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

40ar 39ar thermochronology of mesoproterozoic metamorphism in the colorado front range abstract a low pressure metamorphic episode in the colorado front range has been identified by the presence of staurolite andalusite cordierite and garnet porphyroblasts overprinting earlier assemblages the overprinting assemblages and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding 525 c twenty eight 40ar 39ar dates on hornblende muscovite biotite and microcline were used to infer the age and thermal conditions of metamorphism muscovite and biotite 40ar 39ar ages fall mainly in the interval 1400 1340 ma consistent with cooling through the closure temperature interval of micas 400 300 c after about 1400 ma in contrast hornblende apparent ages documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fon

fault reactivation and pseudotachylite generation in the semi brittle and brittle regimes examples from the gavilgarh tan shear zone central india in the sheared and foliated granitoids of the proterozoic gavilgarh tan shear zone gtsz in central india two types of pseudotachylite pt m and pt c are recognized pt m layers are interbanded with mylonite and ultramylonite show strong internal plastic deformation and buckle folding concurrent with the host rocks and appear to have formed within the greenschist facies 300 400 c in the brittle plastic transitional semi brittle regime pt c layers show sharp contacts with the host rock exhibit abundant coeval cataclasis preserve no evidence of subsequent plastic deformation and formed at shallower depths at temperature

reconstructing the avalon continent marginal to inner platform transition in the cambrian of southern new brunswick a west to east marginal to inner avalonian platform transition comparable to that in southeast newfoundland and southern britain is present in the cambrian of southern new brunswick the saint john caton s island hanford brook area lay on the marginal platform and its thick uppermost precambrian lower lower cambrian is unconformably overlain by trilobite bearing upper lower cambrian an inner platform remnant is preserved in the cradle brook outlier 60 km northeast of saint john in contrast to the marginal platform sequences the cradle brook outlier has a very thin lower lower cambrian and has middle lower cambrian strata bonavista group not present on the marginal platform the cradle brook lower cambrian closely resembles inner platform successions in eastern massachusetts and trinity and placentia bays southeast newfoundland a limestone with camenella baltica zone fossils on cradle brook seems to be the peritidal limestone cap of the subtrilobitic lower cambrian known in avalonian north ame

volume loss model for trace element enrichments in mylonites mylonites from three widely separated localities along the fries ductile deformation zone in the blue ridge province show substantial enrichments in trace and minor elements tio sub 2 p sub 2 o sub 5 zr y and v and depletions in rb and sr relative to the protolith two end member hypotheses can explain the enrichments one involves partitioning to and from an infiltrating fluid and assumes that all the elements were mobile the second hypothesis assumes the high field strength cations were largely immobile and that their enrichment is due to large volume losses in the mylonites modeling indicates that ti is immobile for fluid rock ratios as high as 10 sup 4 and petrographic and modal data support a volume loss mechanism volume loss was accommodated by loss of sio sub 2 and alkalies during feldspar dissolution calculated fluid rock volume ratios required to produce the observed sio sub 2 losses assuming ti mobility range from 70 to 600 variable enrichments in p zr y and v are attributed to selective mobility of these elements during fluid infiltration

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

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

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

the generation of ductile and brittle shear bands in a low angle mylonite zone in the saint barthelemy massif french pyrenees a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies the last major structures formed in the zone are isolated shear bands divided into three types anastomosing inhomogenous ultramylonitic shear bands type i are subparallel to the mylonitic foliation in the gneiss sg most of these bands developed by ductile deformation processes only planar homogeneous ultramylonite bands type ii are usually oblique to sg they generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation type iii shear bands are planar and oblique to sg they consist of pseudotachylyte weakly affected by ductile deformation type i ii and iii bands seem to represent progressively younger structures on a local scale linked to falling p t conditions the systematic variation in orientation of the different shear bands with respect to sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy

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

numerical model for nonstationary shallow water wave spectral transformations a nonstationary wave transformation model is developed based upon a finite difference scheme this model computes spatial and temporal distributions of wave spectral density in shallow water areas with irregular bottom topography current knowledge of the dynamic and kinematic processes is incorporated into the model which include the usual wave refraction and shoaling effects as well as nonlinear dissipative processes breaking bottom friction and a constraint on total energy growth in a depth limited situation and local wind wave generation processes a combined phillips miles model another important feature of the model is the inclusion of current effects spatial and temporal current distributions can be introduced as input conditions based upon this model a number of sample computations are given to examine the characteristics of nonstationary wave transformation processes finally one set of wave data collected in the north sea during the marsen project is used to compare with numerical model computations

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

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

first middle ordovician biota from southern new brunswick stratigraphic and tectonic implications for the evolution of the avalon continent a limestone boulder in the triassic lepreau formation near saint john new brunswick has yielded the first diverse marine fauna from the sub caradoc ordovician of the western avalon continent this fauna includes the first arenig conodonts recovered from avalon and represents an unexposed interval in southern new brunswick association of the conodonts drepanoistodus and baltoniodus and the trilobites neseuretus nileus and stapeleyella emphasizes the faunal dissimilarity of avalon and laurentia through the late middle ordovician extension of the ranges of neseuretus cf neseuretus parvifrons and stapeleyella from britain into new brunswick further emphasizes that eastern and western avalon were confluent parts of a unified insular avalon continent that originated in the latest precambrian this fauna correlates with the lower amorphognathus lenodus variabilis zone kundan stage of baltica and the terminal arenig upper middle ordovician lower darriwilian stage of avalonian britain available evidence suggests that an arenig cover sequence with local shallow water hematitic iron ore quartz arenite and rare limestone extended across the avalonian marginal and inner platforms from eastern newfoundland to the boston massachusetts region this western avalonian arenig shows the greatest similarity with the arenig of the welsh borderlands phosphatic fossils from the boulder have a thermal alteration index much lower than that of nearby lower paleozoic outcrops and suggest derivation of the boulder from a weakly heated avalonian succession brought into the bay of fundy region by post ordovician transcurrent faulting

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

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

fabric anisotropy controls faulting in the continental crust the influence of pre existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust e g nanga parbat massif nw himalayas where earlier brittle structures have been annealed the kinematics of young faults formed in a single continuing tectonic regime nnw compression are distinctly different depending upon the orientation of the early ductile foliations around them faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping thus structural anisotropy does control faulting in the continental crust a result with important implications for seismogenesis fluid flow and basin evolution

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

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

the nature and significance of pseudotachylite from the nason terrane north cascade mountains washington abstract pseudotachylite has been recently discovered at numerous localities in the nason terrane of the north cascade mountains washington the pseudotachylite is widespread not associated with any single major fault or shear zone and tends to occur in narrow foliation parallel zones single pseudotachylite veins range in thickness from less than 0 1 to 10 cm veins and networks of veins occur in six distinct types and these form the basis of a suggested classification scheme multiple generations of pseudotachylite are present in some samples host lithologies include quartz diorite semipelitic schist tonalite amphibolite and especially graphitic pelitic schist glassy veins are rare most pseudotachylite is apparently holocrystalline spherulites vesicles and dendritic microphenocrysts are present and indicate a frictional melt origin the ubiquitous occurrence of cataclasite xenoliths within the pseudotachylite along with other textural observations indicates that in the nason terrane the development of cataclasite zones was the precursor to pseudotachylite generation it is inferred that cataclastic deformation of the schist provided the strain softening which resulted in faulting and pseudotachylite generation the water rich and ultra fine grained matrix of the cataclasite was an ideal starting material for pseudotachylite generation based upon bulk chemistry of host lithologies and microprobe analyses of the pseudotachylite it appears that the pseudotachylite formed as a result of a virtually complete melting of the pelitic i e non quartz vein component of the schist where pseudotachylite occurs within pelitic schist alone the composition of the pseudotachylite is very similar to the pelitic component of the schist while the composition of veins cutting tonalite gneiss in addition to pelitic schist appears to define a mixing line between these two lithologies this suggests either participation of the tonalite during initial melting or subsequent assimilation of gneiss into the melt

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

pseudotachylyte generated in the semi brittle and brittle regimes bench canyon shear zone central sierra nevada pseudotachylyte cuts cretaceous plutonic and volcanic rocks in the bench canyon shear zone central sierra nevada california as documented by optical and scanning electron microscopy the pseudotachylyte displays evidence for a friction melt origin including vesicles amygdules crystallites flow fabrics and embayed crystal fragments based on fault related rock association vein morphology and inferred crustal level of generation two distinct types of pseudotachylyte are documented in the shear zone pseudotachylyte associated with cataclasite was generated at shallow crustal levels 1 10 km via unstable velocity weakening abrasive wear friction pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels 10 15 km most likely via stable velocity strengthening adhesive wear friction this pseudotachylyte underwent ductile deformation in the solid state interlayered pseudotachylyte and mylonite records cyclic aseismic seismic slip at a transitional crustal level between the semi brittle field of quartzofeldspathic rocks and the base of the seismogenic zone the cyclicity is attributed to strain rate fluctuation effected by strain resistant phacoids pore fluid pressure variation ductile instabilities or downward fault propagation the mode of deformation within the shear zone progressed from i production of mylonite during amphibolite to greenschist facies conditions to ii generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi brittle field 300 400 c to iii generation of pseudotachylyte through seismic events in the brittle 300 c during uplift exhumation and cooling some of the latter may have been generated at shallow enough crustal levels 4 km to allow vesicle formation the exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi brittle flow to seismogenic rupturing

ultramylonite bands derived from cataclasite and pseudotachylyte in granites northeast japan abstract small scale ultramylonite and cataclasite bands millimeters to tens of centimeters thick are developed in granitic rocks west of the hatagawa fault zone hfz in the abukuma belt northeast japan they occur as single or paired bands with sharp planar boundaries trending nne ssw and often form networks and conjugate sets the very small s c angle and the high displacement thickness ratio of the bands suggest that the shear strain is high the ultramylonite bands are commonly associated with cataclasite bands and mineral veins and rarely with pseudotachylyte some cataclasite bands contain mylonitized layers in which quartz fragments are strongly deformed and dynamically recrystallized on the other hand some ultramylonites are fractured producing fragments that have rotated during later cataclasis the major element content of the ultramylonite bands is similar to that of the surrounding granitic rocks strongly suggesting that the ultramylonite bands have formed through in situ deformation of the granitic protolith without significant mass transfer mineralogy and microstructures of some ultramylonites suggest the strong possibility that they are derived from pseudotachylyte the ultramylonite bands are interpreted as forming in the 10 15 km deep cataclastic plastic transition zone under greenschist facies conditions where co seismic fracturing and aseismic plastic flow have alternated

transient discontinuities revisited pseudotachylyte plastic instability and the influence of low pore fluid pressure on deformation processes in the mid crust abstract occurrences of pseudotachylyte cyclically introduced as melts into the outer hebrides thrust are demonstrably synkinematic with crystal plastic mylonites classical interpretations of these rock types as respectively products of pressure dependent frictional melting and thermally activated intracrystalline deformation create a paradox in that these processes are to a large extent mutually exclusive detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy the ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 c and stresses of 140 210 mpa on which were imposed much higher transient stresses in response to heterogeneous non uniform flow the latter conditions are sufficient for the development of plastic instabilities in feldspar rich crust if pore fluid pressures are sufficiently low the latter is consistent with the absence of hydration during exhumation observed in the rocks under study low pore fluid pressure during thrust exhumation of deep crust enables activation of high strength ductile processes dominated by dislocation glide which may be a prerequisite to instability whereas other studies have demonstrated the possible occurrence of such melt generating instabilities it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure temperature conditions grain size reduction to produce ultramylonites dominated by grainsize sensitive flow is achieved by both deformation induced dynamic recrystallization and crystallization of instability generated melts

pseudotachylyte and the development of ultramylonite bands in the saint barthelemy massif french pyrenees abstract in mylonitic gneisses of the saint barthelemy massif in the french pyrenees evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte in the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip these acted as weak zones in which ductile deformation was concentrated a microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands

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

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

wave spectral transformation measurements at sylt north sea abstract wave spectra were measured in the nearshore region extending approximately 900 m from the shore at the island of sylt north germany in the north sea the measurement covered two periods one in may 1976 when the sea condition was considered normal in this region and the other period was during a storm in september 1978 the field results are presented and compared with numerical computations for energy transformation in shallow water based on a numerical model developed by shiau and wang 1977 the field data supported the validity of the numerical model the results suggest that prior to wave breaking the transformation of wave components is mainly influenced by shoaling and refraction the bottom frictional effect could also be important in the energy containing range after breaking turbulent generation due to wave instability seems to be the dominant mechanism of energy dissipation

fault plane processes and mesoscopic structure of a strong type seismogenic fault in tonalites adamello batholith southern alps abstract the gole larghe fault is an exhumed paleoseismic fault crosscutting the adamello tonalites italian southern alps ambient conditions of faulting were 9 11 km in depth and 250 300 c in the study area the fault accommodates 1100 m of dextral strike slip over a fault thickness of 550 m displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons faults1 2 3 fault displacement is in the range of few centimeters faults3 to a maximum of a few tens of meters in major faults1 faults1 2 nucleated on pre existing joints whereas faults3 are newly generated fractures produced during slip along faults1 2 each fault within the gole larghe fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production pseudotachylytes are usually generated at the host rock cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins pseudotachylytes and cataclasites have a similar chemical composition which is enriched in loss on ignition k rb ba u and fe3 compared to host rock we envision two models for the evolution of the gole larghe fault in both models synkinematic fluid rock interaction along a fault causes fault hardening by precipitation of abundant k feldspar epidote and minor chlorite in the cataclasite matrix conducive to final production of pseudotachylyte in the first model induration occurs progressively by differential precipitation related to fabric evolution in cataclasites in the second model induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir whatever the model the gole larghe fault represents a strong fault where hardening processes resulted in a low displacement fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement fault thickness ratios

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

conodont biozonation in the upper arenig of sweden conodont faunas from the arenigian lanna volkhov stage in baltoscandia have been the subject of intensive study particularly during the last decade and different biozonations based on them have been proposed the application of some of these zones is discussed here the present investigation is mainly based on 64 samples from ten sections in sweden but conodont collections from bornholm denmark and tamsalu estonia have also been studied lindstrom s 1971 concept of the upper arenigian microzarkodina parva zone is essentially replaced by a revised baltoniodus norrlandicus zone the zone corresponds closely to bagnoli stouge s 1997 recently distinguished b norrlandicus and lenodus antivariabilis zones which can be recognized in sections where a more detailed subdivision and correlation is possible under the names of the t quadrangulum subzone and the l antivariabilis subzone respectively

trilobite systematics biostratigraphy and palaeoecology of the lower ordovician komstad limestone and huk formations southern scandinavia

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

effects of lithology cataclasis and melting on the composition of fault generated pseudotachylytes in lewisian gneiss scotland abstract petrological studies of fault generated pseudotachylytes formed during coseismic slip need to consider cataclastic processes as well as rapid closed system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host rocks whole rock analyses of pseudotachylytes and their gneissic host rocks from the lewisian complex of northwest scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host rock composition but also to its mineralogy and fabric in anisotropic e g gneissic rocks selective cataclastic and melting effects may both contribute to determining the whole rock composition of pseudotachylytes electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases such as biotite and amphibole and to a lesser extent feldspar relative to quartz the pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host rocks this interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host rocks

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

faunal composition and dynamics in unconsolidated sediments a case study from the middle ordovician of the east baltic the volkhov regional stage middle ordovician in the east baltic preserves diverse fossil assemblages dominated by epibenthic suspension feeders brachiopods ostracodes conodonts echinoderms and bryozoans are the main components of palaeocommunities obtained from clay horizons in the putilovo section st petersburg region russia whereas trilobites machaeridians hyolithids graptolites benthic foraminifers and gastropods are rare or occur sporadically brachiopod bioclasts volumetrically dominate the debris of the studied sediments quantitative faunal data are used to assess species diversity patterns as expressed by the species richness total number of species in the standardized sample size and by the evenness or equitability of the community the numerical abundance of particular taxa in each standardized sample was used to evaluate the density of the fauna the communities in the volkhov stage in putilovo quarry reveal a remarkable stability throughout the studied interval and are characterized by high density 1000 6000 specimens per 100 g relatively moderate species richness 10 15 species and a moderately variable equitability 0 3 0 7 for the dominant fossil groups conodonts ostracodes and brachiopods ostracodes significantly increase in numbers within the upper part of the section this confirms a shallowing of the basin during the late volkhov interval small scale variability of the diversity estimates does not correlate with the small scale sea level changes reconstructed for this part of the basin it may be connected with error in diversity measurement or the result of undetected environmental parameters variability estimates for different faunal groups are poorly correlated because particular groups have different environmental tolerances

measurements of the evolution of ocean wave spectra due to bottom friction data from a field experiment in which seven wave measurement instruments were placed in a linear transect across the great australian bight are analyzed to determine the spectral decay which can be attributed to bottom friction since other processes such as atmospheric input nonlinear interactions whitecap dissipation refraction and shoaling will also be active the spectral wave model wam is used as an analysis tool explicitly taking these additional processes into account in the analysis the data show marked spectral decay across the shelf analysis reveals that the bottom friction coefficient c is not constant as often assumed in operational wave models the bottom friction coefficient varies approximately inversely with the wave induced bed velocity consistent with the predictions of previous laboratory data and eddy viscosity models it is possible that percolation may also be important in the region but the available data do not allow the relative contributions of bottom friction and percolation to be determined

seismological evidence for crustal scale thrusting in the zagros mountain belt iran summary crustal receiver functions rfs computed from the records of 45 temporary seismological stations installed on a 620 km long profile across central zagros provide the first direct evidence for crustal thickening in this mountain belt due to a rather short 14 km average station spacing the migrated section computed from radial rfs displays the moho depth variations across the belt with good spatial resolution from the coast of the persian gulf to 25 km southwest of the main zagros thrust mzt the moho is almost horizontal with slight depth variations around 45 km crustal thickness then increases abruptly to a maximum of 70 km beneath the sanandaj sirjan metamorphic zone between 50 and 90 km northeast of the surface exposure of the mzt further northeast the moho depth decreases to 42 km beneath the urumiehdokhtar magmatic assemblage and the southern part of the central iranian microcontinent the region of thickest crust is located 75 km to the northeast of the bouguer anomaly low at 220 mgals gravity modelling shows that the measured moho depth variations can be reconciled with gravity observations by assuming that the crust of zagros underthrusts the crust of central iran along the mzt considered as a crustal scale structure this hypothesis is compatible with shortening estimates by balanced cross sections of the zagros folded belt as well as with structural and petrological studies of the metamorphic sanandaj sirjan zone

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

direct calculation of rupture depth for an exhumed paleoseismogenic fault from mylonitic pseudotachylyte multiple generations of pseudotachylyte within granulite facies orthogneiss northern norway caledonides are cut by ultramylonitic shear bands that contain the mineral assemblage garnet clinopyroxene plagioclase quartz mineralogic thermobarometry yields pressure p 950 150 mpa at temperature t 625 675 c for equilibration conditions of the ultramylonite assemblage requiring minimum depths of 35 5 km for coseismic slip that generated pseudotachylyte these depths are well below the base of the standard seismogenic zone 15 20 km ambient temperatures are 200 300 c higher than those commonly cited for the onset of ductile creep in quartzofeldspathic rocks but they are similar to those in modern intraplate settings of higher than average heat flow that have experienced deep crustal earthquakes the occurrence of overprinting mylonites and pseudotachylytes requires tens of kilometers of dynamic downward rupture propagation from the brittle frictional regime into the ductile regime plastic instabilities do not explain the relationship between ultramylonites and mylonitized pseudotachylytes in this setting

estimates of large magnitude late cambrian earthquakes from seismogenic soft sediment deformation structures central rocky mountains a variety of unusual early post depositional deformation structures exist in grainstone and flat pebble conglomerate beds of upper cambrian strata western colorado including slide scarps thrusted beds irregular blocks and internally deformed beds thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined 15 to 40 ramp locally producing hanging wall lenses with fault bend geometries thrust plane orientations are widely distributed and in some cases nearly oppositely oriented in close proximity indicating that they did not form as failures acted upon by gravity forces irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces these features represent parts of beds that detached moved up onto and some distances across the laterally adjacent undisturbed bed surfaces deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm induced deformation because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks finally internally deformed beds are characterized by large blocks fitted fabrics of highly irregular fragments and contorted lamination which represent heterogeneous deformation such as brecciation and liquefaction the deformation structures were produced by earthquakes linked to the reactivation of mesoproterozoic crustal scale shear zones in the central rockies during the late cambrian analysis of the deformation structures indicates very large body forces and calculated earthquake generated ground motion velocities of ca 1 6 m sec 1 these correspond to moment magnitudes of ca 7 0 or more and a mercalli intensity of x these are the only known magnitude estimates of phanerozoic other than quaternary large intensity earthquakes for the rocky mountain region and they are as large as or larger than previous estimates of proterozoic earthquakes along these major shear zones of the central rockies

field study of wave attenuation on an offshore coral reef a major field experiment was conducted which obtained measurements of the attenuation and transformation of short gravity waves as they cross the windward edge of an offshore coral reef water level data were collected for over 3000 individual time series during a wide range of environmental conditions at the innermost measurement site which is located on the horizontal reef flat after the completion of wave breaking the upper bound of significant and maximum wave heights is limited to less than 40 and 60 of the reef flat water depth respectively as with significant wave height both the shape and energy level of the reef flat spectra are strongly affected by changes in reef flat water depth for higher tide levels the spectra on the reef flat closely mimic the corresponding incident spectra however the attenuation is greater for both lower frequencies and higher energy portions of the spectra this causes the reef flat spectra to be broader than those measured windward of the reef at lower water levels considerable energy losses due to wave breaking and bottom friction occur most of the energy loss comes from the vicinity of the spectral peak and energy shifts to harmonics of the peak of the spectrum can be seen

ductile reactivation of proterozoic brittle fault rocks an example from the vestfold hills east antarctica abstract brittle fault rocks usually in the form of minor pseudotachylyte veins have been reported from most high grade metamorphic archaean and proterozoic terrains in analyses of local crustal evolution such brittle structures are often ignored brittle faults commonly reflect the very last phases of local deformation during final uplift of the basement to shallow crustal levels and as such they are thought to be of minor or no importance in large scale archaean and proterozoic tectonics new data from archaean and proterozoic high grade terrains in australia and antarctica indicate that minor fault veins may in some cases represent significant early low grade brittle events which have been overprinted and largely erased by subsequent phases of ductile deformation this paper describes such a brittle event in the vestfold hills east antarctica which may represent an important phase of mid proterozoic uplift and crustal extension

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

formation of alpine mylonites and pseudotachylytes at the base of the silvretta nappe eastern alps optical microscopy and transmission electron microscopy tem have been used to investigate deformation induced microstructures in alpine pseudotachylytes mylonites and ultramylonites from the upper austro alpine silvretta nappe at the southwestern margin of the lower engadine window in the eastern alps of switzerland and austria the earliest alpine stage of deformation d1 produced mylonitic gneisses with dislocation glide being the dominant deformation mechanism in quartz in the second stage d2a mylonites were formed as a result of strain softening in quartz manifest by dynamic recrystallization and a lattice preferred orientation tem observations are used to characterize the initial stages of migration and rotation recrystallization pseudotachylytes were generated during d2b when strain hardening due to dislocation tangling was not outweighed by strain softening several generations of mylonites and pseudotachylytes are observed a model for the propagation of seismic fractures initiated in comparatively rigid amphibolites into an otherwise ductile environment is proposed for the pseudotachylytes as such they are formed as transient discontinuities in the mylonites d1 and d2a b are related to the eo alpine detachment of the silvretta nappe subsequent eo alpine metamorphism in the investigated area reached the stilpnomelane zone and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during d3 and peak metamorphism the increased temperature combined with fluid infiltration from the overridden penninic strata favoured grain boundary sliding superplasticity as the dominant deformation mechanism during d3

ductile brittle transition in pre alpine amphibolite facies mylonites during evolution from water present to water deficient conditions mont mary nappe italian western alps in the austroalpine mont mary nappe italian western alps discrete zones of mylonites ultramylonites developed from coarse grained upper amphibolite facies metapelites of pre alpine age the syn mylonitic mineral assemblage is quartz biotite muscovite plagioclase garnet sillimanite ilmenite graphite and formed via the model hydration reaction grt 1 kfs h 2 o bt 2 ilm 2 qtz ms sil grain size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite and by compositional changes of garnet and biotite deformation took place at temperatures of 510 580 c under low pressure conditions 0 25 0 45 gpa and corresponds to the latest stages of pre alpine metamorphic evolution the pre alpine mylonitization conditions were close to the brittle ductile transition as indicated by syn mylonitic generation of pseudotachylytes and high diverential stress inferred from quartz grain size piezometry the brittle ductile behaviour at a relatively high temperature and the absence of annealing textures in quartz aggregates are suggestive of water deficient conditions during mylonitization these were accomplished through progressive consump tion of water by syn kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization

thermobarometry of metamorphosed pseudotachylyte and associated mylonite constraints on dynamic co seismic rupture depth attending caledonian extension north norway abstract the exhumed post caledonian eidsfjord and fiskfjord extensional shear zones of northern norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte geothermobarometric calculations on garnet bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low angle ductile extension average pressures from eidsfjord 570 115 mpa at ca 650 c and fiskfjord 1120 220 mpa at ca 650 c correspond to faulting depths of 21 4 km and 41 9 km respectively the fiskfjord results agree with previous thermobarometric calculations on mylonitic cpx grt bearing pseudotachylyte at fiskfjord the calculated depths are 5 25 km below the depth of the standard seismogenic zone these results demonstrate that low angle normal faults may cut through a large portion of continental crust this occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and or unusually high shear stresses to account for coseismic rupture at such depths implying a direct connection with a seismogenic normal fault in the upper crust

the middle lanna volkhov stage middle arenig of sweden and its conodont fauna in the established european conodont zonation the paroistodus originalis zone represents an interval in the upper middle arenig or middle lanna volkhov stage i have so far found it at 14 swedish localities represented in my collections by more than 116 000 conodont elements from 79 limestone samples presence absence data together with major frequency shifts among conodont taxa allow the recognition of five successive phases within the p originalis zone and the corresponding chron phase 1 occurs in the basal part of the zone where an increase in frequency of triangulodus brevibasis and p originalis distinguishes it from immediately subjacent beds phase 2 corresponds to the transgressive lower middle part of the zone with an influx of periodon and drepanoistodus basiovalis replacing d forceps phase 3 is equivalent to the upper middle part of the zone with generally more than 30 of the zonal denominator and with microzarkodina parva replacing m flabellum phase 4 is a short regressive event where the frequency of paroistodus is drastically lowered and phase 5 corresponds to the upper part where paroistodus again increases the zone is particularly well represented at lanna province of narke in south central sweden where it also seems possible to connect the conodont succession with the standard trilobite zonation thus at lanna as well as at finngrundet in the bothnian bay megistaspis m simon first appears in the basal lower middle part of the p originalis zone

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

lithostratigraphy of the cambrian lower ordovician bell island and wabana groups of bell little bell and kellys islands conception bay eastern newfoundland the upper cambrian arenigian succession of bell little bell and kellys islands conception bay eastern newfoundland consists of at least 1500 m of interbedded micaceous sandstones siltstones shales and ironstones the succession has previously been formally subdivided into only two groups namely a lowermost bell island group and an overlying wabana group the existing lithostratigraphic nomenclature below group level has to date only been informally proposed and is therefore herein formally revised the lowermost bell island group comprises from base to top the kellys island little bell island beach redmans ochre cove and dominion formations of these the ochre cove and dominion formations are newly proposed and the beach formation is considerably revised from its historical usage the beach formation contains two newly proposed members the mcgraw and eastern head members both of which in previous nomenclature were informally referred to as beds the disconformably overlying wabana

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

high strain rate deformation fabrics characterize a kilometers thick paleozoic fault zone in the eastern sierras pampeanas central argentina abstract high strain rate fabrics that transgress a crustal depth range of ca 8 22 km occur within a major paleozoic fault zone along the western margin of the sierras de cordoba central argentina the nnw striking east dipping tres arboles fault zone extends for at least 250 km and separates two metamorphic terranes that reached peak temperatures in the middle cambrian and ordovician respectively exposed fault zone rocks vary from a 16 km thickness of ultramylonite and mylonite in the southern deepest exposures to three transects across the fault zone have been examined in detail in the deepest section newly crystallized sillimanite needles define the foliation and wrap garnet and feldspar theta and delta type porphyroclasts in a biotite rich ultramylonite geothermometry and preserved microstructures in feldspar and quartz indicate deformation at temperatures 520 c reaction enhanced grain size reduction and grain boundary sliding were the predominant deformation mechanisms in these high strain rate rocks ultramylonites in the intermediate depth section also contain evidence for grain boundary sliding and diffusional mass transfer although overprinted by late stage chlorite in the shallowest exposed section rocks were deformed at or near to the brittle ductile transition to produce mylonite cataclasite shear bands and pseudotachylyte the overall structure of the tres arboles zone is consistent with existing fault zone models and suggests that below the brittle ductile transition strain compatibility may be accommodated through very thick zones of high temperature ultramylonite

arenig sedimentation and basin tectonics in the harlech dome area dolgellau basin north wales arenig ordovician clastic sediments crop out in the harlech dome region north wales and are placed in a single stratigraphic unit the allt lwyd formation this unit records a marine transgression onto an erosion surface produced during late tremadoc arc volcanicity four discrete petrofacies are denned and reflect differing proportions of detritus derived from tremadoc type basic intermediate igneous rocks and the local sedimentary basement initial shallow marine siliciclastic sandstones and conglomerates are overlain by extensive deep water mud rich units these generally shallow up into a complex arc apron deposit with sediments derived from the eroding tremadoc arc as well as from similar synchronous volcanics predominantly epiclastic sandstones and conglomerates were deposited in deltaic and tidal environments in an arc apron complex and capped by condensed mudstones and an ironstone deposited as sea level rose across these systems sediments were ponded in north south orientated troughs and derived from uplifted blocks facies and petrofacies distribution were controlled by syn sedimentary north south and northeast southwest faults the allt lwyd formation was ponded in a fault controlled basin the dolgellau basin one of a series of interconnected sub basins flooded by the arenig transgression the sediments preserved reflect deposition during the transgression of a volcanic arc prior to the extrusion of marginal basin type volcanics

rheological evolution of a pseudotachylyte bearing deep crustal shear zone in the western canadian shield abstract crustal scale continental strike slip fault systems are typically thought to be thermally segregated into an upper seismogenic expression and a lower zone of ductile deformation likely extending into the lower crust the ability of the lower crust to sustain stress is fundamental to understanding the mechanical behavior of the lithosphere yet significant debate exists regarding the strength of the ductile crust the cora lake shear zone in the western churchill province of the canadian shield is a 4 7 km wide zone of granulite to upper amphibolite facies mylonite which hosts synkinematic pseudotachylyte suggesting a locally strong lower crust kilometer scale mapping suggests that the shear zone localized at a major lithological and terrane boundary with the pseudotachylyte network centered within the ca 1 km wide more intensely deformed finer grained ultramylonitic core in this contribution we use optical petrography and electron backscatter diffraction to describe microstructure across a transect of the cora lake shear zone by combining these observations with grain size piezometry experimental flow laws and regional pressure temperature paths we conclude that stress amplification from a relatively common type of km scale lithological heterogeneity and cooling history is the most reasonable explanation for the deep crustal pseudotachylyte