the present day stress field of australia abstract the unusual present day crustal stress pattern of the australian continent has been the subject of scientific debate for over 25 years the orientation of maximum horizontal present day stress s hmax in continental australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub parallel to the direction of absolute plate motion previous studies on the stress pattern of australia revealed that the complex stress pattern of the continent is controlled at a first order by the superposition of plate tectonic forces exerted at the plate boundaries however prior analysis of the contemporary australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern australia and has not extensively addressed the numerous smaller scale variations in stress orientation the recent development of unconventional reservoirs in australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly constrained areas in eastern australia in addition stress analysis in conventional hydrocarbon mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the australian stress map asm this study presents the new release of the asm with a total of 2150 stress data records in australia increased from 594 data records in 2003 the 2016 asm contains 1359 data records determined from the interpretation of drilling related stress indicators 650 from earthquake focal mechanism solutions 139 from shallow engineering measurements and two from geological indicators the results reveal four distinct regional trends for the s hmax orientation in australia including a nne ssw s hmax trend in northern and northwestern australia which rotates to a prevailing e w orientation in most western and south australia the orientation of s hmax in eastern australia is primarily ene wsw and swings to nw se in southeastern australia a comparison between the new asm database and neotectonic features further confirms the role of contemporary stress in recent deformation of the australian crust the 2016 asm reveals significant discrepancies between newly observed s hmax orientations and predictions by published geomechanical numerical models for the australian continent forces generated at the boundary of indo australian plate remain the major control on the regional crustal stress pattern in continental australia however the increased data density particularly in eastern australia reveals numerous local perturbations of the stress field that were not previously clearly captured hence the key findings of the new release are that local intra plate stress sources are more significant than previously recognized particularly in eastern australian basins and cause substantial local scale rotations of the present day crustal stress pattern that have not been factored into existing australian stress models

the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

synchronous compressional pulses in extensional basins abstract such paradoxes as synchronous compression and tension in extensional sedimentary basins and synchronous deformation of intractonic basins and eustatic sea level falls can each be related to pulsed changes in curvature of the earth s surface australia is everywhere in a state of significant relatively uniform upper crustal horizontal compression approximately perpendicular to the continental margins and generally directed towards its centre the compression commenced in the pliocene but was absent in the late miocene during the evolution of the gippsland and bass basins under extensional stress from the late jurassic to recent seven pulses of similarly directed compression have been identified from the campanian to recent time equivalent compressional pulses can also be demonstrated in such widely separated areas as northwest and northeast australia northwest europe and central america nine earlier pulses of early triassic to albian age are similarly synchronous global events and another twenty three pulses inferred since the neoproterozoic 800 ma are probably similarly globally synchronous earth expansion with subduction can explain these observations since it causes the surface curvature to decrease resulting in flattening of the semi rigid crust flattening results in lower crustal extension which generates sedimentary basins while contemporaneous upper crustal compression inverts and folds the sediments since globally synchronous compressional pulses may reflect expansion in the ocean basins there is a correlation of such pulses with major eustatic sea level lowstands any site of expansion within the earth is transmitted to the semi rigid crust as a volume increase by the whole gravitating fluid oblate spheroid this results in ubiquitous synchronous compressional pulses in the upper crust detectable on seismic reflection data not only at basin scale but also in individual petroleum producing structures

contribution of gravitational potential energy differences to the global stress field summary modelling the lithospheric stress field has proved to be an efficient means of determining the role of lithospheric versus sublithospheric buoyancies and also of constraining the driving forces behind plate tectonics both these sources of buoyancies are important in generating the lithospheric stress field however these sources and the contribution that they make are dependent on a number of variables such as the role of lateral strength variation in the lithosphere the reference level for computing the gravitational potential energy per unit area gpe of the lithosphere and even the definition of deviatoric stress for the mantle contribution much depends on the mantle convection model including the role of lateral and radial viscosity variations the spatial distribution of density buoyancies and the resolution of the convection model gpe differences are influenced by both lithosphere density buoyancies and by radial basal tractions that produce dynamic topography the global lithospheric stress field can thus be divided into 1 stresses associated with gpe differences including the contribution from radial basal tractions and 2 stresses associated with the contribution of horizontal basal tractions in this paper we investigate only the contribution of gpe differences both with and without the inferred contribution of radial basal tractions we use the crust 2 0 model to compute gpe values and show that these gpe differences are not sufficient alone to match all the directions and relative magnitudes of principal strain rate axes as inferred from the comparison of our depth integrated deviatoric stress tensor field with the velocity gradient tensor field within the earth s plate boundary zones we argue that gpe differences calibrate the absolute magnitudes of depth integrated deviatoric stresses within the lithosphere shortcomings of this contribution in matching the stress indicators within the plate boundary zones can be corrected by considering the contribution from horizontal tractions associated with density buoyancy driven mantle convection deviatoric stress magnitudes arising from gpe differences are in the range of 1 4 tn m 1 a part of which is contributed by dynamic topography the egm96 geoid data set is also used as a rough proxy for gpe values in the lithosphere however gpe differences from the geoid fail to yield depth integrated deviatoric stresses that can provide a good match to the deformation indicators gpe values inferred from the geoid have significant shortcomings when used on a global scale due to the role of dynamically support of topography another important factor in estimating the depth integrated deviatoric stresses is the use of the correct level of reference in calculating gpe we also elucidate the importance of understanding the reference pressure for calculating deviatoric stress and show that overestimates of deviatoric stress may result from either simplified 2 d approximations of the thin sheet equations or the assumption that the mean stress is equal to the vertical stress

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

style and intensity of late cenozoic deformation in the nagoorin basin eastern queensland australia and implications for the pattern of strain in an intraplate setting eastern australia was affected by late cenozoic intraplate deformation in response to far field stress transmitted from the plate boundaries but little is known about the intensity and pattern of this deformation we used recently surveyed two dimensional seismic reflection lines and aeromagnetic data and data from the recently released australian stress map to investigate the structure of the nagoorin basin in eastern queensland the western margin of the nagoorin beds was displaced by the boynedale fault which is a nnw striking sw dipping oblique strike slip reverse fault with a vertical throw of c 900 m and c 16 km sinistral displacement a significant part of this large sinistral displacement is interpreted to have occurred prior to late cenozoic time several low angle 30 thin skinned thrusts with a flat ramp geometry also displaced the nagoorin beds which are interpreted to have developed along detachment surfaces in oil shales and claystone the boynedale fault is a segment within longer nnw striking faults that include the north pine and west ipswich fault systems in eastern queensland these nnw striking faults are potentially active and may accommodate neotectonic thrust movement in response to the present day ne sw orientation of s results of this study in conjunction with previous information on sedimentary basins in eastern australia indicate that cenozoic contractional deformation is stronger at the continental margins possibly due to the presence of pre existing rift related structures

petrology geochemistry and geochronology of the post collisional triassic mangerite and syenite in the gwangcheon area hongseong belt south korea abstract the gwangcheon intrusive rocks occur in the hongseong collision belt of south korea which is thought to represent the eastern extension of the dabie sulu collision zone of china the central part of the gwangcheon intrusive complex consists of orthopyroxene bearing monzonite mangerite and the marginal part is composed of syenite north of the gwangcheon mangerite the post collisional 233 2 ma haemi biotite granite contains syenite enclaves shrimp u pb zircon analysis yields ages of 232 3 ma for the mangerite and 230 3 ma for a syenite enclave within the haemi biotite granite the mangerite syenite complex and syenite enclaves exhibit a shoshonitic affinity with a total alkalinity na2o k2o of 7 38 9 64 wt high k2o mg ba sr cr ni and lree contents and insignificant negative eu anomalies geochemical data indicate that the mangerite syenite intrusion and the syenite enclaves are post collisional igneous rocks formed by the partial melting of an enriched lithospheric mantle the heat for this melting was derived from asthenospheric upwelling following oceanic slab break off the shrimp age data suggest that both the gwangcheon intrusives and the syenite enclaves formed after the triassic continental collision of the north and south china blocks in korea

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

geological processes that control lateral and vertical variability in coal seam moisture contents latrobe valley gippsland basin australia abstract a study throughout the latrobe valley coal measures of coal moisture distribution using downhole bore data and 3d digital models of large bore data sets indicates lateral and vertical moisture variability is controlled by a number of factors these include burial type of overburden age of the seam marine influence coal lithotype and lateral compression on folds and monoclines the same rules appear to apply to the adjacent alberton coal measures in the south gippsland basin suggesting these factors may be widespread in other brown coal basins where moisture contents are still equilibrating through differential compaction in gippsland average moisture down seam decreases by 1 2 per seam and at a rate of approximately 1 every 20 m weight average moisture content for each seam decreases on average at a rate of 0 5 every 1 million years a stepwise decrease in moisture content between the 100 m thick major seams of 1 7 reflects large intervals of time are represented by the relatively thinner 1 10 m thick interseam sediments compression by monoclinal folding can decrease moisture contents up to 7 for any given depth moisture content appears higher on anticlines due to a lowered intensity of compression than in adjacent synclines it is considered likely that coals folded over anticlines are more fractured thereby containing greater amounts of free water use of the lower moisture coals and monoclinal coals may be preferable for future power station developments despite an increase in overburden coal ratios there would be significant savings in a lowered coal volume to calorific value and a consequent reduction in greenhouse gas emission

rotation of horizontal stresses in the australian north west continental shelf due to the collision of the indo australian and eurasian plates there is a 40 rotation of regional maximum horizontal stress hmax orientation between the western end of the australian north west continental shelf carnarvon basin and its eastern end bonaparte basin a total of 625 borehole breakouts covering a cumulative length of 7 7 km in 42 wells in the carnarvon basin indicates a hmax orientation of 090 100 n a total of 616 borehole breakouts over 6 8 km in 46 wells in the bonaparte basin indicates a hmax orientation of 055 n 060 n together with extant data from the world stress map these results indicate that regional hmax orientation is 050 060 n from new guinea westward through the bonaparte basin to the canning basin central north west shelf between the canning basin and the carnarvon basin hmax rotates to 090 100 n the parallelism of hmax orientation in the bonaparte basin to the australia banda arc collisional zone indicates that this collision is not generating significant net push rather the 050 060 n hmax orientation of much of the northern australian margin is probably controlled by the more mature new guinea orogen to which it is approximately orthogonal the observed rotation of hmax can be explained solely by the focusing of the forces balancing ridge push along collisional segments of the northeastern boundary of the indo australian plate such as the new guinea orogen although not required to account for the observed stress rotation a slab pull force from oceanic indo australian plate being subducted beneath the sunda arc cannot be dismissed

petrogenesis of granitoids and associated xenoliths in the early paleozoic baoxu and enping plutons south china implications for the evolution of the wuyi yunkai intracontinental orogen abstract the early paleozoic wuyi yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the yunkai and wugong domains located in the western cathaysia block and in the wuyi domain located in the central part of the cathaysia block in order to investigate the evolution of the wuyi yunkai orogen elemental and sr nd isotopic analyses were performed for granites from the baoxu pluton in the yunkai domain and from the enping pluton in the central part of the cathaysia block the baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite granodiorite and diorite and the enping pluton is mainly composed of massive granodiorite biotite granites 441 5 ma and gneissic granite xenolith 443 4 ma of the baoxu pluton are all weakly peraluminous a cnk 1 05 1 10 they show high sr y and la yb ratios and have negative bulk rock end t values 7 0 to 4 4 which are similar to coeval gneissic s type granites in the yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source granodiorites 429 3 ma from enping and granodiorite xenolith 442 4 ma from baoxu are metaluminous and have ree patterns with enriched light ree and flat middle to heavy ree possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level diorite xenolith from baoxu is ultrapotassic k2o 4 9 wt has high contents of mgo 7 0 wt cr 379 ppm and ni 171 ppm and shows pronounced negative nb ta and ti anomalies this xenolith also has negative end t value 3 6 and low rb ba and high ba sr ratios and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite early paleozoic i and s type granites in the wuyi yunkai orogen mostly have negative end t values and do not have juvenile components consistent with genesis by an intracontinental orogenic event these early paleozoic granites occur near the ancient suture zone between the yangtze and cathaysia blocks and have high la yb and sr y ratios likely due to the existence of residual garnet in the source suggesting the thickened crust at ca 440 ma the 450 440 ma gneissic s type granites near the suture zone are earlier than those in the central part of the cathaysia block 430 ma the crustal thickening along the ancient suture zone at 440 ma propagated into the central part of the cathaysia block as evidenced by the 430 ma granites early paleozoic i type granites near the suture zone clearly show involvement of significant mantle derived materials in contrast to granites in the central part of the cathaysia block the ancient suture zone may have acted as channels for the emplacement of mafic magmas during the collapse of an intracontinental orogen

modes of active intraplate deformation flinders ranges australia 1 the flinders ranges form one of the most seismically active zones within the australian continent with seismogenic strain rates over the last 30 years of 10 16 s 1 active deformation in the region reflects late neogene increases in stress levels in the indo australian plate as a response to increased plate boundary forcing from collision zones with the neighboring asian and pacific plates geological and geophysical observations suggest two modes of active deformation in operation in the flinders ranges over the last several million years 1 low amplitude 200 500 m long wavelength 200 km lithospheric flexure and 2 active reverse faulting along the range front with fault slip rates of up to 50 m myr 1 numerical models are developed to explore the contribution of each of these deformation modes to the observed geophysical signals an elastic mode of deformation is suggested by a distinctive long wavelength positive correlation between gravity and topography in which the flinders ranges are bordered by anomalous topographic and gravity lows now occupied by playa lake systems centered some 50 km outboard of range bounding faults numerical models show that flexural instabilities localized by vertical loads arising from older tectonic structuring produce a first order match with observed topography and gravity numerical models are also used to illustrate how the localized failure evident in the contemporary seismicity and quaternary faulting record within the flinders ranges reflects thermal weakening associated with extraordinary concentrations of heat producing elements in the crust as reflected in modern day heat flows of 90 mw m 2

the late eocene early miocene unconformities of the nw indian intraplate basins and himalayan foreland a record of tectonics or mantle dynamics a well developed late eocene to miocene unconformity termed the base miocene unconformity bmu is found throughout the intraplate basins of north western india and has previously been ascribed to himalayan tectonics this hypothesis is investigated by first describing the nature and age of the bmu in the northwest indian intraplate basins and then reconstructing the location of the bmu relative to the himalayan deformation front at the time it formed we suggest that formation of the bmu in western india cannot be related to himalayan tectonic processes associated with plate loading and flexure unless the indian plate had an elastic thickness of 125 km which is highly unlikely furthermore the resumption of deposition post unconformity rules out inversion due to compression associated with india asia convergence as a cause as these compressive forces are still present we note the coeval nature of the unconformity in the nw indian plate intraplate basins and the himalayan peripheral foreland basin if the unconformities of the himalayan peripheral foreland basin and the nw indian intraplate basins formed by a common process uplift due to circulation in the mantle is the only possible regional scale mechanism such circulation could be the result of the intrinsically time dependent high rayleigh number convection in the mantle which has resulted in well documented unconformities elsewhere or be the result of subducting slab break off beneath the himalaya

the world stress map database release 2016 crustal stress pattern across scales abstract knowledge of the present day crustal in situ stress field is a key for the understanding of geodynamic processes such as global plate tectonics and earthquakes it is also essential for the management of geo reservoirs and underground storage sites for energy and waste since 1986 the world stress map wsm project has systematically compiled the orientation of maximum horizontal stress shmax for the 30th anniversary of the project the wsm database has been updated significantly with 42 870 data records which is double the amount of data in comparison to the database release in 2008 the update focuses on areas with previously sparse data coverage to resolve the stress pattern on different spatial scales in this paper we present details of the new wsm database release 2016 and an analysis of global and regional stress pattern with the higher data density we can now resolve stress pattern heterogeneities from plate wide to local scales in particular we show two examples of 40 60 shmax rotations within 70 km these rotations can be used as proxies to better understand the relative importance of plate boundary forces that control the long wave length pattern in comparison to regional and local controls of the crustal stress state in the new wsm project phase iv that started in 2017 we will continue to further refine the information on the shmax orientation and the stress regime however we will also focus on the compilation of stress magnitude data as this information is essential for the calibration of geomechanical numerical models this enables us to derive a 3 d continuous description of the stress tensor from point wise and incomplete stress tensor information provided with the wsm database such forward models are required for safety aspects of anthropogenic activities in the underground and for a better understanding of tectonic processes such as the earthquake cycle

post orogenic thermotectonic history of the mount buffalo region lachlan fold belt australia evidence for mesozoic to cenozoic wrench fault reactivation abstract the early devonian mount buffalo granite is exposed today as an inselberg at the dissected western margin of the southeastern highlands of victoria australia it dominates a block of deeply incised ordovician turbidites that appears structurally isolated from the surrounding highlands by treating these highlands as an elastic plate subject to flexure or warping previous workers have proposed that this region of the lachlan fold belt has experienced a relatively simple thermotectonic and landscape evolution history following shallow level intrusion of the granite however results of apatite and zircon fission track analyses from the granite indicate the occurrence of major previously unrecognized episodes of 1 heating burial prior to 246 ma early triassic and prior to 70 ma late cretaceous and 2 rapid cooling denudation at some time between 246 and 220 ma early to late triassic and between 70 and 50 ma late cretaceous to early tertiary while previous interpretations of the landscape history of the region suggest the prolonged subaerial exposure of the granite following shallow crustal residence our results clearly indicate this region of the lachlan fold belt has experienced a dynamic post paleozoic history thus this new dataset forces a radical re interpretation of the landscape evolution of the mount buffalo structural block which has significant implications for our understanding of the mesozoic and cenozoic geomorphic and structural development of southeastern australia we therefore propose a model incorporating reactivation of wrench faults that may explain the complex thermotectonic history while accommodating the detailed observations of previous workers in this area

stress induced seismic azimuthal anisotropy in the upper crust across the north west shelf australia seismic azimuthal anisotropy saa is observed in many areas of the earth and fast polarized directions have been mapped using earthquake surface waves and teleseismic s wave splitting over large regional and tectonic scales higher resolution petroleum exploration data including 3 d seismic surveys and dipole shear logs often exhibit azimuthal anisotropy however we are unaware of any published studies mapping saa from exploration scale data to study large scale regional and tectonic saa trends the physical mechanisms causing saa in earthquake data are a subject of great interest comparing regional trends to the higher resolution exploration trends may help understand these mechanisms we present a saa analysis using seismic exploration data across the north west shelf nws of australia we map the fast polarization directions from 34 dipole shear logs and two 3 d seismic surveys and compare them to in situ maximum horizontal stress orientations h our results show that the and h trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology these results suggest that differential horizontal stress is the likely mechanism causing saa at a wide range of spatial scales on the nws of australia we also show that saa is not observed at exploration scales in some areas of the nws and propose a lithology dependent stress sensitivity model in which saa is strongest in clean unconsolidated quartz dominated sandstones and weaker or nonexistent in well consolidated cemented rocks and shale dominated sediments

paleostress field evolution of the australian continent since the eocene 1 although the low order present stress field of most continents is fairly well established information on paleostress fields is generally sparse knowledge of paleostresses is crucial for understanding brittle tectonic reactivation through time the indian australian plate lends itself well to a reconstruction of paleostresses as it has undergone enormous changes in plate driving forces through the tertiary and there is a rich record of fault reactivation from sedimentary basins we reconstruct the plate boundary configuration and age area distribution of ocean crust around australia through time to obtain estimates for ridge push slab pull and collisional forces acting on the indian australian plate since the eocene other model parameters we explore are the effects of the australian antarctic discordance and the mechanical strength of the australian continental margin we apply these constraints to model the orientation of the maximum horizontal compressive stress shmax regime for the present early miocene and early eocene using the commercial software abaqus along with the optimization software nimrod o we use an elastic two dimensional plane stress finite element model with a resolution of 0 2 in both longitude and latitude realistic elastic parameters representing different rock types and geologic provinces for the australian continent have been included to model the stress field of a heterogeneous plate we show that spatially significant rotations of shmax directions can be modeled as a consequence of perturbations of shmax in areas of juxtaposed rigid and compliant rheologies the absence of the collisional papua new guinea boundary in the miocene and reduced ridge push force from the south result in stress directions considerably different from the present stress directions over the northern australian continent in the early miocene in particular show large disparity with present stress directions stress orientations for the australian plate during the early eocene are controlled predominantly by ridge push forces arising from spreading in the wharton basin in the indian ocean and vary substantially with stress directions in the early miocene and the present because of the drastically different plate geometry and boundary configurations fault reactivation histories observed on the northwest shelf of australia and in the bass strait region are consistent with modeled changes in stress directions through time

the tectonic stress field evolution of india since the oligocene abstract a multitude of observations suggest neotectonic deformation in and around india but its causes and history are unknown we use a 2 dimensional finite element model with heterogeneous elastic strengths in continental regions to model the regional stress field orientation and relative magnitudes since the oligocene the large scale geological structure of india is embedded in our model by using published outlines of cratons fold belts and basins associated with estimates of their relative strengths enabling the modelling of stress field deflections along interfaces between relatively strong and weak tectonic elements through time at 33 ma a roughly nnw sse oriented band of relatively high maximum horizontal compressive stress s hmax straddled india s west coast while india s east coast and the adjacent wharton basin were characterized by relatively low intraplate stresses between 20 ma and the present growing collisional boundary forces combined with maturing mid ocean ridge flanks result in the establishment of an arcuate belt with anomalously high intraplate stress that stretches from india to the wharton basin intersecting the continental shelf roughly orthogonally and crossing the 85 east and ninetyeast ridges this results in a compressive tectonic regime favouring folding and inversion northeast of the godavari graben on india s east coast as observed in seismic reflection data whereas no tectonic reactivation is observed on the continental margin further north closer to the mahanadi graben or further south our stress models account for these differences via spatial variations in modelled horizontal stress magnitudes and intersection angles between margin paralleling pre existing basement structures and the evolving neogene stress field the models further account for fracture zone strike slip reactivation offshore sumatra and lithospheric folding along india s west and southeast coast and can be used to estimate the onset of these deformation episodes to at least the oligocene and miocene respectively

episodic intraplate deformation of stable continental margins evidence from late neogene and quaternary marine terraces cape liptrap southeastern australia the waratah fault is a northeast trending high angle reverse fault in the late paleozoic lachlan fold belt at cape liptrap on the southeastern australian coast it is susceptible to reactivation in the modern intraplate stress field in southeast australia and exhibits late pliocene to late pleistocene reactivation radiocarbon optically stimulated luminescence osl and cosmogenic radionuclide crn dating of marine terraces on cape liptrap are used to constrain rates of displacement across the reactivated waratah fault six marine terraces numbered qt6 tt1 youngest to oldest are well developed at cape liptrap with altitudes ranging from 1 5 m to 170 m amsl respectively on the lowest terrace qt6 barnacles in wave cut notches 1 5 m amsl yielded a radiocarbon age of 6090 5880 cal bp and reflect the local mid holocene sea level highstand qt5 yielded four osl ages from scattered locations around the cape ranging from 80 ka to 130 ka it formed during the last interglacial sea level highstand mis 5e at 125 ka inner edge elevations approximate paleo high tide line for qt5 occur at distinctly different elevations on opposite sides of the waratah fault offsets of the inner edges across the fault range from 1 3 m to 5 1 m with displacement rates ranging from 0 01 mm a to 0 04 mm a the most extensive terrace tt4 yielded four early pleistocene cosmogenic radionuclide crn ages two apparent burial ages of 0 858 ma 0 16 ma and 1 25 ma 0 265 ma and two apparent exposure ages of 1 071 ma 0 071 ma 10be and 0 798 ma 0 066 ma 26al allowing for muonic production effects from insufficient burial depths the depth corrected crn burial ages are 1 8 ma 0 56 ma and 2 52 ma 0 88 ma or late pliocene a late pliocene age is our preferred age offsets of tt4 across the waratah fault range from a minimum of 20 m for terrace surface treads to a maximum of 70 m for terrace bedrock straths calculated displacement rates for tt4 range from 0 01 mm a to 0 04 mm a using a late pliocene age 2 ma identical to the rates calculated for the last interglacial terrace qt5 this indicates that deformation at cape liptrap has been ongoing at similar time averaged rates at least since the late pliocene the upper terraces in the sequence tt3 110 m amsl tt2 140 m and tt1 180 m are undated but most likely correlate to sea level highstands in the neogene terraces tt1 tt4 show an increasing northward tilt with age the waratah fault forms a prominent structural boundary in the lachlan fold belt discernible from airborne magnetic and bouger gravity anomalies seismicity and deformation are episodic episodic movement on the waratah fault may be coincident with sea level highstands since the late pliocene possibly from increased loading and elevated pore pressure within the fault zone this suggests that intervals between major seismic events could be on the order of 100 ka

maximum horizontal stress orientations in the cooper basin australia implications for plate scale tectonics and local stress sources summary borehole breakouts and drilling induced tensile fractures ditfs were interpreted in 61 wells in the cooper basin indicating an average maximum horizontal stress orientation of 101 n a total of 890 borehole breakouts and 608 ditfs were interpreted in the cooper basin the approximately east west maximum horizontal stress orientation is consistent over much of the basin except in the patchawarra trough where maximum horizontal stress rotates to a northwest southeast orientation this rotation in maximum horizontal stress orientation is consistent with in situ stress data to the northwest of the cooper basin the stress field in the cooper basin appears to mark the apex of a major horseshoe shaped rotation in maximum horizontal stress direction across central eastern australia finite element modelling of the in situ stress field of the indo australian plate iap using a range of plate scale tectonic forces is able to match the regional maximum horizontal stress orientation over most of australia reasonably well including the mean east west maximum horizontal stress orientation in the cooper basin however plate boundary scale modelling does not adequately match the horseshoe shaped stress rotation across central eastern australia the average east west maximum horizontal stress orientation in the cooper basin indicates that stresses from tensional forces acting along the tonga kermadec subduction zone are not transmitted into the interior of the australian plate the majority of the tensional forces associated with the tonga kermadec subduction zone are most likely accommodated along the numerous spreading centres within the lau havre backarc basin a number of more localized stress anomalies have also been identified these cannot be explained by plate scale tectonic forces and are possibly a result of geological structure and or density contrasts locally perturbing the stress field

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

mangerite magmatism associated with a probable late permian to triassic hongseong odesan collision belt in south korea abstract the odesan area in the eastern gyeonggi massif south korea consists principally of migmatitic and porphyroblastic gneisses intruded by mangerite mafic mangerites with sio 2 contents from 52 40 to 54 20 wt have higher feo mgo 14 98 18 28 wt and cao contents 5 80 7 84 wt but lower total alkali contents 4 74 2 o k 2 o 2 2 o k 2 o

structural sequences and styles of subsidence in the michigan basin subsidence in the michigan basin produced 5 km of sedimentation over a period of more than 200 m y during paleozoic time utilizing well log correlations and constrained by compaction corrections and estimates of paleobathymetry we recognize four different styles of subsidence in the basin trough shaped regional tilting narrow basin centered and broad basin centered subsidence began as a trough shaped northerly extension of the illinois basin during late cambrian to early ordovician time this was followed by narrow basin centered subsidence in early to middle ordovician time basin centered subsidence ceased for 30 m y during long wavelength 1000 km eastward tilting in middle to late ordovician time a pattern incompatible with thermal contraction subsidence models basin centered subsidence resumed in silurian time but with a broader distribution a second episode of narrow basin centered subsidence occurred in latest silurian through middle devonian time and was replaced by broad basin centered subsidence at the end of middle devonian time the geometry of upper devonian and younger paleozoic deposits suggests another eastward tilting event but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry in addition to these subsidence patterns two distinct unconformity styles are present basin wide and marginal erosion there is no evidence for significant basin centered unconformities as predicted by purely thermal mechanisms a history of episodic subsidence reactivations is interpreted as the result of a stress induced crustal weakening mechanism for the narrow basin centered subsidence whereas broad basin centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow subsidence episodes recently proposed dynamic topography related to initiation of ordovician subduction provides a driving mechanism for long wavelength eastward tilting together with a temporal correlation to appalachian tectonism these mechanisms provide a plate tectonic framework for the history of the michigan basin

evaluating slab plate coupling in the indo australian plate distributed seismicity in the central indian ocean affords a unique opportunity to evaluate the extent of slab plate coupling in the indo australian plate the mix of reverse fault and strike slip mechanisms in this region with northwest southeast to north south maximum horizontal stress s hmax implies that the effective slab pull is no more than 10 of the total negative buoyancy operating on the subducting slab numerical models of the intraplate stress field predict a slab pull component along the sumatra and java boundary segments of 2 82 0 82 and 0 89 0 35 10 12 n m 1 respectively mantle tomographic constraints coupled with insights from analogue modeling suggest that the differences relate to variations in the depth extent of the slabs and the degree of slab support provided by the transition zone these results help resolve apparent contradictions between insights from intraplate stress fields and plate dynamics i e although plate motion is dominated by subduction slab pull is only poorly expressed in the intraplate stress field because of low slab plate coupling

enhanced intraplate seismicity along continental margins some causes and consequences abstract in australia much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression shmax intriguingly along the margin side of one such zone a near optimally oriented crustal scale structure the darling fault has no recorded historical seismicity to explain these enigmatic features we explore how the compositional and thermal structuring of continental passive margins may lead to variations in mechanical response to imposed forcing using forward modelling we show how lateral heat flow across such margins can produce increases in moho temperatures by several 10s of degrees relative to the continental interior extending up to 200 km inboard of the ocean continent transition provided the step in thermal lithosphere thickness into the continent is greater than 30 km for smaller lithospheric steps moho temperatures beneath continental margins will be cooler than beneath interiors our mechanical models show that such thermal effects can elevate strain rates along the margin by several factors relative to continental interiors when subject to far field forcing in the shallow seismogenic realm the strain rate maximum is concentrated at around 100 150 km inboard of the ocean to continent lithospheric step providing a plausible explanation for the pattern of seismicity observed in some of australia s marginal seismic zones this interpretation requires that strength changes associated with moho temperature variations of several 10s of degrees are sufficient to override the reactivation potential of near optimally oriented crustal penetrative structures with thermal processes providing an important switch controlling reactivation potential of existing structures it also provides an insight into the mechanism of formation of continental ribbons during rifting of thermally mature margins

analysis of the south american intraplate stress field the first order south american intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources the finite element mesh consisted of 3100 nodes in a network of 5993 equal area triangular elements which provided a spatial resolution of about 1 at the equator an important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the mid atlantic ridge e g ridge push the continental margins along the east coast of brazil and argentina and the elevated continental crust e g the andean cordillera predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated pinned collisional boundaries and applied collisional boundary forces constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress shmax provided by 217 stress indicators from the world stress map project as well as by shmax magnitude estimates and torque information from previous investigations our modeling results demonstrate that the first order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate the predicted intraplate stress field is characterized by a nearly uniform e w shmax orientation throughout most regions of the plate with stress magnitudes generally less than 20 mpa averaged over a 100 km thick lithosphere significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the andes although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained we estimate a plausible upper bound on the force per unit length acting along the peru chile trench to be about 2 5 1012 n m 1 while some of our models are consistent with a driving basal drag to balance the torques acting on the plate the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force

in situ u pb and lu hf isotopic studies of zircons from the sancheong hadong amcg suite yeongnam massif korea implications for the petrogenesis of 1 86 ga massif type anorthosite abstract isotopic and geochemical characteristics of proterozoic anorthosite mangerite charnockite granite amcg suite have long been used for tracing the mantle crustal source and magmatic evolution we analyzed lu hf isotopic compositions of zircon from the sancheong hadong amcg complex yeongnam massif korea in order to understand tectonomagmatic evolution of the paleoproterozoic amcg suite occurring at the southeastern margin of the north china craton ncc the anorthositic rocks in this complex associated with charnockitic and granitic gneisses were recrystallized to eradicate magmatic features in situ shrimp sensitive high resolution ion microprobe u pb analyses of zircon from a leuconorite and an oxide bearing gabbroic dyke yielded weighted mean 207 pb 206 pb ages of 1870 2 ma and 1861 6 ma respectively charnockitic granitic and porphyroblastic gneisses yielded weighted mean 207 pb 206 pb zircon ages of 1861 6 ma 1872 6 ma and 1873 4 ma respectively these crystallization ages together with our previous geochronological data for anorthosites 1862 2 ma are indicative of episodic amcg magmatism over an 10 ma interval initial e hf t values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from 2 1 to 6 1 and 0 3 to 5 4 respectively zircon hf isotopic data in combination with available whole rock sr nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination this crustal component is also reflected in charnockitic granitic magmas produced primarily by the melting of lower crust taken together the amcg magmatism at 1 87 1 86 ga in the yeongnam massif is most likely a late orogenic product of paleoproterozoic ncc amalgamation tectonically linked to assembly of the columbia supercontinent

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

correlation of paleoproterozoic igneous and metamorphic events of the korean peninsula and china its implication to the tectonics of northeast asia abstract correlation of the paleoproterozoic rocks of the korean peninsula and china has been considered to be important for the tectonic evolution of northeast asia yet it is still unclear recently considerable new data have been obtained from the korean peninsula and china providing a better opportunity to correlate the paleoproterozoic rocks of the korean peninsula and china intermediate p t metamorphism and post collisional magmatism have been found to have occurred during ca 1 95 1 83 ga not only in the jiao liao ji belt on the eastern north china craton but also in the nangrim and northern gyeonggi massifs within the korean peninsula representing that these paleoproterozoic rocks can be correlated these events occurred as a result of the collision between the longgang block of the eastern north china craton and the nangrim massifs of the korean peninsula on the other hand the southeastern gyeonggi massif underwent an arc related magmatic and metamorphic events during ca 1 96 1 91 ga suggesting that the paleoproterozoic rocks in southeastern gyeonggi massif cannot be correlated with those in northern gyeonggi massif the yeongnam massif is unlikely correlated to the nangrim and northern gyeonggi massifs and the eastern north china craton because arc related igneous activity occurred during ca 2 00 1 85 ga in the sobaeksan gneiss complex in the central to northeastern yeongnam massif the jirisan gneiss complex in the southwestern yeongnam massif underwent intermediate p t metamorphism that is followed by post collisional magmatic and low p t metamorphic events during ca 1 92 1 86 ga and these events may be correlated to those in the eastern cathaysia block on the south china craton the paleoproterozoic correlation between the korean peninsula and china in this study supports a tectonic model in which the permo triassic dabie sulu continental collision belt in china extends into the hongseong odaesan collision belt in the gyeonggi massif within the korean peninsula

late oligocene rapid transformations in the south china sea lithobiostratigraphic data indicate that the double reflectors on the seismic profile through ocean drilling program odp site 1148 represent two unconformities that coincide respectively with the lower upper oligocene boundary at 488 mcd and oligocene miocene boundary at 460 mcd two other unconformities at 478 and 472 mcd respectively were also identified within the upper oligocene section together they erased a sediment record of about 3 ma from this locality in a period of very active seafloor spreading the existence of 32 8 ma marine sediment at the terminated depth 850 mcd indicates that the initial breakup of the south china sea scs was probably during 34 33 ma close to the eocene oligocene boundary high sedimentation rates of 60 115 m my from the much expanded 350 m lower oligocene section resulted from rifting and rapid subsidence between 33 and 29 ma the mid oligocene unconformity at 28 5 ma which also occurred in many parts of the indo west pacific region was probably related to a significant uplift of the himalayan tibetan plateau to the west and the initial collision between indonesia and australia in the south a narrowed indonesian seaway may have accounted for the late oligocene warming and chalk deposition in the northern south china sea including the site 1148 locality the unconformities and slumps near the oligocene miocene boundary indicate a very unstable tectonic regime probably corresponding to changes in the rotation of different land blocks and the seafloor spreading ridge from nearly e w to ne sw as recognized earlier at magnetic anomaly 7 this 25 ma event also saw the first new guinea terrane docking at the northern australian craton the low sedimentation rate of 15 m my in the early to middle miocene may correspond to another period of rapid seafloor spreading and rapid widespread subsidence that effectively caused sediment source areas to retreat with a rapidly rising sea level the isostatic nature of these late oligocene unconformities and slumps with several major collision uplift events indicate that the rapid changes in the early evolutionary history of the south china sea were mainly responding to regional tectonic reconfiguration including the uplift driven southeast extrusion of the indochina subcontinent

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

an andean type retro arc foreland system beneath northwest south china revealed by sinoprobe profiling abstract in the mesozoic south china was situated along the convergent margin between the asian and pacific plates providing an excellent laboratory to understand the interactions between deformation sedimentation and magmatism in a retroarc environment the crustal architecture of northwest south china is displayed along the 600 km long sinoprobe deep seismic reflection profiles and reveals from east to west 1 highly folded and truncated reflectors in the upper crust of the yangtze fold zone which correspond to thin and thick skinned thrust systems and document large scale intraplate structural imbrication and shortening 2 a crustal scale flat ramp flat structure termed the main yangtze decollement which forms a weak viscous layer to accommodate strain decoupling and material transport in the thin and thick skinned systems and 3 nearly flat lying reflectors in the sichuan basin which support interpretation of the basin as a weakly deformed depocentre the yangtze fold zone and the sichuan basin represent a retro arc foreland basin system that is 800 km away from the continental margin magmatic arc we suggest that tectonic processes across the arc and retro arc systems including arc magma flare up basin sedimentation retroarc thrust propagation lithosphere underthrusting root foundering and extension related magmatism were interrelated and governed mass transfer age data and geological relations link the tectonic processes to evolving geodynamics of the subducting paleo pacific plate

ferride geochemistry of swedish precambrian iron ores chemical analysis for major and trace elements have been performed on 30 swedish precambrian iron ores and on some from iran and chile the swedish ores consist of apatite iron ores quartz banded iron ores skarn and limestone iron ores from the two main ore districts of sweden the bergslagen and the norrbotten province some swedish titaniferous iron ores were also included in the investigation the trace element data show that the swedish ores can be subdivided into two major groups 1 orthomagmatic and exhalative 2 sedimentary within group 1 the titaniferous iron ores are distinguished by their high ti contents from the ferride contents of the kiruna apatite iron ores the ores are considered to be mobilization products of skarn iron ores from the norbotten province

tectonic geomorphology of australia the australian continent is actively deforming in response to far field stresses generated by plate boundary interactions and buoyancy forces associated with mantle dynamics on the largest scale several 10 3 km the submergence of the northern continental shelf is driven by dynamic topography caused by mantle downwelling along the indo pacific subduction system and accentuated by a regionally elevated geoid the emergence of the southern shelf is attributed to the progressive movement of australia away from a dynamic topography low on the intermediate scale several 10 2 km low amplitude c 100 200 m long wavelength c 100 300 km topographic undulations are driven by 1 anomalous smaller scale upper mantle convection and or 2 lithospheric scale buckling associated with plate boundary tectonic forcing on the smallest scale 10 1 km fault related deformation driven by partitioning of far field stresses has modified surface topography at rates of up to c 170 m ma 21 generated more than 30 50 of the contemporary topographic relief between some of australia s highlands and adjacent piedmonts and exerted a first order control on long term 10 4 10 6 a bedrock erosion although australia is often regarded as tectonically and geomorphologically quiescent neogene to recent tectonically induced landscape evolution has occurred across the continent with geomor phological expressions ranging from mild to dramatic australia is one of the lowest flattest most arid and most slowly eroding continents on earth the average elevation of the continent is only c 330 m above sea level asl maximum local topographic relief is everywhere 1500 m defined by elevation ranges with 100 km radii and two thirds of the con tinent is semi arid to arid with the exception of localized upland areas in the flinders and mt lofty ranges and the eastern highlands bedrock

influence of basement structures on in situ stresses over the surat basin southeast queensland the jurassic to cretaceous sedimentary rocks of the surat basin in southeast queensland host a significant volume of coal seam gas resources consequently knowledge of the in situ stress is important for coal permeability enhancement and wellbore stability using wireline log data and direct stress measurements we have calculated stress orientations from 36 wells and stress magnitudes from 7 wells across the surat basin our results reveal a relationship between high tectonic stress and proximity to structures within the underlying basement rocks the influence of tectonic stresses is diminished with depth in areas with thicker sedimentary cover that are relatively far from the basement structures we suggest that this relationship is due to the redistribution of in situ stresses around areas where basement is shallower and where basement structures such as the leichhardt burunga fault system are present this behavior is explained by a lower rigidity in the thickest basin cover which reduces the ability to maintain higher tectonic stress over the entire surat basin a significant amount of variability in in situ stress orientation is observed the authors attribute this stress variability to complex plate boundary interactions on the northern and eastern margins of the indo australian plate

source parameters of seven large australian earthquakes determined by body waveform inversion summary we determine source mechanisms and centroid depths for the seven largest earthquakes in and near the australian continent in the past 20yr using teleseismic body waves six of the earthquakes were thrusts and one was strike slip all five earthquakes that occurred beneath the continent were shallower than 10 km the ranges of centroid depths are constrained to be 1 5 2 4 0 3 5 11 and 6 10km and indicate nearly pure thrust faulting the three shallowest events were associated with surface faulting while the published first motion solutions for these three events included large amounts of strike slip our thrust mechanisms are more consistent with displacements observed at the surface of the earth the range of possible centroid depths for a strike slip earthquake beneath the continental shelf off the northwest coast is 23 29 km and that for a thrust earthquake in oceanic lithosphere beneath the tasman sea is 22 26 km below sea level the pressure axis is consistently near horizontal and in the western half of the continent five earthquakes indicate that the azimuth of the maximum compressive stress is e ene azimuthal range of 66 91 on the basis of one earthquake we conclude that the p axis is oriented nnw beneath the tasman sea

apatite monazite relations in the kiirunavaara magnetite apatite ore northern sweden the magnetite apatite ores in the kiruna area northern sweden are generally considered to be of magmatic origin formed in a subvolcanic volcanic environment during the early proterozoic they are thought to have crystallised from volatile rich iron oxide magmas derived by immiscibility in calc alkaline to slightly alkaline parental magmas three major morphological types of the magnetite apatite ore primary brecciated and banded have been investigated for textural relations and mineral chemistry using transmitted light back scattered electron imaging bse electron microprobe analysis empa and laser ablation inductively coupled plasma mass spectrometry la icpms in all three types th and u poor monazite is present as small inclusions in the apatite larger up to 150 m recrystallised monazite grains both along apatite grain boundaries and intergrown with magnetite and silicate minerals are present in the brecciated and banded samples primary apatite grains without monazite inclusions are generally enriched in light rare earth elements lrees together with na and si petrological and mineralogical evidence suggest that the kiruna magnetite apatite ore experienced successive stages of fluid rock interaction the first stage occurred under high temperature conditions 700 800 c shortly after emplacement and crystallisation of the ore magmas and involved concentrated probably cl dominated brines expelled from the magma this fluid is held to be responsible for the nucleation of the numerous small monazite inclusions within the apatite due to high temperature leaching of na and si while the lrees were concentrated in the monazite the large monazite grains in the brecciated and banded samples are proposed to be the product of recrystallisation from the much smaller monazite inclusions during greenschist facies metamorphism t 300 400 c fluids from the surrounding country rock caused strong lree na si depletion along apatite grain boundaries and cracks in the apatite lrees were either redeposited as monazite grains along apatite grain boundaries or were flushed out of the ore this fluid interaction also introduced the silicate components responsible for the interstitial formation of allanite talc tremolite chlorite serpentine muscovite quartz and carbonates along apatite grain boundaries

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

geochemistry zircon u pb ages and hf isotopic compositions of precambrian gneisses in the wonju jechon area of the southern gyeonggi massif implications for the precambrian tectonic evolution of korea and northeast asia abstract understanding the precambrian tectonic evolution of the gyeonggi massif in south korea can provide important insights into the tectonic evolution of korea and northeast asia and the geological relationship between korea and china the wonju jechon area is located in the southeastern part of the gyeonggi massif and is dominated by precambrian gneisses the protoliths of the granitic gneiss and migmatite in this area were emplaced in a collisional and or post collisional tectonic setting at ca 2 5 ga before the intrusion of the protolith of the two mica granitic gneiss in an arc related tectonic setting at 1 94 1 91 ga the intrusion of the two mica granitic gneiss was contemporaneous with the first stage of metamorphism of the granitic gneiss and migmatite in this area the protolith of the porphyritic gneiss then intruded into these granitic gneiss migmatite and two mica granitic gneiss in a post collisional tectonic setting at ca 1 79 ga the final major event recorded by these rocks is the second stage regional metamorphism at ca 285 ma zircons within the precambrian rocks in the wonju jechon area have e hf t values and isotopic model ages t dm2 that provide evidence of two stages of crustal growth at ca 3 3 and 2 7 ga and three periods of crustal reworking at ca 2 5 1 9 and 1 8 ga with generation of the protoliths of the two mica granitic and porphyritic gneisses in this area by the remelting of ancient crustal material the tectonic evolution of the southeastern gyeonggi massif differs from that of the northern gyeonggi and nangrim massifs both of which record continental collision related magmatism and metamorphism at 1 93 1 91 ga post collisional magmatism and metamorphism at ca 1 88 1 85 ga and within plate magmatism at ca 1 84 1 83 ga in comparison the tectonic evolution of the southeastern part of the gyeonggi massif is more similar to that of the northern yangtze block in the south china craton although more research is needed to confirm this relationship

the in situ stress field of the perth basin australia a total of 17 wells in the perth basin were interpreted to have 114 breakouts covering a combined length of 3 2 km and a mean hmax orientation of 108 n the inferred mean orientation of hmax in the perth basin is broadly consistent with that in the adjacent basement yilgarn block the new data confirm that hmax orientation in the region is consistent with other stress indicators different depths and different geological provinces the new data also confirm that the orientation of hmax does not parallel the nne direction of absolute plate velocity anomalous north south hmax orientations identified in a number of wells in the perth basin can be attributed to local structural effects

on the forces driving plate tectonics inferences from absolute plate velocities and intraplate stress summary absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress the absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar while the global pattern of absolute velocity is probably close to those predicted by such models the absolute motions do not therefore provide a strong test of the driving mechanism comparison of predicted intraplate stress with the long wavelength features of the global stress field however as determined by in situ measurements earthquake mechanisms and stressinduced geological structures does prove to be a powerful test of possible driving forces all absolute velocity models have several interesting properties lithosphere in the equatorial half of the earth is moving significantly faster than lithosphere in the polar half some connection to the earth s rotation is implied since this statement is demonstrably untrue for co ordinate poles much different from the geographic pole subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate confounding attempts to explain the dip angles of benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere such rapid migration may be a necessary but is not a sufficient condition for ridge jumps force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere from the rms absolute velocities of individual plates there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere the predicted intraplate deviatoric stress cannot match the well determined stress fields in north america and europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system the mid plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities

tectonism and eustasy across a late miocene strandplain the loxton parilla sands murray basin southeastern australia abstract sediments of the loxton parilla sands form a progradational strandplain that stretches for over 350 km across the murray basin in southeastern australia the strandplain comprises over 600 individual ridges with stratigraphic and strontium isotope data indicating these strandlines are late miocene to early pliocene in age with deposition beginning before 7 2 ma and terminating at approximately 5 0 ma post pliocene uplift along the padthaway high due to regional compressive intraplate tectonism was responsible for altering the orientation of strandlines and drainage systems as well as contributing to increased erosion across the strandplain this uplift both restricted and starved the coastline of sand sized sediment changing the depositional environment from one of progradation to active erosion this produced a series of scarps along the southwestern murray basin and ultimately contributed to the formation of the kanawinka escarpment a large erosional cliff feature whose formation was not exclusively the result of fault related tectonism this evidence for pliocene tectonism along southeastern australia together with the depositional ages inferred from strontium isotope analysis indicates that contrary to previous work the strandlines of the loxton parilla sands do not represent an unbroken record of late miocene pliocene glacioeustatic change we estimate that approximately 3 ma of depositional history may be missing from the sedimentary record between the southwestern most loxton parilla sands strandlines and those of the pleistocene bridgewater formation

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

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

alteration paragenesis and mineral chemistry of the tjarrojakka apatite iron and cu au occurrences kiruna area northern sweden the northern norrbotten area in northern sweden is an important mining district and hosts several deposits of fe oxide cu au type one of the best examples of spatially and possibly genetically related apatite iron and copper gold deposits in the region is at tjarrojakka 50 km wsw of kiruna the deposits are hosted by strongly sheared and metamorphosed intermediate volcanic rocks and dolerites and show a structural control the tjarrojakka iron deposit is a typical apatite iron ore of kiruna type and the tjarrojakka copper occurrence shows the same characteristics as most other epigenetic deposits in norrbotten the host rock has been affected by strong albite and k feldspar alteration related to mineralisation resulting in an enrichment of na k and ba fe and v were depleted in the altered zones and added in mineralised samples ree were enriched in the system with the greatest addition related to mineralisation y was also mobile associated with albite alteration and copper mineralisation the tjarrojakka iron and copper deposits show comparable hydrothermal alteration minerals and paragenesis which might be a product of common host rock and similarities in ore fluid composition or overprinting by successive alteration stages mineralogy and mineral chemistry of the alteration minerals apatite scapolite feldspars amphiboles and biotite indicate a higher salinity and ba k ratio in the fluid related to the alterations in the apatite iron occurrence than in the copper deposit where the minerals are enriched in f and s the presence of hematite barite and in so4 in scapolite suggests more oxidising rich conditions during the emplacement of the tjarrojakka cu deposit from existing data it might be suggested that one evolving system created the two occurrences with the copper mineralisation representing a slightly later product

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

numerical simulation of present day tectonic stress across the indian subcontinent in situ measurements of maximum horizontal stress shmax in the indian subcontinent are limited and do not present regional trends of intraplate stress orientation the observed orientations of shmax vary considerably and often differ from the plate velocity direction we have simulated orientation and magnitude of shmax through finite element modeling incorporating heterogeneities in elastic property of the indian continent and plain stress approximation to understand the variability of shmax four different scenarios are tested in simulation 1 homogeneous plate with fixed plate boundary 2 homogeneous plate with boundary forces 3 heterogeneous plate with fixed boundary 4 heterogeneous plate with boundary forces the estimated orientation and magnitude of shmax with a heterogeneous plate with boundary forces in the himalayan region and an eastern plate boundary comprising the indo burmese arc and andaman subduction zone are consistent with measured maximum horizontal stress this study suggests that plate boundary force varies along the northern indian plate margin and also provides a constraint on the intraplate stress field in the indian subcontinent

the devonian back arc basin and triassic arc continent collision along the imjingang belt in the korean peninsula and their tectonic meaning abstract the imjingang belt which is located between the nangrim and gyeonggi massifs is divided into the imjin group in the north and the samgot unit in the south the southeastern part of the imjin group which is located within south korea is called the jingok unit the samgot unit consists of amphibolite psammitic schist meta alkali granite and meta calc silicate rock the amphibolite experienced prograde metamorphism from 648 550 c 13 11 kbar to 715 c 11 kbar and was then retrograded to ca 579 569 c 6 7 6 3 kbar the psammitic schist was metamorphosed at ca 710 c 11 kbar and then retrograded to ca 670 660 c 11 10 kbar these data indicate that the samgot unit underwent clockwise prograde metamorphism with peak metamorphism conditions of 715 710 c 11 kbar which is confirmed by pseudosection analysis the results of this and previous studies indicate that the metamorphic grade increases southward from the garnet and staurolite zones 630 500 c 9 3 kbar through the kyanite zone 710 665 c 9 kbar in the jingok unit to the samgot unit 715 710 c 11 kbar of the imjingang belt this metamorphic pattern indicates that the imjingang belt underwent regional intermediate p t metamorphism the shrimp u pb age dating of zircons from the amphibolite and meta alkali granite yields intrusion ages of ca 371 361 ma and 373 ma respectively from the psammitic schist ages of ca 401 370 ma are obtained from the youngest detrital zircons and ages of ca 251 249 ma are obtained from the metamorphic zircons in addition the shrimp u pb age dating of titanite from the amphibolite yields a retrograde metamorphic age of ca 229 214 ma the whole rock chemical composition of the amphibolite plots in the back arc basin basalt field on the tectonic discrimination diagrams these data indicate that the imjingang belt including the samgot unit consists of devonian sedimentary rocks that were deposited in a back arc basin and intruded by mafic and felsic igneous rocks at ca 373 361 ma finally the imjingang belt underwent intermediate p t metamorphism during the triassic 251 249 ma instead of high p t metamorphism indicating that the imjingang belt is not the extension of the dabie sulu collision belt but a back arc basin within the north china craton that underwent intermediate p t metamorphism due to the closure of the back arc basin arc continent collision during the collision between the north and south china cratons along the hongseong odesan collision belt within the gyeonggi massif

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

a new genetic model for the triassic yangyang iron oxide apatite deposit south korea constraints from in situ u pb and trace element analyses of accessory minerals abstract the yangyang iron oxide apatite deposit in south korea has undergone multiple episodes of igneous activity deformation hydrothermal alteration and iron oxide apatite ioa mineralization the iron orebodies occur as concordant to discordant layered lenticular or massive magnetite and or magnetite pyrite ores the iron mineralization occurs along a n s trending shear zone within the yangyang syenite which experienced both early ductile and later brittle deformations alteration was caused mainly by the injection of hydrothermal fluid through the shear zone leading to fe p mineralization we recognize multiple stages of alteration in the yangyang deposit based on a paragenesis that is defined by distinct mineral assemblages including na ca k alteration phases e g albite diopside actinolite and biotite and accessory minerals containing high field strength elements e g apatite sphene allanite and monazite the alteration around the magnetite ore body shows an evolutionary trend from ca na alteration through k to phyllic alterations the fe p mineralization is associated with the ca k and k alteration products the iron orebodies are hosted by deformed and altered syenite which intruded the paleoproterozoic gneiss complexes at 233 1 ma shrimp u pb zircon age in a post collisional tectonic setting la icp ms u pb dating of ree rich sphene and apatite from the iron ores and alteration products yields fe mineralization ages of 216 3 ma sphene and 212 13 ma apatite this is the first time which ioa type mineralization in the korean peninsula was dated as triassic age related to post collisional magmatism within the gyeonggi massif south korea the u pb system was subsequently reset 208 3 ma sphene and 151 13 ma apatite by jurassic and cretaceous magmatism this unique geological evolution was responsible for mesozoic metal enrichment and remobilization into suitable structural traps in the yangyang district

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

application of in situ titanite u pb geochronology to volcanic hosted magnetite deposit new constraints on the timing and genesis of the zhibo deposit western tianshan nw china abstract the awulale metallogenic belt within the western tianshan orogenic belt of northwestern china includes four large iron oxide deposits with a total resource of 1000 million metric tons mt fe among these the zhibo deposit is a large 337 mt at 26 68 wt fe volcanic hosted magnetite deposit where massive ti poor magnetite ores are hosted in the carboniferous volcanic and volcaniclastic sequences here we use in situ u pb analyses of titanite and zircon by laser ablation icp ms to place tight constraints on the timing and genesis of iron mineralization at zhibo titanite in the magnetite ore are closely associated with magnetite and ca alteration assemblages consisting of actinolite epidote and calcite the dated titanite exhibit strongly fractionated ree patterns with heavy ree enrichment neutral to negative eu anomalies and have low th and u concentrations and low th u ratios the textural and geochemical characteristics indicate that the titanite are hydrothermal in origin and coeval with magnetite in the paragenetic sequence titanite from three magnetite ores yield weighted mean 207pb corrected 206pb 238u ages of 310 3 1 8 ma mswd 0 17 310 1 1 8 ma mswd 0 30 and 315 3 2 5 ma mswd 0 26 constraining the iron mineralization at zhibo to a time interval between 315 ma and 310 ma magmatic zircon from a host andesite sample yield u pb age of 316 3 3 4 ma mswd 0 079 the overlapping ages for magnetite ores and the host volcanic rocks confirm a genetic relationship between them and are consistent with a magmatic contribution to the mineralization system as also indicated by fe and o isotope data of magnetite in previous work these new u pb results are also consistent with age estimates for mineralization and igneous activity in other major magnetite deposits in the awulale iron metallogenic belt indicating a significant iron mineralization event related to the ca 315 300 ma volcanism combined with previous geological and geochemical evidence we conclude that the zhibo magnetite deposit was formed mainly by iron rich fluids derived from a mafic to intermediate magma in a volcano plutonic structure

late tertiary tectonic subsidence on the northeast australian passive margin response to dynamic topography abstract the carbonate platforms and adjacent troughs off northeast australia record a phase of accelerated tectonic subsidence in the late miocene pliocene previous attempts to analyse tectonic subsidence in this area were hampered by poor core recovery and dating uncertainties at some of the odp ocean drilling program leg 133 sites on the queensland plateau in particular at sites 812 and 814 in order to establish a general chronostratigraphy for these sites we have integrated geophysical logs biostratigraphic lithologic and seismic reflection data to identify and correlate miocene hardgrounds on the queensland plateau to third order eustatic sea level lowstands on the haq et al haq b u hardenbol j vail p r 1987 chronology of fluctuating sea levels since the triassic science vol 235 pp 1156 1167 curve this enables us to estimate ages where biostratigraphic information is poor we have used sediment backstripping to compute tectonic subsidence histories for sites on the queensland plateau 811 825 812 and 814 the marion plateau 815 and the queensland trough 823 in order to determine regional subsidence patterns results of these calculations show that although the leg 133 sites are located on a passive margin approximately 1000 km south of the pacific australian plate boundary they record a greater amount of subsidence than can be predicted from simple elastic models post 9 ma subsidence of 1300 200 m in the queensland trough 650 200 m on the western margin of the queensland plateau post 5 ma subsidence of 500 30 m on its southern margin and 660 50 m on the northern margin of the marion plateau these models show that it is difficult to account for observed subsidence either by means of thrust loading in papua new guinea or by a combination of the latter and in plane stresses originating from collision along the australian pacific plate boundary alternatively shear wave tomography van der hilst r d kennett b l n shibutani t 1998 upper mantle structure beneath australia from portable array deployments in brown j dooley j goleby b van der hilst r klootwijk c eds structure and evolution of the australian continent geodynamics series 26 pp 39 57 displays a nnw sse trending band of anomalously high velocities in the upper mantle stretching from the queensland plateau to indonesia these anomalies likely reflect subducted slab material originating from late eocene late oligocene subduction north of papua new guinea thus we suggest that the observed post 9 ma tectonic subsidence of the queensland and marion plateaus and queensland trough is largely caused by dynamic surface topography due to australia s northeastern margin overriding a slab burial ground modulated by flexural deformation resulting from collision tectonics north of australia the observed rates of tectonic subsidence are much slower than those of third order sea level changes and can thus be differentiated from glacial eustasy

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

evolution of the mesozoic yuljeon basin in south korea and its tectonic implication abstract the mesozoic yuljeon basin yb is a small scale non marine sedimentary basin in the northeastern part of the gyeonggi massif in the korean peninsula the yb consists of two lithostratigraphic units the munamdong and yuljeon formations based on an integrated study of stratigraphy structural geology geochronology and geochemistry major episodes of the yb have been investigated in the late triassic after 227 ma the yb was formed in a tectonic setting wherein crust was extended and uplifted which resulted in deposition of the munamdong formation in the latest early jurassic circa 180 ma the yuljeon formation was deposited in association with volcanic activity in a continental magmatic arc setting on entering the middle jurassic circa 174 ma extension in the yb stopped and then basin inversion occurred due to contractional deformation the middle jurassic thermal pulse in the area generated various scale igneous rocks and induced hydrothermal circulations which created suitable conditions for fault activity with help of fluid pressure this study demonstrates that the yb formed with sedimentation in a post collisional extensional setting related to the songrim orogeny and then it was enlarged with volcanic related sedimentation in a continental arc setting related to the daebo orogeny and lastly it was followed by basin inversion with arc magmatism in the same continental arc setting reflecting the temporal and spatial changes in regional tectonics moreover the general provenance characteristics for the early mesozoic strata in the eastern north china block gyeonggi massif of south korea and southwest japan can be geochronologically correlated the sedimentary rocks record the distinctive tectonothermal histories of east asia such as the middle paleoproterozoic orogens in the eastern north china block the permo triassic continental collision between north and south china blocks and the subduction of paleo pacific plate during the mesozoic

application of swir spectrometry to the determination of biotite compositions in hydrothermally altered units of the yangyang iron oxide apatite ioa deposit south korea abstract the yangyang iron oxide apatite deposit in south korea has gone through multiple events for igneous activity structural evolution hydrothermal alteration and mineralization trioctahedral mica is indicative of potassic alteration in the yangyang ioa deposit and its mineral chemistry is quite variable the variability can be described in terms of biotite colors and short wavelength infrared swir reflectance spectra brown to greenish brown biotite displays lower values of mg 0 48 0 58 than green biotite 0 62 0 78 whereas colorless phlogopite related to fe p mineralization has the highest values of mg 0 85 0 95 to compare mg values with the positions and intensities of absorption in the trioctahedral micas we used three types of spectrum reflectance hull quotient and gaussian brown biotite produces a short mg oh peak near 2340 nm green biotite has a peak near 2330 nm and colorless phlogopite displays an absorption peak near 2310 nm these positional variations are consistent with compositional changes a mica alteration index mai represents the ratio of fe oh and mg oh peak absorption intensities for trioctahedral micas from brown to green biotite and colorless phlogopite the mai values gradually increase with decreasing fe oh intensity correlation coefficients for plots of mg oh absorption values vs mg vary from 0 89 for the reflectance method to 0 93 for the hull quotient method and 0 93 for the gaussian method and for plots of mai vs mg they vary from 0 91 for the reflectance method to 0 92 for the hull quotient method and 0 92 for the gaussian method analyses using our new swir spectroscopic methods confirm that the abundance of indicator minerals and the fe mg ratios in trioctahedral micas vary systematically as a function of hydrothermal fluid chemistry and mineral composition within the altered lithologies swir spectroscopy of trioctahedral micas could provide a useful tool for mapping vectors of alteration in the yangyang ioa system

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

structural and lithological controls on iron oxide copper gold deposits of the southern selwyn mount dore corridor eastern fold belt queensland australia the eastern fold belt of the proterozoic mount isa inlier has undergone numerous episodes of deformation hydrothermal alteration and iron oxide copper gold mineralization the structural and lithological controls on the location and geometry of five iron oxide copper gold deposits starra mount elliott swan mount dore and lady ella in the selwyn mount dore corridor were studied through geologic mapping core logging structural analysis and examination of 3 d grade distribution all the deposits in this area occur in close proximity to the selwyn shear a long lived regional structure which developed at the contact between carbonate rich and siliciclastic dominated rock packages this fault and a series of subparallel structures have been the focus of multiple reactivation events and have controlled the distribution of alteration and mineralization which occurred in this area at 1595 1570 1530 1515 1500 and 1485 ma ironstone hosted mineralization at starra developed in a preexisting zone of sodic and iron oxide alteration along the n striking starra shear kinematic indicators along the starra shear reveal that it has been reused numerous times early d2 thrusting post d2 sinistral shearing and pre d4 syn d3 dextral brittle faulting were all associated with sodic sodic calcic and iron oxide alteration either the post d2 or pre d4 strike slip faulting events can explain the steeply plunging nature of the orebodies at starra however the dextral pre d4 event produces dilational structures in ironstones and is associated with regionally important sodic and iron oxide alteration along ne to ene trending faults which intersect the starra shear close to the main orebodies we consider this the more likely main mineralizing event at starra both strike slip faulting events correspond with a change from e w oriented compression during the main phase of the isan orogeny to transpression that may have been driven by crustal overthickening during orogenesis at mount elliott and swan mineralization is associated with post d4 dextral and reverse movement across a sinistral fault jog along the nnw striking mount elliott fault during transpression mineralization alteration and veins at these deposits clearly overprint all ductile structural features overall the orebody geometry is controlled by the orientation of the mount elliott fault but high grade ore shoots are controlled by breccia formation in elongate pods which are parallel to preexisting f2 fold axes in contrast gently plunging grade shells at the slightly younger mount dore and lady ella deposits formed in response to renewed e w compression and reverse displacement along n to ne striking fault segments of the mount elliott fault and newly formed subparallel faults examination of the structural setting of mount dore and lady ella also demonstrates that strain partitioning between mechanically heterogeneous rock sequences metasedimentary rocks metabasalts granite and strongly silicified rock types exerted strong controls on the location of later mineralized systems the important episodes of iron oxide copper gold mineralization in the eastern fold belt between 1595 and 1485 ma appear to be associated temporally with compressive or transpressive structural regimes these events developed in response to far field tectonic stresses that evolved in response to the collision of laurentia with the proto australian continent during the isan orogeny late in the orogenic history the emplacement of voluminous a type batholiths associated with hot spot plume activity likely generated localized transient shortening events which are also associated with iron oxide copper gold mineralization this unique geodynamic evolution was responsible for the large scale metal enrichment and remobilization into suitable chemical and structural traps over significant time periods in the eastern fold belt iron oxide copper gold district

geochronology of the kiruna iron ores and hydrothermal alterations the magnetite apatite iron ores at kiruna are hosted in a suite of alkaline volcanic and subvolcanic rocks that was formed in a short time interval of about 20 m y between 1900 and 1880 ma u pb dating of titanite from magnetite titanite dikes in the footwall of the luossavaara orebody indicates that the age of the magnetite apatite ores is 1888 or 6 ma titanite actinolite calcite assemblages in amygdules are related to the hydrothermal and metasomatic alteration of the wall rocks and the ore the u pb titanite age of the amygdule fill is 1876 or 9 ma the geographic distribution of the alteration suggests that it is not related to the formation of the iron ores the youngest major magmatic units in the kiruna area are 1792 or 4 ma syenites that intruded the supracrustal rocks the kiruna area underwent a weak metamorphic overprint during the caledonian orogeny our data are inconsistent with a hydrothermal event at ca 1 5 ga as has been suggested earlier

localization of intraplate deformation through fluid assisted faulting in the lower crust the flinders ranges south australia abstract in this paper we present a hypothesis for localized intraplate deformation in the continental crust of south central australia that involves fluid assisted reactivation of faults in the mid to lower crust using data from a temporary seismometer deployment in the flinders ranges we show that earthquakes relocated in a 3d velocity model cluster in elongated low vp vs anomalies that extend to depths exceeding 20 km and are aligned with the axis of the flinders ranges in the northern flinders ranges these low vp vs anomalies can be interpreted as fractured neoproterozoic to cambrian sediments that separate two cratonic blocks the gawler craton to the west and the curnamona province in the east previous studies of helium isotopes in springs to the north of the area provide evidence of mantle derived fluids that may influence faulting at depth our focal mechanism and stress inversion results show a regionally compressive stress field that provides no evidence for stress concentration we also argue that mechanisms for localized faulting such as thermal weakening and isostatic rebound also fail to account for the occurrence of earthquakes at mid to lower crustal depth in this area of high heat flow and that the focused seismicity can only be explained by high pore fluid pressure in the lower crust

planktic foraminiferal biostratigraphy and datum planes in the neogene sedimentary sequence in taiwan abstract the neogene sequences are well developed in western taiwan and they form paleontologically the best known interval of the sedimentary column of the island the succession of planktic foraminifera datums and epoch boundaries in the taiwan neogene are discussed with reference to a generalized range chart and the lithostratigraphic units biostratigraphic ranges of these species in the neogene sections of western taiwan allow establisment of eighteen foraminiferal datum planes useful for intraregional as well as international stratigraphic correlations

quantitative resolution of the debate over antiquity of the central australian landscape implications for the tectonic and geomorphic stability of cratonic interiors we report the first measure of long 100 myr and short 1 myr term denudation rates from key geologically stable landforms in the davenport range central australia these landforms have previously been assigned a cambrian age which arguably places them amongst the oldest persistent landforms on the continent if not on earth our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using 10 be and 26 al show that while average exhumation rates are low the denudation history for this cratonic region is incompatible with extreme sub aerial longevity and long term tectonic and geomorphic stability our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the mesozoic followed by a phase of kilometre scale exhumation that was largely complete by the beginning of the cainozoic we suggest that a similar process of burial and exhumation has probably been responsible for the sub aerial preservation of seemingly ancient landforms elsewhere in australia

cause and evolution of intraplate orogeny in australia abstractintraplate orogeny remains an enigmatic process in terms of plate tectonics regional continental stress elds agree well with stress orientations predicted by the main forces driv ing and resisting plate motion however regional continental stress elds fail to explain major localized compressive plate deformation in continental interiors far away from plate bound aries global or plate wide intraplate stress models typically assume a laterally homogeneous plate rheology and are con ned to modeling the present without addressing the antiquity of the current stress eld an understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time dependent plate geometry and driving forces here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far eld plate boundary forces has caused intra plate orogenesis and tectonic reactivation in southeastern australia during the tertiary our ndings are contrary to the opinion that continental interiors are insensitive to compression at plate margins and help to explain the mechanisms causing intraplate orogenesis keywords

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

the relationship between systematic metamorphic patterns and collisional processes along the qinling sulu odesan collisional belt between the north and south china cratons

topo oz insights into the various modes of intraplate deformation in the australian continent abstract as the fastest lowest flattest and amongst the most arid of continents australia preserves a unique geomorphic record of intraplate tectonic activity evidencing at least three distinct modes of surface deformation since its rapid northward drift commenced around 43 million years ago at long wavelengths several 1000s km systematic variations in the extent of neogene marine inundation imply the continent has tilted north down southwest up at intermediate wavelengths several 100s km several undulations of 100 200 m amplitude have developed on the 1 10 myr timescale at still shorter wavelengths several 10s km fault related motion has produced local relief at rates of up to 100 m myr over several million years the long wavelength north down tilting can be related to a dynamic topographic effect associated with australia s northward drift from the geoid low dynamic topography low now south of the continent to the geoid high dynamic topography low centred above the south east asian and melanesian subduction zones the short wavelength fault related deformation is attributed in time to plate wide increases in compressional stress levels as the result of distant plate boundary interactions and in space in part to variations in the thermal structure of the australian lithosphere at the intermediate wavelengths transient low amplitude undulations can be ascribed to either lithospheric buckling or the development of instabilities in the thermal boundary layer beneath the lithosphere in the latter case topographic asymmetries suggest the australian lithosphere is moving north with respect to the mantle beneath providing a unique attribution to the progressive alignment of seismic anisotropy and absolute plate motion observed near the base of the australian lithosphere

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important