pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

seismic slip in the lower crust inferred from granulite related pseudotachylyte in the woodroffe thrust central australia pseudotachylytes generated in granulite facies rocks are found in the woodroffe thrust central australia petrologic evidence and structural and textural features show that these pseudotachylytes contain typical melting origin features including injection occurrence rounded and embayed clasts and microlites within the fine grained matrix which formed contemporaneously with the wall granulite facies rocks under conditions of 8 kbar and 650 700 c these granulite related pseudotachylyte g pt veins are overprinted in subsequent deformation stages by three other types of pseudotachylytes including mylonite related m pt and ultramylonite related um pt pseudotachylytes that are then transcut by cataclasite related pseudotachylytes c pt the overprinting occurred following uplift of the lower crust where the g pt veins formed through the brittle plastic transition zone where the m pt and um pt veins occurred within the brittle dominated regime of the c pt generation zone in the upper crust the coexistence of multiple generations of large voluminous c pt m pt um pt and g pt veins indicates that numerous large earthquakes accompanying distinct seismic slip which produced the pseudotachylytes occurred over an extended period of seismicity and various depths of the crust within the woodroffe thrust zone the spatial distribution and structural features of the multiple stage pseudotachylytes suggest that coseismic slipping caused by large earthquakes which nucleated in the brittle dominated seismogenic zone propagated downward through the brittle plastic transition zone into the plastic dominated granulite facies depth from the upper to lower crust

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

the structural setting and deformation associated with pseudotachylite occurrences in the palala shear belt and sand river gneiss northern transvaal abstract two occurrences of shear related pseudotachylite from the limpopo metamorphic province are described in particular the structural setting as well as deformation textures in pseudotachylite inclusions and host rocks are presented for these occurrences in gabbroic anorthosite from the palala shear belt and dioritic to granodioritic rocks from the sand river gneiss locality the palala shear belt is a 13 km wide zone of intensely mylonitized rocks of the beit bridge and bushveld complexes situated at the southern boundary of the central zone of the limpopo metamorphic province the sand river gneiss comprises a bimodal suite of high grade dioritic and granodioritic rocks which were subjected to at least three periods of deformation in the context of comparisons between pseudotachylites from different geological settings i e cryptoexplosion structures meteorite craters and tectonic settings the major results are as follows 1 1 the deformation phenomena associated with pseudotachylite formation as recorded in quartz and feldspar occurring as inclusions or at the host rock contact are undulatory extinction in quartz brecciation mechanical twinning and kinking of plagioclase and characteristic formation of irregular and planar vredefort type fractures the latter occurring in quartz 2 2 frequency distributions of crystallographic orientations of planar fractures and their annealed manifestation planar fluid inclusion trails are very similar for the vredefort structure the sudbury structure the johannesburg dome the witwatersrand basin and the limpopo pseudotachylites it is therefore concluded that planar fractures in quartz are the typical microdeformation associated with pseudotachylite irrespective of the geological setting of the latter planar fractures alone do not provide unambiguous information on the genesis of the structure in which they are found

pseudotachylite generated as a result of a drilling burn in abstract material similar to naturally occurring pseudotachylite was generated in proterozoic lava as a result of frictional overheating of a diamond drill dh 5000 impregnated crown at a depth of 2 4 km below surface bit rotation equation to slip rates of about 1 m s is thought to have caused severe overheating to last for 3 10 s estimates show that melting occurred in the temperature range 800 1100 c under wet conditions and at comparatively low effective normal and shear stresses of about 1 9 107 and 9 7 106 pa respectively associated breccia formed as a result of thermal decrepitation rather than dynamic comminution and as a corollary it is suggested that some cataclasites associated with pseudotachylite bearing faults may also form by the mechanism this implies that the presence of both cataclasite and pseudotachylite on a fault plane does not necessarily indicate reactivation

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

holocene sea level and climate change in the black sea multiple marine incursions related to freshwater discharge events abstract repeated marine invasions of the black sea during the holocene have been inferred by many eastern scientists as resulting from episodes of marine inflow from the mediterranean beneath a brackish outflow from the black sea we support this scenario but a fundamental question remains what caused the repeated marine invasions we offer an hypothesis for the repeated marine invasions of the black sea based on 1 the overall similarity of sea level curves from both tectonically quiescent and active margins of the black sea and their similarity to a sequence stratigraphic record from the us mid atlantic coast the similarity of the records from two widely separated regions suggests their common response to documented holocene climate ocean atmosphere reorganizations coolings 2 the fact that in the modern black sea freshwater runoff from surrounding rivers dominates over evaporation so that excess runoff might have temporarily raised black sea level although the black sea would have remained brackish following the initial invasion of the black sea by marine mediterranean waters through the marmara sea in the early holocene repeated marine incursions were modulated or perhaps even caused by freshwater discharge to the black sea climatic amelioration warming following each documented ocean atmosphere reorganization during the holocene likely shifted precipitation patterns in the surrounding region and caused mountain glaciers to retreat increasing freshwater runoff above modern values and temporarily contributing to an increase of black sea level freshwater to brackish water discharges into the black sea initially slowed marine inflow but upon mixing of runoff with more marine waters beneath them and their eventual exit through the bosphorus marine inflow increased again accounting for the repeated marine invasions the magnitude of the hydrologic and sea level fluctuations became increasingly attenuated through the holocene as reflected by black sea level curves

selective melting processes as inferred from experimentally generated pseudotachylytes abstract high speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1 0 1 5 mpa and at slip rates up to 2 m s using a rotary shear high speed frictional testing machine the experimentally generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub angular to rounded fragments these locally occur as injected network veins in the fractured rock x ray diffraction analysis has revealed the presence of as much as 40 70 wt glass the mineral contents of clasts in the experimental pseudotachylyte in granite as determined by x ray diffraction analysis indicate that quartz is the most resistant to melting biotite the least and feldspar is intermediate thus the sio2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting

pan african displaced terranes in the tuareg shield central sahara concepts developed in the recently published model of the air region eastern tuareg shield niger africa as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the tuareg shield 500000 km 2 the 23 terranes recognized have their own lithological metamorphic magmatic and tectonic characteristics and are separated by subvertical strike slip megashear zones that can be traced for hundreds of kilometres or by major thrust fronts some of these boundaries have ophiolitic assemblages or molassic deposits the tuareg shield was shaped and partly accreted during the pan african orogeny 750 550 ma but not as a homogeneous body

geochemistry and isotope sr nd o study of al mg granulites from the in ouzzal archaean block hoggar algeria the in ouzzal al mg granulites are found within sedimentary units deposited after 2 7 ga the whole association being metamorphosed under extreme temperature conditions c 1000 c at 2 ga the al mg granulites are interlayered with other metasediments including metapelites quartzites and magnetite bearing quartzites forsterite spinel marbles and a few meta igneous rocks mainly pyroxenites they do not occur at a specific position in the sedimentary suite and they do not reflect any particular structural control the major and trace element compositions of al mg granulites especially the high cr ni co contents show that their peculiar refractory chemistry is more compatible with premetamorphic sedimentary characteristics rather than with metasomatic metamorphic or partial melting processes sedimentary admixtures of a common mature detrital component coming from the weathering of the local acidic igneous crustal protoliths normal pelitic component with an extremely immature component derived from reworking of basic ultrabasic lithologies al mg cr co ni rich chloritic component is consistent with the geochemistry of such rocks as in other instances the quartz garnet oxygen isotopic thermometer here records an apparent temperature close to the peak metamorphism c 1000 c although the persistence of pre existing 18o variations on a small scale during the metamorphism does not support a major pervasive fluid flow during metamorphism it does not rule out the presence of syn to post metamorphic co2 the low 18o c 5 to 6 of the most typical al mg granulites indicate that the chloritic component in these rocks was derived from hydrothermally altered mafic ultramafic protoliths rather than dominantly from palaeosols it is suggested that the presence of such al mg cr co ni rich sediments is indirect evidence for the presence of greenstone belts in the local crust of the in ouzzal at 2 6 2 7 ga

propagation of seismic slip from brittle to ductile crust evidence from pseudotachylyte of the woodroffe thrust central australia abstract the woodroffe thrust central australia is a 1 5 km wide mylonitized shear zone marked by large volumes of mm to cm scale pseudotachylyte veins the pseudotachylytes display typical melt origin features including rounded and embayed clasts spherulitic and dentritic microlites and flow structures within a fine grained matrix three types of pseudotachylyte are identified on the basis of deformation texture vein morphology and host rock lithology cataclasite related c pt mylonite related m pt and ultramylonite related um pt the m pt and um pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization these pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine grained vein matrix and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite these observations constrain the timing and environment of m pt and um pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust the m pt and um pt veins as well as the host mylonite are overprinted by cataclasis and multiple generations of late stage c pt veins that were generated in the brittle dominated regime of the upper crust during uplift and exhumation of the shear zone the coexistence of multiple generations of voluminous c pt m pt and um pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the woodroffe thrust the timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle dominated seismogenic zone and propagated down through the brittle ductile transition into the ductile dominated regime of the crust

frictional melting of sedimentary rock during high speed diamond drilling an analytical sem and tem investigation abstract high speed drilling at 900 rpm with a downhole motor driven natural diamond drag bit has resulted in the extreme alteration of sedimentary rock in an offshore canadian wildcat well these effects as revealed in the rock cuttings retrieved from the drilling fluid at surface include discolouration darkening cataclasis slickensiding and partial fusion calcareous siltstones are the most susceptible to alteration especially melting whereas sandstone and limestone cuttings mainly exhibit fracturing and surface glazing analytical scanning electron microscopy reveals that the calcareous siltstones have undergone decalcification via decarbonation and the loss of cao to the drilling fluid decarbonation combined with dehydration resulted in the vesiculation and explosive fragmentation of certain cuttings analytical transmission electron microscopy shows that glass is present in the calcareous siltstone cuttings most of the modifications exhibited by the sedimentary rocks can be attributed to the effects of high strain rate deformation and frictional heating at the bit rock interface that melting took place is supported by thermal calculations which indicate that for a coefficient of friction of 0 5 temperatures of approximately 1200 c were realized at diamond rock contacts such high temperatures can accelerate the wear of diamond and its substrate and hence shorten bit life as well as obliterate potentially important geological information in the rock cuttings

petrology of phlogopite sapphirine bearing al mg granulites from ihouhaouene in ouzzal hoggar algeria an example of phlogopite stability at high temperature al mg granulites with cordierite garnet sapphirine orthopyroxene sillimanite spinel phlogopite k feldspar plagioclase and variable quartz from ihouhaouene in ouzzal algeria display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite and of garnet the succession of parageneses suggests that the p t t evolution corresponds to decompression with cooling from peak conditions of about 950 c and 10 kbar this decompression path is obtained from the paragenetic analysis in the fmas system however according to current kfmash grids this p t t evolution should take place outside the stability field of phlogopite quartz yet this assemblage is probably stable during most of the p t evolution notably during peak metamorphism this discrepancy is interpreted as the effect of the high content of f in phlogopite which should shift its stability limit towards higher temperature the consequences of this shift on the phase relationships in the kfemash system are investigated and it is concluded that a topological inversion could exist in the f bearing system

a review of the in ouzzal granulitic terrane tuareg shield algeria its significance within the pan african trans saharan belt the in ouzzal terrane iot or in ouzzal granulite unit iogu is an elongated palaeoproterozoic block within the neoproterozoic pan african belt of north west africa the granulites derive from archaean protoliths that include a large volume of metasediments which were deposited on a 3 2 ga gneissic basement near peak granulite facies conditions between 2 17 and 2 ga were estimated at p 10 kbar and t rising from 800 to 1000 c premetamorphic orthogneisses were intruded at 2 5 ga and followed by the emplacement of syn to late kinematic charnockites syenites and carbonatites at around 2 ga cooling of the granulites occurred till 1800 ma shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas the iot was buried beneath late palaeoproterozoic and neoproterozoic cover sequences and then behaved as a rigid block both margins are lithospheric faults as evidenced by the occurrence of shear zone related mafic and felsic plutons pan african tectonothermal events were negligible in the north but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern pan african terranes the cambrian molasse associated with widespread alkaline volcanism and subvolcanic granites is horizontal in the north the iot which was part of a larger continental mass including its counterpart in northern mali is interpreted as an exotic terrane which may have docked during pan african plate convergence and lateral collision the unchanged pediplain since c 1 7 ga in the north suggests that the iot is underlain by thick palaeoproterozoic lithospheric mantle whereas its southern part is probably allochthonous and overlies pan african structural units

transcrystalline shear fracturing and pseudotachylite generation in a meta anorthosite harris scotland abstract the meta anorthosite is locally deformed by brittle shear fracturing which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures the intervening rock being highly deformed in both a plastic and brittle way in most cases an e w compression on gently dipping to steep reverse shear planes occurs which we relate to a caledonian thrust zone in places the highly deformed rock is cut by pseudotachylite veins which locally form networks the pseudotachylite is generally intrusive but does not appear to be related to movement on major slip surfaces very locally it may have formed in situ pseudotachylite only occurs in highly deformed rock is only very occasionally deformed itself and thus generally represents at each locality the last stage of a complex deformation history as if its presence welded the rock and prevented further deformation these striking differences from the country rock gneisses in which pseudotachylite occurs on well developed fractures in very slightly deformed rock are considered to be due to the low anisotropy of the meta anorthosite to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase the source of the heat necessary to generate the pseudotachylite melt is not clear it may come from crack propagation as well as frictional sliding

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

a multi proxy paleolimnological reconstruction of holocene climate conditions in the great basin united states article i nfo article history received 11 february 2009 available online 14 august 2009 a sediment core spanning 7000 cal yr bp recovered from stella lake a small sub alpine lake located in great basin national park nevada was analyzed for subfossil chironomids non biting midges diatoms and organic content estimated by loss on ignition loi subfossil chironomid analysis indicates that stella lake was characterized by a warm middle holocene followed by a cool neoglacial period with the last two millennia characterized by a return to warmer conditions throughout the majority of the core the stella lake diatom community composition is dominated by small periphytic taxa which are suggestive of shallow cool alkaline oligotrophic waters with extensive seasonal ice cover a reconstruction of mean july air temperature mjat was developed by applying a midge based inference model for mjat two component wa pls consisting of 79 lakes and 54 midge taxa rjack 2 0 55 rmsep 0 9 c comparison of the chironomid inferred temperature record to existing regional paleoclimate reconstructions suggests that the midge inferred temperatures correspond well to regional patterns this multi proxy record provides valuable insight into regional holocene climate and environmental conditions by providing a quantitative reconstruction of peak holocene warmth and aquatic ecosystem response to these changes in the great basin a region projected to experience increased aridity and higher temperatures

geochemistry and 40ar 39ar geochronology of pseudotachylyte associated with uhp whiteschists from the dora maira massif italy abstract in situ uv laser ablation 40 ar 39 ar geochronological and geochemical data together with rock and mineral compositional data have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the uhp whiteschists of the dora maira massif italy several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation whole rock xrf data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss the pseudotachylyte contains newly crystallized grains of biotite and k feldspar in a matrix of glass with partially fused grains of quartz zircon apatite and titanite electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer scale changes in mineralogy uv laser ablation icp ms traverses across the mylonitic gneiss pseudotachylyte contact are consistent with cataclastic communition of ree carriers such as epidote monazite allanite zircon and apatite before melting as an efficient mechanism of ree homogenization in the pseudotachylyte the 40 ar 39 ar data from one band of pseudotachylyte indicate formation at 20 1 0 5 ma when the mylonitic gneisses were already in a near surface position the variable effects of top to the west shear deformation within outcrops of the coesite bearing unit are reflected in localized zones of protomylonite cataclasite ultracataclasite and pseudotachylyte preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the uhp rocks it is speculated that deeper crustal seismic events potentially played a role in the unroofing of the uhp rocks at earlier stages in their exhumation history

generation of pseudotachylite under granulite facies conditions and its preservation during cooling discontinuous ultramylonite zones cut proterozoic granulite facies gneisses in macrobertson land east antarctica and preserve evidence of ductile non coaxial flow and reverse sense of shear cross cutting relationships indicate that ultramylonite deformation involved overthrusting to the east but progressively rotated to involve overthrusting to the north rotation of the principal compressive stress axes is inferred extensive pseudotachylite developed during ultramylonitization the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation neoblastic sillimanite indicates ultramylonitization occurred at 520 c on the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at 700 c and 7 3 0 5 kbar at ah2o 0 3 and low aco2 comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the rayner complex experienced a late increase in pressure of 1 2 kbar during ultramylonitization the p t ah2o conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates most of the pseudotachylite veins are undeformed the mechanism s of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite further strain having been mostly accommodated by a new or subsidiary shear zone renewed stress at reduced strain rates or renewed stress in zones in which the proportion of pseudotachylite was significantly higher could have led to the rare occurrences of deformed pseudotachylite the preservation of fine grained pseudotachylite is dependent on it remaining dry

sidewall ripouts in strike slip faults abstract distinctive splay fault configurations in wallrocks associated with pseudotachylyte bearing strike slip fault structures at three different localities are recognized as sidewall ripouts sidewall ripout structures occur as asymmetric doubly tapered fault bounded lenses or slabs adjacent to a dominant planar fault surface scallop shaped lens ripouts develop in isotropic or weakly anisotropic host rocks and elongate slab ripouts develop in strongly anisotropic host rocks the tapered ends of these structures consist of leading contractional p type ramp structures at a low angle to the dominant planar fault surface and trailing extensional r or x type ramp structures at higher angles to the dominant fault surface the formation of ripout structures is attributed to transient variations in the coefficient of friction that cause adhesion between fault surfaces leading to rupture of the sidewalls ripouts are interpreted as large outcrop scale examples of adhesive wear during rupture of the sidewalls developed along planar fault surfaces during slip the asymmetry of the ripout structures in these exposures allows their use as a kinematic indicator where the narrow tapered leading end points in the direction of movement of the ripout lens or slab during adhesion to the opposing fault block wall

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

pseudotachylite associated with a bedding parallel fault zone between the witwatersrand and ventersdorp supergroups south africa a proterozoic bedding parallel fault zone is described from the witwatersrand basin in south africa the fault zone is dominated by pseudotachylite the youngest tectonite present but also contains quartz veins and cataclasites which post date the spatially associated mylonites both tectonic eliminations and duplications are caused by the fault zone which is dominated by a northerly to westerly overthrusting the fault rocks probably did not form during a single event but as a result of a minimum of two periods of activation of the fault zone separated in time by at least the period required to deposit in excess of 4 km of sediments belonging to the lower part of the transvaal sequence

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

large volumes of anhydrous pseudotachylyte in the woodroffe thrust eastern musgrave ranges australia a mylonitic thrust zone at least 1 5 km thick forms a sharp contact between granulite and amphibolite facies gneisses in the eastern musgrave ranges central australia the thrust dips gently to the south and is interpreted as an extension of the woodroffe thrust which was formed about 550 ma ago mylonites at the base of the thrust grade upwards into ultramylonites which pass abruptly into a pseudotachylyte bearing zone approximately 1 km thick containing approximately 4 of pseudotachylyte veining the orientation of the veins appears to be random pseudotachylytes occur only in the granulite facies rocks and their precursors are felsic pyroxene and or garnet granofelses rotated blocks of ultramylonite are present in some of the pseudotachylytes and some pseudotachylyte veins have been plastically deformed suggesting nearly contemporaneous semiductile and brittle behaviour the matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites also present are glass devitrification microstructures spherulites evidence of liquid flow and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material these features confirm that the pseudotachylyte formed by melting in anhydrous conditions the matrix of the pseudotachylyte veins is less siliceous than the host rocks owing to non equilibrium melting of pyroxene garnet and plagioclase the igneous assemblages of the melt notably the crystallization of pigeonite are consistent with rapid cooling from very high temperature 1000 c melting and quenching is probably due to very local short lived rises in temperature accompanied by dilation

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

pseudotachylyte from an ancient accretionary complex evidence for melt generation during seismic slip along a master decollement pseudotachylyte is present in the shimanto accretionary complex in shikoku japan the pseudotachylyte occurs as a vein in a fault zone and displays flow structure tio 2 microlites and other melting related textures the mineral assemblage of the pseudotachylyte suggests that the temperature was raised at least 450 c above the background levels in the host rocks this is the first direct evidence of seismogenic fault rock from an accretionary complex the pseudotachylyte bearing fault is a roof thrust above underthrusted melange which is overlain by an offscraped package of coherent units the geologic setting of the pseudotachylyte is consistent with melt generation during seismic slip along the master decollement in the subduction zone

p t x relationships in the precambrian al mg rich granulites from in ouzzal hoggar algeria the al mg rich granulites from the in ouzzal craton algeria show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the feo mgo al2o3 sio2 system the sequence of mineral reactions can be subdivided into three distinct stages 1 a high p stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel bearing assemblages 2 a high t stage characterized by the development of sa qz bearing assemblages and 3 a later stage in which garnet bearing assemblages are replaced by more or less fine symplectites involving cordierite during the course of early mineral reactions the distribution coefficient kd between the various ferromagnesian phases decreased significantly whereas al2o3 in pyroxene increased concomitantly these observations when combined with topological constraints clearly indicate that the high p stage 1 was accompanied by a significant rise in temperature estimated at 150 50 c under near isobaric conditions in agreement with the reaction textures by stage 2 pressure and temperature were extreme as evidenced by the low kd value between orthopyroxene and garnet kd 2 06 1 99 the high alumina content in pyroxene up to 11 8 and the high magnesium content in garnet 100 mg mg fe 60 6 mineral thermometry based on fe mg exchange between garnet and pyroxene and on al solubility in pyroxene gives temperatures close to 970 70 c at 10 1 5 kbar these results are in agreement with the development of sa qz assemblages on a local scale late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar except for local re equilibration of mg and fe at grain boundaries there is no evidence for further reactions below 700 c we interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c 2 ga because of the lack of evidence for further uplift after the thermal relaxation which occurred at c 6 kbar it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation

petrology of pseudotachylytes from the alpine fault of new zealand abstract the petrology of pseudotachylytes derived from contrasting host rocks within the alpine fault of new zealand a well known dextral shear zone is presented the structural observations indicate that parallel synthetic shears formed first in a non dilational system at low angles to the alpine fault then between these shears fracturing and frictional heating occured according to a new dilational riedel system accompanied by listric faults the observations suggest that pseudotachylyte melt is mainly formed in the listric faults and channelled into neighbouring areas of extension pseudotachylyte results from in situ melting and its nature depend strictly on the parent rock mineralogy the microlitic nature of the pseudotachylites is confirmed and the variability of the textures explained in terms of degrees of undercooling the morphology and composition of the minerals in the pseudotachylyte plagioclase biotite and amphibole are documented they crystallized at a higher temperature and pressure than their parent rock minerals the melting process is believed to be governed by the mechanical properties of mafic minerals in a fluid free environment at the beginning of the melting mafic minerals provide water which allows the development of the pseudotachylyte

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

les pseudotachylites en conditions granulitiques de la zone de cisaillement paleoproterozoique de tikachitine terrane d in ouzzal bouclier touareg algerie le terrane paleoproterozoique d in ouzzal appartenant au hoggar occidental affleure au sein de la chaine panafricaine trans saharienne du nw de l afrique il montre les parties les plus profondes d une croute continentale formee durant l archeen et recyclee a l eburneen sous les conditions de facies granulite de tres haute temperature les orthogneiss charnockitiques archeens impliques dans la zone de cisaillement eburneenne de tikachitine montrent des veines d injection de pseudotachylites fentes de tension et echelon de structures en pull apart ces pseudotachylites presentent des contacts francs avec l encaissant charnockitique et des matrices sombres parfois litees incluant de nombreux clastes de pyroxene feldspath quartz et rarement de biotite les associations minerales presentes dans les matrices et en aureoles autour des clastes sont de type facies granulite et similaires a celles des orthogneiss charnockitiques excepte l absence de mesoperthite elles sont composees de tres fins crist

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process

microlite morphology and chemistry in pseudotachylite from the fuyun fault zone china microlites in pseudotachylite veins from the fuyun fault zone northwest china can be divided into four morphological groups simple skeletal dendritic and spherulitic from the margin to the center of microcrystalline and microlitic pseudotachylite veins the size of single crystals or crystal fibers becomes progressively larger from a few m to as much as 15 m the morphologies of microlites vary from simple shapes such as acicular and globular forms to more complex forms such as skeletal dendritic and coarse spherulitic forms in glassy pseudotachylites only simple and fine spherulitic microlites generally smaller than 5 6 m occur these microlites have a varied mineralogy but are mainly made up of plagioclase an20 70 and alkali feldspar including sanidine and anorthoclase as well as clinopyroxene grossular garnet biotite hornblende and some magnetic minerals the biotite and hornblende microlites have a variable ti content generally higher than that of biotite and hornblende frag