the three dimensional morphology of arrays of echelon and sigmoidal mineral filled fractures data from north cornwall arrays of mineral filled echelon sigmoidal fractures in cobbles of sandstone from the crackington formation of north cornwall were examined by cutting serial sections in three dimensions each array consists of a set of blade like mineral filled fractures these blades project as a fringe from the single but composite fracture that forms through their linkage linkage takes place as the strips of rock bridges between blades fail through cross fracture cross fractures give rise to the steps that mark the margins of the composite fractures so formed bridges show little penetrative deformation arrays are not zones of high ductile strain displacement is a consequence of dilatation echelon fractures were formed and opened by intruding hydrothermal fluid

the role of lithospheric flexure in the landscape evolution of the wilkes subglacial basin and transantarctic mountains east antarctica reconstructions of the bedrock topography of antarctica since the eocene oligocene boundary ca 34 ma provide important constraints for modelling antarctic ice sheet evolution this is particularly important in regions where the bedrock lies below sea level since in these sectors the overlying ice sheet is thought to be most susceptible to past and future change here we use 3d flexural modelling to reconstruct the evolution of the topography of the wilkes subglacial basin wsb and transantarctic mountains tam in east antarctica we estimate the spatial distribution of glacial erosion beneath the east antarctic ice sheet and restore this material to the topography which is also adjusted for associated flexural isostatic responses we independently constrain our post 34 ma erosion estimates using offshore sediment stratigraphy interpretations our reconstructions provide a better defined topographic boundary condition for modelling early east antarctic ice sheet history we show that the majority of glacial erosion and landscape evolution occurred prior to 14 ma which we interpret to reflect more dynamic and erosive early ice sheet behaviour in addition we use closely spaced 2d flexural models to test previously proposed hypotheses for a flexural origin of the tam and wsb the pre 34 ma topography shows lateral variations along the length of the tam and wsb that cannot be explained by uniform flexure along the front of the tam we show that some of these variations may be explained by additional flexural uplift along the south western flank of the wsb and the rennick graben in northern victoria land

the mechanics of intersecting echelon veins and pressure solution seams in limestone abstract many studies that describe the formation of echelon vein arrays relate the causative stresses implicitly to the deformation reliant on simple shear kinematics such that the vein to array angle and the array width are the primary physical quantities in contrast we identify twelve physical quantities to describe echelon veins in two dimensions including coeval vein intersecting pressure solution seams a finite element method is used to reproduce vein shapes in linear elastic and elastic perfectly plastic model limestone model vein geometries are designed using values within the range of geometries measured from echelon veins at raplee anticline and comb monocline utah four physical quantities are significant for describing echelon vein shapes vein spacing vein array angle limestone elastic stiffness and closing of orthogonal pressure solution seams pressure solution seam closing influences the mechanical interaction between adjacent veins and for a range of conditions causes a nearly linear vein opening distribution triangular shapes and encourages straight vein propagation both of which approximate field measurements model results show that small spacing of veins with seams and large vein array angles promote straight vein traces in limestone with stiffness typical of laboratory measurements given the physical geologic conditions inferred from the burial history of the limestone strata

late quaternary climate change from δ18o records of multiple species of planktonic foraminifera high resolution records from the anoxic cariaco basin venezuela seasonal trade wind induced upwelling along the southern margin of the caribbean sea occurs in response to the annual migration of the intertropical convergence zone laminated high deposition rate sediments of the cariaco basin a small anoxic basin on the venezuelan continental shelf clearly record large changes in the past intensity of this upwelling because sediments of the cariaco basin are largely unbioturbated they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate oxygen isotope data 18o from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr the 18o data from globigerina bulloides after correction for nonequilibrium precipitation are used as a monitor of sea surface conditions during the winter spring upwelling season the 18o data from white globigerinoides ruber are used as a measure of annual average conditions in the near surface while pink g ruber data are consistent with use as an index of endmember conditions during the summer fall nonupwelling season data from the deeper dwelling neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline during the last glacial 18o data from g ruber and generally reduced interspecific differences indicate cooling of surface waters over the cariaco basin by up to 4 c this longer term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger caribbean superimposed on this pattern between 12 6 and 10 ka is a convergence of 18o data between g bulloides and n dutertrei implying much stronger upwelling during the last deglaciation this scenario is consistent with other evidence for high productivity at this time at 14 ka a sharp 18o depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern caribbean suggesting either higher regional rainfall or the influence of glacial melting in the andes minimum 18o values of pink g ruber around 6 7 ka record warmer summer sea surface temperatures and or decreased salinity in the mid holocene

pull aparts shear fractures and pressure solution vein arrays are often composed of pull aparts which are linked by shear fractures good examples of which occur in the lower jurassic limestones of somerset southwestern england such pull apart arrays have displacement distance characteristics which are similar to fault zones with maximum displacement indicated by the largest pull apart widths near the centre of the array and with displacement decreasing towards the tips pull apart arrays usually die out into enechelon or pinnate veins evidence for pressure solution along the shear fractures which connect pull aparts include their dark and braided nature their obliquity to the displacement direction the high dihedral angles often 90 between conjugate shear fractures and the dissolution of earlier structures a range of geometries occurs with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression there is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases there is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays the concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults

surface ocean variability at galapagos from 1936 1982 calibration of geochemical tracers in corals a variety of geochemical tracers has been developed in reef corals as a means of reconstructing the natural variability of the tropical surface ocean over wide ranging time frames the purpose of this paper is to calibrate the performance of five of these tracers in a modern coral colony over an extended period a 47 year growth interval 1936 1982 in a colony of pavona clavus taken from san cristobal island galapagos islands was sectioned into 187 quarterly intervals and analyzed for 18o 13o ba ca cd ca and mn ca the resulting time series are compared with eastern pacific sea surface temperatures one of few instrumental data bases available for calibration over this time period linear least squares regressions of four of the tracers 18o 13o ba ca cd ca against peruvian sst show highly significant correlations p 0 0001 over the annual biennial and enso 3 8 years frequency bands cross spectral comparisons show even stronger coherency with sst frequently accounting for 70 80 of the variance in these tracers these results as well as the phase relationships determined from the spectral analyses suggest that the coupling between sst and 18o and the nutrientlike tracers ba and cd is controlled principally by seasonal upwelling cycles and interannual interruptions of these cycles brought about by el nino and anti el nino phenomena an inverse relationship between sst and 13c has its primary origins in the co occurrence of the warm sst phase with high light levels during the first half of the calendar year the association with sunlight derives from the photosynthetic response of algal symbionts which results in fractionation of light and heavy carbon isotopes between soft tissues and caco3 skeleton the fifth tracer the transition metal mn exhibits poorer correlations against both sst and the other tracers slight shifts in phasing of the historical mn ca annual cycle suggest that the general water column distribution of this tracer which is the reverse of a nutrient may not be stable the erratic behavior of mn illustrates how complex hydrography near galapagos may influence tracers with diverse source signatures

measurement of geometry and linkage in vein arrays abstract veins in an array are characterised by measurement of their length l width w and orientation and by the number 0 1 or 2 of adjacent veins to which they are linked data from a single array 129 veins and two regions comprising 146 and 64 arrays have been measured and analysed the system for measurement provides a sound basis for the quantitative assessment of both the array geometry and the effects of interaction of adjacent veins linkage is achieved by a breaching of the bridges of wall rock between veins either by extension fractures shear failure or the development of stylolites it is primarily controlled by the strain as measured by vein width the size and shape of the veins is found to be controlled by two factors the degree of linkage and the kinematics of the array transtension transpression interplay of these produces variability both within and between arrays and this is examined for two large datasets from the lower jurassic of somerset and the lower carboniferous of the gower peninsula south wales

spatial variations of effective elastic thickness over the ninetyeast ridge and implications for its structure and tectonic evolution we present new data on the strength of oceanic lithosphere along the ninetyeast ridge ner from two independent methods spectral analysis bouguer coherence using the fan wavelet transform technique and spatial analysis flexure inversion with the convolution method the two methods provide effective elastic thickness t e patterns that broadly complement each other and correlate well with known surface structures and regional scale features furthermore our study presents a new high resolution database on the moho configuration which obeys flexural isostasy and exhibit regional correlations with the t e variations a continuous ridge structure with a much lower t e value than that of normal oceanic lithosphere provides strong support for the hotspot theory the derived t e values vary over the northern higher t e similar to 10 20 km central anomalously low t e similar to 0 5 km and southern low t e similar to 5 km segments of the ner the lack of correlation of the t e value with the progressive aging of the lithosphere implies differences in thermo mechanical setting of the crust and underlying mantle in different parts of the ner again indicating diversity in their evolution the anomalously low t e and deeper moho similar to 22 km estimates of the central ner between 0 5 degrees n and 17 degrees s are attributed to the interaction of a hotspot with the wharton spreading ridge that caused significant thermal rejuvenation and hence weakening of the lithosphere the higher mechanical strength values in the northern ner north of 0 5 degrees n may support the idea of off ridge emplacement and a relatively large plate motion at the time of volcanism the low t e and deeper moho similar to 22 km estimates in the southern part south of 17 degrees s suggest that the lithosphere was weak and therefore younger at the time of volcanism and this supports the idea that the southern ner was emplaced on the edge of the indian plate c 2013 elsevier b v all rights reserved

elastic thickness structure of south america estimated using wavelets and satellite derived gravity data abstract we used a wavelet formulation of the classical spectral isostatic analysis to invert satellite derived gravity and topography bathymetry for elastic thickness te over south america and its surrounding plates to provide a homogeneous representation of the gravity field for this vast region we corrected free air anomalies derived from a combination of terrestrial marine gravity data with data from the grace and champ satellite missions model eigen cg03c by a simple bouguer slab using a smoothed representation of surface relief wavelengths 125 km the resulting bouguer anomaly compares well with terrestrial data acquired in the central andes and allows te to be confidently estimated for values greater than 10 km the te map resolves regional scale features that are well correlated with known surface structures and shows maximum values of 100 15 km over the archean neoproterozoic core of the continent decreasing to less than 30 km around continental margins several regions of the oceanic plates and continental margins have an elastic thickness less than 10 km we performed a quantitative analysis by comparing the elastic thickness with the thermal structure predicted from the age of oceanic crust and igneous metamorphic rocks this demonstrates that oceanic plates have been weakened by thermal interaction with hotspots and locally by fracturing and hydration near the trench we observe that only the nucleus of the continent has resisted the thermomechanical weakening induced by the rifting of africa and south america along the passive margin and the andean orogeny along the active margin this latter region shows along strike variations in te that correlate with the geotectonic segmentation of the margin and with the pattern of crustal seismicity our results reveal that the rigidity structure follows the segmentation of the seismogenic zone along the subduction fault suggesting a causal relationship that should be investigated in order to improve the understanding and predictability of great earthquakes and tsunamis

models of fracture orientation at oblique spreading centres three models are proposed for the orientation of extensional faults and dykes at mid ocean ridges based on their relationship to the ridge axis and the relative plate separation vector these models predict four ridge geometries allowing them to be tested by orientation data from 17 different sites within the earth9s oceans a transtensional model is shown to be generally applicable to the structure of mid ocean ridges with special conditions required for a departure from it spreading rate influences ridge geometry with intermediate and fast spreading ridges more likely to have an orthogonal spreading geometry which is an end member case of the transtensional model

the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method 1 we model the lithosphere as a uniform elastic plate overlying an inviscid fluid and loaded with both surface and subsurface fractal loads to generate synthetic topography and gravity data to simulate data having low topographic signal to gravity noise ratio we use an algebraically larger exponent for the subsurface load in the spectral synthesis fractal algorithm the gravity power spectrum then decays less rapidly than that of topography the spectra resembling those for central australia we find that the coherence method using multitaper spectral estimation yields significant underestimates of plate thickness for both low and normal signal to noise ratio data unless the data window is larger than several times the true flexural wavelength we quantify this bias for the parameters used here and apply it as a correction to an effective elastic thickness estimate for central australia obtaining a value of 115 25 km index terms 8110 tectonophysics continental tectonics general 0905 8159 tectonophysics rheology crust and lithosphere 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 citation swain c j and j f kirby the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method geophys res lett 30 11 1574 doi 10 1029 2003gl017070 2003

elastic thickness of the himalayan tibetan orogen estimated from the fan wavelet coherence method and its implications for lithospheric structure abstract this paper is concerned with the spatial variation of the elastic thickness t e in the himalayan tibetan orogen and its implications for the understanding of lithospheric structure and deformation new t e maps of the himalayan tibetan orogen are generated from the new combined satellite terrestrial gravity eigen6c3stat and topography etopo1 models using the fan wavelet coherence technique the high t e values 60 km are observed in the indian shield and the tarim basin while the low t e values 40 km are prevailing over most of the tibetan plateau we also find that the t e distributions estimated using the high and low central wavenumber i e k 0 wavelets are different in southern tibet by combining constraints from the current seismic and thermo rheological modeling this difference may reflect that the strength of the lithosphere in southern tibet mainly resides in the lithospheric mantle within most parts of central and northern tibet the t e values are relatively low 30 km indicating that the entire lithosphere is weak our analysis finds that the t e variations are probably associated with changes in lithosphere structure we interpret the abrupt changes in t e between southern and northern tibet to indicate the location of the underthrusting strong indian lithosphere beneath tibet our results suggest that the northern limit of the underthrusting indian plate reaches as far north as the hindu kush and the southwestern margin of the tarim basin near 70 80 e and extends along the bangong nujiang suture in the central 80 87 e eastward between 87 e 93 e it may move to the indus tsangpo suture indicating a delamination of indian mantle lithosphere there further to the east it may underthrust northward beneath the eastern qiangtang block

variations of the effective elastic thickness over the ross sea and transantarctic mountains and implications for their structure and tectonics abstract the effective elastic thickness t e is a proxy for lithospheric strength and it depends primarily on the thermal gradient and composition of the lithosphere accordingly spatial variations in t e reflect changes in lithospheric properties and can be used to better understand the structure and tectonics of particular regions in this paper we investigate the ross sea and transantarctic mountains in terms of t e using gravity and topographic data and the fan wavelet transform technique the results reveal that relatively high t e values dominate in the extensional basins of the ross sea and the hinterland of transantarctic mountains whereas very low t e values occur along the transantarctic mountain front and in the deep ocean basin with the lowest t e values are found the vicinity of ross island and onshore in northern victoria land in addition the spatial variations in t e correlate well with lithospheric structure at the regional scale by combining these findings with published seismic and heat flow data we conclude that the presence of a zone of anomalously low t e values parallel to the coast indicates that the lithosphere beneath the transantarctic mountain front is extremely weak due to cenozoic volcanism and extension the t e values increase from the transantarctic mountain front 7 km toward the center of the continent 80 km which indicates that the continental lithosphere underlying east antarctica belongs to the classic gondwanan craton the increase in t e indicates that the transantarctic mountain front marks the continent continent boundary between east antarctica and west antarctica the t e values in the other extensional basins of the ross sea exhibit little variation and average approximately 35 km the relatively high t e values are interpreted to indicate that the lithosphere cooled and became mechanically stronger between late cretaceous extension and eocene neogene deposition

a mid twentieth century reduction in tropical upwelling inferred from coralline trace element proxies abstract the cariaco basin is an important archive of past climate variability given its response to inter and extratropical climate forcing and the accumulation of annually laminated sediments within an anoxic water column this study presents high resolution surface coral trace element records montastrea annularis and siderastrea siderea from isla tortuga venezuela located within the upwelling center of this region a two fold reduction in cd ca ratios 3 5 1 7 nmol mol is observed from 1946 to 1952 with no concurrent shift in ba ca ratios this reduction agrees with the hydrographic distribution of dissolved cadmium and barium and their expected response to upwelling significant anthropogenic variability is also observed from pb ca analysis observing three lead maxima since 1920 kinetic control of trace element ratios is inferred from an interspecies comparison of cd ca and ba ca ratios consistent with the sr ca kinetic artifact but these artifacts are smaller than the environmental signal and do not explain the cd ca transition the trace element records agree with historical climate data and differ from sedimentary faunal abundance records suggesting a linear response to north atlantic extratropical forcing cannot account for the observed historical variability in this region

effective elastic thickness of south america and its implications for intracontinental deformation 1 the flexural rigidity or effective elastic thickness of the lithosphere te primarily depends on its thermal gradient and composition consequently maps of the lateral variability of te in continents reflect their lithospheric structure we present here a new te map of south america generated using a compilation of satellite derived grace and champ missions and terrestrial gravity data including egm96 and sagp and a multitaper bouguer coherence technique our te maps correlate remarkably well with other proxies for lithospheric structure areas with high te have in general high lithospheric mantle shear wave velocity and low heat flow and vice versa in this paper we focus on the te of the stable platform we find that old cratonic nuclei mainly archean and early middle proterozoic have in general high te 70 km while the younger patagonian phanerozoic terrane has much lower te 20 30 km suggesting that te is related to terrane age as has already been noted in europe within cratonic south america te variations are observed at regional scale relatively lower te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures rift zones and sites of hot spot magmatism today these low te areas are surrounded by large cratonic nuclei they concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth this implies that intracontinental deformation focuses within relatively thin hot and hence weak lithosphere that cratonic interiors are strong enough to inhibit tectonism and that the differences in lithospheric rigidity structure and composition between stable cratons and sites of intracontinental deformation are not transient and may have been maintained in some cases for at least 500 m y

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

cenozoic landscape evolution of the convoy range to mackay glacier area transantarctic mountains onshore to offshore synthesis on the basis of geomorphic mapping we reconstruct the landscape evolution of the convoy range to mackay glacier area of the rifted margin of the transantarctic mountains and compare the land record with that obtained from offshore marine sedimentary rocks in the ross sea three landform assemblages reflect 1 fluvial planation and dissection 2 local glaciation under temperate conditions and 3 overriding by the east antarctic ice sheet overall landscape evolution is typical of a passive continental margin contrasts in morphology between the convoy range and the adjacent dry valleys and royal society mountains reflect the varying location of the initial drainage divide in relationship to the rifted coast in a wider synthesis we draw the following conclusions for the 260 km long mcmurdo sector of the transantarctic mountains denudation since rifting at ca 55 ma has removed a wedge 4 7 km thick at the coast declining inland to 1 km most denudation occurred in the eocene from planation and incision of river valleys near the coast a subsequent pulse of denudation most rapid at 34 31 ma and declining until ca 17 ma coincided with further crustal extension and a change from cool temperate to polar climate during the same interval there was a progressive decrease in landscape modification by warm based glaciers and or rivers between 14 8 and 13 6 ma the maximum overriding antarctic ice sheet flowed northeastward across the mountains leaving meltwater features crossing high altitude saddles and areally scoured bedrock near the coast since 13 6 ma the landscape has seen little change under a hyperarid polar climate either by local glaciers or subaerially fragile middle miocene surficial deposits still survive there has been tectonic stability since the middle miocene

on the recovery of effective elastic thickness using spectral methods examples from synthetic data and from the fennoscandian shield there is considerable controversy regarding the long term strength of continents te while some authors obtain both low and high te estimates from the bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength others obtain estimates of only 25 km using the free air admittance and suggest that the mantle is weak at the root of this controversy is how accurately te can be recovered from coherence and admittance we investigate this question by using synthetic topography and gravity anomaly data for which te is known we show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free air admittance we reformulate the admittance method and show that it can recover synthetic te estimates similar to those recovered using coherence in light of these results we estimate te in fennoscandia and obtain similar results using both techniques te is 20 40 km in the caledonides 40 60 km in the swedish svecofennides 40 60 km in the kola peninsula and 70 100 km in southern karelia and svecofennian central finland independent rheological modeling using a xenolith controlled geotherm predicts similar high te in central finland because te exceeds crustal thickness in this area the mantle must contribute significantly to the total strength te in fennoscandia increases with tectonic age seismic lithosphere thickness and decreasing heat flow and low te correlates with frequent seismicity however in proterozoic and archean lithosphere the relationship of te to age is ambiguous suggesting that compositional variations may influence the strength of continents copyright 2004 by the american geophysical union

a process oriented approach to modeling the gravity signature of continental margins the gravity anomaly at a continental margin can be regarded as the result of all the processes that have shaped it through time these include rifting sedimentation and magmatic underplating this article discusses a new process oriented approach to gravity interpretation which uses seismic constraints on sediment thickness and the underplate at continental margins to estimate the gravity anomaly that is associated with rifting sedimentation and magmatic underplating by comparing the calculations to the observed free air gravity edge effect anomaly we have been able to determine the elastic thickness and crustal structure and to provide an estimate of the location of the ocean continent transition since crustal structure is a proxy for the amount of heating that occurred at the time of rifting the approach has important implications for the deep water hydrocarbons industry

effective elastic thickness of zealandia and its implications for lithospheric deformation abstract zealandia was once located at the active gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary here we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness te using the topography sediment thickness and gravity data the fan wavelet coherence method is employed to recover te values the effects of dynamic topography and gravity in the subduction zone on the estimated te are considered and removed the final results reveal that the te variation pattern generally coincides well with the tectonic regimes relatively high te values up to 22 km are observed in north island and the southernmost part of south island while central and northern south island are dominated by smaller te values 15 km similar to those of the surrounding submerged plateaus we interpret the relatively high te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone to the north along the tonga kermadec subduction zone the lithosphere surprisingly exhibits anomalously high te values ranging from 22 to 36 km which contradicts the geological evidence of normal faults developed in the incoming pacific plate and the magmatism in the overlying australian plate that generally contribute to low lithospheric strength we speculate that the high te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non isostatic factors finally we find widespread low te values covering the entire arcs and oceanic basins in northern zealandia which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

non linear growth kinematics of opening mode fractures abstract fracture aperture is commonly assumed to be a linear function of fracture length stress and elastic material properties under constant stress fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non elastic deformation processes to investigate the kinematics of fracture opening and its dependence on length and height growth we reconstructed the opening history of three opening mode fractures that are bridged by crack seal quartz cement similar crack seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement using scanning electron microscope cathodoluminescence imaging we determined the thickness and number of crack seal cement layers as a function of position along fracture length and height observed trends in crack seal cement layer thickness and number of fracture opening increments are consistent with non linear fracture growth kinematics consisting of an initial stage of fast fracture propagation relative to aperture growth followed by a stage of slow propagation and pronounced aperture growth consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40 50 m y we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution precipitation creep and concurrent slow possibly subcritical fracture propagation

simple constructions for deformation in transpression transtension zones abstract a simple geometrical construction is demonstrated for determining the orientations of the principal axes of the infinitesimal strain ellipsoid in tectonic zones where boundaries obliquely converge or diverge the construction may also be used to determine the relative magnitudes of the principal axes and the ellipsoid shape for infinitesimal and finite transpressional transtensional strains the analysis leads to a tightening of the definitions of the loosely used terms compression wrench and extension applications of the simple construction and convenient graphical solution are illustrated by brief examples

a reassessment of spectral te estimation in continental interiors the case of north america 1 conventional spectral te studies use the real part of the admittance between gravity anomalies and topography or alternatively the square of the magnitude of the coherency i e coherence here we show the utility of treating both the admittance and coherency as complex quantities inverting the real parts to estimate te we use the imaginary parts to tell if the inversion is biased by noise one method inverts the square of the real coherency with the internal to total load ratio f derived as a function of wave number directly from the gravity and topography the other method inverts the real part of the admittance assuming that f is wave number independent we test the methods using synthetic elastic plate models loaded at the surface and moho in such a way that the final relief is the actual north american topography in some of the models we add gravity noise generated by a model having both surface and internal loads such that the final topography is zero and find that both methods are susceptible to noise application of the two methods to north america gives te maps showing substantial agreement except in regions affected by noise but these are not a dominant part of the total area given the suggested mechanisms by which noise might arise it is not surprising that it is not a more widespread feature of the north american craton importantly both methods show that large parts of the canadian shield are characterized by te 100 km

the long wavelength admittance and effective elastic thickness of the canadian shield the strength of the cratonic lithosphere has been controversial on the one hand many estimates of effective elastic thickness te greatly exceed the crustal thickness but on the other the great majority of cratonic earthquakes occur in the upper crust this implies that the seismogenic thickness of cratons is much smaller than te whereas in the ocean basins they are approximately the same leading to suspicions about the large te estimates one region where such estimates have been questioned is the canadian shield where glacial isostatic adjustment gia and mantle convection are thought to contribute to the long wavelength undulations of the topography and gravity to date these have not been included in models used to estimate te from topography and gravity which conventionally are based only on loading and flexure here we devise a theoretical expression for the free air gravity topography admittance that includes the effects of gia and convection as well as flexure and use it to estimate te over the canadian shield we use wavelet transforms for estimating the observed admittances after showing that multitaper estimates which have hitherto been popular for te studies have poor resolution at the long wavelengths where gia and convection predominate compared to wavelets our results suggest that te over most of the shield exceeds 80 km with a higher te core near the southwest shore of hudson bay this means that the lack of mantle earthquakes in this craton is simply due to its high strength compared to the applied stresses

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

tropical atlantic sea surface temperature variability and its relation to el nino southern oscillation past analyses of tropical atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics across the intertropical convergence zone itcz by analyzing an improved 43 year 1950 1992 record of sst smith et al 1996 and other data derived from the comprehensive ocean atmosphere data set coads it is shown that the regions north and south of the itcz are statistically independent of each other at the seasonal to interannual timescales dominating the data confirming the conclusions of houghton and tourre 1992 some dipole behavior does develop weakly during the boreal spring season when there is a tendency for sst anomaly west of angola to be opposite of that in the tropical north atlantic it is further shown that tropical atlantic sst variability is corre lated with pacific e1 nifio southern oscillation enso variability in several regions the ma jor region affected is the north atlantic area of ne trades west of 40ow along 10on 20on and extending into the caribbean there about 50 80 of the anomalous sst variability is associ ated with the pacific enso with atlantic warmings occurring 4 5 months after the mature phases of pacific warm events an analysis of local surface flux fields derived from coads data shows that the enso related atlantic warmings occur as a result of reductions in the surface ne trade wind speeds which in turn reduce latent and sensible heat losses over the region in ques tion as well as cooling due to entrainment this enso connection is best developed during the boreal spring following the most frequent season of maximum enso anomalies in the pacific a region of secondary covariability with enso occurs along the northern edge of the mean itcz position and appears to be associated with northward migrations of the itcz when the north at lantic warmings occur although easterly winds are intensified in the western equatorial atlantic in response to pacific warm events they do not produce strong local changes in sst contrary to expectations from studies based on equatorial dynamics these teleconnected wind anomalies do not give rise to significant correlations of sst in the gulf of guinea with the pacific enso as the teleconnection sequence matures strong se trades at low southern latitudes follow the de velopment of the north atlantic sst anomaly and precede by several months the appearance of weak negative sst anomalies off angola and stronger positive anomalies extending eastward from southern brazil along 15o 30os

effective elastic thickness and mechanical anisotropy of south china and surrounding regions abstract south china and surrounding regions extend from the eastern tibetan plateau through the tectonically stable sichuan basin and the broad mesozoic magmatic and fold belt to the trench arc basin system in the western pacific which provide an ideal place to study deformation of the continental lithosphere under long term magmatism and oceanic subduction we obtained the effective elastic thickness t e and its anisotropy of south china and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods the t e values of the study area vary from 2 to 75 km with relatively low t e values 30 km along the tectonic boundaries the north south gravity lineament nsgl and seismic zones and in the regions with high surface heat flow the evenly low t e values in the lower yangtze region and the cathaysia block can be attributed to the long lived subduction of the paleo tethys and the paleo pacific ocean basins beneath the south china block scb the nsgl in the scb may separate the unmodified high t e and thermally weakened low t e continental lithosphere due to oceanic subduction despite different distances to the tectonic boundaries earthquakes occur more frequently in regions with t e values of 10 30 km implying strain concentration in the low t e regions a positive correlation between seismic activity and the magnitude of t e anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere the poor correlation between the weak axis of t e anisotropy and the dynamic indicators of the present tectonic regime the shear wave splitting direction the maximum horizontal compressive stress direction confirms that t e anisotropy mainly reflects tectonic inheritance of the continental lithosphere

extensive retreat and re advance of the west antarctic ice sheet during the holocene to predict the future contributions of the antarctic ice sheets to sea level rise numerical models use reconstructions of past ice sheet retreat after the last glacial maximum to tune model parameters 1 reconstructions of the west antarctic ice sheet have assumed that it retreated progressively throughout the holocene epoch the past 11 500 years or so 2 4 here we show however that over this period the grounding line of the west antarctic ice sheet which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf retreated several hundred kilometres inland of today s grounding line before isostatic rebound caused it to re advance to its present position our evidence includes first radiocarbon dating of sediment cores recovered from beneath the ice streams of the ross sea sector indicating widespread holocene marine exposure and second ice penetrating radar observations of englacial structure in the weddell sea sector indicating ice shelf grounding we explore the implications of these findings with an ice sheet model modelled re advance of the grounding line in the holocene requires ice shelf grounding caused by isostatic rebound our findings overturn the assumption of progressive retreat of the grounding line during the holocene in west antarctica and corroborate previous suggestions of ice sheet re advance 5 rebound driven stabilizing processes were apparently able to halt and reverse climate initiated ice loss whether these processes can reverse present day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity parameters that are difficult to measure and to incorporate into ice sheet models

spatial arrangement of faults and opening mode fractures spatial arrangement is a fundamental characteristic of fracture arrays the pattern of fault and opening mode fracture positions in space defines structural heterogeneity and anisotropy in a rock volume governs how faults and fractures affect fluid flow and impacts our understanding of the initiation propagation and interactions during the formation of fracture patterns this special issue highlights recent progress with respect to characterizing and understanding the spatial arrangements of fault and fracture patterns providing examples over a wide range of scales and structural settings five papers describe new methods and improvements of existing techniques to quantify spatial arrangement one study unravels the time evolution of opening mode fracture spatial arrangement which are data needed to compare natural patterns with progressive fracture growth in kinematic and mechanical models three papers investigate the role of evolving diagenesis in localizing fractures by mechanical stratigraphy and nine discuss opening mode fracture spatial arrangement two papers show the relevance of complex cluster patterns to unconventional reservoirs through examples of fractures in tight gas sandstone horizontal wells and a study of fracture arrangement in shale four papers demonstrate the roles of folds in fracture localization and the development spatial patterns one paper models along fault friction and fluid pressure and their effects on fault related fracture arrangement contributions address deformation band patterns in carbonate rocks and fault size and arrangement above a detachment fault three papers describe fault and fracture arrangements in basement terrains and three document fracture patterns in shale this collection of papers points toward improvement in field methods continuing improvements in computer based data analysis and creation of synthetic fracture patterns and opportunities for further understanding fault and fracture attributes in the subsurface through coupled spatial size and pattern analysis

spatial variations of flexure parameters over the tibet quinghai plateau we investigate the tibet quinghai plateau and the tarim basin in terms of spatial variations of the elastic thickness te in the frame of the thin plate flexure model the method of investigation makes use of a convolutive method which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance coherence methodologies we study the relation between the topographic and subsurface loads and the observed crust mantle interface cmi undulations the latter having been obtained from gravity inversion the gravity data used for the inversion are a unique set of high quality data available over the chinese part of the plateau and constitute the highest resolution grid today available in this impervious area the gravity inversion is constrained by results from the study of the propagation of seismic waves the two extensive sedimentary basins the tarim and the qaidam basins are modeled by forward gravity modeling the oscillations of the cmi obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads it is found that the modeling of the sedimentary basins is essential in the flexure analysis the spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau most of the tibet plateau has low te varying in the bounds 10 30 km with lower values in the qiangtang terrain where the te reaches 8 km the tarim and the qaidam basins precambrian platforms overlain by sediments are rigid and have a te of up to 110 km and 70 km respectively the flexural analysis distinctly discerns the tibet plateau with thick crust part of which is molten from the cratonic areas the tarim and qaidam basins which though of thinner crust act as undeformable rigid blocks

effective elastic thickness of the arabian plate weak shield versus strong platform the fan wavelet method has been employed to calculate high resolution maps of variations of the effective elastic thickness eet for the arabian plate and surroundings as the initial data we use high resolution gravity field topography and recent models of sedimentary basins the western part of the plate is generally characterized by low to midvalues of eet 10 30 km while the eastern one by high values 50 km and more in the core this finding confirms that the pronounced asymmetry of the plate is rather associated with fundamental structural differences of the lithosphere than with a forced tilt of the plate due to the rifting in the west southwest and subduction in the northeast therefore the high topography in the western part of the plate is likely supported by relatively hot mantle that is also responsible for the decrease of eet these results are generally in agreement with recent seismic tomography models

global and local isostatic coherence from the wavelet transform 1 a method to compute the variations in lithospheric elastic thickness te has been developed using the wavelet transform the technique which uses a superposition of two dimensional morlet wavelets in a geometry named a fan wavelet is designed to yield isotropic yet complex wavelet coefficients for the co and cross spectra of gravity and topography data these are then used to compute a spatially varying isostatic coherence from which both global and local estimates may be obtained we applied the method to synthetic gravity and topography generated for a thin elastic plate of uniform thickness 20 km yielding an apparent spatially variable te of 24 5 3 5 km the estimated global coherence for this model appears to fit the theoretical prediction as well as fourier transform based estimates and is smoother than these we also computed the wavelet coherence and hence spatially varying te for a plate of non uniform thickness yielding a difference with the model of 2 0 1 7 km

circulation mechanisms related to northeast brazil rainfall anomalies this study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere with focus on the late boreal winter and extreme climatic events in northeast brazil data sources comprise rainfall records in the nordeste surface ship observations of the sea surface temperature sst pressure and wind fields coads period 1948 1990 satellite observations of tropical convection hrc 1971 1988 and upper air analyses of the european centre for medium range weather forecasts ecmwf 1980 1991 all for the tropical atlantic sector interhemispheric sst gradients are most strongly associated with north south contrasts in pressure and the meridional wind component whereas the inverse sst pressure relations in situ are less close enhanced northward temperature increase in the tropical atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component which is representative of a northward displaced intertropical convergence zone itcz with the latter in turn leading to drought in northeast brazil interhemispheric contrasts in the 850 1000 mbar layer mean temperature are similar to those of sst and account for most of the meridional surface pressure gradient during wet as compared to dry years in northeast brazil the tropical north atlantic is cooler and the south atlantic warmer and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator resulting in increased reduced surface pressure over the north south atlantic as well as reduced southerly surface wind stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the nordeste related to a southward displaced near equatorial convection belt the surface and upper air evidence thus indicates the ways in which the interhemispheric sst gradients exert a hydrostatic control on the lower tropospheric thickness pattern and thus force the south north surface pressure gradients and the surface meridional wind component and hence modulate the latitude position of the itcz and nordeste rainfall

variations of the effective elastic thickness over china and surroundings and their relation to the lithosphere dynamics abstract the effective elastic thickness te characterizes response of the lithosphere to a long term tectonic loading as a proxy of the lithospheric strength te can be used to address mechanical behavior and deformation of the blocks with complex geological structure here we use the multitaper coherence method to determine spatial variations of te in china and surroundings based on the topography and bouguer gravity anomaly data the results show that the te values are high 70 km over the cratons e g india craton the siberian craton north china block and also variations of these values are significant over these blocks low te corresponds to the young orogens e g himalayas altun shan qilian shan longmenshan qinling dabie and daxing anling taihang mountains etc combined with other data the lateral variations of te within the north china and south china blocks and the tibetan plateau indicate that the lithospheric strength in china mainland depends on both lithospheric structure and mantle dynamics we guess that during the mesozoic cenozoic period the strength of the lithosphere might have been significantly altered by the thermodynamic processes associated with the india eurasia collision in the southwest and the subduction of the pacific plate in the east the te variations also play a major role in the lithospheric evolution and deformation since the mesozoic

a new analytical solution estimating the flexural rigidity in the central andes summary we present a new 2 d analytical solution of the fourth order differential equation which describes the flexure of a thin elastic plate the new analytical solution allows the differential equation for an elastic plate to be solved for any irregular shaped topography with a high spatial resolution we apply the new method to the central andes the flexural rigidity distribution calculated by this technique correlates well with tectonic units and the location of fault zones for example the central andean gravity high correlates with the presence of a rigid high density body