ice divide flow at hans tausen iskappe north greenland from surface movement data surface strain rates around the southeastern dome of hans tausen iskappe in peary land north greenland 82 5 n 27 5 w are determined from global positioning system surveys of a strain net average longitudinal surface strain rate increases towards the dome from 1 4 0 2 x 10 4 a 1 at 5 10 ice thicknesses from the divide to 2 4 1 0 x 10 4 a within 1 ice thickness from the divide analysis of the data shows that the ice cap is presently building up within the strain net with an average rate of 0 04 0 02 m a assuming a uniform thickening the shape factor of the horizontal velocity the ratio between the vertically averaged horizontal velocity and the horizontal surface velocity decreases towards the dome from 0 9 at a distance of 10 ice thicknesses from the dome to 0 5 at the dome based on application of the continuity equation our results indicate that a region with anomalous flow is formed around the dome supporting recent indications reported by vaughan and others 1999 it is not possible from our data to constrain parameters of the flow law because there is no independent estimate of the significant present thickening of the central part of the ice cap and its pattern around the dome

strain analysis of heterogeneous ductile shear zones based on the attitudes of planar markers using the simple measurement of the attitude of planar features such as foliation shear planes and displaced tabular markers such as veins or dykes fundamental information is obtained on deformation types and rheology in heterogeneous ductile shear zones the method outlined in this study is based on the subdivision of the shear zone in n deformed layers each of them being characterized by approximately homogenous deformation for each layer paired measurements of and are made where is the angle that the foliation forms with the shear plane and is the effective shear strain by means of suitable grids constructed according to strain boundary conditions values of stretch k shear strain strain ratio r and kinematic vorticity number wk are calculated the method has been tested on three ductile wrench zones exposed in a deformed granitoid pluton in the eastern alps italy the data indicate that the shear zones are of dominantly transtensional or transpressional type furthermore the finite shear strain and strain ratio peaked profiles suggest that in all instances shear zone evolution was characterized by strain softening analysis of the kinematic vorticity indicates that strain softening essentially affected the simple shear component of the deformation

nucleation linkage and active propagation of a segmented quaternary normal dextral fault the loma del viento fault campo de dalias eastern betic cordillera se spain active faults from the campo de dalias se betic cordillera allow us to constrain the deformation styles involved in the development of segmented oblique slip faults this sector constitutes the widest outcrop of plio quaternary sediments in the northern boundary of the alboran sea it has emerged since the late pliocene and therefore provides recent deformation markers that are not disturbed by erosive processes the faults started to grow during the pleistocene reactivating previous hybrid joints with a normal dextral slip we present a detailed map of the largest fault in the area the loma del viento fault comprising six onshore segments based on field work and aerial photography the distributions of the contiguous joints have been mapped and the joints reactivated as faults are identified some of these fault segments are hard linked and fault slip enhances toward the linkage sectors between them with associated sedimentary depocenters an electrical tomography profile reveals the wedge geometry of a unit of pleistocene conglomerates and red silts that were coevally deposited during the fault movement long term slip rate in the central part of the fault is estimated at 0 07 0 03 mm y in addition a seismic crisis nucleated close to the loma del viento fault during november 2010 was recorded moment tensor analysis of the two mainshocks mw 3 5 and 4 2 provides a focal solution indicating a n120 e striking right lateral strike slip fault the corrugated morphology of the loma del viento fault may have influenced its seismic behavior some of the fault segments are oblique to the general motion of the fault these oblique segments would provide higher resistance against the general fault motion and could lock the fault leading to accumulate elastic energy

reply to l v dimieri and m m turienzo 2012 comment on fault inversion vs new thrust generation a case study in the malargue fold and thrust belt andes of argentina by j f mescua and l b giambiagi journal of structural geology 35 2012 51 63

control of the geometry of precursor brittle structures on the type of ductile shear zone in the adamello tonalites southern alps italy abstract amphibolite facies ductile shear zones developed during the immediate post intrusive cooling history of the adamello tonalites southern alps italy shear zones include i thin a few mm s thick fault like shear zones that accommodate shear strain values up to several 100 s the dominant type ii mylonitic horizons dm s thick in sharp contact with the undeformed wall rock iii continuous shear zones with sigmoidal shaped s or composite s c foliations a transition between the different types occurs along strike over short distances discontinuous shear zones are also localized on pegmatite dykes and quartz veins and both i and ii are discontinuously outlined by quartz layers detailed surface mapping reveals that most fault like shear zones are arranged en echelon mainly forming contractional steps markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips differential displacement is mainly accommodated at contractional steps by the development of foliated domains geochemical analyses of major and trace elements show that there is no compositional change along strain gradients the overall features are consistent with nucleation of shear zones on former sets of en echelon joints in many cases intruded by pegmatite dykes or filled with quartz reactivation of joints produced strongly localized shear zones whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing these jogs were progressively involved in the accommodation of shear displacement and overall shear zone development

effects of initial rift inversion over fold and thrust development in a cratonic far foreland setting abstract the effects of initial rift inversion over the geometry of a far foreland intracratonic fold and thrust belt are investigated a total of 25 seismic reflection lines and stratigraphic data from 9 wells are used in association to digital elevation and field geological data to investigate the geometric relationship between shallow thrust development and initial positive inversion of buried rift faults in a cratonic environment where inversion tectonics is not common the study case is conducted in the meeting region of the opposed verging brasilia and aracuai foreland fold and thrust neoproterozoic belts located in central brazil formed in the first stages of west gondwana amalgamation seismic reflection sections are used to map buried paleorift faults and trust ramps the best 5 sections are also interpreted in detail in terms of fold and thrust geometry and their relationship to the inverted rifts is analyzed they show high correlation between shallow fold and thrust geometry and the shortening of synrift strata caused by rift inversion results indicate that initial positive rift fault reactivation exerts a determinant control over the locus of thrust ramp nucleation by causing syn rift strata to overfold and drape fold the passive folding propagates upwards forming gentle folds in the decollement plane where thrust ramps tend to nucleate back thrust forethrust pairs tend to form over elevated horsts where higher topographic plateaus are preserved multiple thrusting and lower relief occur over buried rifts with multiple inverted faults strata over non rifted basement remain practically unaffected by thrusting and folding moreover the development of thrust salients and recesses are demonstrated to be conditioned by the morphology of buried rifts the study also contributes with a more precise mapping of the buried rift system and thrust system in the area and brings new insights about the structural evolution of the brasiliano pan africano orogeny affecting the sao francisco craton basement and covers

un coupled thrust belt foreland deformation in the northern patagonian andes new insights from the esquel gastre sector 41 30 43 s the patagonian andes represent a unique natural laboratory to study surface deformation in relation to deep slab dynamics in the sector comprised between latitudes 41 30 and 43 s new apatite u th he ages indicate a markedly different unroofing pattern between the broken foreland area characterized by late cretaceous to paleogene exhumation and the adjacent andean sector to the west which is dominated by miocene pliocene exhumation these unroofing stages can be confidently ascribed to inversion tectonics involving reverse fault related uplift and concomitant erosion late cretaceous paleogene shortening and exhumation are well known to have affected also the thrust belt sector of the study area during a prolonged stage of flat slab subduction therefore the different ages of near surface unroofing documented in this study suggest coupling of the deformation between the thrust belt and its foreland during periods of flat slab subduction e g during late cretaceous paleogene times and dominant uncoupling during periods of steep slab subduction and rollback even when these are associated with high convergence rates i e 4 cm year as those documented in miocene times for the patagonian andes

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

a fractal relationship between vein thickness and gold grade in drill core from la codosera spain analysis of quartz vein thicknesses and gold grades in drill core from western spain indicates that both approximate a power law distribution suggesting an underlying fractal relationship log log plots of cumulative frequency n against vein thickness t show systematic changes in relation to gold grade the exponent d in a power law of the form n c t super d is lower for higher gold grades indicating that a greater proportion of the dilation is taking place on the thicker veins which are interpreted as providing interconnected conduits for gold bearing fluids thus monitoring the vein thickness distribution may provide a means of discriminating mineralized and barren structures in vein hosted mineral deposits

fractal geometries of vein systems and the variation of scalingrelationships with mechanism abstract investigation of vein geometries has identified empirical relationships which allow extrapolation of outcrop scale observation to larger scales measurements of vein shapes and aspect ratios in a number of different environments demonstrate that veins follow similar empirical scaling laws to those of faults geometries of veins are easier to characterise accurately than faults because their thickness is related to the variation in displacement from zero at their tips to a maximum at their thickest point vein sizes obey the empirical relationship l kta where l length in mm t thickness in mm 20 1 this suggests that veins amplify initially by inflation and subsequently by elongation

a permeability model for naturally fractured carbonate reservoirs abstract based on a detailed structural study performed on a reservoir surface analogue a fracture permeability model for carbonate reservoirs is proposed this involves four hierarchical systems each one assumed to convey fluids exclusively to that of immediately higher order new basic equations are then provided simulating hydrodynamic reservoir response the large scale first order structures consist of faults to which high permeability structures damage zones are associated at the meter scale stratabound joint systems together with bedding joints bounding the mechanical layers form a connected network transporting fluids to the fault system at a lower scale down to crystal size non stratabound joints constitute a pervasive and capillary fracture network which conveys fluids within the rock mass the non fractured host rock constitutes the lower permeability system at the crystal scale as no published works include an effective integration of detailed structural and numerical models the present study aims at covering a significant gap in the literature also providing appropriate theoretical basis for subsequent studies on fracture network modeling and reservoir simulation besides its application in the field of reservoir development and management the illustrated model may also be useful in environmental studies involving ground fluids such as e g in the field of special toxic nuclear waste management

modes and timing of fracture network development in poly deformed carbonate reservoir analogues mt chianello southern italy abstract structural and paleostress analyses carried out on a kilometre sized outcrop of allochthonous shallow water carbonate units of the southern apennines allowed us to unravel a superposed deformation pattern associated with plate convergence the reconstructed tectonic evolution involves i early extensional faulting and fracturing associated with bending of the foreland lithosphere during forebulge and foredeep stages including the development of both tangential and radial normal fault and tensile fractures early middle miocene ii large scale thrusting and folding late miocene iii transcurrent faulting including two distinct sub stages characterized by different remote stress fields pliocene early pleistocene and iv extensional faulting late quaternary stage i normal faults generally occurring as conjugate sets and related fractures and veins are variably deformed and overprinted by later horizontal shortening despite having experienced such a long and complex structural history the studied carbonates are characterized by a background fracture network including two joint vein sets orthogonal to each other and to bedding that appears to be associated with the early fault sets that formed during the first foredeep forebulge related deformation stage therefore away from younger late miocene to quaternary fault zones the permeability structure of the studied carbonates appears to be essentially controlled by the early inherited fracture network as a similar fracture network is likely to characterize also the buried apulian platform carbonates representing the reservoir units for major oil fields in southern italy our results also bear possible implications for a better understanding of fluid flow in the subsurface and related hydrocarbon production

potential for stratigraphic folding near ice sheet centers lack of agreement between the deep portions of the greenland icecore project grip and greenland ice sheet project ii gisp2 ice cores from central greenland suggests that folds may disrupt annual layering even near ice divides we use a simple kinematic flow model to delineate regions where slope disturbances wrinkles introduced into the layering could overturn into recumbent folds and where they would flatten leaving the stratigraphic record intact wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties rather than from residual surface structures sastrugi or from open folds associated with transient flow over bed topography if wrinkles are preferentially created in anisotropic ice under divides where the resolved shear stress in the easy glide direction can be weak and variable then the deep intact climate record at dye 3 may result from its greater distance from the divide alternatively the larger simple shear at dye 3 may rapidly overturn wrinkles so that they are not recognizable as folds the ice core record from siple dome may be intact over a greater fraction of its depth compared to the central greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide

basin inversion by distributed deformation the southern margin of the bristol channel basin england several models of basin inversion described in the literature are tested in a study of triassic and early jurassic strata exposed along the southern margin of the bristol channel basin in somerset england that has been exhumed by 95 of them are still in net extension despite widespread kinematic evidence for reverse reactivation when coupled with the general absence of thin skinned thrusts and the widespread occurrence of regional contractional folds it appears that none of three main inversion models the fault reactivation model the thin skinned model and the buttress model are by themselves applicable we erect a new model of basin inversion the distributed deformation model which consists of three stages of basin inversion stage one involved early partial reactivation of large displacement steep normal faults stage two was dominated by folding wherein fault blocks underwent oblique non coaxial shortening by map scale folding accompanied by formation of outer arc normal faults minor cleavage and neoformed thrusts stage three involved reverse reactivation of outer arc normal faults and activation of oblique and strike slip faults that partitioned deformation into compartments

a flow line model for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet a flow line model is presented for calculating the surface profile and the velocity strain rate and stress fields in an ice sheet with given base elevation profile ice thickness at the dome divide flow law parameters mass balance distribution and convergence divergence conditions along the flow line the model which is based on a quasi similarity hypothesis as regards the horizontal velocity depth profiles accounts for changes along the flow line in the depth distributions of temperature normal stress deviators and possible enhanced flow of deep ice of wisconsin origin a curvilinear coordinate system is applied with horizontal axes along flow lines and surface elevation contours respectively the flow equations are reduced to two differential equations one for the surface elevation profile and the other for a profile function that determines the depth distributions of velocities and strain rates the two equations are coupled through a profile parameter that communicates the influence of velocity profile changes to the surface profile equation it is shown that the variation along the flow line of this parameter should also be considered when deriving flow law parameters from ice sheet flow line data for a symmetric dome explicit expressions are derived for the depth distributions of the vertical velocity strain rates and stresses the strain rate profiles display an inflection about half way down the ice sheet and in the case of isothermal ice have surface values 2 2 times their depth averaged values the depth distribution of the vertical velocity indicates that a relatively thick layer of almost stagnant ice is present at the ice sheet base below a dome

mass balance and surface movement of the greenland ice sheet at summit central greenland during the grip deep drilling in central greenland the ice sheet topography and surface movement at summit has been mapped with gps measurements of the surface velocity are presented for a strain net consisting of 13 poles at distances of 25 60 km from the grip site some results are the grip site is located approximately 2 km nw of the topographic summit the surface velocity at the gisp 2 site is 1 7 m yr in the w direction the present mass balance at summit is calculated to be 0 03 0 04 m yr i e close to steady state this result is the best now available for summit a small thinning rate might be a transient response of the greenland ice sheet due to the temperature increase at the wisconsin holocene transition

shortening of the axial zone pyrenees shortening sequence upper crustal mylonites and crustal strength abstract the strength of the lithosphere may be constrained qualitatively by field observations on localized vs distributed modes of deformation and by the mineral assemblages formed during deformation the internal deformation of the bielsa basement unit of the pyrenean axial zone is investigated through structural microstructural and thermometric data in this area shortening is widely distributed as attested by the folded attitude of the interface between the basement and its sedimentary triassic cover shortening is estimated around 1 7 km 13 from a regional balanced cross section and should be considered in pre pyrenean reconstructions shortening probably occurred before strain localization on crustal ramps as suggested by zircon fission track analysis distributed shortening is characterized at small scale by low temperature mylonites and cataclasites in thin section feldspar originally present in the magmatic protolith is partially to totally sericitized this transformation led to significant weakening of the rock and took place in the 250 350 c temperature range sericitization is ubiquitous even in un deformed granodiorites this shows that the weakening effect of sericitization not only occurs in ultra mylonites ultra cataclasites and phyllonites but also more generally in the upper crust early during the shortening history with implications for the shortening style estimates of the geothermal gradient suggest that inherited thermicity may also have influenced the style of shortening we propose that the upper crust was very weak before or at the onset of its shortening due to high thermal gradients and fluid circulation that induced large scale sericitization in greenschist facies conditions this has strong implications on the rheological evolution of the upper crust submitted to metamorphic alteration in the greenschist facies and below

finite strain analysis of a natural ductile shear zone in limestones insights into 3 d coaxial vs non coaxial deformation partitioning abstract in order to obtain new insights into the process of strain localization a ca 50 m thick shear zone has been investigated the shear zone located in the footwall to a major thrust involves limestones and pelites calc mylonites are characterized by a stretching lineation orthogonal to the shear direction and a foliation at an angle of 45 10 to the thrust fault strain analysis and mean foliation analysis have been carried out on 42 samples strain gradients and ellipsoid shape across the shear zone indicate respectively an increase of strain intensity approaching the thrust surface and overall oblate strain apparent flattening increases significantly for intensely deformed rocks theoretical models are proposed to explain the recorded finite strain configuration with a main deformation characterized by simultaneous simple shear and localized pure shear involving lengthening normal to the shear direction a model of asymmetric lateral extrusion is adopted as is consistent with measured horizontal strain gradients this model also takes into account regional geological constraints such as the occurrence of accommodation space along the present day tyrrhenian extensional margin to the sw and of a massive body of platform carbonates forming a rigid buttress to the ne

measurement and characterisation of spatial distributions of fractures abstract a variety of line sample 1 d and map 2 d datasets of faults and joints has been used to investigate the spatial distributions of fractures and to test techniques of fractal analysis the natural fracture datasets have been supplemented by synthetic datasets with known characteristics the fault datasets investigated range in scale from regional to outcrop and the joint datasets are derived from outcrop 1 d datasets were analysed by the spacing population interval counting and fracture number interval counting techniques 2 d datasets were analysed by box counting and fracture number box counting techniques results indicate that fracture spacing can be characterised in line samples using either the 1 d interval counting technique or more simply by measuring the spacing population as the cumulative frequency distribution of spaces between adjacent fractures tectonic faults frequently show a power law spacing population indicating fractal dimensions of between 0 4 and 1 0 but except for outcrop data truncation effects degrade the analysis unrefined joint datasets commonly show negative exponential or lognormal spacing cumulative frequency distributions however single orientation joint sets are characterised by a regular spacing and are therefore non fractal for both fault and joint map data box counting techniques do not yield the power law relationship between box size and the number of boxes that is expected of fractal geometries it is concluded that 2 d box counting techniques are too insensitive to characterise the many different parameters of a fracture array the fracture density technique appears not to discriminate between very different fracture patterns and therefore does not provide useful results a fracture pattern incorporates several attributes e g size distribution orientation linkage density roughness each of which may scale independently and each of which may be fractal with its own fractal dimension full characterisation of fracture patterns will therefore require independent analysis of each of these attributes

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

faulted joints kinematics displacement length scaling relations and criteria for their identification structural geometries and kinematics based on two sets of joints pinnate joints and fault striations reveal that some mesoscale faults at split mountain utah originated as joints unlike many other types of faults displacements d across faulted joints do not scale with lengths l and therefore do not adhere to published fault scaling laws rather fault size corresponds initially to original joint length which in turn is controlled by bed thickness for bed confined joints although faulted joints will grow in length with increasing slip the total change in length is negligible compared to the original length leading to an independence of d from l during early stages of joint reactivation therefore attempts to predict fault length gouge thickness or hydrologic properties based solely upon d lscaling laws could yield misleading results for faulted joints pinnate joints distinguishable from wing cracks developed within the dilational quadrants along faulted joints and help to constrain the kinematics of joint reactivation q 2001 elsevier science ltd all rights reserved

structural controls on jurassic gold mineralization and cretaceous tertiary exhumation in the foreland of the southern patagonian andes new constraints from la paloma area deseado massif argentina abstract fluid rock interaction occurring within high permeability conduits represented by fault fracture meshes is well known to control the development of hydrothermal ore deposits in this study we document the fundamental role of four sets of precursor joints reactivated as dilational faults displaying variable normal dip slip and strike slip components of motion in the formation of epithermal au ag deposits of economic interest paleostress analysis unravels how all the resulting four sets of hybrid extension shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the 3 and the 2 axes such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension and by orthogonal stretching produced by differential downthrow of the hanging wall blocks of laterally terminating jurassic normal faults later fault reactivation appears to be associated with far field propagation of andean stresses marked by superposed strike slip and reverse fault slip components recording roughly e w horizontal compression this late fault reactivation could be related with a late early cretaceous shortening event known to have affected the studied sector of the deseado massif the subsequent tectonic evolution of the study area occurred within the general framework of a slow cretaceous to tertiary exhumation recorded by apatite fission track cooling ages our new thermochronometric results recording steady state cooling at a rate of c 1 c ma during the last 90 ma shed new light onto the post jurassic geodynamic evolution of the foreland of the southern patagonian andes

a comparison between active and passive microwave measurements of the antarctic ice sheet and their association with the surface katabatic winds the intensity of the seasat altimeter return power over antarctica varies in strong correlation with the intensity of model katabatic winds it is also strongly correlated with the polarization of the passive microwave signal at 37 ghz of the nimbus 7 smmr data it is shown that this is most likely the result of the wind induced micro roughness of the ice surface

late permian middle jurassic lithospheric thinning in peru and bolivia and its bearing on andean age tectonics abstract integrated studies and revisions of sedimentary basins and associated magmatism in peru and bolivia 8 22 s show that this part of western gondwana underwent rifting during the late permian middle jurassic interval rifting started in central peru in the late permian and propagated southwards into bolivia until the liassic dogger along an axis that coincides with the present eastern cordillera southwest of this region lithospheric thinning developed in the early jurassic and culminated in the middle jurassic producing considerable subsidence in the arequipa basin of southern peru this 110 ma long interval of lithospheric thinning ended 160 ma with the onset of malm earliest cretaceous partial rift inversion in the eastern cordillera area the lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and or transpressional deformations in particular the altiplano plateau corresponds to a paleotectonic domain of normal lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere the high eastern cordillera of peru and bolivia results from late oligocene neogene intense inversion of the easternmost thinned area

mass balance of the greenland ice sheet at dye 3 the mass balance of the greenland ice sheet a t dye 3 is estimated on the basis of 0 bserva tions of ice thickness accumulation rate surface velocities and surface strain rates the calculations indicate a rate of increase of surface elevation of 3 cm year with 95 confidence limits of 3 cm year and 9 cm year previous estimates of the mass balance of the greenland ice sheet by the same method reported large imbalances these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth averaged quantities the most reliable observations indicate that the interior regions of the greenland ice sheet are at present thickening at a rate of a few centimetres per year a contributing cause for this may be the slow thinning of a bottom layer of relatively soft wisconsin ice resume bilan de masse de l lndlandsis groenlandais a dye 3 le bilan de masse est estime it partir des mesures d epaisseur taux d accumulation vitesses et vitesses de deformation superficielles le calcul don ne un epalss lssement de 3 cm an 1 avec un intervalle de confiance au seuil de 95 allant de 3 cm an 1 it 9 cm an i les estimations anterieures du bilan de l lndlandsis obtenues par la meme methode sont examinees on en deduit que les valeurs de bilan tres eloignees de l equilibre sont certainement dues au manque de donnees preclses et ou a i utilisation de donnees superficielles comme valeurs moyennes sur une

diffuse faulting in the machu picchu granitoid pluton eastern cordillera peru abstract a series of batholiths forming part of the roots of a permo liassic rift system are exposed in the high eastern cordillera of central peru as a result of tectonic inversion shortening of the machu picchu granitoid pluton was accommodated by widespread shear reactivation of primary joints by a process termed here diffuse faulting fault like reactivation of precursor joint surfaces marked by chlorite epidote and quartz shear fibres is locally evidenced by few centimetres offsets within apparently undeformed granite analysis of fault slip data indicates that shear reactivation of different joint sets was kinematically consistent with ene oriented shortening less frequent mylonitic shear zones appear to have evolved from the common brittle precursors apart from rare phyllonitic shear zones fluid rock interaction along the brittle precursors was generally limited and pluton deformation appears to be mainly controlled by the geometry and distribution of primary joints three main sets of reactivated joints can be recognized characterized by oblique slip kinematics with variable reverse and strike slip components of motion theoretical modelling based on quantitative fracture analysis scan line data and different displacement length relationships applied to the main reactivated joint sets yield first order estimates of pluton finite strain the results suggest that bulk finite strain is oblate and essentially coaxial and is characterized by horizontal shortening not exceeding 10 relatively small finite strains integrated over the size of the pluton still result in a few kilometres of crustal shortening

observations of the surface properties of the ice sheets by satellite radar altimetry by comparing modelled and averag ed satellite altimeter return it is demonstra ted that time profiles of altimeter return can be used to provide important information on the surface proper ties of the ice sheets alt imeter ice sheet radar echoes from low altit udes and or rela tively low latitudes are in gene ral domin ated by surface scat tering and in greenl and the area of surface d ominated return broadly coi ncides with the zone of summer melting seasonal variations in the echo wave form shapes are negligible in all regions studied with the possib le exception of an area near the margin of the greenland dry snow zone in general the model expla ins well the observed variations in mean wave form shape but small discrepances between the model wave forms and the recorded wave forms indicate that sub sur face layers may be influ encing the shape of the return the poss ibility of deriving quantitative estima tes of surface proper ties is explored by fitting model returns to averaged altimeter wave forms from the wilkes land plateau in anta rctica surface roug hness can be meas ured unambig uously from the wave form data but estim ations of other parameters such as gra in size snow density and snow temperature are found to be ambiguous because different surface para meters have a similar infl uence on the shape of the retur n despite this the derived estimates compare well with ground b ased observations and suggest that the satellite alti meter may have an important role to play in providing info rmation on the surface properties of the ice sheets

heterogeneous shear zone evolution the role of shear strain hardening softening mathematical models are proposed to describe heterogeneous shear zones characterized by variation thickness with time these models take into account several combinations of deformation type regime simultaneous simple shear pure shear and volume change and degree of strain localization increasing or decreasing shear zone thickness results respectively from strain hardening where deformation progressively diffuses into the host rock and strain softening where deformation gradually localizes within a narrow zone shear zone models are made of a series of layers each characterized by homogenous strain a quantitative evaluation of displacement thickness relationship and finite strain development may be obtained from each model temporal evolution depends on the following parameters i rate of shear strain hardening softening ii initial shear strain increment imposed to the system and iii thickness of homogenously deformed units within a layered shear zone based on data coming from natural shear zones displacement rates may be obtained independently of shear zone thickness

past precipitation rates derived from ice core measurements methods and data analysis ice cores in which annual layers can be identified or particular horizons dated provide a record of past precipitation rates if measured layer thicknesses can be corrected for thinning by flow since deposition total strain is usually calculated from steady state flow models because of lack of information about the ice cap s history an assumption of uniform vertical strain rate is valid at best in the upper half of the ice column complex models are needed to take account of uneven bedrock and variations in precipitation rate upstream of the borehole drilling at an ice divide avoids these complications new data analyses suggest that precipitation at camp century has fluctuated by only plus or minus 10 for the past 8500 years whereas at devon island on the opposite side of baffin bay precipitation has been uniform only for the past 1300 years with significantly higher values before that estimation of pre holocene precipitation rates is especially difficult because that ice may have an effective viscosity different from holocene ice three published estimates of pre holocene precipitation at camp century are unreliable the authors did not allow for thinning and their time scales differ appreciably data from three antarctic stations show some inconsistencies measuring concentrations of 10be in ice cores may be an alternative method of determining past precipitation rates

inference of accumulation rate patterns from deep layers in glaciers and ice sheets the spatial pattern of accumulation rate can be inferred from internal layers in glaciers and ice sheets non dimensional analysis determines where finite strain can be neglected shallow layer approximation or approximated with a local one dimensional flow model local layer approxima tion and where gradients in strain rate along particle paths must be included deep layers we develop a general geophysical inverse procedure to infer the spatial pattern of accumulation rate along a steady state flowband using measured topography of the ice sheet surface bed and a deep layer a variety of thermomechanical ice flow models can be used in the forward problem to calculate surface topography and ice velocity which are used to calculate particle paths and internal layer shapes an objective tolerance criterion prevents over fitting the data after making site specific simplifications in the thermomechanical flow algorithm we find the accumulation rate along a flowband through taylor mouth a flank site on taylor dome antarctica using a layer at approximately 100 m depth or 20 of the ice thickness accumulation rate correlates with ice surface curvature at this site gradients along flow paths critically impact inference of both the accumulation pattern and the depth age relation in a 100 m core

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

small scale ductile shear zones neither extending nor thickening nor narrowing abstract the length thickness and strain gradients of small scale 10 3 10 1 m thick ductile shear zones are pre determined by the presence of surface precursors e g fractures or compositional layers and associated fluid rock interaction fractures with their surrounding host rock damage zone and concurrent tectonic under pressure during dilation provide efficient fluid pathways and networks with diffusive fluid rock interaction at their margins the fluid induced gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types including single homogeneous to heterogeneous shear zones and paired shear zones as a consequence geochemical differences between shear zone and host rock may reflect fluid rock interaction during the precursor brittle history rather than during slip especially considering that shear zones with even a small component of stretch may be over pressured and therefore unable to drain fluids from the surrounding rocks support from field observation for this interpretation is given by 1 the lack of correlation between shear zone thickness and accumulated displacement and 2 the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors most small scale shear zones in massive rocks granitoids are initially neither thickening nor narrowing with increasing strain this concept may also apply to more foliated rocks but does not necessarily hold for larger scale shear zones

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

the microstructure of polar ice part i highlights from ice core research polar ice sheets play a fundamental role in earth s climate system by interacting actively and passively with the environment active interactions include the creeping flow of ice and its effects on polar geomorphology global sea level ocean and atmospheric circulation and so on passive interactions are mainly established by the formation of climate records within the ice in form of air bubbles dust particles salt microinclusions and other derivatives of airborne impurities buried by recurrent snowfalls for a half century scientists have been drilling deep ice cores in antarctica and greenland for studying such records which can go back to around a million years experience shows however that the ice sheet flow generally disrupts the stratigraphy of the bottom part of deep ice cores destroying the integrity of the oldest records for all these reasons glaciologists have been studying the microstructure of polar ice cores for decades in order to understand the genesis and fate of ice core climate records as well as to learn more about the physical properties of polar ice aiming at better climate record interpretations and ever more precise models of ice sheet dynamics in this part i we review the main difficulties and advances in deep ice core drilling in antarctica and greenland together with the major contributions of deep ice coring to the research on natural ice microstructures in particular we discuss in detail the microstructural findings from camp century byrd dye 3 grip gisp2 northgrip vostok dome c edml and dome fuji besides commenting also on the earlier results of some pioneering ventures like the jungfraujoch expedition and the norwegian british swedish antarctic expedition among others in the companion part ii of this work faria et al 2014 the review proceeds with a survey of the state of the art understanding of natural ice microstructures and some exciting prospects in this field of research

application of a flow model to the ice divide region of devon island ice cap canada le modele d ecoulement en regime permanent de reeh est utilise pour etudier les distributions des contraintes de la vitesse de deformation et la variation de l epaisseur de la couche annuelle avec la profondeur dans la region de la ligne de partage d une calotte glaciaire sur un terrain accidente

prediction of macrofracture properties using microfracture information mesaverde group sandstones san juan basin new mexico abstract a better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures the hypothesis that micro and macrofractures represent different size fractions of the same fracture sets and consequently have linked attributes was tested on sandstones of the mesaverde group san juan basin new mexico the use of a cathodoluminescence detector attached to a scanning electron microscope sem allowed observation of microfractures not readily visible using conventional microscopic techniques the orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops fracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies fracture size distributions change at the length scale of the thickness of the mechanical layer typically by increasing the exponent of their power law distribution this change is independent of censoring biases and reflects changes in sampling topology and or fracture growth mechanisms

observations of the antarctic ice sheet with the seasat scatterometer relation to katabatic wind intensity and direction the seasat a satellite scatterometer radar initially designed to measure ocean wind intensity and direction also provided observations on the antarctic ice sheet the signal of the back scatter coefficient decreases strongly from 10 to 20 db when the incidence angle of the observations increases from 0 to 65 an additional 5 db signal is found which is correlated with the direction and intensity of katabatic winds independent of the incidence angle and polarization of the signal by using simplified models of the volume scattering within the snowpack which is mostly sensitive to snow grain size and surface scattering from the air snow interface which depends on roughness it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle and of micro roughness and volume back scatter at incidence angles greater than 25 the instrument therefore provides a means of measuring the direction and intensity of katabatic winds

evolution of the east antarctic ice sheet a numerical study of thermo mechanical response patterns with changing climate an efficient numerical ice sheet model including time dependence and full thermo mechanical coupling has been developed in order to investigate the thermal regime and overall configuration of a polar ice sheet with respect to changing environmental conditions from basic sensitivity experiments in which a schematic east antarctic ice sheet is forced with a typical glacial interglacial climatic shift it is found that i the mutual interaction of temperature and deformation has a stabilizing effect on its steady state configuration ii in the transient mode this climatic transition initially leads to increased ice thickness due to enhanced accumulation after which this trend is reversed due to a warmer base timescales for this reversal are of the order of 103 years in marginal zones and of 104 years in interior regions iii horizontal heat advection plays a major role in damping possible runaway behaviour due to the dissipation strain rate feed back suggesting that creep instability is a rather unlikely candidate to initiate surging of the east antarctic ice sheet the model is then applied to four east antarctic flow lines only the flow line passing through wilkes land appears to be vulnerable to widespread basal melting due to enhanced basal warming following climatic warming time dependent modelling of the vostok flow line indicates that the vostok station area has risen about 95 m since the beginning of the present interglacial due to thermomechanical effects which is of particular interest in interpreting the palaeoclimatic signal of the ice core obtained there

steady flow of a viscous ice stream across a no slip free slip transition at the bed an exact analytic description of the steady downhill flow of a viscous ice stream across a no slip to free slip transition in basal boundary condition suggests ways in which such transitions might be detected by observations of ice stream surface properties we find that the best expression of this transition is in the free surface profile which dips over the point of transition and becomes horizontal far downstream

strain accommodated by brittle failure in adjacent units of the monterey formation u s a scale effects and evidence for uniform displacement boundary conditions abstract extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at arroyo burro beach california the dolostone failed in effective tension whereas the mudstone failed in shear both in response to the same extensional tectonic event in the mudstone unit initial bed length and fault displacement methods document extensional strains of 6 6 0 3 and 6 0 0 3 respectively upon adjusting the displacement estimates according to theoretical fault displacement population analysis the corrected strain becomes 9 7 0 3 for the mudstone measurements of vein apertures in the dolostone document extensional strain that varies according to scale with outcrop vein scanlines indicating a strain of 3 4 0 1 and thin section scanlines yielding a strain of 5 8 0 2 applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture related strain within the dolostone this difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness whereas microscopic veins measured in thin section terminate without regard to a bounding layer the corrected dolostone strain becomes 10 2 1 0 matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies the 10 extensional strain at arroyo burro indicates significant strike parallel nw se extension accommodated by brittle failure during development of the western transverse ranges fold and thrust belt

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization