intermediate depth earthquakes facilitated by eclogitization related stresses eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10 15 one consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing pacific plate and reveal new evidence in favor of this mechanism tensional earthquakes lying 1 km above compressional earthquakes and earthquakes with highly similar waveforms lying on well defined planes with complementary rupture areas the tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate these observations provide seismic evidence for the role of volume change related stresses and possibly fluid related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere

evolution of the grenada and tobago basins and implications for arc migration abstract the tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood this paper presents an integrated stratigraphic and tectonic evolution of late cretaceous to recent volcanic arcs and associated basins in the southeastern caribbean sea using seismic reflection data wide angle seismic refraction data well data and onland geologic data we propose a new tectonic model for the opening of the grenada and tobago basins and the 50 250 km eastward jump of arc volcanism from the late cretaceous aves ridge to the miocene to recent lesser antilles arc in the southeast caribbean based on the mapping of three seismic megasequences the striking similarity of the half graben structure of the grenada and tobago basins that flank the lesser antilles arc their similar smooth basement character their similar deep marine seismic facies and their similar paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single saucer shaped oceanic crust paleogene forearc basin adjacent to the now dormant aves ridge this single forearc basin continued to extend and widen through flexural subsidence during the early to middle eocene probably because of slow rollback of the subducting atlantic oceanic slab rollback may have been accelerated by oblique collision of the southern aves ridge and southern lesser antilles arc with the south american continent uplift and growth of the southern lesser antilles arc divided the grenada and tobago basins by early to middle miocene time inversion of normal faults and uplift effects along both edges of the lesser antilles arc are most pronounced in its southern zone of arc collision with the south american continent the late miocene to recent depositional histories of the grenada and tobago basins are distinct because of isolation of the grenada basin by growth and uplift of the neogene lesser antilles volcanic ridge

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

competitive hydration and dehydration at olivine quartz boundary revealed by hydrothermal experiments implications for silica metasomatism at the crust mantle boundary abstract serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust given that such a fluid has a high silica activity compared with mantle peridotite it is thought that serpentinization and silica metasomatism occur simultaneously at the crust mantle boundary in this study we conducted hydrothermal experiments in the olivine ol quartz qtz h 2 o system at 250 c and vapor saturated pressure under highly alkaline conditions naoh aq ph 13 8 at 25 c to clarify the mechanism of silica metasomatism at the crust mantle boundary composite powders consisting of a qtz layer and an ol layer were set in tube in tube vessels after the experiments the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the ol qtz boundary the mineralogy of the reaction products in the ol hosted region changed with increasing distance from the ol qtz boundary from smectite serpentine smc zone to serpentine brucite magnetite brc zone olivine hydration proceeded in both zones but the total h 2 o content in the products was greater in the brc zone than in the smc zone mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone in contrast the smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions in the smc zone smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration and around the boundary of the smc and brc zones serpentine formation occurred by olivine hydration and brucite dehydration the relative extent of hydration and dehydration reactions controlled the along tube variation in the rate of h 2 o production consumption and the rate of volume increase our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure possibly affecting the mechanical behavior of the crust mantle boundary

contrasting fracture patterns induced by volume increasing and decreasing reactions implications for the progress of metamorphic reactions abstract hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock and some of these reactions proceed with fracturing for this paper we have developed a model for coupled hydraulic chemical mechanical processes using the distinct element method in order to understand the relationships among chemical reactions fluid flow and fracturing during metamorphism the model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure with the help of the model volume decreasing dehydration reactions and volume increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions respectively both types of reaction proceed inwards from drained boundaries or fractures but they show contrasting fracture patterns a volume decreasing dehydration reaction produces tree type fractures with the new fractures generated as branches of a pre existing fracture a volume increasing hydration reaction produces a polygonal network of fractures where new fractures nucleate at sites far from the pre existing fracture and extend to form t junctions these contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient in the case of the tree type fractures the fractures are continuously generated at the reaction front and the reaction proceeds smoothly by positive feedbacks between reaction fracturing and fluid flow in contrast in a volume increasing hydration reaction fracturing initially occurs in response to the irregular boundary shape but as the reaction progresses a compressive stress field is generated which inhibits further fracture generation the compressive stress field also prevents fluid flow by closing the pre existing fractures which slows down the reaction these contrasting feedback systems between volume decreasing dehydration and volume increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved

existence of a seismic belt in the upper plane of the double seismic zone extending in the along arc direction at depths of 70 100 km beneath ne japan 1 we relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the pacific slab beneath ne japan we found a seismic belt parallel to the iso depth contour of the plate interface that is beneath the forearc area at depths of 70 100 km indicating that the distribution of the upper plane earthquakes is non uniform the location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where h2o contents change in the slab crust events in the upper seismic plane have mainly down dip compression type focal mechanisms but several events have normal fault type nf type versions whose spatial distribution appears to correspond to these boundaries these nf events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions

unbending combined with dehydration embrittlement as a cause for double and triple seismic zones 1 using a model of elastic perfectly plastic plate i demonstrate that plate unbending combined with dehydration embrittlement can account for the morphology and stresses of double or triple seismic zones dsz s or tsz s observed in many subducting slabs densifying metamorphic reactions may induce tensile stresses as well as high pore fluid pressures in the slab crust and thus generate the upper seismicity band of the tsz limited dehydration may moderately elevate the fluid pressure in the slab mantle but unbending usually dominates mantle stresses and results in another two seismicity bands dsz s are underdeveloped tsz s

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

an intraslab seismic sequence activated by the 2011 tohoku oki earthquake evidence for fluid related embrittlement 1 using quantitative analysis of waveforms we investigate a seismic cluster that occurred at a depth of 67 km in the philippine sea slab 8 months after the 11 march 2011 megathrust tohoku oki earthquake mw 9 0 the sequence started with an m 4 1 normal fault event on 14 november in the deepest part of the cluster and subsequent earthquakes migrated upward by 6 km along a narrow conduit like zone the earthquakes have stress drops of 0 5 40 mpa and groups of earthquakes with coherent waveforms are observed we explain these observations in terms of fluid related embrittlement and the migration of overpressured fluids the low permeability plate interface of the pacific plate may have been broken by the coseismic or postseismic slips of the tohoku oki earthquake with the fluids subsequently being liberated from the underlying crust of the pacific slab and migrating into the philippine sea slab due to the pore pressure gradient the tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area the heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress the lag of 8 months may represent the time needed for the unsealing of the plate interface the upward migration of fluids and the increase in pore pressure to become sufficient to overcome the lithostatic pressure

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

evidence from high frequency seismic waves for the basalt eclogite transition in the pacific slab under northeastern japan abstract seismic multi pathing effects attributed to a contrast in seismic attenuation between the back arc mantle wedge and subducted crust are detected in central honshu northeastern japan we observe an initial broadened p wave which is followed by a delayed higher frequency p wave signal their discrepant frequencies are best explained by attenuation effects delayed p wave signals travel in the low attenuation oceanic crust and therefore contain more high frequency components the time separation between the initial broadened p waves and the delayed p wave signals are affected by the seismic velocity in the subducted oceanic crust we observe systematic variation in the delay times of the later waves indicating an increase in seismic velocity in the oceanic crust relative to the mantle wedge at 130 150 km depth high frequency seismic simulations incorporating mineral physics derived models show that a 4 vp increase due to the blueschist decomposition and a 9 vp increase associated with the lawsonite talc eclogite transition replicate the observed delay time variation the blueschist breakdown may occur at a depth of 100 km and the lawsonite talc eclogite transition might be linked with the reduced seismicity level at depths greater than 150 km distinct from traditional guided waves the multi pathing effects in this study are mainly controlled by attenuation contrast and therefore may not require the oceanic crust to have low velocity and any special decoupling mechanism the multi pathing effects offer us another important tool to image subducted oceanic crust below back arc mantle wedges especially where guided waves are not observable in this study we demonstrate the value of observing and simulating high frequency seismic waves 20 hz in advancing our understanding of subduction zones

nwa 1152 and sahara 00182 new primitive carbonaceous chondrites with affinities to the cr and cv groups we have investigated the mineralogy petrography bulk chemistry and light element isotope composition of the ungrouped chondrites north west africa nwa 1152 and sahara 00182 nwa 1152 contains predominantly type 1 porphyritic olivine po and porphyritic olivinepyroxene pop chondrules chondrule silicates are magnesium rich fo98 8 1 2 n 36 fs2 3 2 1 wo1 2 0 3 n 23 matrix comprises 40 vol of the sample and is composed of a micron sized silicate groundmass with larger silicate sulfide magnetite and fe ni metal ni 50 wt grains phyllosilicates were not observed in the matrix refractory inclusions are rare 0 3 vol and are spinel pyroxene aggregates or amoeboid olivine aggregates melilite is absent from the refractory inclusions sahara 00182 contains predominantly type 1 po chondrules pop chondrules are less common most chondrules contain blebs of and are often rimmed with fe ni metal and sulfide chondrule phenocrysts are magnesium rich fo92 2 0 6 n 129 fs4 4 1 8 wo1 3 1 1 n 16 matrix comprises 30 vol of the meteorite and is predominantly sub micron silicates with rare larger silicate gains matrix fe ni metal mean ni 5 8 wt and sulfide grains are up to mm scale no phyllosilicates were observed in the matrix refractory inclusions are rare 1 1 vol and melilite is absent the oxygen isotope composition of nwa 1152 falls within the range of the cv chondrites with 17o 3 43 18o 0 70 and is similar to sahara 00182 17o 3 89 18o 0 19 grossman and zipfel 2001 based on mineralogical and petrographic characteristics we suggest nwa 1152 and sahara 00182 show many similarities with the cr chondrites however oxygen isotopes suggest affinity with the cvs thus neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics both samples demonstrate the complexity of inter and intra group relationships of the carbonaceous chondrites whatever their classification nwa 1152 and sahara 00182 represent a source of relatively pristine solar system material

deep fluid release in warm subduction zones from a breached slab seal abstract petrological models and seismic data from subduction zones with geotherms of 7 k km 1 or higher suggest that slabs in these systems dehydrate effectively in the forearc a large fluid flux is nevertheless released from these slabs at and beyond subarc depth suggesting that large amounts of h2o can remain slab bound to much greater depth than expected we propose that this is due to a transient sealing effect exerted by the subducting lower crust to test this concept the petrological and geochemical evolution of such gabbroic crust is investigated through a textural petrological and li chronometric analysis of eclogitized gabbros from an exhumed ultrahigh pressure terrane the samples record pristine transitions from dry rigid gabbro to hydrated eclogite and eclogite mylonite which occurred when these rocks resided at 90 110 km depth the observations characterize step by step the deformation and overstepped mineral reactions that following the influx of external fluids along a developing network of permeable shear zones lithium chronometry indicates that the gabbroic rocks were breached and permeated within a few weeks at a very specific depth within the 90 110 km interval depths where in warm subduction zones large fluid filled channel system emanate from the slab the data support a model in which fluids produced in the deserpentinizing slab mantle are trapped at high pore pressure beneath the slab moho and are ultimately released at subarc depth where the lower crust fails and develops highly permeable fluid vents the subducting lower crust thus may play an important role in regulating h2o and element budgets and controlling slab rheology in warm subduction zones

compressional metamorphic core complexes low angle normal faults and extensional fabrics in compressional tectonic settingscompressional core complexesm searle t lamont metamorphic core complexes mccs are interpreted as domal structures exposing ductile deformed high grade metamorphic rocks in the core underlying a ductile to brittle high strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension extension is supposedly the driving force that has governed exhumation however numerous core complexes notably himalayan karakoram and pamir domes occur in wholly compressional environments and are not related to lithospheric extension we suggest that many mccs previously thought to form during extension are instead related to compressional tectonics pressures of kyanite and sillimanite grade rocks in the cores of many of these domes are c 10 14 kbar approximating to exhumation from depths of c 35 45 km too great to be accounted for solely by isostatic uplift the evolution of high grade metamorphic rocks is driven by crustal thickening shortening regional barrovian metamorphism isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel channel flow following peak metamorphism beneath a passive roof stretching fault in naxos low angle normal faults associated with regional aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening the fact that low angle normal faults exist in both extensional and compressional tectonic settings and can actively slip at low angles c 60 is needed

mafic granulite rheology implications for a weak continental lower crust abstract the jelly sandwich model of continental rheology long the consensus model has recently been challenged the controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust however because of technical issues in experimental rock deformation there are very few data on this problem here we report the rheology and fabrics of a reconstituted fine grained nominally dry i e no hydrous mineral mafic granulite 57 plagioclase 24 clinopyroxene 14 orthopyroxene 5 opaque minerals 0 16 0 28 wt h 2 o deformed at 1 1 1 2 gpa pressure in a modified griggs type deformation apparatus the rheology of this mafic granulite can be described by the constitutive equation of e a 3 2 0 4 exp 244 35 kj mol r t where e is in s 1 in mpa t in kelvin and a 10 2 0 1 6 mpa 3 2 s 1 our results provide new experimental evidence in support of the jelly sandwich lithosphere strength model and imply that mafic granulite with a moderate amount of water 0 05 0 08 wt h 2 o is likely to be a weak layer in the lithosphere both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation the deformation mechanism is dislocation creep with 100 001 and 001 100 010 100 being the dominant slip systems for pyroxenes and plagioclase respectively the low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase a weak continental lower crust has many important implications for geodynamics of crust mantle interactions such as lower crust channel flow and delamination

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate