faulting of rocks in three dimensional strain fields i failure of rocks in polyaxial servo control experiments abstract the dependence of the number of sets of faults and their orientation on the intermediate strain axis is investigated through polyaxial tests reported here and theoretical analysis reported in an accompanying paper in the experiments cubic samples of berea sandstone sierra white and westerly granites and candoro and solnhofen limestones were loaded on their three pairs of faces by three independent mutually perpendicular presses at room temperature two of the presses were servo controlled and applied constant displacement rates throughout the experiment most samples display three or four sets of faults in orthorhombic symmetry these faults form in several yielding events that follow a stage of elastic deformation in many experiments the maximum and the intermediate compressive stresses interchange orientations during the yielding events where the corresponding strains are constant the final stage of most experiments is characterized by slip along the faults

p t t paths of collisional orogens pressure temperature time p t t paths summarize the changes in pressure and temperature imposed on a metamorphic rock during orogenesis they provide a convenient framework for the interpretation of complex metamorphic histories and also offer insight into the thermal and tectonic factors controlling metamorphism in collisional orogens p t t data are acquired through a combination of textural observations thermobarometry and thermochronometry and assembled into a p t t path using geological constraints one dimensional p t t models assuming instantaneous deformation and thermal relaxation by conduction are flexible and useful for testing tectonic models particularly where geochronological constraints are available two dimensional models allow more sophisticated deformation geometries and allow the effects of advection to be incorporated analysis of collisional orogens in terms of critical wedge theory can yield p t t paths that reflect coupling between thickening uplift exhumation erosion and convergence where rates of erosion approach rates of tectonic uplift as is currently happening in the southern alps of new zealand high grade metamorphic rocks can be exhumed rapidly from considerable depth alternatively rapid exhumation may reflect gravity driven extension in an over steepened or thermally weakened orogen

brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

tectonic diffusion estimate of orogenic gold resources age frequency data are used to determine amounts of vertical tectonic displacement experienced by the global population of orogenic gold deposits following their emplacement at a crustal depth of 10 1 km knowledge of this tectonic dispersion allows for an estimate of the total crustal endowment of gold in orogenic gold deposits this estimate is based on a compilation of data on 365 orogenic gold deposits of which gold contents are available for 314 deposits and good age control is available for 126 deposits ages of well dated deposits form three clusters that range in age from 47 to 850 ma primarily phanerozoic 69 deposits 1730 to 2090 ma proterozoic 13 deposits and 2540 to 3107 ma archean 44 deposits gold endowments do not vary significantly among the three age groups and average 155 tonnes t per deposit for the entire population for which data on gold tonnages are available owing to the distinct tripartite nature of the age frequency distribution it is not possible that orogenic gold deposits represent largely invariant rates of ore formation and destruction uninterrupted tectonic dispersion would have resulted in the destruction of essentially all proterozoic and archean deposits separate tectonic diffusion calculations for each of the three deposit age populations were carried out while constraining biases to tectonic diffusion i e the most suitable ratios of uplift and erosion versus subsidence and burial with continental crustal thicknesses on the order of several tens of kilometers the results of these calculations indicated that a total of 25 236 orogenic gold deposits formed over the full span of geologic time that 23 664 of these 94 were destroyed during subsequent uplift and erosion and that 1 572 deposits remain at depth in earth s crust if all 365 known orogenic gold deposits have the same mean gold content as the deposits for which gold contents are available then 706 439 t of gold remain at depth in earth s crust gold in orogenic deposits constitutes about 0 0022 percent of the total gold content of the crust using estimates of gold released by prograde metamorphism of the otago schist gold in the deposits that remain in the crust could have been released from about 1 413 000 km3 of amphibolite facies rocks which constitute only about 12 percent of currently exposed metamorphic rocks if we assume a maximum depth of 3 km for future mining of these deposits the model results indicate that deposits containing almost 113 000 t of gold exist within crustal rocks above this depth discovered deposits make up about 8 percent of this endowment in the context of spatial density the areal concentration of orogenic gold deposits along phanerozoic metamorphic belts is about ten times that in proterozoic terranes and about three times higher than in archean belts this suggests that exploration for the next generation of orogenic gold deposits should be focused on phanerozoic terranes

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

numerical study of dynamic fracture aperture during production of pressure sensitive reservoirs in fractured reservoirs some dynamic behavior such as fracture opening or closure and fracture shear dilation is induced by the variation of pressure and in situ stress which is associated with the fracture aperture and conductivity in this paper we focus on the dynamic behavior of fractures and various kinds of affecting factors including in situ stress lithology and fluid pressure a mathematical model for numerical simulation of the fracture dynamic behavior is developed which consists of a matrix fracture stress strain model and a matrix fracture flow model the stress strain model of fractures is derived which contains two sections of shear dilation before peak dilation and after peak shear dilation then a discrete fracture network dfn technology is used to depict the shape of fractures in reality and the model is solved by the finite element method to investigate what the range ability the fracture aperture presents under the influence of the affecting factors based on this model the effects of pressure distribution lithology anisotropic in situ stress and well production proration are investigated and the specific effects by different affecting factors are analyzed the numerical outcome indicates that the fracture aperture increases as the field pressure rises the anisotropic in situ stress and water injection can trigger large shear dilation which serves as a positive function in fracture opening

a rheological model of a fractured solid abstract experiments to study the behavior of various materials point to the relation that exists between elastic properties and the type of stress the influence of the state of stress on the elasticity of a fractured material will be discussed for a physically non linear model of an elastic solid the strain dependent moduli model of material presented in this paper makes it possible to describe this feature of a solid it also permits to simulate a dilatancy of rocks a damage parameter introduced into the model using a thermodynamical approach allows to describe a rheological transition from the ductile regime to the brittle one and to simulate the rock s memory narrow fracture zone creation and strain rate localization additionally the model enables the investigation of the final geometry of fracture zones and also to simulate their creation process taking into account pre existing fracture zones the process of narrow fracture zone creation and strain rate localization was simulated numerically for single axis compression and shear flow

modeling of large scale accretionary wedge deformation instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory given the assumptions of a perfectly plastic rheology and plane strain conditions after selecting a plastic yield stress and an average density a stress field slip line field can be computed for a wedge of known cross sectional configuration from known stress boundary conditions on the upper surface basal surface tractions are derived without basal boundary conditions the geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress the known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown consequently basal velocity distributions must be assumed to permit the construction of velocity fields these assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge the sunda accretionary wedge west of central sumatra is used to illustrate the plasticity approach a minimum average yield stress of 20 30 mpa and a weaker basal layer of variable effective strength are indicated for this example surface uplift rates may be affected significantly by regions of high basal strain rate finite velocity discontinuities and underplating the known pattern of uplift rates near nias island located on the outer arc ridge is consistent with the constraint that 90 of the total incremental shortening of the wedge is concentrated within 20 km of the trench given a sufficient number of uplift and tilt rate observations across a particular wedge the relative influence of underplating tectonic erosion and basal strain rate variations could be assessed using the perfectly plastic model

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

insights into the damage zones in fault bend folds from geomechanical models and field data abstract understanding the rock mass deformation and stress states the fracture development and distribution are critical to a range of endeavors including oil and gas exploration and development and geothermal reservoir characterization and management geomechanical modeling can be used to simulate the forming processes of faults and folds and predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses this approach enables the development of forward models that incorporate realistic mechanical stratigraphy e g the bed thickness bedding planes and competence contrasts include faults and bedding slip surfaces as frictional sliding interfaces reproduce the geometry of the fold structures and allow tracking strain and stress through the whole deformation process in this present study we combine field observations and finite element models to calibrate the development and distribution of fractures in the fault bend folds and discuss the mechanical controls e g the slip displacement ramp cutoff angle frictional coefficient of interlayers and faults that are able to influence the development and distribution of fractures during fault bend folding a linear relationship between the slip displacement and the fracture damage zone the ramp cutoff angle and the fracture damage zone and the frictional coefficient of interlayers and faults and the fracture damage zone was established respectively based on the geomechanical modeling results these mechanical controls mentioned above altogether contribute to influence and control the development and distribution of fractures in the fault bend folds

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

postmetamorphic unroofing history deduced from petrology fluid inclusions thermochronometry and thermal modeling an example from southwestern new england nonlinear unroofing rates from 10 to 2 and h 2 o rich fluid inclusions require nearly isothermal decompression followed by more isobaric cooling from these conditions comparison of this observed path to those generated by thermal models suggests that decompression was characterized by initial rapid upward en bloc velocity 1 cm yr of brief duration followed by much slower unroofing rates 0 3 mm yr published geochronologic ages in this region constrain the rapid uplift to the middle devonian and uplift slowed dramatically by the end of the devonian this differential uplift has major implications for interpretation of devonian igneous activity and sedimentation in southwestern new england

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

isotope geochemistry of the tieluping silver lead deposit henan china a case study of orogenic silver dominated deposits and related tectonic setting the tieluping silver deposit which is sited along ne trending faults within the high grade metamorphic basement of the xiong er terrane in the qinling orogenic belt is part of an important recently discovered mesozoic orogenic type ag pb belt ore formation includes three stages an early barren quartz pyrite stage e an intermediate polymetallic sulfide ore stage m and a late barren carbonate stage l carbon sulfur and lead isotope systematics indicate that the e stage fluids are deeply sourced the l stage fluids are shallow sourced meteoric water whereas the m stage fluids are a mix of deep sourced and shallow sourced fluids sulfur and lead isotope data show that the ore forming fluids must have originated from a source with elevated radiogenic lead and low 34s values that differs significantly from exposed geologic units in the xiong er terrane the lower crust and the mantle this supports the view that the carbonate shale chert sequences of the guandaokou and luanchuan groups south of the machaoying fault might be the favorable sources although little is known about their isotopic compositions a tectonic model that combines collisional orogeny metallogeny and hydrothermal fluid flow is proposed to explain the formation of the tieluping silver deposit during the mesozoic collision between the north china craton and south china block early mid triassic indosinian orogeny crustal slabs containing the carbonate shale chert sequences of the guandaokou and luanchuan groups locally rich in organic matter carbonaceous shale were thrust northwards beneath the xiong er terrane along the machaoying fault metamorphic devolatilisation of this underthrust slab probably provided the ore forming fluids to develop the ag pb ore belt which includes the tieluping silver deposit fluids and magmas were emplaced during extensional stages related to the jurassic cretaceous yanshanian orogeny

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

geodynamic models of convergent margin tectonics transition from rifted margin to overthrust belt and consequences for foreland basin development a quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads the lithosphere and the associated foreland basin the model treats the lithosphere as a two dimensional nonuniform elastic plate whose strength is controlled thermally the thermal and flexural evolution of a margin is followed from initial rifting and passive margin development through overthrusting and foreland basin deposition to postdeformational erosion our models indicate the following 1 foreland basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting sensitivity decreases the farther the overthrust load is empl ced onto the craton 2 foreland basin depth is strongly influenced by the size of the overthrust load maximum depths in excess of 10 km 6 mi can occur given overthrust topography similar to the present day himalayas 3 a few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained very little of this eroded material is deposited in the foreland basin most must bypass the basin because it also is subjected to erosion and associated flexural uplift 4 thick overthrusts as interpreted from cocorp profiling are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

metamorphism of an early palaeozoic continental margin western baie verte peninsula newfoundland metamorphic mineral assemblages and textures from early palaeozoic continental margin rocks in north western newfoundland indicate that different structural levels have contrasting metamorphic histories rocks of the east pond metamorphic suite which represent the older structurally lower level of the margin experienced an early high pressure low temperature stage of metamorphism 10 12 kbar minimum 450 500 c which produced eclogite in mafic dykes and phengite garnet assemblages in pelites this was overprinted by higher temperature lower pressure amphibolite facies metamorphism 700 750 c 7 9 kbar minimum which produced complex symplectic textures in rocks of all compositions rocks of the fleur de lys supergroup which were deposited in the stratigraphically higher levels of the rifted margin reached pressures of 7 8 5 kbar at about 450 c during the early stages of metamorphism overprinted by assemblages which indicate maximum temperatures of 550 600 c at about 6 5 kbar the metamorphic history of both units is interpreted to be the result of thermal relaxation following initial burial of a continental margin by overriding thrust sheets since there is no evidence that maximum pressures or temperatures within the fleur de lys supergroup were ever as high as those reached in the east pond metamorphic suite these rocks may have followed parallel nested p t t paths with the more deeply buried east pond metamorphic suite subjected to greater thermal relaxation effects quantitative modelling of p t t paths is not possible with the present data owing to both large uncertainties in p t estimates and in the time of metamorphism

earthquake cycle fault zones and seismicity patterns in a rheologically layered lithosphere we study the coupled evolution of earthquakes and faults in a model consisting of a seismogenic upper crust governed by damage rheology over a viscoelastic substrate the damage rheology has two types of functional coefficients 1 a generalized internal friction separating states associated with material degradation and healing and 2 damage rate coefficients for positive degradation and negative healing changes the evolving damage modifies the effective elastic properties of material in the upper crust as a function of the ongoing deformation this simulates the creation and healing of fault systems in the upper seismogenic zone in addition to the vertically averaged thin sheet approximation we introduce a green function for three dimensional elastic half space for the instantaneous component of deformation the formulation accounts in an internally consistent manner for evolving deformation fields evolving fault structures aseismic energy release and spatiotemporal seismicity patterns these developments allow us to simulate long histories of crustal deformation and to study the simultaneous evolution of regional earthquakes and faults for various model realizations to focus on basic features of a large strike slip fault system we first consider a simplified geometry of the seismogenic crust by prescribing initial conditions consisting of a narrow damage zone in an otherwise damage free plate for this configuration the model generates an earthquake cycle with distinct interseismic preseismic coseismic and postseismic periods model evolution during each period is controlled by a subset of physical properties which may be constrained by geophysical geodetic rock mechanics and seismological data in the more generic case with a random initial damage distribution the model generates large crustal faults and subsidiary branches with complex geometries the simulated statistics depend on the space time window of the observational domain the results indicate that long healing timescale h describing systems with relatively long memory leads to the development of geometrically regular fault systems and the characteristic frequency size earthquake distribution conversely short h relatively short memory leads to the development of a network of disordered fault systems and the gutenberg richter earthquake statistics for intermediate values of h the results exhibit alternating overall switching of response from periods of intense seismic activity and the characteristic earthquake distribution to periods of low seismic activity and gutenberg richter statistics

the model of fracture development in the faulted folds the role of folding and faulting abstract tectonic fractures are otherwise known as joints or fissures the density of tectonic fractures is governed by the lithology layer thickness and structures the distribution and density of tectonic fractures in the kuqa depression in the northern tarim basin of northwest china are dominantly controlled by the regional faulted fold belt our purpose of this paper is to analyze how folding and faulting control the development of tectonic fractures based on outcrop and logs of drill data in the folded tight sandstone reservoirs in the kuqa depression the extensional area transition area and shortening area have been separated by two neutral lines which control the distribution and density of tectonic fractures in the buckle folds from the twelve sites of field measurements six logs of drill core data and analysis of the micro texture of tectonic fractures it can be deduced that the fracture development in the sub salt faulted fold belt is related to the neutral lines and related faults the high density tensile fractures in the vicinity of fold hinges are more developed than in the limbs of the fold folding is thus the first level of control over the general pattern of fracture distribution the fractures in the hanging wall of the thrust faults are more developed than those in the footwall further more the fractures in the hinge zone and steeply tilted limbs of folds close to thrust faults are more developed than the gently tilted limbs which are far from faulting this suggests that faulting is a secondary control over the development of local fractures within the faulted fold belt the development of fractures related to faulted folds in the kuqa depression can be also divided into three episodes pre folding syn folding and post folding which are identified by field observation

hybrid fracture and the transition from extension fracture to shear fracture fracture is a fundamental mechanism of material failure two basic types of brittle fractures are commonly observed in rock deformation experiments extension opening mode fractures and shear fractures for nearly half a century it has been hypothesized that extension and shear fractures represent end members of a continuous spectrum of brittle fracture types however observations of transitional fractures that display both opening and shear modes hybrids in naturally deformed rock have often remained ambiguous and a clear demonstration of hybrid fracture formation has not been provided by experiments here we present the results of triaxial extension experiments on carrara marble that show a continuous transition from extension fracture to shear fracture with an increase in compressive stress hybrid fractures form under mixed tensile and compressive stress states at acute angles to the maximum principal compressive stress fracture angles are greater than those observed for extension fractures and less than those observed for shear fractures fracture surfaces also display a progressive change from an extension to shear fracture morphology

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada

dynamic fracturing field and experimental observations abstract we analyzed a system of complex joints in thick dolomite layers that are exposed within the western margins of the dead sea basin these joints display two dominant features tree like branching and a gradual increase of density that leads to local fragmentation the development of this joint system is investigated in laboratory experiments with samples of brittle ductile layered composites the samples were subjected to layer parallel extension and displayed three styles of fracturing planar fractures known from previous tests branching fractures and clustering fractures observed here for the first time in layered composites based on fracture morphology we deduced that the branching and clustering fractures in the experiments and the tree like closely spaced joints in the field propagated at dynamic high velocity growth rates it is proposed that the morphological features described here could be used as field criteria to recognize dynamic rates of rock fracturing

geological characteristics and ore forming process of the gold deposits in the western qinling region china the western qinling belonging to the western part of the qinling dabie sulu orogen between the north china block and south china block is one of the most important gold regions in china isotopic dates suggest that the mesozoic granitoids in the western qinling region emplaced during the middle late triassic and the deposits formed during the late triassic almost all gold deposits in the western qinling region are classified as orogenic carlin type and carlin like gold deposits and they are the products of qinling orogenesis caused by the final collision between the north china block and the south china block the early subduction of the mian lue oceanic crust and the latter collision between south qinling terrane and the south china block along the mian lue suture generated lithosphere scale thermal anomalies to drive orogen scale hydrothermal systems the collision related magmatism also provided heat source for regional ore forming fluids in the carlin like gold deposits orogenic gold deposits such as huachanggou liziyuan and baguamiao lie between the shang dan and mian lue sutures and are confined to wnw trending brittle ductile shear zones in devonian and carboniferous greenschist facies metasedimentary rocks that were highly deformed and regionally metamorphosed these deposits are typical orogenic gold deposits and formed within a late triassic age the deposits show a close relationship between au and ag ores contain mainly microscopic gold and minor electrum and visible gold along with pyrite the ore forming fluids were main metamorphic fluids intensive tectonic movements caused by orogenesis created fluid migrating channels for precipitation locations although some orogenic gold deposits occur adjacent to granitoids mineralization is not synchronous with magmatism that is the granitoids have no genetic relations to orogenic gold deposits as ore forming fluids converged into dilated fractures during the extension stage of orogenesis changes of physico chemical conditions resulted in fluid immiscibility that played a key role in gold and sulfide deposition the geochemical and mineralogical characteristics of the carlin type deposits in the western qinling region are similar to those in the carlin trend nevada usa gold deposits such as la erma and jinlongshan occur mostly in the southeastern margin of the western qinling regionic region whereas some deposits occur in its eastern part these deposits are hosted in slightly metamorphosed cambrian to triassic sedimentary rocks showing structurally and stratigraphically controlled features the deposits mainly contain submicroscopic and microscopic gold in arsenian pyrite and arsenopyrite with characteristic ore forming elements of au as sb ba the ore forming fluids are early stocked formation water and later recharged meteoric water meteoric water apparently evolved in ore forming fluids by circulation indicating the extensional setting and led to the deposition of au and other elements in cool reactive permeable rocks at shallow levels forming the disseminated ores carlin like gold deposits occur between the shang dan suture and the fengxian zhen an fault the host rocks are mainly sedimentary rocks that underwent reconstruction through reworking by structural metamorphism these deposits are structurally controlled by brittle ductile shear zone and occur adjacent to granitoid plutons the most important characteristic that differ to the orogenic and carlin type gold deposits is the genetic relationship with the synchronous magmatism gold occurs mainly as microscopic gold pyrite and arsenian pyrite can be recognized as gold bearing minerals the ore forming fluids are main magmatic water mixed with metamorphic and or formation water similar to orogenic gold deposits fluid immiscibility caused the deposition of gold carlin like gold deposits

temperature time paths in the high p manicouagan imbricate zone eastern grenville province abstract the manicouagan imbricate zone miz eastern grenville province is a series of lithotectonic units assembled from different crustal levels during the grenvillian orogeny structural evidence along with u pb dating suggests that two separate stages of metamorphism occurred at ca 1050 and 1020 ma respectively the first stage is characterised by high pressure and temperature high pt metamorphism up to eclogite facies with a range of maximum recorded pressures between 14 and 20 kbar consistent with variations in burial depth maximum t conditions recorded from apparently different structural levels are rather similar 800 900 c suggesting high heat flow during eclogite facies metamorphism evidence of high temperatures 700 800 c recorded during the early stage of tectonic exhumation indicate near isothermal decompression the second stage is a high t event coeval with the emplacement of syn metamorphic granite and is recorded by rocks in the upper most tectonic slice boundary zone in the south east miz this event may also be responsible for amphibolite facies overprinting across the rest of the miz using the u pb and pt data sets along with published diffusion rates for pb in monazite titanite and rutile tt paths have been modelled for several of the lithotectonic units in most of the units suggested cooling rates appear to have been relatively fast during initial exhuamtion but show a prolonged period of slower cooling 5 10 c ma for 30 40 ma apparently at odds with the steep near isothermal pt paths preservation of eclogitic textures and preservation of garnet zoning in the highest grade rocks on the other hand in the boundary zone the recorded cooling rates are much faster 20 c ma indicative of tectonic exhumation this phase of rapid cooling in the boundary zone corresponds in time with the slow cooling rates at 1030 1000 ma in the rest of the miz thus thrusting of the boundary zone over the rest of the miz appears to have caused a thermal overprint which was superimposed on the tt paths taken as a whole the tt paths for the miz are interpreted as representing two separate metamorphic events which were both relatively short in duration

north west propagation of the grenville orogen grenvillian structure and metamorphism near key harbour georgian bay ontario canada abstract we report structural and metamorphic data from a c 25 km transect across the eastern grenville front tectonic zone gftz to the britt domain at the northern end of georgian bay near key harbour ontario constrasting grenvillian structural and fabric elements characterize the eastern gftz northern britt domain and a narrow transition zone between them moderately to steeply dipping foliations with strong down dip lineations in all three divisions appear to be associated with nw directed thrusting in the transition zone and northern britt domain early s l fabrics with steep lineations are overprinted by younger structures s l with shallow se sse plunging lineations in which sparse dominantly but not exclusively normal sense kinematic indicators are recognized pressure and temperature estimates from grenvillian metamorphic assemblages in metadi abase indicate that conditions of p 12 kbar and t c 800 c were achieved before or during the thrust related deformation with p t t paths that indicate near isothermal decompression to p c 4 kbar and t c 700 c correlation of fabric elements with points on the p t t paths suggests that exhumation occurred during two stages the first associated with thrusting 1035 ma and the second with extension and thrusting pre 1003 ma the gftz contains steeply to moderately dipping thrust related fabrics and lacks shallow extensional structures the latest episode of thrusting in the gftz is inferred to have taken place at 990 980 ma the data are interpreted in terms of a tectonic model involving two stages of propagation of the grenville orogen towards its foreland 1035 ma and 980 ma with an intervening period of extension although the tectonic regime probably remained compressional on the scale of the orogen

growth and retrograde zoning in garnets from high grade metapelites implications for pressure temperature paths a previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies britt domain ontario grenville province some large garnets contain discrete regions of high grossular content as well as variations in the mg mg fe and mn nearly parallel to grain boundaries the latter are superimposed on and cut across regions of high grossular content such crosscutting zoning patterns have not been previously recognized in garnets the ca zoning involves changes of 4 to 10 mol grossular and is interpreted to represent growth zoning whereas the decreasing mg mg fe and increasing mn from core to rim are believed to be a diffusional retrograde effect the ca zoning indicates a pressure drop of ca 5 kbar at 700 50 c

tectonic controls on the uplift of the nanga parbat massif pakistan himalayas cenozoic mountain belts like the himalayas contain regions that experienced rapid uplift in comparison with the areas around them1 whether this uplift occurred by buoyant rebound of previously thickened crust2 or during the crustal thickening process itself is uncertain3 4 here we show however that the 7 mm yr 1 recent uplift5 of the nanga parbat massif pakistan himalayas was accomplished during nw directed thrusting located along its western margin this 3 km wide liachar thrust zone operated from amphibolite facies to the present topographic surface and accommodates substantial crustal shortening the adjacent indus river valley itself cut by seismogenic faulting completely transects the massif showing that erosion keeps pace with geologically rapid thrust uplift6

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

considerations of the dilatancy angle in rocks and rock masses the present study responds to the poor treatment given to dilatancy in classical rock mechanics post failure problems such as tunnel or mine pillar design a comprehensive review of the literature and observations in regard to published test results would indicate that dilatancy is highly dependent both on the plasticity already experienced by the material and confining stress moreover it also appears that scale may play a non negligible role in our article we provide a detailed analysis of published test data with a view to proposing a sufficiently significant but conveniently simple formulation of the dilatancy angle that reflects these dependencies and that can be readily implemented in numerical codes the model is then tested demonstrating that it is capable of representing rock sample strain behaviour in compressive tests finally the model is applied to the resolution of ground reaction curves for tunnels in poor to average quality rock masses showing a good correlation with results obtained using practical rock engineering techniques r 2005 elsevier ltd all rights reserved

microfracturing damage and failure of brittle granites 1 the evolution of stress induced damage and the eventual brittle failure are experimentally analyzed for mount scott granite of oklahoma we quantify the damage intensity in two methods and directly compare model predictions and actual damage the 14 samples of the medium grain size granite were loaded triaxially at dry conditions room temperature and under 41 mpa confining pressure microfractures were mapped in five samples and the majority of them 80 belong to two groups tensile microfractures trending subparallel to the loading axis and shear microfractures trending 11 40 off the loading axis the tensile microfractures dominate the low stress stage and they remain intragranular with a stress increase the relative density of shear microfractures increases with increasing stresses and they formed elongated intergranular zones of coalescing microfractures we compared two independent values of damage intensity 1 the macroscopic experimentally measured reduction of the deformation modulus and 2 the expected reduction of this modulus calculated with several damage models for the density of the mapped microfractures our fracture density data best fit the model of noninteracting cracks of kachanov 1992

damage and plastic deformation of reservoir rocks part 1 damage fracturing in this series of studies we develop a numerical tool for modeling finite deformation of reservoir rocks we present an attempt to eliminate the main limitations of idealized methods for example elastic or kinematic that cannot account for the complexity of rock deformation our approach is to use rock mechanics experimental data and finite element models abaqus to generate realistic simulations the present numerical rheology incorporates the dominant documented deformation modes of rocks 1 rock mechanics experimental observations including finite strength inelastic strain hardening strength dependence on confining pressure strain induced dilation pervasive and localized damage and local tensile or shear failure without macroscopic disintegration and 2 field observations including large deformation distributed damage complex fracture networks and multiple zones of failure our analysis starts with an elastic plastic damage rheology that includes pressure dependent yield criteria stiffness degradation and fracturing via a continuum damage approach using the abaqus materials library we then use experimental results for berea sandstone in two configurations four point beam and dog bone triaxial to refine and calibrate the rheology we find that damage and fracturing patterns generated in the numerical models match the experimental features well and based on these observations we define damage fracturing the fracturing process by damage propagation in a rock with elastic plastic damage rheology in part 2 we apply this rheology to investigate fracture propagation at the tip of a hydrofracture

late variscan exhumation histories of the southern rhenohercynian zone and western mid german crystalline rise results from thermal modeling thermal modeling techniques constrained by published petrological and thermo chronometric data were applied to examine late orogenic burial and exhumation at a variscan suture zone in central europe the suture separates the southern rhenohercynian zone from the mid german crystalline rise and traces the former site of a small oceanic basin closure of this basin during variscan subduction and subsequent collision of continental units were responsible for different tectono metamorphic evolutions in the suture s footwall and hanging wall relative convergence rates between the southern rhenohercynian zone and western mid german crystalline rise can be inferred from the pressure temperature time evolution of the northern phyllite zone during late visean early namurian times horizontal thrusting velocities were at least 20 mm a thermal modeling suggests that exhumation of the mid german crystalline rise occurred temporarily at rates of more than 3 mm a such rapid exhumation cannot be produced by erosion only but requires a substantial contribution of extensional strain exhumation by upper crustal extension occurred contemporaneously with convergence and is explained by continuous underplating of crustal slices and thrusting along faults with ramp flat geometry finally implications for the tectono metamorphic history of the study area and the thermal state of the crust during late variscan exhumation are discussed

the shanggong gold deposit eastern qinling orogen china isotope geochemistry and implications for ore genesis abstract the shanggong au deposit in the xiong er terrane east qinling china has resources of about 30 ton au making it one of the largest orogenic mesothermal au deposits hosted in volcanic rocks of the mesoproterozoic xiong er group three stages of hydrothermal activity are recognized early middle and late of which two early and middle were ore producing and characterized by quartz pyrite and polymetallic sulfides respectively the third and late stage is represented by a carbonate quartz assemblage hydrogen oxygen and carbon isotope systematics of the shanggong deposit from a previous work suggest that the early stage fluids were derived from magmatic and or metamorphic devolatilization of sedimentary rocks at depth this is supported by new c s and published sr and pb isotopic data presented in this paper these new data 13c values ranging from 1 5 for early stage ankerite to 2 2 for late stage ankerite negative 34s values for sulfides from the middle stage 19 2 to 6 3 suggest a contribution from organic matter and that the ore fluid evolved from deeply sourced to shallowly sourced with those of the middle stage representing a mixture of these two fluid systems the comparison of the hydrogen oxygen carbon sulfur lead strontium isotope systematics between the shanggong deposit and the main lithologies in the xiong er terrane shows that neither these nor the underlying lower crust and mantle or combinations thereof could be considered as the source of ore fluids for the shanggong au deposit a likely source was a carbonaceous carbonate sandstone shale chert sequence in the underthrusted guandaokou and luanchuan groups exposed south of the xiong er terrane ar ar and rb sr isochron ages for mineral phases of the early middle and late stages together with geological field data constrain the timing of the hydrothermal activity and au metallogenesis at 242 10 167 7 and 112 7 ma respectively this metallogenesis and associated granitic magmatism can be related to the continental collision between the yangtze and north china cratons that resulted in the formation of the qinling orogen led to the different hydrothermal systems that were responsible for the three stages that formed the shanggong au deposit over a period of about 130 myrs

repeat temperature measurements in boreholes from northwestern utah link ground and air temperature changes at the decadal time scale 1 borehole temperature profiles provide a record of ground surface temperature gst change at the decadal to centennial time scale gst histories reconstructed from boreholes are particularly useful in climate reconstruction if changes in gst and surface air temperature sat are effectively coupled at decadal and longer time periods and it can be shown that borehole temperatures respond faithfully to surface temperature changes we test these assumptions using three boreholes in northwestern utah that have been repeatedly logged for temperature over a time span of 29 years we report 13 temperature depth logs at the emigrant pass observatory borehole gc 1 eight at borehole si 1 and five at borehole dm 1 acquired between 1978 and 2007 systematic subsurface temperature changes of up to 0 6 c are observed over this time span in the upper sections of the boreholes below approximately 100 m any temperature transients are within observational noise we difference the temperature logs to highlight subsurface transients and to remove any ambiguity resulting from steady state source of curvature synthetic temperature profiles computed from sat data at nearby meteorological stations reproduce both the amplitude and pattern of the transient temperature observations fitting the observations to within 0 03 c or better this observational confirmation of the strong coupling between surface temperature change and borehole temperature transients lends further support to the use of borehole temperatures to complement sat and multiproxy reconstructions of climate change

geodynamic settings and tectonic model of skarn gold deposits in china an overview seventy skarn type gold deposits including 1 super large 19 large and 24 medium sized are known from different geotectonic units of china they contain a total resource of approximately 1000 t of gold 625 t in south china and account for 20 of china s gold reserves these skarn deposits are sited in collisional orogenic belts fault controlled magmatic belts and reactivated cratonic margins all of the chinese skarn gold provinces were affected by phanerozoic collisional orogenesis the timing of the metallogenic events and the spatial temporal distribution of the chinese skarn gold deposits indicates that they were formed during ore forming processes linked to the transition from shortening to extension in the geodynamic evolution of a collision orogen and not to subduction systems as is commonly advocated for porphyry copper systems around the pacific rim

a40ar 39ar study of post tectonic cooling in the britt domain of the grenville province ontario abstract we report the results of a 40 ar 39 ar study of rocks from the britt domain northwestern grenville province most of the age spectra obtained for micas and amphiboles have well defined age plateaus typically k feldspars yielded spectra with age gradients and arrhenius plots with kinks or offsets hence some of the data were interpreted in terms of a multi diffusion domain size model this 40 ar 39 ar data base 5 amphiboles 9 micas 4 k feldspar along with some recently acquired uzpb data provide reasonable constraints on the post peak metamorphic cooling history which began at depth in the upper amphibolite facies comparison of the data with exhumation paths generated by thermal models suggest that cooling taking place over 400 ma was for the most part due to erosionally controlled exhumation although the decompression may have initiated with a short period of rapid extensional uplift the entire cooling history may have been erosionally controlled there is an apparent difference in cooling ages between north and south in the 40 ar 39 ar mica and k feldspar data for the britt domain which may be due to somewhat higher exhumation rates in the north and or an elevated geotherm in the south

influence of deformation on pressure temperature paths of metamorphism pressure temperature p t paths in structural terranes dominated by folds and thrust faults can be complex and can vary widely within a single metamorphic belt the complexity and variability of p t paths result from the advection of heat by folding and thrusting conductive heat transfer during and after deformation and the relatively short times for thermal relaxation within folds and thrusts in fold terranes contrasting p t paths are observed between anticlines and adjacent synclines cooling paths are observed within anticlines whereas heating paths are observed within synclines these different p t paths result from the folding of isotherms during deformation and the subsequent relaxation of isotherms after deformation in polydeformed fold terranes noncoaxial folding results in complex p t paths over small distances less than 15 km within a metamorphic belt because of the repetition of the process of folding and relaxation of isotherms during each folding event in structural terranes dominated by thrust faults p t paths differ between major thrust sheets because heat is transferred from the hotter overriding sheet to the cooler lower plate during deformation heat transfer during thrusting causes the upper plate to cool and the lower plate to heat in terranes with multiple thrust sheets p t paths can be complex and will depend upon the relative timing of thrust development and the rate at which heat can be transferred from one thrust sheet to the next the p t paths observed in both the fold and thrust terranes discussed here record relatively short time intervals of 10 5 or 10 6 yr because the mineral equilibria respond to small scale thermal perturbations there may be a wide variability in p t paths over a small distance in a metamorphic belt therefore before p t paths are used to reconstruct tectonic histories both structural and petrologic investigations must be carried out

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

evolution of regional metamorphism in the cape smith thrust belt northern quebec canada interaction of tectonic and thermal processes syn metamorphic re imbrication of the internal part of thrust belts can result in distinct pressure temperature time deformation p t t d pathways for different structural metamorphic domains in the early proterozoic cape smith thrust belt canada an external piggyback sequence thrusting domain is characterized by thermal peak metamorphism occurring after deformation in contrast thermal peak metamorphism in an internal domain occurred during re imbrication by out of sequence thrusting the interactions of tectonic and thermal processes have been studied using three methods i qualitative evaluation of the timing between mineral growth and deformation ii analytical p t paths from growth zoned garnet porphyroblasts and iii numerical modelling of vertical heat conduction derived p t t d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading in contrast paths for the internal domain indicate that the out of sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain this is attributed to thickening by out of sequence thrusting and possibly to extensional faulting at now eroded higher structural levels observations on the timing of metamorphism coupled with numerical modelling suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out of sequence thrusts stack in the internal portion of the thrust belt

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

laboratory measurements of a complete set of poroelastic moduli for berea sandstone and indiana limestone measurements have been completed for eight different poroelastic moduli of water saturated berea sandstone and indiana limestone as a function of confining pressure and pore pressure the poroelastic moduli for indiana limestone are generally consistent to 10 which was verified by a formal inversion procedure for independent moduli from the eight measurements for indiana limestone best fit values were drained bulk modulus 21 2 gpa the undrained bulk modulus 31 7 gpa drained poisson s ratio 0 26 undrained poisson s ratio 0 33 and pore pressure buildup coefficient 0 47 at 20 35 mpa effective stress the poroelastic moduli for berea sandstone are generally consistent to 20 the greater inconsistency is most likely caused by the nonlinear variation of the moduli at different strains for berea sandstone best fit values were drained bulk modulus 6 6 gpa undrained bulk modulus 15 8 gpa drained poisson s ratio 0 17 undrained poisson s ratio 0 34 and pore pressure buildup coefficient 0 75 at 10 mpa effective stress

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

microstructure metamorphism and tectonics of the western cape breton highlands nova scotia microstructural and petrological data from the jumping brook metamorphic suite western cape breton highlands suggest that a single episode of syntectonic prograde metamorphism followed by uplift cooling and associated retrogression affected these rocks during mid palaeozoic times microstructures indicative of progressive crenulation foliation development can be traced from low grade chlorite zone through high grade kyanite zone rocks allowing a clear sequence of porphyroblast growth to be established metamorphic reactions and p t calculations suggest metamorphic conditions of 700 750 c at 8 10 kbar were achieved in kyanite zone rocks although a complete p t t path was not defined combined petrological and geochronological data can be used to constrain computed p t t models these models suggest that a component of post metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift the microstructural and petrological data to not support the interpretation that the high grade rocks represent pre existing crystalline basement indeed the metamorphic history geochronology and computed tectonic models all point to a single short lived episode of silurian devonian volcanism intrusion convergence regional metamorphism and uplift probably resulting from collision tectonics at an irregular continental margin