3d fold growth in transpression abstract geological folds in transpression are inherently 3d structures hence their growth and rotation behavior is studied using 3d numerical finite element simulations upright single layer buckle folds in newtonian materials are considered which grow from an initial point like perturbation due to a combination of in plane shortening and shearing i e transpression the resulting fold growth exhibits three components 1 fold amplification vertical 2 fold elongation parallel to fold axis and 3 sequential fold growth perpendicular to axial plane of new anti and synforms adjacent to the initial fold generally the fold growth rates are smaller for shearing dominated than for shortening dominated transpression in spite of the growth rate the folding behavior is very similar for the different convergence angles the two lateral directions always exhibit similar growth rates implying that the bulk fold structure occupies an increasing roughly circular area fold axes are always parallel to the major horizontal principal strain axis max i e long axis of the horizontal finite strain ellipse which is initially also parallel to the major horizontal instantaneous stretching axis isa max after initiation the fold axes rotate together with max sequential folds appearing later do not initiate parallel to isa max but parallel to max i e parallel to the already existing folds and also rotate with max therefore fold axes do not correspond to passive material lines and hinge migration takes place as a consequence the fold axis orientation parallel to max is independent of convergence angle and viscosity ratio therefore a triangular relationship between convergence angle amount of shortening and fold axis orientation exists if two of these values are known the third can be determined this relationship is applied to the zagros fold and thrust belt to estimate the degree of strain partitioning between the simply folded belt and the bordering strike slip fault system

sm nd chronology and petrogenesis of mesosiderites we have obtained sm nd data from four mesosiderite silicate clasts including three clasts with a variety of textures from the vaca muerta type 1a mesosiderite and one gabbroic clast from the mt padbury mesosiderite the gabbroic vaca muerta pebble 12 and basaltic pebble 16 yield identical 147 sm 143 nd ages of 4 48 0 19 ae and 4 48 0 09 ae respectively while the highly recrystallized pebble 5 gives an age of 4 42 0 02 ae mt padbury yields an age of 4 52 0 04 ae all clasts show a correlation of 142 nd 144 nd with 147 sm 144 nd and provide unequivocal evidence for the presence of live 146 sm at the time of their formation calculated initial 146 sm 144 sm values range from 0 004 pebble 5 to 0 006 pebbles 12 16 and mt padbury and are generally consistent with the 147 sm 143 nd ages however discordance of whole rock leach and residue data and some disagreement between 146 sm 142 nd relative ages and 147 sm 143 nd absolute ages indicate small but significant disturbances to the sm nd systematics the ranges of ages and initial 146 sm 144 sm and 143 nd 144 nd values suggest that each of these silicate clasts underwent a separate protracted evolution on its parent body prior to mixing with metal textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite pebble 12 is extremely lree depleted probably a result of several melt extraction events whereas pebbles 5 and 16 and mt padbury have nearly chondritic sm nd with bulk ree concentrations higher than chondrites by factors of 5 to 15 in general the sm nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation preservation of diverse old ages and the presence of 146 sm imply that metal silicate mixing did not seriously alter the sm nd isotopic memories of these clasts we present a model for metal silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing we show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature depth of burial grain size and diffusion parameters application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing then the clasts must have been buried no deeper than 1 10 m in regolith during the initial high temperature cooling phase in order to prevent the sm nd systems from being extensively reset in order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal silicate mixing and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at 600 700 c the data from this study point to the following history for the mesosiderite parent body 1 differentiation of a silicate parent body within the first 50 m y of solar system history to form diverse parent magmas 2 emplacement of primitive and differentiated mafic magmas near the planetary surface and extensive differentiation in the near surface environment 3 formation and reworking of regolith breccias through impact gardening at the near surface of the body 4 mixing of molten fe ni metal with the regolith followed by rapid cooling 100 150 m y after the origin of the solar system 5 slow cooling from temperatures of 700 c to produce the observed nickel diffusion profiles in the iron phase 6 mild impact metamorphism and brecciation to obtain the much younger k ar ages and 7 recent collisions or perturbations sending mesosiderite fragments into earth crossing trajectories

petrology of alkaline lamprophyres from the coldwell alkaline complex northwestern ontario a suite of alkaline lamprophyre dikes emplaced in centers i and ii rocks of the coldwell alkaline complex is composed of camptonites with calcite ocelli camptonites with quartz macrocrysts amphibole camptonites monchiquites and sannaites the camptonites are characterized by phenocrysts of olivine aluminian pyroxene kaersutite and titanian ferropargasite set in a matrix of magnesian hastingsite augite plagioclase biotite magnetite sphene and minor nepheline quartz macrocrysts occur as corroded euhedral single crystals monchiquites are petrographically similar to the camptonites but are characterized by the presence of an isotropic groundmass sannaites contain aluminian and chromian diopside phenocrysts set in a matrix of ferroan pargasite aluminian diopside biotite albitized plagioclase and epidotized alkali feldspar major element compositions indicate the ocellar camptonites amphibole camptonites and monchiquites have affinities with alkali olivine basalt and that monchiquites and c

formation of mesosiderites by low velocity impacts as a natural consequence of planet formation mesosiderites are an enigmatic class of stony iron meteorites composed of roughly equal amounts of pyroxenitic and basaltic silicates and metallic iron nickel olivine is rare we suggest here that during the period of planet formation mesosiderites originated by the low velocity collisions of large metallic core fragments with the surface of a differentiated asteroid size body relative velocities of the asteroids native to this region were low 1 km s 1 because it was distant from massive protoplanets however a few differentiated asteroids were destroyed by high velocity collisions with interlopers perturbed by protoplanets into highly eccentric orbits these collisions reduced mantles and crusts to small silicate fragments but left cores in the form of large durable metal fragments mesosiderite like pyroxenite basalt metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid the metal contents of other regolithic regions remained low most of the olivine in mesosiderite and howardite breccias we attribute to the mantles of the disrupted parent bodies the low 12 20 mg g 1 olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths

influence of pre existing salt diapirs on 3d folding patterns abstract the 3d detachment folding instability gives rise to a wide variety of fold shapes e g from dome shape structures to long en echelon or straight anticlines as a result of interactions between growing fold segments the 3d growth of these folds as well as the wavelength and lateral propagation of folds is controlled by the physical parameters of a detachment layer and its overburden however the existence of initial heterogeneities such as pre existing salt plugs within the sedimentary cover might affect fold development as well we use numerical modeling to investigate how the fold pattern is affected by pre existing salt structures high resolution 3d folding simulations with and without pre existing salt structures were performed in which we varied the shape height and spacing of pre existing diapirs in a first geometric setup we employed a multilayer setup and synthetic diapir distributions in order to study the influence of diapir spacing on fold spacing and patterns in a second geometric setup we use a diapir distribution that fits the observed exposed diapir distribution in the southeastern zagros results show that the presence of diapirs does not considerably change the wavelength of the folds which is in all cases close to the dominant folding wavelength that develops in the absence of diapirs yet the presence of pre existing structures speeds up the folding instability in those locations and also affects folding patterns as the diapirs localize the initial deformation by accommodating folding above them which results in the diapirs being located in the core of the folds if diapir spacing is much smaller than the dominant folding wavelength diapirs are located in different structural positions such as fold synclines or flanks

open shear fractures at depth in anticlines insights from the kuqa foreland thrust belt tarim basin shear fractures that close under compressive stress regime were described by previous workers here we show the existence of hinge parallel shear opening mode fractures at a certain depth within an anticline structure through core observations formation micro imager fmi log interpretations and production data studied reservoir is a tight sandstone body of cretaceous age which is emplaced in ks2 anticline with depth ranging between 6000 and 8100 m we explore the mechanism of hinge parallel shear fractures by stereographic technique and backtilting as well as show field evidences for opening displacements these shear fractures mainly consist of vertical tension set moderate dipping shear set steeply dipping shear set and sub horizontal shear set crosscutting relationships and surface textures suggest that although the moderate dipping shear set is older and maybe sealed by cements it may have been opened by local stress due to outer arc extension through folding calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress our work documents shear fracture openings in the anticline at depth and fluid flow in subsurface reservoirs

geometry and controls on fracturing in a natural fault bend fold rosario field maracaibo basin venezuela the rosario oil field lies between the perija mountain front and lake maracaibo and produces from fractured cretaceous carbonates and tertiary clastics we interpret the structure as a detached fault bend fold which ramps through cretaceous cogollo and la luna carbonates and flattens into an upper detachment at the base of the upper cretaceous colon shale the structural relief formed primarily during the mid miocene and younger seismic and well control on the three dimensional geometry illustrates the effects of 1 lithology and displacement variation on fold geometry 2 an oblique footwall ramp on hangingwall faulting and 3 fold curvature on fracturing and hydrocarbon production fold geometry at different structural levels is strongly controlled by lithology stiff cogollo and la luna carbonates exhibit kink style folding above the upper fault bend the weak colon shale decouples the faulted carbonates from the concentrically folded tertiary clastics regions of enhanced faulting and fracturing of cretaceous carbonates are a function of structural position we observe normal faults in the hangingwall where the strike of the footwall ramp changes from n20 degrees e to n65 degrees e fold curvature highlights fold hinges yet distributed faulting is seismically imaged in the forelimb suggesting that rocks fracture as they migrate through the ramp upper flatmore fault bend production rates are higher near the forelimb relative to the flat crestal region less

fracture zones constrained by neutral surfaces in a fault related fold insights from the kelasu tectonic zone kuqa depression abstract stress variation associated with folding is one of the controlling factors in the development of tectonic fractures however little attention has been paid to the influence of neutral surfaces during folding on fracture distribution in a fault related fold in this study we take the cretaceous bashijiqike formation in the kuqa depression as an example and analyze the distribution of tectonic fractures in fault related folds by core observation and logging data analysis three fracture zones are identified in a fault related fold a tensile zone a transition zone and a compressive zone which may be constrained by two neutral surfaces of fold well correlation reveals that the tensile zone and the transition zone reach the maximum thickness at the fold hinge and get thinner in the fold limbs a 2d viscoelastic stress field model of a fault related fold was constructed to further investigate the mechanism of fracturing statistical and numerical analysis reveal that the tensile zone and the transition zone become thicker with decreasing interlimb angle stress variation associated with folding is the first level of control over the general pattern of fracture distribution while faulting is a secondary control over the development of local fractures in a fault related fold

the oldest zircons in the solar system abstract here we report the occurrence chemistry and u th pb isotopic systematics of three meteoritic zircon assemblages two from the vaca muerta mesosiderite and one from the simmern h5 chondrite one of the vaca muerta zircons occurs in the mesosiderite proper the other in a eucritic clast associated with chromite ilmenite and tridymite whereas the simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar like terrestrial zircons the meteoritic zircons are enriched in the heavy rare earth elements but unlike terrestrial zircons they do not show a positive ce anomaly this feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment under oxidising conditions ce4 which can substitute for zr4 in the zircon structure is stabilized whereas under relatively reducing conditions ce3 is stable the zircon from simmern has depletions in the relative abundances of tm and yb a characteristic of volatility fractionation of the ree the u concentrations of the two vaca muerta zircons are quite different at 1 and 50 ppm respectively both zircons have extremely low initial lead and give radiogenic ages that are the same albeit with relatively large errors on vm 1 zircon because of its much lower u concentration and hence radiogenic pb concentration the mean of the two analyses made on vm 2 zircon is concordant with a207pb 206pb age of 4563 15 ma 2 the simmern zircon has an exceptionally low u concentration of around 180 ppb and only a poorly constrained 207pb 206pb age of 4100 700 ma could be obtained

geometrical modeling of fault related folds a pseudo three dimensional approach abstract area balanced cross sections of ramp anticlines and tip line folds in thin skinned fold thrust belts can be quantitatively constructed using plane strain fault bend and fault propagation fold models cross sections alone however are somewhat inadequate for interpreting along strike changes in thrust related fold geometry complex interplay between along strike changes in fault slip and fault shape determines a range of geometric configurations which are difficult to visualize without the aid of quantitative three dimensional models we extend conventional two dimensional folding theories into the third dimension by allowing for continuous variations in fault slip and fault shape along strike geometric equations relating map view angles between fold axial surface traces are derived for a case assuming a uniform fault geometry and an along strike displacement gradient the resulting folds possess no unique fold axis but rather are characterized by multiple fold hingelines each corresponding to bends in the fault for typical displacement gradients observed in the field analysis of the models suggests that discernible differences exist between map patterns of fault bend folds and fault propagation folds whereas little difference exists between map patterns of similar and parallel fault bend folds map view angles between axial surface traces for parallel fault bend folds and fault propagation folds are a function of both ramp cutoff angle and fault slip whereas these same angles for similar fault bend folds are solely a manifestation of fault slip examples assuming uniform shortening and continuously varying fault geometry along strike are also presented fault geometry is varied along strike to simulate oblique and lateral ramps depending upon model type and fault shape parameter modified resulting folds exhibit either cylindrical or non cylindrical fold geometries closure is generated by a decrease in fold amplitude where there is a decrease in ramp cutoff angle along strike or where a fault laterally cuts up section in either instance closure develops without varying shortening along strike model map patterns resemble actual thrust related fold geometries and can be compared to maps of real structures to help predict subsurface geology

numerical analysis of fold curvature using data acquired by high precision gps the maximum minimum gaussian and mean curvatures of a folded bedding surface are calculated from a high resolution data set these data were collected using real time kinematic rtk gps and processed using commercially available software the curvature parameters are calculated analytically from the first and second fundamental forms of the folded surface matrix algebra makes this method efficient for large data sets contoured maps of the curvature parameters can then be used as basemaps for the interpretation of other structural information such as fracture densities and orientations this method provides precise analysis of folded surfaces in three dimensions and the data can be incorporated into larger scale data sets obtained from seismic surveys

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

petrogenesis of mesosiderites i origin of mafic lithologies and comparison with basaltic achondrites abstract new petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites combined with existing petrologic and trace element data have served as a basis for interpretation of the petrogenesis of mesosiderite clasts compared with the basaltic achondrites the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrillite and more commonly contain augite as a late magmatic phase their pyroxenes tend to be more mgo rich and have lower fe mn ratios which are positively correlated with the fe mg ratio some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase the basaltic clasts commonly show super chondritic eu sm ratios and slight lree depletions the cumulate gabbro clasts are extremely depleted in lil have very high eu sm ratios and high lu sm ratios all of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt cumulate gabbro metal source region near the parent body surface reducing agents p c s from the metallic fraction reduce feo out of the silicate melt and result in the positive correlation of fe mn with fe mg observed in pyroxene analyses and the modal abundances of tridymite merrillite and augite xenocrystic plagioclase is a natural consequence of remelting a cumulate gabbro rich source as are the unique ree patterns observed

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

outcrop scale tomography insights into the 3d structure of multiphase boudins abstract we analyze the 3d geomorphology of a boudinaged amphibolite layer encased in marble using meter scale tomography by serial sectioning high resolution imaging and 3d reconstruction of a 2 m block from a quarry in the high grade core of the naxos core complex local uncertainties in the model are resolved by dissolution of marble in selected slabs yielding a large highly accurate 3d model results show five generations of brittle extension of the amphibolite in ductile marble although examination of the boudinaged layer on the outside of the block only suggested one generation most prominent are the rotated domino boudins where part of the dilatancy between domino boudins can be shown to be due to later reactivation shear fractures between domino boudins have relatively large throw gradients indicating ductile rupture layer parallel stretch as measured by reconstruction of domino boudins decreases with amphibolite layer thickness although the layer has undergone the same bulk stretch everywhere this suggests that with decreasing layer thickness deformation becomes delocalized offering tantalizing glimpses of rheology under metamorphic conditions

dynamic constraints on the crustal scale rheology of the zagros fold belt iran thin skinned fold and thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer the resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner it is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales here we use the zagros mountains of iran as a case study as it is one of the most spectacular well studied thin skinned fold and thrust belts in the world both analytical and numerical models are employed to study what con trols fold spacing and under what conditions folding dominates over thrusting the models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover deformation is dominated by thrusting which is inconsistent with the data of the zagros fold belt if we instead take into account additional decollement layers that have been documented in the fi eld a switch in deformation mode occurs and crustal scale folding is obtained with the correct spacing and time scales we show that fold spacing can be used to constrain the friction angle of the crust which is 5 the zagros fold belt this implies that on geological time scales the upper crust is signifi cantly weaker than previously thought possibly due to the effect of fl

foreland basement involved structures basement involved structures commonly occur as long irregular chains of uplifts in foreland basins these structures commonly contain significant hydrocarbon accumulations with most major fields located on the broad crests of these structures the search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip and gently dipping frontlimbs and backlimbs fault slip in the basement is usually accommodated in the cover by a triangular widening upward deformation zone on the forelimb with the nature of deformation controlled primarily by the mechanical stratigraphy if the cover contains interlayered competent and incompetent units the incompetent units are characterized by significant penetrative deformation whereas the competent units are faulted after a relatively small amount of penetrative deformation depending on the competency contrast between the basement and cover the nature of basement and the physical conditions of deformation the deformation zone may also propagate downward into the basement gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault respectively many basement faults are characterized by a number of synclinal fault bends within the basement which result in long and gently dipping backlimbs forelimb panels are related to anticlinal bends that typically occur at the basement cover interface as well as at one or more locations in the sedimentary cover case studies of well constrained examples of structures from the bighorn and uinta basins and the central basin platform demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy these models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data

u pb and hf w dating of young zircon in mesosiderite asuka 882023 mesosiderites are unique stony iron meteorites composed of eucrite like silicates and fe ni metals their formation including silicate metal mixing and metamorphisms provide important insights into early planetary processes in the inner solar system this report describes the first in situ u pb and hf w dating of zircon in a mesosiderite asuka 882023 the u pb 4502 75 ma and hf w 4532 8 5 7 10 5 ma ages may represent timing of the zircon formation which is considerably younger than crustal differentiation of the parent body this evidence combined with earlier studies of chronology implies that mesosiderites were reheated at 4530 4520 ma clearly after the silicate metal mixing

age of tertiary volcanic rocks on the west greenland continental margin volcanic evolution and event correlation to other parts of the north atlantic igneous province radiometric ages for undated parts of the volcanic succession and intrusions in west greenland were obtained by the 40ar 39ar incremental heating method acceptable crystallization ages were obtained for 27 samples combined with published results the new data provide a volcanic stratigraphy correlatable throughout the nuussuaq basin the thick onshore volcanic pile consists of four widespread formations 62 5 61 ma picrites vaigat formation 61 60 ma depleted basalts malig t formation and the hellefisk 1 well 60 58 ma less depleted basalts svartenhuk formation and 56 54 ma enriched basalts naqerloq formation two local successions comprise 53 5 ma alkali basalts erqua formation and 38 7 ma transitional basalts talerua member a central volcano developed on ubekendt ejland leading to the sarq ta qaq gabbro granophyre intrusion at 57 55 ma pre break up volcanism took place further south as early as 64 63 ma the offshore volcanic succession most probably comprises the known onshore succession plus some younger lavas the change in spreading direction near the paleocene eocene boundary took place west of the nuussuaq basin c 56 2 ma some tectonomagmatic events are correlatable across the entire north atlantic igneous province a quiescent 58 56 ma period correlates with similar periods in east greenland and the faroes and the naqerloq formation is coeval with the eocene basalts in east greenland the paleocene and eocene tholeiitic basalts are distinguishable chemically in early eocene time mantle typical of the iceland plume seems to have extended beneath the whole west greenland margin as well as the central east greenland margin

thermal history of mesosiderites revisited abstract the results of a fission track study of uranium rich merrillite crystals in a number of mesosiderites are clearly incompatible with cooling rates as low as 0 1 c myr which have previously been inferred for these meteorites classical metallographic techniques thus appear to give erroneous results when applied to this type of meteorites there appears to be no reason to advocate exotic scenarios to explain the thermal history of mesosiderites

lamprophyric lavas in the colima graben sw mexico the colima graben located in sw mexico is one of three grabens which intersect about 50 km ssw of guadalajara forming a triple junction the 90 km long 20 60 km wide colima graben represents a n s rift of the e w trending mexican volcanic belt since the early pliocene the colima graben has served as a locus for the eruption of alkaline lavas the most recent of which are basanites and minettes erupted from late pleistocene cinder cones luhr and carmichael 1981 in this paper we report on older alkaline lavas which crop out in the graben s walls these rocks include phlogopite and hornblende bearing lamprophyres a phlogopite kalsilite ankaratrite and high k andesites these lavas crop out throughout the colima graben area and are intimately associated with calc alkaline lavas in the field compared to these the alkaline rocks are strikingly enriched in the incompatible elements particularly ba sr p and the lree unlike the younger late pleistocene alkaline cinder cone lavas most of the graben wall lamprophyres and the high k andesites represent magmas that appear to have undergone significant evolution since their generation including fractionation crustal contamination and possible magma mixing least squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres the occurrence of these alkaline lavas in an active calc alkaline volcanic arc is unusual and we suggest that they are a manifestation of the rifting processes which produced the colima graben

thermal and shock history of mesosiderites and their large parent asteroid to elucidate the geological evolution of the mesosiderite stony iron meteorites and their 3 4 3 8 gy 40ar 39ar ages we have investigated their shock and thermal histories we have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 gpa shock stage s1 s2 three mesosiderites contain a tiny fraction 0 1 overall of mineral fragments that were shocked to shock stage s3 s6 levels the uniformity of shock features within all fragments shows that these fragments were not shocked in situ our shock data for mesosiderites the absence of evidence of 3 6 3 9 gy impact melt the shock history of impact heated ordinary chondrites and the difficulty in quantitatively removing ar in an impact event all suggest that the mesosiderite parent body did not suffer a major impact event 3 6 3 9 gy ago metallographic cooling rates of 0 03 c my at 400 c were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and fe ni p phase diagram cooling rates of 0 01 c my at 425 325 c were estimated from published compositional data for kamacite grains in four mesosiderites these two techniques and four other semiquantitative metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite we infer that cooling rates at 400 c were 0 02 0 03 c my and certainly less than 0 5 c my the inferred cooling rate is too slow to allow ar closure before 4 gy all of the shock thermal and age data for mesosiderites are consistent with slow cooling at depth 1 my after metal and silicate were mixed around 4 4 gy ago thermal models indicate that the mesosiderites probably cooled in an asteroid some 200 400 km in radius

displacement and deformation associated with a lateral thrust termination southern golden gate range southern nevada u s a abstract the golden gate thrust offers an excellent opportunity to study displacement and deformation at a lateral thrust tip slip across this e vergent thrust is uniformly about 2 4 km for the southern 2 km of exposed strike length and dies out to zero in the northern 5 km constant displacement and uniform hangingwall structure along the southern one third of exposed strike length suggest that along strike displacement variation and consequent deformation are localized near the lateral tip structures located at and north of the lateral tip are consistent with transpression and later uplift as slip accumulated on the thrust fold and fault orientations and calcite twinning strain record possible transpression north of the lateral tip e w striking normal faults at the thrust tip record extension that is interpreted to be the result of the uplift and translation of a structurally continuous rigid block located north of the normal faults this block is connected to and was uplifted with the crest of the hangingwall anticline and was separated by the normal faults from thrust related folding to the south the main hangingwall structure is an anticline that in southern exposures trends n s parallel to the thrust but bends westward and opens into a box fold to the north the hangingwall anticline superficially resembles a classic fault propagation fold formed by a migrating ductile bead however we are forced to reject the ductile bead hypothesis for the fold because 1 the geometry of the hangingwall anticline far from the thrust tip cannot have evolved from the geometry at the thrust tip and 2 twinned calcite strain data from along the thrust suggest that strain was homogeneous and coaxial rather than inhomogeneous and progressively non coaxial as predicted for a migrating ductile bead the three dimensional structural and kinematic relations at the lateral tip of the golden gate thrust appear to be the result of deformation around a pinned tip although the reason that the tip was pinned at its present location is unclear we interpret the thrust to have propagated relatively quickly to its ultimate extent with little initial displacement indicating that it was easier to fault the rocks than to fold them

re os systematics in pallasite and mesosiderite metal re os data on pallasite metal yield a sufficient range in re os to permit the determination of a whole rock isochron if pallasites from different groups are considered together the results are indistinguishable from the data obtained on groups iab iiab iid iiiab iva and ivb iron meteorites pallasite data were obtained on samples of the main group which have been considered related to group iiiab irons and on samples of the distinctive pallasites eagle station finmarken and marjalahti it appears that iron meteorites and pallasites crystallized within a time interval of 20 ma this result is consistent with the evidence from the short lived chronometers 107 pd 107 ag and 53 mn 53 cr it is also consistent with the evidence from 182 hf 182 w that the time interval over which molten feni metal from different iron meteorites equilibrated with and became segregated from silicates was 10 ma the well defined 187 re 187 os whole rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated feni metal and fes and then all crystallized within a 20 ma time interval this conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic the short time scale requires that the irons and pallasites were formed in small parent planets of 10 km radius or near the surface of larger bodies in contrast to the pallasites we find that feni samples from mesosiderites have a narrow range in re os so that it is not possible to determine a whole rock isochron for these samples the mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi stage evolution based on sm nd data on mesosiderite silicate clasts the re os results on mesosiderite metal are compatible with the model of melted or partially molten feni cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 ma after early formation and the feni was splashed onto the surfaces of other small differentiated planetesimals

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

modeling of wind gap formation and development of sedimentary basins during fold growth application to the zagros fold belt iran mountain building and landscape evolution are controlled by interactions between river dynamics and tectonic forces such interactions have been extensively studied however a quantitative evaluation of tectonic geomorphic feedbacks which is imperative for understanding sediments routing within orogens and fold and thrust belts remains to be undertaken here we employ numerical simulations to assess the conditions of uplift and river incision necessary to deflect an antecedent drainage network during the growth of one or several folds we propose that a partitioning of the river network into internal endorheic and longitudinal drainage arises as a result of lithological differences within the deforming crustal sedimentary cover using examples from the zagros fold belt zfb we show that drainage patterns can be linked to the non dimensional incision ratio r between successive lithological layers corresponding to the ratio between their relative erodibilities or incision coefficients transverse drainage networks develop for uplift rates smaller than 0 8 mm yr 1 and low incision ratios 10 20 parallel drainage networks and the formation of sedimentary basins occur for large values of incision ratio r 20 and uplift rates between 1 and 2 mm yr 1 these results have implications for predicting the distribution of sediment depocenters in fold and thrust belts which can be of direct economic interest for hydrocarbon exploration they also put better constraints on the fluvial and geomorphic responses to fold growth induced by crustal scale tectonics copyright 2016 john wiley sons ltd

samarium neodymium evolution of meteorites we have obtained sm nd data on two differentiated meteorites ibitira a eucrite with distinct basaltic texture and with evidence of crystallization and morristown a group 3a mesosiderite as well as on acapulco an unclassified meteorite with chondritic chemical composition and a highly recrystallized texture we have demonstrated the presence of in situ decay of short lived 146 sm in these meteorites with initial abundance of 146 sm 144 sm from 0 009 to 0 007 for the different meteorites precisely defined 147 sm 143 nd internal isochrons were obtained yielding ages of 4 60 0 03 ae for acapulco 4 47 0 02 ae for morristown and 4 46 0 02 ae for ibitira the corresponding initial 143 nd 144 nd values obtained were 0 2 0 9 2 1 0 7 and 1 6 0 8 u epsilon units detailed consideration of the coupled 147 sm 143 nd and 146 sm 142 nd systematics and of the elevated initial 143 nd 144 nd values indicates the presence of inconsistencies in the data for the two sm nd parent daughter pairs in spite of the precise isochrons we present a model which provides an interpretation of the paired sm nd systematics in terms of an impulsive disturbance the abundance of phosphate with large concentrations of rees controls the ree economy plagioclase with low ree concentrations is assumed to be susceptible to exchange pyroxene is considered to be undisturbed because of the low diffusion coefficients of trace elements the experimental results can be reasonably explained by this model the results indicate that three meteorites studied some with very low ree concentrations including a mesosiderite are relatively ancient objects formed within the first 50 to 100 m y of the solar system by planetary differentiation and impact processes and were subjected to late metamorphism the time of late metamorphism is substantially younger than 4 4 ae and possibly as recent as 3 9 ae this metamorphism has resulted in partial very limited reequilibration between plagioclase and phosphate resulting in the observed shifts in initial 143 nd 144 nd sm nd model ages and the 142 nd 144 nd abundances but preserving the major temporal structures of early original formation and later disturbances these results show the need for care in the interpretation of the parent daughter systematics even when they appear well behaved and the mineral phases are viewed as highly resistant to mild metamorphism

the estherville mesosiderite upb rbsr and smnd isotopic study of a polymict breccia abstract a systematic u pb sm nd and rb sr isotopic study shows that the estherville mesosiderite was formed between 4 56 and 4 43 ga observed isotopic heterogeneity is in agreement with multiple generations of meteoritic impacts described in other mesosiderites at least part of the estherville silicate fraction was formed early in solar system history as indicated by the pb pb 4555 35 ma u pb 4560 31 ma rb sr 4542 203 ma and sm nd 4533 94 ma ages mesosiderites therefore present not only petrological but also geochronological similarities with eucrites the pb isotopic composition of the metal phase plots on the same isochron as the silicates indicating formation and subsequent mixing with silicates early in the history of the solar system this is consistent with previous observations indicating that iron was reduced during the silicate magmatic stage most likely a consequence of mixing with metal in addition to these more ancient portions of the estherville breccia other parts were formed later as suggested by the pb pb 4422 50 ma and u pb 4437 11 ma ages observed in a second group of leaches and residues this age is similar to some cumulate eucrite ages and may represent the formation of a second mesosiderite component the sm nd and the rb sr ages obtained on estherville show large errors that may be a consequence of the mixing between the 4 56 and 4 43 ga endmembers estherville like most mesosiderites was affected by a major heating event around 3 5 3 7 ga as shown by the ar ar ages bogard et al 1990 this heating event partially disturbed the rb sr isotopic system rb sr metamorphic ages range between 3 81 and 4 08 ga

impact versus internal origins for mesosiderites mesosiderites are very intimately mixed stony iron breccias including little recrystallized and highly recrystallized varieties i iii and a subgroup iv with melted matrix all mesosiderite breccias contained some material that was hot as indicated by pigeonite that inverted at 1150 c subgroup iv contained silicate and metal liquids at temperatures of at least 1300 c and 1350 c respectively cooling rates are not rigorously defined but are approximately 1 year in the high temperature range and 1 m y below 500 c the latter cooling requires a depth of 40 km in an asteroid with a radius of 400 km or less if porous low conductivity surface material is present cratering with metal projectiles appears incapable of generating mesosiderites a normal impact produces rapidly cooled metal poor breccias impacts on a crust in which metal silicate segregation had taken place could generate stony iron breccias but burial under an insulating blanket would be necessary to obtain slow cooling disruption and reassembly of two colliding bodies and accretion of differentiated planetesimals could form stony iron breccias at depth but as with the previous two models the easiest way to explain the low olivine of mesosiderites is to postulate derivation from olivine poor chondritic material two internal processes turbulent metal crust mixing during core formation and crustal blocks sinking into the core provide a deep location and withhold olivine from the breccia however these mechanisms do not explain how the silicate and metal liquids in subgroup iv otherwise interpreted as impact melt rocks could have failed to segregate in a deep slowly cooled environment no single model for mesosiderite origins is wholly satisfactory

39ar 40ar dating of mesosiderites evidence for major parent body disruption 4 ga ago 39ar 40ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of ar by one or more thermal events less than 4 0 ga ago three samples gave release patterns of constant age whereas the other samples typically showed 200 400 ma increases in 39ar 40ar age with increasing gas extraction temperature these stepwise releases indicate ages of 3 4 3 8 ga 1 ga 109 a averaged ages for individual meteorites are 3 45 3 82 ga no substantial evidence exists in the temperature releases for 39ar 40ar ages older than 4 0 ga and the ar ages appear to be in contradiction to 244pu fission track ages previously reported for a few mesosiderites the ar ages probably date the event that caused disturbance in previously reported rb sr sm nd and pb pb ages of estherville and other mesosiderites data on release of ar as a function of extraction temperature were used in arrhenius plots to estimate activation energies for ar diffusion of 18 52 kcal mol these values were then used to calculate closure temperatures against ar loss of 2 340 c and 180 600 c for assumed cooling rates of 1 c ma and 1 c a respectively the ar data suggest that for certain assumed models of mesosiderite formation the 39ar 40ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of 1 c ma models which call for metal silicate mixing 4 5 ga ago followed by cooling at 1 c ma from temperatures 1150 c seem unlikely single event models which produce the silicate metal mixture at high temperatures rapidly cool the objects to temperatures of 500 c followed by cooling of 1 c ma and are not consistent with the ar data unless the thermal event occurred 500 c and left them deeply buried to undergo slow cooling

the uthpb age of equilibrated l chondrites and a solution to the excess radiogenic pb problem in chondrites abstract u th and pb analyses of whole rock and troilite separates from seven l chondrites suggest that the excess radiogenic pb relative to u and the large variations in pb pb model ages commonly observed in chondritic meteorites are largely due to terrestrial pb contamination induced prior to analyses using the pb isotopic composition of troilite separates to calculate the isotopic composition of the pb contaminants the whole rock data have been corrected for pre analysis terrestrial pb contamination two approaches have been used 1 the chondrite troilite apparent initial pb isotopic compositions were used to approximate the mixture of indigenous intial pb and terrestrial pb in the whole rock sample and 2 a single stage concordant model was applied using the assumption that the excess radiogenic pb in these samples was terrestrial data for l5 and l6 chondrites yield a4551 7my age using the former correction and a4550 5my age using the latter one corrected data for one l4 chondrite tennasilm yield a4552 13my age which is indistinguishable from that of the l5 l6 chondrites however the other l4 chondrite bjurbole yields a4590 6my th u pb data suggest that this older age may be an artifact of the correction procedure and that some of the discordancy of the bjurbole data is the result of either a recent geologic disturbance to the u th pb system or to terrestrial u loss some aliquots of the l5 l6 chondrites also show small amounts of discordancy 10 which are not easily attributable to terrestrial pb contamination the data from the l5 l6 chondrites and tennasilm suggest that there are no more than 15my differences in the ages of l24 l6 chondrites

mapping folds and fractures in basement and cover rocks using uav photogrammetry cape liptrap and cape paterson victoria australia abstract brittle and ductile deformation of alternating layers of devonian sandstone and mudstone at cape liptrap victoria australia resulted in upright folds with associated fold accommodation faults and multiple fracture sets structures were mapped at the fold stack locality at cape liptrap using high resolution aerial photographs acquired by a digital camera mounted on an unmanned aerial vehicle uav subsequent photogrammetric modelling resulted in georeferenced spatial datasets point cloud digital elevation model and orthophotograph with sub cm resolution and cm accuracy which were used to extract brittle and ductile structure orientation data an extensive dataset of bedding measurements derived from the dense point cloud was used to compute a 3d implicit structural trend model to visualise along strike changes of devonian tabberabberan folds at the fold stack locality and to estimate bulk shortening strain this model and newly collected data indicate that first generation shallowly south southwest plunging upright folds were gently refolded about a steeply plunging subvertical fold axis during a devonian low strain north south shortening event this also led to the local tightening of first generation folds and possibly strike slip movement along regional scale faults in order to distinguish fractures associated with devonian compression from those that formed during cretaceous extension and later inversion we compared the five fracture sets defined at cape liptrap to previously mapped joints and faults within the overlying sedimentary cover rocks of the cretaceous strzelecki group gippsland basin which crop out nearby an east southeast trending fracture set that is not evident in the strzelecki group can be linked to the formation of devonian folds additionally hinge line traces extracted from the fold stack dataset are aligned parallel to a dominant fracture set within the overlying cover sediments this suggests that basement structures folds and coeval parallel faults have an important influence on fault and joint orientations within cretaceous cover rocks

fracture abundance and patterns in the subandean fold and thrust belt devonian huamampampa formation petroleum reservoirs and outcrops argentina and bolivia abstract opening mode fractures in devonian sandstones in outcrop and in several fields in the subandean ranges of southern bolivia and northern argentina compose two fracture sets i and ii that strike west northwest and north northeast respectively abundant set i fractures are at a high angle to local andean structural trends and set ii is aligned with fold axes crosscutting relations and quartz textures in fractures suggest that although set i is locally older these sets may have opened partly contemporaneously sets comprise both macroscopic fractures and more abundant millimeter scale microfractures fractures with opening displacement of less than 0 1 mm are typically sealed with quartz but wider fractures are lined with quartz and contain connected fracture porosity microfractures are more abundant than macrofractures and size distributions can be interpreted to approximate power laws microfracture strain is an efficient method of quantifying fracture abundance both sets record small fracture strains of 0 00016 0 0083 in backlimbs set i strain is higher in brittle quartz rich sandstones set ii strain varies markedly and is generally high in hinges and steep forelimbs of asymmetric anticlines for individual samples set i ii strains in fold hinges and forelimbs are comparable consistent with concurrent set i ii growth open fractures that could augment permeability are present both on and off structure but microfractures document fracture abundance that varies with structural position and sandstone quartz content that could account for variations in production outcomes

the lamprophyres of ubekendt ejland west greenland products of renewed partial melting or extreme differentiation the late tholeiitic lavas of southwestern ubekendt ejland 63 54 ma are intruded by lamprophyre dykes 33 ma the compositional and temporal differences would seem to preclude any genetic relationship between the two igneous episodes competing models of renewed partial melting versus extended batch fractionation are considered to explain the origin of the lamprophyres mathematically modelled lamprophyres produced by batch fractionation are very close to observed lamprophyre compositions and this combined with other temporal and spatial evidence suggests that renewed partial melting of the mantle is not the best way to explain such bimodal suites

stony iron meteorites history of the metal phase according to tungsten isotopes abstract the global composition of the early solar system is thought to be roughly chondritic in terms of refractory components and this means that metal and silicate should be present together in early planetesimals to fully understand the metal silicate differentiation process within the eucrite parent body epb it is important to try and identify the metal reservoir that is complementary to the silicate part the isotope 182 of tungsten w a siderophile element is partly formed from the decay of 182 hf and w isotopes are useful for examining metal silicate segregation the w isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites however mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities therefore we undertook the analysis of the metal phase of these stony irons here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body bodies and to assess possible relationships with eucrites all stony iron metals are depleted in radiogenic tungsten by 1 3 to 4 2 e units relative to the terrestrial standard while chondrites for comparison are depleted by 1 9 e units in addition to w isotopic heterogeneity from one stony iron to another there is also w isotopic heterogeneity within individual meteorites a formation model is tentatively proposed where we show that mesosiderites pallasites and eucrites could possibly come from the same parent body several hypotheses are discussed to explain the isotopic heterogeneity the production of cosmogenic tungsten the in situ decay of hafnium present in inclusions and tungsten diffusion processes after metal silicate mixing during the cooling of the meteorites the two latter hypotheses provide the best explanation of our data

late miocene deformation kinematics along the nw zagros fold thrust belt kurdistan region of iraq constraints from apatite u th he thermochronometry and balanced cross sections

lu hf and sm nd evolution in lunar mare basalts lu hf and sm nd data for mare basalts combined with rb sr and total ree data taken from the literature suggest that the mare basalts were derived by small or 10 degrees of partial melting of cumulate sources but that the magma ocean from which these sources formed was light ree and hf enriched calculated source compositions range fromm lherzolite to olivine websterite nonmodal melting of small amounts of ilmenite or 3 in the sources seems to be required by the lu hf data a comparison of the hf and nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the epsilonhf epsilonnd ratios of low ti mare basalts are much higher than in terrestrial oceanic basalts the results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates alternatively the results may imply that the terrestrial mantle has evolved in two or more stages of evolution and that the net effect was depletion of the mantle during the first approx 1 3 b y followed by enrichment during the last 1 2 b y or simply that there is a difference in lu hf crystal liquid partitioning relative to sm nd between the lunar and terrestrial mantles

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

fracture patterns in successive folding in the western sichuan basin china abstract a detailed field survey of fractures was carried out in the pingluoba and adjacent anticlines in the western sichuan basin to discuss fracture development during progressive folding including the relative timing of fracture formation the successive deformation in a rock layer the distribution of fracture sets in folds and the variation of fractures along with the subsequent deformation of rock layers based on a comparison of the diverse fracture patterns in folded strata at different stages of fold growth horizontal low angle dipping high angle dipping and vertical layers a model of fracture development related to fold evolution was built up it is suggested that the strata were initially under uniform compression and two sets of planar conjugate shear fractures were formed the orientations of which would vary along with the subsequent folding of strata then during progressive folding the deformation of rheologically hard layers was controlled by tangential longitudinal strain inducing the development of a set of tensile fractures that struck parallel to the fold axis layer parallel shear was predominant in the soft layers and two sets of bedding oblique dextral shear fractures intersecting with the bedding at angles of 25 40 and 160 were developed in the progressive folding process the shear fractures were developed later than the tensile fractures a polar stereonet of fractures was generated and applied to improve the efficiency and accuracy of fracture identification in the field providing precise evidence for the exact analysis of fracture patterns and rock deformation

stress evolution during 3d single layer visco elastic buckle folding implications for the initiation of fractures abstract buckle folds of sedimentary strata commonly feature a variety of different fracture sets some fracture sets including outer arc tensile fractures and inner arc shear fractures at the fold hinge zones are well understood by the extensional and compressional strain stress pattern however other commonly observed fracture sets including tensile fractures parallel to the fold axis tensile fractures cutting through the limb extensional faults at the fold hinge and other shear fractures of various orientations in the fold limb fail to be intuitively explained by the strain stress regimes during the buckling process to obtain a better understanding of the conditions for the initiation of the various fractures sets associated with single layer cylindrical buckle folds a 3d finite element modeling approach using a maxwell visco elastic rheology is utilized the influences of three model parameters with significant influence on fracture initiation are considered burial depth viscosity and permeability it is concluded that these parameters are critical for the initiation of major fracture sets at the hinge zone with varying degrees the numerical simulation results further show that the buckling process fails to explain most of the fracture sets occurring in the limb unless the process of erosional unloading as a post fold phenomenon is considered for fracture sets that only develop under unrealistic boundary conditions the results demonstrate that their development is realistic for a perclinal fold geometry in summary a more thorough understanding of fractures sets associated with buckle folds is obtained based on the simulation of in situ stress conditions during the structural development of buckle folds

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

control of folding by gravity and matrix thickness implications for large scale folding 1 we show that folding of a non newtonian layer resting on a homogeneous newtonian matrix with finite thickness under influence of gravity can occur by three modes 1 matrix controlled folding dependent on the effective viscosity contrast between layer and matrix 2 gravity controlled folding dependent on the argand number the ratio of the stress caused by gravity to the stress caused by shortening and 3 detachment folding dependent on the ratio of matrix thickness to layer thickness we construct a phase diagram that defines the transitions between each of the three folding modes our priority is transparency of the analytical derivations e g thin plate versus thick plate approximations which permits complete classification of the folding modes involving a minimum number of dimensionless parameters accuracy and sensitivity of the analytical results to model assumptions are investigated in particular depth dependence of matrix rheology is only important for folding over a narrow range of material parameters in contrast strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition our theory is applied to estimate the effective thickness of the folded central asian upper crust using the ratio of topographic wavelength to moho depth phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the jura and the zagros mountains but does control folding in central asia applicability conditions of viscous and thin sheet models for large scale lithospheric deformation derived in terms of the argand number have implications for the plate like style of planetary tectonics

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

asteroids and meteorites origin of stony iron meteorites at mantle core boundaries abstract stony iron meteorites formed at the core mantle interfaces of small asteroidal parents the mesosiderites formed when the thick crust of a largely molten parent body 100 200 km in diameter foundered and sank through the mantle to the core pallasites formed in smaller parent bodies 50 100 km in which olivine crystals from the partially molten mantle sank to the core mantle interface and rafted there subsequent collisions stripped away the rocky mantles of both kinds of parent bodies exposing the stony iron surfaces of their cores to direct impacts which continue to knock off meteorite fragments

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

physical models of regional fold superposition the role of competence contrast abstract physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference rheologic contrast affects strain partitioning between layer parallel shortening and buckling and affects the development of structural anisotropies parallel to first generation folds laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins type 1 interference in addition to folding the layers accommodate shortening by layer parallel thickening preferentially in the fold culminations laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces type 2 interference the less competent layers thicken in the hinges the more competent layers maintain their initial thickness in plan view the f 1 hingelines are refolded in a lobate cuspate to box style whose axial traces form conjugate pairs the models are dynamically scaled to represent folds with km scale wavelengths the evolution of surface structures map patterns at different depths and equal area projections synthesized from structure contour maps were used for analysis comparable to regional analysis common to field studies gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds thereby enhancing the formation of type 2 interference

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

3d numerical modeling of forward folding and reverse unfolding of a viscous single layer implications for the formation of folds and fold patterns abstract the main aims of this study are to show i that non cylindrical three dimensional 3d fold shapes and patterns can form during a single unidirectional shortening event and ii that numerical reverse modeling of 3d folding is a feasible method to reconstruct the formation of 3d buckle folds 3d viscous newtonian single layer folding is numerically simulated with the finite element method to investigate the formation of fold shapes during one shortening event an initially flat layer rests on a matrix with smaller viscosity and is shortened in one direction parallel to the layering forward modeling with different initial geometrical perturbations on the flat layer and different lateral boundary conditions generates non cylindrical 3d fold shapes and patterns the simulations show that in reality the initial layer geometry and the boundary conditions strongly control the final fold geometry fold geometries produced from the forward folding models are used as initial setting in numerical reverse folding models with parameters identical to those of forward models these reverse models accurately reconstruct the initial geometry of forward models with also only one extension event parallel to the previous shortening direction the starting geometry of the forward models is inaccurately reconstructed by the reverse models if a significantly different viscosity ratio than in the forward models is used this work demonstrates that reverse modeling has a high potential for reconstructing the deformation history of folded regions and rheological constraints such as viscosity ratio reverse models may be applied to natural 3d fold shapes and patterns in order to determine if they formed i during a single or multiple deformation events and ii as active buckle folds with a viscosity ratio 1 or as passive kinematic folds without buckling this approach may find much application to fold interference patterns in particular

quantification of the geometrical parameters of non cylindrical folds abstract the geometrical parameters of natural folds are used by structural geologists to estimate finite strain and rheological properties of deformed rocks the relation between geometry and rheology is well understood in cases of cylindrical folds but is still limited for non cylindrical folds although the latter are frequent in nature the sparsity of quantitative geometrical data of non cylindrical folds can be explained by the small number of 3d exposures and by the lack of robust methods to quantify their geometrical parameters in 3d space we present a new workflow which can be used to quantify geometrical parameters of non cylindrical folds 3d fold geometry is described using fold wavelength arc length l and amplitude a as most natural folds do not show ideal shapes but are affected by various types of discontinuities the new procedure is not fully automatic but requires the manual selection of measuring profiles along which the geometrical parameters are constrained the new workflow is tested using natural and experimentally produced non cylindrical folds the geometric parameters obtained can be used to improve our understanding of fold kinematics and fold mechanics and should assist the quantitative analysis of non cylindrical folds present in gneiss and salt domes and in rocks containing reservoirs of hydrocarbons and minerals deposits

triangle zones geometry kinematics mechanics and the need for appreciation of uncertainties abstract triangle zones are important structures found in foreland fold and thrust belts all over the world and are commonly associated with tectonic wedging however tectonic wedging and consequently the formation of passive hinterland verging roof thrusts requires particular mechanic conditions such as pre fractured rocks syntectonic sedimentation or a specific stratigraphic layering with variable mechanically weak and rigid formations in this contribution models of triangle zones and hypotheses regarding their formation are reviewed our results show that the term triangle zone is often used in a contradictory sense and interpretations are affected by large uncertainties passive roof thrusts are not necessarily required in all cases and suggested models of triangle zones even if they are geometrically and kinematically viable are hampered by their mechanical implications with respect to a large number of published triangle zones and associated kinematic as well as mechanic models we present a new definition and classification scheme which is discussed and applied to natural examples worldwide we show that natural examples of triangle zones can be separated into two types 1 detachment dominated and 2 ramp dominated triangle zones both types imply particular mechanic conditions of involved detachments and regional dynamics regarding the large uncertainties associated with triangle zone interpretation we suggest to only use the term triangle zone in a very stringent manner and only if irrefutable evidences for stratigraphic repetitions within a duplex are given the results should then be validated with additional kinematic and mechanic considerations geometric and kinematic uncertainties as well as implications of the proposed model explaining observed triangular structures should be reflected in the nomenclature used

olivine coronas metamorphism and the thermal history of the morristown and emery mesosiderites abstract coronas are present on all millimeter sized mineral clasts of olivine in the emery and morristown mesosiderites and are a manifestation of high temperature t 850 1100 c metamorphism these coronas formed by reaction and diffusion between olivine and a mesosiderite like matrix assemblage the bulk composition of the coronas can be approximated by a mixture of 10 25 wt olivine and as 90 75 wt metal free matrix except for p and cr which are significantly enriched in coronas phosphorus and cr diffused relatively rapidly to coronas and were derived from a large volume of matrix most likely from metal that was originally enriched in these elements prior to metamorphism the coronas in both meteorites show a similar zone sequence but are systematically thicker in emery 800 m wide than in morristown 350 m wide suggesting that emery experienced more grain growth and more intensive metamorphism than morristown textural relationships suggest that corona formation and high temperature metamorphism occurred largely after intensive millimeter scale brecciation and after or during metal silicate mixing a local equilibrium model can explain many features of the coronas but chemical equilibrium was maintained only on a very small scale overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high temperature metamorphism the compositional interface between core and overgrowth plagioclase is extremely sharp suggesting that cooling rates were 0 1 c y at the peak temperature of metamorphism consistent with high temperature metamorphism occurring in a near surface region of the parent body

geomorphic indicators of active fold growth south mountain oak ridge anticline ventura basin southern california south mountain oak ridge near ventura california is an asymmetric anticlinal uplift forming at the present time above the active buried oak ridge reverse fault shortening along the oak ridge fault accumulated largely in quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0 5 m y tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth stream gradient indices are relatively high in the northern eroded fold scarp of the ridge a pattern consistent with the existence of active rapid slip on the oak ridge fault mountain front sinuosity along the northern slope of the anticlinal ridge roughly decreases from 2 to 1 toward the westernmost 10 km of observed surface folding valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from 1 5 to 0 5 values of the hypsometric integral along the northern flank increase significantly from 0 35 to 0 4 maximum 0 55 from east to west drainage density varies from 4 to 6 km km2 along both flanks of the south mountain oak ridge anticline entrenchment of streams into the southern backlimb of the fold along the westernmost 9 km of the structure decreases from 20 m to 1 m from east to west apparent backlimb rotation as measured by dip of strata along the westernmost 7 km of the fold decreases from east to west from 35 to 20 fold growth of the south mountain oak ridge anticline occurred during the past 0 5 ma following deposition of the saugus formation lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried oak ridge fault

influence of surface processes and initial topography on lateral fold growth and fold linkage mode elongation of randomly distributed fold segments and their potential linkage are important for hydrocarbon exploration because it can greatly influence the morphology of the reservoir and both migration and accumulation of hydrocarbons in antiformal traps here we study the effects of surface processes and the presence of a topographic slope on the different linkage modes that can occur and how these parameters affect the required horizontal offset for perturbations to link the proposed numerical model represents a sedimentary cover detached over a much weaker basal decollement layer the upper surface is modified by mass redistribution which is achieved by a combination of fluvial and hillslope processes several series of simulations were performed 1 without surface processes or regional slope 2 with regional slope only 3 with fluvial incision and hillslope processes and 4 with hillslope processes only model results show that the presence of a regional slope reduces the critical distance required for the transition between linkage and no linkage modes whereas erosion and redeposition of sediments on the contrary increase this distance the location of the saddle point where fold segments link and its vertical distance to the crests of the anticlines are different compared to the case without erosion or initial topographic slope which potentially can affect the morphology of hydrocarbon traps moreover both erosion and redeposition of sediments enhance the fold elongation growth along the fold axis once the erosion velocity exceeds the folding velocity model results have been compared to the zagros fold belt

neodymium diffusion in orthopyroxene experimental studies and applications to geological and planetary problems abstract we have determined the nd 3 diffusion kinetics in natural enstatite crystals as a function of temperature f o 2 and crystallographic direction at 1 bar pressure and applied these data to several terrestrial and planetary problems the diffusion is found to be anisotropic with the diffusion parallel to the c axial direction being significantly greater than that parallel to a and b axis also d a is likely to be somewhat greater than d b diffusion experiments parallel to the b axial direction as a function of f o 2 do not show a significant dependence of d nd 3 on f o 2 within the range defined by the iw buffer and 1 5 log unit above the wm buffer the observed diffusion anisotropy and weak f o 2 effect on d nd 3 may be understood by considering the crystal structure of enstatite and the likely diffusion pathways using the experimental data for d nd 3 we calculated the closure temperature of the sm nd geochronological system in enstatite during cooling as a function of cooling rate grain size and geometry initial peak temperature and diffusion direction we have also evaluated the approximate domain of validity of closure temperatures calculated on the basis of an infinite plane sheet model for finite plane sheets showing anisotropic diffusion these results provide a quantitative framework for the interpretation of sm nd mineral ages of orthopyroxene in planetary samples we discuss the implications of our experimental data to the problems of melting and subsolidus cooling of mantle rocks and the resetting of sm nd mineral ages in mesosiderites it is found that a cooling model proposed earlier ganguly j yang h ghose s 1994 thermal history of mesosiderites quantitative constraints from compositional zoning and fe mg ordering in orthopyroxene geochim cosmochim acta 58 2711 2723 could lead to the observed 90 ma difference between the u pb age and sm nd mineral age for mesosiderites thus obviating the need for a model of resetting of the sm nd mineral age by an impulsive disturbance prinzhoffer a papanastassiou d a wasserburg g j 1992 samarium neodymium evolution of meteorites geochim cosmochim acta 56 797 815

lateral fold growth and fold linkage we use a three dimensional finite element model for viscous flow to investigate the lateral linkage mode of two initially isolated fold segments depending on the separation distance between the fold hinge lines four different fold linkage scenarios are observed linear linkage yields a sub cylindrical fold with a saddle at the location where the two initial folds linked oblique linkage produces a fold resembling a type ii refold structure oblique no linkage results in two curved folds with fold axes plunging in opposite directions linear no linkage yields a fold train of two separate subcylindrical folds with fold axes plunging in opposite directions representative numerical models for detachment and matrix folding for both power law and linear viscous rheologies exhibit the same four fold linkage modes the transition from linkage to no linkage occurs when the fold separation between the initially isolated folds is slightly larger than one half of the low amplitude fold wavelength we demonstrate a natural example of lateral fold linkage from the kurdistan region of iraq and point out the economic interest of saddle points linking the initially isolated fold segments

fracture density in the deep subsurface techniques with application to point arguello oil field because monterey formation reservoirs rely on fractures joints for permeability quantitative information on fracture spacing is important to exploration strategies and for understanding reservoir behavior density of joints in cores of four wells from the point arguello reservoir has been determined with a new probability based method and these subsurface joint densities are compared with joint densities measured at nearby outcrops my measure of joint density is the fracture spacing index which is the slope of the trend of layer thickness to joint spacing a single set of extension joints that strike parallel to bedding dip predominates at outcrops likewise the cores contain a single set of extension joints perpendicular to the anticline axis and parallel to present day maximum horizontal compressive stress in core and at outcrop the only lithologic control on joint density is between nonjointed mudstone and harder more brittle jointed rocks within each well the fracture spacing index is the same for all hard rocks though it varies between wells in the reservoir joint density relates to structural position the most densely jointed strata fracture spacing index 0 45 are from the hinge of a minor anticline where its plunge steepens a less steeply plunging anticlinal nose has lower joint density fracture spacing index 0 30 the lowest joint density fracture spacing index 0 095 and 0 12 is in cores from gently dipping areas at outcrops in various structural settings the fracture spacing index is the same approximately 1 29 in chert dolostone and porcelanite and siliceous shale these rocks may be saturated with joints so that differences in brittle strain due to local structural variations have been overwhelmed as joints continued to form during unroofing of these strata chert looks more fractured than other lithologies because of thin bedding fracture spacing index is used to compute such parameters as fracture porosity and volume of fractures that directly contact the well bore these parameters may be important when trying to model the behavior of a petroleum reservoir or when trying to assess the feasibility of strongly deviating wells to improve the performance of a fracture reservoir

growth and lateral propagation of fault related folds in the siwaliks of western nepal rates mechanisms and geomorphic signature 1 we study the controls on drainage development in tectonically active regions using a numerical tectonic surface processes model combining tectonic uplift caused by fault related folding with erosion by fluvial incision hillslope diffusion and landsliding our model shows the fundamental control exerted by the dip of the detachment underlying the folds on drainage evolution when the detachment is horizontal the relative rates of tectonic uplift and fluvial incision control the evolution for a nonzero dip in contrast the lateral displacement gradient associated with fold propagation sets up a lateral slope behind the active structure which deflects the stream network we also demonstrate the importance of landsliding for the attainment of realistic and steady state topography in contrast discontinuous tectonic movements do not appear to influence the system we apply our model to the dundwa fault related fold ridge siwalik nepal we estimate a remarkably low mean propagation rate for different segments of the structure this finding together with the structure and morphology of the ridge leads us to propose that the ridge developed by linkage of several component segments the drainage evolution predicted when modeling this scenario compares favorably with field observations our models provide insights into the dynamics of fault related fold propagation in particular as a result of fault segment linkage estimates of propagation rate of these structures may be strongly scale dependent and the observed morphology and drainage patterns on fault related fold ridges may be controlled by fault geometry rather than by the relative rates of tectonics and surface processes

stress and strain evolution during single layer folding under pure and simple shear abstract folds in rocks are commonly used in geology for strain analysis they contain information about rock rheology and the kinematics and mechanics of deformation the systematic analysis of the evolution of stress and strain during rock folding can provide key information on the mechanical behaviour of rocks and the efficiency of the folding process in order to investigate the evolution of rock rheology during fold development a series of two dimensional simulations of single layer folding are presented here they are run using the finite element method basil integrated within the software platform elle to simulate linear and non linear viscous deformation the kinematics of deformation the competence ratio between the folding layer and surrounding matrix as well as the stress exponent of the power law viscous material are systematically varied the results allow comparing the stages of folding under different deformation kinematic conditions for all simulations the folding amplification process starts when the second invariant of the strain rate tensor in the competent layer deviates from the theoretical strain rate curve for a homogeneous material which corresponds to a viscosity ratio between layer and matrix of m 1 the relative time when the fold amplification starts is determined by the viscosity ratio between the competent layer and its surrounding matrix the initial layer orientation with respect to the shear plane the kinematics of deformation and the stress exponent the folding process is more effective in cases with high viscosity ratio non linear rheology and layers initially oriented at a low angle with respect to the shear plane because the second invariant of the strain rate tensor at the layer deviates earlier from the theoretical curve the results also show differences depending on the boundary conditions where 1 folding a competent layer requires less work in simple shear than in pure shear and 2 the geometrical softening experienced by the competent layer due to fold development is followed by a hardening stage in pure shear and by a major softening phase in simple shear the simulation results suggest that the decrease of stress of a competent layer without decreasing the mechanical strength has a direct influence on the behaviour of a lithospheric layer around the crust mantle boundary which may experience geometrical softening depending on the tectonic settings rather than material softening due metamorphic reactions or grain size reduction

the influence of initial fold geometry on type 1 and type 2 interference patterns an experimental approach abstract experiments on superposed folding were performed in a plane strain pure shear rig using paraffin wax as an analogue for rocks a series of pre formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis type 1 interference geometry roundness tightness and amplitude were varied to investigate the influence of first fold geometry on the interference patterns experimental folding of an initially planar layer oriented parallel to the first fold envelope provides a reference geometry for comparison the results suggest that hinge roundness has less influence on the interference pattern than other factors close initial folds refold into type 2 interference patterns which become even more pronounced as the tightness increases open folds produce type 1 interference patterns folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude folds with relatively large amplitude are refolded into type 2 patterns whereas folds with small amplitude give clear dome and basin structures in a second set of experiments the compression direction also acted parallel to the first fold hinge but the orientations of the intermediate and extension axes were interchanged the interference patterns obtained are very similar to those in type 1 interference geometry suggesting that the major factor in determining whether type 1 or type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation

jabal hafit anticline uae and oman formed by decollement folding followed by trishear fault propagation folding abstract creating three dimensional 3d models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen the jabal hafit anticline in the foreland of the al hajar mountains extends through oman and the united arab emirates and is a relatively simple and well exposed structure surprisingly previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation in this study a structural 3d geological model of the jabal hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two phase model 1 flexural slip folding followed by 2 trishear fault propagation folding the trishear mode occurred due to a west dipping backthrust that propagated from the decollement to accommodate wsw directed shortening the different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late oligocene to early middle miocene this timing supports the recently established age for uplift of the al hajar mountains

fold interaction and wavelength selection in 3d models of multilayer detachment folding abstract many fold and thrust belts are dominated by folding and exhibit a fairly regular fold spacing yet in map view the aspect ratio of doubly plunging anticlines varies considerably from very elongated and sometimes slightly curved cylindrical folds to nearly circular dome like structures in addition the fold spacing often varies significantly around an average value so far it remains unclear whether these features are consistent with a folding instability therefore we here study the dynamics of multilayer detachment folding process by which shortening can be accommodated in thin skinned fold and thrust belts we start by analysing the physics of this process by using both a semi analytical thick plate theory and numerical simulations results show that several different folding modes occur about half of which are affected by gravity and have a wavelength that depends on the background deformation rate non dimensional expressions are derived that predict the dominant wavelength and growth rate of each of these folding modes and mechanical phase diagrams are presented that illustrate the applicability of each of the modes next we perform 3d simulations and compare the results with those of 2d models and analytical theory both 2d and 3d numerical simulations have wavelengths that are in good agreement with the analytical predictions in the high resolution 3d simulations the lateral growth of folds is studied in particular with respect to fold segment interactions and evolution of fold width length aspect ratio the numerical simulations show a number of similarities with the fars region of the zagros fold and thrust belt including a large range of fold aspect ratio and a normally distributed fold wavelength around a dominant one

morphometry of drainage basins and stream networks as an indicator of active fold growth gorm anticline fars province iran abstract the zagros folded belt as one of the most tectonically active areas in the world contains nw se trending whaleback anticlines that are growing vertically and laterally morphometric indices are widely used in tectonic geomorphological research because of their low cost and relative ease of application drainage basins and their networks adapt to tectonically induced changes in surface slope and hence their qualitative and quantitative analysis can reveal aspects of active anticlinal growth this study evaluates the morphometric properties of drainage basins and their networks developed on the fold limbs using this quantitative data to detect lateral propagation of the gorm anticline in the zagros simply folded belt zsfb the gorm anticline was considered to be suitable for this approach because of the variability of drainage basins and their networks along the axis of the anticline and also transversely across its forelimb and backlimb the morphometric properties of stream networks including drainage density dd drainage frequency df ratio of 1st order streams to the total number of streams of all orders n1 n length of 1st order streams l1 spacing ratio r of main streams on flanks and drainage patterns were analyzed in three zones of the anticline southeastern se central and northwestern nw also the morphometric indices of drainage basins such as basin area ba shape bs asymmetry factor af hypsometric integral hi a new proposed crescentness index ci and sinuosity of anticline divide sad were evaluated results show that an increase in n1 n ratio and l1 from the center towards both the southeastern and northwestern fold noses suggests lateral growth of the anticline towards the southeast and northwest the se and nw zones are characterized by the smaller elongated and crescent shaped basins lower elevations lower dip slopes higher hypsometric integral and lower sinuosity of anticline ridge these results collectively suggest that the southeastern and northwestern fold noses are younger less eroded and laterally growing the formation of crescent shaped basins with asymmetric forked drainages developed in the se and nw zones are considered to be useful indicators of lateral growth of the gorm anticline although higher values of ba sad df and s indexes in the central part of the fold can be attributed to the older relative age of this area the activity of the nezamabad fault and the exposure of some weak rocks also play some roles in explaining the increase the values of parameters overall the n1 n l1 ci sad indexes as well as the fan shaped and asymmetric forked drainage pattern are the most useful tectonic geomorphological criteria to detect lateral propagation of anticlines

insights into fold growth using fold related joint patterns and mechanical stratigraphy despite how common folds are as structural features along strike fold propagation has proven elusive to document however if a fold grows laterally along its axis early formed fold related joints may differ significantly in orientation from joints that form later in this paper we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing additionally we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure we present joint data from eight sedimentary beds on the fold terminus at sheep mountain anticline wyoming usa the joint patterns around the terminus show two distinct patterns joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units despite being in the same structural position fracture resistance calculations confirm that the beds with mis oriented fractures are less resistant to fracturing than other units in the study and therefore would have fractured earlier in fold growth history we present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed the joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds which are now positioned behind the inflection line suggesting that the fold grew laterally the combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution

relationship between cooling rate and cooling age of a mineral theory and applications to meteorites we reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system which permits direct retrieval of cooling rate from cooling age when the diffusion parameters grain size and initial temperature are known this formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system the cooling rates of seven h chondrites of the metamorphic types h4 h5 and h6 were retrieved from the available data on the pb pb model ages of the phosphates and the diffusion kinetic data of pb in apatite the results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites we also addressed the problem of 90 ma younger sm nd mineral isochron age defined by orthopyroxene phosphate and plagioclase of the morristown mesosiderite compared to the pb pb age of the estherville mesosiderite it is shown that this younger age could have been a consequence of resetting during cooling instead of an impulsive heating event as suggested earlier

a 3 d lagrangian finite element algorithm with remeshing for simulating large strain hydrodynamic instabilities in power law viscoelastic fluids we present a three dimensional 3 d numerical algorithm pink 3d that is based on the finite element method the algorithm is designed to simulate hydrodynamic instabilities in power law viscoelastic fluids under gravity these instabilities are caused by large and sharp contrasts in mechanical strength and or density between different materials e g folding necking or rayleigh taylor diapirism the instabilities are controlled by the geometry of the material interfaces and the related intralayer stress distribution when amplitudes of the material interfaces are still low the presented algorithm combines a deformable lagrangian mesh with remeshing in order to accurately simulate the low amplitude stages of the emerging instabilities and also to simulate the large strain evolution of the structures emerging from these instabilities the remeshing is based on material interfaces that accurately track the boundaries between materials with strongly varying material properties e g effective viscosity or power law stress exponent we describe here the main technical details of the 3 d algorithm the accuracy of the 3 d algorithm is demonstrated with comparisons between the numerical results and 2 d and 3 d analytical solutions for folding necking rayleigh taylor diapirism and circular inclusions in viscous medium we also benchmark the 3 d algorithm with results of a different 2 d finite element algorithm to test the accuracy of the large strain results with remeshing furthermore two tests are presented that show the accuracy of the viscoelasticity implementation pink 3d is also used to study 3 d necking applied to lithospheric slab detachment and 2 d and 3 d folding applied to fold nappe formation in particular we apply the 3 d code to quantify and visualize the evolution of the 3 d finite strain ellipsoid for the developing 3 d structures

an extremely low source in the moon uthpb smnd rbsr and isotopic systematics and age of lunar meteorite asuka 881757 abstract we have undertaken u th pb sm nd rb sr and 40 ar 39 ar isotopic studies on asuka 881757 a coarse grained basaltic lunar meteorite whose chemical composition is close to low ti and very low ti vlt mare basalts the pb pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 28 ma which is similar to the u pb 3850 150 ma and th pb 3820 290 ma internal isochron ages the sm nd data for the mineral separates yield an internal isochron age of 3871 57 ma and an initial 143 nd 144 nd value of 0 50797 10 the rb sr data yield an internal isochron age of 3840 32 ma 87 rb 1 42 10 11 yr 1 and a low initial 87 sr 86 sr ratio of 0 69910 2 the 40 ar 39 ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 12 ma we interpret these ages to indicate that the basalt crystallized from a melt 3 87 ga ago the sm nd age and an impact event disturbed the rb sr system and completely reset the k ar system at 3 80 ga the slightly higher pb pb age compared to the sm nd age could be due to the secondary pb from terrestrial and or lunar surface pb contamination that remained in the residues after acid leaching alternatively the following interpretation is also possible the meteorite crystallized at 3 94 ga the pb pb age and the sm nd rb sr and k ar systems were disturbed by an impact event at 3 80 ga the crystallization age obtained here is older than those reported for low ti basalts 3 2 3 5 ga and for vlt basalts 3 4 ga but similar to ages of some mare basalts indicating that the basalt may have formed from a magma related to a basin forming event imbrium the age span for vlt basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least 500 million years the impact event that thermally reset the k ar system of asuka 881757 must have been post imbrium perhaps orientale in age the lead isotopic composition of asuka 881757 is nonradiogenic compared with typical apollo mare basalts and the estimated 238 u 204 pb value for the basalt source is 10 3 this source value is the lowest so far measured for lunar rocks a large positive nd value 7 4 0 5 and the time averaged 147 sm 144 nd ratio for the basalt source are similar to those for some apollo 12 15 and 17 basalts suggesting a lree depleted mantle which is consistent with the global magma ocean hypothesis the u th pb sm nd and rb sr data on asuka 881757 suggest that the basalt was derived from a low u pb low rb sr and high sm nd source region mainly composed of olivine and orthopyroxene with minor amounts of plagioclase or clinopyroxene and with sulfides enriched in volatile chalcophile elements the basalt source may be deep in origin and different in chemistry from those previously estimated from studies of apollo and luna mare basalts indicating heterogeneous sources for mare basalts

mesostructural development in detached folds an example from west virginia an alleghanian duplex of three rootless anticlines in the central appalachians containing oriskany sandstone provides an example of mesostructural development in detached folds these mesostructures which include stylolites veins vein arrays joints and faults developed before during and after duplex formation non alleghanian mesostructures are the most ubiquitous joint sets began forming during the late pennsylvanian in response to burial stresses and fluid overpressure subsequent pre alleghanian n s compression reactivated the joint trends to form stylolites and vein arrays again in the presence of large fluid pressures during alleghanian deformation mesostructures developed only locally in the folds in response to layer rotation interlayer slip and hydrofracturing during thrusting during alleghanian and post alleghanian uplift and erosion thermal elastic relaxation and arc length changes reactivated a joint trend to form new joints the resulting varied distribution of mesostructures

toward 4d modeling of orogenic belts example from the transpressive zagros fold belt abstract latest developments in computer power and computational solutions open new ways to envision complex natural systems advances in high resolution 3d geodynamic modeling in particular allow investigating tectonic processes such as transpression which involve normal and tangential kinematics with respect to the bulk orogenic trends we demonstrate that implementing the 4 dimensions space and time provides and constrains new answers to long lasting discussions as case study we have chosen the zagros simply folded belt in iran which exhibits spatial variations of structural patterns from the homogeneously shortened wide fold belt in the fars to the narrower higher elevated area in the izeh high resolution 4d numerical modeling shows how a low viscosity decollement becoming frictional toward the dezful embayment influences the exposed fold patterns results also emphasize the importance of an oblique backstop to produce en echelon arranged folds where convergence related and backstop controlled folds are mingled in the transpressional orogen

recent fold growth and drainage development the janauri and chandigarh anticlines in the siwalik foothills northwest india the active growth of a fault and thrust belt in frontal zones of himalaya is a prominent topographical feature extending 2500 km from assam to pakistan in this paper kinematical analysis of frontal anticlines and spatial mapping of active faults based on geomorphological features such as drainage pattern development fault scarps and uplifted quaternary alluvial fans are presented we analyse the geomorphic and hydrographic expressions of the chandigarh and the janauri active anticlines in the nw india siwaliks to investigate the morphological scenario during the folding process we used spatial imagery geomorphometric parameters extracted from digital elevation models and fieldwork folding between the beas and sutlej rivers gives clear geomorphological evidence of recent fold growth presumably driven by movements of blind thrust faults structural style within the janauri and chandigarh anticlines is highly variable fault propagation folds pop up structures and transfer faults the approach presented here involves analysis of topography and drainage incision of selected landforms to detect growth of active anticlines and transfer faults landforms that indicate active folding above a southwest dipping frontal thrust and a northeast dipping back thrust are described along strike differences in ridge morphology are measured to describe the interaction of river channel patterns with folds and thrust faults and to define history of anticline growth the evolution of the apparently continuous janauri ridge has occurred by the coalescence of independent segments growing towards each other by contrast systematic drainage basin asymmetry shows that the chandigarh anticline ridge has propagated laterally from nw to se

3d fold growth rates geological folds are inherently 3d structures therefore they also grow in three dimensions here fold growth in all three dimensions is quantified by numerically simulating upright single layer folds in 3d newtonian media horizontal uniaxial shortening leads to a buckling instability which grows from a point like initial perturbation in all three dimensions by fold amplification vertical fold elongation parallel to fold axis and sequential fold growth parallel to shortening direction of secondary and further folds adjacent to the initial isolated fold the two lateral directions exhibit similar averaged growth rates leading to bulk fold structures with aspect ratios in map view close to 1 however fold elongation is continuous with increasing bulk shortening while sequential fold growth exhibits jumps whenever a new sequential fold appears and the bulk fold structure therefore suddenly occupies more space compared with the two lateral growth directions fold amplification exhibits a slightly higher growth rate