a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

effect of partial melting on rock deformation experimental and natural evidences on rocks of granitic compositions abstract deformation tests at constant strain rate 10 5 s 1 at 210 250 mpa confining pressure and at 560 900 c are performed on an aplite with varying water contents the mechanical behaviour is correlated with the microstructure after a stage of plastification depending on the temperature and on the water content the rock strength falls dramatically under such confining pressures before melting and before straining the pores of the wet rocks burst as the thermal expansion of water develops a local high pore pressure which causes cracking and fracturing in our opinion this pore bursting will increase the effects of thermal cracking due to the differential expansion of the minerals the rocks exhibit a uniform crack network with increasing temperature melting occurs in the cataclased rock the crack development and melting depend on the initial water content the loss of cohesion obtained either by cracking or by cracking and melting provokes a dramatic weakening axial shortening induces cracking parallel to 1 and melt transfer into extensional regions deformation is concentrated in dilatant shear bands at the highest temperature 900 c a melt film network is rapidly established and induces a very low flow stress melt effects in naturally deformed gneisses suggest that biotite breakdown furnishes a silicate fluid which induces hydraulic fracture and melt dispersion

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

thermal modeling of convergent and extensional tectonic settings for the development of low grade metamorphism in the welsh basin the lower paleozoic welsh basin of the u k caledonides shows a metamorphic transition from zeolite to low greenschist facies for which there are two radically opposing models of development the traditional model is of a syntectonic metamorphism associated with crustal thickening during basin inversion and cleavage development the alternative model links the metamorphism not to traditional ideas of crustal thickening but to thermal anomalies associated with the extensional development of a back arc basin these two models are tested by applying quantitative thermal modeling to crustal thickening and crustal extension tectonic settings specific to the welsh basin this basin has a unique and very well constrained record of its stratigraphic sedimentological and chronologic histories that is used here to provide the framework from which the dynamics of crustal evolution can be modeled crustal thickening models suggest that a higher pressure facies series with a pumpellyite actinolite to greenschist facies transition should be dominant in contrast extensional modeling suggests that a low pressure fades with a transition from prehnite pumpellyite to greenschist facies should be dominant the thermal evolution of the extensional setting is more compatible with the lower pressure metamorphic series that is recorded in metabasites as well as pelitic rocks of the region the metamorphic evolution is regarded as developing initially as a burial style which is then over printed by fabric development while at or near peak p t conditions thus giving an apparent syntectonic style of metamorphism

short paper low grade metamorphism of the welsh basin lower palaeozoic succession an example of diastathermal metamorphism the lower palaeozoic succession of the welsh basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development while mineralogical characters indicate a low pressure facies series in order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting temperatures remained relatively high during final closure associated with deformation of the basin sediments and consequent cleavage development such a model adequately resolves previously conflicting interpretations of burial and deformation related metamorphism

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

frictional melt pulses during a 1 1 ma earthquake along the alpine fault new zealand our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine grain size of frictionally fused rock and the lack of detailed high resolution microscopy studies that describe features at the crystal lattice scale in such a study we here report the complex nature of melting and crystallization in a symmetrically layered pseudotachylyte vein from the alpine fault new zealand two melt pulses are recognized attributed to successive but rapid injections of frictionally generated material the initial injection preserved at the vein margins was proximally derived and contains a high concentration of clasts and a si rich glass this was quickly followed by a second generation of a more distally derived melt dominated fraction which was injected into the weak vein center whereas fragments of wall rock biotite are preserved in the marginal zones neocrystallized microlitic biotite characterizes both margins and center the vein biotite is different in composition microstructure and polytypism from the metamorphic biotite of the wall rock in all melt layers newly formed biotite shows notable signs of syn flow crystallization strain features and erosion at crystal glass contacts with breakdown of neocrystallized microlites along both crystal edges and faces these characteristics imply cyclic pulses of heating melting and crystallization occurred during a single large earthquake episode and probably reflects the stick slip propagation properties of coseismic faulting 40 ar 39 ar total gas ages from the vein center give a 1 11 0 04 ma date for cyclic melting which based on current exhumation rates occurred at a crustal of depth v11 km 2003 elsevier science b v all rights reserved

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

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

metamorphic and post metamorphic fluid flow in the low grade rocks of the harlech dome north wales a combination of fluid inclusion stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during caledonian low grade metamorphism of the harlech dome north wales fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous na k cl brine but in the graphitic clogau formation and in parts of the overlying maentwrog formation immiscible h2o rich and ch4 rich fluids coexisted late stage inclusions are of calcium rich brine and a dilute aqueous fluid the chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz oxygen isotopic compositions show considerable variation between different units both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism after the metamorphic peak there was focused flow of fluid upward through the sequence along fractures in response to end caledonian uplift and unloading where the migrating fluid crossed the graphitic shales interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the dolgellau gold belt subsequent to this mineralizing event there was widespread development of 18o enriched calcites and micas in the case of vein minerals it is possible that these crystallized directly from late stage fluids at lower temperature than the quartz in the same veins alternatively the original vein minerals may have re equilibrated with later 18o enriched or cooler fluid in the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re equilibration of initially light grains with an introduced fluid requiring considerable influx of fluid this event may relate to either of two late stage fluids observed as inclusions

formation and modification of metastable intermediate sodium potassium mica paragonite and muscovite in hydrothermally altered metabasites from northern wales

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

vesicles amygdales and similar structures in fault generated pseudotachylytes amygdales in fault generated pseudotachylytes from the outer hebrides thrust scotland and the ikertoq shear belt west greenland contain mineral assemblages dominated by k spar sphene epidote quartz and carbonates respectively these contrasting assemblages indicate that vesicle infilling took place under differing environmental conditions other filled cavities resembling amygdales in form and mineralogy are recognized as replacement pseudomorphs after porphyroclasts using published data relating vesicle abundance and solubility functions for h2o and co2 to pressure a palaeoseismic depth estimate equivalent to the depth of vesiculation of about 1 6 km is inferred for the greenland pseudotachylytes described in this paper

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

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

ubiquitous burgess shale style clay templates in low grade metamorphic mudrocks despite the burgess shale s british columbia canada paleobiological importance there is little consensus regarding its taphonomy its organic fossils are preserved as compressions associated with phyllosilicate fi lms clay templates debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism meaning that it is important to establish the timing of their formation relative to decay an early diagenetic origin has been proposed based on anatomy specifi c variations in their composition purportedly refl ecting contrasts in decay however we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low grade metamorphism such phyllosilicates may also occur within voids created by volume loss in maturation a process that may have aided their formation in bedding plane assemblages from graptolitic mudrocks different taxa are associated with distinct phyllosilicates this likely refl ects stepwise maturation of their constituent kerogens in an evolving hydrothermal fl uid with different hyllosilicates forming as each taxon progressively underwent maturation these observations provide an analogue for the distribution and composition of phyllosilicates on burgess shale fossils which we interpret as refl ecting variations in the maturation of their constituent tissues thus their clay templates seem unremarkable forming too late to account for exceptional preservation

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

turbidite pathways pore fluid pressures and productivity in the central wales orefield abstract lodes in the central wales orefield are mineralized oblique slip normal faults that postdate the regional joints they developed shortly after the acadian deformation of the welsh basin in the late early devonian and many were reactivated with further mineralization in the early carboniferous although orefield extent is clearly centred on the structural culmination around the plynlimon dome the major areas of lead zinc production are strongly off centered these areas lie on the depositional pathways of three horizons of late ordovician early silurian massive turbidite sandstones it is argued that these basin floor sand bodies are preferential sites of lode nucleation and that their distribution controls the size and abundance of the lodes and hence their chances to link laterally and at depth this in turn controls their ability to capture and transmit mineralizing fluids and thus their chances of hosting large ore deposits the prime cause of high fluid pressure gradients during the first phase of mineralization was rapid uplift of tight9 rock with dehydration fluids from acadian metamorphism second phase fluids probably originated as seawater and were driven by convective overturn at essentially hydrostatic pressures the stratigraphic control of lode productivity has implications for mineral exploration in such terranes

injection veins of crushing originated pseudotachylyte and fault gouge formed during seismic faulting this paper describes the injection veins of crushing originated pseudotachylyte and fault gouge formed along the iida matsukawa fault in granitic cataclasite southern nagano prefecture central japan the pseudotachylyte which shows a dense and aphanitic appearances and fault gouge occur as simple veins fault vein along the main fault plane and as complex network veins injection vein in the neighboring cataclasite locally the injection veins of pseudotachylyte and fault gouge can be traced back to the fault veins powder x ray diffraction patterns and petrological analysis indicate that both of the pseudotachylyte and fault gouge consist entirely of fine grained angular clasts and that the pseudotachylytes and fault gouges have a similar x ray diffraction patterns with that of the host granite the similarity of chemical compositions and distribution patterns of grain size also show that the injection veins of pseudotachylyte and fault gouge have the same source material as that of fault veins field occurrences and petrological characteristics strongly suggest that the injection veins of pseudotachylyte and fault gouge formed during seismic faulting by a rapid intrusion and fluidization of crushed fine grained materials generated in the shear zone in a gas solid fluid system

collision and extrusion at the kuril japan arc junction hokkaido japan is a region where collision tectonics is ongoing at an arc arc junction and where lower crust is presently exposed as a result of the collision of the forearc sliver of the kuril arc with the northeast japan arc deep seated ductile deformation in metamorphic rocks has already been described but more shallow deformation is not sufficiently well observed to clarify a total history of the collision we focus on the shallow deformation in the cape erimo area in central hokkaido and discuss in detail the process of collision at this arc arc junction four phases of deformation are discriminated in the study area as follows 1 ene wsw extension immediately after sediment deposition 2 nw trending dextral strike slip 3 ne sw compression in association with the formation of conjugate sets of strike slip faults and 4 e w compression represented by the formation of low angle reverse faults the first and the second phases of faulting may be a manifestation of the oligo miocene dextral transform plate boundary between the eurasia and okhotsk plates a conjugate system of strike slip faults formed in the third phase newly clarified in this study by ne trending horizontal compression nearly normal to the nw trending dextral strike slip fault zone of the previous phase development of the strike slip faults especially the predominance of sinistral strike slip faults in the southern half of the arc arc junction must be explained by extrusion in association with collision of the forearc sliver of the kuril arc such extrusion may be a common tectonic feature even in the case of a small scale collision at an arc arc junction

geochemical and petrographic variations in pseudotachylyte along the outer hebrides fault zone scotland the islands of the outer hebrides scotland host a c 200 km long fault zone that was active between 1100 and 250 ma during which time a belt of fault rock several kilometres wide was created the fault zone hosts crush breccias phyllonite mylonite and pseudotachylyte the latter unit is often found in major fault zones and within meteorite impact structures although pseudotachylyte is widely believed to be a product of frictional melting within a fault zone there is still controversy over the specific mechanisms of formation pseudotachylyte was collected from the southern block of the outer hebrides fault zone a geochemical mixing model was applied to reconstruct the geochemistry of the pseudotachylyte matrix using the locally available lithologies the results show that the pseudotachylyte could have been derived from the local country rock although some mixing was necessary to provide accurate reconstructions suggesting a degree of mixing between melts derived from different protoliths petrographic observations identified additional characteristics of melt transportation including transported clasts aligned fragments melt segregation and injection of the melt into tensile fractures the latter probably served as the transport mechanism drawing melt into the low pressure tensile structures that developed adjacent to the melt forming zones within the fault plane supplementary material sample site co ordinates and electron microprobe settings are available at https doi org 10 6084 m9 figshare c 4567931

evaluation of asperity scale temperature effects during seismic slip abstract for the common rock forming minerals the joule thompson effect produces cooling on compression and heating on decompression experimental rock friction studies and seismological evidence suggest that the strength of faults is controlled by high stress asperities that represent a small fraction of the total fault area during seismic rupture the area immediately beneath asperities is inferred to undergo adiabatic compressions and decompressions on the order of 1 2 8 gpa joule thompson coefficients are in the range 261 372 c gpa resulting in temperature changes of 336 880 c quartz asperities with a compressive strength of 2 8 gpa can produce a heating or cooling of 880 c immediately beneath an asperity by comparison frictional heating flash temperatures are less than 1000 c for asperity contact areas 2 the joule thompson effect is independent of asperity size and may augment or counteract frictional heating the paucity of pseudotachylytes in the geologic record and the absence of localized heat flow anomalies on active faults might be explained by the joule thompson cooling effect

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

basin thermal history favours an accretionary origin for the southern uplands terrane scottish caledonides there is general agreement that the siluro ordovician southern uplands suspect terrane is an imbricate thrust belt formed adjacent to the laurentian continental margin during closure of the iapetus ocean however there are conflicting models for its tectonosedimentary evolution especially for the initial depositional environment was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism or was it a back arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt to address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated palaeogeothermal conditions in the southern uplands terrane assessed from regional metamorphic patterns clay mineral assemblages and k white mica composition rule out extension and support deposition in a low heat flow plate convergent setting there is no evidence for a change from extensional to convergent basin regimes from north to south as required in some models instead throughout the terrane there is a close temporal and spatial relationship between deposition burial metamorphism and tectonism that is elsewhere typical of accretion

orogenesis vs diagenesis can we use organic rich shales to interpret the tectonic evolution of a depositional basin abstract black shales from the southern appalachian basin and the southwest welsh basin have anomalous u pb and nd model ages suggesting syn and post depositional resetting of the sm nd and u pb isotopic systems this alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic depositional processes that obscure paleo environmental and provenance information recorded prior to and during deposition the trace element and isotopic signatures of these two shale sequences reveal a syn post depositional history that is de coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition both the ordovivian welsh basin and the devonian appalachian basin sequences host ree and u bearing diagenetic phosphate minerals that play a critical role in the whole rock ree and u budgets in the welsh basin shales early diagenetic apatite and a later monazite phase dominate the ree budget and cause the redistribution of ree early in the basin s history ca 460 ma this redistribution is recorded by the sm nd system 450 90 ma and the nd model ages that are anomalously old by as much as 20 t dm 2 0 ga this early history is complicated by a permo triassic fluid event affecting the whole rock u budget and resetting the u pb isotopic system at 193 45 ma the appalachian basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the sm nd isotopic system 392 76 ma and the pb isotopic system 340 50 ma at about the time of deposition ca 365 ma these two sequences suggest a pattern of diagenetic disturbance common to black shales these processes are unique to black shales and must be considered when interpreting provenance and paleo environmental information from the black shale sequences although these rocks are susceptible to alteration the alteration may provide extensive information on the post depositional history of the basin while still retaining some primary depositional information if black shale processes are considered during the interpretation of isotopic and trace element signatures from organic rich shales it may be possible to recover an extensive basin history

chemical and oxygen isotope composition of natural and artificial pseudotachylyte role of water during frictional fusion abstract pseudotachylyte samples from the long ridge fault north carolina the homestake shear zone colorado the fort foster fault maine and experimentally produced frictional melt were analyzed for major elements and o isotopes using an electron microprobe and laser microprobe respectively the pseudotachylytes display a 18o range of 5 9 10 7 standard mean ocean water smow and are both isotopically lighter and heavier than their respective host rocks in general the isotopic composition of bulk pseudotachylyte pt in the absence of water rock interaction can be modeled as 18 o bulk pt x frag 18 o frag x min 18 o min where xfrag refers to the volume fraction of unmelted mineral fragments and xmin refers to the volume fraction of minerals that undergo melting mineral melt isotopic fractionation is considered to be small and less than 1 the majority of pseudotachylyte isotopic compositions can be explained as a disequilibrium partial melt of the protolith rock comprising melted feldspar and micas plus a small proportion 10 of unmelted quartz clasts in contrast the long ridge fault samples are substantially depleted in 18o by up to 4 relative to their host rock which is best explained by closed system water rock interaction of a pore fluid at 100 c with a melt at 1000 c and a water rock weight ratio of 0 2 this fluid is also inferred to have caused oxidation of the pseudotachylyte melts producing the hematite characteristic of this location

electron microscopy study of hyalomylonites evidence for frictional melting in landslides abstract hyalomylonites from langtang nepal and kofels otz valley austria were investigated by transmission electron microscopy tem the matrix is glassy at both localities rarely it is crystallized and devitrified to dendrites and skeletal microlites of plagioclase alkali feldspar and biotite the matrix is chemically heterogeneous with schlieren and formed by partial to almost complete melting of host rocks of granitic to granodioritic composition locally glasses with pure quartz plagioclase and alkalifeldspar composition were found from sio 2 glass minimum temperatures of 1520 c are estimated at both hyalomylonite localities melting appears to be the result of frictional heating on the gliding plane of a large landslide the host rocks were deformed in a brittle mode microstructures typical of plastic deformation at high temperatures are displayed by inclusions and were probably inherited from previous tectonic events

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

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

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

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

surface deformation and tectonic setting of taiwan inferred from a gps velocity field 1 we have determined the present day surface deformation of taiwan by computing the velocity gradient field and fault slip from 143 gps velocity vectors in southern taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south in northern taiwan we infer the onset of gravitational collapse which is induced by the on land extension of the okinawa trough in the eastern central range the observed inverted nw se extension is consistent with geological observations and high heat flow measurements this could be the result of exhumation of crustal material the model further shows a significant decrease in slip rate northward along the longitudinal valley fault at 23 7 n the northern coastal range shows high strain rates and two oppositely rotating blocks by combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present day tectonic activity both seismicity and tomography show further evidence for active southward propagating exhumation of a crustal slice in the eastern central range offshore east taiwan we deduce strong evidence of a southward propagating crustal tear fault accommodating most of the philippine sea plate eurasian plate convergence the tear is the crustal response to incipient northwestward subduction of the philippine sea plate thus the ryukyu trench is bending southward becoming almost perpendicular to the convergence direction while subduction of the philippine sea plate continues in this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

the illite crystallinity technique a critical appraisal of its precision analysis of the precision of the illite crystallinity technique shows that machine errors are 0 8 it is necessary that contours should be at intervals of 0 1 2 which is equivalent to subdivision of the anchizone into upper and lower units where previous interpretations have relied upon an isocryst method of contouring at less than 0 1 2 the conclusions must be regarded as unsubstantiated centrifuge separation of clay fractions based on a stokes law application gives separations in which a significant but variable percentage of grains have long axes greater than the size calculated for the typical 2 m fraction utilized some 20 of grains lie in the 2 4 m range although the proportion is not believed to have a significant effect upon crystallinity values the formula is applicable for grain sizes down to 0 5 m illite crystallinity values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing the differences are minimal at the epi anchizone level but increase to some 10 at the anchizone diagenetic level the effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded

compositional controls on coexisting prehnite actinolite and prehnite pumpellyite facies assemblages in the tal y fan metabasite intrusion north wales implications for caledonian metamorphic field gradients the tal y fan intrusion is a 110 m thick sub concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of ordovician age in north wales despite low grade metamorphism primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization this has resulted in a vertical sequence of secondary mineral assemblages through the intrusion during early hydrothermal alteration k feldspar replaced plagioclase micropheno crysts in the marginal and contact zones and olivine in the central zone was replaced by saponite subsequent regional metamorphism resulted in the development of metastable prehnite pumpellyite epidote assemblages in two sub zones characterized by high fe2o3 elsewhere the assemblage prehnite actinolite epidote developed except in the contact and marginal zones where activity of co2 suppressed both prehnite and pumpellyite both assemblages contain excess albite quartz and chlorite and on the basis of uniform mineral compositions over the area of an individual thin section are considered to represent buffered equilibrium assemblages indicative of prehnite pumpellyite and prehnite actinolite facies conditions a metamorphic temperature of 310 c at 1 85 kbar is obtained using the p t x grid of liou maruyama cho 1985 which implies a field gradient of 44 c km 1 assuming that metamorphism relates to burial an overburden thickness of 7 km is indicated total maximum thicknesses however of ordovician silurian and lower devonian strata in the area do not exceed 6 km indicating a field gradient of 52 c km 1 these relatively high gradients may possibly be related to concealed late caledonian intrusions or alternatively may result from high heat flow as a consequence of crustal thinning rapid sedimentation and intense magmatic activity in a marginal basin setting

the influence of strain lithology and stratigraphical depth on white mica illite crystallinity in mudrocks from the vicinity of the corris slate belt wales implications for the timing of metamorphism in the welsh basin low and very low grade metamorphism in a mudrock dominated succession ranging from upper cambrian to wenlock is characterized by the white mica illite crystallinity technique kubler indices indicate that grade ranges from late diagenetic to low epizonal the prograde reaction most easily observed by xrd study is the progressive ordering of k na micas diagnostic 2m 1 polytypic reflections become recognizable at 0 4 2 and strengthen as grade advances grade generally correlates with thickness of overburden in the 9 km thick succession however strain exercises additionally a profound influence on grade and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates the distribution of metamorphic grade implies that the lower palaeozoic succession was augmented by several kilometres of lower devonian old red sandstone strata at the metamorphic culmination the culmination was syntectonic and represents an acadian rather than an early diastathermal event subsequent dextral movement on the bala fault displaced the regional isocryst pattern by c 4 km retrogressive smectite appears to be localized to the vicinity of the bala fault and is probably of variscan age

chemical heterogeneity in an enderbite hosted pseudotachylite eastern india evidence for syn deformation multi reaction melting in pseudotachylite the origin of chemical and mineralogical heterogeneity in tens of microns wide layers and domains in enderbite hosted couple of centimeters wide pseudotachylite vein is examined based on the results of bse and x ray element imaging and electron probe microanalyses of major elements of host rock minerals clasts microphenocrysts and pseudotachylite matrix the pseudotachylite layers and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz k feldspar plagioclase and garnet clasts the clasts are chemically modified along margins and intra clast pseudotachylite injections the chemical modifications are extensive in smaller clasts 5 m diameter at least three chemically distinct layers and domains in the pseudotachylite and their fine grained matrices plot in sharply defined well segregated and non overlapping fields in feo mgo al2o3 sio2 feo cao mgo and cao na2o k2o and feo vs feo feo mgo diagrams the compositions of the layers and domains smeared between a feldspar quartz component and a ferromagnesian component of garnet fe ti oxides orthopyroxene possibly correspond to fractionated quenched melts or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed the compositional variations are incompatible with deformation driven crystal fractionation in melt instead the layers and domains possibly are crystal melt mushes produced by syn deformation ultra high temperature 1 250 1 375 c melting reactions involving variable proportions of host rock minerals determined by time transient local phase aggregates experiencing strain the similar element variation trends in pseudotachylite examined here and those reported from anorthosite metapelite and charnockite elsewhere suggests local phase aggregate controlled multi reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

glassy pseudotachylyte veins from the fuyun fault zone northwest china glassy pseudotachylytes occur in granitic mylonites from the fuyun fault zone northwest china powder x ray diffraction data indicate that the fine grained matrices of the fuyun pseudotachylytes vary from almost crystalline to complete glass a few to 90 wt glass powder x ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains microlites displaying quenched or rapid cooling textures vesicles and amygdules flow structures and rounded fragments are also common in the fuyun pseudotachylytes the average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks but the glass matrix has a low sio2 component which is 5 15 wt lower than that of the host rocks the pseudotachylytes contain 2 3 wt water h2o using experimental data which relate the solubility of water to pressure approximately 400 bars lithostatic pressure corresponding to approximately 1 5 km depth of pseudotachylyte formation is estimated a minimum temperature estimate of 1450 c can be estimated due to the presence of the pure sio2 glass the x ray diffraction and chemical data indicate that the fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

a multi kilometer pseudotachylyte system as an exhumed record of earthquake rupture geometry at hypocentral depths colorado usa abstract a system of pseudotachylyte bearing fault zones preserved along the proterozoic homestake shear zone in the southern rocky mountains provides an avenue for investigating earthquake processes at the hypocenter the results of detailed field mapping suggest that pseudotachylyte may serve as a dynamic indicator of rupture directivity and yield general estimates of some earthquake source parameters when examined at the multi kilometer fault system scale pseudotachylyte fault veins are primarily exposed within eight ne striking sub vertical fault zones that have a cumulative length of more than 21 km the fault zones are mapped for 7 3 km along strike and fan to the northeast from a 170 m wide outcrop belt to a maximum cross strike width of 2 3 km pre existing structural control on rupture geometry is indicated by concordance between foliation and fault veins as well as spatial coincidence between the limbs of map scale rootless isoclinal folds and the location of most fault zones the central portion of the longest fault zone exhibits evidence for dextral oblique slip that involved more than 2 1 m of strike slip offset between four parallel fault veins that are interpreted to have formed in response to a single rupture event in addition an along strike continuity and systematic distribution of fault zones a progressive northeastward decrease in pseudotachylyte volume and maximum vein thickness and a relative scarcity of cross cutting relationships further suggests that the majority of the frictional melt in the system may have developed in response to one or several multi kilometer ruptures as opposed to hundreds of shorter ruptures the similarity of kilometer scale relationships observed along the homestake pseudotachylyte system with the subsurface slip distribution and surface geometry of present day strike slip earthquakes is interpreted to indicate that frictional melting occurred within a concentrated zone of moment release or an earthquake hypocenter during one or more m w 6 3 earthquakes m 0 6 9 10 25 dyn cm that involved northeastward rupture propagation

strain related differences in the crystal growth of white mica and chlorite a tem and xrd study of the development of metapelitic microfabrics in the southern uplands thrust terrane scotland tem and xrd techniques were used to study crystal growth characteristics of the fabric forming phyllosilicates which developed in response to low grade metamorphism and tectonic imbrication in part of the southern uplands thrust terrane prograde regional metamorphism ranging from late diagenesis through the anchizone to the epizone was accompanied by the development of a slaty cleavage which is commonly bedding parallel tem measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with xrd measured crystallinity indices analytical tem data show that prograde changes in composition lead to a net loss of si ca and minor fe from the fabric forming phyllosilicates white micas are paragonite poor phengites with a mean b lattice parameter of 9 037 a and indicate an intermediate pressure series of metamorphism with a field gradient of 25 c km chlorite compositions evolved from diabantite with intergrown corrensite to ripidolite over an estimated temperature range of 150 320 c field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane during late diagenesis crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions and resulted in similar size distributions which resemble typical ostwald ripening curves under anchizonal and epizonal conditions white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development as a result crystal thickness distributions are not typical of ostwald ripening in contrast chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates these retained dislocations reduce the crystallite thicknesses detected by tem and xrd compared with those of white mica these differences in strain induced crystal growth indicate that white mica illite and chlorite crystallinity indices are likely to show significant differences where low grade metamorphism is closely associated with tectonic fabric development

a fault controlled depositional model for the aberystwyth grits turbidite system the aberystwyth grits group was the first in a series of southerly supplied sandstone rich turbidite systems that invaded the welsh basin during the telychian stage late llandovery silurian structural and stratigraphical evidence indicates that deposition of the sand dominated parts of the system was largely restricted to the east by a zone of syndepositional faulting now marked by a major vergence divide downfaulting and reverse drag along the fault hangingwall created a bathymetric low which focused deposition of high density sand carrying turbidite flows and flow components allowing substantial thicknesses of sediment to accumulate in contrast low density mud rich turbidites were deposited over a wider area enhancing the topographic relief generated by uplift above the fault footwall to form a feature which restricted the spread of the coeval easterly supplied turbidites of the devil s bridge formation

excess argon 40are uptake during slate formation a 40ar 39ar uv laserprobe study of muscovite strain fringes from the palaeozoic welsh basin uk syn kinematic pure muscovite strain fringes grew around pyritized graptolites in response to the early devonian acadian closure and uplift of the peri gondwanan welsh basin they record ar isotope information on the timing of low temperature deformation and the mudrock to slate transition here we have applied three different 40ar 39ar laserprobe dating methods to strain fringes from the central welsh basin to test for variations in age and to explore the technical limitations of the method infrared laserprobe step heating analyses yielded only occasional plateau ages varying between ca 403 ma and 422 ma and strong evidence for discordance in the ar system likewise ages from infrared laserprobe total fusion analysis are scattered and range from ca 390 ma to 651 ma by contrast ultraviolet laserprobe intra fringe age profiles revealed a complex pattern of ages across individual strain fringes this is the highest spatial resolution technique available and the resulting age profiles consistently showed an oscillatory pattern comprising troughs or baseline ages that are all within error of ca 397 ma and peaks or apparent ages ranging up to ca 550 ma these apparent ages we interpret as the results of repeated periodic assimilation of excess 40ar 40are during deformation and the mudrock to slate transition here basinal fluids carrying 40are are driven through bulk mudrock during contractional phases of the acadian orogen and 40are is taken up by muscovite strain fringes the concentration of 40are varies with metamorphic grade and is inversely proportional to the temperature of metamorphism and we suggest is controlled by the solubility of ar in water which has potentially quite severe implications for 40ar 39ar dating of k bearing minerals in any mudrock these results provide a first record of ar isotope information in strain fringes and a first record of orogenically driven fluid flow through bulk deforming mudrock they also demonstrate how the high spatial resolution ultraviolet laserprobe 40ar 39ar method can access ar isotope information that is obscured by lower spatial resolution laser analysis or bulk sampling methods

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

an argument against the crush origin of pseudotachylytes based on the analysis of clast size distribution abstract the origin of pseudotachylytes has been controversial since wenk cast doubt on the melt origin of the matrix of pseudotachylytes in 1978 the matrix of this rock is so fine grained that the crush origin of pseudotachylytes revived by wenk cannot easily be denied this paper presents a new line of argument based on the size analysis of clasts contained in pseudotachylytes in felsic granulite from the musgrave range central australia these clasts are definitely crush products produced during the pseudotachylyte generation their sizes as measured on photomicrographs of thin sections obey the size distribution n n r d where n is the cumulative number of clasts with sizes greater than r d is the fractal dimension and n is a constant that depends on the number of measurements d was found to be 1 5 0 05 for the size ranges of 10 2000 m if the matrix of the pseudotachylytes consists mostly of ultrafine crush products they must have formed simultaneously with those coarse crush products the proportion of fine products relative to the coarse clasts can be estimated assuming that a similar size distribution also holds for the fine products the estimated area occupied by fine products in a thin section is of the order of only several percent whereas the measured area of the matix is about 60 thus the major part of the matrix of the pseudotachylytes cannot be regarded as crush products it is also shown that the number of clasts smaller than about 5 m becomes very small perhaps as a result of nearly complete dissolution of fine clasts in a melt however if the ultrafine grained matrix of the pseudotachylytes had formed by crushing during seismogenic fault motion the grain size refinement during the crushing should have occurred jumping the size range of at least 1 5 m this is quite unreasonable and disproves the crush origin for the matrix of pseudotachylytes

organic carbon deposition and coastal upwelling at mid latitude during the upper ordovician late katian a case study from the welsh basin uk a lack of stratigraphical sedimentological and geochemical data for sediment accumulation rates and indicators of productivity and anoxia means that causative models for ancient black shales are largely inferred from modern settings coastal upwelling has been suggested as a general hypothesis for ordovician black shale deposition within the iapetus ocean but has not been directly tested despite anchizone metamorphism we utilize a suite of geological and geochemical environmental proxy data toc wt 13corg ba al p to elucidate the mechanism for the origin of a single grey black shale cycle within the upper katian succession of the welsh basin here we interpret organic carbon oc rich deposition to be suggestive of a period of high photic productivity higher toc wt with 12c and ba enrichment comparable to high productivity events in modern coastal upwelling systems productivity proxy values are consistent with those from margins where sedimentation rates are high e g gulf of california inter bedded grey shales have low toc wt more positive 13corg and marginally lower ba and are interpreted as low productivity events thalassinoides planolites and chondrites ichnofacies indicate changing seafloor oxygen levels these show predominantly dysoxic conditions even during the deposition of oc poor grey shales during an oc rich laminated hemipelagite event oxygen levels declined at the sea floor before the deposition of the oc rich layers full anoxia was established early on in the deposition of oc rich layers and the return to more oxic conditions was rapid though fluctuating and coincident with the return of grey shale deposition this pattern suggests oc accumulation at the seafloor resulted from a complex interaction of productivity and preservation none of the widely used trace element redox proxies are reliable in anchizone metamorphic rocks climate sensitive detrital proxies k al and ti al indicate arid temperate conditions in the basin hinterland during the deposition of the oc rich layers the welsh basin was situated on the southern margin of the iapetus ocean in 30 s beneath the prevailing se trade winds we interpret the occurrence of oc rich laminated hemipelagite events to represent the intensification of upwelling prior to the hirnantian glaciation possibly having resulted from a strengthening of the trade winds associated with stepped changes in ice volume and a more arid local climate

very low and low grade metamorphism in the trinity peninsula group permo triassic of northern graham land antarctic peninsula the permo triassic trinity peninsula group is a widespread regionally metamorphosed metasedimentary sequence in northern graham land antarctica which forms the local basement to the mainly jurassic cretaceous antarctic peninsula magmatic arc the metamorphic grade thermal evolution and pressure series of this major tectono stratigraphical unit are largely unknown determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases mainly in rare volcanic beds however diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas using white mica illite crystallinity of interbedded mudrocks the trinity peninsula group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest outwith thermal aureoles near plutonic intrusions the alteration temperatures ranged mainly from 250 to 325 c exceeding 300 c in the highest grade epizone greenschist facies parts of the sequence the facies series k white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under moderate geothermal gradients 35 c km corresponding to burial depths of c 7 10 km unroofing and substantial erosion of the trinity peninsula group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern graham land the geological setting of the trinity peninsula group is ambiguous and could have been a foreland or back arc basin or the mid to upper levels of an accretionary prism

synchronous very low grade metamorphism contraction and inversion in the central part of the welsh lower palaeozoic basin a metamorphic map white mica crystallinity based on 663 data points has been generated for the recently published british geological survey 1 50 000 geological sheets 178 llanilar and 179 rhayader epizone upper anchizone lower anchizone and sub anchizone are delineated metamorphic cross sections are drawn and superimposed on structural cross sections assumptions made in order to draw metamorphic cross sections are 1 the anchizone spans 100 c 2 the metamorphic field gradient was 36 ckm 1 3 metamorphic surfaces were initially generated parallel to the contemporary ground surface restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults development of back thrusts footwall short cuts and limited growth of precursor folds had taken place metamorphism continued locally during later stages of the contractional phase as a strain related phenomenon finally tensional faulting followed the cessation of folding and metamorphism

calibration of the anchizone a critical comparison of illite crystallinity scales used for definition instrumental settings for determination of illite muscovite crystallinity half height width of the 10 a x ray diffraction peak and the limits of the anchimetamorphic zone adopted by various authors fall into several groups the variation in the limiting peak widths between the authors that have adopted kubler s neuch tel boundary values of 0 42 and 0 25 2 can be interpreted in terms of variation in the instrumental settings the choice of time constants higher than those given by the formula causes peak broadening this effect is particularly marked at high scan rates the peak broadening is by constant increments that are virtually independent of the absolute peak width the differences between the kubler neuch tel derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings many of these scales are not equivalent in particular the limits adopted by dunoyer de segonzac 1969 and subsequent workers at strasbourg are too broad their anchizone represents a range of grades of metamorphism lower than that of kubler widely overlapping the latter s diagenetic zone those adopted by some other mainly french authors are too narrow the limits of the anchizone should be calibrated by inter laboratory standards and the instrumental settings should be specified in full

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

a seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite facies rocks of calabria southern italy pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses metamonzodiorites as well as felsic and mafic granulites at the base of the section of the hercynian lower crust exposed in calabria southern italy the crustal section is tectonically superposed on lower grade units ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses whereas sparse fault related pseudotachylytes and thin mylonite ultramylonite bands occur in granulite facies rocks the presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high temperature conditions preceded the formation of pseudotachylytes we have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust different protoliths were selected to test how lithology controls pseudotachylyte composition and textures in migmatites and felsic granulites euhedral or cauliflower shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition this indicates that pseudotachylytes originated at deep crustal conditions 0 75 gpa in mafic protoliths quenched needle to feather shaped high alumina orthopyroxene occurs in contact with newly crystallized plagioclase the pyroxene crystallizes in garnet free and garnet bearing veins the simultaneous growth of orthopyroxene and plagioclase as well as almandine suggests lower crustal origin with pressures in excess of 0 85 gpa the existence of melts of different composition in the same vein indicates the stepwise non equilibrium conditions of frictional melting melt formed and intruded into pre existing anisotropies in mafic granulites brittle faulting is localized in a previously formed thin high temperature mylonite bands migmatitic gneisses are deformed into ultramylonite domains characterized by s c fabric small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high strain rate leading to seismic events and melting

fault rocks within the blueschist metabasalts of the diamante terranova unit southern italy potential fossil record of intermediate depth subduction earthquakess vitale et al fault rocks within blueschist metabasalts

eye and sheath folds in turbidite convolute lamination aberystwyth grits group wales eye and sheath folds are described from the turbidites of the aberystwyth group in the silurian of west wales they have been studied at outcrop and on high resolution optical scans of cut surfaces the folds are not tectonic in origin they occur as part of the convolute laminated interval of each sand mud turbidite bed the thickness of this interval is most commonly between 20 and 100 mm lamination patterns confirm previous interpretations that convolute lamination nucleated on ripples and grew during continued sedimentation of the bed the folds amplified vertically and were sheared horizontally by continuing turbidity flow but only to average values of about 1 the strongly curvilinear fold hinges are due not to high shear strains but to nucleation on sinuous or linguoid ripples the aberystwyth group structures provide a warning that not all eye folds in sedimentary or metasedimentary rocks should be interpreted as sections through high shear strain sheath folds highlights eye folds occur in turbidite convolute lamination viewed down the palaeoflow direction the convolutions formed during bed deposition and are overturned down flow by current shear the eye folds are sections through non cylindrical sheath folds due to this shear average shear strains are less than 1 0 lower than generally necessary to generate sheath folds instead sheath folds nucleate on already sinuous current ripples lower in the turbidite bed

a transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence north wales uk high resolution transmission electron microscopy hrtem measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate crystallite thickness measured normal to 001 increases as grade increases whereas the xrd measured 10 a peak profile the kubler index decreases the mode of the tem measured size population can be correlated with the effective crystallite size n 001 determined by xrd the results indicate that the kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent x ray scattering domains these changes conform to the scherrer relationship between xrd peak broadening and small crystallite size lattice strain broadening is relatively unimportant and is confined to white mica populations in the diagenetic mudstone rapid increases in crystallite size occur in the anchizone coincident with cleavage development changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain growth by ostwald ripening the kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone

transition from diagenesis to metamorphism in extensional and collision settings areas in which there is a transition from diagenesis to metamorphism occur in a variety of tectonic settings some of which are of an extensional character rather than the collision setting normally associated with regional metamorphism in collision settings the diagenetic to metamorphic transition is typically followed by a continuum into the highest grades of metamorphism the baric type is usually of an intermediate pressure variety and metamorphism occurs at a late stage in the tectonic cycle in extensional settings such as marginal basins a restricted facies series is developed that ranges from diagenetic levels up to a low greenschist facies maximum the metamorphism is normally of a low pressure type and develops early in the tectonic cycle these characteristics suggest that it is possible to subdivide broadly the field of low grade regional metamorphism into compressional orogenic and extensional settings

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

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

numerical modeling of mountain building interplay between erosion law and crustal rheology 1 coupling between erosion and tectonics is thought to play a determinant role in orogenic evolution here we investigate the interplay in this coupling between the assumed erosion law and the crustal rheology at the margin of a collisional plateau like the himalaya of central nepal lithospheric deformation is calculated over a time scale of 100 kyr by a 2d finite element model that incorporates the rheological layering of the crust and the main features of the convergence across the range for the upper boundary condition two surface processes were tested a linear diffusion model and a 1d1 2 integrative model including fluvial incision along the fluvial network and hillslope erosion by landsliding model results and their sensitivity to the chosen combinations of erosion law and crustal properties are discussed in light of the constraining geologic and geomorphologic observations in contrast with the conclusions of cattin and avouac 2000 where a compliant quartz rich crustal rheology and diffusion law were required we combine a composite quartz diabase rheology for the crust with fluvial incision erosion law to account for erosion and elevation profiles across the himalaya of central nepal more generally it is proposed that because of the interplay between the dominant denudation conditions and the rheology of the crust both well documented erosion rates and processes can provide significant constraints on crustal properties within an active orogen

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

low grade metamorphism of the manx group isle of man a comparative study of white mica crystallinity techniques studies have been made in two laboratories on metapelites from the manx group isle of man showing the distribution of white mica crystallinity using the kubler index 2 and the weber index hb rel two isocryst maps which show broadly similar distributions of metamorphic grade are illustrated and a correlation between the laboratories is presented the mineralogy of the fraction in relation to crystallinity has been determined the distribution of metamorphic grade over central and eastern parts of the isle of man as depicted by the kubler index isocryst map is related to subjacent granite plutons but these contact metamorphic effects overprint an earlier largely anchizonal regional metamorphism localized late retrograde effects are related to low temperature hydrothermal mineralization and to cold re working in a large fault zone differences between the maps are largely in the degrees of resolution which stem directly from the different sampling intervals employed comparisons between the maps suggest a sampling interval of 1 2 km is adequate in regions of simpler geology but that an interval of the changes in the mineralogy of the

generation and crystallization of an amphibolite shear melt an investigation using radial friction welding apparatus a cylinder of amphibolite comprising the assemblage amphibole clinozoisite albite has been frictionally melted using radial friction welding apparatus this was achieved by rotating a steel ring at 750 rpm and a force of 98 kn for 10 s against a stationary steel casing which housed the rock sample the ring penetrated the casing then proceeded to rotate and compress the sample to 95 of its original volume until it cracked at right angles to its length this generated a whole rock silicate melt which injected the crack and on cooling produced glass crystallites and vesicles melting occurred in two stages an initial low pressure melting event with crystallization to augite fe rich anorthite followed by a high pressure melting event with crystallization to fassaitic clinopyroxene it is estimated that pressures of 0 5 gpa rising to 1 gpa were realized at the ring rock interface under these conditions fassaite superseded augite anorthite crystallization due to the increased solubility of ca tschermak s component in clinopyroxene the high pressure event provides a crude analogue for the frictional melting of basic rock at depths of 15 30 km in a seismogenic fault a situation realized along the slip zone between cold descending lithosphere and overlying mantle during subduction

viscosity determinations of some frictionally generated silicate melts implications for fault zone rheology at high strain rates analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths the data show that the matrices of the friction melts which cool to form pseudotachylytes are relatively basic and hydrous even when their protoliths are intermediate to acid this is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points not the formation of minimum melts through equilibrium mineral interaction this means that hydrous ferromagnesian minerals e g micas and amphiboles melt preferentially to form the liquid matrix while feldspars and especially quartz more readily survive as clasts pseudotachylytes generated by frictional melting are therefore not bulk melts and as clast melt suspensions they cannot be considered as simple newtonian fluids the calculated viscosities of the friction melts are low for example at 1200 c most friction melts possess zero shear suspension viscosities of 102 104 dpa s 1 dpa s 1 p this is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature these viscosities are maximum determinations because as clast melt suspensions friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates i e during slip on a fault surface contrary to earlier suggestions where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding this work shows that most pseudotachylytes are partial melts possessing low viscosities the formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation this would be manifest as an increase in high frequency seismic wave radiation and vibrational

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

incipient metamorphism in the lower palaeozoic marginal basin of wales a diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the lower palaeozoic marginal basin of wales by examination of variations in phyllo silicate mineralogy illite crystallinity and bo parameter of white micas this transition represents a temperature range from 150 c to 400 c and the metamorphism is of a low pressure facies series type with a geothermal gradient of 40 ckm 1 variations in grade can be correlated largely with the original basin and shelf form suggesting a depth related metamorphism however in areas closer to the site of caledonian plate collision an increasingly syn tectonic metamorphic event is apparent correlation of pelite data with metabasite assemblages is variable the most consistent relationship being between epizone crystallinity values andepidote actinolite greenschist facies assemblages diagenetic clay mineral assemblages are found associated with prehnite pumpellyite assemblages in metabasites and it is suggested that the latter represent non buffered and therefore non diagnostic assemblages