constraints on the deep structure and dynamic processes beneath the alps and adjacent regions from an analysis of gravity anomalies summary gravity anomalies including terrain corrections over the western alps and the molasse basin are within 30 mgal of those calculated assuming local airy isostatic equilibrium but isostatic anomalies over regions nw and se of the alps exceed 50 mgal therefore mass excesses and deficits that must be supported by the strength of the lithosphere or by dynamically induced stress in the asthenosphere appear to be greater beneath the areas surrounding the alps than beneath the alpine chain itself attempts to account for the gravity gradient over the molasse basin and sub alpine chains and the small bouguer anomalies measured over the elevated areas of the vosges and black forest by the flexure of an elastic plate require an absurd set of parameters the high elevations of the vosges and the black forest are presumably due to high temperature in the underlying uppermost mantle but the deviations from isostatic equilibrium as well as the high temperature probably result from active upwelling in the asthenosphere beneath this area the absence of large isostatic anomalies over the alps and the failure of an elastic model to account for the gravity anomalies over the molasse basin suggest that the dynamic processes that flexed the european plate down to form the molasse basin and that built the alpine chain have waned the late cenozoic uplift of the molasse basin and the alps might be a consequence of a diminution or termination of downwelling of mantle material beneath the alps

assembling and reactivating the proterozoic capricorn orogen lithotectonic elements orogenies and significance abstract the capricorn orogen was initiated during palaeoproterozoic suturing events that brought together the archaean yilgarn and pilbara cratons to form the west australian craton the orogen comprises palaeoproterozoic plutonic and medium to high grade metamorphic rocks of the gascoyne complex a series of palaeoproterozoic volcano sedimentary and sedimentary basins including the ashburton blair yerrida bryah padbury and earaheedy basins and the deformed margins of the pilbara and yilgarn cratons major pulses of deformation and metamorphism took place during the ca 2200 ma ophthalmian orogeny the 2000 1960 ma glenburgh orogeny the 1830 1780 ma capricorn orogeny and a more localized unnamed event at the end of the palaeoproterozoic 1670 1620 ma the orogen has been the site of repeated intracratonic reactivation with renewed basin formation magmatism and orogeny during the mesoproterozoic and the neoproterozoic the exhumed palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic mesoproterozoic edmund and collier basins bangemall supergroup which have been deformed during the neoproterozoic edmundian orogeny the ophthalmian and glenburgh orogenies affected the northern and southern margins of the orogen respectively the former resulted in the development of a foreland basin along the pilbara craton margin whereas the latter is associated with accretion of an allochthonous element the glenburgh terrane of the gascoyne complex to the northwestern margin of the yilgarn craton effects of the capricorn orogeny extended across the entire orogen and provide a younger limit for amalgamation of the yilgarn and pilbara cratons a number of basins developed along both the northern and southern margin of the orogen during this event and were filled with sediment eroded from the yilgarn craton the gascoyne complex and a presently unexposed early palaeoproterozoic terrane the capricorn orogeny corresponds with a major phase of continental collision both in australia and world wide which appears to mark a major phase of supercontinent assembly mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings including the world class iron orebodies of the hamersley basin

gravity anomalies crustal structure and isostasy associated with the proterozoic capricorn orogen western australia abstract present knowledge of the proterozoic capricorn orogen in western australia is largely based on surface geological studies geophysical studies of the crust of the orogen are limited to a few regional seismic refraction profiles and regional potential field interpretations the aim of this paper is to present a simple gravity constrained crustal model for the capricorn orogen anomalous lows immediately north of the yilgarn craton are prominent features of the capricorn orogen gravity field and geoid these anomalous lows do not correlate with surface geology or elevated topography their character suggests that they reflect thickened crust in the core of the capricorn orogen preserved in a state of mechanical equilibrium for about two billion years the location of the anomalous lows and associated crustal thickening south of the inferred location of the pilbara yilgarn suture suggest that the pilbara craton was subducted southwards beneath the previously combined yilgarn craton and southern gascoyne complex in a simple two dimensional crustal model features such as the bangemall bryah and yerrida basins are assumed to be dominantly supported by the rigid lithosphere of this region effective elastic thickness 50 100 km the yerrida basin and adjacent marymia inlier are modelled as north dipping features the bangemall basin as a southward deepening basin and the lower boundary of the gascoyne complex is best modelled as a north dipping contact at the northern margin of the orogen the sylvania inlier appears to lie at shallow depths 1 km beneath the eastern ashburton basin the rocks of the south eastern hamersley province reach a maximum thickness of about 7 km and thin toward the north a notable feature of the model is that it does not maintain isostatic equilibrium the prolonged erosion of this region and the prominent upper crustal density anomalies mean that it is difficult to develop gravity models that maintain isostatic balance a simple isostatic analysis suggests that the present day topography of the capricorn orogen is over compensated this over compensation can be explained by resistance of the rigid lithosphere of western australia to rebound in response to mesozoic or cainozoic erosion mantle flow acting to pull the crust downward from its equilibrium position could also lead to the apparent lack of isostatic equilibrium

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

australia s moho a test of the usefulness of gravity modelling for the determination of moho depth abstract in general seismic methods provide a reliable way to image the crust mantle interface which is marked by a rapid increase in seismic velocity the moho however the coverage provided by seismic networks is necessarily limited due to access difficulties and the cost and labour involved in collecting data gravity data provide an alternative way to model the depth to the moho and provide more consistent and broader coverage we discuss the usefulness of gravity data to model moho depth and the advantages and disadvantages of several gravity modelling methods as an example a model of australia s moho is generated through seismically constrained gravity inversion including an estimate of modelling uncertainty the inversion results demonstrate that gravity inversion is generally useful but that its usefulness is subject to the following limitations 1 gravity inversion cannot spontaneously generate thick high density crust nor thin low density crust and unless constrained will not generate a correct moho where such crust exists 2 major errors in the definition of the a priori density structure in particular features that are fixed during inversion will influence the moho results 3 applying a broad range of inversion parameters is necessary to characterise uncertainty model variability maps for australia show that the average error is less than 5 km there is a general relationship with seismic coverage but the areas of highest uncertainty are not necessarily those with the lowest seismic estimate density comparison with previous seismic and seismic gravity models of australia s moho indicates that low seismic data density limits usefulness due to higher uncertainty in the gravity inversion high seismic data density also limits usefulness because moho depth is largely known and there is little scope for change the usefulness of gravity inversion is maximum under conditions where seismic coverage is moderately dense but estimates are well distributed

effects of loads in the upper crust on estimates of the elastic thickness of the lithosphere summary the admittance function and coherence relating gravity and topography are important constraints on the elastic thickness te of the lithosphere forsyth 1985 modelled them with a lithosphere loaded both externally by topography and internally by density contrasts such as initial undulations on the moho his approach has yielded thicknesses as high as 150 km for the continental lithosphere mckenzie fairhead 1997 argued that this method can only supply an upper bound on te because the response is biased by noise due to density variations in the upper crust we have used forsyth s approach to analyse the effect of shallow heterogeneities such as sedimentary basins and igneous intrusions simulating them by a thin layer with a laterally variable density the information content of the admittance and coherence is concentrated in the wavenumbers at which the two responses roll off to half their zero wavenumber values these two parameters are used to explore the interdependence of the retrieved elastic thickness and internal external loading ratio f provided the internal load is less than 10 per cent of the external it does indeed bias the response so that te is overestimated however for predominantly internal loads the reverse is the case and the value of te obtained by assuming equal internal and external loads is actually a lower bound it is particularly important to allow for the flexure due to upper crustal loads when the lithosphere is old and topography subdued due to prolonged erosion application of our approach to mckenzie fairhead s 1997 western australia data shows that the loading ratio depends strongly on wavenumber but that it is still possible to use the response data to constrain te which must be at least 90 km if the results obtained by conventional spectral estimation are accepted at least 40 km using the multitaper method

insights into the crustal structure and geodynamic evolution of the southern granulite terrain india from isostatic considerations the southern granulite terrain of india formed through an ancient continental collision and uplift of the earth s surface was accompanied by thickening of the crust once the active tectonism ceased the buoyancy of these deep crustal roots must have supported the nilgiri and palani cardamom hills here the gravity field has been utilized to provide new constraints on how the force of buoyancy maintains the state of isostasy in the southern granulite terrain isostatic calculations show that the seismically derived crustal thickness of 43 44 km in the southern granulite terrain is on average 7 8 km more than that required to isostatically balance the present day topography this difference cannot be solely explained applying a constant shift in the mean sea level crustal thickness of 32 km the isostatic analysis thus indicates that the current topography of the southern granulite terrain is overcompensated and about 1 0 km of the topographic load must have been eroded from this region without any isostatic readjustment the observed gravity anomaly an order of magnitude lower than that expected 125 mgal however shows that there is no such overcompensation thermal perturbations up to pan african present day high mantle heat flow and low te together negate the possible resistance of the lithosphere to rebound in response to erosional unloading to isostatically compensate the crustal root compatible to seismic moho a band of high density 2 930 kg m 3 in the lower crust and low density 3 210 kg m 3 in the lithospheric mantle below the southern granulite terrain is needed a relatively denser crust due to two distinct episodes of metamorphic phase transitions at 2 5 ga and 550 ma and highly mobilized upper mantle during pan african thermal perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography

u pb age and hf isotopic constraints of detrital zircons from the himalayan foreland subathu sub basin on the tertiary palaeogeography of the himalaya the subhimalaya of nw india exposes a well developed marine to continental foreland sequence preserving sediments shed from the tectonically evolving cenozoic himalayan orogen detrital zircon u pb ages and their hf isotopic compositions of seven samples were analyzed from the subathu sub basin foreland formations to constrain sediment provenance the detrital zircon u pb age spectra yielded a similar pattern for all foreland formations with major peaks clustered at 500 650 ma 900 1000 ma and 2500 ma corresponding to indistinguishable tethyan himalaya and higher himalayan crystalline sources and those at 1800 1900 ma corresponding to lower a lesser himalayan arc source comparison of the foreland detrital zircon u pb age spectra with himalayan lithotectonic source units in conjunction with their hf isotopes and previously determined sediment petrography and age constraints suggest that the sediments to the flysch marine subathu formation 65 to 41 ma were sourced from a positive relief area of the tethyan himalaya mixed with detritus from lower lesser himalaya and additional cratonic northern indian precambrian rocks no cretaceous to eocene aged detrital zircons contributed by eroding ladakh batholith north of the indus tsangpo suture zone were recorded in the palaeogene aged foreland samples the younger synorogenic continental dagshai formation 30 to 22 ma sediments were sourced from the very low grade tethyan himalaya whereas the alluvial facies kasauli formation 22 to 13 ma began to receive sediments additionally from erosion of the medium grade metamorphic rocks of the higher himalayan crystalline and the lower lesser himalaya the lower siwalik formation sediments 13 11 5 ma were derived from the low grade rocks of the tethyan himalaya both the upper and lower lesser himalaya and additionally from the eroding high grade migmatitic gneisses of the higher himalayan crystalline thus from the similar detrital zircon age spectra of the foreland formations we infer that differences in the continental sediment detritus into the foreland basin reflect a straightforward sediment mixture of variable proportions from fixed himalayan lithotectonic sources which progressively shifted towards the foreland with development of the orogen

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

characterization of a proposed palaeoproterozoic suture in the crust beneath the curnamona province australia abstract we use constrained forward and inverse modeling of bouguer gravity data to investigate a regional positive gravity anomaly that defines but is not confined to the eastern margin of the curnamona province best fit results imply that the positive gravity values are sourced from a relatively dense terrane 0 04 mg m 3 with respect to surrounding crust located at middle crustal levels 16 27 km depth which is likely dismembered and offset into discrete slivers by later tectonism consideration of pre 1500 ma plate reconstructions of the australian continent correlates this terrane with an interpreted buried suture in the mount isa inlier of northern australia where allochthonous crust was amalgamated to the australian margin at ca 2 20 1 85 ga bouguer gravity forward and inverse modeling also delineates a shallow west dipping crustal scale structure that possibly represents a primary boundary between precambrian basement of australia and the eastern fold belts accreted to the precambrian margin during the palaeozoic

seasonality of the north atlantic oscillation abstract monthly sea level pressure slp data from the national centers for environmental prediction reanalysis for 1948 99 are used to develop a seasonally and geographically varying mobile index of the north atlantic oscillation naom naom is defined as the difference between normalized slp anomalies at the locations of maximum negative correlation between the subtropical and subpolar north atlantic slp the subtropical nodal point migrates westward and slightly northward into the central north atlantic from winter to summer the naom index is robust across datasets and correlates more highly than eof coefficients with historical measures of westerly wind intensity across north atlantic midlatitudes as measured by this mobile index the nao s nodes maintain their correlation from winter to summer to a greater degree than traditional nao indices based on fixed stations in the eastern north atlantic azores lisbon iceland when the naom index is extended back to 1873 its annual values during

seismic structure of the crust and uppermost mantle of the capricorn and paterson orogens and adjacent cratons western australia from passive seismic transects abstract the seismic structure of the proterozoic capricorn and paterson orogens and adjacent archaean yilgarn and pilbara cratons western australia is determined using a passive seismic approach we use recordings of distant earthquakes made along two transects of 3 component broadband stations the stations were deployed for approximately 1 year mid 2006 2007 during which time 70 earthquakes were recorded at each station with a suitable signal to noise ratio for receiver function analysis and hence the s wavespeed profiles of the crust and uppermost mantle beneath each recording station are determined we investigate the deep crustal constraints on terrane boundary locations the patterns of seismic discontinuities in the crust and the variations in the depth and character of the moho this broad scale information regarding the present day crustal architecture and hence the crustal evolution of western australia complements previous surface geological and other geophysical studies western australia is an exceptionally large well preserved region of ancient crust and hence this work also adds to the body of knowledge regarding proterozoic orogenic processes in general the new passive seismic work shows a region of double crust or upper mantle discontinuities beneath the glenburgh terrane adjacent to the northwest yilgarn craton the upper crust of the orogens is always layered whereas the cratons have a simple upper crust both the capricorn and paterson orogens are characterised by deeper moho discontinuities with a lesser wavespeed contrast than the very sharp discontinuity observed beneath the adjacent pilbara and yilgarn cratons this is consistent with the weaker orogenic crust of the capricorn and paterson orogens accommodating most of the horizontal deformation during assembly and reworking of the west australian craton while the pilbara and yilgarn cratons acted as rigid crustal blocks

moho geometry gravity inversion experiment moggie a refined model of the australian moho and its tectonic and isostatic implications abstract at the continent scale models of moho depth based on seismic estimates alone can be inadequate due to irregular or sparse data gravity based moho modelling provides better coverage however the methods used are typically hampered by an inability to explicitly honour seismic constraints and are also limited by over simplistic model conditions e g laterally homogenous layering i present a new method to generate a continent scale moho model based on the constrained inversion of free air gravity data this method explicitly honours seismic moho estimates and accounts for a laterally heterogeneous crust and mantle resolution and sensitivity testing shows that for wavelengths greater than 200 km crustal density and moho depth are recovered with reasonable accuracy 30 kg m 3 and 3 km respectively moggie uses a six layer model incorporating ocean sedimentary basin upper crust lower oceanic crust eclogitised crust and mantle inversion variables were the density of the crustal layers constrained by a standard density model and the depths to intra crustal boundaries and the moho constrained by 230 seismic depth estimates the results demonstrate that a balanced approach to seismically constrained gravity inversion has the capability to generate detailed and well constrained models of the moho and crustal density at the continent scale for australia this is a clear improvement on the sparse and irregular resolution of the moho provided by seismic estimates of crustal thickness which fail to resolve short wavelength features newly defined tectonic features include extensive magmatic underplates crustal scale shear zones and the boundaries between tectonic blocks isostatic analysis reveals that little of the continent is close to isostatic equilibrium with isostatic disequilibria preserved at multiple scales from hundreds of kilometres to the entire continent these disequilibria are interpreted to indicate long wavelength flexure of highly competent continental lithosphere

evidence for solar forcing on the indian monsoon during the last millennium abstract solar influence on the intensity of the indian monsoon is demonstrated using a sediment core from the eastern arabian sea dating back to 1200 yr through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity the intensity of the indian monsoon is found to have decreased during periods of solar minima during the last millennium periodicities of 200 20 105 15 and 60 10 yr are observed in the proxy records coinciding with those known for solar cycles the 60 yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the indian monsoon on a multi decadal time scale evidence for the presence of a coupled atmospheric forcing for the indian and east african monsoons on a centennial time scale is also seen

the northern margin of the capricorn orogen western australia an example of an early proterozoic collision zone abstract the early proterozoic 2200 1600 ma capricorn orogen is a major zone of deformation metamorphism and granite emplacement between the archaean yilgarn and pilbara cratons of northwest australia capricorn orogen structures are recognized along the southern margin of the pilbara craton deforming the archaean granite greenstone terrain of the sylvania inlier together with rocks of the hamersley ashburton and blair basins in the sylvania inlier deformation is characterized by large scale shear zones interpreted as nne directed thrusts in the hamersley basin orogenic deformation forms the ophthalmia fold belt and can be divided into two groups of structures in the southwest large scale dome and basin folds pre ashburton basin are present in the southeast folds and faults are part of a younger syn ashburton basin n directed foreland fold and thrust belt directly linked to deformation in the sylvania inlier the deformed ashburton basin and adjacent parts of the hamersley basin form the ashburton fold belt two periods of deformation are recognized early recumbent folding d1a post ashburton basin pre blair basin and later dextral wrench faulting and associated folding d2a syn or post blair basin capricorn orogen structures are interpreted as the result of an oblique continent continent collision between the pilbara and yilgarn cratons along the southern pilbara margin collision first occurred in the southeast and migrated northwest with the ashburton basin evolving as a foreland basin late stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

identifying the lithospheric structure of a precambrian orogen using magnetotellurics the capricorn orogen western australia abstract a 300 km long magnetotelluric mt survey was carried out across the capricorn orogen in western australia the capricorn orogen includes reworked crust of the archaean yilgarn and pilbara cratons and the allochthonous latest archaean to palaeoproterozoic glenburgh terrane in the southern gascoyne complex and the palaeoproterozoic northern gascoyne complex the survey aimed to increase our understanding of the juxtaposition of these crustal elements by obtaining information about their electrical structure with depth phase tensor analysis of the data showed that 2d mt inversion could only be carried out for stations in the centre of the profile crossing the gascoyne complex the resulting model showed that there is no electrical distinction between the glenburgh terrane and the northern gascoyne complex suggesting that the glenburgh terrane may form basement to the whole of the gascoyne complex 3d forward modelling was carried out for the entire survey line incorporating the basement blocks as well as the surrounding ocean and sedimentary basins station data are most closely reproduced by a model in which the margin between the glenburgh terrane and the yilgarn craton the errabiddy shear zone dips south with crust of the glenburgh terrane wedged beneath the northern yilgarn craton forward modelling results also suggest that the main boundary between the gascoyne complex and the pilbara craton to the north is a steeply dipping structure beneath the edmund and collier basins that may correlate with the talga fault

geology and tectonic evolution of the palaeoproterozoic bryah padbury and yerrida basins formerly glengarry basin western australia implications for the history of the south central capricorn orogen abstract the palaeoproterozoic bryah padbury and yerrida basins are situated along the northwestern margin of the archaean yilgarn craton central western australia these basins form part of the capricorn orogen which developed between 2 0 and 1 8 ga as a result of the collision between the archaean pilbara and yilgarn cratons the bryah padbury and yerrida basins which at the present day cover a total area of ca 20 000 km2 were formerly considered as one geological entity the glengarry basin these three basins are characterized by distinct stratigraphy igneous activity structural and metamorphic history and mineral deposit types igneous activity only affected the bryah and yerrida basins with voluminous eruptions of tholeiitic magma in the bryah basin tholeiitic volcanic rocks are mg rich and have mixed morb to oceanic island chemical signatures but with a boninitic subduction related component in the yerrida basin tholeiites are fe rich and have chemical signatures that suggest a mixed tectonic environment ranging from oceanic to continental it is considered possible that this tholeiitic magmatism is related to a mantle plume two models for the tectonic evolution of the bryah padbury and yerrida basins are proposed 1 the bryah and yerrida basins were formed in a back arc setting whilst the padbury basin developed as a retro arc foreland basin over the bryah basin and or 2 strike slip transtension during and following the pilbara yilgarn collision created the bryah and yerrida strike slip pull apart basins a change in regional stress regime resulted in the inversion of the basins and the development of a foreland basin in the northwest padbury basin

shrimp u pb geochronology of magmatism and thermal events in the archaean marymia inlier central western australia zircons from porphyry and granitoid samples collected in and around the marymia gold mine in the marymia inlier western australia record a complex history the results of u pb studies confirm that the plutonic well greenstone belt and the surrounding granitoid envelope including a 2 721 6 ma intrusion represent an archaean terrain which was intruded by high level felsic to intermediate porphyries at 2 694 7 ma and potentially also at 2 660 4 ma zircon xenocrysts ca 3 35 2 93 and 2 74 ga indicate that there was older crust within or below the greenstone belt at the time of porphyry emplacement zircons from the granitoid envelope and intrusions within the greenstone belt record subsequent metamorphism and or hydrothermal activity coeval with magmatism in the late archaean ca 2 66 2 63 ga and peak metamorphism magmatism and gold mineralisation in the yilgarn block a later period of metamorphism and hydrothermal activity at ca 1 72 ga is coeval with orogenesis in the southern capricorn orogen both the late archaean and palaeoproterozoic thermal events have altered zircons redistributed trace elements and caused zircon recrystallisation which is distinctive in its isotope chemistry in particular th u ratios 1 and morphology e g homogeneous in transmitted light and back scattered electron images but sector zoned in cathodoluminescence

structural metamorphic evolution of the palaeoproterozoic bryah and padbury groups during the capricorn orogeny western australia abstract volcano sedimentary successions of the bryah and padbury groups were deposited and deformed along the northern margin of archaean yilgarn craton western australia during the ca 2 0 1 8 ga capricorn orogeny this orogeny is interpreted to record convergence and collision of the yilgarn craton with the pilbara craton to the north the bryah group with its extensive mafic and ultramafic volcanic rocks represents a short lived proto oceanic backarc basin whereas the sedimentary padbury group records the filling in of a retro arc foreland basin the two groups were deformed as a single package and are now everywhere in faulted contact with older palaeoproterozoic rocks or underlying archaean granite greenstone four events are recognized in a progressive compressional deformation regime during d1 subhorizontal high strain shear zones and tectonic interleaving developed between the volcanosedimentary succession and reworked archaean crustal fragments that were underthrust or underplated during initial basin closure movement directions were mainly north to south d2 n s shortening resulted in upright e w folds dominated by two regional basement cored anticlines which are interpreted as fault bend folds above a crustal scale frontal thrust ramp uplifted deeper crustal rocks are now represented by zones of higher grade metamorphic assemblages in the core of these anticlines d3 folds are upright n s striking and restricted to a zone depression between the two basement cored anticlines or domes d2 and d3 folds are largely mutually exclusive with little or no evidence for overprinting and are regarded as contemporaneous the d3 structures are interpreted to have formed above lateral ramps in the crustal scale thrust fault during continued n s shortening d4 is a minor event of nne ssw compression and did not influence the regional geometry this deformation history records the closure and deformation of marginal back arc and foreland basins

patterns of low frequency monthly sea level pressure variability 1899 1986 and associated wave cyclone frequencies abstract spatial patterns of low frequency sea level pressure slp variability are identified by performing rotated principal component analysis rpca on a long term 1899 1986 northern hemisphere gridded dataset the analysis is limited to the region 160 e eastward to 40 e due to missing data early in the century the objective is to identify a comprehensive set of highly recurrent slp teleconnection patterns to examine some aspects of their seasonality and to identify the associated mean winter pressure fields and cyclone frequencies occurring at times of opposite eigenvector polarity the results are further described in the context of the southern oscillation and known midtropospheric teleconnection patterns four low frequency variability patterns are identified over the atlantic european sector including i the north atlantic 0scillation nao and spatial patterns with slp variability centers over ii the eastern atlantic eatl iii southern europe and the northern mediterranean basin s

a physical mechanism for north atlantic sst influence on the indian summer monsoon a link between the atlantic multidecadal oscillation amo and multidecadal variability of the indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking atlantic climate and monsoon has been identified the amo produces persistent weakening strengthening of the meridional gradient of tropospheric temperature tt by setting up negative positive tt anomaly over eurasia during northern late summer autumn resulting in early late withdrawal of the south west monsoon and persistent decrease increase of seasonal monsoon rainfall on inter annual time scales strong north atlantic oscillation nao or north annular mode nam influences the monsoon by producing similar tt anomaly over eurasia the amo achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong nao nam events this mechanism also provides a basis for explaining the observed teleconnection between north atlantic temperature and the asian monsoon in paleoclimatic proxies citation goswami b n m s madhusoodanan c p neema and d sengupta 2006 a physical mechanism for north atlantic sst influence on the indian summer monsoon

winter surface pressure anomalies over eurasia and indian summer monsoon in this study the spatial relationships between the monthly mean sea level pressure mslp anomalies and indian summer monsoon rainfall ismr have been examined using the gridded mslp data gmslp2 for 46 years 1949 1994 winter mslp anomalies over nw europe south europe are negatively positively correlated with ismr during the strong deficient monsoon years there is enhanced weakened poleward pressure gradient over eurasia the europe pressure gradient index which is an indicator of the winter poleward pressure gradient is significantly and positively correlated with ismr this relationship also suggests a link between the north atlantic oscillation and ismr on inter annual time scale the winter pressure gradient over eurasia has physical linkage with indian summer monsoon rainfall through winter snow cover anomalies and springtime mid latitude activities like upper tropospheric blocking highs and stationary wave anomalies

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

delineation of a large ultramafic massif embedded within a major sw pacific suture using gravity methods abstract gravity studies have delineated the largest ultramafic massif in new zealand embedded within a buried major sw pacific crustal suture zone this suture records terrane collision onto the gondwana margin during the mesozoic and separates a forearc terrane from an outboard accretionary prism terrane it can be traced throughout the length of new zealand as the junction magnetic anomaly and contains the permian dun mountain ophiolite belt which in the south island of new zealand is characterized by a string of isolated ultramafic massifs in a sheared matrix of serpentinite and sediment our analysis reveals a steep gravity gradient at the suture boundary which is attributed to a newly recognised density contrast 0 1 mg m 3 between terranes of the forearc and the accretionary prism the massif itself is marked by the occurrence of a strong elongate residual gravity anomaly 120 g u extending 50 km along the suture and coincident with the junction magnetic anomaly it is modelled at its southern end as a dense 15 km wide source body extending to at least 6 km in depth in conjunction with detailed aeromagnetic data this modeling indicates the presence of a spindle shaped ultramafic massif analogous to but larger than similar bodies found within the dun mountain ophiolite belt elsewhere this fabric of sheared serpentinites enclosing ultramafic massifs therefore extends at least the length of new zealand and probably beyond in part it may result from accretion of asperities in the subducting plate but it is also due to disruption of larger ultramafic bodies during subsequent strike slip motion which caused the remarkable linearity of the dun mountain belt given the common occurrence of the plate tectonic processes involved it is likely that such structures can be found in other regions around the world using similar geophysical potential field methods