seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

a pressure solution creep law for quartz from indentation experiments 1 indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions this deformation mechanism contributes to earth s crust geodynamics controlling processes as different as fault permeability strength and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism indenter experiments have been performed at 350 c and 20 120 mpa during months with differential stress varying from 25 to 350 mpa several experimental parameters were varied nature of quartz synthetic or natural nature of fluid manner in which the solid solution solid interface was filled and orientation of the indented surfaces versus quartz crystallographic c axis significant strain rates could only be obtained when using high solubility solutions naoh 1 mol l 1 displacement rates of the indenter were found activated by differential stress with exponential dependence as theoretically predicted the mean thickness of the trapped fluid phase below the indenter was estimated in the range 2 10 nm moreover the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution the indenter displacement rate was driven by differential stress and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface conversely marginal strain energy driven dissolution was observed around the indenter and its kinetics was controlled by free surface reaction these experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones providing characteristic time scales for postseismic transitory creep and sealing processes in quartz rich rocks

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

13c and 18o compositions of carbonates from a cyclic carbonate evaporite rock sequence evidences for meteoric water input abstract twenty five carbonate rock samples from a core section composed of seven thick rhythmic carbonate evaporite sequences lower mcknight formation early cretaceous south texas u s a were analyzed for carbon and oxygen isotopic compositions results show the carbonate rocks to have 13 c 0 38 to 1 68 and 18 o 4 60 to 2 48 vs pdb consistent with the reported world wide values for carbonate rocks of cretaceous age despite this low 2 overall variation 18o shows more variability than 13c throughout the core section studied furthermore 13c and 18o are covariant becoming progressively lighter toward the overlying evaporites within each carbonate evaporite sequence the tendency toward lighter isotopic compositions together with the covariance between 13c and 18o in these carbonates is thus not reconcilable with a simple model of carbonate deposition in a marginal evaporitic basin upon continuous evaporation of seawater instead the data imply contributions from isotopically light meteoric waters

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

oxygen 18 and carbon 13 in the carbonates of the salina formation of southwestern ontario the oxygen and carbon isotopic composition of the carbonates of the upper silurian salina formation of the michigan basin was investigated to aid in interpretation of depositional environments 13c results indicate that a change from generally anoxic bottom conditions to oxic conditions occurred during deposition of the b evaporite unit the organic rich a carbonate units were deposited in a shallow water evaporitic setting most likely adjacent to a sabkha type environment a positive water balance maintained the anoxic conditions and buffered the carbon isotopes above the b evaporite the isotopic composition suggests that the development of a similar depositional environment a subaerial prograding sabkha occurred over wide areas of the basin 18o results support the conclusion that silurian oceans were depleted in 18o with respect to modem oceans by 5 6

effect of cooling on the water content and mechanical behavior of metamorphosed rocks calculations of the decline in f h2o in equilibrium with typical metamorphic assemblages during cooling indicate that there will be a large drop in pore water pressure within 100 of the metamorphic maximum the loss of pore fluid and the generation of large effective pressures across grain boundaries will inhibit rock deformation by those mechanisms that depend on the presence of an aqueous fluid phase in many metamorphic belts extensive deformation occurs below the metamorphic maximum during heating that is while fluid is being generated whereas rocks do not deform in the same ductile manner on cooling even while temperature is high this may reflect changing mechanical properties of the rocks rather than merely changes in regional stresses mobile belts could be made rigid in this way in about 20 m y

late orogenic doming in the eastern betic cordilleras final exhumation of the nevado filabride complex and its relation to basin genesis the geometry timing and kinematics of late orogenic extension in the betic cordilleras pose the problem of a decoupling of upper crustal and lower crustal deformation regimes perpendicular directions of extension in metamorphic domes and nearby sedimentary basins remain unexplained this paper puts kinematic constraints on the final exhumation of the nevado filabride complex focusing on the formation of metamorphic domes and their relations with the adjacent basins structural fabrics and kinematic indicators below the main shear zones as well as their relations with both published changing metamorphic p t conditions and geochronological data were studied our approach describes 1 a consistent top to the west shear parallel to dome axes of during d2 i e during decompression with distributed ductile flow and the onset of strain localization along major shear zones 2 further strain localization along the major shear zones under greenschist facies conditions during d3 leading to s c mylonites formation accompanied with a rock strong thickness reduction 3 the divergence of shear direction on either limbs of domes during d3 showing the appearance of the dome geometry and 4 a local evolution toward n s brittle extension d4 in the upper plate and formation of sedimentary basins continuous ductile to brittle top to the west shear is compatible with the slab retreat hypothesis from the miocene the formation of domes which adds gravitational forces responsible for the final stages of exhumation is thus characterized by important kinematics changes necessary to explain coeval n s opened basins later from the upper tortonian a contractional event d5 amplified the earlier domal structures forming the present north vergent folds

textural and mechanical evolution with progressive strain in experimentally deformed aplite determining meaningful flow laws for the polyphase aggregates that characterize the crust is difficult because the strength of such aggregates depends on the strengths volume proportions and geometrical arrangement of their constituent phases and all of those factors may change with progressive strain in order to investigate some of these factors we have performed a series of axial compression experiments on a fine grained granite aplite over a range of shortening strains 20 85 and temperatures 700 900 c at 1 5 10 6 s 1 and 1400 mpa we have also done comparison experiments on pure quartz and feldspar aggregates the aplite is composed of 1 3 quartz and 2 3 oligoclase and microcline with minor biotite and magnetite the quartz grains are dispersed in a continuous matrix of feldspar optical and tem scale microstructures indicate that the deformation mechanisms of the quartz and feldspar grains in the aplites are similar to those of pure quartz and feldspar aggregates deformed at the same conditions the feldspars deform by semi brittle flow at 700 and 800 c and by recrystallization accommodated dislocation creep at 900 c while quartz deforms by recrystallization accommodated dislocation creep at 700 c and by climb accommodated dislocation creep at 800 and 900 c pure albite aggregates are stronger than quartz aggregates over this range of conditions however in the aplite an apparent strength reversal is inferred from grain strain measurements and changes in phase distribution with increasing temperature at 700 c the dispersed quartz grains in the aplite remain less deformed than the feldspar grains because the scarcity of quartz quartz grain boundaries limits recovery by grain boundary migration recrystallization causing the quartz strength to rise above that of feldspar however the aplite is weaker than a pure feldspar aggregate because cracking is enhanced by stress concentrations at the quartz grains at 800 c the two phases undergo approximately equal strain because easy dislocation climb in quartz allows individual grains to strain homogeneously the aplite strength is close to that of a pure feldspar aggregate at 900 c the initially continuous feldspar grains deform inhomogeneously and undergo microboudinage while the quartz grains deform homogeneously and gradually become more interconnected and undergo higher strain than the feldspars the aplite strength is initially closer to that of a feldspar aggregate but with increasing strain it becomes closer to that of a quartz aggregate the aplite undergoes strain softening over the whole range of conditions at 700 and 800 c the weakening is due to grain boundary migration recrystallization of feldspar in the fine grained crush products along grain scale faults at 900 c the weakening is due to a combination of grain boundary migration recrystallization of the feldspars plus increasing interconnection of the weaker quartz grains these results demonstrate that the textural evolution of a polyphase aggregate with increasing strain is determined by both the phase distribution and the deformation mechanism of each phase and that very high strains may be required to approach microstructural and mechanical steady state therefore measuring or calculating a flow law appropriate for granite at mid crustal conditions is not simple

rheological evolution of a mediterranean subduction complex abstract we use field and microstructural observations coupled to previously published p t time histories to track the rheological evolution of an intracontinental subduction complex exposed in the betic cordillera in the western mediterranean region the body of rock we focus on known as the nevado filabride complex nfc was originally part of the upper crust of the iberian margin it was subducted into hot asthenospheric mantle then exhumed back toward the surface in two stages an early stage of fast exhumation along the top of the subducting slab in a subduction channel and a late stage of slower exhumation resulting from capture by a low angle detachment fault rooted at the brittle ductile transition each stage of deformation in the nfc was punctuated by changes in the dominant deformation mechanism deformation during initial subduction of the complex was accommodated by pressure solution creep in the presence of a fluid phase the grain sizes stress magnitudes and estimated strain rates for this stage are most consistent with a thin film model for pressure solution in which the diffusion length scale is controlled by the grain size during the early stages of exhumation within the subduction channel deformation transitioned from pressure solution to dislocation creep due to increases in temperature which resulted in increases in both water fugacity and grain size each of which favor the dislocation creep mechanism differential stress magnitudes for this stage were 10 mpa and are consistent with simple models of buoyancy driven channel flow with continuing subduction channel exhumation deformation remained within the dislocation creep field because sequestration of free water into hydrous retrogressive minerals suppressed the pressure solution mechanism differential stresses progressively increased to 100 mpa near the mouth of the channel during cooling as the rocks moved into mid crustal levels during the final core complex stage of exhumation deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid crustal detachment fault localization was promoted by a transition from dislocation creep to dislocation creep accommodated grain boundary sliding at temperatures of 350 380 c grain sizes of 4 m and differential stress magnitudes of 200 mpa peak differential stress magnitudes of 200 mpa recorded just below the brittle ductile transition are consistent with byerlee s law in the upper crust assuming a vertical maximum principal stress and near hydrostatic pore fluid pressures overall the distribution of stress with temperature coupled to independent constraints on strain rate from field observations and geochronology indicate that the naturally calibrated hirth et al 2001 flow law for wet quartzite accurately predicts the rheological behavior of mid crustal rocks deforming by dislocation creep

formation of diagenetic dolomite in coastal sabkha along arabian persian gulf aragonitic intertidal sediments are being dolomitized in the subsurface supratidal environment of a sabkha along the south shore of the arabian persian gulf as a result of the percolation of wind driven marine derived brines the development of abundant diagenetic dolomite results from an optimum combination of high mmg mca ratio fluids a rapid flow rate which is related to the flooding frequency and sediment permeability and a shoreline configuration that maintains the proper conditions for a sufficient length of time strongly dolomitized areas are localized along the flanks of remnant channels and in the lee of islands that have been engulfed by the prograding sabkha the dolomitizing process takes about 1 000 to 1 500 years most diagenetic dolomite forms at the expense of primary aragonite according to the reaction mg 2caco3 arag rarr camg co3 2 ca dolomite forms under the following conditions a chloride concentration between 3 25 and 3 75 mcl kg b mmg mca greater than about 6 c ph between 6 3 and 6 9 d minimum pco2 of 10 2 to 10 3 atm e temperature between 25 and 40 c f saturation with respect to gypsum and g reducing environment in association with hydrogen sulfide other recent dolomite occurrences in the bahamas florida and bonaire are significantly different in terms of stratigraphy degree of lithification and evaporite mineral association from those in the arabian gulf

crystal plasticity and superplasticity in quartzite a natural example plastically deformed quartzites from the betic movement zone betic cordilleras spain exhibit microstructures indicative of crystal plasticity on a mineral grain scale quartzites with dynamically recrystallized grain sizes larger than 10 m have strong crystallographic preferred orientations narrow grain boundaries little creep damage and an inverse proportionality of dislocation density and grain size mylonites with grain sizes smaller than 10 m have low crystallographic preferred orientations wide grain boundaries up to 1000 a abundant creep damage and decreasing dislocation density with diminishing grain size this is thought to reflect a clear cut shift in deformational regimes from dislocation creep to superplastic flow at 10 m grain size superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 m during an initial dislocation creep stage dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain boundary sliding in the mylonites it seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens in the mylonites dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms and an estimate for late stage stresses is provided by unbound dislocation densities in both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization

titaniq a titanium in quartz geothermometer titanium is one of many trace elements to substitute for silicon in the mineral quartz here we describe the temperature dependence of that substitution in the form of a new geothermometer to calibrate the titaniq thermometer we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt at temperatures ranging from 600 to 1 000 c at 1 0 gpa the ti contents of quartz in ppm by weight from 13 experiments increase exponentially with reciprocal t as described by text log left x text qtz text ti right 5 69 pm 0 02 frac 3765 pm 24 t k application of this thermometer is straightforward typically requiring analysis of only one phase quartz this can be accomplished either by epma for crystallization temperatures above 600 c or by sims for temperatures down to at least 400 resulting temperature estimates are very precise usually better than 5 c potentially allowing detailed characterization of thermal histories within individual quartz grains although calibrated for quartz crystallized in the presence of rutile the thermometer can also be applied to rutile absent systems if tio2 activity is constrained

silicon diffusion in a natural quartz aggregate constraints on solution transfer diffusion creep abstract silicon bulk diffusion rates were determined in a fine grained 1 2 m diameter natural quartz aggregate novaculite from hot springs ar the diffusion experiments were done at 600 800 c and 150 mpa confining pressure hydrothermal and at 800 1100 c and 1 atm in a stream of dry n2 the technique employed an isotopic tracer 30si and standard step scan analysis using an ion microprobe sims the temperature dependence of silicon bulk diffusion in the novaculite is described by the arrhenius parameters d0 bulk 3 7 10 10 m2 s and q 137 18 kj mol and d0 bulk 6 2 10 9 m2 s and q 178 38 kj mol for the hydrothermal and dry experiments respectively at 800 c the silicon bulk diffusion rate at 150 mpa hydrothermal is about a factor of 10 greater than at 1 atm dry for an assumed effective grain boundary width of 2 nm the silicon grain boundary diffusion rates in novaculite are a factor of two to three less than oxygen diffusion rates for similar hydrothermal conditions and more than 12 orders of magnitude greater than anhydrous silicon volume diffusion rates extrapolated to the temperature of these experiments the results of this study provide valuable constraints on the role of diffusion accommodated deformation in quartz rich rocks

diagenesis of ordovician carbonates from the north east michigan basin manitoulin island area ontario evidence from petrography stable isotopes and fluid inclusions middle to late ordovician subtidal carbonates in the manitoulin island area of ontario are predominantly limestone in composition but non ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase integration of field petrographic geochemical 6i3c 6 o and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis with much of the alteration controlled by regional fracturing and hydrothermal influences aqueous type 1 fluid inclusions in early calcite pre dolomite and dolomite are saline 29 wt nacl eq solutions with ca and or mg in excess of na and display homogenization temperatures with modes of 95 and 101 c respectively these temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient which also is not well supported the fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata type 1 inclusions in late post dolomite calcite are less saline 19 wt nacl eq and have a bimodal distribution of homogenizatign temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183 c a small proportion of methane and light hydrocarbon bearing fluid inclusions type 2 are present in all stages of carbonate dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the michigan basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the manitoulin island area were dolomitized alternatively migration of dolomitizing brines downward from the overlying pervasively dolomitized silurian sequence into fractures in the ordovician carbonates may have occurred integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin nevertheless high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear the alteration styles of the ordovician carbonates in the manitoulin area are similar to those of ordovician hydrocarbon reservoirs described from other parts of the michigan basin they indicate that fracture related diagenesis occurred on a basin wide scale and that hydrothermal effects were important

linked silurian carbon cycle perturbations bursts of pinnacle reef growth extreme sea level oscillations and evaporite deposition michigan basin usa abstract integration of sedimentologic chemostratigraphic biostratigraphic and geochronologic data identifies the profound influence that sudden bursts of carbonate sedimentation sea level oscillation and hypersalinity associated with ireviken and mulde carbon cycle perturbations had on the evolution of the michigan basin during the silurian conodonts and carbon isotope 13ccarb stratigraphy of the lockport and engadine dolomites constrain the rapid build up and progradation of a thick biohermal wedge along the basin margins to the sheinwoodian age ireviken excursion a recently identified period of global reef growth this event significantly changed the hydrologic architecture of the basin ultimately resulting in restricted seawater exchange that drove evaporite deposition zircon u pb ages of 428 ma from bentonites near the base of the evaporite bearing salina group together with 13ccarb profiles transecting the basin constrain the onset of hypersalinity to initiation of the late homerian mulde excursion importantly the double 13ccarb peak of this event parallels a complex pattern of dramatic sea level oscillations on the order of 100 m recorded within the a 0 to a 2 evaporites and associated pinnacle reefs ascending 13ccarb values were accompanied by sea level fall karstification and the formation of evaporites in the basin center intra excursion sea level rebounds triggered exceptionally rapid upward pinnacle reef growth subsequent sea level fall again exposed the ramp and pinnacle reefs resulting in basin wide restriction and massive salt deposition that ultimately encased the reefs in salt the return to baseline 13ccarb values was accompanied by flooding and carbonate sedimentation our integrated approach with high resolution 13ccarb stratigraphy from closely spaced intra basinal sections as a central component enables finely tuned chronostratigraphic correlation recognition of unconformity bound facies packages i e systems tracts and constrains variations in sea level and sedimentation rates these findings have immediate applications to predictive static reservoir models for the evaluation of the reefs as repositories for anthropogenic co2 sequestration gas storage reservoirs and hydrocarbon exploration ultimately our findings demonstrate that a cascade of rapid environmental changes including pulses of carbonate sedimentation high amplitude sea level oscillations and evaporite deposition were associated with the ireviken and mulde perturbations of the silurian carbon cycle

formation of 18o depleted dolomite within a marine evaporitic sequence triassic reichenhall formation austria dark grey bituminous dolostones interbedded with marine derived anhydrite horizons occur in the triassic reichenhall formation of western austria fossils are rare and indicate a hostile hypersaline depositional environment the dolomites are finely crystalline fairly stoichiometric well ordered and non ferroan closely spaced samples 94 in total of individual dolomite units have been analysed for their carbon and oxygen isotopic composition the data indicate surprisingly low 18o values 5 7 to 2 1 0 pdb whereas the 13c values are comparable to the contemporary triassic seawater 0 2 to 2 6 0 pdb sedimentological evidence including i lack of any evidence for extensive dissolution ii distinct oxygen and carbon isotope ratios of individual dolomite units iii covariance of carbon and oxygen isotopes within some dolomite layers and iv inclusions of celestite in dolomite indicates a nearly closed system after early diagenesis combining this information with water rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation a secondary factor may be dolomite recrystallization at elevated temperatures during burial

geochemistry of cambro ordovician arbuckle limestone oklahoma implications for diagenetic δ18o alteration and secular δ13c and variation abstract isotopic analyses of 227 limestone samples from the cambro ordovician arbuckle group oklahoma document slow secular changes in the chemistry of the limestones from late cambrian to early ordovician the 18o values of the limestones increase from 10 to 7 pdb 13c values decrease from 0 to 2 pdb and 87 sr 86 sr ratios decrease from 0 7091 to 0 7088 the light 18o values suggest that all arbuckle limestones underwent diagenetic alteration probably caused by meteoric water recharged during the development of the overlying pre middle ordovician unconformity the gradual 18o increase from late cambrian to early ordovician reflects reduced 18o depletion with decreasing burial temperature during alteration although the presence of additional primary secular 18o variation cannot be ruled out the 13c and 87 sr 86 sr variations in accord with 13c and 87 sr 86 sr variations in the literature represent primary secular variations the variations indicate that the 13c value and 87 sr 86 sr ratio of early paleozoic surface seawater decreased from late cambrian to early ordovician the 13c variation during this time period seems to correlate with sea level variation specifically during sea level fall an increase in the rate of oxidation of organic matter caused 13c depletion of inorganic bicarbonate in seawater as a result early ordovician carbonates probably deposited during the regression stage of the latest precambrian to latest early ordovician cycle became 13c depleted relative to late cambrian carbonates the decrease of seawater 87 sr 86 sr ratio from late cambrian to early ordovician may have resulted from decreased riverine sr input caused by decreased rate of continental weathering

accurate differential stress measurement using the molten salt cell and solid salt assemblies in the griggs apparatus with applications to strength piezometers and rheology abstract calibrations have been developed for the molten salt cell and solid salt assemblies used in tri axial griggs type piston cylinder deformation apparatus griggs apparatus that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell elastic response of the apparatus was characterized by loading alumina and wc rods and flow strengths were measured for high purity nickel and molybdenum and a ti zr mo alloy in replicate experiments in a griggs apparatus and a heard tri axial gas apparatus gas apparatus at identical pressures temperatures and strain rates the elastic response of the griggs apparatus is nonlinear at low total loads 1 760 mpa on a 5 mm diameter sample but becomes linear at loads greater than 15 kn plastic deformation experiments on water insensitive metals indicate that differential stresses measured in the molten salt cell msc and solid salt ssa assemblies in the griggs apparatus are greater than those measured in the gas apparatus at all conditions but vary systematically and reproducibly a simple linear calibration gas apparatus 0 73 griggsmsc 10 mpa where differential stress 1 3 can be applied to the differential stresses obtained using the msc in the griggs apparatus to obtain sample flow strength a similar linear calibration gas apparatus 0 73 griggsssa 48 mpa 30 mpa can be applied to the differential stresses obtained using the griggs apparatus with a solid salt assembly to obtain accurate sample flow strength these calibrations have been tested by applying them to mechanical data for carbonates deformed in the griggs apparatus and gas apparatus the calibrations are also applied to data from previous studies performed in the griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus when the experimental starting materials and conditions are the same the recalculated flow strengths measured in the griggs apparatus experiments agree with those measured in the gas apparatus however discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

stable isotopes in sulphate minerals from the salina formation in southwestern ontario the oxygen and sulphur isotopic compositions of anhydrite and minor gypsum from the salina formation in the michigan basin of southwestern ontario were investigated to obtain information on depositional environments the data base was also used to compare the isotopic compositions of these late silurian sulphates with global age curves for evaporites the morphological and petrographical appearance of the sulphates suggests a shallow water possibly sabkha type environment of deposition of the salina anhydrites throughout most of the basin 18o values are rather consistent throughout the salina but with short and substantial isotopic shifts in specific horizons these rapid changes reflect the importance of redox cycling in shallow restricted environments no significant correlation between 18o and 34s is observed in massive and bedded anhydrites but is recognized in nodular and some chickenwire anhydrites biological reduction of sulphate in the environment of deposition must account for this comp

recent cross formational fluid flow and mixing in the shallow michigan basin ground water with total dissolved solids tds ranging from 10 to 50 g l exists at depths between 90 and 140 m in devonian oil bearing formations in the shallow michigan basin of southwestern ontario these formations comprise mainly limestone and dolomite and the sources of the saline ground water have not been identified previously isotopic and major ion data from ground water from devonian oil bearing formations in southwestern ontario indicate that the highest salinity fluids were emplaced locally from depths of several hundred meters during the later stages of or following the wisconsinan glaciation saline water and petroleum were also emplaced into overlying pleistocene clay rich glacial deposits isostatic rebound leading to fracturing and enhanced formation permeability due to matrix expansion was probably the primary mechanism that enabled these saline fluids to migrate into discrete areas of the devonian and pleistocene formations the geochemical data presented here support a model involving cross formational fluid flow from depth occurring vertically on the scale of several hundred meters since glaciation the hydrogeologic regime in this shallow basinal system must therefore be viewed as dynamic rather than static to depths of at least 500 m over a time frame of 100 000 yr 54 refs 10more figs 2 tabs less

p and s wave upper mantle seismic velocity structure beneath the northern apennines new evidence for the end of subduction orogeny in the geologically young northern apennines is explained by the eastward retreat of a convergence zone in which adriatic lithosphere subducts to the west it is unclear however whether all of the lithosphere or only the lower portion has subducted since 15 ma when the convergence zone lay near corsica we combine teleseismic p and s wave arrival time data from the retreat seismic network 2003 2006 and surrounding permanent stations to estimate tomographic images of the upper mantle structure beneath the northern apennines we image a vertically oriented slab that that extends to only 300 km depth our slab termination is shallower than previous studies but is confirmed by resolution tests furthermore our images resolve the southern edge of the northern apennines slab at 43 n with no deep continuity with any slab segment to the south as earlier proposed our results suggest that only a 300 400 km strip of lithosphere has subducted since 15 ma beneath the northern tyrrhenian sea a length roughly equivalent to the distance from the present day west coast of corsica to the crest of the northern apennines although not a definitive indicator the largely aseismic vertical slab configuration and its limited extent coupled with other volcanic and geophysical indicators suggests the delamination scenario of bird 1979 in which only the lower portion of the continental lithosphere subducts

orthogonal folding of extensional detachments structure and origin of the sierra nevada elongated dome betics se spain 1 a close relationship of the kinematics and timing between low angle extensional faulting and upright folding is established for the miocene detachment systems in the hinterland of the betics in southeastern spain folding accompanied tectonic denudation developing elongated domes with fold axes both parallel and perpendicular to the direction of extension the geometry kinematics and tectonic evolution of two major sequentially developed extensional fault systems have been characterized in several e w elongated mountain ranges of the central betics on the basis of new cartographic and structural data and a comprehensive revision of other available geological and geophysical observations the extensional systems have an average wsw direction of extension and led to exhumation of the two lower metamorphic complexes of the betics the nevado filabride and the alpujarride during the middle and upper miocene serravallian to tortonian the extended domain contains a core complex with distal and proximal antiformal hinges separated by around 60 km and with a fold amplitude of 6 km measured parallel to the direction of extension the total amount of extension across the core complex is about 109 116 km corresponding to a stretching factor of 3 5 3 9 estimated using the distance between fold axial surfaces and a geometrical model accomplishing footwall deformation during tectonic denudation via subvertical simple shear the elongated domal and basinal geometry of the detachments and their respective footwalls is due to the interference between two sets of orthogonal large scale open folds trending n s and e w longitudinal n s trending folds are interpreted as isostatic folds i e folds that developed in response to differential unloading of the extensional detachment footwalls inducing ductile flow in the middle crust these folds formed in a rolling hinge anticline in which rotation migrates westward through the footwall as it is progressively unroofed in contrast e w trending folds being subparallel to the direction of extension have a contractional origin and progressively affect to the west the isostatically readjusted segments of the detachments once they are inactive orthogonal folding occurred since the serravallian to lower pliocene extension is still active from the rolling hinge anticline in sierra nevada to the west the middle miocene to pliocene tectonic evolution of the sierra nevada elongated dome at the core of the betic hinterland is another example of the coexistence of extension and contraction during continued overall convergence and mountain building

microbially induced dolomite precipitates in eocene lacustrine siliciclastic sequences in the dongying depression bohai bay basin china evidence from petrology geochemistry and numerical modeling this study investigates the origin of dolomite by examining thick lacustrine sequences of evaporites mudstones and deep water sublacustrine fan sandstones in the lower unit of the fourth member of the eocene shahejie formation es4x in bohai bay basin china this petrographic geochemical and numerical modeling study recognizes evidence for microbially mediated dolomite precipitates in the es4x sandstones including the following 1 spherical or elliptical dolomite aggregates consisting of submicrometer sized crystals 2 negative 13cvpdb vpdb refers to vienna peedee belemnite values of the dolomite cements 6 56 to 3 35 3 relatively low precipitation temperatures of dolomite cements 39 c 65 c 102 f 149 f based on 18ovpdb values from 8 76 to 4 65 and 4 negative 34svcdt vcdt refers to vienna canon diablo troilite values of associated framboidal pyrite 3 9 to 5 7 with respect to an anhydrite precursor 21 2 to 37 8 one dimensional reactive transport modeling incorporating biodegradation kinetics was conducted to evaluate the link between biodegradation of organic matter and geochemical reactions of inorganic minerals during microbial sulfate reduction msr microbial metabolism in the model creates favorable geochemical conditions for dolomite precipitation by increasing ph carbonate alkalinity and mg2 ca2 ratio the modeling results reproduce petrographic observations associated with the msr process and indicate that the kinetics of microbial growth can significantly modify the geochemistry of simulated waters and enhance microbially mediated dolomite precipitation therefore this study illustrates the effect of msr in dolomite precipitation in a semiclosed and reducing continental lacustrine basin

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

order from chaos a field based estimate on bulk rheology of tectonic melanges formed in subduction zones abstract many tectonic hp lt melanges exhibit block in matrix structures in contrast to the pervasively deformed matrix the blocks of various lithologies and pt histories usually appear undeformed if we accept that an hp lt melange reflects persistent high strain deformation in a deep 30 km subduction channel the record of these rock associations can provide insight into stress state rheology and flow patterns along a plate interface the bulk rheology of a block in matrix structure is controlled by a the dominant deformation mechanism of the matrix b the volume fraction of blocks engulfed in that matrix and c the geometry of the blocks the microstructures of the matrix indicate distributed deformation by dissolution precipitation creep dpc while crystal plastic deformation is subordinate dpc is grain size sensitive controlled by the type of interfaces and requires an aqueous pore fluid at quasi lithostatic pressure the rheology of a rock body undergoing deformation by dpc is believed to be newtonian viscous matrix viscosities are estimated to be on the order of 10 19 pa s or less here we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology using stereology we determine the block size distributions as well as the total block volume fractions for block in matrix structures from several regions the volume fractions are found to range from 2 to 70 with typical values between 5 and 50 using published mixing laws we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50 the viscosity increase is less than one order of magnitude consequently the influence of blocks on the bulk viscosity of a subduction channel is minor instead bulk rheology is primarily controlled by the deformation behaviour of the matrix material

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

exhumation rates of high pressure metamorphic rocks in subduction channels the effect of rheology 1 exhumation of high pressure metamorphic rocks can take place with typical plate velocities of cm year this is consistent with a model of forced flow in a subduction channel the micro structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep according to a power law rheology instead deformation appears to be localized in low strength shear zones and is dominated by dissolution precipitation creep or fluid assisted granular flow implying a newtonian rheology 1d modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation when the subduction flux either equals or exceeds the return flux the maximum exhumation rate for newtonian behavior of the material is at least twice as high 1 3 of the subduction burial rate compared to that for power law creep 1 6 of the subduction burial rate

shear band formation and strain localization on a regional scale evidence from anisotropic rocks below a major detachment betic cordilleras spain regional scale deformation taking place in a strongly anisotropic yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact i e the filabres shear zone a major detachment in the betic cordilleras spain the density of c3 shear bands or extensional crenulation cleavage reworking the earlier s2 schistosity increases exponentially towards the contact in parallel with the decrease in the size of the shear domains systematic variations in angles and shape ratios are also reported deformation and age patterns however suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time this fossilized shear strain gradient was thus produced somewhat diachronously such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

metallogenic epoch of chagangnuoer iron deposit in western tianshan mountains xinjiang information from garnet sm nd isochron age the large size chagangnuoer iron deposit is hosted in andesite and andesitic volcaniclastic rocks in the middle upper part of carboniferous dahalajunshan formation with widespread high hydrothermal alteration represented by garnet distributed extensively around the uppermost ore body fei the precise formation age of high hydrothermal alteration is the key to judging whether iron metallogeny was related to volcanism or intrusive activity garnet which had a closely genetic relationship with magnetite was formed simultaneously with or slightly earlier than magnetite garnet sm nd isotope analyses reveal that 147sm 144nd values vary between 0 2792 and 0 5481 while 143nd 144nd ratios vary between 0 512950 and 0 513501 data of seven isotope samples form a good linear fitting relationship yielding the sm nd isochron age of 316 8 6 7 ma which represents the formation epoch of high temperature hydrothermal alteration the result indicates that magnetite intergrown with garnet was formed at the late stage of early carboniferous period therefore the iron metallogeny and high hydrothermal alteration might have resulted from the metasomatism between the post magmatic hydrothermal solution derived from eruption of dahalajunshan formation volcanic rocks and the underlying marble rather than the skarnization caused by the reaction between magmatic hydrothermal solution from permian intrusion and marble in the ore district

the record and stratigraphie implications of organic walled microfossils from the ediacaran terminal proterozoic of south australia two assemblages of organic walled microfossils have been recognized in drillcore samples from the late proterozoic rodda beds in the eastern officer basin south australia the fossils include tube like remains and large simple and sculptured acritarchs lithostratigraphic studies and seismic information in conjunction with previous albeit limited acritarch finds allow local correlation of the rodda beds with ediacaran or terminal proterozoic sequences in the northern adelaide fold belt site of the nominated ediacaran stratotype the new palynofloras are comparable with acritarch assemblages in the amadeus basin of central australia and suggest tentative correlations with sequences in china and the u s s r the presence of isotopically heavy marine carbonate in the lower fossiliferous horizons of the rodda beds pcpdb 3 to 6 0 is consistent with isotopic data from the equivalent interval in china in contrast the upper fossiliferous strata occur higher in the rodda beds where carbonate is significantly lighter 13 cpdb 1 to 3 o

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

crustal structure a key constraint on the mechanism of ultra high pressure rock exhumation abstract the distribution of ultra high pressure uhp metamorphic rocks demonstrates that burial to 100 km and rapid exhumation 1 cm a 1 of continental crust is a normal part of early 10 ma continental collision currently there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper crustal structures resulting from early collisional uhp rock exhumation characteristic features requiring explanation include structural domes that are cored by uhp nappes associated medium to high pressure nappes displaying a distinct pressure gap overlying lower grade rocks including suture zone ophiolites and coeval foreland directed thrust faults and syn exhumation normal faults we present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge competition between down channel shear traction and up channel buoyancy forces expressed as the exhumation number e controls burial and exhumation leading to rapid up channel flow when e 1 exhuming uhp material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge driving thrusting and extension this solution is compelling because it explains both the geology and the petrology of the tso morari and other uhp complexes and because it demonstrates that pulse like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures this places constraints on the exhumation mechanism and provides a test of alternative models other proposed mechanisms such as continuous circulation in a lithospheric scale wedge or overpressured subduction channel predict different types of upper crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of uhp terranes

experimental intracrystalline plastic flow in hot pressed synthetic quartzite prepared from brazilian quartz crystals samples of synthetic ultrafine grained quartzite were prepared by hot pressing aggregates of crushed clear brazilian quartz of mean grain size 0 4 m at 300 mpa and 1373 and 1473 k the samples displayed rapid grain growth to ca 12 20 m with 2 porosity at 1473 k facilitated by the 0 6 wt of water adsorbed onto the grain boundaries during sample preparation this water could be driven off by preheating thereby preventing grain growth sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile these were deformed experimentally in the quartz field using an argon gas medium apparatus at 300 mpa confining pressure at temperatures mainly between t 1273 and 1473 k ductile flow was described by the flow law 10 4 9 3 2 9 7 f h 2 o exp 242000 rt with stress a in mpa and grain size d in microns and strain rate in s 1 r is the gas constant and f h 2 o is water fugacity based on observation of grain flattening optical strain features shapes and densities of dislocation arrays and flow law parameters samples deformed dominantly by dislocation creep with some contribution from grain boundary sliding bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the phase transition extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction