aftsolve a program for multi kinetic modeling of apatite fission track data aftsolve is a computer program for deriving thermal history information from apatite fission track data it implements a new fission track annealing model that takes into account the known kinetic variability among different apatite species to fully utilize this model a fission track worker must obtain data that can be used to infer the kinetic characteristics of each apatite grain from which a measurement was taken such data can consist of etch figure lengths or chemical composition the benefit of this overall approach is that it allows useful information to be derived from previously unusable analyses extends the practical range of geological temperatures constrained by fission track analyses and increases overall confidence in model predictions aftsolve also incorporates the effects of fission track orientation relative to the apatite crystallographic c axis variation in initial track length and the biasing effect of 2 5 2 cf irradiation for enhancing confined horizontal track length detection aftsolve is written for windows operating systems and has a graphical interface that allows interactive input of thermal histories and real time generation of estimates for fission track length distributions and ages for up to six simultaneously modeled kinetic populations it also includes procedures for estimating the range of time temperature histories that are statistically consistent with a data set and constraints entered by the user

strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

the exponential flow law applied to necking and folding of a ductile layer summary the uniaxial exponential law has been applied to experimental results for steady state creep including that of wet and dry olivine pyroxenite and carbonates at a deviatoric stress greater than 100 mpa such stress levels likely occur in the upper mantle lithosphere and middle crust in a layered rock in layer parallel extension or shortening high deviatoric stress can occur in the stiffest layers for example a dolomite layer in fine grained marble under assumptions of isotropy and incompressibility the uniaxial law yields where j2 sijsij 2 is an invariant of the deviatoric stress tensor sij linearization about a homogeneous basic state of flow yields equations identical in form to those obtained for the familiar power law this establishes expressions for an effective viscosity and stress exponent where is an invariant of the basic state deformation rate these results allow application of existing analytical folding and necking solutions for a rock layer of power law fluid to a rock layer with an exponential flow law the effective stress exponent for the exponential flow law increases with decreasing temperature through the dependence of c on the latter and weakly with increasing deformation rate for dry olivine effective stress exponents are between 10 and 30 for temperatures between 400 and 600 c with little dependence on deformation rate finite element simulations employing full non linear forms of the flow laws show that large strain necking is nearly identical for power law and exponential flow laws the results suggest that the instability in necking and folding in ductile rock layers can be considerably stronger than inferred from results based on flow laws representing diffusion and dislocation creep the large values of the effective stress exponent ne 15 that may be attained for exponential flow laws can account for observed outcrop scale ductile necking

the limitations of three dimensional kinematic vorticity analysis abstract the kinematic vorticity number wk can be calculated for three dimensional as well as two dimensional geologic deformations for steady state deformations wk can be correlated to and analyzed in terms of finite strains the analysis shows that assumptions commonly made for two dimensional deformations are not applicable to three dimensional deformations a single wk describes an infinite number of three dimensional deformations further even knowledge of flow apophyses orientation instantaneous stretching axes orientation and or wk are not sufficient to describe deformation three dimensional deformations also require knowledge of the deformation type or boundary conditions of deformation e g transpression hence in addition to being difficult to estimate the value of knowing wk for three dimensional deformations is greatly reduced compared with plane strain the most useful methods of determining wk from naturally deformed rocks are presented

finite deformation in and around a fluid sphere moving through a viscous medium implications for diapiric ascent abstract different types of geodynamically active regions like mountain belts hot spots or areas of salt tectonics are characterized by diapirism one key for the reconstruction of the dynamic history of such structures is the progressive strain which can sometimes be determined from field observations successful interpretation of such observations requires a quantitative model of the finite strain in and around a rising diapir a constant viscosity fluid sphere of radius r rising through another isoviscous fluid is assumed to approximate the buoyant motion of a diapir known analytical solutions for the velocity fields are used to numerically evaluate the finite deformation in and around the sphere regions of very high strains are found in a tube with a radius 1 2 r behind the sphere and in a shell of thickness of 0 1r around the sphere the following three dimensional strain regimes can be identified in the half space above the sphere down to its equator finite strains are oblate behind the sphere they progressively change to plane strain in the diapiric source region they are prolate viscous drag at the sphere s surface leads to an internal circulation with one overturn after 10r of rise if the sphere has a relatively low viscosity finite strains within the fluid sphere show a continuous increase with superimposed cyclic straining and unstraining after several body radii of rise the strains become highly inhomogeneous inside the sphere except along the vertical axis and just inside the sphere s surface where strong prolate and oblate strains are observed respectively finite strain determinations in a falling ball experiment cruden 1988 are compared with the theoretical results at horizontal distances of a few body radii from the fall axis the effect of confining container walls is clearly seen in the experimental strain data the results are compared briefly with available strain data from the field which seem to be significantly lower than predicted proposed explanations include a short memory of the rock fabric and a lack of recognition of the strain concentration which would be expected for a temperature dependent rheology it might also be possible that few natural diapirs rise more than a few radii in the solid state

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

the evolution of deformation and topography of high elevated plateaus 1 model numerical analysis and general results a temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus such plateaus are products of both continent continent collision tibetan plateau and ocean continent collision the altiplano and develop in the overriding continental plate in this study we emphasize the mechanically simpler case of oceanic collision because it does not involve mass transfer between the two plates the lithosphere is deformed in response to tectonic and buoyancy forces the tectonic forces arise from subduction of an oceanic plate or underthrusting of continental lithosphere that horizontally indents and vertically shears the overriding lithosphere the buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust the time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere the model produces dynamically supported near trench topography and inland mountain topography that is isostatically supported by a thick crust a finite region of localized deformation thick crust and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust however only thermally perturbed lithosphere generates a plateau topography the shape and size of the calculated plateau depend on the wavelength of the thermal perturbation grashof number and density contrast between the crust and mantle the time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves during early stages compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone at later stages as the crust thickens buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland this migration does not affect the location of the mantle deformation which remains in the thermally weak region but it is accompanied by a significant shear deformation in the weak lower crust the separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus this demonstrates the limitations of models that ignore changes of deformation with depth such as plane stress or thin sheet models

a fundamental problem with the kinematic interpretation of geological structures abstract non steady flows are ubiquitous in nature not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non steady flow histories in modern kinematic analysis flow apophyses the instantaneous stretching axes and the vorticity vector collectively referred to as the eigen directions of deformation are used in place of bruno sander s kinematic axes for deformation with a steady flow history this practice is well justified and has led to great advances in structural geology but for non steady flow histories the geometrical relationships among eigen directions flow pattern vary with time this makes it inappropriate to correlate an association of structures and fabrics with certain time invariant flow patterns and hence raises the question how should we interpret geological structures and fabrics kinematically without the assumption of homogeneous and steady deformation we suggest that the answer lies primarily in forward modeling of deformation based on a knowledge of rock properties

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

structural features of some younger granitoids in central sweden and implications for the tectonic subdivision of granitoids abstract structural relationships are described along parts of the eastern margin of the 1 76 1 84 ga old batholith known as the transscandinavian igneous belt tib they demonstrate that the svecofennian structures of the country rocks are highly dependent on the shape of the younger granitoids which themselves have margins folded in the same svecofennian structures this pattern is attributed to an interference between forces created by the diapiric intrusion and those following the last lateral movements of the svecofennian the tib would then be syn tectonic late svecofennian local cross cutting relationships alone are insufficient criteria for the postorogenic label commonly applied to these granitoids

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

flow and finite deformation of surface elements in three dimensional homogeneous progressive deformations rocks are heterogeneous materials deformation structures and fabrics in the ductile lithosphere can result from the history of the three dimensional flow field or the progressive deformation of some rheologically distinct elements embedded in the 3d flow field or a combination of both the flow and progressive deformation of a rheologically distinct element is generally non steady even if the hosting 3d flow field is steady vorticity decomposition is the key to following the progressive history of the deformation of specific elements a general theoretical background to the problem is presented flow and progressive deformation of surface and layer like elements in 3d homogeneous flows are investigated specifically analytical procedures to calculate the sectional vorticity number and the sectional dilatancy number on a general section in a 3d flow field are introduced the theoretical analysis is applied to various examples of sectional flow embedded in different types of 3d homogeneous and steady flow fields implications of this investigation for kinematic analysis of natural rock deformation are discussed all types of material line histories are expected in sectional flows of layer like elements shortened boudins can be formed in a single progressive deformation in sequences of rocks with a well developed transposition foliation common in large scale high strain zones each layer unit may have undergone a unique deformation history during its rotation to the final shear zone boundary parallel orientation in kinematic analysis of such areas it is important to ascribe the correct kinematic significance to structural associations and fabrics

anatomy of mushroom shaped diapirs we produced more than 100 mushroom shaped diapirs in eight centrifuged models under acceleration of 1200 g the modelling results allow natural examples to be recognized mushroom diapirs have an overhanging bulb fringed by one or more skirts which can curl inward to form vortices capable of entraining cover material to various degrees most of the internal folds are downward facing the oldest buoyant unit occurs in the skirt and in the diapiric core younger buoyant units or their immediate cover are infolded in crescentic patterns external mushroom diapirs have skirts that infold cover material with greater maturity the skirts can curl inward to form an external vortex entraining cover in contrast internal mushroom diapirs have skirts confined entirely within the intrusion again with greater maturity these confined skirts may curl inward but they entrain only diapiric material internal mushroom structure results from toroidal circulation confined within the diapir where isoviscosity prevails external mushroom structure results where source and entrained cover have similar effective viscosities entrained inclusions of more permeable country rock threaten the integrity of a mined cavern by creating a plumbing system

deformation of the assegaai supracrustals and adjoining granitoids transvaal south africa abstract the structures in archaean quartzites of the assegaai greenstone belt in the transvaal of south africa can be interpreted in terms of three major deformation events earliest d1 involved thrusting along brittle ramp and flat crush zones rising temperatures then led to the mobilization of sio2 and the development of syn d1 stylolites and omnidirectional quartz veins in the quartzites increasingly ductile behaviour results in some of the early d1 crush zones being overprinted by late d1 mylonitic shear zones while eastward verging recumbent folds formed about n s axes in the layered quartzites the assegaai supracrustals were then separated from a contiguous basement of ancient gneiss complex agc by the intrusion of a granitoid sheet about 3 km thick along their sheared subhorizontal contact either late in d1 or in the interval between d1 and d2 the supracrustals were then refolded about steep nne trending axial surfaces by concentric f2 folds on a variety of scales such folds were the second major structures to affect the supracrustals but the first to deform the granitoid sheet the older of two suites of quartzo feldspathic veins in the agc were probably generated penecomtemporaneously with the syn d1 quartz veins in ther assegaai quartzites this interpretation can be tested by removing the intense d2 constriction and 30 volume increase recorded by a later network of syn granitoid quartzofeldspathic veins in the agc from the d1 d2 flattening recorded by the older veins this procedure can be carried out by geometric vector subtraction on a hsu diagram the result reveals that the d1 strains in the assegaai supracrustals and the then contiguous agc were identical penetrative subhorizontal flows prior to the intrusion of the intervening granitoid the last significant strain in the region is represented by a brittle nne trending strike slip fault and locally in the quartzites disharmonic conjugate folds

transcurrent shearing granite sheeting and the incremental construction of the tabular 3 1 ga mpuluzi batholith barberton granite greenstone terrane south africa structural petrographic and geochronological studies show that the tabular 3 1 ga mpuluzi batholith in the barberton granite gneiss terrane in south africa was emplaced via a combination of external and internal processes external structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a ne ene trending system of dextral transcurrent synmagmatic shear zones internally the mpuluzi batholith is essentially made up of granite sheets the structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping variably deformed dykes and sheets these granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3 13 ma the spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting the transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets

palaeoproterozoic intraplating exposed by resultant gravity overturn near kiruna northern sweden abstract the tectonic history of the kiruna area is reexamined on the basis of four newly available digitised geophysical maps a literature survey and some first hand field observations a basement complex of archaean gneisses was unconformably overlain by a palaeoproterozoic sequence of metabasalt andesite and keratophyric porphyries with interstratified tuffs and sediments the cover was then detached from its basement by an orogeny with a northerly tectonic grain a pile of ductile nappes separated by mylonites in the east pushed westward an imbricate fold thrust belt that may have decoupled over an evaporite unit of regional extent conglomerates and quartzites accumulated in a foredeep migrating in front of a wedge of emergent thrusts the allochthonous cover then developed a vertical north trending foliation in high t low p conditions as it foundered among buoyant diapirs of remobilised granitoid basement the near constant lateral spacing of the centres of these diapirs their mushroom shapes and the absence of contemporaneous volcanic rocks suggest that these diapirs rose in the solid state the region continued to distort episodically even after diapir emplacement ductile shear zones evolved through vein systems to faults as the region cooled and was uplifted before the cambrian the youngest fault movements clearly displace 9000 years old glacial deposits post or late orogenic plutons in the kiruna area appear to be contemporaneous with syn svecofennian plutons further south we therefore suggest reinstating the label svecokarelian for the orogeny in the kiruna area heat advected by melts from the mantle is invoked to account for the archaean basement risking in two convective stages the first as porphyritic lavas the second as solid state diapirs melts rose rapidly and erupted before the svecokarelian orogeny while the slower diapirs ended it we attribute the current southwestern dip of the diapir zone to extension of svecokarelian crust before the svecofennian magmatic arc was built above it

constraining the strain ellipsoid and deformation parameters using deformed single layers a computational approach assuming pure shear and isotropic volume change if the deformation history of individual layers in a deformed rock mass can be appropriately analysed and categorised then the surface of no elongation snoe and the surface of no infinitesimal elongation snoie may be identified a computational approach to automatically identify the polar snoe and snoie is presented and tested on synthetic and natural examples additionally the equations that allow extraction of parameter values which quantify the deformation are derived here two fundamental assumptions are made 1 the progressive deformation is pure shear and 2 the associated volume change is isotropic the proposed method is tested using data generated from known deformation sequences and is found to work reasonably well however as is the case with most methods of analysis suitable data with a wide range of orientations is required for reliable results

deformation around a rising diapir modeled by creeping flow past a sphere a new experimental method for determining the pattern of flow and three dimensional progressive finite deformation of fluid elements in low reynolds number re viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder this is intended as a model analogue of natural rock deformation associated with magmatic and solid state diapirs rising through ductile crust the kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures the experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent newtonian test fluid used sgm36 are discussed in detail the progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis the progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere the results compare favorably with the external strain pattern associated with post tectonic mesozonal diapiric intrusions

forward modeling of non steady state deformations and the minimum strain path reply structural analyses of shear zones often rely on an assumption of steady state behavior i e that the ratio of pure shear strain rate s to simple shear strain rate s remains fixed throughout deformation however geological deformations are not necessarily steady state non steady state deformation paths can be theoretically modeled if certain deformation parameters such as strain or offset are specified we have analyzed a two dimensional case of specified offset and geometry termed the minimum strain path the minimum finite strain needed to produce a fixed offset across a shear zone is neither simple shear nor pure shear but a combination of the two the minimum strain path if this deformation accumulates with a steady state deformation the kinematic vorticity number w of the minimum strain path varies with the amount of finite offset although w approaches 0 7 at high offset values because of this relation between w and finite offset the minimum strain path is better modeled as a non steady state deformation in which case deformation history starts close to simple shear but rapidly changes to a more pure shear dominated deformation it is expected that the minimum strain path is applicable to geological deformation zones with relaxed boundary conditions such as basal parts of spreading nappes or extensional detachment systems 0 1997 elsevier science ltd

formation and deformation of subglacial intrusive clastic sheets in the dwyka formation of northern natal south africa abstract an arenaceous diamictite rests on fissile silty mudstone and argillaceous diamictite and is overlain by a laminated muddy siltstone east of helpmekaar in the dundee district of northern natal the argillaceous deposits originated either in the subglacial or proglacial zone of a partly grounded marine ice sheet where sediment was supplied by debris laden floating ice meltwater plumes and turbidity currents the arenaceous diamictite is the product of melt out debris released close to the grounding line of the ice sheet where it was subjected to overriding by the ice and slumping and redeposition by sediment gravity flow the laminated siltstone represents sediment that was deposited in a low energy setting by turbidity currents and suspension settling downslope from a retreating g ounding line dispersed ice rafted clasts occur in these fine grained deposits the upper surface of the arenaceous diamictite displays regularly spaced linear undulations that are parallel to the former iceflow direction as are corrugations in discrete subhorizontal slip planes in its upper part dike like bodies of this diamictite are found in the underlying mudstone and argillaceous diamictite these dikes or wedge shaped sheets of sandy diamictite are nearly vertical in their upper parts but at depth curl to the southeast to form sill like bodies these intrusive diamictites record a novel and previously undescribed form of subglacial bed shear by repeated hydraulic injections which shuffled the upper layers of the substrate to the south southeast bed compaction and shear beneath the grounded ice is recorded by folds in steepened portions of the clastic intrusive sheets and by pinch and swell and folded pinch and swell of the clastic sills displacement of steep folded sheet elements along subhorizontal slicken sided slip planes indicate that bed shear continued along spaced planes of slip during the process of compaction the corrugated and planar slip surfaces and the formation of the listric or spoon shaped intrusions are all shown to be different expressions of bed shear induced by traction from the overriding ice flowing to the southeast when it was temporarily grounded the base of an ice sheet flowing from land into the sea undergoes flow acceleration and forms a ramp where it decouples from the high friction substrate the arenaceous diamictite mound most likely to have developed beneath this ramp is inferred to have a structural core of sediments that have been squeezed and sheared and have slipped from beneath the ice in the tidal grounding zone

delimitation of a cryptic eocene tectono thermal domain in the eastern cordillera of the bolivian andes through k ar dating and 40ar 39ar step heating new k ar ages and 40 ar 39 ar age spectra for samples from granitoid rocks and meta sediments of the cordillera real bolivia reveal a complex mid tertiary tectono thermal history for this segment of the central andean eastern cordillera previous radiometric dating of the cordillera real intrusions defined two areally distinct plutonic domains in the n upper triassic lower jurassic in the s upper oligocene lower miocene our new k ar ages many from deformed intrusions fall between these two episodes for the variably foliated zongo massif there is a marked discordance between biotite and muscovite k ar dates each decreasing consistently from sw to ne across this 8 km wide plutonic centre the age gradients are developed within the foliated and unfoliated facies of the zongo intrusions but do not continue ne into the palaeozoic meta sedimentary rocks for which k ar dates are erratic 40 ar 39 ar step heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux 210 and 39 ma whereas those with intermediate ages exhibit disturbed spectra a plateau date for a biotite from the ne margin of the zongo massif shows that final cooling through c 300 c occurred in the late eocene the new data reveal that a marked thermal discontinuity existed 39 ma ago at the ne contact of the zongo intrusion additional discordant dates for the yani and illampu intrusions to the nw indicate that the discontinuity was of regional scale there is no direct relationship between this tectono thermal event which affected at least 2000 km 2 and the upper palaeozoic foliation of several of the plutons involved and thus it is cryptic on outcrop and microscopic scales we propose that the thermal discontinuity resulted from late eocene crustal shortening sw verging thrust faulting and attendant cordilleran uplift at the inner eastern boundary of the andean orogen coincident with the incaic deformation defined in peru

lessons from the first 100 minimum strain ellipsoids constrained in gneisses deformed at high metamorphic grade abstract the talbot 1970 method for constraining minimum strain ellipsoids has recently been updated by a computational approach and theory that can automatically constrain more complete strain ellipsoids with sufficient data it is thus worth appreciating the lessons learned from the 100 minimum strain ellipsoids constrained mainly in gneisses deformed at high metamorphic grades published over the last four decades in essence the method measures the strikes and dips of either competent or incompetent active single layers with shortened or extended deformation structures on scales of several limitations expected on outdated theoretical grounds were overcome so that treating deformation fields as paths on a variety of deformation plots allowed recognition of multiple deformations rotations volume changes and even the distinction of contemporary pure and simple shears by restricting measurements to single layers with length thickness ratios 10 the competence contrast between the markers and their country rocks does not affect the ellipsoids but can be seen to affect the structures that develop on intervening and larger scales

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

the influence of mechanical anisotropy on the behaviour of the lower crust abstract the aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension the state of stress existing there and how the finite strain pattern is developed from a consideration of the theoretical analyses of layer normal compression analogue models and outcrops of rocks from the lower crust it is concluded that the extended lower crust contains units with a significant competence contrast has a high mechanical anisotropy and experiences bulk rheological changes during deformation based on a consideration of hydraulic fracturing in anisotropic materials it is argued that depending on the magnitude differential stress fluids will move through and out of the lower crust by fluid fracturing along the planar horizontal stretching fabric and gently inclined normal kink bands extensional shears or by the formation of vertical fractures

mechanical evolution of transpression zones affected by fault interactions insights from 3d elasto plastic finite element models abstract the mechanical evolution of transpression zones affected by fault interactions is investigated by a 3d elasto plastic mechanical model solved with the finite element method ductile transpression between non rigid walls implies an upward and lateral extrusion the model results demonstrate that a transpression zone evolves in a 3d strain field along non coaxial strain paths distributed plastic strain slip transfer and maximum plastic strain occur within the transpression zone outside the transpression zone fault slip is reduced because deformation is accommodated by distributed plastic shear with progressive deformation the 3 axis the minimum compressive stress rotates within the transpression zone to form an oblique angle to the regional transport direction 9 10 the magnitude of displacement increases faster within the transpression zone than outside it rotation of the displacement vectors of oblique convergence with time suggests that transpression zone evolves toward an overall non plane strain deformation the slip decreases along fault segments and with increasing depth this can be attributed to the accommodation of bulk shortening over adjacent fault segments the model result shows an almost symmetrical domal uplift due to off fault deformation generating a doubly plunging fold and a positive flower structure outside the overlap zone expanding asymmetric basins subside to negative flower structures on both sides of the transpression zone and are called transpressional basins deflection at fault segments causes the fault dip fall to less than 90 86 89 near the surface 1 5 km this results in a pure shear dominated triclinic and discontinuous heterogeneous flow of the transpression zone

strains and vorticity beneath a tabular batholith in the zambesi belt northeast zimbabwe structures on scales of cm to dm in formerly universally orientated quartz veins are used to analyse deformation in part of the pan african e w zambezi orogen by integrating their orientations over length scales of six orders of magnitude 2 cm to 20 km sixty two strain ellipsoids on locality scales 20 200 m and six on district scales 2 20 km demonstrate that each of the constituent lithologies together with the metasedimentary cover sequence as a whole strained as homogeneous materials after the quartz veins developed related to orogenic structures these homogeneous strains demonstrate that major folds in the cover initiated as buckles on e w axes before the quartz veins were inserted but tightened passively as the metamorphic grade rose from greenschist to upper amphibolite facies by contrast post vein f2 flow folds originally on ne axes amplified actively at the same time but without noticeable shortening such f2 folds are interpreted to have been driven by the gravitational stabilization of the relatively dense proterozoic cover and its less dense archaean basement the instability due to this primary density inversion was aided significantly by a secondary density inversion induced by the orogenic thermal gradient while the metamorphic gradient was below 30 c km the basement rose in conformable upright ridges on the same ne trends preserved in the archaean rocks in the kalahari craton to the south the early e w trending folds and later ne trending folds of the zambezi orogeny interfered to generate class ic patterns of elongate domes and basins throughout the cover sequence when the regional thermal gradient rose above 30 c km the basement rose diapirically into or through the cover as domes of migma migmatite or magma on centres 5 10 km apart melts of the deep basement eventually drained up the stems of earlier migma diapirs and presumably fed a tabular batholith in the epizone that has since been removed by erosion diapirs of basement or remobilized basement displaced the earlier class ic fold interference pattern in the cover in all directions this led to uniaxial radial flattening and vorticity down to scales of 100 m in the cover within about 2 km of the basement contact meanwhile away from the immediate influence of the diapirs the cover sequence continued to undergo vorticity about a nnw shortening axis on district scales but only spun on locality scales the mean post vein n s orogenic shortening of the cover sequence is found to be 60 on the locality scale linear strain fields for data from appropriate districts are interpreted in terms of plane strains with about 50 60 of the total shortening being due to uniaxial nnw volume loss of sio2 and water on both locality and district scales all the post vein shortening measured in the orogen can be attributed to lateral displacement of the cover by diapirs of a sialic basement which may not have shortened on the regional scale at any stage pre vein shortening of the cover by thrusts and ductile flow has not been quantified but n s shortening due to early buckles is calculated to have been about 17 the upward loss of sio2 in solution and either solid or molten basement from levels still exposed to others since eroded appears to be a far more characteristic aspect of the eastern part of the zambezi orogen than lateral shortening on the regional scale

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

strain ellipsoids from incompetent dykes application to volume loss during mylonitization in the singo gneiss zone central sweden we describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes our method is demonstrated by using metabasaltic dykes to

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

hercynian deformation and metamorphism in the cordillera oriental of southern bolivia central andes abstract in southern bolivia the very thick ordovician siliciclastic rocks of the cordillera oriental show folds and a slaty cleavage of pre cretaceous age these deformations had previously been attributed to either the late ordovician ocloyic or to the late devonian to early carboniferous eohercynian chanic orogenies now we present k ar age determinations from phyllosilicates of the ordovician slates which have been interpreted in relation to illite crystallinity data high anchizonal to epizonal metamorphism is indicated for most of the investigated samples the majority of the samples have provided ages within the 320 290 ma interval late carboniferous to early permian indicating a late hercynian orogeny traces of synchronous orogenic processes are known from different parts of the central andes this points to late hercynian orogenic activities in that region

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

deformation path in high strain zones with reference to slip partitioning in transpressional plate boundary regions abstract the current status of the kinematics and strain geometry of high strain zone studies is briefly summarized a general high strain zone has a triclinic deformation path and monoclinic shear zones are special end member cases fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion non steady deformation paths remain difficult to deal with and may ultimately rely on a realistic mechanical treatment of high strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved an examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area trench parallel high strain zone is generally triclinic the alpine fault in the south island of new zealand provides an example of a currently active triclinic shear zone the southern knee lake shear zone of manitoba canada provides an archean example of a triclinic shear zone

obliquely foliated dikes as deformed incompetent single layers the structures that develop in deformed competent single layers are well known in comparison to the results of the deformation of incompetent single layers it is argued that the oblique foliations within some sets of deformed igneous sheet intrusions are the most common indication that such sheets deformed as single layers with less competence than their country rocks previous workers have implicitly assumed this situation when interpreting examples of such foliated sheets as having sheared between rigid country rocks while still hot this is taken here as a special case of a much more general situation in which strains in the sheets are induced by strains of the more competent country rocks as in previous models the foliation within sheets is interpreted as being oblique to their contacts because of layer parallel shears along the sheets in a departure from earlier approaches such shears are assumed here to arise from the rotation of the sheets due to the bulk strains of their country rocks this idea is explored using the patterns of the structural movement paths traced by their poles during strain each type of bulk strain leads to a different pattern of structural movement paths as the rotation of the sheet contacts drives the rotation of the foliation between them their poles follow colinear but opposed movement paths and are taken to lie at different positions along what will be called apparent structural movement paths this working hypothesis is empirically tested using data from two field areas sets of apparent structural movement paths for foliated intrusive sheets in two areas are found to compare favorably with theoretical structural movement paths expected if the sheets deformed as incompetent single layers in homogeneously deforming country rocks the approach developed here can be used to determine empirically the homogeneity of strain of the rocks from which the data have been derived the orientations and relative values of the principal strains and an approximation of the shape of the strain ellipsoid however no attempt is made to quantify the amount of strain recorded in the field examples quoted here

melanges intrusive and extrusive sediments and hydraulic arcs water carrying sediment may intrude with or without associated extrusion wherever the fluid pressure exceeds the intrusion strength of the surrounding materials it has recently been suggested that thrust sheets moving toward continental interiors act as giant squeegees driving fluids through their substrates ahead of them active ice sheets and sedimentary prisms can also squeeze fluids through rocks such fluids potentially inject hydraulic melanges along their plumbing systems but these are difficult to distinguish from other melanges and from broken formations or olistostromes because of surficial slumping or extrusion miniature examples of melanges pumped through a marine slope by the tidal grounding of an ice sheet of permian carboniferous dwyka age in south africa are used to illustrate that there are even more categories of melange than previously suspected instead of being clarified by recent work the problem of deducing the origin of individual melanges becomes more difficult this is because melanges are formed by the mixing of thick hydrous slurries in any confined conditions and are not diagnostic of subduction nonetheless the results of hydraulic activity within a subglacial marine slope suggest that the sedimentary volcanoes developed on many accretionary prisms overlie relatively cold hydraulic arcs oceanward of the hot magmatic arcs behind subduction zones

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

empirical paths of poles to planes eppps constrain the kinematics of geological shear zones ductile shear zones are tabular bodies of deformed rocks bound by less deformed wall rocks this work introduces a simple empirical approach to analysing the 3d kinematics of shear zones the orientations of pre shear planar markers distorted across natural shear zones by local strains are systematically measured and plotted as poles on lower hemisphere equal area projections that constrain smooth empirical paths of poles to planes eppps such eppps recording local strain gradients are used to fix a reference frame to the plane of greatest shear in any homogeneous bulk strain assuming that space can be taken as a proxy for time the curvatures of pre shear planar markers across shear zones are interpreted as the records of the 3d bulk strain histories of shear zones the sig or zig moidal symmetries of sheared markers record different amounts of the same general strain within the same overall movement pattern i e in a constant flow field whatever its geometry or history in effect eppps represent the strain memories of shear zones with successively inward readings recording successively younger shearing in planes other than the bulk xy great circle eppps indicate simple shear while hyperbolic eppps indicate pure shear eppps for suites of shear zones in proterozoic gneisses in sweden exhibit the parabolic shapes indicative of pure rather than simple shear

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

dykes cups saucers and sills analogue experiments on magma intrusion into brittle rocks magma is transported in the crust by blade like intrusions such as dykes sills saucers and also collects in thicker laccoliths lopoliths and plutons recently the importance and great number o

quantification of weathering processes in an acid brown soil developed from tuff beaujolais france the petrographical and mineralogical composition of tuff in the studied area was determined by optical microscopy x ray analysis bulk chemical analysis and mean chemical composition of each mineral obtained from the electron microprobe the tuff stones sampled on the surface and in the pits of soil are usually surrounded by a bleached rind from the results of chemical weathering mass balance isovolumetric calculations the chemical and mineralogical transformation are quantified during the formation of this weathered rind this tuff from the upper visean was composed of about 45 5 phenocrysts quartz andesine albite and biotite and about 55 5 devitrified groundmass spherolites quartz k feldspar albite andesine and biotite this series was a broad monotonous series although some facies diversity occurred i e colour and density of plagioclase phenocrysts evidence of fluidality a local heterogeneity in the tuff mineralogy and a variation in its chemical composition k2o cao na2o and loss on ignition have been induced by devitrification of the groundmass and hydrothermal alteration the contact metamorphism seemed to have caused the groundmass devitrification and recrystallization of biotite phenocrysts as small biotite crystals 0 1 mm hydrothermal alteration has led to biotite chloritization transformation of plagioclase albite and andesine into white mica and k feldspar and a few veins quartz chlorite epidote in the montagne des aiguillettes tuff weathering appeared as a thin bleached rind around all stones the weathering gradient inside rinds corresponded to a progressive dissolution of tuff minerals quartz k feldspar white mica biotite albite apatite andesine the weathering processes were characterized by a strong dissolution of plagioclases and apatite increase of voids by about 12 and to a lesser extent by the transformation of micas biotite into hydroxy interlayered vermiculite hiv and mica hiv mixed layer minerals and by kaolinite ka precipitation the results of the petrographical study were consistent with the calculated mass balance the weight losses were about 18 and corresponded to a strong exportation of cao na2o sr p2o5 al2o3 and sio2 74 62 51 42 17 and 25 respectively and an increase in the water content eighty two percent of andesine 41 of apatite 40 of albite and biotite 3 were dissolved the increase in the total water content corresponded to the formation of hydrated weathered minerals 2 6 of hiv and 4 3 of ka

sediment accumulation on top of the andean orogenic wedge oligocene to late miocene basins of the eastern cordillera southern bolivia a large volume of tertiary synorogenic sediment accumulated on top of the eastern cordillera of southern bolivia as the andean orogenic wedge was shortened thickened and uplifted oligocene to upper miocene strata were deposited in five basins that were separated by active north trending fold thrust structures of the then frontal part of the orogenic wedge these coarse grained deposits recorded accumulation in the most proximal sector of the andean foreland basin system the wedge top depozone analyses of depositional systems sediment dispersal patterns and clast provenance of 0 6 2 3 km thick oligocene to upper miocene wedge top successions demonstrate that faultpropagation and fault bend folds commonly isolated individual basins while serving as primary sediment sources growth strata which formed by progressive fold limb rotation indicate thrust fault displacement and related folding concurrent with deposition alluvial fans defined most basin margins whereas braided streams or small lakes occupied basin axes diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins growth strata and crosscutting and onlapping relationships between contractional structures and wedge top strata delineate the chronology of fold thrust deformation in the eastern cordillera five new 40 ar 39 ar dates and previously published k ar dates ranging from 30 to 8 ma define an oligocene phase of west vergent backthrusting followed by primarily east vergent thrusting during miocene time timing of displacement on two east vergent thrusts is determined by 40 ar 39 ar ages of tuffs within adjacent growth strata sequences of the tupiza formation conglomerate 16 14 0 06 ma and oploca formation 13 33 0 15 ma 15 7 2 4 ma these ages combined with basin depositional histories demonstrate synchronous and out of sequence thrust displacement during miocene shortening upper crustal contractional deformation and wedge top deposition terminated in the eastern cordillera during late miocene time as the thrust front propagated eastward into the subandean zone continued thrust front migration produced the present day configuration in which eastern cordillera wedge top basins originally developed above the toe of the orogenic wedge at relatively low elevations are now 250 km west of the active thrust front and at 3 km elevation the modern wedge top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrustbounded subandean zone basins and sediment overlying blind structures beneath the westernmost chaco plain in general wedge top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted nevertheless eastern cordillera wedge top deposits have been preserved for 10 30 m y in southern bolivia such long term preservation may reflect the inability of drainage systems to remove sediment mass from this low precipitation region of the central andes retention of sediment mass within the orogenic belt may promote critical thrust wedge conditions in which propagation of the thrust front is favored

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

inclined transpression in the neka valley eastern alborz iran three major nappes in the neka valley in the eastern alborz mountains of iran allow the cimmerian to present convergence following the oblique collision between iran and the southern margin of eurasia this work reports the identification of an inclined transpression zone recognized by field investigations and strain analyses of the geometries of formations and detailed mesoscopic structural analyses of multiple faults folds and a cleavage the main structures encountered include refolded recumbent asymmetric fold nappes highly curved fold hinges in a transpression zone that dips 37 to the nw between boundaries thrusts striking from n050 to n060 the angle the angle between the zone boundary and direction of horizontal far field shortening is about 80 the north west and south east boundaries of this zone coincide with the haji abad thrust and the shah kuh thrust respectively fold axes generally trend ne sw and step to both right and left as a result of strike slip components of fault displacements strain analyses using fry s method on macroscopic ooids and fusulina deformed into oblate ellipsoids indicate that the natural strain varies between 2 1 and 3 14 the estimated angle between the maximum instantaneous strain axis isamax and the transpression zone boundary is between 6 and 20 the estimated oblique convergence angle therefore ranges between 31 and 43 the average kinematic vorticity number wk is 0 6 in a zone of sinistral pure shear dominated inclined triclinic transpression these results support the applicability of kinematic models of triclinic transpression to natural brittle ductile shear zones

reconstruction of deformation and flow parameters from deformed vein sets abstract in any homogeneously deformed material sets of material lines with a similar stretch history occupy sectors with a scale and orientation that depend on finite strain area change rate and vorticity of the parent flow if the size and shape of these material line sectors are known they can be used to determine the sense of shear and to calculate deformation and flow parameters even if actual stretches along material lines are unknown in rocks which have undergone near homogeneous deformation on a large scale the orientation of sets of boudinaged folded and folded boudinaged or boudinaged folded veins can be used for such calculations a general theoretical background to the problem is presented and a mohr circle construction to carry out the deformation and flow analysis is introduced among other statements the theory predicts that shortened or folded boudins must develop in non coaxial monophase deformation with considerable area change in the plane of observation or in polyphase deformation

some uncertainties in the derivation of rates of denudation from thermochronologic data low temperature thermochronology such as that provided by apatite fission track analysis provides a valuable means of establishing the timing of major denudational events and associated rates of denudation over geological time scales of 106 108 ma care must be taken however in deriving denudation rates from the crustal cooling histories documented by thermochronologic techniques especially in rapidly eroding terrains since in such cases apparent denudation rates derived from thermochronologic data will usually overestimate true rates if the advective effect of denudation is not included this is likely to be resolvable where the rate of denudation exceeds 300 m ma 1 and when the depth of denudation occurring at these rates exceeds several kilometres prior to the sample cooling below the appropriate closure temperature because the time at which a sample cools below a particular closure temperature is relatively insensitive to advection the initiation of denudation can be accurately established even given uncertainties in the estimation of depths and rates of denudation where thermal events originate from a source within or below the lower crust the cooling through denudation will dominate the low temperature history of the shallow crust if denudation occurs coevally with the subsurface heating 1997 by john wiley sons ltd

geophysical exploration linking deep earth and backyard geology solid earth sciencebig science came to solid earth studies when the 400 million earthscope program offered a sharper geophysical view of the interior that could help geologists the proposed four pronged program would point a geophysical telescope inward to bring the subsurface of north america into unprecedentedly sharp focus the satellite radar component never came to pass and the drill hole through the san andreas fault fell far short of its goals but imaging with a traveling seismic network and taking the pulse of the plate boundary are revealing all manner of processes shaping surface geology from a plume fueling yellowstone volcanism to drips off the continent s bottom

origin of the high plateau in the central andes bolivia south america the bolivian altiplano in the central andes of south america is part of the second largest high plateau on earth it is an elongate region of subdued relief 1 2 105 km2 and 4 km above sea level bounded by the eastern cordillera and volcanic arc western cordillera here the crust is up to 75 km thick we describe the cenozoic geological evolution of this region using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure crustal shortening and magmatic addition and locally sedimentation are the main mechanisms of cenozoic crustal thickening leading to nearly 4 km of surface uplift since the paleocene addition of mafic melts appears to be a first order mechanism of cenozoic crustal growth contributing 40 of the crustal thickening beneath the volcanic arc removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind arc mafic volcanism at 23 ma and 0 5 ma contributing to the surface uplift the altiplano originated as a sedimentary basin several hundred kilometers wide between the proto western cordillera and a narrow zone of uplift proto eastern cordillera farther east the latter zone formed by inversion of the center of a wide lacustrine or marine cretaceous paleocene basin close to sea level at 45 ma a thickness of 2 4 km of paleogene continental elastics accumulated in the proto altiplano basin subsequently in the oligocene we estimate that this region and the western margin of the eastern cordillera were technically shortened 22 65 km resulting in 9 km of average crustal thickening the altiplano basin was rejuvenated at 25 ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting eastern and western cordilleras between 25 and 5 ma folding and thrusting in the western margin of the eastern cordillera migrated westward into the center of the altiplano basin essentially terminating deposition except in local subbasins and accommodating 13 30 km of shortening and an estimated 7 km of average crustal thickening subsequently there has been strike slip deformation and limited local thrusting 5 km of shortening geomorphological and geochronological evidence for 1 5 2 km of surface uplift of this region since the late miocene suggests 14 km of lower crustal thickening beneath an essentially rigid lid and can be explained by 100 150 km of underthrusting of the brazilian shield and adjacent regions beneath the eastern margin of the central andes the present subdued relief in the altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin

structural geology meets micromechanics a self consistent model for the multiscale deformation and fabric development in earth s ductile lithosphere abstract earth s lithosphere is made of rheologically heterogeneous elements of a wide range of characteristic lengths a micromechanics based self consistent multiorder power law approach is presented to account for lithospheric deformations and the accompanying multiscale fabric development the approach is principally based on the extended eshelby theory for the motion of a power law viscous ellipsoid in a power law viscous matrix and the idea of embedding inhomogeneities within inhomogeneities the extended theory provides a general means for investigating deformation partitioning in heterogeneous rocks the inhomogeneities within inhomogeneities method allows multi hierarchical levels of flow field partitioning and hence multiscale deformations to be investigated partitioned flow fields are used to investigate fabric development being based fully on micromechanics the approach generates model predictions of both kinematic quantities strain strain rates and vorticity and stress histories the former can be directly compared with field and laboratory structural observations while the latter can help to understand the physics of natural deformations the self consistent and multiscale approach is applied to a natural example of the cascade lake shear zone in the east sierra nevada of california the modeling shows that the fabrics are most consistent with a steeply dipping transpression zone with a convergence angle of 20 and a strike slip displacement about 26 km further the strength evolution of the model zone confirms that a transpression zone is a weakening system with respect to the simple shearing component and a hardening one for the pure shearing component this is consistent with slip partitioning in obliquely convergent plate boundaries boundary normal convergence tends to spread over a broad area whereas boundary parallel shear tends to localize in major strike slip zones

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

upb columbite chronology of post kinematic palaeoproterozoic pegmatites in sweden rare element pegmatites of the lithium cesium tantalum lct family originate from crustal melts derived from orogenically thickened continental crust and yield a minimum estimate for the age of orogenic thickening pegmatites of this type from the palaeo

structural pattern and emplacement mechanism of the neka valley nappe complex eastern alborz iran the neka valley nappe complex is exposed in the south of gorgan county in the eastern alborz fold and thrust belt we use the results of a regional survey of the structural data and their patterns to interpret the mechanisms that emplaced the unmetamorphosed nappes in the foreland fold and thrust belt of the alborz mountains most of the strain magnitudes are low in the study area but increase slightly towards what are probably their proximal ends strain ellipsoid is dominantly oblate with xy aligned along and across the belt or the nappe complex the average kinematic vorticity number w k 0 6 which indicates most of the strain partitioning resulted in a general shear most of flinn s k values and the stretch along the shear plane values are lower than 1 structural indicators such as orthogonal extensional joints pinch and swell structures anastomosing cleavages and listric normal and growth faults developed by push from the rear large scale thrust complexes with opposed dips such as triangle zones as well as k and values 1 are compatible with the shear flow diverging distally and streamlines expected of the rear compression emplacement mechanism together with a later minor brittle deformation these major ductile strains appears to provide a general model suitable for the emplacement of the nappes studied in a thin skinned fold and thrust belt where the sedimentary cover strata shortened and imbricated in the upper crust

strain distribution in a fold in the west norwegian caledonides abstract strain has been measured from clasts within a deformed conglomerate layer at 17 localities around an asymmetric fold in the rundemanen formation in the bergen arc system west norwegian caledonides strain is very high and a marked gradient in strain ellipsoid shape exists to either side of the fold strain within the conglomerate bed is of the extreme flattening type in the fold especially on the lower fold closure the strain is constrictional mathematical models of perturbations of flow in glacial ice have produced folds of the same geometry as this fold with a strikingly similar pattern of finite strain the fold geometry and strain pattern as well as other field observations suggest that the fold developed passively as the result of a perturbation of flow in a shear zone where the strain was accommodated by simple shear accompanied by extension along y

active low angle normal faulting in the woodlark extensional province papua new guinea a physical model 1 in eastern papua new guinea the seismicity geomorphology and results of seismic reflection profiling drilling and structural geological analysis reveal a zone of active low angle normal faulting the only clear known active example of this process worldwide this woodlark extensional province is 250 km long in the east west direction located within the continental crust to the south of the solomon sea ocean basin it adjoins the young oceanic spreading center farther east in the woodlark basin and since 4 ma has accommodated northward extension at rates of in excess of 10 mm yr 1 on low angle normal faults that are rooted to the north this geometry and the region s earlier history are assessed in the light of modern ideas for the mechanics of low angle normal faulting to derive a working hypothesis for the existence of this process in this region the extension in the woodlark extensional province and oceanic spreading in the woodlark basin are thus interpreted as consequences of the slab pull force due to northward subduction of the solomon sea floor which has caused the small solomon sea plate to separate from the australian plate it is suggested that this plate geometry developed by 6 5 ma following the ending of southward subduction of the solomon sea floor beneath what is now the woodlark extensional province the double thickness of lithosphere that formerly existed locally cooled the continental crust making it brittle to the moho following the end of this subduction the base of the continental lithosphere became progressively exposed to the asthenosphere causing dramatic heating of the overlying continental crust this resulted in transient thermal disequilibrium inducing transient horizontal lower crustal flow the shear tractions imparted on the base of the brittle layer by such flow combined with the stress components that develop after some of the lower crust has flowed from beneath a locality requiring it to be supported laterally rather than from below will rotate the principal axes of the stress tensor sufficiently to enable low angle normal faults to form and it is estimated that this sense of deformation was initiated around 4 ma the geometry of this study region means that this process will create north dipping low angle normal faults as are observed this scheme can also account for a great deal of other observational evidence including the available thermochronologic evidence and the observed vertical crustal motions to explain this instance of active low angle normal faulting thus requires knowledge of its relationship to earlier subduction and of effects of temperature dependent changes to continental crustal rheology in crust that was initially cold

extreme channelization of fluid and the problem of element mobility during barrovian metamorphism geochemical profiles across chemically altered zones selvages surrounding amphibolite facies quartz kyanite veins were investigated to determine major minor and trace element mass transfer and volume strain due to channelized fluid flow the three profiles are perpendicular to two veins cutting the wepawaug schist connecticut u s a which underwent barrovian style metamorphism during the acadian orogeny selvages are highly aluminous with considerably more kyanite staurolite and garnet and less quartz plagioclase and mica than surrounding wallrocks kyanite crystals increase in size toward veins and reach several centimeters in length within veins mass balance analysis indicates 50 silica loss and 34 volume loss from selvages on average silica transferred locally from selvages to veins accounts for 40 to 80 of the vein silica the remainder must have been precipitated from fluids that flowed through the veins during regional devolatilization fluid flow also transported elements into the rock mass which were concentrated in the selvages gains of fe mn y and hree were due to the growth of selvage garnet fe zn and li were sequestered into staurolite kyanite staurolite and garnet growth resulted in al mass gains destruction of mica particularly muscovite and of plagioclase in the selvages resulted in losses of k na ba pb sn and volatiles and losses of na sr and eu respectively thin na and sr enrichment zones associated with increased modal plagioclase are found along the margins between selvages and less altered wallrock and may represent either chemical self organization produced during diffusive mass transfer and reaction or are relics from a possible earlier period of na enrichment in the selvages simple two dimensional numerical modeling of flow in a fractured porous medium indicates that fluxes vary significantly over short distances 1 m adjacent to veins fluids channelized into the high permeability fracture conduits carry the bulk of the fluid flow mass transfer to and from selvages adjacent to the conduits occurred by some combination of diffusion and flow in contrast areas distal to the conduits are impoverished in fluid and undergo much more limited infiltration as a consequence different workers can come to vastly different conclusions about the magnitude of fluid fluxes and element transfer depending on the part of a fractured outcrop studied this extreme spatial variability due to channelization can largely explain contrasting views that have arisen in the literature regarding the nature and intensity of non volatile element mass transfer during barrovian metamorphism determining average time integrated fluid fluxes and levels of element transport across outcrops remains as important research challenges due to the spatial variability of flow

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

compaction fabrics of pelites experimental consolidation of kaolinite and implications for analysis of strain in slate abstract compaction of clay and shale results in large reductions in volume as pore water is expelled preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the march owens model this relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the x ray pole figure goniometer and scanning electron microscope sem initial porosity corresponds to the onset of the strain recorded by the preferred orientation and the values of 0 78 for dispersed slurries and 0 76 for flocculated slurries reflect the electrostatic forces between the clay platelets initial porosities of recently deposited fine silt and clay are in the range of 0 60 0 90 and are a function of grain size and mineralogy loss of this initial porosity has a large effect on the subsequent development of slaty cleavage matrix methods were used to model deformation paths for slates in the welsh slate belt preferred orientation of mica and ellipsoidal shapes of reduction spots were simulated for one locality by loss of a 0 60 initial porosity a 6 tilt of the beds and horizontal shortening involving plane strain strain determinations for shales and slates should include the large reduction in volume

tectonic pumping of pervasive granitic melts finite element 2d models are used to study how tectonic stresses pump pervasive granitic melts within migmatites we start by assuming elliptical melt filled veins of different orientations interconnected by the assumption that they share the same pressure the melt is then redistributed in the vein array by the application of lateral compression or extension the effective permeability of vein networks is thought to be up to 13 orders of magnitude higher than the matrix permeability of 10 19 m2 this allows us to approximate pressure equilibration and neglect irreversible compaction in the matrix this simple configuration allows solution of constitutive equations for the equation of state in the viscoelastic matrix the melt in the vein array and the mechanical equilibrium along with force balance on the walls of the deforming veins a rectangular box is pressurized by a constant load applied to one side and continuously deformed by shift of another side with constant velocity new veins cannot initiate and pre existing veins cannot migrate bodily down pressure gradients however potential flaws are cut in line with the tip of every pre defined vein this means that veins not only close open and shear as they rotate but can also undergo limited propagation at rates dictated by the bulk deformation of the matrix we use constant viscosities s in the range 1017 1018 pa s deformation rates e vary between 10 10 and 10 9 s 1 and a total strain up to 4 5 isolated veins parallel and normal to 1 have melt pressures between p0 and p0 4 se the mean vein pressure differs from these extremes and is equilibrated by driving melt from shrinking veins into veins parallel to 1 which widen and lengthen the above assumptions result in an asymptotic pattern of stress distribution and the amount of melt redistributed at given strain approaching the kinematic limit with time this occurs whatever the viscosity of the matrix and the strain rate multilayered systems are modeled by pre defining a single vertical vein crossing the planar horizontal boundary between two uniform media compression parallel to the layering expels melt from the part of the vein in the more viscous layer to open and extend that part of the same vein in the less viscous layer melt moves in the opposite direction during lateral extension if sheet like bodies become sufficiently tall to become buoyant horizontal sheets resulting from lateral compression are likely to rise to higher crustal levels as diapirs while vertical sheets resulting from lateral extension are more likely to ascend as dikes

setting of zn cu au ag massive sulfide deposits in the evolution and facies architecture of a 1 9 ga marine volcanic arc skellefte district sweden the skellefte mining district occurs in an early proterozoic mainly 1 90 1 87 ga svecofennian magmatic province of low to medium metamorphic grade in the baltic shield in northern sweden the district contains over 85 pyritic zn cu au ag massive sulfide deposits and a few vein au deposits and subeconomic porphyry cu au mo deposits the massive sulfide deposits mainly occur within and especially along the top of a regional felsic dominant volcanic unit attributed to a stage of intense extensional continental margin arc volcanism from facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow water areas surrounded by deeper seas all the major massive sulfide ores occur in below wave base facies associations however some ores occur close to stratigraphic intervals of above wave base facies associations and the summits of some volcanoes that host massive sulfides emerged above sea level intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism uplift resulting in subregional disconformities and then differential uplift and subsidence resulting in a complex horst and graben paleogeography uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels a few massive sulfide ores formed within the basal strata of this second stage the horst and graben system was filled by prograding fluvial deltaic sediments and mainly mafic lavas and during this stage the skellefte district was a transitional area between renewed arc volcanism of more continental character to the north and subsidence and basinal mudstone turbidite sedimentation to the south this whole volcanotectonic cycle occurred within 10 to 15 m y we define 26 main volcanic sedimentary and intrusive facies in the skellefte district the most abundant facies are 1 normal graded pumiceous breccias which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris 2 porphyritic intrusions and 3 mudstone and sandstone turbidites facies associations define seven main volcano types which range from basaltic shields to andesite cones and rhyolite calderas despite this diversity of volcano types most massive sulfide ores are associated with one volcano type subaqueous rhyolite cryptodome tuff volcanoes these rhyolite volcanoes are 2 to 10 km in diameter 250 to 1 200 m thick at the center and are characterized by a small to moderate volume rhyolitic pyroclastic unit intruded by rhyolite cryptodomes sills and dikes massive sulfide ores occur near the top of the proximal near vent facies association the remarkable coincidence in space and time between the ores and this volcano type indicates an intimate genetic relationship between the ores and the magmatic evolution of the volcanoes many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits we suggest that massive sulfides in the skellefte district span a range in ore deposit style from deep water sea floor ores to subsea floor replacements to shallow water and possibly subaerial synvolcanic replacements facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes

left lateral transpressive deformation and its tectonic implications sveconorwegian orogen baltic shield southwestern sweden abstract the mylonite zone mz is a major ductile deformation zone in the sveconorwegian orogen baltic shield of southwestern sweden and southeastern norway which has a strike length of over 400 km and an across strike width which often exceeds 5 km it is an orogen parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher grade structures in the surrounding crustal units the mz marks a conspicuous metamorphic break in the area south of lake vanern and a distinct lithological break in the area north of this lake regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north structural studies in three areas north of lake vanern varmlandsnas kil and torsby charlottenberg suggest that the mz displays coeval left lateral strike slip and reverse dip slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist facies conditions in the northernmost torsby charlottenberg area at relatively shallow crustal levels the mz splays out to form two left lateral contractional strike slip duplexes which together define a positive flower structure regionally the mz defines the western flank of an east verging thrust system which formed in the eastern part of the sveconorwegian orogen the structures within this thrust system preceded the mz contain a subordinate left lateral strike slip component of movement and were rotated into the sveconorwegian frontal deformation zone sfdz in the frontal part of the orogen the mz is inferred to have formed in an extrusive dispersive tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a wnw ese direction the initial stages of this collision produced the east verging thrust system and shortening was absorbed at this stage by crustal thickening late possibly out of sequence thrusting with a right lateral strike slip sense of shear along the sfdz marked the waning stages of crustal shortening in the sveconorwegian orogen

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

facies analysis of a 1 9 ga continental margin back arc felsic caldera province with diverse zn pb ag cu au sulfide and fe oxide deposits bergslagen region sweden the bergslagen mining district in south central sweden has been a major metal producer for more than 1 000 years in geologic terms bergslagen is the intensely mineralized part of a large early proterozoic mainly 1 90 1 87 ga felsic magmatic region of mainly medium to high metamorphic grade in the baltic shield the district contains a diverse range of ore deposit types including banded iron formation magnetitecalc silicate skarn manganiferous skarn and carbonate hosted iron ore apatite bearing iron ore stratiform and strata bound zn pb ag cu au sulfide ores and w skarn most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns this study is a field oriented regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration deformation and metamorphism the region is interpreted as an extensional probably back arc active continental margin magmatic region it evolved through stages of intense magmatism thermal doming and crustal extension followed by waning extension waning volcanism thermal subsidence reversal from extension to compressional deformation regional metamorphism and structural inversion the volcanic successions comprise interfingering proximal near vent medial volcano flanks and distal volcano margin facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite rhyolite complexes nonwelded to poorly welded pyroclastic flow and fallout units and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies subvolcanic porphyritic intrusions and cryptodomes are also abundant the pyroclastic debris was shed from mainly shallow water and subaerial proximal areas to shallow and moderately deep water above and below storm wave base distal areas most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash siltstone limestone and vitric crystal sandstone and breccia these mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base but locally above the storm wave base water depths are interpreted to have been mainly 10 to 500 m with local deeper areas although the ore deposits occur in medial to distal facies associations many occur above or laterally adjacent to subsided volcanic vent complexes this suggests a spatial temporal and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers except for the apatite bearing iron deposits it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes this is possible especially for the base metal sulfide deposits which have intense localized footwall alteration in felsic volcanic rocks however an important role of the volcanic centers may have been to provide strong positive pertubations or magmatic hydrothermal input to the regional convective geothermal system the main zn pb ag cu au ore deposits span a range between two end member types stratiform ash siltstone hosted sea floor deposits that have similarities to some stratiform sediment hosted zn pb ag deposits especially the broken hill type deposits and strata bound volcanic associated limestone skarn subsea floor replacement deposits that more closely resemble felsic volcanic associated massive sulfide deposits facies models are presented for both the banded iron formations are interpreted as chemical suspension deposits rhythmically interbedded with rhyolitic ash siltstone the skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements apatite bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite rhyolite volcanic center

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

ductile nappe extrusion in constrictive strain at the origin of transverse segments of the allochthon boundary thrust in the manicouagan imbricate zone central grenville province quebec abstract separating the parautochthonous and allochthonous belts the allochthon boundary thrust abt is a major lithotectonic boundary that generally runs parallel with the length of the grenville province but with some transverse segments in this contribution we investigate one of these segments the manicouagan imbricate zone miz by adding new field based data supplemented by new u pb deformation ages to a compilation of previous research conducted on both sides of the abt in the region our results reveal striking differences between the footwall and hanging wall of the abt in terms of strain and structures and concludes that the transverse orientation of the abt is not related to folding an updated strain analysis allows us to determine that deformation of the miz was dominated by a constrictive type of strain we build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca 990 ma the insertion of the hot ductile nappe into partially molten metasediments of the surrounding parautochthonous belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter

diapir driven crustal convection decompression melting renewal of the magma source and the origin of nested plutons abstract this paper studies numerically the rise of diapirs and the consequences of diapir driven crustal flow the ascent of diapirs imposes a convective flow pattern concentrated mostly in the low viscosity lower crust in which the rocks immediately surrounding the diapir are dragged upwards and rocks a few radii to the sides of the diapir are pushed downwards to fill the gap left by the rising mass there are two main consequences of this flow one warm and water undersaturated rocks may undergo decompression melting when dragged upwards this is shown to enhance the ability of a diapir to intrude the crust because the partially molten wall rocks gain buoyancy and add to the total buoyancy of the diapir which regains part of the energy it spent on heating and dragging the surroundings two the downward flow of rocks renews the magma source with potentially fertile rocks which may undergo melting and give rise to a new diapir which will follow the same path as the first one and repeat the process giving rise to a sequence of diapirs whereas source renewal is a direct result of diapirism magma transport through dykes leads to the accumulation of refractory restite in the source which will eventually prevent further melting diapirism and decompression melting may lead to strong recycling of the lower crust and give rise to large volumes of melt sequential diapirism may explain nested plutons and multipulse mid to upper crustal batholiths