joints and en echelon cracks in middle eocene chalks near beer sheva israel abstract planar vertical joints and straight en echelon cracks are closely associated in an erosional exposure of middle eocene chalks in a syncline south of beer sheva the two dominant joint sets are oriented 357 n3 w and 031 n31 e and both sets occur in two separate outcrops 400 m apart an en echelon crack array accompanies each joint set the individual en echelon cracks are oriented 337 and 053 respectively radial plumose markings are present on the surfaces of the en echelon cracks they imply that fracture occurred normal to a local tensile stress axis a gradual rotation of the azimuths of the joints and cracks occurs between the two outcrops this rotation implies jointing due to nne ssw compression and en echelon cracking due to unloading of this compression

a succession of quartz veins in archean metaturbidites yellowknife bay slave province the style of extensional quartz veins changed during deformation and metamorphism of greywacke mudstones near yellowknife with successive types of veins accompanying a progression from steeply plunging predominantly macroscopic f1 and f2 folds to mesoscopic f3 folds and subvertical s3 axial planar cleavages a early stage multiple bedding parallel veins are confined to pelitic units some form saddle reefs around hinges of f1 and a few later folds whereas others are unrelated to fold hinges emplacement of most bedding veins during or before f1 folding is suggested b echelon veins pre dominantly lie stratabound within pelites folding about s3 cleavage indicates a pre f3 or early f3 origin c foliation veins more commonly cross sandstones than pelites and follow s3 or f3 axial surfaces boudinage of the veins indicates emplacement before s3 was fully developed d sporadic veins discordant to bedding and cleavage vary from undeformed to folded about s3 evidently these veins were emplaced to

the origin of deep buried channels of elsterian age in northwest germany buried channels cut to a depth of over 100m into the underlying substratum of north germany are almost exclusively of elsterian age evaluation of geophysical well logs in the hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles it can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence fining from proximal to distal channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet the channel infill reflects these episodic outbursts of meltwater during deglaciation the channel remnants became infilled with fine sediments the channels ceased to exist as morphologically detectable forms at the end of the holsteinian stage

oxygen isotope record of fluid rock sio 2 interaction during variscan progressive deformation and quartz veining in the meta volcanosediments of belle ile southern brittany abstract belle ile in the south armorican domain is composed of palaeozoic volcano detrital sequences with sericite phyllites and porphyroids fine banded and folded meta tuffites microquartzites and graphitic quartzites occur in the basal part at plage de bordardoue phengite compositions constrain that variscan metamorphism did not exceed 430 c 4 5 kbar four generations 1 4 of centimeter thick quartz veins were precipitated during variscan progressive deformation and recorded changing fluid compositions values of 26 18osmow in vein 1 quartz exceed high 18osmow in the host rocks decrease of quartz 18osmow from margins to centers can be observed from the syntaxial veins younger veins 4 have lower 18o their inclusions indicate lower salinities and traces of ch4 in the fluid when compared with veins 1 veins 1 were overprinted by shearing and fissuring subgrain rotation recrystallization occurred along briquette structures and subgrain boundaries the initial isotope values have been preserved larger domains with small grained quartz can be identified by lower values of 18o homogeneous isotopic compositions are found in hinges of folded veins 2 with grain boundary migration recrystallization the small scale oxygen isotope variations and the changing fluid compositions point to a locally hosted fluid system with a limited contribution of meteoric water during multiphase deformation and vein formation

stress configurations in conjugate quartz vein arrays abstract arrays of quartz gash veins in small angle 40 conjugate shear zones type 1 arrays of beach are well developed in the upper devonian merrimbula sandstone of the south coast region of new south wales australia we argue that vein and cleavage geometry support an origin of the veins as tension fractures in a rotated secondary stress field rather than a primary shear origin as advocated by beach we also conclude that the veins develop in dilational shear zones under high fluid pressure

displacement features associated with fault zones a comparison between observed examples and experimental models abstract fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures tension cracks r or p type fractures which give the brittle shear zone a width translation along these faults and related features can occur by sliding on the elementary planes and or by solution deposition processes with opening of transtension zones and possible dilatation of the shear zone in order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations displacement along an analogue fault model was examined under conditions of direct shear the experimental fracture patterns were then compared with the natural features the structural elements of certain faults created by testing and showing transpression and transtension zones were found to be similar to natural domino structures bounded by elementary shears and could be compared with computed situations it can be inferred that stresses are reoriented inside the shear zone the angle between elementary fractures depends on their order of development transpression and transtension zones occur systematically and the shear zone undergoes dilatancy under low normal stresses

geometry and kinematics of convergent conjugate vein array systems abstract conjugate arrays of quartz veins in the neranleigh fernvale turbidite beds at norries head eastern australia occur in configurations in which the trends of the veins in a principal section converge towards the acute bisector of the conjugate arrays a convergent configuration such a configuration is common in vein arrays and has been attributed to initiation of the veins as antithetic shear fractures based on geometric arguments the morphology of the veins in this study indicates that the veins are hosted by extension fractures which formed by en echelon breakdown of faults previous models of extension fracture arrays emphasise that all fractures lie parallel to the bisector of conjugate arrays and that arrays develop after initiation of extension fractures and concurrent with fracture propagation sigmoidal vein shapes have been attributed to concurrent shear strain and fracture propagation an alternative model is proposed for the geometry of conjugate arrays formed by en echelon breakdown of faults the conjugate angle between parent faults is established before the en echelon extension fractures are formed the fracture array angle depends on the local displacement of the parent fault thus there is no necessity for the fractures in different arrays to be parallel if the fracture array angle is greater than half the conjugate angle between parent faults a convergent configuration of fractures is produced the kinematics of opening of veins in this study involved bending of rock bridges between fractures producing a gradation from planar to sigmoidal shapes in serial sections without evidence of concurrent fracture propagation

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

en echelon vein array development in extension and shear a subdivision of en echelon vein arrays is proposed based on the geometry of sets of transcurrent arrays from the lower carboniferous rocks in the se midlands of ireland the parameter vein zone angle is proportional to the array orientation for arrays developed through secondary failure in shear zones shear arrays for arrays developed during propagation of an extension vein extension arrays is proportional to the overlap of veins in the array p o criteria for field distinction together with the different methods of kinematic interpretation that are required for the two types of arrays are described for shear arrays the average trends of the two members of the conjugate set are calculated and the axis of principal shortening strain z bisects these the average trend of extension arrays is parallel to that of extension veins and is therefore normal to the maximum extensional strain axis x

paleobiogeography of africa how distinct from gondwana and laurasia abstract although africa was south of the tethys sea and originally belonged to the gondwana its paleobiogeographical history appears to have been distinct from those of both gondwana and laurasia as early as the earliest cretaceous perhaps the late jurassic this history has been more complex than the classical one reconstructed in the context of a dual world gondwana vs laurasia geological and paleobiogeographical data show that africa was isolated from the mid cretaceous albian aptian to early miocene i e for ca 75 million years the isolation of africa was broken intermittently by discontinuous filter routes that linked it to some other gondwanan continents madagascar south america and perhaps india but mainly to laurasia interchanges with gondwana were rare and mainly out of africa dispersals whereas interchanges with laurasia were numerous and bidirectional although mainly from laurasia to africa despite these intermittent connections isolation resulted in remarkable absences poor diversity and emergence of endemic taxa in africa mammals suggest that an african faunal province might have appeared by late jurassic or earliest cretaceous times i e before the opening of the south atlantic during isolation africa was inhabited by vicariant west gondwanan taxa i e taxa inherited from the former south american african block that represent the african autochthonous forms and by immigrants that entered africa owing to filter routes nearly all or all immigrants were of laurasian origin trans tethyan dispersals between africa and laurasia were relatively frequent during the cretaceous and paleogene and are documented as early as the earliest cretaceous or perhaps late jurassic i e perhaps by the time of completion of the tethys between gondwana and laurasia they were permitted by the mediterranean tethyan sill a discontinuous route that connected africa to laurasia and was controlled by sea level changes interchanges first took place between southwestern europe and africa but by the middle eocene a second eastern route the iranian route involved southeastern europe and southwestern asia the iranian route was apparently the filtering precursor of the definitive connection between africa and eurasia the relationships and successive immigrations of mammal mostly placental clades in africa allow the recognition of five to seven phases of trans tethyan dispersals between africa and laurasia that range from the late cretaceous to the eocene oligocene transition these dispersal phases involve dispersals toward laurasia and or toward africa immigrations the immigrations in africa gave rise to faunal assemblages the african faunal strata afss all successful and typical african radiations have arisen from these afss we recognize four to six afss each characterized by a faunal association even major old african clades such as paenungulata or the still controversial afrotheria which belong to the oldest known afs involving placentals ultimately originated from a laurasian stem group africa was an important center of origin of various placental clades their success in africa is probably related to peculiar african conditions endemicity weak competition although strongly marked by endemicity the african placental fauna did not suffer extinctions of major clades when africa contacted eurasia the present geographic configuration began to take shape as early as the mid cretaceous at that time the last connections between africa and other gondwanan continents began to disappear whereas africa was already connected to eurasia by a comparatively effective route of interchange

stress fields about strike slip faults inferred from stylolites and tension gashes abstract in general the long axis of the tension gashes and stylolitic columns developed in limestones during a single phase of compressional deformation occur parallel to the direction of the maximum compressive stress 1 this is the case in the languedoc for structures developed in the jurassic limestones during the n s pyrenean compression exceptionnally however these microstructures turn in direction and become oblique even orthogonal one to the other probably as a consequence of a variation in intensity and direction of the stress field at the end of a microfault this mechanism also occurs in a larger scale structure involving segments of pre tectonic joints that act as en echelon microfaults in a brittle kink band equivalent to a peculiar type of potential wrench fault

the association of folds and veins in shear zones abstract paired hook shaped asymmetric folds with shared limbs cut by axial planar veins dikes or fractures are common in glaciers and rocks the sense of offset across the vein or fracture is characteristically opposite to that expected from the sense of fold asymmetry the kinematics of development of the folds and the relationship of the folds to the fractures or veins can be understood by considering the nature of glacial flow and by utilizing simple clay experiments in zones of simple shear dilational fractures may form at about 45 to the shear zone boundaries the perturbation of strain around such a fracture during its development and subsequent rotation and closure under continued deformation causes layers or pre existing foliation cut by the fracture to become folded thus folding is a consequence of the fracturing and not vice versa the precise shape of the fold depends on the nature of the perturbation and particularly whether or not the fracture remains for a time open or fluid filled and thus has walls unable to sustain a shear stress during vein rotation paired hook folds provide evidence for non coaxial strain and are good sense of shear indicators however not all folds with veins parallel to their axial surfaces form in the way described in this paper in some cases vein emplacement occurs late during fold development or post dates folding in these cases offsets along the veins are consistent with fold asymmetry

phyllonitization and development of kilometer size extension gashes in a continental scale strike slip shear zone north goias central brazil abstract several km scale vertical extension gashes occur in a low metamorphic grade strike slip shear zone in central brazil these mega gashes show many of the characteristics commonly found in en echelon extension gashes of cm and outcrop scale reflecting a wide range of scale invariance for this phenomenon the gashes are filled with quartz veins which commonly host gold mineralization microstructures show a progressive deformation of the original cavity infilling vein structures towards the gash margins logarithmic plots of length vs thickness for gashes from thin section outcrop and air photograph scales define a power law l 11 4 t 0 96 logarithmic plots of cumulative frequency define curves with power law segments whose slopes indicate fractal dimensions d around 1 4 1 5 for both length and thickness the phyllonite zones adjacent to the mega gashes are interpreted to exert a crucial role in their development calculations show that the amount of quartz depleted in the phyllonite zones correspond to the amount of quartz precipitated in the mega gashes 7 10 6 m 3 volumes of fluid in the order of 10 10 m 3 must have been channelled through the opened fractures to precipitate such an amount of quartz we conclude that these mega gashes have developed from continuous propagation and opening of tension fractures in zones relatively preserved from phyllonitization protomylonites it is suggested that the development of kilometric extension gashes in the non phyllonitic domains produces a volume gain in response to the volume loss produced in the phyllonite zones the whole shear zone is envisaged therefore as an isovolumetric system with alternating lateral zones of volume loss phyllonites and volume gain extension gashes

insights into the mechanics of en echelon sigmoidal vein formation using ultra high resolution photogrammetry and computed tomography abstract two novel techniques photo based reconstruction photogrammetry and computed tomography ct are used to investigate the formation of an exceptional array of sigmoidal veins in a hand sample from cape liptrap southern victoria and to provide constraint on models for their development the accuracies of the photogrammetric models were tested by comparison with a laser scan generated three dimensional 3d model the photogrammetric model was found to be accurate to at least 0 25 mm and substantially more detailed than the laser scan a methodology was developed by which 3d structural measurements could be extracted from the photogrammetric model this was augmented with the ct model which through its capacity to elucidate internal structure was used to constrain the geometry and linkage of structures within the rock volume the photogrammetric and ct data were then combined with detailed photomicrographs to evaluate the evolution of the sigmoidal veins in the sample the angle between the sigmoidal vein margins and an inferred shear zone as well as the orientations of the crystal fibres were found to imply a rotation of 27 however coeval pressure solution seams and older veinlets in the rock bridges between the veins were only found to have rotated by 10 an observation not easily explained using existing models for sigmoidal vein formation a new model is proposed in which a significant component of sigmoidal vein geometry is due to localised dilation caused by slip on the pressure solution seams the process involves strain partitioning onto pressure solution seams which leads to exaggeration of sigmoidal vein geometries if not accounted for the apparent vein rotation due to slip partitioning introduces errors into calculations of simple shear and volume strain based on sigmoidal arrays of this type furthermore the ct data demonstrated that in 3d the veins are continuous and channel like implying a far higher degree of connectivity and fluid transport than is suggested by their 2d form

tension fractures and boudinage oblique to the maximum extension direction an analogy with luders bands a study has been made of the orientation of boudin necklines and extension fractures with respect to the maximum extension direction x determined by infilling fibre growth several localities within the variscan belt of western europe the pyrenees and the alps have been investigated in detail it has been generally accepted that boudinage and extension fracturing occur perpendicular to x in the rock at the time of their formation however we have shown that is not the case boudin necklines and extension fractures occur at an angle between 45 90 to x with the most frequent orientations between 65 70 and 80 85 angles of exactly 90 are rare in order to explain this obliquity an analogy is made with the phenomenon of luders bands localised regions of plastic deformation developed in thin metal plates under tensile testing because the thickness of a layer is negligible compared with its lateral extent we will only be concerned here with the case of thin metal plates

inter array and intra array kinematics of en echelon sigmoidal veins in cross bedded sandstone merimbula southeastern australia abstract the kinematics of en echelon arrays of quartz veins hosted by sandstone of the upper devonian worange point formation are strongly influenced by bedding and cross bedding bulk deformation recorded by the vein arrays inter array kinematics was influenced by the anisotropy of the host rock such that three types of arrays are observed 1 east verging arrays parallel to bedding 2 east verging arrays oblique to bedding with veins parallel to cross bedding and 3 west verging arrays parallel to cross bedding with veins parallel to bedding these three array types correspond to flats ramps and backthrusts respectively and represent the earliest stages of thrust fault formation flat and ramp arrays form a staircase geometry and conjugate ramp and backthrust arrays form pop up structures the interpretation of the morphology of veins and wall rock intra array kinematics indicates rotation of bridges was the primary kinematic process in the arrays the opening of vein space by rotation of bridges which maintain constant thickness is predicted from measurements of the orientations of vein wall rock interfaces at vein tips and at the array centre comparison of the predictions with direct measurement of the proportion of vein to wall rock along the centreline of arrays demonstrates the applicability of the bridge rotation model which has been generalised to include the thickening or thinning of bridges during rotation

shear strain and volume change associated with sigmoidal vein arrays in the boda claystone abstract the boda claystone formation a potential host rock for high level nuclear waste in hungary contains numerous sigmoidal vein arrays with angular vein terminations these veins are excellent strain markers and can be used in the interpretation of palaeo environments sigmoidal vein arrays were investigated in the baf 2 well which penetrates the formation to depths above 900 m with nearly 100 core recovery the veins have antitaxial growth morphologies and are filled by calcite anhydrite and barite celestine phases to estimate the volume change in shear zones we used geometric mass balance and density change calculations these methods exhibit different volume loss rates the geometric approach indicates a large amount of volume loss whereas the isocon method and density change approach suggest that volume loss in the shear zones was less than 10 we propose that the volume loss in the shear zone occurred because of tectonic compaction vein shapes indicate formation under compressional stress fields based on their intersections with other vein generations the sigmoidal en echelon vein generation is determined to be one of the youngest structures in the well which probably formed during a late cretaceous compression event

kinematic analysis of an en echelon continuous vein complex an array of sigmoidal tension gashes from the idaho wyoming thrust belt changes laterally into a continuous vein detailed mechanical twin analysis was used to determine the strain variation in the optically and chemically homogeneous blocky calcite filling in the continuous portion of the vein complex the shortening axes are parallel to the vein boundary however the orientation of the shortening axes in the tip areas of the sigmoidal gashes are at an angle of approximately 35 to the vein boundary and are parallel to the trend of the tips twinning patterns in the central portions of the gashes record two principal strain axes of shortening of nearly equal magnitude with the maximum perpendicular to the vein trend everywhere in the vein complex the orientation of the maximum extension axis is parallel to the twist axis of the gashes the petrofabric strain results show that the vein filling has largely recorded local strains the pattern of variation in orientation of the principal strains in the vein complex is in close agreement with the theoretically determined stress distribution in similar structures our results show that the sigmoidal gashes were formed at the leading edge of a propagating vein and that the sigmoidal shape reflects changes in the local strain field rather than a remote shear the orientation of these local strains closely corresponds to the orientation of the local stresses

lithological and structural controls on the form and setting of vein stockwork orebodies at the mount charlotte gold deposit kalgoorlie the mount charlotte quartz vein gold deposit comprises a series of steeply plunging pipelike vein stockwork orebodies in massive metagabbro the orebodies are strata bound to the most differentiated unit of the host sill and are typically adjacent to major steeply dipping faults that cut the sill the stockworks have two sets of veins with a dihedral angle of about 50 that developed as hydraulic fractures filled simultaneously and are generally approximately equally developed veins crosscut major faults and parallel minor faults of two sets but are cut along reactivated fault surfaces the fault sets were inactive during mineralization and are neither veined nor are loci of zones of intense alteration rare faults of a third set are in part veined and are loci of zones of mineralization two interpretations of the stress regime during vein formation are based on different models of fracture formation for both stress regimes the major fault sets are relatively unfavorably oriented for slip or dilation and predicted movement vectors do not fit fault plane lineations the lack of fault activity during ore fluid flow promoted formation of vein stockworks at mount charlotte rather than shear zone or fault hosted veins fluid flow paths and orebody siting are controlled by stress guide effects due to the rheology of the host gabbro and by the three dimensional geometry of impermeable faults and of fault bounded blocks of rock

genesis of the batinah melange above the semail ophiolite oman abstract the batinah melange which overlies the late cretaceous semail ophiolite in the northern oman mountains comprises mostly sedimentary rocks of deep water facies alkalic lavas and intrusives all of continental margin affinities together with smaller volumes of semail ophiolitic and metamorphic rocks four intergradational textural types of melange can be recognized sheet melange has large 1 km intact sheets either with little intervening matrix or set in other melange types and with an organised sheet orientation fabric slab melange is finer textured 100 m and more disrupted block melange has smaller m blocks with some matrix and a weak to random block fabric clast melange is matrix supported rudite with a weak depositional clast fabric structural relationships particularly the absence of tectonic fabrics the decreasing strength of fragment fabrics with increasing fragmentation and the abundance of brittle fragmentation suggest that these melange types formed by either gravity driven sedimentary processes or superficial sliding or thrusting of individual rock slabs in the slab melange long sequences can be pieced together passing up from upper triassic mafic sub marine extrusives and sediments into radiolarian cherts hemipelagic and redeposited limestones and terminating in non calcareous radiolarities with mn deposits of early cretaceous age mafic sills are numerous these sequences can be matched with sub ophiolite rocks now exposed in fault corridors through the semail these sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying melange which then thins away from the corridors we argue that during late cretaceous emplacement over the arabian margin active fault corridors split the semail slab and acted as conduits up which sub ophiolite rocks were supplied to the ophiolite surface there the rocks were redisributed by superficial processes

true and apparent geometric variability of en echelon vein arrays abstract the wide range of vein array angles reported for en echelon vein arrays can be attributed to both true and apparent geometric effects a survey of published figures supports a continuum of orientations rather than distinct populations related to different fracture mechanisms the results of the survey indicate a modal veinarray angle of less than 45 which indicates area increase as described by a displacement vector oriented outward from the array a stereometric analysis of the effects of exposure plane orientation on the apparent vein array angle is developed examples of vein arrays from the nambucca slate belt of eastern australia exposed on two differently oriented surfaces are shown these examples include non profile exposures with apparent vein array angles which are 1 less than 2 greater than 3 approximately equal to and 4 of opposite shear sense to the true vein array angle