late paleozoic closure of the ob zaisan ocean along the irtysh shear zone nw china implications for arc amalgamation and oroclinal bending in the central asian orogenic belt the largest accretionary orogen in the world the central asian orogenic belt has evolved through the assembly of various oceanic and continental blocks understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes one such major suture zone which records the collisional history of the siberian marginal arc system chinese altai with intra oceanic arc systems east west junggar is the irtysh shear zone in nw china the spatial continuity and the tectonic nature of this shear zone are still poorly understood but its development has supposedly made a significant impact on the architecture of the western central asian orogenic belt and the formation of the kazakhstan orocline here we provide new insight into the evolution of this shear zone based on detrital zircon ages hf isotope composition structural data and ar ar age constraints on the timing of deformation our results show a major discrepancy in detrital zircon populations and hf model ages across the southern chinese altai and the northern east west junggar thus allowing us to map the exact location of the tectonic boundary detrital zircon data constrain the initial closure of the ob zaisan ocean to the late carboniferous 323 ma and new structural and ar ar geochronological data shed light on the subsequent collisional processes we propose that the collisional zone between the chinese altai and the east west junggar was initially subjected to crustal thickening at ca 323 295 ma followed by orogen parallel extension ca 295 ma and transpressional deformation ca 286 253 ma the closure of the ob zaisan ocean in nw china postdates the initial phase of oroclinal bending in the western central asian orogenic belt thus indicating that oroclinal bending initiated during subduction based on our new constraints and other available geological data we suggest that the early stage of oroclinal bending was likely driven by slab rollback

fluid infiltration and transport of major minor and trace elements during regional metamorphism of carbonate rocks wepawaug schist connecticut usa geochemical alteration of metacarbonate layers due to fluid infiltration during greenschist and amphibolite facies metamorphism was investigated based on 149 whole rock analyses of the wepawaug schist usa ankerite albite ank ab rare ankerite oligoclase ank ol biotite bt amphibole amp and diopside di index mineral zones developed during progressive metamorphism acadian orogeny and devolatilization co2 h2o s loss ank ab ank ol and bt zone rocks were altered along some lithologic contacts and in reaction selvages around veins mass changes included gains of na y and in many cases p and fe k rb and ba were typically lost and sr was lost except where calcite precipitated in and around calcite bearing veins na gain and k loss reflect muscovite destruction and albite growth in response to infiltration of fluids from surrounding metapelitic and metapsammitic metaclastic rocks at higher metamorphic grades k rb and ba were lost when biotite broke down to form amphibole or diopside these elements were transported out of metacarbonate layers down chemical potential gradients at lithologic contacts and in selvages na was lost as plagioclase reacted to form clinozoisite zoisite czo zo calc silicates cm to dm scale thickness rich in amphibole and czo zo amp zones or diopside and zo di zones formed at layer margins and in selvages via extreme metasomatism including major 1 addition of al si and in many rocks fe 2 destruction of calcite and 3 loss of ca and volatiles volatile losses near 90 percent relative to low grade precursors demonstrate that calc silicate formation resulting from al si metasomatism is an important source of co2 to be considered when assessing regional devolatilization al mass gains for calc silicates indicate that large quartz veins and some lithologic contacts were loci for massive time integrated fluid fluxes of 105 m3 m 2 heavy rare earth elements hree were gained in the ank ab through the amp zones and a di zone example gained light and mid ree including sm and nd the ree gains coincide with gains of y and in most cases p suggesting that p and y complexes transport ree volume was lost progressively from the unveined interiors of metacarbonate beds attaining 20 to 30 percent loss in the di ii zone mass addition due to veining reduced volume losses and in some cases produced volume gains reaction transport modeling indicates that fluid flow was mostly layer parallel was focused along lithologic contacts fractures and permeable horizons and was accompanied by diffusion and or mechanical dispersion at high angles to the flow directions the metasomatic fluids infiltrated from surrounding metaclastic and or syn metamorphic igneous rocks thus interaction of fluids derived from different rock types must be considered when assessing mass transfer in lithologically diverse mountain belts

time scales of deformation and exhumation in extensional detachment systems determined by high spatial resolution in situ uv laser 40ar 39ar dating abstract once dated with adequate precision intra grain recrystallization microstructures in deformed minerals provide valuable information about the tectonic and kinematic history of shear zones and entire orogens we present combined in situ uv laser 40ar 39ar geochronology and stable isotope data to decipher the relationships between intra grain recrystallization microstructures and 40ar 39ar ages in muscovite fish from extensional mylonite of the porsgrunn kristiansand shear zone southern norway texturally controlled uv laser 40ar 39ar dating of isotopically and compositionally distinct cores rims and tips of muscovite fish resolves intra grain age variations on the micrometer scale twenty eight in situ uv laser 40ar 39ar analyses from two muscovite cores yield a weighted mean age of 891 2 2 9 ma interpreted to date synkinematic neocrystallization of muscovite the integrated 880 2 2 8 ma n 30 analyses weighted mean age of rims and tips is in good agreement with observed recrystallization relationships which suggest that rims and tips of the muscovite fish record the final increments of the ductile deformation history diffusion considerations based on published argon diffusion rates and peak temperature estimates from oxygen isotope thermometry show that the 40ar 39ar ages are unlikely to be disturbed during cooling the 11 0 4 0 my core to rim age difference together with the stable isotope data are consistent with protracted ductile deformation extensional detachment faulting and meteoric fluid infiltration occurring over this time interval this approach combining x ray element mapping stable isotope geochemistry and in situ uv laser 40ar 39ar dating allows well characterized microstructures to be analyzed with high precision and permits direct determination of the time scales of deformation in crustal scale shear zones

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

how is strain localized in a meta granitoid mid crustal basement section spatial distribution of deformation in the central aar massif switzerland abstract this study investigates strain distribution in granitoid rocks formerly in the middle crust in the central aar massif switzerland and places the deformation behavior in the tectonic framework of the alpine orogeny strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones sz 80 sz km with sz thicknesses the majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 c in the north to 450 c in the south of the aar massif the shear zones initiated at about 18 20 km depths during a stage of crustal thickening handegg phase subsequent deformation reactivated some shear zones with a gradual transition from reverse dip slip over oblique slip to strike slip shear zones under local transpressional conditions oberaar phase

crustal architecture of central australia based on deep seismic reflection profiling abstract the crustal architecture of central australia is interpreted from deep seismic reflection profiling conducted by the australian geological survey organisation in two surveys in 1985 and 1993 the seismic traverses oriented normal to the main surface structures ran north south in central australia and crossed parts of the arunta block amadeus basin musgrave block and officer basin the present crustal fabric was set in place by the end of the mesoproterozoic by about 1100 ma reactivation of the structures took place in a continental intraplate setting mainly during the middle late palaeozoic alice springs orogeny but also during other orogenic events in the arunta block the crust is dominated by major north dipping planar structures interpreted as thick skinned thrusts many of these thrusts cut deep into the crust and at least one the redbank thrust zone appears to cut and offset the moho in contrast in the northern musgrave block limited field mapping and teleseismic data suggest that the major crustal scale planar structures are south dipping in the central to southern amadeus basin deformation is essentially thin skinned with north directed thrusting confined to the sedimentary succession thus the deep seismic profiles in central australia show a present day crustal architecture that is the response of the crust to mesoproterozoic terrane amalgamation and to later reactivation by intraplate deformational events therefore central australia is a model for intraplate cratonic deformation that occurs in continental crust that is cold thick and strong

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

the p t deformation path for a mid proterozoic low pressure terrane the reynolds range central australia the proterozoic low pressure high temperature lpht terrane of the reynolds range occurs in a 130 km long nw trending belt in the central part of the arunta block central australia the reynolds range has been affected by two mid proterozoic tectonic cycles di and dii associated with two metamorphic events mi and mii di mi effects are restricted to the older of two sedimentary successions the lander rock beds which are separated from the younger reynolds range group by an angular unconformity the dominant structural metamorphic features formed during dii mii affected both sedimentary successions and the various granites that intruded them and reworked most di mi effects the dii deformation history can be subdivided into one prograde two peak and one retrograde stage average p t calculations in the southeastern half of the range indicate a peak metamorphic pressure of 4 1 0 3 kbar because the calculated values are derived from the same stratigraphic level corresponding to the base of the reynolds range group which is exposed throughout the area it is likely that pressures were similar in the entire range in fact however the peak metamorphic temperature shows a dramatic increase from greenschist facies c 400 c in the northwest to granulite facies 740 60 c in the southeast indicating that mii was associated with anomalously high heat flows the p t path is anticlockwise with isobaric cooling from the metamorphic peak indicated by corona textures however the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures peak metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation suggesting they formed in response to the same thermal event if this is true the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time 110 ma based on zircon dating as it is not clear whether proterozoic asthenosphere active thermal perturbations operated for this long the alternative interpretation must be considered namely that the peak metamorphic events are separate from the shear zone event associated with reheating of the area

dynamics of intensified downwelling circulation over a widened shelf in the northeastern south china sea abstractthis study reveals the dynamics behind the intensified downslope cross isobath transport over a widened shelf narrowing downwave in the northeastern south china sea nscs during a downwelling event utilizing a three dimensional numerical model over an idealized nscs shelf this study identified the forcing process and derived an analytical understanding of the invoked dynamics for the intensified downslope cross isobath transport this study found that the transport was formed by an amplified geostrophic transport because of an increased positive along isobath pressure gradient force pgf py and by an enhanced bottom ekman transport due to converging flow over the widened shelf based on the depth integrated vorticity dynamics py was linked with net frictional stress curl in the water column and the curl was primarily associated with the shear vorticity field induced by downwelling jet the increased positive vorticity seaside of the jet provided positive py for the downslope geostrop

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

crustal scale ductile fault systems in the arunta inlier central australia detailed geologic observations along three 100 km long traverses across the early proterozoic arunta inlier central australia indicate that paleozoic ductile shear zones are organized into crustal scale structural systems which control the map pattern and distribution of metamorphic zones two provinces can be distinguished 1 1 a southern one where shear zones form a series of hinterland dipping and south verging imbricate reverse faults in the west redbank area grading eastward into a foreland dipping duplex in the arltunga nappe complex directly north of this complex flat lying paleozoic south directed ductile shear affects a large portion of the crust a point which has been underestimated so far 2 2 the northern province is characterized by both south and north dipping shear zones showing reverse motion to produce a crustal scale pop up structure presumably resulting from a more coaxial deformation history strike slip shear zones become predominant in the eastern part of this province stratigraphic and cross cutting relationships indicate that the southern systems developed first the northern systems probably developed in response to the lock up stages of overthrusting in the southern province one important result of this work is that the distribution of granulite facies rocks and major geophysical anomalies throughout the arunta inlier is best explained by their position relative to the paleozoic ductile fault systems described above there is no need to invoke extensive proterozoic uplift of these lower crustal materials

dating mylonitic deformation by the 40ar 39ar method an example from the norumbega fault zone maine strongly mylonitic rocks associated with the regionally extensive norumbega fault zone in south central maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite 40ar 39ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined early carboniferous plateau ages in contrast to these nonmylonitized samples all age spectra for muscovite from the mylonites are highly discordant they are characterized by young ages at low extraction temperatures which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older relict muscovite porphyroclasts and fine grained white mica aggregates that recrystallized during mylonitic deformation total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size indicating a larger component of the recrystallized grains in the finer grain size fractions although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra their initial increments do coincide at approximately 290 ma this indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation these data combined with kinematic analysis reveal that the segment of the norumbega fault zone studied the sandhill corner fault is a late carboniferous early permian dextral strike slip fault a lack of significant offset in regional early carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km this study demonstrates that 40ar 39ar dating methods can be used to date deformational events effectively as long as several important criteria are met first and foremost samples must be well characterized prior to analysis dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral finally the effects of excess argon must be negligible

neoproterozoic stratigraphy of the centralian superbasin australia abstract the basement of central australia includes the arunta block and the 1200 1100 ma musgrave block succeeded by 1075 1000 ma bimodal volcanic rifts and dolerite dykes a regional hiatus at 1000 800 ma interpreted as reflecting a central post volcanic underplated upland was followed at 800 ma by a second swarm of dykes in the musgrave block gawler craton and stuart shelf associated with the crustal sagging that initiated the centralian superbasin the centralian superbasin is defined here to encompass the neoproterozoic fill of the amadeus georgina ngalia officer and savory basins the intra cratonic superbasin was disrupted internally 540 600 ma ago petermann ranges orogeny by a central uplift and associated thrusts and by mid carboniferous 320 ma tectonism alice springs orogeny to form the structural basins listed above the stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions new results from acritarch biostratigraphy and isotope chemostratigraphy in conjunction with conventional lithostratigraphy and sequence analysis allow better stratigraphic resolution than previously possible field observations in the recently discovered savory basin in western australia provide a basis for correlations with the adjacent basins

constraining deformation stages in brittle ductile shear zones from combined field mapping and 40ar 39ar dating the structural evolution of the grimsel pass area aar massif swiss alps abstract 40 ar 39 ar dating of syn kinematic white mica phengite and biotite is used to constrain the age of shear zones within the granodiorites of the grimsel pass area aar massif central alps three sets of steeply dipping shear zones have been mapped stage 1 shear zones are very broad 200 m wide zones of biotite bearing rocks with a progressive increase in intensity of ductile deformation from rim to core and have dip slip kinematics subsequent stage 2 shear zones are narrower contain phengite rich mylonites and show a gradual change from dextral strike slip to dip slip kinematics across strike strain localisation during the formation of stage 2 shear zones is interpreted to have been aided by reaction weakening during extensive fluid rock interaction brittle precursors of stage 2 shear zones are marked by biotite rich cataclasites preserved along the northern contact of the overprinting main stage 2 shear zone biotite from a stage 1 shear zone yields an age of 21 1 0 2 ma phengites in stage 2 mylonites in the grimsel pass area have a very narrow age range 13 8 12 2 ma which is interpreted to bracket the duration of ductile deformation during stage 2 deformation at mid crustal levels however the biotite rich cataclasites at the margin of a stage 2 shear zone may have localised fluid flow up to 2 ma prior to the onset of main stage 2 ductile deformation stage 3 shear zones are brittle strike slip faults containing cataclasites breccias and clay fault gouges earlier studies have established that these late faults formed at

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

the flow of surface derived fluids through alice springs age middle crustal ductile shear zones reynolds range central australia the south east reynolds range central australia is cut by steep north west trending alice springs age c 334 ma shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement amphibolite facies 550 600 c 500 600 mpa shear zones cut metapelites while greenschist facies shear zones 420 535 c 400 650 mpa cut metagranites the sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow altered granites within greenschist facies shear zones have gained si and k but lost ca and na relative to their unsheared counterparts suggesting that the fluid flowed down temperature and hence probably upward through the shear zones time integrated fluid fluxes calculated from silica addition are up to 2 1 1010 mol m 2 c 4 2 105 m3 m 2 similar time integrated fluid fluxes are also estimated from changes in k and na the sheared granitic rocks locally have 18o values as low as 0 which is much lower than the 18o values of the adjacent unsheared granites 7 to 9 implying that the fluid which flowed through these shear zones was derived from the surface for the estimated time integrated fluid fluxes the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres the flow of surface derived fluids into the ductile middle crust with subsequent expulsion upwards through the shear zones may have been driven by seismic activity accompanying the alice springs deformation

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

south arunta block the internal zones of a proterozoic overthrust in central australia a detailed structural and microstructural analysis indicates that the chewings range area south arunta block was deformed during a north directed shearing event the chewings range quartzite and metasediments together with their granitic basement were involved in a proterozoic ductile deformation event responsible for a flat lying foliation axial planar to n s trending isoclinal folds within the foliation a n s oriented lineation developed parallel to a plane strain shearing or constrictive stretching direction in all rock types a ductile stretching of 100 150 was achieved during this strain history shear criteria observed in the gneisses are consistent with the asymmetric pattern of quartz c axis fabrics in the quartzite and indicate a south over north shear component strain is concentrated in narrow high strain zones which delineate a stack of complexly folded thrust sheets these results provide a new interpretation for the early to middle proterozoic of central australia the south arunta block may represent the internal zones of a north directed proterozoic overthrust correlation between structural elements of the south arunta block and the granulite belt to the north is unclear until this problem is resolved by careful dating it is suggested that d2 did not develop in the granulitic rocks owing to an intracrustal detachment above them

a simple method for settlement evaluation of loess pile foundation collapsible soils have been regarded to be a significant engineering concern for foundation constructions in northwest china given that the collapsible soils also known as loess cover most of no

shrimp ionprobe dating of short lived proterozoic tectonic cycles in the northern arunta inlier central australia structural and metamorphic evidence from low p high t metamorphic terranes within the northern arunta inlier central australia indicates that two discrete tectonic cycles affected the region each cycle was associated with compressive deformation coincident with peak granulite facies metamorphism and granite intrusion shrimp u th pb analysis of zircons from early syn and late tectonic granitoids allows the age of the two tectonic cycles to be directly measured the age of the first cycle d1 is determined by emplacement of the mount stafford granite dated at or before 1818 15 ma after the first two foliations of the cycle had formed but before the axial plane cleavage associated with later upright folding the regional extent of this tectono magmatic event is confirmed by an emplacement age of 1818 8 ma measured on the large discordant harverson granite 40 km away the second cycle d2 consisted of four distinct deformation stages d2a 2d the ages of which are constrained by 1785 1775 ma old early syn and late tectonic granitoids the warimbi granite emplaced at or before 1785 22 ma and the granitic protolith to the napperby gneiss inferred to be 1780 10 ma pre date or formed during the earliest foliation s2a zircon considered to have grown at peak granulite facies metamorphic conditions during generation of the dominant s2b foliation gives an age of 1774 6 ma in the possum creek charnockite leucogranite in the napperby gneiss which post dates s2b and large scale upright nw se trending f2d folds yields an age of 1775 12 ma and confirms that the entire tectonic cycle d2a 2d formed in a rapid 10 ma period younger zircons which formed at 1659 6 ma in the napperby gneiss cannot be related to any local structural event but are similar in age to a major magmatic metamorphic episode in the southern arunta black and shaw 1992 150 km away they may be the peripheral manifestation of that event further zircon growth at 1600 ma in the napperby gneiss and possum creek charnockite was caused by hydrothermal fluid fluxing associated with post tectonic pegmatite emplacement these dykes were deformed by amphibolite facies retrograde shear zones ion probe dating of zircon stratigraphy from the granitoids has confirmed the discrete successive tectonic cycles identified by earlier structural and metamorphic studies for the northern arunta inlier at least those cycles or orogenies are associated with short lived 10 ma thermal pulses related to granite magmatism not prolonged thermal perturbations spanning 100 ma or more cf dirks and wilson 1990 dirks and hand 1991 such short lived pulses are intimately associated with granite intrusion and may be a general feature of low p high t metamorphism

assessing the geochemical and tectonic impacts of fluid rock interaction in mid crustal shear zones a case study from the intracontinental alice springs orogen central australia the reynolds anmatjira ranges central australia form part of a high grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the ordovician carboniferous alice springs orogeny unlike typical retrograde structures associated with discrete fluid flow the mid crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration to address these issues we describe two detailed traverses across deformed and metasomatized basement rocks in this region and interrogate their record of fluid rock interaction from various perspectives both traverses combine structural and petrological observations with zr in rutile and ti in quartz thermobarometry oxygen and hydrogen stable isotope analysis and major trace and rare earth element mobility trends each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas providing a strategic framework for the general investigation of fluid affected shear zones ultimately the integrated data sets specify pressure temperature conditions of 530 c and 4 5 kbar implying average apparent thermal gradients of 29 36 c km 1 and depths of 14 18 km other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived non equilibrium fluid this is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin with estimated 18o and d values as low as 2 3 and 59 8 respectively we propose that these surficial fluid signatures are imposed on the mid crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre orogenic basin formation progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle ductile transition promoting extensive hydration metasomatism and reaction softening at the locus of stress transmission from plate boundary sources the sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long term strength of the lithosphere providing strong impetus for the initiation and advancement of intracontinental orogenesis

nucleation of ductile shear zones in a granodiorite under greenschist facies conditions neouvielle massif pyrenees france abstract the problem of ductile shear zone nucleation under greenschist facies conditions is approached from the example of small scale shear zones developed in the neouvielle granodioritic pluton pyrenees france by means of field and microstructural observations and chemical analyses these shear zones are not related to pre existing fractures and exhibit networks of numerous conjugate fairly parallel and regularly spaced centimetre scale brittle ductile shear zones involving diffuse localization mechanisms although the mode of deformation depends on the minerals present deformation in the shear zones is basically controlled by hydration processes hydration and consequent fluid controlled alteration and deformation are related to fluid migration towards developing cracks all these cracks are extensional and formed on the scale of at most a few grains and frequently single grains a comparison of chemical compositions of undeformed and sheared granodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems we propose a sequence of mechanisms by which the nucleation of a small scale brittle ductile shear zones spreading out within a granodioritic rock may occur in these mechanisms shear zone nucleation occurs independently of pre existing fractures and results from the heterogeneous character of the polymineralic rock in the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain scale hydraulic fracturing fluid focusing generates instabilities by local softening and subsequently shear zone nucleation

contact metamorphism of layered carbonate shale sequences in the oslo rift ii migration of isotopic and reaction fronts around cooling plutons abstract intrusion of felsic plutons of permian age into lower paleozoic shale carbonate sequences caused extensive contact metamorphism in the oslo rift oxygen and carbon isotope data from both carbonates and shales are presented to demonstrate that the progress of metamorphic reactions in the contact aureole around the drammen granite was to a large extent controlled by the infiltration of water rich fluids of a magmatic oxygen and with a carbon isotope signature highly integrated fluid fluxes were confined to shale rich lithologies massive limestones were relatively impermeable to fluid flow the spatial distribution of isotopic and reaction fronts in the aureole and coupled 18 o 13 c isotope variations of carbonates from interlayered shale carbonate units fits well to a 1 d transport model with fluid flow away from the intrusive contact whereas the same isotope variations in limestones containing thin shale intercalations do not the high fluid fluxes estimated from 1 d models 2000 m 3 m 2 require either focused flow of magmatic fluids in the study area or large scale convective flow possibly involving fluids of meteoric origin limited kinetic broadening of the oxygen isotopic front indicates fluid fluxes of 8 m s fluid flow must have persisted for 10 4 yr to have moved the oxygen front 1000 m from the intrusive contact simple 2 d transport models involving dominant fluid flow along aquifers or fractures and involving diffusion as well as advection explain most of the coupled 18 o 13 c trends reported for carbonates from contact aureoles

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

p t t estimation of deformation in low grade quartz feldspar bearing rocks using thermodynamic modelling and 40ar 39ar dating techniques example of the plan de phasy shear zone unit brianconnais zone western alps pressure temperature time p t t estimates of the syn kinematic strain at the peak pressure conditions reached during shallow underthrusting of the brianconnais zone in the alpine subduction zone was made by thermodynamic modelling and 40ar 39ar dating in the plan de phasy unit se of the pelvoux massif western alps the dated phengite minerals crystallized syn kinematically in a shear zone indicating top to the n motion by combining x ray mapping with multi equilibrium calculations we estimate the phengite crystallization conditions at 270 50 c and 8 1 2 kbar at an age of 45 9 1 1 ma combining this p t t estimate with data from the literature allows us to constrain the timing and geometry of alpine continental subduction we propose that the brianconnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision

apatite and monazite an effective duo to unravel superimposed fluid flow and deformation events in reactivated shear zones abstract mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid driven metasomatic retrogression and reactivation however the relationships between fluid rock interaction retrogression deformation and mylonitisation and the timing thereof are often cryptically recorded this study focuses on the granulite facies boothby orthogneiss from the reynolds range central australia which contains a large scale mylonitic shear zone with an apparent record of structural inheritance fluid infiltration and reactivation the chosen site provides an ideal natural laboratory in which to investigate the timing of deformation associated fluid flow and mass transport u pb isotope analyses of monazite indicate an average pb recrystallization age of c 1560 ma demonstrating that the orthogneiss fabric developed during the mesoproterozoic chewings orogeny 1590 1550 ma structural mapping suggests that this shear zone represents a riedel branch of larger structures that were subsequently reactivated during the paleozoic alice springs orogeny 450 300 ma the timing of reactivation and fluid flow is constrained by u pb dating of apatite which is present as a stable u bearing mineral in both orthogneiss and mylonite modelling of apatite radiogenic pb retention ages accounting for a wide potential range in common pb compositions demonstrates at least some growth and or recrystallization at c 1500 ma and c 400 ma confirming apatite precipitation during alice springs shearing and the reactivation of chewings age structures in addition alice springs aged apatite is found along pre existing fabrics in the orthogneiss in the vicinity of the shear zone indicating pre kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected the combined datasets demonstrates that integrated apatite and monazite u pb geochronology is an effective method to unravel the record of superimposed fluid flow and deformation events this includes the detection of an inverse younging relationship where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone such effects are potentially common where deformation is driven by pre kinematic fluid rock interaction with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks

the geological evolution of the reynolds range central australia evidence for three distinct structural metamorphic cycles the reynolds range is affected by three distinct structural metamorphic cycles di mi dii mii anddiii miii which represent the local expressions of a series of orogenic events that affected large sections of the arunta block central australia the tectonic cycles recognized in the reynolds range correlate extremely well with similar events in the nearby anmatjira range di mi took place around 1820 ma and only affected the nw reynolds range dii mii occurred between 1760 and 1650 ma and involved ne sw shortening with identical timing relationships between the deformations and metamorphism along the entire reynolds range dii commenced with the formation of a fabric sii1 preserved as inclusion trails in porphyroblasts sii1 formed early during a progressive event dii1 2 which involved the formation and tightening of upright nw se trending isoclinal folds a penetrative subvertical nw se trending fabric sii2 developed axial planar to these folds the general symmetry of dii2 microstructures suggests that fii2 folds were formed during progressive coaxial deformation under plane strain conditions dii3 structures are associated with conjugate sets of crenulation bands within the macroscopic crenulation bands large sections of earlier folds and fabrics are rotated to a near recumbent position the dominant set of crenulation bands is upright and trends ene wsw while the conjugate to this set trends nw se no penetrative fabric formed during dii3 dii4 is associated with a system of amphibolite grade ne dipping thrusts interconnected by numerous smaller shear zones the sense of movement is ne over sw during dii essentially penetrative coaxial deformation dii2 evolved into essentially localized non coaxial deformation dii3 dii4 dii deformation occurred concomitant with a low pressure granulite facies metamorphic event mii 1730 ma peak metamorphic sillimanite cordierite orthopyroxene ilmenite spinel garnet k feldspar quartz assemblages define sii2 oriented sillimanite biotite assemblages that formed early during dii3 and unoriented andalusite chlorite muscovite assemblages that formed late during dii3 indicate that cooling of mii was initially isobaric during dii4 thrusting staurolite garnet biotite quartz and biotite kyanite quartz assemblages defining the mylonitic fabric overprint the late dii3 andalusite assemblages dii4 shear movement is therefore accompanied by a slight rise in pressure dii4 shear zones gradually increase in grade eastward along the reynolds range this trend coincides with the variation in peak metamorphic grade attained during mii indicating that the rocks had not yet fully cooled when dii4 thrusting commenced diii miii is late devonian to carboniferous in age and caused minor differential movement 100 m and retrogression along dii4 shear zones

geochronological constraints on orogenic events in the arunta inlier a review early workers subdivided the arunta inlier central australia into three major tectonic provinces classified the principal tectonostratigraphic units into three divisions and separated the tectonomagmatic history into five discrete events ranging in age from 1800 ma to 300 ma a review of the evidence for tectonic boundaries within the arunta inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces but that the redbank thrust zone is at least in part a province boundary the tectonostratigraphic division concept also requires revision as division 3 rocks of the northern and southern arunta cannot be time equivalents division 1 rocks grade into division 2 in the northern region and division 3 of the northern region may be equivalent to division 1 in the central region rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made compilation of rb sr and 40ar39ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 ma to 1000 ma but age clusters exist at 1400 ma 1100 ma and 400 300 ma which have geological significance subdivision of older ages into discrete tectonic events was not justified from this data alone but more recent u pb isotopic analysis of zircons in rocks designated to particular structural metamorphic events has revealed a complex tectonic history in the 1880 1600 ma interval the northern and central provinces underwent major tectonism between 1780 1730 ma but much of the southern province was not deformed until 1600 ma the revised tectonic history nomenclature is proposed for the arunta inlier following deposition of a widespread turbiditic unit at or before 1880 ma possibly on thinned archaean crust the northern and central provinces underwent localized tectonism magmatism at 1880 ma yuendumu tectonic event and 1820 ma stafford tectonic event prior to the widespread strangways orogeny which is subdivided into an early phase at 1780 1770 ma and late phase at 1745 1730 ma it is not clear whether the strangways orogeny is a single arunta wide event or a series of separate short lived smaller scale events although the major deformation in all areas appears to have involved w to sw directed thrusting in the southern province the major events occurred at 1680 ma argilke tectonic event and 1600 ma chewings orogeny the latter involving n directed thrusting that was penecontemporaneous with post tectonic pegmatite intrusion in the northern province suggesting linkage of both regions at this stage the younger history of the inlier is characterised by several stages of uplift and isolated thermal magmatic events major mesoproterozoic crustal exhumation anmatjira uplift phase has been interpreted from widespread 1500 1400 ma ages including regional scale closure of 40ar 39ar systems in hornblende from mafic granulite outcrop scale rb sr total rock isotopic systems in mylonitized granitoids and sm nd garnet hornblende pairs in mylonitized amphibolites a 1150 ma thermal event in the southern part of the inlier teapot magmatic event is recorded by intrusion of granite pegmatites and the alkaline mordor igneous complex and by isotopic resetting of rb sr mineral systems dolerite dyke injection at 1050 ma reflects limited extensional tectonism that may have related to formation of the amadeus basin the second major uplift event was the alice springs orogeny at 400 300 ma an unusual intracratonic thick and thin skinned thrust event that involved limited retrograde amphibolite facies but extensive greenschist facies metamorphism and restricted pegmatite intrusion

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

high resolution geochemical record of fluid rock interaction in a mid crustal shear zone a comparative study of major element and oxygen isotope transport in garnet garnet grains from an intensely metasomatized mid crustal shear zone in the reynolds range central australia exhibit a diverse assortment of textural and compositional characteristics that provide important insights into the geochemical effects of fluid rock interaction electron microprobe x ray maps and major element profiles in situ secondary ion mass spectrometry oxygen isotope analyses and u pb and sm nd geochronology are used to reconstruct their thermal temporal and fluid evolution these techniques reveal a detailed sequence of garnet growth re equilibration and dissolution during intracontinental reworking associated with the ordovician carboniferous 450 300 ma alice springs orogeny a euhedral garnet porphyroblast displays bell shaped major element profiles diagnostic of prograde growth zoning during shear zone burial coexisting granulitic garnet porphyroclasts inherited from precursor wall rocks show extensive cation re equilibration assisted by fracturing and fragmentation oxygen isotope variations in the former are inversely correlated with the molar proportion of grossular suggesting that isotopic fractionation is linked to ca substitution the latter generally show close correspondence to the isotopic composition of their precursor indicating slow intergranular diffusion of o relative to fe2 mg and mn peak metamorphism associated with shearing 550 c 5 0 6 5 kbar occurred at c 360 ma followed by rapid exhumation and cooling progressive mn enrichment in rim domains indicates that the retrograde evolution caused partial garnet dissolution accompanying intra mineral porosity production then stimulated limited oxygen isotope exchange between relict granulitic garnet grains and adjacent metasomatic biotite resulting in increased garnet 18o values over length scales 200 m spatially restricted oxygen interdiffusion was thus facilitated by increased fluid access to reaction interfaces the concentration of ca in channelled fracture networks suggests that its mobility was enhanced by a similar mechanism in contrast the intergranular diffusion of fe2 mg and mn was rock wide under the same p t regime as demonstrated by a lack of local spatial variations in the re equilibration of these components the extraction of detailed reaction histories from garnet must therefore take into account the variable length and time scales of elemental and isotopic exchange particularly where the involvement of a fluid phase enhances the possibility of measureable resetting profiles being generated for slowly diffusing components such as ca and o even at low ambient temperatures and relatively fast cooling rates

dating of lithospheric buckling 40ar 39ar ages of syn orocline strike slip shear zones in northwestern iberia abstract orogenic curvature is a ubiquitous feature of mountain belts and the plate tectonic and geodynamic setting responsible for the development of curved orogens is a subject of debate in order to distinguish between different models of orocline formation it is necessary to tightly constrain the absolute timing of oroclinal development however determining the absolute isotopic timing of oroclinal bending is challenging the most direct method available to constrain the time interval during which an orocline formed is to obtain absolute age constraints on fabrics generated within syn orocline strike slip shear zones that accommodated rotations around a vertical axis during the development of crustal or lithospheric scale orogenic curvature here we present a data set of 40ar 39ar ages obtained from five shear zones some of which display curved traces parallel to the cantabrian orocline structural grain in nw iberia and are interpreted to have been generated coevally to it the 40ar 39ar ages were obtained by laser rastering induced step heating on single muscovite crystals that grew synkinematically during shearing all five samples yielded ages that cluster tightly at 308 3 ma providing direct evidence for the age of oroclinal bending this age is consistent with the age constraints obtained from paleomagnetic and structural data that place the onset of oroclinal bending in moscovian times our 40ar 39ar age determinations therefore provide a reliable absolute constraint on the age of buckling of the variscan orogen around a vertical axis and provide further evidence that oroclinal bending is unrelated to variscan convergence collision or the subsequent extensional collapse of the mountain belt

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

u pb monazite geochronology of granitic rocks from maine implications for late paleozoic tectonics in the northern appalachians crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern maine constrain the timing of final ductile motion of a major shear zone to approximately 280 ma u pb monazite ages of granite at brunswick 278 pm 2 ma and evolved granitic pegmatites in topsham 275 269 ma provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation these granitic rocks were evidently generated during the final high temperature event experienced by this portion of the appalachian orogen during this time the host rocks record initial rapid cooling c 16 c m y followed by slower cooling c 4 c m y into the mesozoic the similarity of ages of granite in brunswick and pegmatites in topsham permit the rocks to be related to the same magmatic system the spatially associated sebago batholith the largest exposed pluton in new england has a concordant u pb monazite age of 293 pm 2 ma thus the sebago batholith cannot be

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

relations between progressive deformation and fluid rock interaction during shear zone growth in a basement cored thrust sheet sevier orogenic belt utah variations in microtextures strain whole rock chemistry mineralogy fluid inclusion characteristics and fracture network properties record complex interactions between deformation and fluid rock processes during progressive growth of a shear zone within crystalline basement rocks this shear zone formed at a depth of about 15 kilometers t 350 c and at elevated fluid pressures the shear zone sz has a 10 meter thick core of highly deformed phyllonite and is bounded by a 3 to 8 meter thick transition zone tz of variably fractured chloritic gneiss which grades outward into relatively undeformed granitic gneiss granitic gneiss consists mostly of coarser grained feldspar and quartz chloritic gneiss consists of varying amounts of quartz feldspar and fine grained micaceous matrix produced by a mixture of cataclastic plastic and alteration processes and phyllonite consists mostly of very fine grained quartz mica rich matrix produced by pervasive plastic deformation and alteration processes estimated strain ratios are 10 1 within the sz whole rock chemical compositions record significant depletion of ca and na and enrichment of mg and h2o during progressive alteration of granitic gneiss into phyllonite similar average contents of al ti and fe as well as si indicate that alteration was about isovolumetric at outcrop scale although variations between individual samples record up to 20 percent local volume change alteration produced significant changes in mineral abundances and compositions with conversion of feldspar to muscovite mafic minerals to mg rich chlorite and dissolution precipitation of quartz fluid inclusion characteristics and mineral compositions indicate that sz fluids were moderately saline and became depleted in mg during alteration changes in mineralogy and fluid composition record large influxes of reactive fluids with geochemical fluid rock ratios on the order of 102 to 103 variably deformed cross cutting veins fractures and microcracks record repeated episodes of cataclasis fluid influx and sealing along complex networks fluid flow appears to have been concentrated within the sz along grain scale and vein networks with a component of outward flow into the tz along fracture and microcrack networks estimated average fluid fluxes are on the order of 10 7 to 10 10 m s in the sz and 10 9 to 10 12 m s in the tz consistent with average permeabilities of 10 14 to 10 17 m2 in the sz and 10 16 to 10 19 m2 in the tz for moderate fluid pressure gradients these average permeabilities are consistent with observed grain scale and fracture network properties for a range of sealing and fluid pressure histories permeabilities may have been transiently greater during episodes of very high fluid pressure but decreased as fluid pressure gradients equilibrated and fractures sealed strain softening in the sz was likely produced by reaction softening grain size reduction and hydrolytic weakening of quartz a model of shear zone growth involves 1 initial fracturing of relatively strong coarser grained quartz feldspar rich rock 2 episodic influx of reactive fluids during periods of fracturing and high fluid pressure with intervening periods of sealing and reduced fluid pressure 3 progressive alteration of feldspar and mafic minerals to micas and recrystallization of quartz to form relatively weak fine grained matrix and 4 concentrated deformation and focused fluid flow in a growing sz core