cretaceous stress field evolution and origin of the jiaolai basin eastern north china abstract the jiaolai basin to the east of the tan lu fault zone tlfz is a cretaceous ne trending basin in eastern north china and has a multi episodic evolution different from other contemporary nw trending en echelon graben systems west to the tlfz the origin of the jiaolai basin three episodes k1l 135 120 ma k2q 120 105 ma k2w 88 65 ma cannot be simply explained by the sinistral displacement of the tlfz at the beginning of the early cretaceous around ca 135 ma the purpose of this paper is to investigate the critical factors e g bounding faults tectonic setting stress field driving the development of the jiaolai basin and constraint its evolution three episodes of two dimensional finite element models were generated in order to determine the roles of the boundary conditions and to reveal the tectonic development of the basin when compared to the calculated maximum horizontal compressive stress hmax trajectories and the observed data along with a comparison between the von mises stress u von and the depocenters of the basin the calculated results can be well proved by the geological evidence both in stress field evolution and in the basin patterns our analysis show that the three episodes of the maximum principle stress trajectories changed from ne sw earlier early cretaceous k1l nne ssw later early cretaceous k1q then e w late cretaceous k2w moreover the results show a good consistency between the u von and the depocenters in the basin through geological cross sections via models we suggest that the origin and evolution of the jiaolai basin were controlled by three episodes of tectonic events i the reactivated bounding faults and the post orogenic extension of the sulu orogen during the earlier early cretaceous ii the lithospheric delamination of the thickened lithosphere during the later early cretaceous and iii the likely far field effects produced by the collision between the kohistan drags arc complex and asia during the late cretaceous the jiaolai basin evolved from extension to pull apart experiencing volcanic activity during the three episodes of the cretaceous

channel flow and fault segmentation with implications for the generation of earthquakes in the longmenshan fault zone eastern tibetan plateau abstract the ms 8 0 wenchuan earthquake occurred in the longmenshan fault zone lfz of the eastern margin of the tibetan plateau tp the tectonic activities in this region have been a focus of extensive studies in the past two decades two different models involving oblique stepwise rise and channel flow respectively are proposed and can be used to explain the generation of earthquakes the oblique stepwise rise model emphasizes more about the fault of the upper crust whereas the channel flow model concerns more about the different crustal rheological structures currently because of the confirmation of geophysical data on the rheological structure of the tp more researchers agree with the channel flow model to investigate the generation of earthquakes in the lfz this study establishes a three dimensional viscoelastic model based on the heterogeneity of the lithospheric structure and the segmentation of faults using ansys finite element software to simulate the evolution of stress and strain the calculated results are consistent with focal mechanism solutions fault striations and topography the best fit stress field suggests that stress localization in the longmenshan range is mainly controlled by the channel flow and fault segmentation the differing viscosities and relative movement between the channel flow and the sichuan basin induce a concentration of stress in the lfz the stress was most concentrated in segment ii closest to the leading edge of the channel flow and wherein the wenchuan earthquake occurred additionally the higher locking degree of segment ii found before the wenchuan earthquake induced higher stress levels at shallow depths corresponding to higher seismic activity in segment ii thus the combined action of the channel flow and fault segmentation controls the distribution of stress and causes the generation of earthquakes in the lfz

late paleoproterozoic extension and a paleostress field reconstruction of the north china craton abstract mafic dyke swarms and aulacogens are major anorogenic extensional events in the late paleoproterozoic north china craton ncc the n nnw mafic dyke swarms are widespread in the ncc whose ages span between 1 83 and 1 77 ga the similar ages and orientations of 1 8 ga dyke swarms in the ncc demonstrate that the amalgamated ncc experienced widespread extension at this time based on the width statistics of dyke swarms on ten survey lines an average crustal extension ratio of 0 35 was found for the ncc the small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures and indicates that the ncc had become a brittle plate prior to the emplacement of the mafic dyke swarms precisely dated mafic dyke swarms when used as paleostress indicators can be employed in the paleostress field reconstruction of precambrian cratons two dimensional finite element modeling 2 d fem of the ncc in which the various blocks were assigned densities and elastic constants shows that north south compression favors dyke intrusion along generally n nw lines and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust the best fitting model can be considered a plausible representation of the tectonic force acting on the ncc that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms the results of modeling of the late paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the central orogenic zone western and east blocks of the ncc the results also show that the north south tectonic forces play an important role in determining the paleostress field in the ncc the widespread extension of the ncc resulted from the north south tectonic forces which may be related to the break up of the late paleoproterozoic supercontinent the paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break up

prediction of tensile fractures in ks2 trap kuqa depression nw china abstract tectonic fractures that generally controlled by tectonic stress field especially tensile fractures are crucial for tight reservoirs that are characterized by low porosity and low permeability prediction of fracture distribution is extremely important for further exploration and development of such reservoirs tensile fractures are developed in the upper bashijiqike formation in kuqa depression in northwest china the subject of this study was the ks2 faulted fold and the characteristics of the tensile fractures were analyzed in detail a three dimensional finite element elastic model of the faulted fold was constructed and the two factor method was applied to predict the distribution of tensile fractures in the upper bashijiqike formation in the faulted fold distributions of predicted tensile fractures reveal that positions relative to the fold axis and faults in the hinge zone both have significant influence on the distribution of tensile fractures the predicted tensile fractures are developed in the hinge zone in the faulted fold particularly in the area around fault tips which is the region with the most potential for oil and gas exploration in faulted folds

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

post glacial sea level rise from a coral record at huon peninsula papua new guinea dating of coral reef terraces can provide a record of changes in sea level which should be pronounced during the transition between glacial and interglacial periods cores drilled from coral reefs at barbados in the equatorial west atlantic1have revealed the sea level changes that occurred during the younger dry as event at the end of the last glacial maximum 11 000 yr bp it has not been known however whether pacific coral reefs can grow at a rate sufficient to keep up with the rise in sea level during such a transition here we report results obtained from a 52 m drill core from the post glacial reef at huon peninsula papua new guinea spanning the interval from 7 000 to 11 000 14c yr bp which show that coral growth kept pace while the relative sea level rose by 50m although the tectonic environment is very different from that at barbados the two records compare well when corrections are made for local tectonic uplift showing that sea level rise was similar at both locations the rate of rise was greatest between 9 000 and 10 00014c yr bp corresponding to the time of the younger dryas

a revised relaxation time spectrum for fennoscandia summary the fennoscandian relaxation time spectrum rts first derived by mcconnell 1968 is a classic data set in studies of glacial isostatic adjustment gia we outline a new method for estimating an rts from a set of strandline data which is based on a damped least squares solution for spherical harmonic coefficients associated with the strandline heights in contrast to the hankel transform approach outlined by mcconnell 1968 the method does not require interpolation or extrapolation of the data nor does it use the assumption that peripheral deformations are zero we begin by applying the new approach to a suite of synthetic strandlines these synthetic calculations quantify the effect on the rts estimates of the common assumptions of free decay uplift an axisymmetric fennoscandian deformation field and the uncertainty introduced by limited spatial and temporal sampling of this field recently the accuracy of the sauramo 1958 strandline data upon which the mcconnell 1968 rts was based has been questioned wolf 1996 accordingly we apply our new approach to a set of more robust strandline data published by donner 1964 1969 1980 1995 to compute a revised rts for fennoscandia at high harmonic degrees above 50 our new rts is characterized by weak constraints whereas mcconnell s 1968 rts suggests a significant reduction in relaxation times relative to the values at low degrees this reduction was the basis for mcconnell s 1968 inference of an elastic lithosphere in contrast to this we conclude that the trend is an artefact of the observational data set he adopted at lower degrees mcconnell s 1968 relaxation time estimates lie at the lower end of the range implied by the present analysis to complete our study we apply the techniques of linearized bayesian inference to invert our newly derived rts for mantle viscosity we find that the rts provides an estimate of 5 1020 pa s for the volumetric mean viscosity in a region extending from the base of the lithosphere to about 550 km depth target regions which extend from the transition zone to 1200 km depth are less well constrained however the average viscosity from the base of the lithosphere to 1200 km depth is consistent with the classic haskell 1935 value of 1021 pa s for mantle viscosity finally we demonstrate that viscosity models within the class inferred by mitrovica forte 1997 from joint inversions of postglacial relaxation times associated with gia and mantle convection observables simultaneously fit the revised fennoscandian rts

40ar 39ar and k ar geochronology of the dykes from the south indian granulite terrain abstract 40ar 39ar and conventional k ar data on mafic dykes and a felsic dyke together with previous data have been used to constrain the ages of the dyke magmatism in the south indian granulite terrain the age of the mainly ene wsw trending agali anaikatti swarm is estimated to be 1980 25 ma with a very limited occurrence of upper cretaceous 80 90 ma dykes in the area the nw se dharmapuri swarm is dated as at least 1800 ma possibly 1855 9 ma both the nw se and the ne sw dyke swarms of tiruvannamalai are dated at 1650 10 ma only a few dykes in north kerala are of middle proterozoic ca 1700 1650 ma age furthermore the 40ar 39ar results of dharmapuri dykes exhibit age spectrum characteristic of partial degassing they provide strong evidence of latest proterozoic overprinting by a thermal event probably related to the pan african orogeny most of the phanerozoic dykes are confined to the coastal region in kerala province the dolerites of central kerala and the majority of north kerala dolerites intruded during the 70 65 ma period whereas a large gabbro and a felsic dyke in central and north kerala respectively are of upper cretaceous age ca 85 ma and the mean 81 1 5 ma age from these dykes may represent cooling age the oldest phanerozoic upper jurassic 144 6 ma dyke intrusions have a restricted occurrence in south kerala very close to the southern tip of india the proterozoic dykes are interpreted as being lateral intrusions from under plating igneous bodies beneath the eastern shield region the geological record of the terrain does not show indications of significant pre magmatic extension and rifting but dyke magmatism may have led to extension rifting and the consequent development of the india antarctica sri lanka proterozoic high grade terrain the data are also interpreted to suggest that the south indian granulite terrain is an early proterozoic tectono metamorphic terrane with strong thermal imprints of neoproterozoic early palaeozoic age it is difficult to reconcile its extensions into proterozoic sri lanka or antarctica where tectono metamorphism is dated to be of neoproterozoic age the late phanerozoic dykes are predominantly related to the deccan trap event and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the seychelles and the indian continent the few older ca 145 ma dykes in south kerala may be related to the rift tectonics that preceded the separation of australia antarctica and india

palaeoproterozoic 1 9 ga extension and breakup along the eastern margin of the eastern dharwar craton se india new sm nd isochron age constraints from anorogenic mafic magmatism in the neoarchean nellore greenstone belt abstract the neoarchean nellore greenstone belt located along the eastern margin of the eastern dharwar craton was the site for younger igneous activity the timing of this magmatism and the host metabasalt critical for constraining extensional events along the deformed southeast margin of the eastern dharwar craton was conducted by the sm nd method samples from the large differentiated gabbroic kandra igneous complex a subset kandra group a define an isochron age of 1926 110 ma nd i 0 51006 12 mswd 0 98 n 5 e nd 1 7 and the kandra group b subset yielded a model 3 isochron age of 1882 120 ma nd i 0 50994 14 mswd 1 8 n 4 e nd 5 1 whereas the three samples from north pamuru gabbro anorthosite yielded an isochron age of 1882 160 nd i 0 51012 18 mswd 0 064 e nd 1 7 the kandra group a and north pamuru samples regressed together yielded a preferred isochron age of 1911 88 ma nd i 0 51008 10 mswd 0 62 n 8 e nd 1 6 for the emplacement of gabbros in contrast host greenstone belt metabasalt from chundi yielded a sm nd isochron age of 2693 94 ma nd i 0 50920 8 mswd 1 4 n 4 e nd 1 2 whereas regressing the chaganam and chundi samples yielded an isochron age of 2654 100 ma nd i 0 50923 9 mswd 1 5 n 11 e nd 0 7 basal metapelite underlying the metabasalt yields old nd model ages between 2888 and 3054 ma the results of this study show that the 2 7 ga nellore greenstone belt was intruded by enriched morb type gabbros at 1911 ma it is proposed that this palaeoproterozoic magmatism formed in response to a major extensional event along the eastern dharwar craton margin that likely initiated breakup from the southeastern margin of the north china craton this exposed craton margin later evolved as the site for collisional events during the late palaeoproterozoic to early mesoproterozoic

mechanics of the giant radiating mackenzie dyke swarm a paleostress field modeling 1 the 1 27 ga mackenzie dyke swarm of the canadian shield is a giant radiating dyke swarm that gradually swings in orientation from n s in the focal area to nw se trends in peripheral areas in this paper we propose a new model the plug model that accounts for the paleostress contribution to the mechanism of emplacement for the mackenzie dyke swarm in the canadian shield the 1 27 ga stress field on the canadian shield calculated by the plug model explains the radiating nature of the mackenzie dyke swarm around the coppermine river lava field by local stress concentrations the parallel nature of the dyke swarm at distance more than 1000 km from the focal source can be explained by the existence of a regional tectonic stress field created by ridge push acting on the southeast margin of the canadian shield from the grenville ocean the thin elastic plate and two dimensional cross section modeling suggest that the interaction between stresses from a mantle upwelling and the grenville ocean spreading play an important role in the intrusion mechanism of the mackenzie dyke swarm the change in dyke orientation from n s trending to nw se trending is caused by coupling between resistance from the focal area plug area and a grenville ocean ridge push

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

evidence for 1 8 ga extension of the eastern block of the north china craton from shrimp u pb dating of mafic dyke swarms in shandong province abstract a large mafic dyke from the nnw trending dyke swarms in the eastern block of the north china craton was dated by shrimp u pb age analysis this dyke occurs near mt taishan in shandong province it cuts neoarchean tonalite adamellite and diorite plutons two types of zircons were found in the dyke type 1 zircons were inherited from the host rocks and gave a shrimp u pb age of 2544 24 ma with a weighted mean 207pb 206pb age of 2523 18 ma type 2 zircons were crystallized from the dyke magma and yielded a concordant shrimp u pb age of 1830 17 ma with a weighted mean 207pb 206pb age of 1837 18 ma indicating that the large dyke at taishan was emplaced at 1830 ma the mafic dyke swarm provides evidence of a phase of extension in the eastern block of the north china craton at 1 8 ga this dyke has a similar age to dyke swarms in the western block and in the central orogen belt of the north china craton this lithospheric extension led to the opening of the yanliao and zhongtiao aulacogens the similar ages and orientations of 1 8 ga dyke swarms throughout the north china craton demonstrate that at that time the eastern and western blocks were united and that the amalgamated craton experienced widespread extension

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

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

modelling postglacial sea levels with power law rheology and a realistic ice model in the absence of ambient tectonic stress previous studies of postglacial rebound with power law rheology and simple ice models over laurentia have found that sea level data in the transition zone e g boston cannot be easily explained unless non linear rheology is restricted to lie within a thin layer between the lithosphere and the upper mantle the reason is the existence of a viscously stationary zone which is characteristic of non linear mantles in this study the simple ice model is replaced by a realistic ice history adapted from the ice 3g model which takes into account important effects such as retreating ice margins and eustatic ocean loading a 3 d finite element model is used to calculate the deformation and the relative sea levels in eastern canada and along the us east coast the purpose is to investigate whether the realistic load model allows sea level data near the centre of rebound and along the atlantic east coast of canada and the usa to be explained by non linear rheology simultaneously the assumptions made are that 1 postglacial rebound sees the steady state creep and not the transient creep 2 either ambient tectonic stresses do not interact with rebound stress or the ambient stress level is low 3 lateral heterogeneity of rock properties and anisotropic flow can be neglected results indicate that the retreating ice margin does make it easier for non linear rheology in the mantle to explain the sea level data in the transition zone around boston however reconciling all the sea level data near the centre of rebound and along the atlantic east coast simultaneously is still difficult for a non linear rheology so far two types of models with non linear rheology have been found to be able to explain the sea level data inside and outside laurentia simultaneously they are 1 earth models with a thin 270 km non linear zone overlying a 1021 pa s lower mantle 2 earth models with a linear upper mantle with an average viscosity of 1021 pa s overlying a non linear lower mantle with a 3 10 35 pa 3 s 1 and n 3

space geodetic constraints on glacial isostatic adjustment in fennoscandia analysis of global positioning system gps data demonstrates that ongoing three dimensional crustal deformation in fennoscandia is dominated by glacial isostatic adjustment our comparison of these gps observations with numerical predictions yields an earth model that satis es independent geologic constraints and bounds both the average viscosity in the upper mantle 5 x 10 20 to 1 x 10 21 pascal seconds and the elastic thickness of the lithosphere 90 to 170 kilometers we combined gps derived radial motions with fennoscandian tide gauge records to estimate a regional sea surface rise of 2 1 0 3 mm year furthermore ongoing horizontal tectonic motions greater than 1 mm year are ruled out on the basis of the gps derived three dimensional crustal velocity field

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

late pleistocene and holocene sea level change along the australian coast abstract tectonically the australian continent is relatively stable and as such it provides a good platform for studying late pleistocene and holocene sea level change the observations indicate that present sea level was reached at about 6000 years ago and that since then level has remained constant to within a few metres considerable spatial variability in the amplitude of the emergence and in the time at which sea levels first reached their present values have been observed spatial variability has also been observed in the sea level during the last glacial maximum and during the late phase of the holocene transgression much of this variaiblity is the result of the earth s adjustment to the glacial melting in the late pleistocene early holocene time and the concomitant distribution of this meltwater into the oceans conclusions that are drawn from matching observations with this glacio hydro isostatic model include the following 1 the mantle viscosity increases with depth the average upper mantle viscosity is about 2 3 10 20 pa s and the average lower mantle defined as below 670 km depth viscosity is about 10 22 pa s the average lithospheric thickness for the coastal and offshore region is about 70 80 km lateral variations in these mantle parameters are indicated 2 the addition of melwater into the oceans did not cease 6000 years ago but continued at a reduced rate up to more recent times adding an amount of water equivalent to a 3 m sea level rise in this interval 3 the amplitude of the holocene emergence is a function inter alia of coastal geometry and significant variations occur over distances of a few hundred kilometres and more holocene highstands of 1 2 m are generally well developed at the continental margins but are less well developed or do not developed at all at offshore islands and reefs 4 variations in the time of maximum emergence also occur being earlier at upstream tidal river sites than at coastal sites at offshore reefs or islands the sea level curve lags the coastal curves by 1000 years or more 5 considerable spatial variability occurs in the sea levels at the time of the last glacial maximum with variations of up to 30 m occurring across the continental shelves 6 there is generally no need to invoke tectonic movements to explain the sea level data along the australian coast but possible exceptions occur for the perth region and for tasmania

insights into the damage zones in fault bend folds from geomechanical models and field data abstract understanding the rock mass deformation and stress states the fracture development and distribution are critical to a range of endeavors including oil and gas exploration and development and geothermal reservoir characterization and management geomechanical modeling can be used to simulate the forming processes of faults and folds and predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses this approach enables the development of forward models that incorporate realistic mechanical stratigraphy e g the bed thickness bedding planes and competence contrasts include faults and bedding slip surfaces as frictional sliding interfaces reproduce the geometry of the fold structures and allow tracking strain and stress through the whole deformation process in this present study we combine field observations and finite element models to calibrate the development and distribution of fractures in the fault bend folds and discuss the mechanical controls e g the slip displacement ramp cutoff angle frictional coefficient of interlayers and faults that are able to influence the development and distribution of fractures during fault bend folding a linear relationship between the slip displacement and the fracture damage zone the ramp cutoff angle and the fracture damage zone and the frictional coefficient of interlayers and faults and the fracture damage zone was established respectively based on the geomechanical modeling results these mechanical controls mentioned above altogether contribute to influence and control the development and distribution of fractures in the fault bend folds

late quaternary variations in relative sea level due to glacial cycle polar wander growth and decay of continental ice sheets can excite significant motion of the earth s rotation pole and cause a complex spatio temporal pattern of changes in relative sea level these two effects have generally been considered separately but may interact in important ways in particular a simple model of the melting of the laurentide ice sheet causes a uniform eustatic sea level rise of 55 m and also induces a motion of the rotation pole by 0 1 to 1 degree depending on viscosity structure in the mantle this motion produces a secular pole tide which is a spherical harmonic degree 2 order 1 component of the relative sea level pattern with peak to peak amplitude of 20 to 40 m the maximum effect is along the great circle passing through the path of the pole and at latitudes of 45 this secular pole tide has been ignored in most previous attempts to estimate ice sheet loading history and mantle viscosity from global patterns of relative sea level change it has a large influence along the east coast of north america and the west coast of south america and significantly contributes to present day rates of relative sea level change

triassic shoshonitic dykes from the northern north china craton petrogenesis and geodynamic significance zircon u pb ages major and trace element geochemistry and sr nd and pb isotope compositions of diorite and diorite porphyry dykes from the jinchanggouliang jcgl gold ore field on the northern margin of the north china craton ncc were studied to investigate their sources petrogenesis and geodynamic significance la icp ms zircon u pb dating reveals three major age groups of 2500 ma n 2 253 7 ma n 5 and 227 1 ma n 9 the inherited ages of 2500 ma contemporary with the archaean ncc continental growth imply that crustal material was involved in the magma source the igneous zircons with a concordia age of 227 1 ma may record the emplacement age of the jcgl dykes both diorite and diorite porphyry exhibit a wide range of sio 2 and mgo contents and are characterized by high concentrations of na 2 o k 2 o and al 2 o 3 and low abundances of p 2 o 5 and tio 2 they are enriched in large ion lithophile elements and light rare earth elements without significant eu anomalies and depleted in high field strength elements all are categorized as shoshonitic rocks all samples show a narrow range of sr isotope compositions with initial 87 sr 86 sr ratios from 0 70394 to 0 70592 variable e nd t values 1 1 to 12 0 and t dm2 ages of 913 1972 ma their pb isotope compositions form continuous variation trends and plot in the fields between enriched mantle 1 em1 and lower continental crust lcc the above results suggest that the jcgl dykes studied could have been derived from mixing of lower crust lithospheric mantle of the ncc and ascending asthenospheric melt in a post orogenic extensional geodynamic setting these shoshonitic dykes together with the geochronological data of regional ene trending retrograded eclogites ophiolites continental arc magmatic belt a type granite alkaline intrusions and metamorphic core complex from the northern ncc and central asian orogenic belt caob suggest that closure of the palaeo asian ocean i e stage of pre collision to collision had completed during latest permian to earliest triassic time and that the caob was subsequently tectonically dominated by post orogenic extensional regimes the involvement of asthenospheric melt in the magma source implies that the sub continental lithospheric mantle sclm of the ncc had been modified and the onset of lithospheric destruction and thinning beneath the northern ncc may have occurred in middle late triassic time as a result of post orogenic subducting slab detachment and lithospheric delamination

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

the siberian lithosphere traverse mantle terranes and the assembly of the siberian craton abstract the kimberlite fields scattered across the ne part of the siberian craton have been used to map the subcontinental lithospheric mantle sclm as it existed during devonian to late jurassic time along a 1000 km traverse ne sw across the archean magan and anabar provinces and into the proterozoic olenek province 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe major elements and proton microprobe trace elements these data and radiometric ages on the kimberlites have been used to estimate the position of the local paleo geotherm and the thickness of the lithosphere and to map the detailed distribution of specific rock types and mantle processes in space and time a low geotherm corresponding approximately to the 35 mw m2 conductive model of pollack and chapman tectonophysics 38 279 296 1977 characterised the devonian lithosphere beneath the magan and anabar crustal provinces the devonian geotherm beneath the northern part of the area was higher rising to near a 40 mw m2 conductive model areas intruded by mesozoic kimberlites are generally characterised by this higher but still cratonic geotherm lithosphere thickness at the time of kimberlite intrusion varied from ca 190 to ca 240 km beneath the archean magan and anabar provinces but was less 150 180 km beneath the proterozoic olenek province already in devonian time thinner devonian lithosphere 140 km in parts of this area may be related to riphean rifting near the northern end of the traverse differences in geotherm lithosphere thickness and composition between the devonian toluopka area and the nearby mesozoic kimberlite fields suggest thinning of the lithosphere by ca 50 60 km related to devonian rifting and triassic magmatism a major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data extended from outcrops in the anabar shield represent major lithospheric sutures which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock type distribution between 100 and 250 km depth the presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the magan and anabar provinces is concordant with their archean surface geology the lack of harzburgitic garnets and the chemistry of the lherzolitic garnets beneath most of the other fields are consistent with the proterozoic surface rocks mantle sections for different terranes within the archean portion of the craton show pronounced differences in bulk composition rock type distribution metasomatic overprint and lithospheric thickness these observations suggest that individual crustal terranes of both archean and proterozoic age had developed their own lithospheric roots and that these differences were preserved during the proterozoic assembly of the craton data from kimberlite fields near the main archean proterozoic suture the billyakh shear zone suggest that reworking and mixing of archean and proterozoic mantle was limited to a zone less than 100 km wide

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

estimating past continental ice volume from sea level data we predict sea level change since the last glacial maximum lgm at four far field sites barbados bonaparte gulf huon peninsula and tahiti using a revised theoretical formalism this formalism includes a time varying shoreline geometry an accurate treatment of sea level variations near regions of ice retreat and the influence of glacial cycle perturbations in earth rotation we elucidate the physics of far field sea level change by de constructing the predicted signal into spatially uniform versus spatially varying components as well as isolating contributions due to the ice load the ocean load and the rotational potential we demonstrate that the sum of these three contributions plus the spatially uniform sea level fall associated the with retreat of grounded marine ice sheets can produce a significant difference between predictions of sea level change at far field sites and the eustatic or meltwater curve associated with the adopted ice model this difference is site and time dependent for example the total sea level rise since the lgm predicted using our ice earth model can be either smaller e g barbados or larger e g tahiti than the eustatic sea level change finally we review procedures that have been applied to estimate continental ice volume from far field sea level observations and apply these procedures to data from barbados and bonaparte gulf applying an ice earth model that is tuned to fit the barbados data we estimate a change in grounded ice volume from the lgm to the present of 43 5 51 106 km3 based on barbados data and an lgm ice volume estimate of 51 106 km3 based on bonaparte gulf data our results for the bonaparte gulf data are consistent with the recent study by yokoyama et al nature 406 2000 713 these lgm ice volume estimates map into a eustatic or meltwater sea level rise of 115 135 m taking into account plausible variations in the adopted radial earth model introduces uncertainties in the range of 1 5 106 km3 for ice volume estimates based on the barbados data

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

a group of rifting events in the terminal paleoproterozoic in the north china craton abstract rifting events of the terminal paleoproterozoic are well preserved in the north china craton they include an aulacogen and cratonic marginal rift basins mafic dyke swarms two anorthosite suites and rapakivi granites all of these events took place after metamorphism deformation and magmatism of the late paleoproterozoic between 2 0 ga and 1 85 ga or 1 80 ga the rifting events belong to an anorogenic event group reflecting breakup of a late paleoproterozoic united north china craton an intracratonic rift basin in the center of the north china craton is named the yanshan taihangshan aulacogen a stratigraphic thickness of 9193 m was deposited within the basin including three groups and twelve formations with time range ca 1 8 ga to 0 85 ga the oldest changcheng group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates terrestrial clastic rocks and shallow marine sandstone and carbonates with super high potassium basalt trachyte the marginal basins occur around the northern western and southern margins of the north china craton respectively located in langshan baiyun obo helanshan mountain and the adjacent regions between henan shanxi and shaanxi provinces the stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part with a thick volcanic sequence with u pb age of 1840 14 ma exposed in the southern marginal regions based on stratigraphic sequences isotopic data and volcanic features the initial rifts took place at about 1 85 ga to 1 80 ga mafic dyke swarms are largely developed in central parts of the north china craton where they intrude archean granulite and high amphibolite rocks a new single zircon u pb isotopic age of 1769 2 5 ma has been reported from a mafic dyke in east henshan mountain northwestern shanxi province an anorthosite suite agms is exposed along two deep fault belts in northern hebei province isochron ages of 1683 193 ma by rb sr and 1735 239 ma by sm nd were measured from an anorthosite body the rapakivi granites crop out along the miyun qiangzilu xinglong fault in an e w direction and are characterized by megacrysts of alkali feldspars mantled by plagioclase thus showing many of the petrographic features of classic rapakivi granites worldwide a typical rapakivi body preserved in shachang of miyun county beijing has a u pb age of 1716 31 ma super high potassium volcanics located in the upper changcheng group have been dated at 1683 67 ma and 1625 6 ma the time range for all magmatic events is between 1 72 ga and 1 62 ga

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

glacial isostatic adjustment on a rotating earth summary we extend and complete previous work to compute the influence of perturbations to the rotation vector on a suite of observables associated with glacial isostatic adjustment gia we emphasize observables relevant to present and future geodetic missions for example present day 3 d crustal motions relative sea level change and geoid or absolute sea level variations our calculations adopt spherically symmetric self gravitating maxwell viscoelastic earth models while incorporating realistic mass ice plus ocean load and rotation variations the predicted rotation induced signals are dominated by the influence of true polar wander tpw the spatial geometry of the tpw induced relative sea level geoid and radial velocity fields is primarily that of a degree two order one surface spherical harmonic the spatial variation of the horizontal velocity vectors is given by the gradient of this harmonic the peak radial and horizontal velocities are of the order of 0 5 mm yr 1 however we show that this value is sensitive to the adopted profile of mantle viscosity we also demonstrate that an accurate prediction of tpw induced sea level and 3 d crustal deformation rates requires that a realistic number of glacial cycles be incorporated into the ice load history we conclude that geodetic observations of the gia process should be analysed using a gia theory valid for a rotating planet finally we also consider variations in rotation driven by simple present day polar melting scenarios and predict the influence of these variations on a suite of geophysical observables we find that the rotational feedback associated with greenland melting is capable of significantly perturbing both relative and absolute sea level variations

on postglacial geoid subsidence over the equatorial oceans we develop two new spectral formalisms for the gravitationally self consistent solution of the sea level equation which governs the redistribution of glacial meltwater on a visco elastic earth the first is a purely spectral technique based on an extension of theory outlined by dahlen 1976 for determining the equilibrium oceanic tide on an elastic planet using this technique with a feasible spherical harmonic truncation level l 30 we can obtain convergence in the computed sea level variations within the degree range l 10 we also develop a second pseudospectral technique however which permits the construction of gravitationally self consistent solutions to much higher degree and order l 100 the pseudospectral formalism is employed here to develop a comprehensive physical explanation for the global pattern of present day sea level variations due to ongoing glacial isostatic adjustment in particular we focus upon a mechanism which we term equatorial ocean syphoning that acts to draw water toward the oceanic portion of the collapsing peripheral bulge that encircles previously glaciated regions the collapse of the forebulge induces a flow of water which is required to maintain hydrostatic equilibrium the syphoning mechanism dominates the relative sea level rsl variation in oceans in the far field of the ice sheets that is beyond the peripheral bulge during periods such as the interglacial of the past 4000 years during which the volume of the ice sheets has not apparently changed appreciably while at the same time isostatic adjustment persists the rsl change in the near field is in contrast dominated by the local vertical displacement of the solid surface finally comparison of gravitationally self consistent predictions of rsl change in the far field with the observational data at small pacific island sites has provided an upper bound of 1 to 2 m for the eustatic sea level rise produced by any recent last 3000 years melting of the antarctic ice sheet

corrigendum to global ice volumes at the last glacial maximum and early lateglacial earth planet sci lett 181 2000 513 527 in the above article an erroneous distinction was made between the ice volume equivalent sea level vje t de ned by eq 3 and eustatic sea level vjeus t in the absence of other factors contributing to sea level change thermal expansion melting of mountain glaciers not included in the ice models or changes in groundand surfacewater storage vje t corresponds to eustatic sea level eqs 4 and 5 state that the two are diierent but this is incorrect since the averages of the terms over the oceans must vanish in order to conserve mass as has been repeatedly stated in earlier papers by the authors the error was a result of a programming error introduced in the output of the sea level code whereby at the end of each iteration the mean sea level change is computed in our formulation sea level change is non zero over land so as to permit the calculation of change along narrow waterways the tilting of inland lakes and the changing elevations of inland barriers or sills of lakes thus the appropriate areal integral is over the ocean surface whereas the erroneous quantity calculated at this stage was the average over the entire globe because this was found to be diierent from that estimated from the ice volumes directly and because of the similarity in magnitude it was seen as an explanation for the distinction sometimes made between sea level equivalent and eustatic sea level e g denton and hughes 13 p 74 rather than as being incompatible with the condition of conservation of mass the integration of the sea level change over the oceans requires a knowledge of the location of the shoreline at each time step of the grounding line of the ice and of the part of the shelf ice that oats or grounds as a result of sea level change and shelf ice thickness change the mistake was introduced in 1998 modi cations to the code when this treatment of the shelf ice was improved fortunately this erroneous estimate of the mean sea level change at no stage feeds back into the main part of the sea level calculation because there the correct estimate based on the ice volume change is used thus the relative sea level predictions are unaiected and the relation between vje t and ice volume eq 3 below remains correct none of the conclusions about ice volume change are aiected in the paper it was noted that some inconsistency has crept into the usage of the term eustatic sea level to which the senior author has contributed unfortunately the senior author contributed further to this confusion by allowing himself to be convinced that the wrong argument to explain the diierence was correct hopefully that was a temporary lapse we thank w r peltier for drawing this lapse to our attention

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

yields of soviet underground nuclear explosions from seismic surface waves compliance with the threshold test ban treaty abstract magnitudes of the larger soviet underground nuclear weapons tests from the start of the threshold test ban treaty in 1976 through 1982 are determined for short and long period seismic waves yields are calculated from the surface wave magnitude for those explosions at the eastern kazakh test site that triggered a small to negligible component of tectonic stress and are used to calibrate body wave magnitude yield relationship that can be used to determine the sizes of other explosions at that test site the results confirm that a large bias related to differential attenuation of p waves exists between nevada and central asia the yields of the seven largest soviet explosions are nearly identical and are close to 150 kilotons the limit set by the threshold treaty

the model of fracture development in the faulted folds the role of folding and faulting abstract tectonic fractures are otherwise known as joints or fissures the density of tectonic fractures is governed by the lithology layer thickness and structures the distribution and density of tectonic fractures in the kuqa depression in the northern tarim basin of northwest china are dominantly controlled by the regional faulted fold belt our purpose of this paper is to analyze how folding and faulting control the development of tectonic fractures based on outcrop and logs of drill data in the folded tight sandstone reservoirs in the kuqa depression the extensional area transition area and shortening area have been separated by two neutral lines which control the distribution and density of tectonic fractures in the buckle folds from the twelve sites of field measurements six logs of drill core data and analysis of the micro texture of tectonic fractures it can be deduced that the fracture development in the sub salt faulted fold belt is related to the neutral lines and related faults the high density tensile fractures in the vicinity of fold hinges are more developed than in the limbs of the fold folding is thus the first level of control over the general pattern of fracture distribution the fractures in the hanging wall of the thrust faults are more developed than those in the footwall further more the fractures in the hinge zone and steeply tilted limbs of folds close to thrust faults are more developed than the gently tilted limbs which are far from faulting this suggests that faulting is a secondary control over the development of local fractures within the faulted fold belt the development of fractures related to faulted folds in the kuqa depression can be also divided into three episodes pre folding syn folding and post folding which are identified by field observation

paleostress reconstruction from kinematic indicators in the oslo graben southern norway new constraints on the mode of rifting abstract the oslo graben southern norway is a n s trending carboniferous permian rift system characterized by major mafic to silicic magmatism n s trending faults reactivation of preexisting precambrian faults and formation of half grabens magmatism is expressed by the presence of lavas dyke injections cauldron formation and the intrusion of batholiths paleostress analyses mainly based on slickensides have been performed in the area both within and outside the rift structure combined with the current tectono magmatic model of the oslo graben area the analyses show the following stress evolution from the caledonian orogeny with its compressional tectonics in the silurian to the late stages of the carboniferous permian rifting nw se compression occurred in silurian times due to the continental collision of the caledonian orogeny after a long period of a missing geological record the area was affected by n s compression during the late carboniferous a shallow sedimentary basin developed indicative for the pre and proto rift phases the transition from the proto rift phase to the initial rift phase is marked by a transition from a transpressional regime into a transtensional regime during the early permian the stress regime changed from pure extension to radial extension we suggest that the oslo graben can not simply be explained in terms of or passive or active rifting but that a combination of both evolving through time is more suitable to explain the observations we propose a model in which the oslo graben is initially triggered by far field stresses in the latest carboniferous and earliest permian which infers opening due to passive rifting these far field stresses are suggested to be linked to the hercynian orogeny active to the south in central europe in carboniferous times and the reorganisation of the pangea supercontinent in early permian times the stress regime caused radial extension indicative for uplift and a change to an active mode of rifting simultaneously with the radial extension large volumes of magma are emplaced at near surface levels despite all alternative models an anomalous thermal gradient is necessary to create these large volumes of magmatic material we propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active plume related although far field stresses might still be present

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

some effects of lateral heterogeneities in the upper mantle on postglacial land uplift close to continental margins summary we investigate the effects of lateral heterogeneities in the upper mantle on the calculation of postglacial land uplift for the model calculations we use a commercial finiteelement code which enables us to solve the equations governing a layered isotropic incompressible maxwell viscoelastic half space with laterally varying layer thicknesses and physical properties following previous investigations performed by sabadini yuen portney 1986 and gasperini sabadini 1989 we extend their results using a more realistic loading history and different earth models we then focus our attention on the question whether lateral heterogeneities in the upper mantle can be modelled correctly using a set of homogeneous earth models to this end a comparison of model calculations using both laterally homogeneous and heterogeneous earth models is performed we find that lateral heterogeneities in the upper mantle significantly influence the calculated postglacial land uplift the resolving power of relative sea level observations for the prescribed lateral heterogeneities used in this study is mainly focused on observations around the load margin and outside the glaciated areas where differences in predicted land uplift between individual models are large enough to be resolved by observations we can qualitatively determine lateral heterogeneities in the upper mantle using a set of laterally homogeneous earth models if the geological structure for example a continental margin is known however in order to infer the correct values of lithospheric thickness and asthenospheric viscosity we need to use laterally heterogeneous models

stratigraphic and sedimentary records of the rift to drift evolution of the northern north china craton at the paleo to mesoproterozoic transition article i nfo this paper presents a stratigraphic and sedimentary study of late paleoproterozoic to early mesoproterozoic successions of the northern margin of the north china craton including the changcheng and nankou groups a reconstruction of the tectono sedimentary history of these two groups demonstrates that the northern north china craton experienced a two stage evolution as recorded by the changcheng group early synrift subsidence and sedimentation were controlled by normal faults and accompanied by volcanism in contrast a late postrift stage was characterized by broad subsidence and widespread occurrence of shallow marine clastic and carbonate rocks a transgressive unconformity is identified and together with its time equivalent conformable contact clearly separates the synrift and postrift sequences the diachronous unconformity can be related to continental rifting and the separation of the north china craton from the adjacent india craton at roughly 1600 ma and is thus interpreted as a breakup unconformity this age of ca 1600 ma is constrained by newly reported precise shrimp u pb zircon ages of transgressive deposits in the overlying mesoproterozoic dahongyu and gaoyuzhuang formations just coincident with the paleoproterozoic to mesoproterozoic transition the 1600 ma time estimate for the tectonic departure of the north china craton from the india craton is comparable with the timing for the breakup of columbia a pre rodinia supercontinent containing almost all of the earth s continental blocks

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

sedimentary sequence and evolutionary history of the proterozoic basin at the southern margin of the north china craton abstract aulacogen rift system deposits and tectonic magmatic activities can be linked to the breakup of the columbia supercontinent and have developed in the north china craton ncc the xiong er basin at the southern margin of the ncc consists of late palaeoproterozoic neoproterozoic sedimentary strata and can be divided into three sedimentary sequences sedimentary sequence i 1 83 1 75 ga consists of bimodal volcanic sedimentary rocks of the xiong er group with continental marine facies and alluvial fan deposits of the bingmagou formation with local development it represents the evolution of the intracontinental fault from initiation to decline sedimentary sequence ii 1 75 1 6 ga developed after the southern margin of the ncc entered a stable continental margin stage is dominated by clastic rocks interbedded with local carbonate deposits and fills the interior of the basin as continuous transgression led to a transition in the sedimentary environment sedimentary sequence iii 1 6 1 4 ga is dominated by carbonate strata that began to develop afterwards this sequence is the caprock deposit in the xiong er basin subsequently tectonic uplift events occurred at the southern margin of the ncc along with the start of exposure and denudation which lasted for hundreds of millions of years moreover abundant microbially induced sedimentary structures miss with different morphological characteristics developed in clastic strata across different areas of the xiong er basin these structures effectively indicate sedimentary facies and their evolution which facilitates the establishment of regional stratigraphic isochronous correlation and connection for example based on the time limit for the development of molar tooth structures mts the hejiazhai formation belongs to the neoproterozoic strata with a deposition time of 1 0 0 72 ga in conclusion the study of the precambrian sedimentary succession in the xiong er basin in response to the supercontinental breakup can be useful in exploring the coupling mechanisms between various geological events

mantle viscosity and ice age ice sheet topography ice age paleotopography and mantle viscosity can both be inferred from observations of earth s response to the most recent deglaciation event of the current ice age this procedure requires iterative application of a theoretical model of the global process of glacial isostatic adjustment results demonstrate that the iterative inversion procedure converges to a paleotopography that is extremely close to that from the ice 4g model the accompanying mantle viscosity profile is furthermore shown to reconcile the requirements of aspherical geoid anomalies related to the mantle convection process thus resolving a fundamental issue concerning mantle rheology the combined model also explains postglacial sea level histories for the east coast of the united states

reconstruction and interpretation of giant mafic dyke swarms a case study of 1 78 ga magmatism in the north china craton

geochronology geochemistry and sr nd hf isotopes of mafic dikes in the huicheng basin constraints on intracontinental extension of the qinling orogen abstract the diabase dikes intruded the lower cretaceous sandstone in the huicheng basin are key to understanding the mesozoic tectonic evolution of the qinling orogen la icp ms zircon u pb dating yields a crystallization age of 107 1 ma for them the dikes have low contents of sio2 42 46 50 16 wt mgo 3 47 5 59 wt with low mg of 49 59 and tio2 1 35 1 63 wt high tfe2o3 8 15 9 36 wt al2o3 14 75 17 23 wt and k2o 0 87 3 61 wt their ni and cr contents are in range of 16 8 111 and 45 7 315 ppm respectively they are significantly enriched in light rare earth elements and large ion lithophile elements e g cs pb and ba and depleted in rb k p and ti the dikes show relatively high whole rock initial 87sr 86sr ratios 0 7071 0 7079 and negative end t values 1 5 to 4 8 with single stage nd model ages of 941 1186 ma in addition they have low zircon ehf t values 8 6 to 3 5 with single stage hf model ages of 674 1117 ma both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution they were derived from partial melting of an enriched lithospheric mantle source together with regional geological data these results suggest that the qinling orogen experienced an intracontinental extension during the late early cretaceous

the effect of spatially non uniform water loads on prediction of sea level change summary numerical modelling of postglacial rebound predics a spatially variable pattern of sea level change yet many previous models predicting late pleistocene sea level change either neglect the load of water added to the oceans or assume a spatially uniform time dependent load over the whole ocean this is a poor approximation of the load near the former ice sheet since sea level change varies geographically in addition the coastline goemetry changes through time a spectral technique has been used to solve the sea level equation on an earth model with axisymmetric distribution of ice and oceans and also for a realistic ice ocean configuration the axisymmetric model is useful for demonstrating the fundamental physics involved in sea level changes caused by changes in surface loads in particular it can be seen that the elastic and gravitational response to the load is spread over a great distance whereas the viscous response is more localized a realistic ice ocean configuration has been used to demonstrate that neglect of the melt water load underestimates the amount of uplift in fennoscandia and other regions where rebound occurs in the ocean as previously noted by wu peltier 1983 in hudson bay and that movement of the coastlines and spatially varying water loads must both be included in calculations for the north sea and other near field regions with shallow seas sea level calculations including these effects and careful analysis of the sea level and ice melting record are required to determine modifications to the viscosity profile and ice model inferred by previous inaccurate models of the ocean load in the near field

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

stress orientations during oligocene volcanism in trans pecos texas timing the transition from laramide compression to basin and range tension orientations of dikes and hydrothermal veins indicate that east northeast compression occurred during the main pulse of volcanism in west texas 39 to 32 m y ago direction of compression was essentially the same as that during eocene laramide deformation the observations support a continental arc tectonic setting the beginning of extension between 32 and 30 m y ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in texas and to bimodal but dominantly rhyolitic volcanism in northern mexico

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

the influence of time dependent ocean continent geometry on predictions of post glacial sea level change in australia and new zealand we describe predictions of post glacial relative sea level rsl variations in australia and new zealand that incorporate a realistic evolution of the ocean continent margin and compare these to predictions which assume that the ocean continent interface is fixed to the present day coastline geometry we find that the predicted amplitude of holocene sea level highstands in this region can be increased by more than 2 m when a time dependent ocean continent margin is incorporated into the modeling procedure future attempts to refine both mantle viscosity and late holocene melting events based on the prediction of a set of these highstands should adopt a theory that includes this extension

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

palaeoproterozoic assembly of the north china craton the basement of the north china craton consists of the eastern and western blocks separated by the central zone both the eastern and western blocks are dominated by late archaean tonalitic trondhjemitic granodioritic gneiss complexes interdigitated with minor supracrustal rocks metamorphosed at 2 5 ga with anticlockwise p t paths the central zone is composed of reworked late archaean components and palaeoproterozoic juvenile crustal materials that underwent regional metamorphism at 1 85 ga with clockwise p t paths involving isothermal decompression as a result of collision between the eastern and western blocks which resulted in the final assembly of the north china craton

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

the tectosphere and postglacial rebound large lateral variations in the thickness of the lithosphere and viscosity of the mantle may be associated with continental roots the effects of continental roots or the tectosphere on postglacial rebound are calculated using an axisymmetric flow model in which both the mantle and lithosphere are described as newtonian fluids continental roots are shown to have a measurable effect on postglacial rebound in particular on the evolution of the peripheral bulge effects of continental roots are concentrated near the edge of the tectosphere and thus often near ocean continent boundaries where many relative sea level histories are measured the pattern and distribution of tectonic and seismic activity associated with deglaciation are affected by a tectosphere

effects of rock mechanical properties on local stress field of the mahanadi basin india results from finite element modelling 1 two dimensional 2 d finite element models fem are generated using two available seismic sections across the mahanadi basin mb at the east coast of india this permits to study the interactive effect between applied stress in the direction of maximum horizontal compressive stress sh and variations in rock mechanical properties of several litho units present in the basin the sh orientation for six offshore wells in the mb known from our previous studies chatterjee and mukhopadhyay 2000 have been used as constraints for fem local stress modifications due to contrast in the young s modulus between sedimentary in fill and the crust down to 20 km depth are investigated for two more orthogonal seismic profiles traversing the basin from onland to offshore fem derived stress vectors clearly respond to variations in elastic properties of rocks corresponding to lithological interfaces in the basin and its underlying crust rotation of the principal stress is prominent where the stress concentration is high further the stress concentration reflects the basinal configuration

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

natural fractures in carbonate rich tight oil reservoirs from the permian lucaogou formation southern junggar basin nw china insights from fluid inclusion microthermometry and isotopic geochemistry abstract natural fractures within tight reservoirs form preferred pathways for hydrocarbon charge and provide space for hydrocarbon preservation however the mechanism that controls the formation of natural fractures in tight reservoirs remains controversial this study investigates natural fractures within carbonate rich tight oil reservoirs from the permian lucaogou formation in the jimsar sag southern junggar basin drill core and image log observations show that fractures in the lucaogou formation can be categorized into two types bedding parallel fractures and inclined fractures the carbon and oxygen isotopic compositions of calcite cements in the bedding parallel fractures range from 0 6 to 6 0 13cpdb and from 15 5 to 8 9 18opdb respectively which are similar to those of the organic rich host rock this suggests that the bedding parallel fractures likely formed in a closed fluid system fluid inclusion analyses of the aqueous inclusions in calcite cements from the bedding parallel fractures record homogenization temperatures of 91 5 c 130 4 c indicating that they formed at the cretaceous to cenozoic during when the lucaogou formation reached peak oil generation stage therefore the bedding parallel fractures were most likely generated by hydrocarbon generation overpressure the presence of hydrocarbon inclusions in bedding parallel fracture cements supports natural fracturing by high pore fluid pressure the 13cpdb and 18opdb of calcite cements within the inclined fractures vary from 3 6 to 9 6 and from 11 0 to 2 9 respectively which are close to those of the carbonate rich parent rock this also supports a closed fluid system scenario the relatively lower fluid inclusion trapping temperatures 70 5 c 109 7 c suggest that they were likely generated by decrease of confining stress during uplift processes therefore both an increase in pore fluid pressure and a decrease in confining stress contribute to the formation of natural fractures in the lucaogou formation tight oil reservoirs

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

late devensian and holocene relative sea level changes in northwestern scotland new data to test existing models abstract pollen diatom lithostratigraphic and radiocarbon data from five sites in northwestern scotland provide new data from an area previously devoid of reliable and precise information on late devensian and holocene sea level changes the sites cover a range of palaeoenvironments indicative of diversity in coastal evolution since deglaciation for each site and palaeoenvironment the reference water tide level indicative range age and tendency of sea level movement of all sea level index points are quantified to enable correlation of the diverse coastal environments the data record patterns of relative sea level change and tendencies of sea level movement from 12 ka bp to 1 ka bp this is the longest and most comprehensive published record of relative sea level change from the area the information is used to test the accuracy of existing models of relative sea level change the results are only broadly consistent with a quantitative rebound model and there is significant disagreement with empirical models during the late devensian and the early holocene

glacial rebound of the british isles ii a high resolution high precision model summary observations of ice movements across the british isles and of sea level changes around the shorelines during late devensian time after about 25 000 yr bp have been used to establish a high spatial and temporal resolution model for the rebound of great britain and associated sea level change the sea level observations include sites within the margins of the former ice sheet as well as observations outside the glaciated regions such that it has been possible to separate unknown earth model parameters from some ice sheet model parameters in the inversion of the glaciohydro isostatic equations the mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km the transition zone down to 670 km depth and the lower mantle no evidence is found to support a strong layering in viscosity above 670 km other than the high viscosity lithospheric layer models with a low viscosity zone in the upper mantle or models with a marked higher viscosity in the transition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary in contrast a marked increase in viscosity is required across this latter boundary the optimum effective parameters for the mantle beneath great britain are a lithospheric thickness of about 65 km a mantle viscosity above 670 km of about 4 5 lo2 pa s and a viscosity below 670 km greater than 4 x lo2 pas

quantitative prediction of lower order faults based on the finite element method a case study of the m35 fault block in the western hanliu fault zone in the gaoyou sag east china

the relation between differential stress driving pressure ratio tensile strength and reservoir failure in a magmatic system towards eruption forecasting abstract in this study a relation is established between differential stress d driving magma pressure ratio r and tensile strength t of a rock mass in a volcanic system it is shown that in d vs r space the t curve follows equation of rectangular hyperbola and for dyking or volcanic eruption to occur this t curve must be exceeded it is proposed that this theoretical relation can be exploited to understand magma reservoir failure and for volcanic eruption forecasting applicability of this concept is demonstrated on santorini volcanic system greece for which considerable background information is available from previous studies published dyke orientation and aspect ratio data from santorini yield d and r values of 8 04 mpa and 0 22 respectively considering a weak quality host rock mass in santorini tensile strength curve for t 1 5 mpa is plotted in d vs r space considering magma reservoir volume and rock physical property estimates to be valid and assuming d and absolute stress to be constant it is argued that change in reservoir dynamics would be controlled by variations in volume of magma released from the deeper magma reservoir to shallow reservoir vr in santorini values of r and vr for the events of non dyking in santorini are also calculated minimum r and vr required to induce dyking and eruption are estimated and it is concluded that the proposed theory can be generally applied for eruption forecasting

polar wandering and the forced responses of a rotating multilayered viscoelastic planet changes in the earth s rotation produced by the waxing and waning of large mobile ice sheets can be used to infer the viscosity of the mantle from a comparison with the recent secular polar drift as revealed in the international latitude service data and also in the estimates of the nontidal acceleration of the length of the day we have developed by using an analytical approach a physical model of a rotating earth consisting of an elastic lithosphere a viscoelastic mantle and an inviscid core forcings from the two major ice sheets laurentide and fennoscandia have been considered sensitivity analysis of the parameters governing the external forcings such as the length and the time of termination of the deglaciation phase show in general that there exist two families of mean mantle viscosities 0 1022 p and 0 1023 p which can match the data satisfactorily however we find it an improvement to incorporate some amount of shrinkage from the antarctic ice sheet obtaining simultaneously a better fit with the two rotational data sets to the lower viscosity solutions hence forcings from the southern hemisphere may be able to remove the ambiguity associated with the extraction of mantle viscosity from rotational data we have also solved the initial value problem of the components of instantaneous angular velocity by means of the laplace transform for applied loads characterizing the continental ice sheets which since the mid pliocene have periodically expanded and contracted over the northern hemisphere in consequence of this continual forcing polar wander of around 5 10 degrees can take place for a mean mantle viscosity 0 1022 p recent reanalysis of paleomagnetic data in conjunction with hot spot tracks have revealed that several degrees of true polar wander could have occurred in the last 5 m y these calculations thus provide a physical explanation for the observed movement of the rotation axis membrane stresses of 0 10 bars can develop from this secular polar motion

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

the sensitivity of glacial isostatic adjustment predictions to a low viscosity layer at the base of the upper mantle abstract recent inferences of mantle viscosity based on convection related observables long wavelength nonhydrostatic geoid and free air gravity harmonics indicate the presence of a relatively thin low viscosity region at the base of the upper mantle we explore the sensitivity of observables associated with glacial isostatic adjustment gia to the viscosity and thickness of this region these observables include post glacial relative sea level rsl variations within previously glaciated areas and in the far field of the late pleistocene ice sheets as well as anomalies associated with the rotational state of the planet the latter include present day secular variations in the geopotential harmonic j 2 or equivalently variations in the rotation rate and polar wander speed we find in contrast to previous suggestions that the gia observables with the exception of polar wander speed are sensitive to fine scale low viscosity structure at the base of the upper mantle for example j 2 predictions can be altered by as much as 2 10 11 yr 1 by the inclusion of a low viscosity zone this signal is sufficient to significantly affect both inferences of mantle viscosity and constraints on the present day melting of large polar ice sheets antarctic greenland that are derived using this observable indeed if a low viscosity region exists then models that do not include it can overestimate recent polar melting events by as much as 0 5 mm yr of equivalent eustatic sea level rise