant lid convection is established

rheology of a coaxial shear zone in the virginia blue ridge wet quartzite dislocation creep at 250 280 c abstract cambrian quartzite along the western margin of the blue ridge near front royal virginia records significant penetrative strain under subgreenschist facies conditions strain analyses of the quartzite indicate general flattening and octahedral shear strain values up to 1 25 x z up to 5 7 1 samples with x z as low as 1 5 1 have crystallographic preferred orientations that record a dominance of basal slip and patterns consistent with coaxial flattening in most samples quartz has undergone minor amounts of dynamic recrystallization with mean grain sizes 4 5 m locally the quartzite is brittlely deformed and subgrains and recrystallized bulges are present along fractures and extinction bands fluid inclusion planes and fluid inclusions along deformation lamellae are abundant and quartz ftir analyses yield average h 106si values of 1100 3300 indicating fluid flow and high intragranular water content during deformation higher strain samples generally record lower water contents which may indicate that fluids migrated to grain boundaries during deformation dissolution microstructures are rare in high strain samples and we interpret most strain to have been accommodated by dislocation creep thermal modeling of zircon low temperature thermochronology data coupled with quartz c axis opening angle thermometry suggest deformation temperatures were likely 250 280 c these deformation temperatures are lower than what is typically considered the brittle plastic transition in quartz we attribute localization of this high strain zone and dislocation creep at these temperatures to hydrolytic weakening and a relatively low strain rate estimated to be 10 15 s 1

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

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

dating fault reactivation by ar ar laserprobe an alternative view of apparently cogenetic mylonite pseudotachylite assemblages apparently cogenetic mylonite pseudotachylite assemblages are commonly reported in major fault zones they represent two very incompatible modes of deformation and complex formation mechanisms have been proposed to explain this paradox we report here one such assemblage from central norway in which apparently synchronous mylonite and pseudotachylite formation is separated by 100 ma mean muscovite ar ar laserprobe spot ages from mylonite and pseudotachylite matrix are 406 11 ma and 290 10 ma respectively in preference to a complex cogenetic model we invoke fault reactivation as a viable alternative and suggest that assumed contemporaneity in such assemblages may be invalid in many cases

ultramylonite bands derived from cataclasite and pseudotachylyte in granites northeast japan abstract small scale ultramylonite and cataclasite bands millimeters to tens of centimeters thick are developed in granitic rocks west of the hatagawa fault zone hfz in the abukuma belt northeast japan they occur as single or paired bands with sharp planar boundaries trending nne ssw and often form networks and conjugate sets the very small s c angle and the high displacement thickness ratio of the bands suggest that the shear strain is high the ultramylonite bands are commonly associated with cataclasite bands and mineral veins and rarely with pseudotachylyte some cataclasite bands contain mylonitized layers in which quartz fragments are strongly deformed and dynamically recrystallized on the other hand some ultramylonites are fractured producing fragments that have rotated during later cataclasis the major element content of the ultramylonite bands is similar to that of the surrounding granitic rocks strongly suggesting that the ultramylonite bands have formed through in situ deformation of the granitic protolith without significant mass transfer mineralogy and microstructures of some ultramylonites suggest the strong possibility that they are derived from pseudotachylyte the ultramylonite bands are interpreted as forming in the 10 15 km deep cataclastic plastic transition zone under greenschist facies conditions where co seismic fracturing and aseismic plastic flow have alternated

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

resolving the lithosphere asthenosphere boundary with seismic rayleigh waves summary seismic surface wave measurements offer tight constraints on shear wave speed values within the lithosphere and asthenosphere it is still a matter of debate however how accurately and under what conditions surface waves can resolve the depth and thickness of the lithosphere asthenosphere boundary lab we investigate the sensitivity of rayleigh waves to lab properties and find that if the lab is associated with a 2 3 per cent shear speed reduction at a depth less than 100 km then 10 20 km variations in its depth translate into phase velocity changes of up to 1 per cent variations in the thickness of the lab from zero to a few tens of kilometres cause much smaller phase velocity changes 0 1 per cent although the signal of the lab thickness variations is enhanced by a distinct frequency dependent pattern of phase velocity increases and decreases for lab depths increasing beyond 100 km larger absolute variations in the lab depth and thickness are needed to generate response of the same amplitude in phase velocities we introduce a grid search inversion of phase velocity curves for the lab depth and thickness defining them as the middle and the width respectively of a depth interval with a linear shear speed decrease in it the inversion comprises dense sampling of the lab depthlab thickness parameter plane and non linear gradient search inversions at every point it accountsforanytrade offsofthelabparameterswithshearspeedvariationsaboveandbelow the lab inversions of phase velocity curves for the lab depth and thickness show that the two parameters have uncorrelated uncertainties with the lab depth constrained better than the lab thickness random errors in phase velocity measurements have a limited effect on lab depth measurements causing errors of only a few kilometres measurements of the lab thickness are less robust and are not possible in the presence of large 1 per cent random errors substantial systematic biases in the measurements errors that persist over broad period ranges of tens of seconds can affect both lab depth and lab thickness estimates phasevelocity curves with such biases are not suitable for inversions for lab properties resolving the lab however in particular the lab depth should already be possible with some of the accurate broad band phase velocity measurements available today applying our method to phase velocity data measured in phanerozoic west central germany we find best fitting lab depths in the 60 80 km range the lab depth always understood to be the middle of the shear speed reduction depth range with minimum misfits achieved near the 70 km lab depth beneath the archean guyana shield we determine a 140 175 km lab depth range with a 160 km best fitting mid lab depth

the lithosphere asthenosphere boundary and cratonic lithospheric layering beneath australia from sp wave imaging sp and ps scattered wave receiver functions were calculated for nineteen stations across australia and the island of tasmania in order to image the lithosphere asthenosphere boundary and layering within the lithosphere within phanerozoic eastern australia and the eastern margin of the south australia craton prominent sp phases from a negative velocity contrast were found at depths of 61 11 km to 131 9 km consistent with the lithosphere asthenosphere boundary depth range from surface wave tomography these phases imply significant velocity drops over depth ranges of 30 40 km or less and thus cannot be explained by a lithosphere asthenosphere boundary that is controlled by temperature alone rather they imply that the asthenosphere is hydrated with respect to a drier depleted lithosphere or contains a small amount of partial melt the shallowest sp phases have the largest amplitudes and occur in regions with the most recent voluminous volcanism strengthening the link to partial melt at the base of the lithosphere in contrast no significant negative sp phases were found at the base of the thick cratonic lithosphere at the stations in central and western australia implying that the cratonic lithosphere asthenosphere velocity gradient is distributed over more than 50 70 km in depth this gradient may be purely thermal in origin although gradational changes in composition or melt content cannot be ruled out a negative sp phase was observed at depths of 69 8 km to 85 14 km at stations in central and western australia indicating the presence of a drop in velocity internal to the lithosphere this interface within the lithosphere may be a relic of cratonic mantle formation or the result of alteration by melt and metasomatism

influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization 1 small scale convection ssc rolls below the oceanic lithosphere have the tendency to align with the large scale shearing direction and thus with the plate motion direction relative to the deep mantle understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the pacific such as gravity lineations small scale seismic velocity variations and intraplate volcanism that cannot be explained by hot spots in this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed as suggested by the kink in the hawaii emperor seamount chain using three dimensional numerical flow models we investigate the realignment of ssc rolls after a change in plate motion direction from the nature of the ssc it is expected that rheological parameters dominate the characteristics of this realignment our results show that this is indeed the case we find that 1 using constraints from onset timing of ssc realignment of rolls can occur as fast as within 20 ma but might also take much longer dependent on the rheology 2 the realignment period is strongly correlated to the sum of large scale shear stress induced by plate motion and small scale shear stress from the ssc itself 3 in a mantle deforming by dislocation creep realignment occurs faster than by diffusion creep because dislocation creep ssc is more vigorous and 4 activation energy has little influence on the realignment time possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

a multi kilometer pseudotachylyte system as an exhumed record of earthquake rupture geometry at hypocentral depths colorado usa abstract a system of pseudotachylyte bearing fault zones preserved along the proterozoic homestake shear zone in the southern rocky mountains provides an avenue for investigating earthquake processes at the hypocenter the results of detailed field mapping suggest that pseudotachylyte may serve as a dynamic indicator of rupture directivity and yield general estimates of some earthquake source parameters when examined at the multi kilometer fault system scale pseudotachylyte fault veins are primarily exposed within eight ne striking sub vertical fault zones that have a cumulative length of more than 21 km the fault zones are mapped for 7 3 km along strike and fan to the northeast from a 170 m wide outcrop belt to a maximum cross strike width of 2 3 km pre existing structural control on rupture geometry is indicated by concordance between foliation and fault veins as well as spatial coincidence between the limbs of map scale rootless isoclinal folds and the location of most fault zones the central portion of the longest fault zone exhibits evidence for dextral oblique slip that involved more than 2 1 m of strike slip offset between four parallel fault veins that are interpreted to have formed in response to a single rupture event in addition an along strike continuity and systematic distribution of fault zones a progressive northeastward decrease in pseudotachylyte volume and maximum vein thickness and a relative scarcity of cross cutting relationships further suggests that the majority of the frictional melt in the system may have developed in response to one or several multi kilometer ruptures as opposed to hundreds of shorter ruptures the similarity of kilometer scale relationships observed along the homestake pseudotachylyte system with the subsurface slip distribution and surface geometry of present day strike slip earthquakes is interpreted to indicate that frictional melting occurred within a concentrated zone of moment release or an earthquake hypocenter during one or more m w 6 3 earthquakes m 0 6 9 10 25 dyn cm that involved northeastward rupture propagation

imaging crustal and upper mantle structure beneath the colorado plateau using finite frequency rayleigh wave tomography a new 3 d shear velocity model of the crust and upper mantle beneath the colorado plateau and surrounding regions of the southwestern united states was made with finite frequency rayleigh wave tomography using earthscope usarray data the goal of our study is to examine the colorado plateau lithospheric modification that has resulted from cenozoic tectonism and magmatism we have inverted for the isotropic vs model from a grid of rayleigh wave dispersion curves obtained by a modified two plane wave method for periods from 20 to 167 s we map the lithosphere asthenosphere boundary under the colorado plateau by identifying the middle of the shallowest upper mantle negative vs gradient the depths of the lithosphere asthenosphere boundary inferred here agree well with receiver function estimates made independently the strong lateral heterogeneity of shear velocity can be mainly attributed to 200 400 k variations in temperature together with 1 partial melt fraction in the shallow upper mantle the resulting vs structures clearly image the upper mantle low velocity zones under the colorado plateau margins that are associated with magmatic encroachment these upper mantle low velocity zones resulted from the convective removal of the colorado plateau lithosphere that had been rehydrated by subduction released water refertilizing and destabilizing it this convective erosion by the asthenosphere at the low viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended basin and range since the mid cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast south and northwest moreover the rayleigh wave tomography model images parts of a high velocity drip in the western colorado plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography the widespread edge convective erosion which the regional delamination style downwelling processes are a 3 d manifestation of could provide additional buoyancy sources to support the excess uplift at the margins of the plateau

intraplate volcanism with complex age distance patterns a case for small scale sublithospheric convection 1 many volcano chains in the pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions one possible explanation for non hot spot intraplate volcanism is small scale sublithospheric convection ssc and we explore this concept using 3 d numerical models that simulate melting with rheology laws that account for the effects of dehydration ssc spontaneously self organizes beneath relatively mature oceanic lithosphere whenever this lithosphere is sufficiently young and thin ssc replaces the shallow layer of harzburgite which was formed by partial melting at the mid ocean ridge with fresh peridotite this mechanism enables magma generation without any preexisting thermochemical anomalies however the additional effect of melting induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous ssc overturn of the compositional stratification and related magmatism the intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of ssc into very shallow and cooler levels on the one hand such a restriction precludes high degrees of melting but on the other hand it slows asthenospheric cooling and thus prolongs the duration of melting to 25 ma volcanism over such an elongated melting anomaly continues for at least 10 20 ma and occurs on seafloor ages of 20 to 60 ma these seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid ocean ridge melting the long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains ssc thus gives an explanation for previously enigmatic volcano ages along the line islands and the gilbert and pukapuka ridges as well as along the individual subchains of the wakes marshalls and cook australs

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

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

imaging the seismic lithosphere asthenosphere boundary of the oceanic plate 1 the seismic lithosphere asthenosphere boundary lab or g discontinuity a seismologically characterized abrupt drop in wave speed in the uppermost mantle is one of the key issues in current geodynamics although plate tectonics started as a theory for the ocean reports on lab for normal oceanic regions are scarce due to paucity of seismic data and whether or not the oceanic lab grows with age is the key issue to be resolved we conduct a systematic survey for the oceanic lab using s to p converted seismic waves along three margins of oceanic plates whose crustal age ranges from 10 myr to 130 myr and we observe laterally continuous oceanic lab images the thickness of the oceanic plate estimated from labs increases with the plate age though scattered suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic lab represents a boundary that grows with age

influence of variable uncertainties in seismic tomography models on constraining mantle viscosity from geoid observations the radial viscosity structure of the earth is explored on the basis of the geoid observations the variations of uncertainty in seismic tomography models are accounted for when finding the radial viscosity structure the new methodology we propose attempts to fit more closely those features of the geoid that are better constrained by tomography models and avoids to fit those features that are poorly constrained this approach is particularly important because the error of geoid predictions caused by uncertainties in seismic tomography models is overwhelmingly larger than the noise in the geoid measurements the synthetic tests indicate that the viscosity structures obtained by disregarding the uncertainty variations in seismic tomography models can be biased depending on the geoid spectral band and on the input seismic tomography model when the uncertainty variations in seismic models are considered in the inversion process results do not indicate a viscosity in the transition zone lower than in the upper mantle a robust feature found with the new method is a viscosity in the upper mantle two orders of magnitude smaller than in the lower mantle the error covariance of seismic tomography models is critical for the method we suggest a covariance matrix rigorously derived by seismologists should help to even more reliably infer the viscosity structure and relation between anomalies in density and seismic velocities from surface observations such as the geoid and thus lead to a better knowledge of the earth interior

cyclic ductile and brittle deformation related to coseismic thrust fault propagation structural record at the base of a basement nappe preveli crete 1 the structural record at the base of a basement nappe preveli nappe crete greece thrust upon sedimentary rocks is investigated aimed on understanding mechanisms which result in decoupling of the thrust sheet from its original substratum we identify several superimposed deformation stages each with characteristic structural style and indications of episodic deformation at initially high differential stress the final stage involves formation of a matrix supported breccia transected by pseudotachylytes comprising the lowermost 30 m of the nappe brecciation and pseudotachylyte formation occurred in a single event and structures were not modified afterward complete induration of breccia and composition of phengite crystallized during devitrification of pseudotachylytes place the sequence of events into the middle crust we propose a model relating episodic deformation and cyclic stress history to propagation of a thrust fault in a limited number of seismic events terminal brecciation and frictional fusion record passage of the fault front beneath the site of observation and decoupling of the thrust sheet absence of discernible further deformation is consistent with negligible basal friction during transport as a nappe brecciation and pseudotachylyte formation mark the switch from a history of repeated coseismic loading and postseismic stress relaxation in the plastosphere driven by seismic events on the approaching thrust fault to passive transport with deformation localized in a weak thrust plane for a sequence of superimposed ductile to brittle structures our model provides an alternative to progressive cooling and exhumation concomitant with deformation over millions of years

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

three dimensional numerical simulations of mantle flow beneath mid ocean ridges we perform three dimensional 3 d numerical simulations of an oceanic lithosphere cooling above a convective mantle to investigate mantle flow geometry and its associated lithospheric features a constant horizontal velocity half spreading velocity of 2 or 4 cm yr is applied at the box surface to mimic plate motion in all cases because of the temperature dependent viscosity a rigid conductive lithosphere cools progressively beneath the two plates separated by the ridge after 16 40 ma cold downgoing instabilities develop at the base of the lithosphere small scale convection is then superimposed on the large scale circulation it produces and then interacts with a slowly evolving short wavelength isotherms topography at the base of the lithosphere the influence of the imposed ridge geometry on the mantle flow appears by comparison of simulations with case a a linear ridge perpendicular to the plate motion case b a linear ridge strongly oblique to the plate motion and case c a ridge with transform faults along the spreading center with a mean orientation strongly oblique to the plate motion all simulations reveal complex interaction between the isotherm topography within the base of the lithosphere the small scale flow generated at or just below the base of the lithosphere and the large scale flow dominating in the mantle below the large scale flow appears always controlled by the mean ridge axis direction and thus may be oblique to the imposed plate motion direction it is enhanced by the instabilities development we argue that the large scale flow orientation results from the interaction with the small scale velocity field which flows down the isotherm topography at the base of the lithosphere the latter is either inherited lithospheric cooling transform faults or develops as boundary layer instabilities

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

seismically observable features of mature stagnant lid convection at the base of the lithosphere some scaling relationships small scale convection transfers heat from the asthenosphere into the base of the lithosphere scaling constraints are obtained on features associated with such convection that may be resolved by seismic studies compact notation arises by simply representing rheology viscosity changes by a factor of e with a temperature increase of t and the minimum viscosity in the asthenosphere is 0 the geothermal gradient in the lithosphere and the conductive heat flow are constrained by xenolith seismic and heat flow studies geoid studies constrain the viscosity um in the deep asthenosphere to 1020 pa s seismic studies have resolved cool downwellings at the edge of the colorado plateau the total anomaly of these downwellings temperature contrast times width depends on the square root of the ambient viscosity at depth the observed anomalies are reasonable if partial melt causes seismic velocity to vary rapidly with temperature convection produces scalloped relief on the lithosphere asthenosphere boundary of 20 km beneath stable continents if t is small less than 65 k shear associated with plate motions aligns convection cells convection is likely to strongly align cells if vigorous convection in fact exists beneath the pacific plate modest alignment is predicted beneath slowly moving ocean and continental plates seismic studies have yet to resolve scalloping and cell alignment

deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte bearing brittle faults a matter of a few seconds abstract tectonic pseudotachylytes i e quenched friction induced silicate melts record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites in this study we document the occurrence of recrystallization of quartz to ultrafine grained grain size 1 2 m aggregates along microshear zones 50 150 m thick in the host rock adjacent to pseudotachylytes from two different faults within quartzite schneeberg normal fault zone eastern alps and tonalite adamello fault southern alps in the brittle crust the transition from the host quartz to microshear zone interior includes i formation of high dislocation densities ii fine 0 3 0 5 m polygonization to subgrains defined by disordered to well ordered dislocation walls iii development of a mosaic aggregate of dislocation free new grains the crystallographic preferred orientation cpo of quartz towards the microshear zone shows a progressive misorientation from the host grain by subgrain rotation recrystallization to a nearly random cpo possibly related to grain boundary sliding these ultrafine aggregates appear to be typically associated with pseudotachylytes in nature we refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip thermal models indicate that the process of recrystallization including recovery processes occurred in a time lapse of a few tens of seconds

identifying deformed pseudotachylyte and its influence on the strength and evolution of a crustal shear zone at the base of the seismogenic zone abstract on a strand of the norumbega fault system a paleozoic subvertical seismogenic fault system in northeastern new england usa we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite phyllonite layers we consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone the norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of 10 15 km some of which preserve evidence for mutually overprinting pseudotachylyte and mylonite along one of these the sandhill corner shear zone all stages of the pseudotachylyte to ultramylonite phyllonite transformation are preserved from 1 primary pseudotachylyte structures to 2 initial mineral crystallization 3 grain coarsening and reactions and 4 viscous deformation our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin using these features we estimate that 5 50 of the rock volume in the sandhill corner shear zone mean 30 locally 50 is deformed pseudotachylyte suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone the sandhill corner shear zone localized along the contact between two rheologically contrasting units mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core indicating that seismic rupture also localized there a decrease in grain size promoting grain size sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers the formation of multiple generations of weak deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core

linear stability analysis of richter rolls presence of vertical shear rayleigh numbers up to 10 6 are considered the transition rayleigh number over which longitudinal rolls are unstable is derived as a function of surface velocity effects of nonlinear vertical shear are shown to be insignificant unless nonlinearity becomes extreme our results are in good agreement with previous studies except at high rayleigh numbers where the meaning of roll stability becomes ambiguous owning to inherently time dependent convection when applied to small scale convection in the upper mantle our results suggest that the roll stability is very sensitive to plate velocity if the upper mantle viscosity is relatively high 10 20 pa s richter rolls may be expected only beneath fast moving plates such as the pacific plate index terms 3010 marine geology and geophysics gravity 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8121 tectonophysics dynamics convection currents and mantle plumes 8180 tectonophysics tomography citation korenaga j and t h jordan linear stability analysis of richter rolls geophys res lett 30 22 2157 doi 10 1029 2003gl018337 2003

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

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

interactions of scales of convection in the earth s mantle abstract the existence of undulations of the geoid gravity and bathymetry in ocean basins as well as anomalies in heat flow point to the existence of small scale convection beneath tectonic plates the instabilities that could develop at the base of the lithosphere are sufficiently small scale 60 ma they form networks that are shaped by the plate evolution slabs plumes and the geometry of continental boundaries plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere they create a wake in which new instabilities form downstream these wakes form channels that are about 1000 km wide and thus are possibly detectable by seismic tomography beneath fast plates cold sinking instabilities are tilted in the direction opposite to plate motion while they sink vertically for slow plates these instabilities are too small to be detected by usual seismic methods since they are about 200 km in lateral scale however this preferred orientation of instabilities below fast plates could produce a pattern of large scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models

internal tide radiation from mendocino escarpment abstract strong semidiurnal internal tides are observed near mendocino escarpment in full depth profile time series of velocity temperature and salinity velocity and density profiles are combined to estimate the internal tide energy flux divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment a baroclinic energy flux of 7 kw m 1 radiates from the escarpment corresponding to 3 of the 220 kw m 1 fluxing poleward in the surface tide energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site 90 km north of the escarpment coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection beyond the first surface reflection north of the escarpment the energy flux drops by 2 kw m 1 and beams are no longer discernible turbulence as inferred from finescale parameterizations is elevated by over two orders of magnitude relative to th