reaction induced embrittlement of the lower continental crust field observations and geophysical data reveal a causal link between brittle seismic failure and eclogitization of the lower continental crust we present results from experimental deformation of plagioclase rich samples at eclogite facies conditions and quantify the link between rock rheology and the kinetics of the eclogitization reactions the deformation was ductile both in the absence of reaction and when the progress of eclogitization was fast compared to the imposed strain rate however when the reaction rate was relatively slow the breakdown of plagioclase into nanocrystalline reaction products triggered embrittlement highlighted by a high acoustic emission activity fluid induced plagioclase breakdown under eclogite facies conditions is an exothermic reaction accompanied by a negative change in solid volume this is similar to other mineral transformations that are known to trigger transformational faulting we demonstrate that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

dynamic rupture processes inferred from laboratory microearthquakes we report macroscopic stick slip events in saw cut westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory experiments were conducted under triaxial loading 1 2 3 at confining pressures 3 ranging from 10 to 100 mpa a high frequency acoustic monitoring array recorded particle acceleration during macroscopic stick slip events allowing us to estimate rupture speed in addition we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane is almost total in the breakdown zone for normal stress 75 mpa while the friction f recovers to values of f 0 4 within only a few hundred microseconds enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our post mortem microstructural analysis relationships between initial state of stress rupture velocities stress drop and energy budget suggest that at high normal stress leading to supershear rupture velocities the rupture processes are more dissipative our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes a power law scaling of the fracture energy with final slip is observed over eight orders of magnitude in slip from a few microns to tens of meters

archaean flow top alteration zones formed initially in a low temperature sulphate rich environment the 3 500 3 400 myr onverwacht group in the southern part of the barberton greenstone belt south africa includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1 komatiitic lavas showing spinifex textures chrome spinels and high ni and cr contents are common throughout the sequence2 the rocks have been subject to polyphase archaean deformation and low grade metasomatism2 4 at several positions within the onverwacht group fig 1 the tops of komatiitic lavas underlying chert beds are marked regionally by 1 50 m thick zones of silicification carbonatization and k2o enrichment fig 2 because these zones have been variously interpreted to represent the felsic tops of mafic to felsic volcanic cycles weathering horizons and shear zones their origin is germane to any interpretation of the stratigraphy structure and evolution of the barberton belt here we present evidence that they formed inlarge part by low temperature near surface alteration of komatiitic lavas during intervals of volcanic quiescence they do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault bounded structural slices

cataclastic deformation of garnet a record of synseismic loading and postseismic creep abstract microstructural evidence for episodic deformation of the uppermost plastosphere related to failure of the schizosphere in major seismic events has been previously reported from the sesia zone western alps with inferred extraordinarily high stresses and strain rates for the early stages of creep during stress relaxation at temperatures of about 300 350 c in some of these rocks crystals of the high strength mineral garnet embedded in a quartz matrix are intensely fragmented detailed optical electron microprobe and scanning electron microscopic study combined with electron backscatter diffraction techniques suggests that fragmentation of garnet took place quasi instantaneously at very high peak stresses subsequent deformation during stress relaxation caused displacement of the garnet fragments by frictional sliding controlled by dislocation creep in the surrounding quartz matrix in a final stage after restoration of a near lithostatic pore fluid pressure the garnet fragments were pulled apart and the opening cracks were filled by quartz and feldspar precipitated from the pore fluid this record appears to be consistent with the proposed model of synseismic loading and postseismic creep and the related pore pressure changes

elasticity of glaucophane seismic velocities and anisotropy of the subducted oceanic crust abstract upon subduction the oceanic crust transforms to blueschists and eclogites with seismic properties that gradually become similar to those of the surrounding mantle in order to evaluate the anisotropy of blueschists and glaucophane bearing eclogites the elastic constants of glaucophane single crystal plates from the sesia lanzo zone aosta valley western alps were measured using brillouin spectroscopy at ambient conditions the mean p and s wave velocities are 7 8 and 4 6 km s 1 respectively and the anisotropy is high 38 1 av p and 27 3 av s glaucophane develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 110 poles concentrated sub perpendicular to the foliation in both blueschist and eclogite rocks the measured lpo is in good agreement with viscoplastic self consistent numerical models seismic properties of glaucophane bearing blueschist and eclogite are calculated by combining measured lpo and the single crystal elastic moduli of glaucophane with the other main mineral constituents of the rock mostly epidote for blueschist and omphacite and garnet for eclogite blueschists present stronger anisotropies av p 16 1 and av s 10 3 than eclogites av p 2 9 and av s 1 7 the shear wave splitting and resulting delay times for a 7 km thick layer of eclogite or blueschist are low for the eclogite

garnet yield strength at high pressures and implications for upper mantle and transition zone rheology 1 garnet helps control the mechanical behavior of the earth s crust mantle and transition zone here measurements are presented suggesting that garnet long considered to be a high viscosity phase is actually weaker than the other dominant components in the transition zone the mechanical behavior of garnet at high pressures was examined using radial diffraction techniques in the diamond anvil cell the yield strength of grossular garnet was inferred from synchrotron x ray measurements of differential lattice strains the differential stress was found to increase from 1 3 0 6 gpa at a hydrostatic pressure 5 8 1 1 gpa to 4 1 0 4 gpa at 15 7 1 0 gpa where it was level to 19 gpa the strength results are consistent with inferred strength values for majorite garnet from measurements in the diamond cell normal geometry bolstering the idea that garnet structured materials may all have similar strengths in this low temperature high differential stress regime garnet is shown to be significantly weaker than anhydrous ringwoodite and to have a strength similar to hydrous ringwoodite this result suggests that the presence of water in the transition zone may not be required to explain a weak rheology and therefore models of transition zone behavior built assuming that garnet is the high strength phase may need to be revised

brittle precursors of plastic deformation in a granite an example from the mont blanc massif helvetic western alps abstract in the mont blanc helvetic massif granites record mesoscale alpine structures which include joints veins cataclastic to mylonitic shear zones and foliated granites a detailed structural analysis indicates that brittle deformation predates plastic strain joints never pass through and veins are offset by cataclastic shear zones and mylonites the mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions plastic deformation is first localized in the brittle discontinuities and the fine grained matrix of cataclasites then it involves the granite within brittle shear zones and this is initially accomplished mainly by flow of reaction softened aggregates of sericite widely replacing the strain supporting magmatic plagioclase the brittle to plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity and these characteristics are derived from reactivation of and focusing along pre existing brittle discontinuities in addition mylonites may inherit high angles of intersection and may contain granite porphyroclasts these features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

zaccagnaite 3r a new zn al hydrotalcite polytype from el soplao cave cantabria spain we have recently discovered significant amounts of zaccagnaite a natural zn al co 3 hydrotalcite in the el soplao cave north spain the el soplao zaccagnaite is speleothemic i e formed in the cave and therefore it represents a new cave mineral the origin of el soplao zaccagnaite is related to the diagenesis of zn and al rich ferromanganese speleo stromatolites where it occurs as a pore filling cement that likely precipitated at low temperature 11 c in some stromatolite layers the abundance of zaccagnaite crystals is large enough to enable their physical separation this has allowed us to obtain its x ray powder diffraction pattern infrared spectrum and differential thermal thermogravimetric profiles the cell parameters of the el soplao zaccagnaite refined from x ray powder diffraction data are a 3 06616 1 and c 22 6164 1 a 90 120 v 184 139 1 a 3 z 3 consistent with a new trigonal polytype of zaccagnaite zaccagnaite 3 r besides the el soplao zaccagnaite shows some features previously unknown in natural hydrotalcites such as octahedral like morphologies and fluorescence zoning electron microprobe analyses revealed that the el soplao zaccagnaite 3 r has an unusual chemistry for natural hydrotalcites as it is significantly more rich in al zn 2 al 3 1 6 than the hexagonal 2 h polytype zn 2 al 3 2 0 the simplified chemical formula deduced from electron microprobe analysis is zn 0 6 al 0 4 oh 2 co 3 0 2 0 5h 2 o where c and water were calculated by stoichiometry the carbon content calculated by stoichiometry 2 2 wt is in good agreement with that measured with the electron microprobe on gold coated samples 2 5 wt the presence of interlayer water and co 3 groups was confirmed by thermogravimetric analysis coupled to mass spectroscopy and by the analysis of the infrared spectrum

petrography and isotopic geochemistry of bedding parallel fibrous gypsum veins in the neogene qingshuiying formation of the ningnan basin north china insights for growth model of antitaxial fibrous veins abstract bedding parallel fibrous gypsum veins which are widely developed in the neogene qingshuiying formation of the ningnan basin a tertiary basin located in the northeastern margin of the tibetan plateau provide valuable information on stress field fluid origin and host rock deformation during their formation the systematic petrographic observation and isotope analysis of both of veins and wall rocks were carried out to investigate the formation mechanism of bedding parallel fibrous gypsum veins the distribution of fibrous gypsum veins shows vertical heterogeneity and mainly occurs in the middle and lower part of the qingshuiying formation with relatively high gypsum content single fibrous gypsum veins consist of a median zone and two fibrous zones the median zone is characterized by blocky gypsum crystals and multiple host rock fragments formed by crack seal events the fibrous zone is composed of parallel gypsum fibres a high angle to the vein walls indicating shear component vein opening is considered to be the result of fluid overpressure caused by horizontal compression in low permeability mudstone during basin inversion fibrous gypsum crystals grow continuously without obvious growth competition between adjacent crystals this indicates that growth of gypsum fibres occurs in limited space even without fractures the main driving force for vein dilation is force of crystallization of gypsum fibre growth and fluid pressure also has a certain contribution the sulfur and strontium isotopes of gypsum veins exhibit a high degree of consistency with wall rocks which indicates the material of the fibrous gypsum veins is derived from the wall rock a suitable growth model of bedding parallel fibrous gypsum veins in the ningnan basin is established which explains the process of vein interaction and the formation of v shaped or u shaped solid inclusions

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

coseismic damage runs deep in continental strike slip faults abstract coseismic off fault damage and pulverization significantly influence the mechanical and transport properties and in turn the rupture dynamics of faults although field based laboratory and numerical studies help elucidate the structure of damage zones adjacent to modern strike slip faults the vertical extent of these zones remains an open question to address this question we analyzed particle size distribution and microfracture density of fragmented garnets from the sandhill corner shear zone a strand of an ancient seismogenic strike slip fault system exhumed from frictional to viscous transition depths 400 500 c the shear zone has mutually overprinting pseudotachylyte and mylonite and juxtaposes quartzofeldspathic and schist units the inner parts of the quartzofeldspathic and schist units 63 m and 5 m wide from the lithologic contact respectively have two dimensional d values 1 5 which indicates dynamic fragmentation during rupture propagation similar to the particle size distribution analysis microfracture density data from the garnets show progressive but asymmetric increase toward the core in each unit our results suggest that coseismic damage extends down to the base of the seismogenic zone in mature strike slip faults and the asymmetric distribution of damage may indicate preferred rupture directivity as proposed for some modern strike slip faults

energy balance from a mantle pseudotachylyte balmuccia italy in the balmuccia massif nw italy a pseudotachylyte vein network n068 trending in a spinel lherzolite is interpreted as the product of frictional melting during a single m w 6 earthquake the subvertical fault underwent a metric dextral coseismic displacement raking 60 degrees sw the average width of the main slip surface is similar to 5mm a dense network of thin 20 200m injection and ultramylonite like veins decorates the fault walls in the injection veins raman microspectrometry mapping reveals pockets of still preserved amorphous silicate containing approximate to 1 of structurally bound h2o in the ultramylonite like veins electron backscattered diffraction mapping reveals that ultrafine 0 2 2m olivine grains exhibit a strong fabric with 010 planes parallel to shearing consistent with temperatures above 1250 degrees c during deformation and suggesting fast recrystallization from the frictional melt the veins also exhibit pyroxene and recrystallized spinel which proves that the earthquake occurred at a minimum depth of 40km the energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding i e dynamic friction coefficient s we argue that lubrication was only transient as the melt could rapidly flow into tensile fractures which led to rapid cooling and promoted strength recovery and sliding arrest combined together our observations suggest that this pseudotachylyte is the frozen record of a deep 40km earthquake of 6 m w 7 its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite like veins and deciphered by electron backscattered diffraction plain language summary in active mountain ranges rocky massifs gradually rise to the surface under the combined effect of convergence and erosion it is then possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep tens of millions of years ago thus it can be seen that contrary to what we experience on the surface the rock melts when it tears a magma forms and facilitates relative ground displacement the traces discovered by the geologists are the solidified remains of this magma here by field observation and detailed analysis of samples in the laboratory we describe the traces of an earthquake that occurred more than 40 km deep in the earth s mantle its magnitude was greater than 6 the sliding on the fault greater than 1 m and yet the trace is thinner than 5 mm and against all odds the most talkative witnesses are extremely small micrometric crystals which formed from the magma upon the rupture record crucial information unveiling its focal mechanism we still do not entirely know what an earthquake exactly is and nobody will ever be able to see what happens on the fault at depth during an earthquake hence these fossils are key

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

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

lithological control on fracture cementation in the keuper marl triassic north somerset uk the spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the triassic keuper marl formation uk a caprock for hydrocarbon reservoirs and co 2 sequestration the marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins the nodular gypsum in evaporite horizons provides excess gypsum for nodule rooted horizontal gypsum veins our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls we interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks based on stratigraphic correlations and much lower sr concentrations than gypsum nodules gypsum was transported to adjacent veins mainly through diffusion in the low permeability marls the localization of gypsum veins and varied sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non evaporite beds this results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements the study implies that the cementation of natural fractures in low permeability rocks can highly depend on the presence of cement minerals in the host rock

disappearing ink how pseudotachylytes are lost from the rock record abstract melt origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record however reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed in this paper we document the processes whereby pseudotachylytes are overprinted destroyed and otherwise removed from the rock record we present examples of recrystallized altered and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults based on these observations we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes our results imply that pseudotachylytes are vastly under reported due to their vulnerability to destruction and the resulting difficulty in identification as a consequence the significance of frictional melting is underestimated the criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record

eclogite breccias in a subducted ophiolite a record of intermediate depth earthquakes understanding processes acting along the subduction interface is crucial to assess lithospheric scale coupling between tectonic plates and mechanisms causing intermediate depth seismicity despite a wealth of geophysical studies aimed at better characterizing the subduction interface we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones we herein report the finding of eclogite breccias formed at 80 km depth during subduction in an almost intact 10 km scale fragment of exhumed oceanic lithosphere monviso ophiolite western alps these eclogite breccias correspond to meter sized blocks made of 1 10 cm fragments of eclogite mylonite cemented by interclast omphacite lawsonite and garnet and were later embedded in serpentinite in a 30 150 m wide eclogite facies shear zone at the mineral scale omphacite crack seal veins and garnet zoning patterns also show evidence for polyphased fracturing healing events our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust accompanied by the input of externally derived fluids we also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab intermediate depth earthquakes at 80 km depth

early pleistocene volcanism in the emile baudot seamount balearic promontory western mediterranean sea the recovery of calcareous breccia and other carbonate cemented rocks from mont dels oliva in the balearic promontory indicates that both this seamount and the adjacent mont ausias marc east of ibiza and formentera islands are of continental origin recent multibeam echosounder mapping and high resolution seismic reflection profiles indicate that the third high emile baudot seamount is located within a field of more than 118 pinnacles based on their magnetic signature and the recovery of basalt in the vicinity of the pinnacles suggest that the features are of volcanic origin a basalt sample dredged from emile baudot seamount indicates an igneous origin for this seamount the limited sampling the chemistry of the rock recovered and the size of the seamount itself 18 by 3 km however does not preclude the possibility that emile baudot may be a fractured uplifted continental block intruded by the basalt the k ar age of 1 46 0 18 my yielded by the basalt sample verifies the earlier proposal by acosta et al mar pet geol 128 2001a 349 that the volcanism that created part or all of emile baudot seamount and the southwest majorca volcanic field associated with the seamount took place in the pleistocene this volcanism is probably related to decompression resulting from the normal faulting that led to the subsidence of the low central depression between ibiza and majorca balearic islands

tidal asymmetry and residual circulation over linear sandbanks and their implication on sediment transport a process oriented numerical study received 12 january 2007 revised 24 july 2007 accepted 17 september 2007 published 22 december 2007 1 a series of process oriented numerical simulations is carried out in order to evaluate the relative role of locally generated residual flow and overtides on net sediment transport over linear sandbanks the idealized bathymetry and forcing are similar to those present in the norfolk sandbanks north sea the importance of bottom drag parameterization and bank orientation with respect to the ambient flow is examined in terms of residual flow and overtide generation and subsequent sediment transport implications are discussed the results show that although the magnitudes of residual flow and overtides are sensitive to bottom roughness parameterization and bank orientation the magnitude of the generated residual flow is always larger than that of the locally generated overtides also net sediment transport is always dominated by the nonlinear interaction of the residual flow and the semidiurnal tidal currents although cross bank sediment transport can occur even in the absence of a cross shore residual flow on the other hand net sediment divergence convergence increases as the bottom drag decreases and as bank orientation increases the sediment erosion deposition is not symmetric about the crest of the bank suggesting that originally symmetric banks would have the tendency to become asymmetric

faulting of natural serpentinite implications for intermediate depth seismicity abstract the seismic potential of serpentinites at high pressure was investigated via deformation experiments on cored natural serpentinite samples during which micro seismicity was monitored by recording acoustic emissions aes deformation was performed at pressures of 3 5 gpa using a deformation dia device and over a wide range of temperatures both within and outside antigorite s stability field below 400 c serpentinite deformation involves silent semi brittle mechanisms even in cases where strain localization is observed at high temperature i e above 600 c despite conditions propitious to dehydration embrittlement i e fast strain rates and reaction kinetics joint deformation and dehydration lead to ductile shear without generation of aes brittle behavior was observed in a narrow temperature window ca 500 c in this latter case aes are consistently observed upon faulting and extremely sharp strain localization is observed in recovered samples the resulting microstructures are consistent with the inverse ductile to brittle transition proposed by proctor and hirth 2016 in antigorite this may therefore be a source of seismicity in subducting slabs at mantle pressures and temperatures from 500 to 600 c however the acoustic signal observed here is orders of magnitude weaker than what is obtained at low pt conditions with brittle failure consistently with low radiation efficiency of serpentinite faulting prieto et al 2013 and suggests that other mechanisms are responsible for large intermediate depth earthquakes in fact the present results are in line with a recent study ferrand et al 2017 that suggests that intermediate earthquakes are likely induced by mechanical instabilities due to dehydration in partly hydrated peridotites

fibrous gypsum and fibrous anhydrite in veins fibrous calcium sulphate occurs in nature mainly as gypsum satinspar in extensional veins several mechanisms have been put forward to account for these veins and their fillings but a number of features remain unexplained or are not in accord with the interpretation s given this paper describes anhydrite as a fibrous vein precipitate which shows most features typical of satinspar subhorizontal attitude of the vein s vertical fibres a parting separating a lower from an upper seam and incorporated wall rock fragments new observations include the stacked nature of the fibres pressure shadows next to wall rock fragments and parabolic alignment of the latter these features suggest that the veins opened due to vertically oriented tensile stress the vein filling crystals grew centrifugally outwards from the initial plane of rupture which now forms the parting keeping pace with the dilation the mode of incorporation of wall rock fragments in the fibres requires repeated differential opening of the fissure thereby either wall rock fibre interface ruptured at a time and statistically the ruptures alternated so that trace element trends across the vein s are roughly symmetrical to the parting the source of the calcium and sulphate ions may have been adjacent evaporite beds but an external source is indicated where no evaporite beds are present in contrast to previous interpretations hydration of anhydrite to gypsum in the host rocks is not a prerequisite for the formation of satinspar veins the exact causes for the vertical dilation remain somewhat speculative and may have been different in various locations the vertically oriented tensile stress may have resulted from lateral tectonic compression or hydraulic overpressure or a combination of the two

laboratory earthquakes triggered during eclogitization of lawsonite bearing blueschist abstract the origin of intermediate depth seismicity has been debated for decades a substantial fraction of these events occurs within the upper plane of wadati benioff double seismic zones believed to represent subducting oceanic crust we deformed natural lawsonite rich blueschist samples under eclogite facies conditions 1 5 p 3 5 gpa 583 k t 1121 k using a d dia apparatus installed at a synchrotron beamline continuously monitoring stress strain phase changes and acoustic emissions aes two distinct paths were investigated i heating during deformation at pressures 2 5 gpa to maximum temperatures ranging from 762 to 1073 k during which lawsonite and glaucophane became gradually unstable while entering the stability field of lawsonite eclogite and the breakdown reaction of lawsonite was only crossed in case of the highest final temperature ii heating while deforming at a pressure

genesis of natural hydraulic fractures as an indicator of basin inversion abstract satin spar fibrous gypsum veins which occur in evaporite basins worldwide provide significant insights into host rock deformation and fluid flow although the genetic mechanism remains obscure satin spar veins in the red marls of the triassic mercia mudstone of the bristol channel basin were characterized in the context of regional and local setting the vein network in the keuper marl lower mercia exhibits a lack of systematic cross cutting between three distinct vein sets two sets of veins are observed in the overlying tea green marl with one set clearly crossing the other the gypsum veins commonly contain a blocky median zone of multiple thin bands of host rock inclusions and alabastrine gypsum crystals exhibiting crack seal patterns fibrous zones on either side of the median zone consist of pure parallel aligned gypsum fibres that are oblique to vein walls indicating a hybrid shear extensional mode of vein widening veins developed within reverse reactivated faults contain fibre lineations in the median zones and also on vein surfaces suggesting a minimum of two phases of fault slip the veins are interpreted to have formed as a result of overpressure in the low permeability mudstones by tectonic compression during basin inversion giving rise to the median zone growth of gypsum fibres commenced when the initial fractures were completely sealed the timing of vein formation is suggested to be miocene based on cross cutting relationships with folds formed during the latest phase of basin inversion because the fractures initiated in response to fluid overpressures they provide a useful analog to subsurface hydraulic fracture systems in low permeability rocks subjected to tectonic compression the fibrous widening of the veins post dates their initiation and led to continued propagation and increased connectivity

global rupture parameters for deep and intermediate depth earthquakes we investigate the global rupture parameters for deep and intermediate depth earthquakes from measurements of rupture duration and radiated seismic energy we estimate stress drop apparent stress and radiation efficiency and obtain a detailed earthquake energy budget from scaling of the source parameters we highlight differences between crustal and deep seismicity with the latter showing larger fracture energies the observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate depth events differs in agreement with previous studies we observe along strike variability of rupture properties for deep and intermediate depth earthquakes correlating with slab morphology plate age or presence of volcanic structures

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

how ductile are ductile shear zones three premises of ductile deformation in the middle and lower crust are widely accepted 1 rocks flow with power law viscous rheology 2 localization in ductile shear zones involves strain softening and 3 fluid flows into and is channelized within ductile shear zones ductile viscous shear zones should therefore initially develop along planes of maximum shear stress strain soften and rotate as material planes into the field of progressive extension however the mean stress or pressure within a weak elongate viscous band being stretched is higher than the surrounding matrix and this is difficult to reconcile with premise 3 in contrast mohr coulomb brittle faults always have lower pressure within the fault zone flow of fluid and melt into high temperature shear zones therefore implies that ductile shear zones are not perfectly viscous but have a pressure dependent viscoplastic rheology the continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger scale discrete fractures

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

dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust intermediate depth earthquakes in cold subduction zones are observed within the subducting oceanic crust as well as the mantle in contrast intermediate depth earthquakes in hot subduction zones predominantly occur just below the mohorovicic discontinuity these observations have stimulated interest in relationships between blueschist facies metamorphism and seismicity particularly through dehydration reactions involving the mineral lawsonite here we conducted deformation experiments on lawsonite while monitoring acoustic emissions in a griggs type deformation apparatus the temperature was increased above the thermal stability of lawsonite while the sample was deforming to test whether the lawsonite dehydration reaction induces unstable fault slip in contrast to similar tests on antigorite unstable fault slip that is stick slip occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed microstructural observations indicate that strain is highly localized along the fault r1 and b shears and that the fault surface develops slickensides very smooth fault surfaces polished by frictional sliding the unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate a thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones indicating the potential for unstable frictional sliding within natural lawsonite layers

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

experimental evidence for wall rock pulverization during dynamic rupture at ultra high pressure conditions abstract the mechanisms triggering intermediate and deep earthquakes have puzzled geologists for several decades there is still no consensus concerning whether such earthquakes are triggered by brittle or ductile mechanisms we performed a deformation experiment on a synthetic lawsonite bearing blueschist at a confining pressure of 3 gpa and temperatures from 583 to 1 073 k after deformation the recovered sample reveals conjugated shear fractures garnet crystals are dissected and displaced along these narrow faults and reveal micro and nanostructures that resemble natural pulverization structures as well as partial amorphization formation of such structures at low confining pressures is known to require high tensile stresses and strain rates and is explained by the propagation of a dynamic shear rupture the absence of shearing in the pulverized wall rock is taken as evidence that these structures pre date the subsequent heat producing frictional slip in analogy to observations at low pressure we infer that the garnet structures in our experiment result from rapid propagation of a shear fracture even at the high pressure exerted on the sample and thus suggest that brittle deformation is possible at lower crustal to upper mantle depths

rock pulverization at high strain rate near the san andreas fault rocks near the san andreas fault are pervasively crushed at distances of up to 400 m from its core laboratory experiments and calculations suggest that the rocks were pulverized at high strain rates 150 s 1 associated with a supershear rupture a rupture propagating at a velocity equal to greater than that of seismic shear waves

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

repeated events of hardground formation and colonisation by endo epilithozoans on the sediment starved pontine continental slope tyrrhenian sea italy abstract rock samples dredged offshore the pontine archipelago 400 2000 m water depth were studied by palaeontological analyses of planktic foraminifera and endo epilithozoan faunas colonising fe mn crusts and by petrographic analyses of volcanic samples palaeontological and petrographic analyses have revealed a complex plio pleistocene depositional history controlled by hemipelagic sedimentation and volcanic activity with episodic non depositional phases the sedimentary deposits are associated with ferromanganese hardgrounds more or less colonised by endolithic and epilithozoan organisms the presence of foraminifera typical of tropical and subtropical waters such as globigerinoides ruber globigerinoides trilobus pulleniatina obliquiloculata and sphaeroidinella dehiscens indicates that the micritic deposition occurred during warm climatic periods whilst micrite cementation and hardground formation are here attributed to colder climates the depositional processes recognised on the pontine continental slope reveal a sediment starved condition that locally resemble those of seamount environment the slope sedimentation found in the pontine archipelago can be helpful in the interpretation of ancient deposits from seamounts and continental slope depositional environments characterised by hardground and hiatuses

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

textural and chemical characteristics of diagenetic and hydrothermal alteration in glassy volcanic rocks examples from the mount read volcanics tasmania the mount black and sterling valley volcanics comprise a complex cambrian succession of originally glassy felsic to mafic submarine volcanic facies in western tasmania the mount black and sterling valley volcanics have been modified by the combined effects of diagenesis hydrothermal alteration and regional metamorphism metamorphosed diagenetic and hydrothermal alteration facies have been distinguished using a combination of alteration mineralogy overprinting relationships texture distribution intensity and whole rock geochemistry alteration facies that are interpreted to be the regionally metamorphosed equivalents of diagenetic clays and zeolites are regionally extensive and overprinted by the regional cleavage although widespread their distribution is patchy and their intensities are highly variable reflecting the complexity of the original textures in glassy volcanic rocks thin films of sericite carbonate and chlorite sericite hematite replace clays that coated original glassy surfaces at the onset of diagenesis prior to compaction feldspar quartz sericite chlorite sericite and chlorite sericite hematite alteration facies replace zeolites and clays that filled pore space and altered glass prior to and synchronous with compaction the final diagenetic alteration facies chlorite epidote began after compaction and continued during metamorphism diagenetic alteration involved significant mineralogical and textural changes but only minor changes in chemistry the chemical changes associated with diagenetic alteration facies are consistent with the interaction of rhyolitic and basaltic glass with seawater during burial alteration facies that are interpreted to be hydrothermal are local in distribution crosscut stratigraphic boundaries and involve significant changes in texture mineralogy and composition early hydrothermal alteration facies quartz sericite and calcite quartz hematite predated compaction and were synchronous with diagenesis chlorite calcite magnetite and chlorite pyrite alteration facies were synchronous with regional deformation and are associated with tectonic breccias faults and shear zones chlorite quartz calcite alteration facies postdated regional deformation