flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

geology of part of a long lived dynamic plate margin the coastal cordillera of north central chile latitude 30 51 31 s a complex composed of ultrabasic and basaltic lavas chert arkose and conglomerate was assembled in the coast of north central chile lat 30 30 31 s prior to 200 ma the character of and relationships between the rock formations exposed here are consistent with an autochthonous evolution of this part of chile in the last 200 ma three major episodes of deformation and metamorphism have been observed in this area the first episode f1 produced a compositional layering s1 and amphibolite facies metamorphism coeval with the intrusion of an extensive igneous complex between 220 and 200 ma a second episode of deformation f2 locally formed reverse faults and tight recumbent folds in s1 units in which f2 folds are well developed yield k ar and rb sr ages between 163 and 140 ma at between 140 and 126 ma upright open to tight folds f3 formed with sharp hinges and axial planes that strike north and dip steeply east structures of similar age style and orientation have been reported as far sou

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

syntectonic emplacement of the middle jurassic concon mafic dike swarm coastal range central chile 33 s abstract the concon mafic dike swarm cmds consists of basaltic to andesitic dikes emplaced into deformed late paleozoic granitoids during the development of the jurassic arc of central chile the dikes are divided into an early group of thick dikes 5 12 m and a late group of thin dikes 0 5 3 m two new amphibole 40ar 39ar dates obtained from undeformed and deformed dikes constrain the age of emplacement and deformation of the cmds between 163 and 157 ma based on radiometric ages field observations ams studies and petrographic data we conclude that the emplacement of the cmds was syntectonic with the jurassic arc extension and associated with sinistral displacements along the nw trending structures that host the cmds the common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously the early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep seated coastal rocks in the early cretaceous around 100 ma the remanent magnetization in late thin dikes is mainly retained by small amounts of low ti magnetite at high temperature and pyrrhotite at low temperature the magnetization in these dikes appears to be primary in origin paleomagnetic results from the thin dikes also indicate that the whole area was tilted 23 to the nnw during cooling of the cmds the nnw sse extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the middle late jurassic of northern chile pointing to major heterogeneities along the margin of the overriding plate during the mesozoic or differences in the mechanisms driving extension during such period

magma flow in the east greenland dyke swarm inferred from study of anisotropy of magnetic susceptibility magmatic growth of a volcanic margin summary volcanic passive margins vpms are characterized by large volumes of melt emplaced within the lithosphere during break up processes several data and a recently proposed conceptual model of volcanic margin development suggest that vpms are fed from localized crustal zones of magma storage underlying large polygenetic volcanoes localized above diapir like instabilities of the asthenosphere we investigated the magma flow pattern within the coastparallel dyke swarm of the east greenland vpm which is the only outcropping vpm over a distance of 125 km the 44 sampled dykes are representative of the successive families of intrusions igneous petrofabrics are constrained by the measurements of the anisotropy of magnetic susceptibility the magnetic fabrics are of medium to low anisotropy p 0 flow related fabrics are recorded in 75 per cent of the sampled dykes we infer the flow directions from the imbrication geometry of the magnetic foliation planes at the dyke margins and check the results by measuring the preferred orientation of plagioclase in thin sections cut in the magnetic principal planes due to probable fabric superposition the magnetic lineation represents the zone axis for the distribution of magnetic foliation plane we obtained 23 reliable flow directions that are predominantly horizontal and directed away from identified crustal reservoirs this flow pattern supports the proposed model of vpm growth and emphasizes the localized nature of the magma sources in the mantle the entire flood basalt sequence appears to have been fed by a restricted number of crustal reservoirs and associated dyke swarms

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

paleogene clockwise tectonic rotations in the forearc of central andes antofagasta region northern chile 1 for the central valley of northern chile antofagasta region a paleomagnetic analysis of data from 108 sites mainly in mesozoic and paleogene volcanic rocks has yielded stable remanent magnetization directions for 86 sites from these data we infer clockwise tectonic rotations of up to 65 within the forearc domain of the central andes the apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the incaic orogenic event of eocene early oligocene age a few paleomagnetic results obtained in neogene rocks do not show evidence of clockwise rotations hence the development of the bolivian orocline during late neogene time cannot be explained by simple bending of the whole margin these results demonstrate that tectonic rotations within the forearc and pre cordillera are key elements of early andean deformation which should be taken into account by kinematic models of mountain building in the central andes

displacement history of the atacama fault system 25 00 s 27 00 s northern chile in the cordillera de la costa of the central andes in northern chile mesozoic arc complexes are cut by a trench parallel strike slip fault system the atacama fault system brittle faulting in the atacama fault system is superposed on steeply dipping foliations in ductile shear belts between 25 s and 27 s the western part of the fault system was active in early cretaceous time as an upper amphibolite facies down to the east dip slip ductile shear zone in the eastern part of the fault system ductile deformation is of similar early cretaceous age but occurred under lower grade metamorphic conditions at the greenschist lower amphibolite facies transition the mylonites in the eastern part of the fault system were formed by sinistral strike slip displacement the dip slip and sinistral strike slip displacements are contemporary with the development of a magmatic arc and they imply that the tectonic environment in this part of the arc was transtensional the ductile deformation was partitioned spatially into a dip slip component associated with the emplacement of magmas and a sinistral strike slip component brittle fault zones in the el salado segment of the atacama fault system define large scale sidewall ripout structures subhorizontai slickenlines ripout asymmetry and s c type fabrics in fault gouge indicate that brittle deformation involved sinistral strike slip displacements the transition from ductile to brittle sinistral strike slip displacements may have occurred due to cooling in mid cretaceous time when the magmatic arc was abandoned

characteristics of salt inclusions in polar ice from dome fuji east antarctica 1 we studied the depth profile of the chemical properties number concentration and average diameter of micro inclusions in antarctic ice from dome fuji analysis of the data indicated that the distribution of inclusions was not significantly modified during ice sheet flow although a main constituent na2so4 10h2o changed into another solid phase in the deep ice in the chemical form sodium sulfate and magnesium sulfate were dominant in the interglacial and early glacial ice but were not detected in the glacial maximum ice instead the sulfate in the glacial maximum ice was mainly in the form of calcium sulfate this is strong support for the hypothesis that the neutralization regime of acid aerosols in the atmosphere and snowpack dramatically changed from chemical reaction with sea salt to that with dust during glacial maxima

jurassic and early cretaceous island arc volcanism extension and subsidence in the coast range of central chile more than 2000 km 3 of acid and 9000 km 3 of basic volcanic rocks formed during the jurassic and early cretaceous in the coast range of central chile between 32 30 s and 34 s these rocks which constitute the major part of an 15 km thick pile of alternately marine and continental deposits issued from volcanic arcs situated between a land area with paleozoic basement in the west and a marginal sea in the east asthenospheric upwelling led to extension and bimodal volcanism the volcanic products were deposited in intra arc basins subsiding at high rates 100 300 m m y the source of the magmas became more depleted with time due to an increase in degree of partial melting and their compositions were modified by subduction related fluids and contamination with a progressively thinner and younger crust the basic lavas are of high k calc alkaline to shoshonitic affinity chemically resembling the lavas found in some mature island arcs in the western pacific the extension and subsidence resulted in a low relief topography close to sea level in contrast with the present day convergent type of andean volcanism at the same latitude where calc alkaline intermediate lavas erupt from volcanoes at great height above a thick crust

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

the kinematic and geodynamic significance of the atacama fault zone northern chile abstract the atacama fault zone afz is the dominant feature in the structure of the north chilean coastal cordillera left lateral displacement took place along its system of longitudinal faults during the jurassic and early cretaceous this development was contemporaneous with arc magmatism and was later reactivated resulting in a steep normal fault strike slip movements along the afz consist of two sets of ductile shear zones of different ages one jurassic formed under amphibolite facies conditions the other early cretaceous with greenschist facies mylonites structural asymmetries point to a sinistral sense of shear in both sets the afz can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate trench linked strike slip fault the sinistral sense of shear is consistent with reconstructions of late jurassic to early cretaceous plate configurations in the se pacific

oblique opening and noncoaxial emplacement of the jurassic independence dike swarm california abstract several features of the independence dike swarm ids indicate that dikes opened obliquely to their walls in a sinistral sense the ids is largely latest jurassic in age 148 ma and has been traced for more than 600 km along its nw se strike from east central to southern california field and petrographic observations from the ids in the sierra nevada indicate that i some and perhaps all of the dikes initially opened perpendicular to their margins and then were sinistrally sheared and ii sinistral displacement occurred during and shortly after dike injection dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west side down reverse we suggest that the ids intruded dilatant fractures within a regional sinistral shear system the swarm is located within or along the eastern flank of the cordilleran subduction related magmatic arc the ids and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique late jurassic subduction

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

time interval between volcanism and burial metamorphism and rate of basin subsidence in a cretaceous andean extensional setting 40ar 39ar ages were obtained from basaltic flows belonging to a 9 km thick sequence generated in an extensional ensialic setting of an arc back arc basin type during the early cretaceous and presently exposed along the coastal range of central chile the basalts have been affected by very low to low grade burial metamorphism mostly under prehnite pumpellyite facies age values obtained from primary volcanic and secondary metamorphic minerals permit to quantify the time interval between volcanism and burial metamorphism a plateau age of 119 1 2 ma from primary plagioclase represents the best estimation of the age of the volcanism whereas adularia in low variance assemblages contained in amygdules gave a plateau age of 93 1 0 3 ma which is interpreted as the age of the metamorphism considering the p t conditions estimated for this metamorphic event the c 25 ma time interval between volcanic emplacement and prehnite pumpellyite facies metamorphism the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150 180 m ma

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time