thermochronological investigation of fault zones abstract the timing of faulting episodes can be constrained by radiometric dating of fault zone rocks fault zone material suitable for dating is produced by tectonic processes such as 1 fragmentation of host rocks followed by grain size reduction and recrystallization to form mica and clay minerals 2 secondary heating melting of host rocks by frictional fault motions and 3 mineral vein formation as a result of fluid advection associated with the fault motions the thermal regime of fault zones consists primarily of the following three factors a regional geothermal structure across the fault zone and background thermal history of studied province bounded by fault systems b frictional heating of wall rocks by fault motions and c heating of host rocks by hot fluid advection in and around the fault zone thermochronological methods widely applied in fault zones are k ar 40ar 39ar fission track and u th methods for which methodological principles as well as analytical procedures are briefly described the thermal sensitivities of individual thermochronological systems are then reviewed which critically control the response of each method against the thermal processes based on the knowledge above representative examples as well as key issues are highlighted to date fault gouges pseudotachylytes mylonites and carbonate veins placing new constraints upon geological geomorphological and seismological frames finally the nojima fault is presented as an example for multiple applications of thermochronological methods in a complex fault zone

the structure of an exhumed intraplate seismogenic fault in crystalline basement abstract the 600 m thick gole larghe fault zone glfz is hosted in jointed crystalline basement and exposed across glacier polished outcrops in the italian alps ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes solidified frictional melts formed at 9 11 km depth and ambient temperatures of 250 300 c previous work focused on the southern part of the fault zone here we quantitatively document fault zone structure across the full width of the glfz and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike these new datasets indicate that the glfz has a broadly symmetric across strike damage structure and contains distinct southern central and northern zones distinguished by large variations in fracture density distribution of pseudotachylytes volume of fault rock materials and microfracture sealing characteristics the c 100 m wide central zone is bound by two thick 2 m and laterally continuous 1 km protocataclastic to ultracataclastic horizons within and immediately surrounding the central zone fracture density is relatively high due to cataclastic fault fracture networks that reworked earlier formed pseudotachylytes the fault fracture networks were associated with pervasive microcracking and fluid rock interaction resulting in the development of a c 200 m thick alteration zone delimited by lobate fluid infiltration fronts in the c 250 m thick southern and northern zones fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre existing magmatic cooling joints analysis of the structure of the glfz suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 au sable forks intraplate earthquake sequence in the northeastern usa including seismicity distributed across a fault zone 500 1000 m thick and large 100 mpa static stress drops associated with frictional melting

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

estimating thermal conductivity in sedimentary basins using lithologic data and geophysical well logs a method for estimating in situ thermal conductivity profiles in oil and gas wells is advanced to rectify a major shortcoming in thermal analyses of sedimentary basins thermal conductivity estimates are made in a two stage procedure and are based on a model for the conductivity of mixtures and input data from lithological and geophysical logs first rock matrix conductivity for an arbitrary depth interval i e drill cuttings sample interval which is about 3 m is determined from the laboratory calibrated conductivities and volumetric representation of its individual lithologic components using a geometric mean model in situ conductivity is then estimated by a second application of the model correcting for temperature and porosity determined from geophysical logs the method is illustrated for three uinta basin utah wells that penetrate a series of tertiary sandstones shales and muddy carbonates detailed lithologic descriptions together with sonic and neutron logs were digitized and used for estimating in situ conductivity the validity of the method was tested by comparing the prediction against laboratory measurements on 565 samples from the same wells rock matrix thermal conductivity ranges from 1 6 to 6 8 w m k and is predicted within 20 of the actual measurement for 90 of the samples both in situ conductivity values and variations for a given lithologic unit are reduced at increased porosity and increased temperature thermal conductivity nomograms are presented as useful tools to predict directly the in situ thermal conductivit of a formation from a well log signal

crystal fractionation in the friction melts of seismic faults alpine fault new zealand compositional variations are documented in friction melts along the hari hari section of the alpine fault new zealand with multiple stages of melt injection into quartzo feldspathic schists intermediate to felsic melts were heterogeneous in composition but all fractions show a common trend with a tendency for the younger melt layers and glasses to be more alkali na k and si enriched while being depleted in mafic fe mg mn components these changes are attributed primarily to crystal fractionation of the melt during transport farther traveled molten layers were on the whole less viscous mostly due to a higher melt to clast ratio however compositional change together with a decrease in volatile content produced a progressively more viscous liquid melt with time the glass phase is interpreted as a remnant of this high viscosity felsic residual melt that was preserved during final quenching following initial failure the formation of largely phyllosilicatederived volatile rich lower viscosity melt corresponds with a phase of fault weakening subsequent rapid crystal fractionation during melt transport the loss of volatiles and freezing of residual melt contributed to the strengthening of the fault during seismic slip d 2005 published by elsevier b v

melting and dissolution kinetics application to partial melting and dissolution of xenoliths kinetics of melting and dissolution rates and textures in magmatic systems is reviewed and applied to partial melting and dissolution of xenoliths special attention is paid to melting dissolution of solid solution crystals in which characteristic cloudy textures are formed and diffusion of elements in the crystals plays an important role due to compositional change of the crystals based on the above discussion model calculations are made for xenolith digestion using the haplogabbro system diopside plagioclase xenolith digestion can proceed by partial melting of the xenolith itself as well as dissolution in this case xenolith size is not important the digestion rates suggest that many xenoliths could survive only if they were incorporated into a host basaltic magma just prior to eruption natural partial melting found in a spinel lherzolite xenolith from kilbourne hole new mexico is also examined with additional heating experiments it is concluded that spongy clinopyroxene in the spinel lherzolite was formed by partial dissolution reaction between clinopyroxene and partial melt during partial melting at depth for a short run duration a month or less both the model calculations and the heating experiments show that even mafic to ultramafic xenoliths can easily undergo partial melting or react with basaltic melts and many xenoliths found in basaltic rocks could be incorporated into the basalts just prior to eruptions

geochemistry and thermodynamics of an earthquake a case study of pseudotachylites within mylonitic granitoid abstract pseudotachylites are melts produced by frictional heating during seismic slip understanding their origin and their influence on slip behavior is critical to understanding the physics of earthquakes to provide insight into this topic we conducted a case study in the proto mylonitic to mylonitic asbestos mountain granitoid in the eastern peninsular ranges batholith california which records both ductile mylonites and brittle deformation features pseudotachylites and ultracataclasites u pb chronology and zr thermometry of titanite porphyroblasts in the mylonites indicate that mylonitization of the plutons occurred at near solidus conditions 750 c over a 10 ma interval from 89 to 78 ma mylonitization resulted in recrystallization of quartz plagioclase and biotite with the biotite concentrated into biotite rich foliation planes subsequent brittle deformation is superimposed on the ductile fabrics micro xrf elemental mapping and in situ la icp ms analyses on these brittle deformation products show that the pseudotachylites are more mafic lower si but higher fe and k rich than the host mylonite while the ultracataclasites are intermediate between the host and the pseudotachylites inverse mass balance calculations show that both brittle deformation products are depleted in quartz but enriched in biotite with the pseudotachylites showing the most significant enrichment in biotite indicating preferential involvement of biotite during brittle deformation we suggest that biotite rich layers generated during ductile deformation may have been the preferred locus of subsequent brittle deformation presumably because such layers represent zones of weakness frictional heating associated with slip along such planes results in melting which causes a decrease in viscosity in turn leading to further strain localization during the short time span of an earthquake frictional melting appears to be a disequilibrium process in which the minerals are melted in order of their melting points from biotite 800 c to plagioclase 1400 c and finally to quartz 1700 c rather than by equilibrium melting which results in silicic eutectoid melts at lower temperatures 650 c thus with progressive slip melt composition should evolve from mafic to felsic eventually approaching the bulk composition of the host rock the mafic composition of the pseudotachylites thus indicates that they formed between the melting point of biotite and plagioclase 800 1400 c our chemical and modeling analyses on the pseudotachylites suggest that the chemical composition of pseudotachylites can potentially be used to constrain the thermodynamic conditions in the shear zone as well as earthquake source mechanics

fusion by earthquake fault friction stick or slip field observations of pseudotachylites and experimental studies of high speed friction indicate that melting on a slipping interface may sig nificantly affect the magnitude of shear stresses resisting slip we investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer thermodynamics of phase transitions and fluid mechanics results of laboratory experiments conducted at high order of m s slip velocities but low order of mpa normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength presumably due to viscous effects however extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses when transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction resulting in a nearly complete stress drop thermal runaway the amount of energy dissipation associated with the formation of pseudo tachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting clasts from a coarse grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of non equilibrium overheating the maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus via viscous braking and slip arrest or liquidus via thermal runaway and vanishing resistance to sliding our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz rich rocks especially at shallow 5 7 km depths where the driving shear stress is relatively low

centrifuge modelling of the evolution of low angle detachment faults from high angle normal faults abstract centrifuge models composed of a ductile layer overlain by a semi brittle layer are used to study how deformation localised by a high angle normal fault promotes detachment faulting during lateral extension driven by centrifugal force localised extension along a pre existing fault initiated localised isostatic upwelling of the denser lower layer where the lower ductile layer was significantly less dense than the semi brittle upper layer localised extension along the prescribed cut initiated upwelling of the ductile lower layer based on model results we argue that the transition from high angle normal faulting to low angle detachment faulting is an inevitable consequence of localising extension provided that there is viscous coupling between the extending upper layer and the upwelling lower layer in models with a lower layer of equal density or a denser lower layer this rotation takes place at the later stages of localised thinning in the upper semi brittle layer whereas in models with a less dense lower layer the rotation takes place earlier due to the buoyant rise of the ductile lower layer in areas of distributed crustal stretching e g rift basins where extension of the upper layer is accommodated by numerous steep faults distributed over a wide area and upwelling of lower ductile materials is distributed across the area normal faults remain more planar despite a large amount of extension models show that distributed extension along several closely spaced normal faults encourages rotation of blocks rather than their distortion to form listric faults we further conclude that the only configuration whereby localisation of extension would not result in detachment style faulting is when the upper and lower layers were completely decoupled

modelling the heat pulses generated on a fault plane during coseismic slip inferences from the pseudotachylites of the copanello cliffs calabria italy abstract a pseudotachylite vein network crosscutting late hercynian foliated tonalites can be observed along the copanello cliffs calabria southern italy pseudotachylites formed during the oligocene miocene at intermediate crustal levels ca 10 km they show variable thickness ranging from few mm up to 10 cm as observed in injection veins branching from the fault plane microscopic observations indicate that pseudotachylite matrix mainly consists of plagioclase an46 an58 and biotite microlites rounded clasts of quartz plagioclase or of plagioclase quartz lithic fragments are disseminated in the matrix intergranular flow and spherulitic textures are commonly observed microstructural features are consistent with rapid crystallisation from melt eds analyses of rare and tiny glass veins indicated a trachyandesite or an50 plagioclase melt composition the conditions for pseudotachylite formation were reproduced by an analytical model taking into account the heat released by friction along a horizontal fault plane during a seismic event the model is based on a three stage rupture history that includes nucleation propagation and stopping in addition by means of a numerical approach the model reproduces cooling that follows the stopping stage according to previous studies the thermal perturbation induced by fault displacement is very intense in fact temperatures exceeding the tonalite and even an50 plagioclase liquidus 1470 c are reproduced by small amount of slip 6 cm in suprahydrostatic regime on the other hand the thermal perturbation is strongly localised and of short duration peak temperatures abruptly decrease at a short distance from the fault plane typically in few millimetres in these conditions a thin film of melt can be produced therefore the presence of cm scale pseudotachylite veins can be only explained assuming an efficient and fast melt migration towards dilatant sites such as pull apart structures and injections veins results of the model may be useful to predict the thermal disturbance produced by earthquakes of low intensity

evidence of dmisteinbergite hexagonal form of caal2si2o8 in pseudotachylyte a tool to constrain the thermal history of a seismic event the determination of the maximum temperature achieved by friction melt tmelt in pseudotachylytebearing faults is crucial to estimate earthquake source parameters e g earthquake energy budgets coseismic fault strength on a geological basis here we investigated the mineralogy of a pseudotachylyte from the gole larghe fault italian alps by using x ray powder diffraction micro raman spectroscopy and eds equipped field emission scanning electron microscopy in particular we report the presence of the hexagonal polymorph of caal2si2o8 dmisteinbergite in a pseudotachylyte published experimental work shows dmisteinbergite can crystallize at 1200 1400 c by rapid quenching therefore the presence of dmisteinbergite in pseudotachylyte could be a reliable geothermometer for friction melts for which tmelt has only as yet been estimated

fault welding by pseudotachylyte formation during earthquakes melt produced by frictional heating can accumulate on slip surfaces and dramatically weaken faults by melt lubrication once seismic slip slows and arrests the melt cools and solidifies to form pseudotachylytes the presence of which is commonly used by geologists to infer earthquake slip on exhumed ancient faults field evidence suggests that solidified melts may weld seismic faults resulting in subsequent seismic ruptures propagating on neighboring pseudotachylyte free faults or joints and thus leading to long term fault slip delocalization for successive ruptures we performed triaxial deformation experiments on natural pseudotachylyte bearing rocks and show that cooled frictional melt effectively welds fault surfaces together and gives faults cohesive strength comparable to that of an intact rock consistent with the field based speculations further shear is not favored on the same slip surface but subsequent failure is accommodated on a new subparallel fault forming on an off fault preexisting heterogeneity a simple model of the temperature distribution in and around a pseudotachylyte following slip cessation indicates that frictional melts cool to below their solidus in tens of seconds implying strength recovery over a similar time scale

the destiny of a clast within a molten pseudotachylyte vein pseudotachylytes are important markers that can indicate the thermal state during a coseismic slip failure and provide indirect information about the level of stress and sliding velocities attained during that time window on the other hand survivor fragmented clasts embedded in the quenched material are also very impor tant in that they provide information about the energy spent to create new fracture surfaces in this paper i study the temperature evolution of clasts subjected to the heat dissipated by a just formed molten pseudotachylyte pt vein in particular i find the analytical solutions for the temperature evolution within the pt vein the surrounding undamaged host rock and inside a clast according to the proposed model the numerical results show that the clasts tend to preferentially melt in the inner part of the pt vein i e they are completely assimilated by the pt in contrast some clasts can survive far from the pt vein center my solutions although based upon a simplified model can provide a theoretical framework to predict the maximum size of the survived clasts at a given distance from the pt center the distribution of these survivor clasts follows a power law relation in terms of their radius and its features generally agree with field and laboratory observations

friction and roughness of a melting rock surface we acknowledge takehiro hirose for providing sample hvr 687 stefan nielsen was granted by the miur project fumo and giulio di toro and stefan nielsen were granted by the european research council starting grant project 205175 usems and ca ri pa ro we thank leonardo tauro for thin section preparation luca peruzzo steven smith and piergiorgio scarlato for sem facilities and toshi shimamoto for his constant support finally we are grateful to nick beeler and chris marone for their constructive reviews

pseudotachylite from the main zone of the hidaka metamorphic belt hokkaido northern japan pseudotachylite veins have been found in the mylonite zone of the hidaka metamorphic belt hokkaido northern japan they are associated with faults with wnw ese to ene wsw or ne sw trends which make a conjugate set cutting foliations of the host mylonitic rocks with high obliquity the mylonitic rocks comprise greenschist facies to prehnite pumpellyite facies mineral assemblages the mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near surface environments an analysis of the deformational and metamorphic history of the hidaka main zone suggests that the ambient rock temperature was 200 300 c immediately before the formation of the hidaka pseudotachylite three textural types of veins are distinguished cryptocrystalline microcrystalline and glassy the cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline type veins the microcrystalline type is largely made up of quench microlites of orthopyroxene clinopyroxene biotite plagioclase and opaque minerals with small amounts of amphibole microlites the interstices of these microlites are occupied by glassy and or cryptocrystalline materials the presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km the bulk chemical composition of the pseudotachylite veins is characterized by low sio2 and a high water content and is very close to that of the host mylonitic rocks this indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high al contents the presence of pigeonite and high al pyroxene microlites of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins the melt temperatures were estimated to be up to 1100 c using a two pyroxene geothermometer using published data relating water solubilities in high temperature andesitic magmas to pressure a depth estimate of about 4 km is inferred for the hidaka pseudotachylites evidence derived from pseudotachylites in the hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

experimental generation of volcanic pseudotachylytes constraining rheology volcanic systems are highly dynamic environments that comprise rocks and magmas which in the process of strain localisation e g in catastrophic flank collapse or dome building events are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes we evaluate the frictional behaviour of a plagioclase two pyroxene and glass bearing andesite and introduce an approach to constrain the rheology of frictional melts frictional slip at a rate of 1 3 m s under an axial stress of 1 5 mpa induces heating at a rate approximating 130 c per meter of slip and frictional melting of the andesite occurs at 1000 c which corresponds to a peak in shear stress with continuing displacement the shear stress decreases by 25 from the peak strength and stabilises while the temperature equilibrates to 1230 1290 c the shear stress for the system is congruent with a non arrhenian temperature dependent frictional melt rheology exhibiting a non newtonian viscosity evolving from approximately 104 2 to 103 4 pa s post experiment analysis show a gradation from 1 the host rock to 2 an extraordinary zone of unmelted yet viscously deformed material to 3 a thin outer region of chemically heterogeneous protomelts and to 4 an inner region of chemically homogeneous frictionally generated modified melt in the core of the slip zone we discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass bearing volcanic materials

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

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

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

mantle and crustal structure in phoebe regio and devana chasma venus 1 we use gravity and topography data to study crust and mantle structure in phoebe regio and devana chasma on venus we find that phoebe is a region of thickened crust and hotter than normal mantle these results are inconsistent with a cold downwelling model and may support a mantle plume origin for phoebe however the pattern of thermal anomalies is unusual with temperature maxima on the periphery of the highland rather than in its center this may reflect development of thermal boundary layer instabilities as the plume head interacts with the lithosphere the hot mantle anomaly beneath devana chasma is discontinuous near latitude 10 north coinciding with a 600 km offset in the rift zone trend devana should be regarded as two separate rifts one propagating south from beta regio and the other propagating north from phoebe regio our results support the view that venus remains a geologically active world

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

structural modelling of regional depth profiles in the voring basin implications for the structural and stratigraphic development of the norwegian passive margin interpretation of key regional seismic lines across the voring basin has identified a number of seismic megasequences that have provided a framework for further basin analysis these megasequences have been classified as pre syn and post rift with respect to the late cretaceous early tertiary phase of extension forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post rift megasequences reverse thermal subsidence modelling flexural backstripping of these lines to the base tertiary was performed using a laterally varying beta factor across the sections a number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections the backstripped templates were then forward modelled using the flexural cantilever model the forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques the modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models fault heaves and the extension needed to produce geologically acceptable backstripped profiles this observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the norwegian continental margin

superheated friction induced melts in zoned pseudotachylytes within the adamello tonalites italian southern alps pseudotachylytes and cataclasites are present along a strike slip fault zone in the tonalites of the adamello intrusion italian southern alps ambient conditions during faulting were 0 25 0 3 gpa and 250 300 c pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain the thickness of the microlitic domains x increases linearly with the half total thickness a of the pseudotachylyte vein commonly 2 cm according to the relation x 0 29 0 12 a the spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts a numerical model explains zoning as the result of the different cooling rates and clast melt interactions at the center and periphery of thick veins zoning is compatible with the injection of a single pulse of superheated friction induced melt tmelt 1450 c melt temperatures estimated by the clast matrix ratio o hara 2001 are considerably lower 316 577 c it is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites pseudotachylytes

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

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