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

corumbella an ediacaran grade organism from the late neoproterozoic of brazil abstract the morphology and phylogenetic affinities of corumbella werneri hahn g hahn r leonardos o h pflug h d and walde d h g 1982 korperlich erhaltene scyphozoen reste aus dem jungprakambrium brasiliens geologica et paleontologica 16 1 18 type species of the genus is reinterpreted based on new material from the tamengo formation corumba group upper neoproterozoic of western mato grosso do sul brazil corumbella secreted a narrow elongate tetraradially symmetrical tube reinterpretation of tube morphology and new evidence of reproduction by means of budding indicate close affinities with the present day coronate scyphozoan cnidarian stephanoscyphus and possibly the conulariids corumbella species were distributed between the amazon craton and laurentia corumbella is interpreted as a sessile predator and provides evidence of the importance of predation in animal evolution during the late neoproterozoic

the ages and tectonic setting of the faja eruptiva de la puna oriental ordovician nw argentina abstract the ordovician faja eruptiva de la puna oriental is a magmatic predominantly intrusive belt in the puna of northwestern argentina with a n s extension of ca 400 km scarce isotope geochemical ages and biostratigraphic data on some of the folded faja eruptiva country rocks assign the magmatism either to the lower and lower middle ordovician or to the latest ordovician interpretations of origin and tectonic framework of the faja eruptiva are controversial and vary between arc back arc and collisional orogenic settings we present high resolution la icp ms u pb age and hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200 km in the northern argentinian puna the xenocrystic and magmatic zircon age data have a wide spread between 2700 ma and 440 ma concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460 ma and between 453 and 444 ma and constrain the time of the last intrusions at 444 3 ma and at 445 2 ma thus defining this last and main phase of intrusion at 444 ma ehf t values define a main vertical trend centered at 500 ma with ehf t values between 3 and 16 indicating significant mixing of juvenile early paleozoic melts with paleoproterozoic crustal components a second trend is formed by zircons with ages between 1 1 ga and c 500 ma and predominantly positive ehf values between 8 and 3 and originates in juvenile mantle compositions between 1 6 and 1 1 ga the spread of the zircon and hf data document that the faja eruptiva intrusives have experienced large scale contamination by the hosting crustal basement it follows that the basement of the puna is formed either by the upper proterozoic lower cambrian puncoviscana formation as an erosional product of the proterozoic orogenic belts of sw amazonia or that the puna including its puncoviscana basement is underlain by a crust shaped by these orogenies the main intrusive event at 444 ma has been linked to the ocloyic orogeny in the late ordovician the plutons intruded very likely in a sinistral strike slip regime after the main folding phase of the ocloyic orogeny had deformed the ordovician sedimentary country rocks

seismic investigation of an active fault off kobe another disaster in the making on 17 january 1995 a magnitude 7 2 earthquake struck the kinki district of southwestern japan this earthquake called the hyogo ken nanbu earthquake but most familiarly known as the kobe earthquake killed more than 6000 people injured about 40 000 people and destroyed about 100 000 houses the hypocenter was about 16 km beneath the akashi strait between akashi city and the northern awaji island figure 1 its focal mechanism is right lateral slip trending northeast southwest which is produced by a regional compressive stress field that trends east west the aftershocks were aligned northeast southwest from north of kobe to the northern awaji island surface ruptures appeared along the nojima fault on the northwest coast of awaji island but no surface ruptures were observed in the kobe area

multiple migmatite events and cooling from granulite facies metamorphism within the famatina arc margin of northwest argentina the famatina margin records an orogenic cycle of convergence metamorphism magmatism and extension related to the accretion of the allochthonous precordillera terrane new structural petrologic and geochronologic data from the loma de las chacras region demonstrate two distinct episodes of lower crustal migmatization the first event preserves a counterclockwise pressure temperature path in kyanite k feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at 12 kbar and 868 c at 461 1 7 ma the shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive ordovician migmatization throughout the famatina margin one dimensional thermal modeling coupled with regional isotopic data suggests ordovician melts remained at temperatures above their solidus for 20 30 ma following peak granulite facies metamorphism throughout a time period marked by regional oblique convergence the onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of precordillera terrane accretion at 436 ma the second migmatite event was regionally localized and occurred at 700 c and 12 kbar between 411 and 407 ma via vapor saturated melting of muscovite migmatization was synchronous with extension exhumation and strike slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the chilenia terrane

mid to late cambrian docking of the rio de la plata craton to southwestern gondwana age constraints from u pb shrimp detrital zircon ages from sierras de ambato and velasco sierras pampeanas argentina abstract the early palaeozoic stratigraphy and tectonic history of the eastern sierras pampeanas of central argentina are complicated by metamorphism and deformation resulting from the pampean 545 510 ma and famatinian 490 440 ma orogenies we report u pb sensitive high resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at quebrada de la cebila c 28 459s 66 259w the ambato and the la cebila metamorphic complexes the ambato zircons record age peaks corresponding to pampean 530 10 ma brasiliano c 570 and c 640 ma grenville c 950 to c 1025 ma and minor neoarchaean ages similar peaks are also apparent in the la cebila sample but it additionally contains palaeoproterozoic zircons c 2 1 ga corresponding to the age of the rio de la plata craton from which they are considered to have been sourced our interpretation is that the protolith of the ambato complex was deposited prior to juxtaposition with the craton and is older than the early ordovician la cebila metamorphic complex we infer that the craton reached its current relative position in the mid to late cambrian after the main pampean tectonothermal event 530 520 ma and before deposition of the la cebila protolith and the achavil formation sierra de famatina which contain comparable detrital zircon populations

oxygen isotopes in iron iii oxides abstract quartz sio 2 and hematite fe 2 o 3 possess considerably different gram formula weights and different stoichiometric coefficients for their oxygen these differences mean that the numerical values of sio 2 and fe 2 o 3 compositions when reported as weight percent are not the same as the numerical values of x o i e oxygen in the mineral as a mole percent of the total oxygen in a binary quartz hematite system consequently end member quartz and hematite 18 o values which are determined from linear extrapolations of 18 o vs weight percent fe 2 o 3 data from binary mixtures can be in error by as much as several per mil extrapolation to end member 18 o values in binary quartz hematite systems should always be performed using mole fraction of oxygen as the compositional variable the hematite water fractionation curve of yapp and the quartz water curve of knauth and epstein were used together with the kinetic isotope exchange model of criss et al to constrain the depositional environment of the late proterozoic quartz hematite banded iron formation deposits of urucum brazil the depositional temperatures permitted by the model assumptions employed here range from 0 to 35 c or 54 c the permitted range of depositional water 18 o values is from 6 6 or 8 9 to 0 0 while the range of permitted depositional conditions is rather large it does encompass the possible cool temperatures and brackish or fresh waters which are indicated by sedimentologic evidence in these deposits

mid crustal deformation in a continental margin orogen structural evolution and timing of the famatinian orogeny nw argentina new structural data from a mid crustal segment in the eastern sierras pampeanas coupled with geochronological methods and p t estimates reveal polyphase contractional deformation and metamorphism during the famatinian orogeny over a long period of time peaks of metamorphic monazite and zircon ages are recorded at c 500 ma between 484 and 465 ma and at c 440 ma between 484 and 465 ma the region attained high temperature ht low pressure lp conditions that resulted in widespread partial melting peak at c 470 ma a contractional phase occurred during this event as suggested by syn anatectic structures as well as folding at subsolidus conditions renewed contraction under subsolidus conditions is evidenced by reverse ductile shearing and folding at c 440 ma thrusting along the la chilca shear zone caused metamorphic inversion the consistent orientation of folds in the quebrada del molle metamorphic complex el portezuelo metamorphic igneous complex and the la chilca shear zone as well as wsw directed thrusting at the later shear zone indicate uniform wsw ene directed shortening this late deformation records the ocloyic tectonic phase and brought the famatinian orogeny to an end in the late ordovician to early silurian supplementary material analytical methods methods and description of the calculated p t pseudosection diagrams bulk compositions and electron microprobe u pb and 40ar 39ar data are available at https doi org 10 6084 m9 figshare c 4697189

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

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

maximum and minimum water contents of granitic melts implications for chemical and physical properties of ascending magmas new results obtained by the investigation of liquidus and solidus phase relations in the system qz ab or and by the determination of h2o solubility in haplogranitic melts are used to discuss the evolution of granitic quartzo feldspathic magmas during their ascent in the crust particular attention is given to the role of h2o and its effect on physical properties of ascending magmas the evolution of melt fraction h2o content of the melt and viscosity of melt and magma during ascent are discussed for several examples adiabatic ascent ascent with cooling ascent with crystal fractionation it is shown that decompression melting processes may be very important significant increase of the melt proportion especially at low pressure in magmas ascending along adiabats or with slow cooling rates the comparison of the discussed examples shows that melts formed at low t are significantly less viscous than at high t although the degree of melting may remain very low in the case of dehydration melting at low temperature the low viscosity of the generated melts may enhance the extraction of these melts from the source area to produce leucogranites

some characteristics of the stress field of the 1995 hyogo ken nanbu kobe earthquake we investigate the characteristics of the stress field associated with the 1995 hyogo ken nanbu kobe earthquake we use for the study a tomographic model of the fault slip inferred from the numerous near field recordings and we calculate the space and time evolution of shear stress on the fault during the earthquake we show that the spatial distribution of stress drop is very heterogeneous the apparent strength of the fault at the onset of the earthquake is low of the order of 1 mpa or less which indicates that the pre earthquake tectonic shear stress was close to the static friction over most of the fault at many locations the stress drop rotates significantly during sliding as was originally proposed by spudich 1992 we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake the pre earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 mpa average values of the initial and final shear stresses over the fault are 3 3 and 1 6 mpa respectively indicating that about half of the pre earthquake tectonic stress was released during the earthquake there is a relatively strong correlation between the initial and final stress distributions which suggests that intrinsic fault properties not modified by the earthquake control the spatial distribution of tectonic stress over the fault we show that on average over the fault the dynamic friction coefficient is equal to about 40 of the static friction coefficient diagrams depicting the evolution of shear stress as a function of slip are consistent with slip weakening behavior but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion

the neoproterozoic cambrian paraguay belt central brazil part i new structural data and a new approach on the regional implications abstract together with the araguaia and brasilia belts the paraguay belt forms in central brazil the tocantins province that is one of the largest orogens of western gondwana the corumba area occupies the site where the northern and southern parts of the paraguay belt form together with the chiquitos tucavaca aulacogen stretching e w in the adjacent bolivian territory an r r r basin system opened filled in the 700 650 540 ma interval within the amazon rio apa paleo continent the sedimentary volcanic rocks of the jacadigo and corumba groups found around the corumba city record part of the neoproterozoic cambrian passive margin precursor of the paraguay belt our pioneer structural analysis reveals that these rocks experienced progressive deformation phases d 1 d 2 d 3 and low grade metamorphism during the brasiliano cycle 540 513 ma the crystalline basement was also involved according to structural data and k ar ages in the literature the paleo passive margin was thickened during the d 1 d 2 deformation and was lately shortened d 3 in two orthogonal directions se nw d 3p and sw ne d 3t developed co axially and verging to nw d 1 d 2 d 3p structures record the closure of the basin precursor of the paraguay belt whereas d 3t structures seem related to the inversion of the aulacogen although the tectonic transport to nw as observed in the corumba area matches the reported transport of paraguay belt s supracrustal rocks towards the eastern margin of the rio apa block and araguaia belt s rocks towards the amazon craton the transport direction is opposite in other parts of the paraguay belt our comprehensive discussion of these facts brings to light profound regional implications

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

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

pampean deformation in the sierra norte de cordoba argentina implications for the collisional history at the western pre andean gondwana margin 1 in the sierra norte de cordoba argentina at the inferred western pre andean gondwana margin structural analyses show that upper proterozoic lower cambrian strips of metamorphic clastic rocks underwent two stages of a compressive deformational history the d1 deformation led to the formation of an s1 cleavage foliation and affected an older granite it is related to f1 folding around n s to ne sw trending axes that partly record a steep plunge reconstructed fold structures display maximum wavelengths and amplitudes on a scale of hundreds of meters to kilometers a steep to overturned dip of thick packages of metaconglomerates and metasandstones is related to this folding event also because of variably oriented l1 lineations and dextral shearing structures of the d1 deformation are assigned to different structural domains interpreted as the result of strain partitioning during dextral transpression strips of post d1 granite and clastic mylonites with a dextral sense of shear are attributed to the second stage deformational event the late stage dextral shearing and mylonitization locally affected intrusions of granites and dacitic porphyries in the country rocks it took place along preexisting s1 planes or locally developed s2 c2 planes related to f2 fold structures with steeply plunging axes the regional metamorphism accompanying the deformation was in the range of the subgreenschist facies greenschist facies transition and greenschist facies the emplacement of synkinematic intrusions presumably followed active dextral shear zones it cannot be excluded that during compressive deformation older normal fault lines were reactivated and caused strain partitioning and later dextral shearing after cessation of compressive deformation granites and granodiorites intruded the deformed clastic succession and led to contact metamorphism of variable extent younger high level intrusions record chilled margins also at angular xenoliths isotopic dates from granitoids dacites and metaclastic rocks in the sierra norte area and surroundings suggest that compressive deformations and metamorphism took place during the early cambrian pampean orogeny the deformational and metamorphic history can be attributed to either ridge subduction collision or collision of the pampean terrane with the western pre andean gondwana margin the two stage kinematics suggests that oblique collision was followed by a change to dextral displacements in the upper plate during the final state of and after collisional tectonics the sierra norte area represented the upper eastern plate as supposed part of the rio de la plata craton and its cover overriding the lower plate that consisted of an accretionary complex in the west

neoproterozoic backarc basin sensitive high resolution ion microprobe u pb and sm nd isotopic evidence from the eastern pampean ranges argentina the eastern pampean ranges comprise high grade supracrustal sequences with linear belts of mafic ultramafic bodies representing ophiolite remnants new u pb and nd isotopic data suggest that the tectonic evolution of the pampean ranges started ca 640 ma with the deposition of supracrustal sequences in a backarc basin between a neoproterozoic magmatic arc to the east and the pampia terrane to the west ophiolite remnants of this backarc basin yielded a whole rock isochron indicating the age of 647 77 ma 2 and nd initial time t of 5 2 sensitive high resolution ion microprobe u pb data for detrital metasediments show provenance patterns with two main age populations the older 1 1 0 9 ga and the younger between ca 0 7 and 0 6 ga the neoproterozoic population is relatively more abundant in sediments of the easternmost units of the eastern pampean ranges and becomes less abundant toward the west depleted mantle ages show a similar pattern with ages generally increasing from east 1 42 ga to west 1 76 ga suggesting the presence of neo proterozoic sources to the east of the ranges the provenance data do not support previous evolution models for the eastern pampean ranges according to which the supracrustal sequence represents the passive margin of the rio de la plata craton early cambrian collision and high grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc alkaline metaluminous and peraluminous granitic magmatism ca 530 514 ma the results suggest that the geological evolution of the eastern pampean ranges took place between ca 640 and 514 ma coeval with other brasiliano orogens in brazil e g the paraguay and araguaia fold belts

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

basement evolution of the sierra de la ventana fold belt new evidence for cambrian continental rifting along the southern margin of gondwana u pb sensitive high resolution ion microprobe data together with geochemical and nd isotope analyses obtained in the basement complex of the sierra de la ventana fold belt indicate that the early palaeozoic passive margin history of the basin followed cambrian magmatism related to rifting in a 600 ma neoproterozoic crust the cambrian episode started with intrusion of 531 4 and 524 5 ma a and i type granites derived from a dehydrated infracrustal source e nd 530 3 1 to 5 9 and culminated with eruption of high zr peralkaline spherulitic rhyolites derived from an undepleted lithospheric mantle 509 5 ma e nd 509 0 5 to 1 0 these rift related magmatic rocks were covered by shelf sediments deposited along a once continuous passive margin encompassing the sierra de la ventana fold belt the cape fold belt the falkland malvinas microplate and the ellsworth mountains block in antarctica the cambrian rifting event defined the outline shape of the southern part of gondwana and can be regarded as the initiation of the supercontinent stage which lasted until jurassic break up the conjugate continental fragments separated from gondwana during the cambrian rifting could be the source for microcontinents with c 1000 ma basement rocks that collided with the proto andean margin during ordovician silurian times

fault rocks and past to recent fluid characteristics from the borehole survey of the nojima fault ruptured in the 1995 kobe earthquake southwest japan the mineralogy fluid inclusions and distribution of fault rocks of the nojima fault were examined in the core recovered from a borehole drilled by the geological survey of japan gsj 12 months after the 1995 kobe hyogo ken nanbu earthquake mjma 7 2 in southwest japan the borehole was drilled across a slipped portion of the fault to a depth of 746 7 m nearly continuous coring between 152 2 and 746 7 m recovered granodiorite protolith porphyry dikes and fault related rocks the fault zone was intersected at 426 2 m and is characterized by a greater intensity of brittle deformation and or hydrothermal alteration than typical host granodiorite the fault core consists of three types of fault gouge and occurs at the depth range of 623 1 to 625 3 m the fault normal thicknesses of the fault core and the fault zone are 0 3 m and 46 5 m respectively three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions the first type is characterized by chloritization of mafic minerals at 200 c and occurred prior to the fault activity during the intrusion and cooling of the granodiorite the second type occurred during faulting and is recorded by zeolite mineralization at 200 c the third type is recorded by carbonate mineralization related to recent fluid flow although most of the second type of alteration occurred prior to the third type repeated mineralization is recorded by mutually crosscutting relationships between zeolite and carbonate veins and between zeolite vein and carbonate precipitated fault gouge this may record repeated changes in fluid chemistry within the fault zone in connection with the seismic cycle although the nojima fault slipped in the 1995 earthquake ancient fault related textures and mineral alteration are well preserved in the fault rocks

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

timing and rate of granulite facies metamorphism and cooling from multi mineral chronology on migmatitic gneisses sierras de la huerta and valle fertil nw argentina abstract migmatitic paragneisses of the valle fertil la huerta ranges at the western margin of the sierras pampeanas are composed of garnet cordierite plagioclase biotite quartz bearing units that experienced peak metamorphic conditions of ca 800 c at 6 7 kbar based on petrological studies pseudosection modeling and petrographic observations an anticlockwise p t path with a small pressure increment is proposed rare earth element la icp ms patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high t at pressures above the ilmenite rutile transition u pb dating of zircon rims from the migmatites indicates two distinct metamorphic u pb ages of 525 9 ma and 478 9 ma the older age is suggested to record an amphibolite facies event of the pampean orogeny the younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478 4 ma we suggest that the prograde high t metamorphic famatinian event is associated with the emplacement of large magmatic bodies in which large scale magmatic activity gave rise to an increased geothermal gradient of about 35 c km sm nd garnet ages of 447 3 ma indicate a time span of around 30 ma for which temperatures above the garnet closure temperature prevailed using u pb sm nd and rb sr isotope systems a cooling rate of 3 to 6 c myr is inferred

a cambrian mixed carbonate siliciclastic platform in sw gondwana evidence from the western sierras pampeanas argentina and implications for the early paleozoic paleogeography of the proto andean margin the western sierras pampeanas wsp of argentina record a protracted geological history from the mesoproterozoic assembly of the rodinia supercontinent to the early paleozoic tectonic evolution of sw gondwana two well known orogenies took place at the proto andean margin of gondwana in the cambrian and the ordovician i e the pampean 545 520 ma and famatinian 490 440 ma orogenies respectively between them an extensive continental platform was developed where mixed carbonate siliciclastic sedimentation occurred this platform was later involved in the famatinian orogeny when it underwent penetrative deformation and metamorphism the platform apparently extended from patagonia to northwestern argentina and the eastern sierras pampeanas and has probable equivalents in sw africa peru and bolivia the wsp record the outer deepest part of the platform where carbonates were deposited in addition to siliciclastic sediments detrital zircon u pb shrimp ages from clastic metasedimentary successions and sr isotope compositions of marbles from the wsp suggest depositional ages between ca 525 and 490 ma the detrital zircon age patterns further suggest that clastic sedimentation took place in two stages the first was sourced mainly from re working of the underlying neoproterozoic metasedimentary rocks and the uplifted core of the early cambrian pampean orogen without input from the paleoproterozoic rio de la plata craton sediments of the second stage resulted from the erosion of the still emerged pampean belt and the neoproterozoic brasiliano orogen in the ne with some contribution from the rio de la plata craton an important conclusion is that the wsp basement was already part of sw gondwana in the early cambrian and not part of the exotic precordillera cuyania terrane as was previously thought

a review of the famatinian ordovician magmatism in southern south america evidence of lithosphere reworking and continental subduction in the early proto andean margin of gondwana abstract along the proto pacific margin of gondwana from venezuela to northeastern patagonia the early middle ordovician famatinian orogeny was the first orogenic event following assembly of the supercontinent previous isotope studies of the igneous and meta sedimentary rocks of southwestern gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin to interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern south america including the 700 600 km type sector of the orogen in the sierras pampeanas 27 33 s the precordillera and northeastern patagonia previous geological geochemical and geochronological results are reviewed together with new u pb shrimp crystallization ages 177hf 176hf and 18o 16o data for dated zircon and whole rock sr and nd isotope compositions four geological domains are recognized in the sierras pampeanas western central eastern and foreland famatinian domains magmatism is mostly restricted to the interval 463 4 to 486 7 ma with the most intense period of emplacement between 468 and 472 ma constituting a magmatic flare up granitoid emplacement in both northeastern patagonia and the cordon de lila puna altiplano chile was effectively synchronous with that in the sierras pampeanas defining a continuous belt combined geochemical and isotopic data whole rock sr nd hf o in zircon indicate that the source of calcic metaluminous suites is the subcontinental lithosphere both mantle and mafic lower crust with variable contamination by the early paleozoic metasedimentary country rocks the lithospheric mantle involved is assumed to underlie the outcropping 1330 1030 ma age basement of the western domain which exhibits tectonic characteristics of active continental margin in the north and oceanic arc back arc in the south the latter sector is the potential source of some minor famatinian igneous rocks with less evolved isotopic compositions although a restricted asthenospheric addition cannot be discarded in this case minor peraluminous granites are spatially associated with the metaluminous sequence but major highly peraluminous batholiths occur on the eastern flank of the central domain field relations and geochemical isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post early cambrian metasedimentary sequence comprising the host country rocks episodic tectono magmatic evolution of the famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector ca 474 486 ma roll back stage this is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly peraluminous batholiths in the central and eastern famatinian domains trondhjemite plutons with an adakitic signature were emplaced in the foreland domain ca 468 472 ma slab break off stage steepening of the oceanic slab and arc migration to the southwest ended with slab break off due to subduction of continental crust during continental collision with the precordillera terrane this stage produced voluminous metaluminous magmatism at the western edge of the central domain the flare up episode k bentonites in the precordillera leucogranites in the western domain and scattered metaluminous and peraluminous plutons in all famatinian domains both slab roll back and break off stages developed during a high t regime typical of hot orogens although asthenospheric mantle was a necessary heat source for lithospheric melting its material contribution to the growth of early paleozoic crust was apparently very minor recycling of mesoproterozoic lithosphere including the subcontinental mantle coupled with crustal melting of early paleozoic metasedimentary sequences accounts for most famatinian magmatism comparable results from the central andes and east antarctica confirm that the early stages of the terra australis orogen in sw gondwana were dominated by lithospheric reworking processes

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

structural and chemical characterization of shear zones in the freshly activated nojima fault awaji island southwest japan behavior and role of each shear zone in a shallow fault zone of granitic origin during seismic cycles are revealed by comprehensive examinations of petrographic and chemical characterization on a fault zone in the geological survey of japan gsj drill core penetrating the nojima fault which was activated during the 1995 hyogoken nanbu earthquake m 7 2 the gsj core consists of granodiorite and porphyritic intrusive rocks including a nojima fault zone which involves seven thin shear zones main shear zone msz 625 m depth upper cataclasite zone ucz upper shear zone usz lower shear zones lsz 1 and lsz 2 and lower cataclasite zones lcz 1 and lcz 2 these shear zones are generally surrounded by weakly pulverized and altered fault related rocks wpar which generally show volume gain the fault zone architecture is clarified as follows 1 total thickness of the nojima fault zone is 70 m 2 all shear zones except the older cataclasite zones ucz 1 lcz 1 and lcz 2 were evolved from wpar indicating that pulverization and alteration of recent activity were more diffused at the initial stage of faulting and gradually localized to each shear zone 3 the msz 2 m thick can be regarded as a high velocity frictional zone with accompanying volume loss compaction and possibly with heat generation during coseismic periods 4 the lsz 1 3 6 m thick located just beneath the msz and typically showing explosion brecciation texture is also regarded as a coseismic shear zone this zone could function as a trap zone for fluid or gas during postseismic interseismic periods 5 the lsz 2 2 7 m thick located around 710 m depth contains foliated fault gouge enriched with clay minerals and characterized by a large degree of volume gain possibly a result of slow velocity motion or creep during the interseismic and or postseismic periods

urucum neoproterozoic cambrian manganese deposits ms brazil biogenic participation in the ore genesis geology geochemistry and depositional environment abstract the urucum region mato grosso do sul brazil contains neoproterozoic cambrian manganese ore in the layers mn 1 mn 2 and mn 3 which are interbedded with iron formations ifs from the santa cruz formation jacadigo group and surrounded by carbonate rocks from the bocaina and tamengo formations corumba group these rocks fill an ancient graben with sediments from the santa cruz formation overlying fluvial deposits from the urucum formation and limestones from the bocaina and tamengo formations deposited in the shallow marginal regions as the graben deepened it was flooded with microbe rich seawater and hydrothermalism began iron and manganese precipitated within the pores in arkosic sandstones thereby forming the clastic lower mn 1 ore layer the remaining manganese was simultaneously deposited on the basin floor with clastic clay and silt from turbidity currents thereby forming the upper mn 1 clastic chemical ore layer the sedimentation of mn 1 and its host ifs was controlled by seawater saturation and homogeneous oxidation which caused manganese and iron sedimentation and the death of microorganisms generating mangano ferriferous sediments with high p co and ni contents mn 2 formed during periods of tectonic quiescence through water influxes that carried abundant microbes fe ii mn ii and sio 2 under uv radiation these microbes oxidized mn ii and fe ii to form hydroxides and died thereby forming biomass that was enriched in p co and ni and minor carbonate adjacent to and below the oxycline these microbes also formed kremydilites oblate structures that resemble manganese stromatolites within the biomass and produced ch 4 and or h 2 gas bubbles during early diagenesis wormlike organisms inhabited the biomass and probably separated iron from manganese to produce cryptomelane microbialites which agglutinated to form manganese micronodules that coalesced to form the massive layer mn 2 this process ended with the disappearance of microorganisms and the sedimentation of the ifs and a new water influx generated layer mn 3 via the same process the final seawater upwelling precipitated ifs in the basin depocenter whereas limestone from the tamengo formation was deposited along the basin s edges the diagenesis initially transformed the manganese oxides into cryptomelane and then into hollandite

isotope sr c and u pb shrimp zircon geochronology of marble bearing sedimentary series in the eastern sierras pampeanas argentina constraining the sw gondwana margin in ediacaran to early cambrian times abstract the sierras de cordoba metasedimentary series consists of marbles and metasiliciclastic rocks of ediacaran to early cambrian age ca 630 and 540 ma that underwent high grade metamorphism during the collisional pampean orogeny in the early cambrian the ages of the marbles were determined from the sr isotope composition blind dating of screened samples of almost pure calcite marble and were further constrained with c and o isotope data and u pb shrimp detrital zircon ages of an interbedded paragneiss two groups of samples are recognised with sr isotope composition ca 0 7075 and 0 7085 that are considered stratigraphically significant the first is inferred early ediacaran the second late ediacaran to early cambrian the sierras de cordoba metasedimentary series is correlated for the first time with marble bearing metasedimentary series in several sierras to the west and north of sierras de cordoba e g the difunta correa sedimentary sequence and the ancajan series implying that all were probably parts of an originally extensive sedimentary cover these series bear evidence of sedimentary sources in the mesoproterozoic and paleoproterozoic basement of the western sierras pampeanas part of the large mara continental block and farther west laurentia in terms of the age of limestones marbles and detrital zircon patterns the sierras de cordoba metasedimentary series differs strongly from the older section of late ediacaran to early cambrian puncoviscana formation of northwestern argentina which mostly outcrops in northern sierra chica and sierra norte with sedimentary input from western gondwana sources the sierras de cordoba metasedimentary series and the puncoviscana formation were probably juxtaposed during the pampean orogeny along a complex suture zone that was further folded and or imbricated at mid crustal depths the peak of metamorphism was attained at 527 2 ma according to the evidence found here most of the sierras pampeanas to the west of the sierras de cordoba were part of the lower colliding plate during the final amalgamation of sw gondwana

stratigraphy and mineralogy of the manganese ores of urucum mato grosso brazil preliminar results of geological mapping and microscopic studies are presented the mineral deposits of urucum are clearly different from the rest of the banded iron formations in brazil urucum is essentially younger and probably late proterozoic the sequence is unmetamorphic two main types of manganese ore exist in urucum namely

late diagenetic changes in manganese ores of the upper proterozoic penganga group india sedimentary manganese oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive upper proterozoic marine sequence of the penganga group andhra pradesh india this sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed the manganeseoxide ores show unusual coexistence of the primary sedimentary minerals todorokite and birnessite with their diagenetic conversion products manganite braunite and bixbyite the mineralogical characters and chemical compositions of the important phases as well as their texture are discussed braunite occurs in two generations one earlier or contemporaneous with bixbyite and the other later than bixbyite the different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved the general observations suggest that bixbyite was also produced by conversion of todorokite birnessite pyrolusite and cryptomelane conventionally considered as diagenetic products of mn oxide hydroxide precursors deposited as primary sediments are here rarely present and then only as supergene derivatives this documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese oxide ores that abound in the precambrian

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

early evolution of the proto andean margin of south america from a detailed study of a 500 km transect in the sierras pampeanas central west argentina two pre silurian tectono magmatic episodes are recognized and defined each culminating in micro continental collisions against the proto andean margin of gondwana the pampean orogeny started in early cambrian time with short lived subduction indicated by ca 535 ma calc alkaline granitoids following pampean terrane collision burial to granulite facies conditions ca 9 kbar generated widespread migmatites and ca 520 ma highly peraluminous granites in the eastern sierras pampeanas after brief quiescence a second major episode the famatinian orogeny started with subduction ca 490 ma forming a wide continental arc and ensialic backarc basin this heralded the approach of laurentia to gondwana during which the precordillera terrane separated from the southern appalachian region finally colliding with gondwana in silurian devonian time

structural evolution of a composite middle to lower crustal section the sierra de pie de palo northwest argentina 1 the sierra de pie de palo of northwest argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during ordovician accretion of the precordillera terrane to the gondwana margin new structural petrologic and geochronologic data from a 40 km structural transect reveals that the sierra de pie de palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence top to the west thrusting occurred intermittently on discrete ductile shear zones from 515 to 417 ma and generally propagated toward the foreland with progressive deformation ordovician crustal shortening and peak metamorphic temperatures in the central portion of the sierra de pie de palo were synchronous with retro arc shortening and magmatic flare up within the famatina arc accretion of the precordillera terrane resulted in the end of arc flare up and the onset of synconvergent extension by 439 ma continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the precordillera terrane the results support models of cordilleran orogens that link voluminous arc magmatism to periods of regional shortening the deformation metamorphic and magmatic history within the sierra de pie de palo is consistent with models placing the region adjacent to the famatina margin in the middle cambrian and not as basement to the precordillera terrane

lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of new caledonia trough and transient uplift of lord howe rise during eocene and oligocene initiation of tonga kermadec subduction western pacific 1 we use seismic reflection and rock sample data to propose that the first order physiography of new caledonia trough and norfolk ridge formed in eocene and oligocene time and was associated with the onset of subduction and back arc spreading at the australia pacific plate boundary our tectonic model involves an initial cretaceous rift that is strongly modified by cenozoic subduction initiation hence we are able to explain 1 complex sedimentary basins of inferred mesozoic age 2 a prominent unconformity and onlap surface of middle eocene to early miocene age at the base of flat lying sediments beneath the axis of new caledonia trough 3 gently dipping variable thickness and locally deformed late cretaceous strata along the margins of the trough 4 platform morphology and unconformities on either side of the trough that indicate a phase of late eocene to early miocene uplift to near sea level followed by rapid oligocene and miocene subsidence of 1100 1800 m and 5 seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of new caledonia trough by 1800 2200 m since eocene time the cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab this created a deep 2 km enclosed oceanic trough 2000 km long and 200 300 km across in eocene and oligocene time as the lower crust detached with simultaneous uplift and local land development along basin flanks disruption of late cretaceous and paleogene strata was minimal during this cenozoic phase and involved only subtle tilting and local reverse faulting or folding basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into norfolk basin we suggest that delamination of the lithosphere with possible mixing of the lower crust back into the mantle is more widespread than previously thought and may be commonly associated with subduction initiation such as cenozoic events in the mediterranean and western pacific

the pacific gondwana margin in the late neoproterozoic early paleozoic detrital zircon u pb ages from metasediments in northwest argentina reveal their maximum age provenance and tectonic setting abstract u pb detrital zircon ages are reported from puncoviscana formation late neoproterozoic early cambrian and meson group late cambrian greywackes of northwest argentina to constrain provenance and depositional environment the new data are combined with previously published detrital zircon ages to show that puncoviscana formation age patterns contain two broad groups late mesoproterozoic early neoproterozoic 1150 850 ma and late neoproterozoic early cambrian 650 520 ma with their relative proportions varying inversely with youngest component age the 1150 850 ma age components are dominant in greywackes with oldest late neoproterozoic components 600 ma the former diminish considerably when late neoproteozoic components become dominant and younger to 520 ma a northernmost greywacke sample from purmamarca jujuy is distinctive whilst its zircon age pattern partly resembles other puncoviscana formation samples it contains no cambrian late neoproterozoic ages the youngest ages being early neoproterozoic this may reflect an early neoproterozoic passive margin depocentre for the formation or an older early neoproterozoic succession within it which may predate the brasiliano orogeny in brazil the youngest age components c 520 ma in a greywacke from rancagua cachi salta province dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late early cambrian depocentre detrital zircon age patterns of the meson group lizoite formation have major cambrian latest neoproterozoic components resembling those of the puncoviscana formation but its mesoproterozoic component is diminished and there are no significant age components of this age small youngest components at c 500 ma suggest a maximum late cambrian stratigraphic age the puncoviscana formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised extensive late mesoproterozoic orogen with minor paleoproterozoic and archean precursors and a more variable late neoproterozoic orogen containing an evolving sequence of less extensive subcomponents a direct relationship with the brazilian shield is suggested with sediment supplies originating within active margin orogens of the interior and collisional orogens at the suture between african and south american cratons but ultimate deposition in passive margin environments of western gondwanaland

effects of frictional heating on the thermal hydrologic and mechanical response of a fault the mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating in this study we formulate a fault model which incorporates the effects of frictional heating on the thermal hydrologic and mechanical response of a small patch of the failure surface this model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant the problem has two time scales a characteristic slip duration and a characteristic time for thermal pressurization the slip duration is set by the fault geometry the characteristic time for thermal pressurization is set by the slip rate the friction coefficient and the thermal and hydraulic characteristics of the medium the response of the fault depends on the relative magnitude of these two times results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response for earthquakes occurring across zones of low porous medium compressibility 10 9 pa 1 and permeability 10 18 m2 the characteristic time for thermal pressurization is small in this case frictional heating can cause fluid pressures to approach lithostatic values the shear strength to approach zero and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault moderate slip events where shear strains exceed two cause substantial strain weakening and consequently large stress drops accelerations and displacements thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones both the dynamic stress drop and total displacement decrease for zones with larger compressibility permeability or width if the compressibility or permeability exceeds 10 8 pa 1 or 10 14 m2 or the shear strain is less than one then the effects of frictional heating may be negligible and the fault will exhibit no strain weakening characteristics consequently the patch acts as barrier that halts or resists further fault motion extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface explaining many of the features of active faulting e g barriers nonuniform slip rupture stoppage random ground accelerations strong motions and frequency magnitude relations

fast sediment underplating and essentially coeval juvenile magmatism in the ordovician margin of gondwana western sierras pampeanas argentina metasedimentary high pressure upper amphibolite facies gneisses 1 2 0 1 gpa and 780 45 c at las chacras sierra de valle fertil are tectonically juxtaposed to the westernmost parts outboard of the famatinian early ordovician magmatic arc which underwent syn plutonic middle crust high grade metamorphism at lower pressure u pb shrimp zircon data suggest that the gneisses contain famatinian igneous detritus so that their sedimentary protoliths were probably deposited in a forearc basin and then rapidly underthrust and accreted to the lower crust of the arc essentially coevally with arc magmatism at 468 4 ma chemically and isotopically juvenile garnetiferous amphibolites within the gneisses are recognised as representing the most primitive magmas so far observed in this belt which has often been considered to be a continental arc derived from isotopically mature sources this is consistent with the idea that at least part of the dominant famatinian magmatism originated in depleted mantle but was heavily contaminated by crustal components

neoproterozoic to early cambrian crustal evolution of the pan african saldania belt south africa abstract the saldania belt forms part of a system of neoproterozoic mobile belts that surround and weld older cratons on the african continent it is a poorly exposed low grade orogenic belt composed of a number of inliers unroofed in mega anticlinal hinges of the permo triassic cape fold belt along the southern tip of africa deformed meta volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep rooted sinistral dextral strike slip fault zones which display evidence of prolonged reactivation sedimentary rocks and volcanics of the boland terrane malmesbury group lower kango group and gamtoos group are considered to be distal facies of rift successions overlying the mid proterozoic crystalline basement on the southwestern edge of the kalahari craton these lithologies have been deposited in semi pull apart basins that formed in response to the breakup of rodinia and progressive opening of the proto atlantic adamaster ocean from 780 750 ma fragments of juvenile pan african crust with wpb morb characteristics occur in the bridgetown formation swartland terrane malmesbury inlier and attest to ocean floor spreading in this belt the thick turbidite successions of the swartland and tygerberg terranes malmesbury group reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean continental margin basin marked by increasing sea levels and reduction of continental freeboard reversal of spreading and closing of the adamastor ocean possibly initiated by the opening of iapetus occurred in the period 600 570 ma although this movement vector caused sinistral transpressional reactivation of the gariep and saldanian margins there is no proper collision orogen developed in the saldania belt the bridgetown formation represents the suture between the southern rifted margin succession of the kalahari craton and a poorly developed southern orogenic belt represented in part by the swartland and tygerberg terranes this geosuture is similar to that of the south atlantic which formed in the analogous gariep belt between 575 and 543 ma multiphase s i and a type granites of the cape granite suite intruded the saldania belt in a pervasive transpressive regime between 550 and 510 ma comparing well with the timing of peak metamorphism collision in the gariep belt 545 ma the absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the saldania belt seem to suggest that the cape granite suite was generated in a strike slip regime a discrepancy of approximately 50 ma between the age of the collision granites of the dom feliciano belt and cape granite suite suggests that they are genetically unrelated similarities in structural styles magmatism and dated events between the saldania ross delamarian chain of orogens however indicate a common history the evolution of this chain and that of the gariep damara dom feliciano belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of laurentia from south america and opening of the iapetus ocean loading and depression of the western and southern kalahari craton margin by thrust stacks of the late neoproterozoic orogens led to the formation of syn to post orogenic peripheral foreland basins such as those represented by the nama and vanrhynsdorp groups lowering of sea level and increased continental freeboard deposition of the unconformity based upper kango group and possibly the molassic franschhoek formation occurred in intra orogen pull apart basins which possibly developed between 530 and 510 ma in the saldania belt

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

coeval formation of cataclasite and pseudotachylyte in a miocene forearc granodiorite southern kyushu japan abstract cataclastic rocks and pseudotachylytes are exposed along the uchinoura shear zone a normal fault zone cutting the middle miocene 14 ma osumi granodiorite in southern kyushu japan cataclastic rocks include non foliated clast supported to matrix supported cataclasite and foliated clast supported cataclastic granodiorite in these rocks fracturing and comminution played a major role but dissolution and recrystallization of quartz and plastic deformation of quartz and biotite were also active processes especially in foliated granodiorite two types of pseudotachylyte are distinguished a foliated type characterized by a planar arrangement of clasts and microlites and a spherulitic type characterized by clasts surrounded by microlite overgrowths both types are of melt origin as attested by the presence of microlites and rounded or embayed clasts and by the scarcity of biotite clasts unlike spherulitic type pseudotachylyte which solidified without being deformed the foliated type pseudotachylyte underwent flow before complete solidification this deformation is thought to reflect post seismic strain accommodation immediately following the main slip episode kinematic indicators which consist of riedel type secondary fractures branching on primary fractures shear bands offsetting the foliation of foliated granodiorite or asymmetrical porphyroclast systems within pseudotachylyte veins show that all fault rocks were generated during n s to nw se directed extensional deformation pseudotachylyte is closely associated both in time and space with cataclastic rocks thus indicating that the behaviour of the uchinoura fault zone alternated between comminution and frictional melting given the slow strain rates which characterize dissolution and recrystallization processes detected in cataclasites the juxtaposition of pseudotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone

sr c and o isotope geochemistry and stratigraphy of precambrian and lower paleozoic carbonate sequences from the western sierras pampeanas of argentina tectonic implications abstract sr c and o isotope data are presented for carbonate rocks from the sierra de pie de palo and other crystalline outcrops of the western sierras pampeanas these are used to distinguish three groups of rocks of quite different ages within the nappe pile that constitutes the sierra de pie de palo carbonates from the grenville age ophiolitic unit at the bottom of the pile probably resulted from the interaction between hot seawater and contemporaneous oceanic crust these carbonates are tentatively classified as ophicalcites carbonates from the difunta correa sedimentary sequence are part of a cover sequence to a grenvillian basement together forming the upper nappes isotope stratigraphy suggests a middle to late neoproterozoic age 580 720 ma for these carbonates this cover might be correlated with cover sequences to the archean and early to middle proterozoic cratons that are well preserved over large areas of brazil this fact together with the apparently absence of an equivalent neoproterozoic carbonate bearing cover in the appalachian margin of laurentia suggests that the western sierras pampeanas which are considered part of the exotic argentine precordillera terrane of allegedly laurentian derivation could be autochthonous or para autochthonous to gondwana other geochemical and geological arguments reinforce this hypothesis the caucete group carbonates underlie the nappe pile and are separated from it by a first order thrust the pirquitas thrust these latter carbonates are cambrian in age and isotopically similar to the carbonate platform of the precordillera we thus conclude that the pirquitas thrust is the boundary between the exotic precordillera terrane and the autochthonous or para autochthonous western sierras pampeanas

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 provenance study of the paleozoic ventania system argentina transient complex sources from western and eastern gondwana abstract the u pb and lu hf isotopic analyses of the different sedimentary sequences of the ventania system an old paleozoic orogenic belt exposed in the southern region of the rio de la plata craton in the province of buenos aires argentina provide new evidence for the understanding of the tectonic evolution of the western sector of the gondwanides mountain belt these ranges formed as result of the late paleozoic collision of the patagonia terrane against the continental margin of gondwana the provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the tandilia system a paleoproterozoic mountain belt formed during the amalgamation of the rio de la plata craton at about 1800 2200 ma and incorporated to western gondwana during the brasiliano orogeny at 550 530 ma the local dominant source at the base of the early paleozoic changed to more distant supplies toward the top of the sequences when an increasing participation of detritus from first cambrian 560 520 ma zircons from the pampean orogen and later on ordovician 480 460 ma zircons from the famatinian orogen is recorded the detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early paleozoic stratigraphy the balcarce formation an early paleozoic sedimentary cover of the tandilia metamorphic and igneous basement shows striking differences when compared with the new data from the ventania system the two data sets reveal different sources for the two regions the late paleozoic foreland basin deposits mark an abrupt change of 180 in the paleocurrent directions in the petrographic composition of the sediments and in the provenance of detrital zircons these data indicate a southern provenance with the first evidence of carboniferous and permian magmatic zircons the oldest archean zircons together with the finding of clasts with archeocyathids support the provenance from patagonia which was derived from eastern gondwana the u pb ages of the ash fall tuffs in the tunas formation confirm the early permian age of the eurydesma fauna in the ventania system the u pb data together with the lu hf isotopic data enhance the comprehension of the tectonic evolution of the ventania system as part of the larger gondwanides belt that amalgamated to western gondwana during late permian times with some independent pieces derived from eastern gondwana

fluid inclusions in pseudotachylytes from the nojima fault japan pseudotachylytes i e rocks formed by frictional melting have been observed in the nojima fault that was penetrated by the hirabayashi borehole drilled 1 year after the 1995 hyogo ken nanbu kobe earthquake these rocks display millimeter scale banding defined by different pseudotachylyte layers the nature of unmolten crystal fragments k feldspar albite calcite and or quartz allows us to infer a minimum melting temperature of 1200 c the glass has a 8 3 volatile content and a higher cao content than that of the parent granodiorite thus suggesting that pseudotachylyte formation occurred in an already altered calcite bearing and hydrated fault zone fluid inclusions have been observed in the glass and are filled with a dense low salinity 4 3 1 2 wt eq nacl co2 h2o fluid characterized by steep isochoric curves the intersection between the isochores and the measured 24 c km geothermal gradient indicates a minimum 15 km depth for the pseudotachylyte formation such a depth suggests that an important uplift has brought the studied pseudotachylytes to their present position and that these pseudotachylytes are probably pre miocene in age the calculated thermal evolution of a typical millimeter scale pseudotachylyte layer indicates that cooling does not last more than a few seconds this indicates that related seismic processes such as deceleration of fault movement and healing of the fault were both very rapid the geometry and thermal budget of the millimeter thick pseudotachylytes suggest that seismic events of magnitude 6 to 7 are responsible for their formation

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

geodynamic evolution of the eastern sierras pampeanas central argentina based on geochemical sm nd pb pb and shrimp data whole rock geochemical analyses using major and trace elements in combination with the sm nd and pb pb isotope systems together with shrimp age dating on metasedimentary rocks from the sierras de chepes the sierras de cordoba the sierra norte and the san luis formation in the sierra de san luis have been carried out to unravel the provenance and the geodynamic history of the eastern sierras pampeanas central argentina the geochemical and the sm nd data point to a slightly stronger mafic and less fractionated material in the provenance area of the sierras de cordoba when compared to the other units the tdm model ages from the sierras de chepes 1 82 ga and the sierra norte 1 79 ga are significantly older than the data from the sierras de cordoba 1 67 ga the pb data are homogeneous for the different units only the 208pb 204pb ratios of some samples from the sierras de cordoba are higher a late pampean detrital zircon peak around 520 ma from the sierras de chepes is in accordance with the new data from the san luis formation this is similar to the literature data from the famatina belt located to the northwest of the sierras de chepes and also fits the detrital zircon peaks in the meson group these maximum depositional ages were also reported from some locations in the puncoviscana formation but are absent in the sierras de cordoba an improved model for the development of the eastern sierras pampeanas in the area between the sierras de cordoba and the puncoviscana formation is provided this gives new insights into the late pampean development of the sierra de san luis and the complex development of the eastern sierras pampeanas this new model explains the younger detrital ages in the puncoviscana formation compared with the older ages of the sierras de cordoba another model of the sierra de san luis explains the younger depositional ages of the pringles metamorphic complex and the san luis formation when compared to the nogoli metamorphic complex and the conlara metamorphic complex additionally the rather fast change of the high grade metamorphic conditions in the pringles metamorphic complex and the low grade metamorphic conditions in the san luis formation is explained by extension the ascent of ultra mafic material and later folding and erosion

experimental decrepitation and re equilibration of fluid inclusions in synthetic quartz abstract strong discrepancies between densities of the fluids trapped in fluid inclusions f i and p tp couples deduced from other geological considerations are common so it was decided to test experimentally the possible f i re equilibrations when bringing the host crystal in p tp conditions different from trapping conditions runs were conducted on synthetic quartz in a gas apparatus and in a griggs solid medium apparatus the following modifications of the f i were observed 1 1 drastic shape changes of the primary f i and appearance of clusters or trails of small secondary inclusions sometimes related to a crack 2 2 lowering of the fluid densities such a re equilibration is a decrepitation phenomenon first hydraulic fracturing then sealing of the cracks with appearance of secondary inclusions when using the gas apparatus the new measured densities are in very good accordance with the nominal p tp conditions when using the solid medium apparatus the new densities are surprisingly low relative to the nominal p t conditions probably due to a strongly anisotropic stress state in the apparatus

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

the allochthony of the argentine precordillera ten years later 1993 2003 a new paleobiogeographic test of the microcontinental model abstract recent and new faunal data from the cambrian to silurian rocks of the precordillera famatina and northwest argentina basins are used to discriminate between different paleogeographic models and especially to establish to what extent they are compatible with a previous conclusion that the precordillera is a laurentian derived microcontinent there is no paleontological evidence to support a para autochthonous gondwanan origin of the precordillera the strong differences in the cambrian trilobite faunas and lithologic successions preclude a common origin of the precordillera terrane eastern antarctica and south africa recent discoveries of brachiopods and organisms of the phylum agmata strengthened laurentian affinities during the cambrian the latest cambrian early ordovician faunas that inhabited the autochthonous northwest argentina basin including the western puna volcaniclastic successions are mostly peri gondwanan the early ordovician brachiopods ostracods and trilobites display mixed laurentian baltic and avalonian biogeographical links supporting a drifting of the precordillera across the iapetus ocean increasing gondwanan elements during the llanvirn along with varied geological evidence indicate that the first stages of collision may have begun at that time involving a major change in the plate kinematics the distribution of facies and faunas basin development and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the precordillera terrane with the gondwana margin the high level of endemism of caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike slip movement of the colliding precordillera plate with respect to the gondwana margin suggested relationships between facies distribution geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late cambrian to silurian

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

the neoproterozoic early paleozoic metamorphic and magmatic evolution of the eastern sierras pampeanas an overview the eastern sierras pampeanas were structured by three main events the ediacaran to early cambrian 580 510 ma pampean the late cambrian ordovician 500 440 ma famatinian and the devonian carboniferous 400 350 ma achalian orogenies geochronological and sm nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event ediacaran dividing into two mesoproterozoic major crustal blocks source of the grenvillian age peaks in the metaclastic rocks this event would be coeval with the development of arc magmatism along the eastern margin of the eastern block closure of this eastern margin led to a cambrian active margin sierra norte arc along the western margin of the eastern block in which magmatism reworked the same crustal block consumption of a ridge segment input of oib signature mafic magmas which controlled granulite facies metamorphism led to a final collision pampean orogeny with the western mesoprotrozoic block sm nd results for the metamorphic basement suggest that the tdm age interval of 1 8 1 7 ga which is associated with the less radiogenic values of end 540 6 to 8 can be considered as the mean average crustal composition for the eastern sierras pampeanas increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the sierras de cordoba is associated with a younger tdm age and a more positive end 540 value pampean pre 540 ma granitoids form two clusters one with tdm ages between 2 0 and 1 75 ga and another between 1 6 and 1 5 ga pampean post 540 ma granitoids exhibit more homogenous tdm ages ranging from 2 0 to 1 75 ga ordovician re activation of active margin along the western part of the block that collided in the cambrian led to arc magmatism famatinian orogeny and related ensialic back arc basin in which high grade metamorphism is related to mid crustal felsic plutonism and mafic magmatism with significant contamination of continental crust tdm values for the ordovician famatinian granitoids define a main interval of 1 8 1 6 except for the ordovician ttg suites of the sierras de cordoba which show younger tdm ages ranging from 1 3 to 1 0 ga in devonian times achalian orogeny a new subduction regime installed west of the eastern sierras pampeanas devonian magmatism in the sierras exhibit process of mixing assimilation of depleted mantle signature melts and continental crust achalian magmatism exhibits more radiogenic end 540 values that range between 0 5 and 4 and tdm ages younger than 1 3 ga in pre devonian times crustal reworking is dominant whereas processes during devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism

the southern border of the guapore shield in western brazil and bolivia an interpretation of its geologic evolution abstract the geology of the southeastern and southwestern margins of the guapore shield is described the shield rocks comprise high grade granulite and low grade greenschist sequences these rocks are poorly known oldest recorded ages include transamazonian datings 2 ma the margins of the shield show the development of a mobile belt formed by rocks of upper proterozoic to cambrian age within this mobile belt the cuiaba group of uncertain but possibly upper proterozoic age outcrops to the east and shows tectonic contacts with younger rocks that span the vendian upper proterozoic to cambrian age the rocks of vendian to cambrian age form three isolated elongated basins which show evidence of fault control the axes of these basins are equally separated by angles of 120 and meet at a triple junction near corumba the stratigraphy of the vendian to cambrian basins is correlated from bolivia to brazil based on tectonic continuity sedimentological similarity and fossil content the central location at corumba shows instability since the middle proterozoic when important igneous activity is recorded at rincon del tigre bolivia and in brazil this igneous activity is interpreted as being related to hot spot activity in an intercratonic setting from which the younger upper proterozoic basins originated