the leka opholite complex central norwegian caledonides field characteristics and geotectonic significance the leka ophiolite complex loc of lower ordovician age u pb zircon age of 497 2 ma contains the following components 1 a lower strongly deformed harzburgite unit representing the depleted upper mantle this unit becomes progressively richer in dunite towards 2 a sequence of ultramafic cumulates unconformably overlying the harzburgite tectonite the crystallization sequence in the ultramafic cumulates is olivine chromite clinopyroxene orthopyroxene in the south western part of the ultramafic cumulate sequence layers of metagabbro appear interlayered with banded dunite wehrlite the above mentioned rock sequences may be in tectonic contact with 3 layered metagabbros which give way upwards to laminated and vari textured metagabbro within the laminated vari textured metagabbro is the first appearance of 4 metabasalt dykes which increase upwards in abundance from scattered single dykes to a 100 sheeted dyke swarm associated with the vari textured metagabbro and metabasalt dykes are minor acid intrusions within the upper part of the dyke swarm is the first appearance of 5 pillow lavas which are associated with thick volcaniclastic metasediments at apparently higher stratigraphic levels geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite iat to morb character and typical metaboninites are also present this range of geochemical composition as well as the crystallization sequence of the cumulates strongly suggests that the first stages in the formation of the loc was by spreading above a subduction zone the pillow lavas associated with volcaniclastic rocks are of morb or wpb type and these probably represent the later spreading stages of the loc in a back arc basin but at sufficient distance to be unaffected by the geochemical fingerprints of the subduction process

localized slip controlled by dehydration embrittlement of partly serpentinized dunites leka ophiolite complex norway abstract dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure a process referred to as dehydration embrittlement however the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate in the layered ultramafic sections of the leka ophiolite complex in the central norwegian caledonides prograde metamorphism of serpentinite veins led to local fluid production and to the growth of mg rich and coarse grained olivine with abundant magnetite inclusions and 18o values 1 0 1 5 below the host rock embrittlement associated with the dehydration caused faulting along highly localized

influence of pore pressure on the event size distribution of induced earthquakes 1 during an enhanced geothermal system egs experiment fluid is injected at high pressure into crystalline rock to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted the fracturing of the basement caused by these high pore pressures is associated with microseismicity however the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates and thus pore pressure is unknown here we show how pore pressure can be linked to the seismic frequency magnitude distribution described by its slope theb value we evaluate the dataset of an egs in basel switzerland and compare the observed event size distribution with the outcome of a minimalistic model of pore pressure evolution that relates event sizes to the differential stress d we observe that the decrease of b values with increasing distance of the injection point is likely caused by a decrease in pore pressure this leads to an increase of the probability of a large magnitude event with distance and time

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

stratiform platinum group element mineralizations in the ultramafic cumulates of the leka ophiolite complex central norway three types of stratiform platinum group element pge enrichment have been documented within olivine cumulates of the layered series in the leka ophiolite complex 1 os ir ru enrichments associated with chromite rich rocks 2 pt dominated enrichments and 3 pd dominated enrichments associated with discrete 10 to 50 cm thick sulfide rich horizons drill cores across the pt and pd enriched horizons have yielded 0 5 and 1 m averages of ca 1 ppm pge au and hand samples from one of the horizons yielded 3 to 4 ppm pge the horizons can be traced laterally for more than 1 5 km beta autoradiography of core samples shows that the enrichments may be only a few cm thick and that they form well defined horizons rich in discrete pge bearing minerals mainly alloys of pt fe ni and cu au pd tellurides and antimonides the pge enrichments are associated with the bases of macrorhythmic units and seem to have formed during or shortly after the influx of primitive magma into the chamber the pt pd enriched horizons are displaced relative to chromite horizons thus explaining why stratiform chromitites in ophiolites in contrast to chromitites in many intracratonic layered intrusions are depleted in pt and pd relative to os ir and ru the study demonstrates that ophiolites should not be disregarded as potential sources for pt and pd

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

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

imaging b value variation beneath the pamir hindu kush region the frequency magnitude distribution b value for the seismicity of the pamir hindu kush region from august 2008 to june 2010 is analyzed we used a very precisely located earthquake dataset containing 9532 events with a magnitude of completeness of 2 9 we attempted both 1d and 2d approaches to resolve the b value variation as a function of depth several pockets of high b value were detected during the cross sectional analysis that may reflect the changes in the physical properties of the subducting slab with depth under the hindu kush sector a high b value 1 2 is identified at the depth of 150 km but under pamir this zone b 0 9 is apparent at shallower depth an earlier detailed seismic tomography study revealed a low velocity zone beneath hindu kush where the high b value is imaged in the present study this observation is corroborated by the existence of fluid due to the dehydration and increase in pore pressure within the lower portion of the indian lithosphere the relatively high b value at shallower depth could be associated with the higher slab temperature as caused by the slower subduction under pamir

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

spinel inclusions in olivine of peridotite xenoliths from tubaf seamount bismarck archipelago papua new guinea evidence for the thermal and tectonic evolution of the oceanic lithosphere olivine in spinel peridotite xenoliths from the bismarck archipelago northeast of papua new guinea which were transported to the surface by quaternary basalts shows spinel inclusions up to 25 m long and 200 nm wide these inclusions mainly occur as inhomogeneously distributed needles and subordinately as octahedral grains in olivine of veined metasomatic peridotites as well as peridotites without obvious metasomatism the needles very often occur in swarms with irregular spacing in between them similar spinel inclusions in olivine have only previously been reported from ultramafites of meteoritic origin composition and orientation of the spinel inclusions were determined by transmission electron microscopy tem and analytical electron microscopy aem both the needles and the grains display a uniform crystallographic orientation in the host olivine with 001 o1 1 10 spl and 100 ol 111 spl the needles eare elongated parallel 010 in olivine which is the same in all olivine grains as these needles have no relation to the metasomatic sections in the peridotite it is concluded that they are primary features of the rock although the composition of the spinel needles is often very similar to the large chromian spinel octahedra in the matrix the small octahedral spinel inclusions in olivine are in part mg rich aluminous spinel and sometimes almost pure magnetite the spinel needles are suggested to have formed by exsolution processes during cooling of al and cr rich high temperature olivine during the initial formation of the lithospheric mantle at the mid ocean ridge the al rich spinel octahedra probably formed by the breakdown of an al rich phase such as phlogopite or by metasomatism whereas the magnetite was generated by oxidizing fluids these oxidizing fluids may either have been set free by dehydration of the underlying subducted plate or by the quaternary magmatism responsible for the transport of the xenoliths to the seafloor

simultaneous acoustic emissions monitoring and synchrotron x ray diffraction at high pressure and temperature calibration and application to serpentinite dehydration abstract we have successfully developed an experimental setup that allows collecting in situ x ray powder diffraction and simultaneously recording full waveform acoustic emissions ae at high pressure and temperature pt in a dia multi anvil device this setup is a powerful tool for investigating rock embrittlement at high pt due to phase transitions and or mineral reactions since both reaction progress and kinetics and ae triggering can be simultaneously monitored the dehydration of natural serpentinite samples antigorite rich under deviatoric stress has been investigated by this method since antigorite dehydration is believed to trigger intermediate depth earthquakes through dehydration embrittlement we performed beforehand a series of tests on the cold compression of reference materials with contrasted mechanical behaviors quartz beads and kaolinite powder due to grain crushing cold compression of quartz gave rise to numerous ae events several hundreds which were located within the sample cold compression and heating of kaolinite a ductile material yielded no aes demonstrating that the pressure assembly is noiseless unexpectedly antigorite rich serpentinite samples produced no detectable aes in the course of their dehydration under the differential stress imposed by alumina waveguides the only aes that were recorded occurred during cold compression sample microstructures indicate that conjugate faults inherited from the cold compression stage are activated during or after dehydration of the sample the aseismic slip along these faults could be attributed to the presence of talc or a talc like phase or of fine grained materials dehydration reaction products in the fault gouge furthermore ae triggering can also be influenced by hydraulic diffusivity and the differential stress level on the sample two parameters that are not controlled in conventional multi anvil experiments our results highlight the fact that coupling between dehydration reactions and seismicity might not be as straightforward as previously thought in fact fast reaction kinetics or high reaction extent may inhibit the nucleation of mechanical instabilities through rapid stress relaxation of the solid matrix

pulverized fault rocks and damage asymmetry along the arima takatsuki tectonic line japan abstract we present field and laboratory data on pulverized rocks at the hakusui kyo outcrop of the arima takatsuki tectonic line attl which is a dextral strike slip fault with 17 km displacement juxtaposing granite to the south against rhyolite to the north the majority of slip at the surface is localized to a clay rich gouge fault core 8 10 cm in width surrounded by a coarsening outwards fault breccias up to 3 m wide fault damage is highly asymmetric with respect to the slipping zone the granite south of the fault has a pulverized damage zone up to 200 m wide while the rhyolite to the north has only about 3 m wide non pulverized fault breccia the degree of pulverization in the granite decreases approximately logarithmically with normal distance from the slip zone the highly fractured pulverized rocks exhibit several distinct textural characteristics in thin section grains appear to be highly comminuted but the original grain shapes and margins are recognizable microfractures tend to be tensile in no preferred orientation grain fragments display little to no rotation and lack evidence of in situ shear consequently at macroscale the rocks appear to preserve original granitic textures despite being highly fractured and friable the observed pulverization and rock damage asymmetry are most consistent with generation mechanism involving ruptures on a bimaterial interface with statistically preferred propagation direction leading to damage primarily on the side with higher seismic velocity at depth this is supported by laboratory measurements of p wave ultrasonic velocities on intact samples which indicate that the granites have consistently higher velocity than the rhyolite with increasing confining pressure

frictional sliding in serpentine at very high pressure abstract using a new four channel system for detecting acoustic emissions in a multianvil apparatus we have assessed the pressure temperature range for such emissions as well as the role of dehydration by deforming samples of extensively serpentinized peridotite we show that in the absence of dehydration and for samples initially faulted at low pressure acoustic emissions occurred well outside the expected pressure temperature field of unassisted brittle failure emissions were also detected during and after dehydration of serpentine microstructures of post run specimens revealed fault slip with offsets up to 500 m regardless of whether or not dehydration took place dehydration appears to effectively stop slip on pre existing faults and create new ones analysis of p wave travel times from the four sensors confirmed that the acoustic emissions originated within the specimen during fault slip these observations suggest that earthquakes can be triggered by slip along an existing fault containing serpentine under significantly higher pressure and temperature conditions than previously thought possible without dehydration

u pb ages of ophiolites and arc related plutons of the norwegian caledonides implications for the development of iapetus u pb zircon ages are reported for four ophiolites and three crosscutting arc related plutons from the norwegian caledonides plagiogranite differentiated from gabbro of the karmoy ophiolite is dated at 493 7 4 ma whereas arc related trondhjemite cutting this ophiolite crystallized at 485 2 ma a crosscutting clinopyroxene phyric gabbro intrusion is dated at 470 9 5 ma by near concordant magmatic titanite sphene and discordant u rich 2903 6677 ppm zircon lower intercepts of 247 68 and 191 70 ma defined by the plagiogranite and clinopyroxene phyric gabbro best fit lines may reflect a real low t alteration rift related event

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

discrimination of estuarine marsh subenvironments san francisco bay california u s a using a multivariate statistical calibration of abiotic sediment properties abstract multivariate analysis of a suite of sediment characteristics provides a novel inexpensive and time efficient approach to characterizing estuarine depositional subenvironments especially in the absence of diagnostic microfossils and macrofossils the development calibration and implementation of a statistical model are discussed using data collected from the san francisco bay estuary california forty seven surface sediment samples were collected from three marshes within the san francisco bay estuary and from six different subenvironments ranging from tidal salt marsh below mean higher high water mhhw to tidal freshwater marsh sediments located above mhhw sediment samples were analyzed for percent organic content percent water content percent clay content sediment density and iron concentration standard statistical analysis of the data indicates that changes in sediment characteristics exist within and between the individual marshes however individually these characteristics are insufficient to distinguish among marshes and subenvironments multivariate analysis therefore is employed to consider the array of sediment characteristics simultaneously the individual marsh research sites are treated as idealized reference types the results indicate a strong degree of and statistically significant predictive capability these surface data and analyses are used in the interpretation of stratigraphic cores from three sites in san francisco bay the results using this method were broadly consistent with interpretations using traditional methods however this method determined more subtle variability than was previously recognized

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

dynamic ruptures on a frictional interface with off fault brittle damage feedback mechanisms and effects on slip and near fault motion the spontaneous generation of brittle rock damage near and behind the tip of a propagating rupture can produce dynamic feedback mechanisms that modify significantly the rupture properties seismic radiation and generated fault zone structure in this work we study such feedback mechanisms for single rupture events and their consequences for earthquake physics and various possible observations this is done through numerical simulations of in plane dynamic ruptures on a frictional fault with bulk behavior governed by a brittle damage rheology that incorporates reduction of elastic moduli in off fault yielding regions the model simulations produce several features that modify key properties of the ruptures local wave propagation and fault zone damage these include 1 dynamic generation of near fault regions with lower elastic properties 2 dynamic changes of normal stress on the fault 3 rupture transition from crack like to a detached pulse 4 emergence of a rupture mode consisting of a train of pulses 5 quasi periodic modulation of slip rate on the fault and 6 asymmetric near fault ground motion with higher amplitude and longer duration on the side with reduced elastic moduli the results can have significant implications to multiple topics ranging from rupture directivity and local amplification of seismic motion to near fault tremor like signals

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

gouge formation by dynamic pulverization during earthquake rupture it is proposed here that fault gouge forms by rock pulverization within the tip region of a fast propagating earthquake due to the intensity of stress and rates of strain in this zone to examine this mechanism we calculate the deformation conditions of a dynamic shear fracture propagating close to the limiting rayleigh wave velocity the mechanical conditions close to the fracture tip are extreme tensile stresses approach 5 gpa volumetric strain rates exceed 105 s 1 and volumetric expansion alternates with volumetric contraction it is expected that a localized zone a few millimeters wide will be pulverized under these conditions this mechanism of dynamic gouge formation provides new insight to recent observations of the texture of gouge from the san andreas fault zone and from rupture zones in south african mines further as such extreme conditions develop only during fast propagating earthquakes presence or absence of pulverized gouge may serve as an indicator of earthquake propagation velocity

olivine friction at the base of oceanic seismogenic zones 1 we investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault triaxial compression tests were conducted on dry olivine powder grain size 60 m at effective confining pressures between 50 and 300 mpa using argon as a pore fluid temperatures between 600 c and 1000 c and axial displacement rates from 0 06 to 60 m s axial strain rates from 3 10 6 to 3 10 3 s 1 yielding shows a negative pressure dependence consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments a combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes sample strengths were pressure dependent and nearly independent of temperature localized shear zones formed in initially homogeneous aggregates early in the experiments the frictional response to changes in loading rate is well described by rate and state constitutive laws with a transition from velocity weakening to velocity strengthening at 1000 c microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 c consistent with the focal depths of earthquakes in the oceanic lithosphere

geochemistry of a transitional ne trachybasalt q trachyte lava series from patmos dodecanesos greece further evidence for fractionation mixing and assimilation trace element and preliminary sr and o isotopic data are reported for a transitional alkaline sub alkaline lava series mvs from patmos greece the lava types belonging to this series are ne trachybasalt hy trachybasalt hy trachyandesite and q trachyte rb sr and ba contents as well as k rb ratios of the ne trachybasalts differ from those of alkali basalts of oceanic islands and those of k rich alkaline lavas of continental regions and are consistent with the occurrence of these volcanics in a destructive plate margin environment qualitatively the variations shown by many trace elements throughout the mvs are explicable in terms of magma evolution via fractional crystallization involving removal of the observed phenocryst phases cross cutting ree patterns can be explained by removal of small amounts of apatite however certain features of the data cannot be reconciled with the operation of fractional crystallization alone these are a the compatible behavior of ba throughout the mvs b the moderately as opposed to highly incompatible behavior of zr rb and nb relative to th and c the significant decrease of k th rb th zr th zr nb nb th yb th ta th u th and zr ta ratios especially but not exclusively in the mafic part of the series quantitative modeling indicates that the hy trachybasalts are anomalously enriched in both highly incompatible and highly compatible elements and these lavas are shown to be hybrids formed by mixing of ne trachybasalt and hy trachyandesite mixing proportions of the end members calculated from incompatible element abundances 19 ne trachybasalt differ from those calculated from compatible element abundances 62 ne trachybasalt and are inconsistent with proportions calculated from published mineral chemical data in addition mixing cannot account for the observed variations in incompatible element ratios and this is taken as evidence for the simultaneous operation of assimilation isotopic variations 87sr 86sr from 0 7049 to 0 7076 and 18o 16o from 4 7 to 8 6 and the positive correlation of isotope ratios with sio2 and th contents provide conclusive proof that assimilation occurred calculations show that the isotopic characteristics and the concentrations of many trace elements in the q trachytes can be explained by fractional crystallization of ne trachybasalt combined with assimilation of average continental crust 87sr 86sr 0 710 and that large amounts of assimilation are not necessary ma mc 0 55 ree data are not well explained by this model and suggest a crustal end member enriched in lree relative to the average crust zr and hf data are also not well explained and indicate that the assimilant was depleted in hfse relative to average crust or that hfse are held back in relatively insoluble phases such as zircon in the restite during assimilation nevertheless the results of the modeling demonstrate that ba concentrations may decrease during afc processes and that high sr contents 1500 ppm in the mvs ne trachybasalts do not render mafic parental magmas immune to the effects of assimilation in terms of their 87sr 86sr ratios the results of this study confirm conclusions based upon major oxide and mineral chemical data for the mvs lavas but more importantly show that careful analysis of trace element data allows the various processes involved in magma evolution to be identified and quantified even in the absence of major oxide and isotopic data finally it is reiterated that magma mixing and assimilation may be coupled processes in the magma chambers beneath many volcanic centers and recognition of this fact has profound implications for studies of magmas erupted at continental margins and through continental crust