the neogene to recent rallier du baty nested ring complex kerguelen archipelago taaf indian ocean stratigraphy revisited implications for cauldron subsidence mechanisms 1 ups cnrs fre 2566 orsayterre department des sciences de la terre universite de paris sud f 91405 orsay cedex france bbonin geol u psud fr 2 cnrs umr 6524 magmas et volcans department de geologie petrologie geochimie universite jean monnet f 42023 saint etienne cedex 2 france 3 cnrs ep 125 department des sciences de la terre universite de nice sophia antipolis f 06108 nice cedex 2 france 4 department of geology university college dublin belfield dublin 4 eire 5 gemoc macquarie university sydney australia

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

magnetization of 0 26 5 ma seafloor at the ultraslow spreading southwest indian ridge 61 67 e 1 we investigate the magnetic signature of volcanic and nonvolcanic seafloor areas along the southwest indian ridge between 61 e and 67 e and analyze their relationship with crustal thickness variations and past to present ridge segmentation this part of the southwest indian ridge is an end member for the global ridge system in terms of low melt supply thin crust and ultraslow spreading rates it is characterized by large expanses of seafloor that show no evidence for a volcanic upper crustal layer we find that variations of intrinsic magnetization and thickness of the basaltic extrusive layer where it is present dominate the present day along axis crustal magnetization off axis the magnetization contrast is on average higher for volcanic seafloor than for smooth nonvolcanic topography indicating that the contribution of the basaltic upper crustal layer to the production of magnetic anomalies remains important however magnetic anomalies that record past magnetic polarity events are found almost everywhere in the survey area even over domains that lack a volcanic upper crustal layer arguing thus for the contribution of other sources we propose that both gabbros and serpentinized peridotites contribute to these anomalies although not systematic and weak over most parts of the survey area an induced component of magnetization is clearly present in some nonvolcanic seafloor domains serpentinized peridotites are the likely carriers of this induced magnetization component

structure and early history of the labuan basin south indian ocean recent multichannel seismic reflection data from the labuan basin in the southern indian ocean are used to reevaluate older single channel data from this region together they throw light on the structure and evolution of this basin situated between the older than 100 ma southern kerguelen plateau and the younger than 43 ma australian antarctic basin the labuan basin is a deep extensive basement depression more than 350 000 km2 in area located adjacent to the eastern margin of the southern kerguelen plateau in contrast to the nearby kerguelen plateau no prominent reflectors are observed within the basement of the labuan basin this suggests a different mechanism of formation from that of the kerguelen plateau which is believed to have been formed by thick sequences of large lava flows the basement surface of most of the labuan basin is presently quite rough as the result of a tectonic event which created prominent tilted block structures and turned it into a large northwest southeast trending syncline there is a 1 1 5 km elevation difference between the labuan basin and the australian antarctic basin created at the southeast indian ridge as well as a large difference in sedimentary thickness between them indicating that the labuan basin is significantly older the boundary of the labuan basin with the kerguelen plateau is generally a steep and somewhat linear feature which appears to be of tectonic origin it seems to result from two extensive tectonic episodes dated at 96 ma and 75 68 ma associated with the prerift phase of plate breakup between australia and antarctica the basement of the labuan basin was created 130 100 ma ago at about the same time than the kerguelen plateau an extensive tectonic episode to the south of the labuan basin initiated the formation of the boundary between the southern kerguelen plateau and the labuan basin this tectonic episode could be linked to the beginning of seafloor spreading between australia and antarctica 96 ma ago sediments were regularly deposited in the basin until 75 68 ma at which time an important extensional tectonic event occurred it involved two large northwest southeast uplifts one centered on the kerguelen plateau affected the western part of the labuan basin while the other of which only the western half is observed affected the eastern part of the labuan basin this tectonic episode seems to correspond to the prerifting episode leading to the breakup between the kerguelen plateau labuan basin and broken ridge diamantina zone at 43 ma after this tectonic episode the sediments of the postuplift megasequence filled topographic lows formed by the normal faults and the tilted blocks the diamantina zone which prior to 43 ma was continuous with the labuan basin can be expected to have a similar tectonic history in particular the rough basement of the diamantina zone can be due to postformation tectonism rather than to slow spreading as previously suggested

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

the crustal role of the agulhas plateau southwest indian ocean evidence from seismic profiling summary its key geographical position near the reconstructed centre of the gondwana break up between antarctica south america and africa has brought attention to the agulhas plateau an oceanic plateau in the southwest indian ocean with regard to its crustal nature and origin the majority of previous studies have suggested a dominantly continental origin as part of the project setarap sedimentation and tectonics of the agulhas ridge and agulhas plateau we conducted an extensive seismic survey over the plateau with the aim of solving the questions about its crustal structure origin and role in a plate tectonic reconstruction context in addition to 1550 km of high resolution seismic reflection profiles we recorded deep crustal large offset and wide angle reflection refraction data from an ocean bottom hydrophone obh profile across the southern plateau the reflection data show clear indications of numerous volcanic extrusion centres with a random distribution we are able to date this phase of voluminous volcanism to late cretaceous time a period when numerous other large igneous provinces formed traveltime inversion of the deep crustal obh records reveals an up to 25 km thick crust with velocities between 7 0 and 7 6 km s 1 for the lower 50 70 per cent of its crustal column we do not find indications for continental affinity but rather a predominantly oceanic origin of the agulhas plateau similar to that inferred for the northern kerguelen and ontong java plateaus in late cretaceous time its main crustal growth was controlled by the proximity of spreading centres and by passage over the bouvet hotspot at 80 100 ma

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

kerguelen plateau a volcanic passive margin fragment the 21 25 km thick crust of the southern kerguelen plateau consists of three units 1 a 2 3 km thick sedimentary cover 2 a 3 6 km thick basaltic layer with velocities ranging from 4 5 to 6 2 km s and 3 a 15 17 km thick lower crust with velocities from 6 6 to 6 9 km s including a 3 6 km thick transition zone located at the base of the crust the low velocity transition zone has an average velocity of 6 7 km s and exhibits several internal wide angle reflections the velocity depth structure of the crust differs significantly from that of other hotspot related oceanic plateaus and suggests that the southern kerguelen plateau may be a fragment of a volcanic passive margin composed of a thinned continental crust overlain by basalt flows

terrace styles and timing of terrace formation in the weser and leine valleys northern germany response of a fluvial system to climate change and glaciation abstract in glaciated continental basins accommodation space is not only controlled by tectonics and sea level but also by the position of ice sheets which may act as a regional base level for fluvial systems although the pleistocene terrace record of major river systems in northwestern europe has been investigated by many authors relatively little attention has been paid to base level changes related to glacier advance retreat cycles and how these regional changes in base level interacted with river catchment processes this study provides a synthesis of the stratigraphic architecture of middle pleistocene to holocene fluvial terraces in the upper weser and middle leine valley in northern germany and links it to glaciation climate and base level change the depositional architecture of the fluvial terrace deposits has been reconstructed from outcrops and high resolution shear wave seismic profiles the chronology is based on luminescence ages 230 th u ages 14 c ages and middle palaeolithic archaeological assemblages the drainage system of the study area developed during the early miocene during the pleistocene up to 170 m of fluvial incision took place a major change in terrace style from strath terraces to cut and fill terraces occurred during the early middle pleistocene before marine isotope stage mis 12 which may correlate with climate deterioration and the onset of glaciation in northern central europe during this time a stable buffer zone was established within which channels avulsed and cut and filled freely without leaving these vertical confines climate was the dominant driver for river incision and aggradation whereas the terrace style was controlled by base level changes during ice sheet growth and decay a major effect of glacio isostatic processes was the post elsterian re direction of the river weser and river leine the middle pleistocene fluvial terraces are vertically stacked indicating a high aggradation to degradation ratio corresponding with a regional base level rise during glacier advance at the beginning of the late pleistocene the terrace style changed from a vertical to a lateral stacking pattern which is attributed to a decrease in accommodation space during glacier retreat the formation of laterally attached terraces persisted into the holocene major incision phases took place during mis 5e 5d 5c and probably early mis 4 early mis 3 and mis 2 lateglacial during mis 5e and the lateglacial the braided river systems changed into meandering rivers indicated by preserved organic rich flood plain and point bar deposits the late pleistocene braided river systems mis 5c to mis 3 are characterized by a high sinuosity which may be a direct effect of an increased downstream gradient after deglaciation when the channel lengthened and the river adjusted to the increased gradient by increasing sinuosity these middle pleniglacial fluvial deposits are unconformably overlain by lateglacial to holocene meandering river deposits which form laterally attached terraces recording millennial scale channel shifts the lack of late pleniglacial deposits might be related to late weichselian forebulge formation

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

on the fit of broken ridge and kerguelen plateau abstract we present revised finite rotation parameters for australia and east antarctica in the early tertiary incorporating new magnetic data off of wilkes land antarctica reconstructions based on these and recently published rotation parameters document a relatively stationary position of broken ridge abutted against central kerguelen plateau between chron 34 83 ma and chron 20 43 ma prior to the commencement of seafloor spreading between the two late cretaceous features these reconstructions are significant as they are the first which are consistent with geological and geophysical constraints in not producing an overlap of broken ridge and kerguelen plateau between chron 34 and chron 24 53 ma and not requiring significant extension or strike slip motion within kerguelen plateau we also find that the finite rotations for chrons 20 to 34 outline the area of rough topography inferred from the satellite derived free air gravity for the diamantina zone and labuan basin a consequence of these rotations is that there is considerable overlap of continental crust in the region of tasmania the south tasman rise and east antarctica in the late cretaceous solutions to this problem may imply either post santonian 79 ma extension or transform motion in the bass strait or late cretaceous or cenozoic extension in wilkes land the reconstructions also agree with prior estimates of roughly 150 km of motion between east and west antarctica since chron 20

equatorial paleosecular variation of the geomagnetic field from 0 to 3 ma lavas from the galapagos islands abstract complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites lava flows from the galapagos islands of santa cruz san cristobal and floreana provide data for the statistical characterization of the time averaged geomagnetic field near the equator for the past few million years estimates of vgp dispersion due to paleosecular variation range from 9 2 to 11 8 depending on site selection criteria our preferred estimate based on 64 site vgps 51 accepted from this study and 13 from the 1971 study by cox is 11 4 95 confidence interval 10 2 13 0 consistent with previous estimates from the galapagos islands as well as paleosecular variation model g and confirming that angular dispersion of vgps near the equator is relatively low the mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the galapagos hotspot preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 t on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time averaged field was about 21 t or only two thirds the present strength in agreement with some other recent estimates

constraining long term denudation and faulting history in intraplate regions by multisystem thermochronology an example of the sudetic marginal fault bohemian massif central europe 1 the rychlebske hory mountain region in the sudetes ne bohemian massif provides a natural laboratory for studies of postorogenic landscape evolution this work reveals both the exhumation history of the region and the paleoactivity along the sudetic marginal fault smf using zircon u th he zhe apatite fission track aft and apatite u th he ahe dating of crystalline basement and postorogenic sedimentary samples most significantly and in direct contradiction of traditional paleogeographic reconstructions this work has found evidence of a large cretaceous sea and regional burial to 6 5 km of the carboniferous permian basement in the late cretaceous 95 80 ma during the burial by sediments of the bohemian cretaceous basin system the smf acted as a normal fault as documented by offset zhe ages across the fault at 85 70 ma the basin was inverted cretaceous strata eroded and basement blocks were exhumed to the near surface at a rate of 300 m ma as evidenced by late cretaceous paleocene aft ages and thermal modeling results there is no appreciable difference in aft and ahe ages across the fault suggesting that the smf acted as a reverse fault during exhumation in the late eocene oligocene the basement was locally heated to 70 c by magmatic activity related to opening of the eger rift system neogene or younger thermal activity was not recorded in the thermochronological data confirming that late cenozoic uplift and erosion of the basement blocks was limited to less than 1 5 km in the study area

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

the geomagnetic field over the past 5 million years summary the modern geomagnetic field is usually expressed as a spherical harmonic expansion although the palaeomagnetic record is very incomplete in both space and time sufficient data are available from a span of ages to generate time averaged spherical harmonic field models with many degrees of freedom here three data sets are considered directional measurements from lavas inclination measurements from ocean sediments and intensity measurements from lavas individual data are analysed as well as siteaverages using the same methods that have been developed for the modern field to give models for the past 5 myr the normal polarity field model has an axial dipole intensity similar to that of the modern day field whilst the equatorial dipole component is very much smaller the field is not axisymmetric but shows flux concentrations at the core s surface under canada and siberia similar to those observed in the field over historical timescales tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples the reverse polarity field model does not show such obvious features but this may be due to the sparsity of the data the patterns observed in the normal polarity field with persistent features in the northern hemisphere and a smooth southern hemisphere could be explained if the present pattern of secular variation is typical of the past several million years this would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation with relatively little change over quieter regions however we have been unable to find any evidence for a geographical pattern of secular variation in the data

do river profiles record along stream variations of low uplift rate environments 1m m yr 1 where climate changes may have erased tectonic features we evaluate the respective contribution of low uplift rate 0 1 mm yr 1 and pleistocene climate oscillations on gradient variations of two comparable river profiles crossing different uplift zones in the southern upper rhine graben we compare the observed points of discontinuity in river profile knickpoints and convex portions knickzones with those predicted by a detachment limited model that includes stochastic short term and cyclic long term variations in climate a bedrock detachment threshold and rock uplift the detachment limited model is chosen as it predicts the development of persistent knickpoints differing values of the shear stress exponent erosion threshold climate variability and uplift pattern have been checked our modeling suggests that climate changes had no significant effects on profiles and that anomalies are more likely due to anticline growth this surprising result arises from the combination of a very low regional uplift rate and the detachment limited assumption the detachment limited model implies an upstream propagation of knickpoints and knickzones generated by uplift at the outlet during dry climate periods of low erosion the greater the uplift rate the larger the variations in river bed elevation thus for high uplift rate knickpoints and knickzones generated by climate oscillations are more likely to hide tectonic features this result seems counterintuitive because it suggests that tectonic knickzones will be better preserved in low uplift rate environments provided that the lithology is homogeneous

the rare holsteinian mazovian interglacial limnic deposits in the ksiąznica outcrop at krzczonow near świdnica sudetic foreland abstract the article presents the results of research on peats muds and sands filling a palaeocavity within alluvial fans of the bystrzyca river valley in the sudetic foreland of sw poland in the 534 cm deep research profile in the central part of a palaeolake 267 samples were collected at an interval of 2 cm and lithologically structural and textural studies palaeobotanically studies of diatoms pollen plant macrofossils and fossil wood palaeozoologically cladocera mollusca and ostracoda geochemically chemical elements and isotopes and palaeomagnetically analyzed the profile consists of three parts organic with mineral layers lower mineral middle and organic upper the palynological diagram was divided into three zones that represent local sets of pollen zones this diagram although incomplete is clearly indicative of the younger part of the holsteinian mazovian pollen iii and iv periods interglacial the upper part of the diagram consists of four local pollen zones that are correlated with stadial and interstadial periods analysis of several samples of fossil wood shows the widespread presence of pinus remains and numerous fragments of rhytidome bark of betula development of the peat layer 1 36 0 68 m depth started with paludification of the forest with the betula forest analysis of cladocera shows the presence of chitin armor components species chydorus sphaericus alona rectangula and ephippia of alona rectangula in three samples lithological cladocera and palaeo magnetic research supported by geochemical studies indicate the existence of at least three main phases the lake originated during the younger part of the holsteinian mazovian intergalcial as an oxbow within the valley where the deposition of variously thick up to 1 5 m depth peats organic and minerogenic silts the first stage in the second phase a closed fairly deep late holsteinian lake continued to exist under relatively warm conditions with mainly mineral deposition from the slope the lake became shallower and wider and then more open and better oxygenated with through flow in the final phase the lake was completely filled by organic sediments under interglacial conditions the absence of diatom molluscan and ostracod communities indicates an acid lacustrine environment at this time the fluvial and limnic deposits are sealed by overlaying saalian odranian till

paleomagnetic directions from mid latitude sites in the southern hemisphere argentina contribution to time averaged field models back arc volcanism located to the east of the andean cordillera was sampled in the argentina provinces of mendoza and neuquen for paleomagnetic time average field and paleosecular investigations the activity ranges from 2 ma to very recent time with a large variety of products from basalts to highly differentiated lavas after removal of sites affected by lightning those with 95 higher than 10 and combining of nearby sites displaying close directions we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs the payun matru and the cerro nevado previous and new k ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka and from 1 9 to 0 9 ma respectively most paleomagnetic samples have nrm intensities between about 1 and 20 a m and depict progressive removal of magnetization components in a consistent fashion during stepwise af or thermal demagnetization nineteen flows yielded a normal direction declination 354 8 inclination 53 0 95 6 8 and 12 flows a reverse direction declination 181 0 inclination 52 3 95 5 9 the combined data yielded a mean direction declination 357 3 inclination 52 8 95 4 6 which is not statistically different from the axial dipole field g10 expected at this latitude 36 s the angular dispersion of virtual geomagnetic poles calculated from flows with normal directions asd 16 5 compares well with the observed value from global datasets for this site latitude but flows with reverse directions display a surprisingly low dispersion asd 12 5 since most reverse directions were sampled from flows ranging between 1 9 and 0 9 ma this can be interpreted as an interval of low paleomagnetic secular variation additional data also with accurate time constraints are obviously needed to better support this observation finally no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 myr

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

paleomagnetism and the nature of the geodynamo records of direct observations of the earth s magnetic field cover less than a ten millionth ofthe known lifetime of the field thus our knowledge of several geomagnetic phenomena critical to our understanding of the geodynamo must come firom the paleomagnetic record a combination of substantial advances during the past decade or so both in dynamo theory previously the domain of the mathematician and in paleomagnetism previously the domain of the geologist has led to provocative models of the earth s magnetic field and a better understanding of the geodynamo

palaeosecular variation in java and bawean islands indonesia during the brunhes chron summary a palaeomagnetic study has been conducted from indonesian lava flows and dykes sampling was concentrated to the merapi and merbabu volcanoes central java the bromo tengger volcanic complex the lurus volcano both east of java and the bawean island numerical dating indicates that the rock units sampled have ages that are well distributed within the brunhes chron only a few reverse polarity flows were probably emplaced during the late matuyama chron alternating field af and thermal demagnetization treatments were routinely used for all sites results from both methods were generally in good agreement demonstrating the reliability and the reproducibility of the palaeomagnetic determinations obtained from 36 independent sites the large dispersion of the directions and the age distribution indicate that a large spectrum of the secular variation has been sampled our best estimate of the mean brunhes field direction in java is characterized by a zero declination and an inclination anomaly of 4 7 with a 95 per cent confidence angle of 5 3 this value agrees with an axial dipolar field model or a model to which a persisting zonal quadrupolar term of a few per cent of the geocentric axial dipole is added this study adds new constraints to recent results which suggested that a large zone of negative inclination anomalies may have persisted in the southwestern pacific ocean over the past 500 kyr and possibly extended toward the eastern indian ocean the present results do not support such an extension they also suggest that the inclination anomaly may have varied over 100 kyr and that it was more pronounced in recent times

channeling at the base of the lithosphere during the lateral flow of plume material beneath flow line hot spots chains of volcanic edifices lie along flow lines between plume fed hot spots and the thin lithosphere at ridge axes discovery and euterpe musicians seamounts are two examples an attractive hypothesis is that buoyant plume material flows along the base of the lithosphere perpendicular to isochrons the plume material may conceivably flow in a broad front or flow within channels convectively eroded into the base to the lithosphere a necessary but not sufficient condition for convective channeling is that the expected stagnant lid heat flow for the maximum temperature of the plume material is comparable to the half space surface heat flow of the oceanic lithosphere two dimensional and three dimensional numerical calculations confirm this inference a second criterion for significant convective erosion is that it needs to occur before the plume material thins by lateral spreading scaling relationships indicate spreading and convection are closely related mathematically the nusselt number ratio of convective to conductive heat flow in the plume material scales with the flux volume per time per length of flow front of the plume material a blob of unconfined plume material thus spreads before the lithosphere thins much and evolves to a slowly spreading and slowly convecting warm region in equilibrium with conduction into the base of the overlying lithosphere three dimensional calculations illustrate this long lasting and hence observable state of plume material away from its plume source a different flow domain occurs around a stationary hot plume that continuously supplies hot material the plume convectively erodes the overlying lithosphere trapping the plume material near its orifice the region of lithosphere underlain by plume material grows toward the ridge axis and laterally by convective thinning of the lithosphere at its edges the hottest plume material channels along flow lines geologically the regions of lithosphere underlain by either warm or hot plume material are likely to extend laterally away from the volcanic edifices whether or not channeling occurs

revised and updated paleomagnetic results from costa rica 1 paleomagnetic results from globally distributed lava flows have been collected and analyzed under the time averaged field initiative tafi a multi institutional collaboration started in 1996 and designed to improve the geographic and temporal coverage of the 0 5 ma paleomagnetic database for studying both the time averaged field and its very long term secular variations paleomagnetic samples were collected from 35 volcanic units either lava flows or ignimbrites in costa rica in december 1998 and february 2000 from the cordilleras central and guanacaste the underlying canas liberia and bagaces formations and from volcano arenal age estimates range from approximately 40 ka to slightly over 6 ma although initial results from these sites were used in a global synthesis of tafi data by johnson et al 2008 a full description of methodology was not presented this paper documents the definitive collection of results comprising 28 paleomagnetic directions 24 normal 4 reversed with enhanced precision and new geological interpretations adding two paleointensity estimates and 19 correlated 40ar 39ar radiometric ages the average field direction is consistent with that of a geocentric axial dipole and dispersion of virtual geomagnetic poles 17 3 4 6 is in general agreement with predictions from several statistical paleosecular variation models paleointensity estimates from two sites give an average field strength of 26 3 t and a virtual axial dipole moment of 65 zam2 the definitive results provide a useful augmentation of the global database for the longer term goal of developing new statistical descriptions of paleomagnetic field behavior

localized rifting at chagos bank in the india capricorn plate boundary zone chagos bank is a region of thick crust in the central indian ocean where a concentration of seismicity indicates that extension is ongoing we have modeled bathymetry and gravity data to show that the crust mantle boundary shallows near a seabed trough consistent with a total extension of 15 km over a zone 50 km broad this estimate is consistent with 5 10 m y of deformation at the present rate of relative india capricorn motion the 15 km of extension is toward the lower limit of the total extension expected from magnetic anomaly reconstructions but would be accumulated in 1 3 m y in a regime having the present day seismicity we attribute the localization of seismicity beneath chagos bank to a weak rheology caused by its thick crust

drainage evolution in the polish sudeten foreland in the context of european fluvial archives detailed study of subsurface deposits in the polish sudeten foreland particularly with reference to provenance data has revealed that an extensive preglacial drainage system developed there in the pliocene early pleistocene with both similarities and differences in comparison with the present day odra oder system this foreland is at the northern edge of an intensely deformed upland metamorphosed during the variscan orogeny with faulted horsts and grabens reactivated in the late cenozoic the main arm of preglacial drainage of this area at least until the early middle pleistocene was the palaeo nysa klodzka precursor of the odra left bank tributary of that name significant preglacial evolution of this drainage system can be demonstrated including incision into the landscape prior to its disruption by glaciation in the elsterian sanian and again in the early saalian odranian which resulted in burial of the preglacial fluvial archives by glacial and fluvioglacial deposits no later ice sheets reached the area in which the modern drainage pattern became established the rivers incising afresh into the landscape and forming post saalian terrace systems issues of compatibility of this record with the progressive uplift implicit in the formation of conventional terrace systems are examined with particular reference to crustal properties which are shown to have had an important influence on landscape and drainage evolution in the region

initial impingement of the yellowstone plume located by widespread silicic volcanism contemporaneous with columbia river flood basalts during the main phase of steens and columbia river united states flood basalt eruptions between 16 7 and 15 0 ma 3900 km 3 of silicic magma erupted from centers dispersed across 25 000 km 2 the largest and oldest silicic centers high rock mcdermitt and lake owyhee caldera complexes lie along a narrow nne trend at the transition between the craton and accreted oceanic crust suggesting that silicic magma generation was greatest where mafic magma intruded more felsic crust less voluminous and younger silicic centers track the spread of mafic dikes to the north the high rock caldera complex forms the western end of the snake river plain yellowstone trend of eastward younging silicic centers we attribute a regional lull in volcanism between ca 23 and 7 ma to lifting of the juan de fuca slab by a mantle plume beginning at ca 25 ma and the distribution of main stage flood basalts and coeval rhyolites to breakthrough of plume material at ca 17 ma to form a smaller plume head centered beneath steens mountain

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

neotectonic development of drainage networks in the east sudeten mountains and monitoring of recent fault displacements czech republic abstract this paper presents results of morphotectonic research carried out in order to determine the neotectonic development of the drainage network in the ne spur of the bohemian massif central europe the area studied comprises the north eastern sector of the rychlebske mts belonging to the sudeten mountains and the adjacent part of the ulovska hilly land in the sudetic foreland czech republic analysis of drainage network characteristics such as cross sections erosion rate longitudinal river profiles stream length gradient index sl and investigation of alluvial fans terraces was performed using detailed geomorphological mapping and field examination and dem data moreover a reconstructed neotectonic evolution was compared with present day fault movements obtained by fault monitoring using the tm71 deformeter the deformeter was installed directly across faults in two karst caves in the study area within the nw se striking sudetic marginal fault smf zone this zone is one of the morphologically most prominent neotectonic structures in central europe separating the sudeten mountains from the sudetic foreland morphotectonic research reveals that segments of enhanced erosion correspond well with increased sl indices changes in valley cross sections and anomalies in the longitudinal profiles the beginnings of the stretches of increased headward erosion rejuvenated erosional phase are concentrated at the foot of marginal slopes of the mountainous sector of the study area which supports the hypothesis that uplift of the mountainous sector is still expressed in its relief alluvial fans terraces of three levels recognized in the adjacent ulovska hilly land are of middle to late pleistocene age saalian 1 240 280 ka saalian 2 130 180 ka and weichselian 10 80 ka respectively they postdate the retreat of the last continental ice sheet which reached the study area in elsterian 2 400 460 ka their relative heights above the river channel are greater than terrace levels of the same age along the main nysa klodzka river the height differences attain 20 m at the highest level 1 at least 8 m at level 2 and up to 2 3 m at level 3 these discrepancies imply post saalian 1 uplift of the ulovska hilly land relative to the topographically lower nysa klodzka valley monitoring of present day tectonic movements in the studied area revealed slow micro displacements hundredths to tenths of millimetres per year the displacements have an aseismic character and the vertical component always prevails over the horizontal one the inferred compressive stress comes generally from the southern sector which would imply dextral transpression in the studied portion of the smf where the northern part is thrusting over the southern one the trend of these present day movements corresponds well with uplift of the studied area north of the smf which is also indicated by analysis of the drainage network it is concluded that in areas of low tectonic activity the detailed study of individual characteristics of the drainage network particularly their spatial relationships as well as monitoring of fault micro displacements can reveal rates and kinematics of ongoing tectonism

subsidence and growth of pacific cretaceous plateaus abstract the ontong java manihiki and shatsky oceanic plateaus are among the earth s largest igneous provinces and are commonly believed to have erupted rapidly during the surfacing of giant heads of initiating mantle plumes we investigate this hypothesis by using sediment descriptions of deep sea drilling project dsdp and ocean drilling program odp drill cores to constrain plateau subsidence histories which reflect mantle thermal and crustal accretionary processes we find that total plateau subsidence is comparable to that expected of normal seafloor but less than predictions of thermal models of hotspot affected lithosphere if crustal emplacement was rapid then uncertainties in paleo water depths allow for the anomalous subsidence predicted for plumes with only moderate temperature anomalies and volumes comparable to the sources of modern day hotspots such as hawaii and iceland rapid emplacement over a plume head of high temperature and volume however is difficult to reconcile with the subsidence reconstructions an alternative possibility that reconciles low subsidence over a high temperature high volume plume source is a scenario in which plateau subsidence is the superposition of 1 subsidence due to the cooling of the plume source and 2 uplift due to prolonged crustal growth in the form of magmatic underplating this prolonged crustal growth and uplift scenario may explain the low and thus submarine relief during plume initiation the late stage eruptions found on ontong java 90 ma and manihiki 70 ma a large portion of the high seismic velocity lower crust and the widespread normal faults observed throughout and along the margins of the three plateaus such late stage underplating may have occurred continuously or in discrete stages over 30 m y and implies lower magmatic fluxes than previously estimated

revised tectonic evolution of the eastern indian ocean published plate tectonic models for the australian antarctic plate pair imply geologically improbable scenarios at either or both ends of the cretaceous rift and spreading system controversy also exists around the location of and motion at the plate boundary extending west of australia antarctica through the kerguelen plateau region we present a plate tectonic model of relative motions among india australia and antarctica from the onset of continental rifting to the establishment of rapid seafloor spreading at 43 ma the model conforms to a wide range of geological geophysical evidence and reconstructs the formation of both the western kerguelen region and the eastern tasman region the incorporation of spatio temporally continuous plate boundaries reveals the presence of a plate boundary beneath the contiguous central kerguelen plateau and broken ridge for 65 million years to investigate the relationship between the plate boundary system and the kerguelen plume we test three alternative absolute reference frames using a fixed hotspot reference frame the indian ocean mid ocean ridge system remains within 500 km of the kerguelen plume while the proximity of the plate boundaries and the plume is more variable with a moving hotspot reference frame proximity between the plume plate boundaries and the central kerguelen plateau broken ridge for 65myrs suggests that these specific features were not formed by a single short lived 5 10 myr pulse of magmatic activity but rather by a 25 myr period of relatively high magma flux followed by 40 myr period of lower volume magmatic activity an interpretation not excluded by the relatively sparse dredge and drill ages

continental crust beneath the agulhas plateau southwest indian ocean the agulhas plateau lies 500 km off the cape of good hope in the southwestern indian ocean acoustic basement beneath the northern one third of this large aseismic structural high has rugged morphology but basement in the south is anomalously smooth excepting a 30 to 90 km wide zone with irregular relief that trends south southwest through the center of the plateau seismic refraction profiles across the southern plateau indicate that the zone of irregular acoustic basement overlies thickened oceanic crust and that continental crust locally thinned and intruded by basalts underlies several regions of smooth acoustic basement recovery of quartzo feldspathic gneisses in dredge hauls confirms the presence of continental crust the smoothness of acoustic basement probably results from erosion perhaps initially subaerial of topographic highs with redeposition and cementation of debris in ponds to form high velocity beds basalt flows and sills also may contribute locally to form smooth basement the rugged basement of the northern plateau appears to be of oceanic origin a plate reconstruction to the time of initial opening of the south atlantic places the continental part of the southern plateau adjacent to the southern edge of the falkland plateau and both abut the western mozambique ridge both the agulhas and falkland plateaus were displaced westward during initial rifting in the early cretaceous formation of an rrr triple junction at the northern edge of the agulhas continental fragment during middle cretaceous time may explain the origin of the rugged thickened oceanic crust beneath the northern plateau as well as the apparent extension of the continental crust and intrusion of basaltic magmas beneath the southern plateau

volcano tectonic segmentation of the northern andes data from landsat images published geologic and geophysical reports and field observations are used to document eight major transverse boundaries that divide the northern andes into segments about 125 km long most of the segment boundaries have no apparent relation to structure on the nazca plate being subducted and may instead represent fundamental and long lasting structures of the continent subduction of the carnegie ridge results in increased uplift of the andes widespread and chemically diverse volcanism and major faulting and seismicity subduction of a highly fractured zone in southern ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style from andesitic stratovolcanoes to rhyolitic ash flow sheets recognition of the segmented structure of the northern andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to subduction

late permian paleomagnetic pole from dikes of the tarim craton china we have obtained a late permian paleomagnetic pole from dikes in the tarim craton of china directions of magnetization from 21 mafic and ultramafic dikes at three localities pass a fold test at the 95 confidence level the new paleomagnetic pole lies at lat 66 n long 181 e n 21 k 61 and 95 3 9 on the basis of a comparison with paleomagnetic data from adjacent tectonic units we conclude that 1 the late permian paleolatitude of the tarim craton differs from that of the sino korean craton suggesting that the two cratons were distinct tectonic units in late permian time 2 there was relative motion between the tarim craton and the siberian craton after late permian time and 3 negligible relative motions have occured between different thrust sheets in the keping bachu area of the north western tarim craton

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

crustal uplift in southern england evidence from the river terrace records abstract much of the past work on the quaternary rivers of northwest europe has been concerned with river terraces which characterise almost every valley while these terraces are undoubtedly striking features of the landscape the incision achieved by quaternary rivers is equally significant in terms of river behaviour and for an understanding of the factors affecting landform development during the quaternary this paper examines the incision achieved during the quaternary by the thames in both its upper and lower catchments and by the hampshire avon in southern england valley incision rates of ca 0 07 0 10 m ka 1 have been identified although in the lower catchment of the thames these have been enhanced by additional incision in response to glacio isostasy and valley shortening a model is proposed in which regional uplift is recognized as the primary cause of incision by these quaternary rivers possible mechanisms for regional uplift are considered

conduit diameter and buoyant rising speed of mantle plumes implications for the motion of hot spots and shape of plume conduits 1 mantle plumes are expected to be affected by large scale flow in the earth s mantle related to plate motions subducted slabs and possibly large scale upwellings motion of plume conduits will depend on both large scale flow and buoyant rising speed of the conduit through the mantle here we present a model of depth dependent plume conduit temperature viscosity radius and buoyant rising speed and use it to compute plume and hot spot motion results support a temperature anomaly of about 500 k at the plume base in this case sublithospheric temperature anomaly is about 150 k for the iceland plume transition zone anomaly is between 150 and 200 k for iceland about 250 k for samoa and about 300 k for hawaii thermal plume radii are about 100 km in the upper mantle increasing to about 200 km in the lower mantle beneath the lithosphere viscosity in the vicinity of plumes is substantially reduced compared to the underlying mantle and fast moving plates deflect plumes by 200 km or less corresponding to a plume conduit buoyant rise time of about 3 myr between 400 and 100 km depth where most of the shear flow due to plate motions occurs during 100 myr plume conduits rise buoyantly from about 1500 2000 km depth in many cases computed hot spot motion agrees with previous computations south southeastward motion of the hawaiian hot spot during the past 100 myr louisville hot spot motion in a similar direction but at slower speed during the past 50 myr and westward motion of the easter hot spot relative to hawaii and louisville however the previously computed substantial southward motion of the kerguelen hot spot is not confirmed if plume sources in the lowermost mantle are assumed to move with large scale flow they are predicted to be displaced toward the two large scale upwellings beneath africa and the pacific relative to hot spot surface locations for fixed sources predicted tilts of the lower part of conduits tend to be opposite to that

modelling the geomagnetic field from syntheses of paleomagnetic data abstract this review examines results from time varying geomagnetic field models that span several thousand years and from variations in dipole moment strength up to million year time scales for the past 400 years twin magnetic flux lobes bordering the inner core tangent cylinder in both northern and southern hemispheres dominate the geomagnetic field and appear more or less fixed in location in contrast the millennial scale view shows that such features are quite mobile and subject to morphological changes on time scales of a few centuries to a thousand years possibly reflecting large scale reorganization of core flow the lobes rarely venture into the pacific hemisphere and average fields over various time scales generally reveal two or three sets of lobes of diminished amplitude thus millennial scale models are suggestive of thermal core mantle coupling generating a weak bias in the average field rather than a strong inhibition of large scale field changes the recovery of variations in dipole moment on million year time scales allows frequency domain analyses to search for characteristic time scales for core dynamics that might be associated with excursion and reversal rate time taken for reversals or any signs of control by earth s orbital parameters the spectrum is characteristically red for the time interval 0 160 ma suggesting non stationarity associated with average reversal rate changes probably reflecting the impact of superchrons and a continually evolving core distinct regimes of power law decay with frequency may reflect different physical processes contributing to the secular variation evidence for non stationarity at shorter time scales is also present in dipole moment variations over 0 2 ma with average growth rate faster than the decay process rates of change of dipole moment and rapid local field variations found in the paleomagnetic record are evaluated in the context of the 400 year historical record and the spectrum of geomagnetic variations for 0 160 ma

full fit palinspastic reconstruction of the conjugate australian antarctic margins 1 despite decades of study the prerift configuration and early rifting history between australia and antarctica is not well established the plate boundary system during the cretaceous includes the evolving kerguelen broken ridge large igneous province in the west as well as the conjugate passive and transform margin segments of the australian and antarctic continents previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history we investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair potential field data are used to define the boundaries of stretched continental crust on a regional scale integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary we then use the prerift plate boundary positions along with additional constraints from geological structures and large igneous provinces within the same australian and antarctic plate system to compute full fit poles of rotation for australia relative to antarctica our preferred model implies that the leeuwin and vincennes fracture zones are conjugate features within gondwana but that the direction of initial opening between australia and antarctica does not follow the orientation of these features rather the geometry of these features is likely related to the earlier rifting of india away from australia antarctica previous full fit reconstructions based on qualitative estimates of continental margin overlaps generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration

kerguelen plateau crustal structure and basin formation from seismic and gravity data we use multichannel seismic data gravity and subsidence modeling in conjunction with plate reconstructions to evaluate the crustal origin and composition of the kerguelen plateau predominantly oceanic crust of the southern and parts of the central kerguelen plateau appears to include continental fragments related to the breakup of india and antarctica these fragments may have been metamorphosed during emplacement of the main plateau the upper crust is basaltic the middle crust is intrusive mafic rock and intruded continental crust and the lower crust is a plagioclase rich metamorphic rock the labuan basin crust is predominantly oceanic with stranded kerguelen plateau fault blocks high density lower crust in the labuan basin is probably composed of serpentinized peridotites formed during slow rifting and spreading plate reconstruction models indicate opening between eastern broken ridge and southern kerguelen plateau at 90 ma heralding the formation of the labuan basin and diamantina zone crustal attenuation and slow accretion of oceanic crust continued until the australian and antarctic plates separated at ci8 time 40 ma plate reconstructions of the free air gravity field indicate that the naturaliste plateau fits against antarctica and that elan bank and india were juxtaposed until 110 ma both naturaliste plateau and elan bank are probable microcontinents a 1 km positive residual depth anomaly in the oceanic basins adjacent to the plateau along with the positive geoid anomaly centered beneath the northern kerguelen plateau imply that the lithosphere is partially dynamically supported by an upwelling hot asthenosphere of the kerguelen hot spot

geomorphometry based detection of enhanced erosional signal in polygenetic medium altitude mountain relief and its tectonic interpretation the sudetes central europe abstract we explore the assumption that uplift drives erosion and hence more dissected eroded terrains may indicate areas subject to more intense uplift than others especially if other factors such as elevation and geology can be demonstrated as inconsistent with the spatial pattern of dissection geomorphometry and statistics are employed to recognize the enhanced erosional signal within a medium altitude mountain terrain of the sudetes in central europe considered as a representative for moderately uplifted areas of complex bedrock diversity this signal is understood as a meso scale reflection of relief properties whereas the recognition of homogeneous terrain units different in terms of strength of erosional signal is carried out by means of multi criteria terrain classification involving principal component analysis and k means grouping four groups covering 45 of the area have been identified they were quantitatively compared with independent variables of altitude lithology structural units and rock strength and also assessed in relation to rainfall pattern it is concluded that their dependence on altitude and geology is only partial then three types of dissected terrain are distinguished and interpreted in terms of uplift intensity rock strength as a factor modulating erosional response to uplift and timescale the spatial distribution of these types of terrain indicates several isolated hot spots of erosional dissection on predominantly strong rocks linked by less elevated areas underlain by rocks of variable strength the former are considered as having longer history of uplift and dissection dated back to the late palaeogene whereas the latter record more recent history of endo exogenic interactions possibly since the late miocene the presence of canyon like valleys incised into planar or gently sloping relief may indicate the most recent pulse of relief rejuvenation the procedure proposed in the paper is suitable to be used in various settings given that good quality topographic data are available

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

the quaternary uplift history of central southern england evidence from the terraces of the solent river system and nearby raised beaches abstract we have used fluvial solent river system and marine terraces to reconstruct the uplift history of central southern england in the case of the former we make the assumption that fluvial incision has been a direct response to surface uplift with its precise timing controlled by climatic forcing of fluvial activity such that height of terrace gravel above modern river is a consequence of uplift since deposition in the case of the marine sequence we take the height of interglacial raised beaches above a calculated contemporaneous sea level as a measure of uplift the calculation involving an adjustment from modern sea level using the deep oceanic oxygen isotope signal as an indication of global ice volume at the time of deposition this exercise requires some degree of dating constraint which is problematic for both environments the solent terraces have yielded little biostratigraphical evidence whereas the south coast raised beaches have either been poorly exposed in recent years or their ages have been controversial because of disputes between biostratigraphy and geochronological data we have supplemented the evidence available from these sources by using key aspects of the archaeological record as dating constraints in particular the first appearances of levallois technique a marker for mis 9 8 and of bout coupe handaxes mis 3 the first of these has been particularly useful in modelling of the middle pleistocene parts of the river terrace staircases of the solent system in undertaking this reappraisal we have noted several inconsistencies and disagreements between past correlation schemes for the terraces of the solent and its various tributaries we find that versions involving shallower downstream gradients in the main solent river are most likely to be correct and that revisions on this basis solve a number of problems in interpretation encountered previously our results show that most of this region has uplifted by 70 m since the late early pleistocene and by 150 m since the middle pliocene there being a high degree of consistency between uplift histories inferred for river terraces and marine terraces uplift rates increase gradually westward such that along the river frome at the western end of the hampshire basin 80 m of uplift since the late early pleistocene is indicated this variation is interpreted as a consequence of a regional scale variation in crustal properties about 80 m of uplift is also indicated on this timescale by raised beaches in the portsdown area and adjacent terraces of the river test and solent in the vicinity of the portsdown anticline to the north of southampton we interpret this as a consequence of 10 m of vertical slip in the past million years on the blind reverse fault beneath this anticline this dataset thus provides the first clear indication of measurable quaternary structural development in crustal basement in the onshore uk the solent has formed more than a single terrace per 100 ka milankovitch cycle leading us to attribute terraces to isotopic substages potentially improving upon the resolution available from sequences in which terraces formed once per cycle athough the first appearence of levallois technique was initially considered to date from the mis 9 8 transition based on evidence from the thames we found that there was a better modelling fit if this was taken as having occurred slightly earlier in mis 9b perhaps in association with the post mis 9e or 9c marine regression which could have permitted immigration into a previously insular britain of people versed in levallois technology

quaternary eruptive history of nevado del ruiz colombia abstract nevado del ruiz has a 1 8 m y long eruptive record that includes alternate construction and destruction of three edifices during three main eruptive periods termed ancestral ruiz older ruiz and ruiz nevado del ruiz is located on a complex intersection of four groups of faults the most significant being the n20 e palestina strike slip fault and the n50 w villamaria termales normal fault ancestral ruiz was a broad stratovolcano built by two eruptive stages of lava flows starting about 1 8 ma ago and ending 1 0 ma ago a partial collapse and formation of a caldera are thought to have occurred between 1 0 and 0 8 ma ago older ruiz was a stratovolcano constructed by lava flows in three stages starting about 0 8 ma ago and ending about 0 2 ma ago extensive and voluminous welded and nonwelded pyroclastic flow deposits that partly fill preexisting valleys record the formation of a young summit caldera between 0 2 and 0 15 ma ago present ruiz is formed by a cluster of composite lava domes that probably filled the summit caldera of older ruiz present ruiz eruptive activity is mostly explosive but also includes dome growth and parasitic dome activity of la olleta and alto la piramide twelve eruptive stages occurred during the last 11 000 years accompanied by rockslide debris avalanches pyroclastic flows or surges and their subsequent interactions with the ice cap as well as by glacial erosion and mass wasting diverse processes within these twelve stages have led to a partial destruction of the summit domes this long and complex pleistocene and holocene eruptive sequence helps to put the november 13 1985 eruption into a broader perspective

comment on major australian antarctic plate reorganization at hawaiian emperor bend time whittaker et al reports 5 october 2007 p 83 presented reconstructions for australia and antarctica showing a change in relative plate motion 53 million years ago coincident with an inferred major global plate reorganization this comment addresses problematic areas in their assumptions and the geological consequences of their reconstructions

first indications of high slip rates on active reverse faults nw of damascus syria from observations of deformed quaternary sediments implications for the partitioning of crustal deformation in the middle eastern region recent research on rates of crustal shortening within the palmyra fold belt pfb in syria has drawn attention to the possibility that reverse faults near the city of damascus which adjoins the sw pfb have significant slip rates we infer that the damascus fault directly adjacent to the city has developed a throw of 2500 m and report the discovery of the en echelon bassimeh fault with a throw of 1000 m this fault being revealed by warping of the local bedrock and of a terrace of inferred late pleistocene age of the river barada we estimate that this set of faults became active circa 0 9 ma synchronous with changes to the pattern of faulting previously reported farther southwest in the northern jordan valley vertical slip rates on the bassimeh and damascus faults of 1 1 and 2 8 mm a 1 respectively are thus estimated we also infer that large historical earthquakes previously attributed to left lateral faulting farther west on the dead sea fault zone dsfz probably occurred on this set of reverse faults these faults thus represent a significant hazard to the city of damascus our observations indicate that as much as half of the northward motion of the arabian plate relative to the african plate may be absorbed by crustal shortening within the pfb potentially explaining the low slip rate recently measured geodetically on the northern dsfz in western syria

change of tectono stratigraphic regime in the australian plate during the 99 ma mid cretaceous and 43 ma mid eocene swerves of the pacific the clockwise bend at 99 ma mid cretaceous in linear volcanic chains in the tropical pacific coincides with a change from pre 99 ma head on chilean type subduction of the pacific plate beneath eastern gondwana to 99 43 ma sinistral oblique mariana type subduction and strike slip breakup by simple sea floor spreading between australia and antarctica and by backarc spreading in the southwest pacific the 99 ma breakup of australia from antarctica is documented by a mid cretaceous unconformity this tectono stratigraphic change founded modern australia with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south the counterclockwise bend at 43 ma mid eocene the emperor hawaiian bend coincides with the onset of structure in the challenger rift of new zealand the eromanga cooper basin of central australia and the oil shale grabens of coastal queensland

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes the hypothesis that a single mushroom like mantle plume head can generate a large igneous province within a few million years has been widely accepted the siberian traps at the permian triassic boundary and the deccan traps at the cretaceous tertiary boundary were probably erupted within one million years these large eruptions have been linked to mass extinctions but recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years this observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal single stage plume model suggests here we present numerical experiments to demonstrate that the entrainment of a dense eclogite derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces the characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations such a process may contribute to multiple episodes of large igneous provinces

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331

structure and tectonic setting of the 77 e and 75 e grabens kerguelen plateau south indian ocean the central kerguelen plateau south indian ocean is characterized by abundant north south striking normal faults which comprise two prominent north south rifts known as the 77 e and 75 e grabens the 77 e graben is a well defined structure which extends over some 800 km from the eastern margin of the kerguelen plateau to about 58 5 s over most of its length it is associated with a 10 30 km wide axial rift and with a 100 150 km wide uplift the 75 e graben is less well documented but the available data suggest that its dimensions and internal structure resemble that of the 77 e graben in the better documented 77 e graben six rift segments 50 100 km long are identified faulting is more developed at the northern and southern ends of the 77 e graben possibly resulting from the interaction with other rifts to the north the 77 e graben abuts the highly faulted eastern margin of the kerguelen plateau and the northern part of an even larger rift zone the plate boundary rift zone which extends along the boundary with the australian antarctic basin to the south the 77 e graben adjoins the northwestern end of the southern kerguelen plateau rift zone the 77 e and 75 e grabens and the other rift zones on the kerguelen plateau appear to have been formed at approximately the same time between 72 and 60 ma they are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the plate boundary rift zone into the southeast indian ridge between 46 and 43 ma the north south trend of the 77 e and 75 e grabens is different from that of the other rift zones which are oriented northwest southeast this geometry suggests that some strike slip motion may have occurred along the north south trending grabens as a result of extension on the northwest southeast trending rifts particularly the southern kerguelen plateau rift zone however since near surface extension estimates for the southern kerguelen plateau rift zone are small the strike slip motion along the 77 e graben must be equivalently small also the available seismic data from this graben do not show typical seismic characteristics of a strike slip environment such as zones of compression or flower structures thus the results of this work are inconsistent with any model for the development of the south indian ocean which requires significant amount of transform motion on the 77 e or 75 e grabens finally the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting we find great similarities between the two processes in the segmentation of the rifts the asymmetric cross section and the associated shoulder uplifts

evaluation of magmatic processes for the products of the nevado del ruiz volcano colombia from geochemical and petrological data abstract trace element concentrations of the eruptive products from the nevado del ruiz volcano indicate that at least for the last 1 ma the major controlling factor in the evolution of the magma has been a simple fractional crystallization the zr hf ratios suggest that the source material is mantle like in origin and whilst subduction related contamination of the source material has taken place there is no evidence that assimilation of continental crust has affected the series no convincing geochemical evidence for magma mixing was found and data on banded pumices of pleistocene historic and the 1985 eruptions strongly support a crystal fractionation model to explain the compositional variation of their glasses therefore if mixing has taken place it can only be a mixing of layers within a stratified or partially stratified magma chamber modeling of fractional crystallization using a stepwise program has been undertaken and least squares approximations with small residuals are commensurate with analyzed samples crystal fractionation models with plagioclase dominating two pyroxenes and iron oxides give good fits for all calculations we conclude that simple crystal fractionation is the process most consistent with major and trace element variations in the ruiz series this closed system model requires at least 77 crystal fractionation of a basaltic parent to generate the observed compositions of dacites

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

plume ridge interaction lithospheric stresses and the origin of near ridge volcanic lineaments 1 in many hot spot ridge systems formation of off axis volcanic chains or lineaments occurs between the hot spot centers and nearby mid ocean ridges in some cases such as galapagos kerguelen and possibly the mid pac mountains and tristan these lineaments appear to meet in a focus zone near the hot spot and fan outward in the direction of the ridge axis the origins of these lineaments are not well known and do not easily fit into typical conceptual models of ridge or hot spot volcanism it has been proposed for the galapagos region that such lineaments are caused by channeling of hot asthenosphere from off axis mantle plumes toward mid ocean ridges where enhanced volcanism initiates island formation morgan 1978 alternatively other workers suggest that these lineaments are controlled by patterns of stress in the lithosphere we examine this latter hypothesis by considering the effects of buoyant uplift and asthenospheric shear on the base of the lithosphere induced by an expanding mantle plume using thin plate theory we calculate the two dimensional plan view pattern of depth integrated stresses in a plate of varying thickness both a straight continuous ridge and a ridge transform ridge system are simulated by a displacement discontinuity boundary element model ridge and transform segments have imposed normal and zero shear tractions and we test different idealized tectonic or far field stress conditions assuming that volcanism is promoted by lithospheric tension and aligns along lineaments parallel to trajectories of least tensile stress calculations reproduce the rough fan shaped pattern of lineaments between the galapagos archipelago and the galapagos spreading center the focus of the fan shape is over the plume center when it interacts with a straight ridge but the fan focus appears closer to a segmented ridge offset by a transform fault if the ridge segments are more tensile than the far field stress this condition provides a plausible explanation for the apparent focus at the galapagos north of the hot spot center the width of the fan pattern along the ridge axis is predicted to increase as the plume ridge separation increases and as the far field stress becomes more tensile parallel to the ridge the observed width along the galapagos spreading center is consistent with a nearly isotropic remote stress models predict the lithospheric tension caused by plumes to promote lineament formation on only young lithosphere in support of this prediction a compilation of data from 23 hot spots shows that lineament formation is common near hot spots with plume ridge separations of 1250 km or on lithosphere of 25 ma but not on older lithosphere

the role of neotectonics and glaciation on terrace formation along the nysa klodzka river in the sudeten mountains southwestern poland the nysa klodzka river drainage basin in the sudeten mts sw poland preserves a complex late cainozoic succession that includes eight fluvial series or terraces and deposits from two glacial episodes as well as local volcanic rocks slope deposits and loess fluvial sedimentation took place during the late pliocene and from the early middle pleistocene cromerian with a long erosion phase gap during the early pleistocene fluvial series are dated to the late pliocene cromerian holsteinian late saalian eemian weichselian and the holocene glacial deposits represent the early elsterian and early saalian stages almost all these stratigraphic units have been observed in all geomorphic zones of the river the mountainous klodzko basin the bardo mts bardo gorge and in the mountain foreland the main phase of tectonic uplift and strong erosion was during the early pleistocene minor uplift is documented also during the post early saalian and probably the post elsterian the post early saalian and post elstrian uplift phases are probably due to glacio isostatic rebound the quaternary terrace sequence was formed due to base level changes epigenetic erosion after glaciations and neotectonic movements the cromerian fluvial deposits terraces do not indicate tectonic influence at all all other quaternary terraces indicate clear divergence and the post early saalian terraces also show fault scarps the fluvial pattern remained stable once formed during the pliocene with only minor changes along the uplifted block along the bardo gorge inferring an antecedent origin for the bardo gorge only during the post glacial times have epigenetic incisions slightly modified the valley

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

active tectonics research using trenching technique on the south eastern section of the sudetic marginal fault ne bohemian massif central europe abstract the nw se striking sudetic marginal fault smf is one of the most conspicuous tectonic structures in central europe it controls the pronounced morphotectonic escarpment of the sudetic mountains for a length of 130 km this paper presents the results of trenching on the smf undertaken in order to assess activity of the fault the trenching technique has not hitherto been applied on either this particular fault system or elsewhere in the bohemian massif however it is the most effective tool for near surface fault investigation in areas that are well vegetated and therefore devoid of fault outcrops moreover the study area is situated in an intraplate region with a low displacement rate which is exceeded by an erosion rate and does not favor the preservation of fault scarps the trench sites were selected from prior dem analyses geomorphological fieldwork and geophysical sounding the trenches exposed a range of lithologies of variscan crystalline rocks and caenozoic sediments at least four phases of faulting have been distinguished based on structural data succession and age of the deformed sediments reverse faulting n160 e which displaced the miocene sediments over the crystalline rocks post dates their deposition 15 ma and pre dates late glacial gelifluction horizontal movements that have affected the miocene deposits along a structure striking n35 e have the same wide time constraint younger reverse faulting which caused a co seismic relief step post dates the late glacial gelifluction but preceded the early holocene colluvium deposition 10 940 140 cal yrs bp normal faults striking n145 e which cut the miocene unit might have been reactivated after the early holocene colluvial sedimentation but before the buried soil 430 120 cal yrs bp moreover based on the identified prehistoric earthquake respective minimum moment magnitude m 6 3 and slip rate 0 03 mm year were estimated

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

late quaternary activity of the feldbiss fault zone roer valley rift system the netherlands based on displaced fluvial terrace fragments abstract the meuse river crosses the feldbiss fault zone one of the main border fault zones of the roer valley graben in the southern part of the netherlands uplift of the area south of the feldbiss fault zone forced the meuse river to incise and as a result a flight of terraces was formed faults of the feldbiss fault zone have displaced the middle and late pleistocene terrace deposits in this study an extensive geomorphological survey was carried out to locate the faults of the feldbiss fault zone and to determine the displacement history of terrace deposits the feldbiss fault zone is characterized by an average displacement rate of 0 041 0 047 mm a 1 during the late pleistocene individual faults show an average displacement rate ranging between 0 010 and 0 034 mm a 1 the spatial variation in displacement rates along the individual faults reveals a system of overstepping faults these normal faults developed by reactivation of paleozoic strike slip faults as fault displacements at the bases of the younger terrace deposits are apparently similar to the tops of the adjacent older terrace the age of these horizons is the same within thousands of years this implies that the model of terrace development by rapid fluvial incision followed by slow aggradation does apply for this area

time averaged field at the equator results from ecuador seventy sites were collected from pliocene to recent lavas in ecuador between 0 19 and 1 1 s latitude lightning affected many of the sites sampled and the final data set consisted of 51 sites with 95 of 10 or less 21 flows yielded normal directions of magnetization declination 354 6 inclination 5 95 7 2 and 30 yielded reverse declination 183 5 inclination 5 6 95 5 these sites pass a reversal test at a high level the combined data give a direction of declination 359 9 inclination 5 4 and 95 4 2 which is just significantly different from the gad field but is in agreement with a gad field plus 5 quadrupole field mean vgps are not significantly different from the geographic poles the asd for the combined sites is 13 3 which is in agreement with model g paleointensity measurements yield values of vadm which range from 1 39 1022 to 22 6 1022 am2 the mean value for the reverse field is 3 2 1022 am2 while for the normal field it is 10 2 1022 am2 the 40ar 39ar dates on selected sites show that they are 2 6 ma and younger the age dates and magnetic polarity indicate that the chacana structure is brunhes matuyama in age

climatically controlled river terrace staircases a worldwide quaternary phenomenon a comparison of fluvial terrace sequences from around the world based on data collected as part of international geoscience programme igcp project no 449 has revealed significant patterns river terraces provide important records of uplift which is essential for their formation and of landscape evolution their cyclic formation however almost invariably seems to have been a response to climatic fluctuation sequences in the european core area of igcp 449 which has the longest and most extensive research history have been used as templates for worldwide comparison there is evidence for a global acceleration of uplift at the time of and perhaps in response to the mid pleistocene revolution when climatic fluctuation switched to 100 kyr milankovitch cycles terraces formed prior to this generally consist of wide aggradational sheets that probably each represent formation over several 41 kyr cycles subsequently river valleys became more steeply entrenched and terraces formed in response to the stronger 100 kyr climatic forcing in many cases at approximately one per cycle this paper uses the new data resource to explore differences between records in different climate zones between sequences with variable numbers of middle late pleistocene terraces and between systems in which the all important incision event has occurred in different parts of climatic cycles key records discussed include european examples from the rhine thames somme dniester dnieper don volga and aguas from asia the gediz turkey and orontes syria from north america the south platte and colorado from south africa the vaal and sundays from australia the shoalhaven and from south america the amazon paraguay and tributaries of the colorado and negro

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

exhumed mantle forming transitional crust in the newfoundland iberia rift and associated magnetic anomalies transitional zones located between iberia and north america formed during continental rifting and mostly consist of exhumed mantle in this study we show that ages of exhumed mantle at ocean drilling program odp sites 1068 and 1070 in the iberia abyssal plain and site 1277 in the newfoundland basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation on the basis of paleomagnetic and geological data and a comparison with a fossil ocean continent transition in the alps we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater and which only affects the upper tens of meters of the exhumed mantle the inversion of magnetic data euler deconvolution shows that magnetic sources are n s trending horizontal cylindrical bodies located within the highly serpentinized upper crust therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading within transition zones sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust but not on its nature oceanic versus exhumed mantle this discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting during rifting the deep distal continental margins and the adjacent transitional zones in the newfoundland iberia rift system were formed by ultraslow extension from early berriasian to late valanginian early hauterivian and by slow extension from early hauterivian to the late aptian early albian boundary therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors