breakup of the newfoundland iberia rift abstract the newfoundland iberia rift is considered to be a type example of a non volcanic rift key features of the conjugate margins are transition zones tzs that lie between clearly continental crust and presumed normal penrose type oceanic crust that appears up to 150 180 km farther seaward basement ridges drilled in the iberia tz consist of exhumed serpentinized peridotite of continental affinity consistent with seismic refraction studies although the boundaries between continental crust and the tzs can be defined with relative confidence there are major questions about the position and nature of the change from rifting to normal sea floor spreading at the seaward edges of the tzs notably drilling of presumed oceanic crust in the young m series anomalies to investigate and potentially resolve these conflicts we analysed the tectonic history and deep pre cenomanian stratigraphy of the rift using seismic reflection profiles and drilling results rifting occurred in two main phases late triassic earliest jurassic and late jurassic early cretaceous the first phase formed continental rift basins without significant thinning of continental crust the second phase led to continental breakup with extension concentrated in three episodes that culminated near the end of berriasian hauterivian and aptian time the first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift respectively suggesting that the rift opened from south to north in a two step process the third episode persisted through barremian and aptian time we suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary and that elevated in plane tensile stress throughout the rift caused intraplate extension primarily within the exhumed mantle this rifting may have been interrupted for a time during the barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the southeast newfoundland ridge and j anomaly ridge and the conjugate madeira tore rise we propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest aptian earliest albian time initiating sea floor spreading this resulted in relaxation of in plane tensile stress i e a pulse of relative compression that caused internal plate deformation and enhanced mass wasting this aptian event produced a strong rift wide reflection that is unconformably onlapped by post rift sediments that were deposited as a stable sea floor spreading regime was established although previously considered to be a breakup unconformity associated with separation of continental crust the event instead marks the final separation of the subcontinental mantle lithosphere our analysis indicates that interpretation of tectonic events in a non volcanic rift must consider the rheology of the full thickness of the continental lithosphere in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift

adaptation of prestack migration to multi offset ground penetrating radar gpr data in adapting the prestack migration technique used in seismic imaging to the inversion of ground penetrating radar gpr from time to depth sections we show that the theoretical integral formulation of the inversion can be applied to electromagnetic problems albeit with three assumptions the first two assumptions concern the electromagnetic characteristics of the medium primarily that the medium must be perfectly resistive and non dispersive and the third concerns the antennae radiation pattern which is taken to be 2d the application of this adaptation of the inversion method is confirmed by migrating actual gpr measurements acquired on the test site of the laboratoire central des ponts et chaussees the results show good agreement with the geometry of the structures in the medium and confirm that the possible departure from the assumption of a purely resistive medium has no visible effect on the information concerning the geometry of scattering and reflecting structures the field experiments also show that prestack migration processing is sufficiently robust with regard to the assumption of a non dispersive medium the assumption of a 2d antennae radiation pattern however produces artefacts that could be significant for laterally heterogeneous media nevertheless where the medium is not highly laterally heterogeneous the migration gives a clear image of the scattering potential due to the geometry of structural contrasts in the medium the scatterers are well focused from diffraction hyperbolae and well localized spatial geometry has limited dimensional accuracy and positions are located with a maximum error equal to the minimum wavelength of the signal bandpass objects smaller than one wavelength can nevertheless be detected and well focused if their dielectric contrasts are sufficiently high as in the case of iron or water in gneiss gravels furthermore the suitability of multi offset protocols to estimate the electromagnetic propagating velocity and to decrease the non coherent noise level of measurements is confirmed our velocity estimation is based on the semblance calculation of multi offset migrated images and we confirmed the relevance of this quantification method using numerical data the signal to noise ratio is improved by summing multi offset results after the addition of random noise on measurements thus the adaptation of prestack migration to multi offset radar measurements significantly improves the resolution of the scattering potential of the medium limitations associated with the methods used here suggest that 3d algorithms should be applied to strongly laterally heterogeneous media and further studies concerning the waveform inversion are necessary to obtain information about the electric nature of the medium

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

new models for evolution of magma poor rifted margins based on a review of data and concepts from west iberia and the alps direct observation and extensive sampling in ancient margins exposed in the alps combined with drill hole and geophysical data from the present day iberia margin result in new concepts for the strain evolution and near surface response to lithospheric rupturing at magma poor rifted margins this paper reviews data and tectonic concepts derived from these two margins and proposes that extension leading to thinning and final rupturing of the continental lithosphere is accommodated by three fault systems each of them characterized by a specific temporal and spatial evolution during rifting of the margin by its fault geometry and its surface response the data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting the place of final break up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres during final break up the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes

magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes serpentinization of ultramafic rocks during hydrothermal alteration at mid ocean ridges profoundly changes the physical chemical rheological and magnetic properties of the oceanic lithosphere there is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults which eventually results in structures known as oceanic core complexes occs however we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of occs since magnetite is a direct product of serpentinization the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere here we present new results from an integrated rock magnetic paleomagnetic and petrological study of variably serpentinized peridotites from the first fossil occ recognized in an ophiolite integration with existing data from mid ocean ridge related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range 0 100 of degrees of serpentinization variations in a range of magnetic parameters with serpentinization and associated paleomagnetic data provide i key constraints on the mechanism s of serpentinization at mid ocean ridges ii insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies and iii evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at occs

structural style formation of low angle normal fault and its controls on the evolution of baiyun rift northern margin of the south china sea abstract recent studies of high resolution seismic reflection profiles in the northern rifted margin of the south china sea revealed that low angle normal faults dominated the structural evolution of deepwater basins rather than high angle normal faults solely as was previously assumed however none of the studies proposed kinematic restorations of the fault system and the relationship to continental margin formation the baiyun rift is located in the slope of the northern margin of the south china sea underlain by extremely thinned continental crust it is filled with a relatively complete sequence of cenozoic sediments up to 10 km and has been considered the best documented deepwater basin basin wide 3d seismic data together with the available deepwater well data have allowed the structural framework to be built and tectono stratigraphic history of the baiyun rift to be traced in detailed from the rift stage a quantitative study of the primary geometry and kinematics of the major faults can be performed consequently results show the inception of the rift is characterized by faults slipped at high angles the later stage mid eocene of intensive extension is dominated by a fault system with large horizontal displacements 10 km and initial low dips

contourite drifts on early passive margins as an indicator of established lithospheric breakup the albian cenomanian breakup sequence bs offshore northwest iberia is mapped described and characterised for the first time in terms of its seismic and depositional facies the interpreted dataset comprises a large grid of regional 2d seismic reflection profiles complemented by industry and odp dsdp borehole data within the bs are observed distinct seismic facies that reflect the presence of a black shales and fine grained turbidites b mass transport deposits mtds and coarse grained turbidites and c contourite drifts borehole data show that these depositional systems developed as mixed carbonate siliciclastic sediments proximally and as organic carbon rich mudstones black shales distally on the northwest iberia margin mtds and turbidites tend to occur on the continental slope frequently in association with large scale olistostromes distally these change into interbedded fine grained turbidites and black shales showing widespread evidence of deep water current activity towards the top of the bs current activity is expressed by intra bs erosional surfaces and sediment drifts the results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after aptian albian lithospheric breakup therefore we interpret the recognition of contourite drifts in northwest iberia as having significant palaeogeographic implications contourite drifts materialise the onset of important deep water circulation marking the establishment of oceanic gateways between two fully separated continental margins as a corollary we postulate the generation of deep water geostrophic currents to have had significant impact on north atlantic climate and ocean circulation during the albian cenomanian with the record of such impacts being preserved in the contourite drifts analysed in this work

the widespread occurrence of low angle normal faults in a rift setting review of examples from thailand and implications for their origin and evolution abstract at least 31 low angle 1 km cenozoic normal faults are identified on seismic reflection data onshore and offshore thailand although some faults have been moderately rotated to lower angles pre rotation dips still indicate formation at low angles about 25 and 35 the dominant east dip of low initial fault dips suggest that pre existing fabrics controlled the fault dip direction in the mergui basin patches of gently dipping basement reflections on seismic reflection data support this observation well data points to a pre rift basement with a widespread component of slaty phyllitic and chlorite schist lithologies that are associated with significant strength anisotropy the subduction zone setting of se asia with a high fluid flux into the crust associated with high levels of co2 has parallels with the setting for lanfs in italy where it is suggested high pore fluid pressures played an important role in lanf reactivation in cross section most lanfs curve from a high angle in sedimentary section to planar or stepped in basement until they reach the brittle ductile transition zone in a few cases lanfs have a convex up geometry some long segmented faults are composed of mixed regions of high and low angle segments the local mix of high and low angle faults indicates that the principal stresses probably were vertical and horizontal during fault formation any rotation of stresses would have to be local not regional lanfs in the mergui basin have very high displacement gradients as low as 2 1 indicating the same fault segments some 20 30 km long have repeatedly slipped while displacements can decline dramatically at key discontinuities the lanf segments are inferred to be very weak and probably slipped mostly aseismically while adjacent lower displacement high angle segments were probably seismically active hence in areas of mixed high and low angle fault segments seismicity may erroneously suggest that high angle normal faults are the dominant fault type the presence of such a high concentration of low and high displacement lanfs in a non metamorphic core complex setting demonstrates that lanfs can form initially at a low angle that isostatic rotation of initially high angle faults to a low angle is not necessary or appropriate to explain the faults and that low angle normal faults are not just features associated with orogenic collapse

structural and thermal constraints on the initiation angle of detachment faulting in the southern basin and range the chemehuevi mountains case study the cenozoic normal fault system exposed in the chemehuevi mountains of the southern cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching there three stacked brittle low angle normal faults that formed at depths as great as 10 12 km cut gently down section northeastward through deformed proterozoic and mesozoic crystalline basement hanging wall blocks are displaced relatively northeastward the upper crust above the chemehuevi detachment fault was pulled apart along high angle normal faults that rotated to more gentle dips through time in contrast rocks of originally mid crustal affinity in the footwall were only gently rotated and accommodated minor extension application of 40 ar 39 ar and fission track thermochronology to rocks in the footwall of the chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting at the onset of extension between 22 and 24 ma granitic rocks exposed in the southwestern and northeastern portions of the footwall were at 200 c and 350 400 c respectively separated by a distance of some 23 km down the known slip direction this gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre extension isothermal surfaces and constrains the exposed part of the chemehuevi detachment fault to have had a regional dip initially of about 15 to 30 the fault system apparently cut gently down through the upper crust to a minimum depth of 10 12 km the deepest exposed parts of the system today and was domed from midcrustal depths and locally denuded during continued slip together the structural and thermochronologic data confirm the suggestion that faults accommodating large magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles that is 30

ground penetrating radar a near face experience from washington county arkansas digital ground penetrating radar gpr is an emerging technology for detecting locating and or mapping features in the shallow subsurface it has application to geology civil engineering environmental science engineering soil science and many other fields this case history describes a high quality gpr survey performed in very close proximity to a vertical limestone rock face at a roadcut site in northwest arkansas the configuration allowed interpretation of the gpr results with the maximum of amount of geologic control the rock face itself our experience suggests that gpr has significant potential as a high resolution tool for mapping shallow structure and stratigraphy in carbonate rocks

multifold borehole radar acquisition and processing the multifold acquisition principle was applied to a borehole radar survey performed in a granitic site grimsel test site switzerland two multifold coverage acquisitions 40 fold and 20 fold were carried out in a subhorizontal borehole instrumental drifts transmission time and sampling frequency fluctuations were corrected in order to remove shifts observed on cmp gathers and to optimize velocity analysis and trace stacking computation of velocity spectra was adapted in order to take into account the features of the medium investigated homogeneous velocity various reflector orientations the nmo velocities were then interpreted as angles between reflectors and the survey line the processing based on the computation of several constant velocity stacked sections performed with different nmo velocities leads to better results than the standard dmo nmo processing the signal to noise ratio of the stacked profile is improved in comparison with the single fold section which results from a standard acquisition from a practical point of view the implementation of a multifold radar survey within a borehole is difficult but a greater investigation range is obtained more reflectors are detected and the mapping of geological discontinuities is improved

spatial and temporal evolution of hyperextended rift systems implication for the nature kinematics and timing of the iberian european plate boundary we focus on the iberian european plate boundary iepb whose nature age and evolution are strongly debated in contrast to previous interpretations of the iepb as a major lithospheric scale left lateral strike slip fault we propose a more complex deformation history the mapping of rift domains at the transition between iberia and europe emphasizes the existence of spatially disconnected rift systems based on their restoration we suggest that the deformation was partitioned between a set of distinct left lateral transtensional rift systems from the late jurassic to early cretaceous a plate kinematic reorganization at aptian albian time resulted in the onset of sea floor spreading in the western bay of biscay and extreme crustal and lithosphere thinning in intra continental rift basins to the east the formation and reactivation of the iepb is interpreted as the result of the polyphase evolution of a diffuse transient plate boundary that failed to localize the results of this work may provide new insights on 1 processes preceding breakup and the initiation of segmented and strongly oblique shear margins 2 the deformation history of nascent divergent plate boundaries and 3 the kinematics of the southern north atlantic and alpine domain in western europe

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

acquisition and processing of wide aperture ground penetrating radar data a 40 channel wide aperture ground penetrating radar gpr data set was recorded in a complicated fluvial aeolian environment in eastern canada the data were collected in the multichannel format usually associated with seismic reflection surveys and were input directly into a standard seismic processing sequence filtering static corrections common midpoint gathering velocity analysis normal and dip moveout corrections stacking and depth migration the results show significant improvements over single channel recordings in noise reduction and depth of penetration by stacking and in spatial positioning and reduction of diffraction artifacts by migration these characteristics increase the potential for reliable interpretation of structural and stratigraphic details thus without having to develop any new software gpr data processing technology is brought to the same level of capability flexibility and accessibility that is current in seismic exploration

the tectono sedimentary evolution of a hyper extended rift basin the example of the arzacq mauleon rift system western pyrenees sw france in this paper we present a sedimentary and structural analysis that together with maps sections and new ar ar data enable to describe the tectono sedimentary evolution of the mauleon hyper extended rift basin exposed in the w pyrenees hyper extension processes that ultimately resulted in exhuming mantle rocks are the result of the subsequent development of two diachronous detachment systems related to two evolutional stages of rifting an initial late aptian early albian crustal thinning phase is first recorded by the development of a crustal necking zone controlled by the north vergent southern mauleon detachment system during a subsequent exhumation phase active faulting migrates to the north with the emplacement of the northern mauleon detachment system that exhumed north section thinned continental crust and mantle rocks this diachronous crustal thinning and exhumation processes are also recorded by the diachronous deposition of syn tectonic sedimentary tracts above the two supra detachment sub basins syn tectonic sedimentary tracts record the progressive exhumation of footwall rocks along detachment systems tectonic migration from the southern to the northern mauleon detachment system is recorded by the coeval deposition of sag deposits above the necking zone basin and of syn tectonic tracts above exhumed rocks north section located on a hanging wall situation related to the mauleon hyper extension structures the arzacq basin also records a major crustal thinning phase as shown by its subsidence evolution so as by deep seismic images the absence of major top basement structures and its overall sag morphology suggest that crustal thinning processes occurred by decoupled extension of lower crustal levels contrasting with the southern mauleon detachment system reconciling observations from the mauleon and arzacq basins we finally propose in this paper that they were the result of one and the same asymmetric crustal thinning and exhumation processes where extension is accommodated into the upper crust in the mauleon basin lower plate basin and relayed in ductile lower crust below the arzacq basin upper plate basin

interpolating wide aperture ground penetrating radar beyond aliasing abstractmost of the data acquisition in ground penetrating radar is done along fixed offset profiles in which velocity is known only at isolated points in the survey area at the locations of variable offset gathers such as a common midpoint we have constructed sparse heavily aliased variable offset gathers from several fixed offset collinear profiles we interpolated those gathers to produce properly sampled counterparts thus pushing data beyond aliasing the interpolation methodology estimated nonstationary adaptive filter coefficients at all trace locations including at the missing traces corresponding positions filled with zeroed traces this is followed by an inversion problem that uses the previously estimated filter coefficients to insert the new interpolated traces between the original ones we extended this two step strategy to data interpolation by employing a device in which we used filter coefficients from a denser variable offset gather to interpolate the missing traces on a few

the structure and formation of diapirs in the yinggehai song hong basin south china sea abstract the occurrence of shale diapirs in the yinggehai song hong ygh sh basin is well documented as is their association with big petroleum fields in order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high resolution two and three dimensional seismic reflection data as well as drilling data that cover the diapirs in ygh sh basin as many as 18 diapirs were identified and are arranged in six n s striking vertical en echelon zones on seismic reflection sections gas chimney structures diapiric faults and palaeo craters are genetically linked with the process of diapirism here we use geophysical and geological observations to propose a three stage model for diapirism initiation emplacement and collapse during these three stages different diapiric structure styles are formed which we describe in detail these include buried diapirs piercing diapirs and collapsed diapirs we link the diapirism to activity on the offshore continuation of the red river fault as shown on our high resolution seismic reflection data which is also related to a high paleogeothermal gradient caused by crustal thinning we also recognize the role of loading by the very large volume of sediment eroded from the edges of the tibetan plateau and delivered by the red river to the basin

frequency wavenumber modelling and migration of 2d gpr data in moderately heterogeneous dispersive media an algorithm for modelling and migrating ground penetrating radar gpr data in moderately heterogeneous dispersive media is presented the method is based on wavefield extrapolation in the frequency wavenumber f k domain from the solution of the 2d maxwell s equations the wavefield is extrapolated by a phase shift technique using a constant relative permittivity k and a quality factor q it is then modified by a correction term to handle the lateral k and q variations the spatial distribution of the k and q factor values representing the given model parameters is introduced into the algorithm by a regular grid parametrization the radar wave dispersion and attenuation induced by relaxation processes are taken into account by a linear frequency dependent q model and expressed by a complex wavenumber in the propagation equation a synthetic case and a field data set illustrate the potential of the method for frequencies of 300 500 and 900 mhz in the first case a typical civil engineering problem is considered the frequency dependence of the wave velocity and attenuation is well illustrated the synthetic data are afterwards migrated using the initial model parameters the results show the importance of using spatially varying model parameters in the migration processes the second case concerns an application of the method to a real data set in order to adjust the model parameters a forward modelling sequence is performed until the best match between the measured and the synthetic data is achieved a depth migration is then applied to the data and the result is compared with the initial model parameters in conclusion we assess the contributions of the method to industrial applications by discussing the performance of the algorithm compared with its limitations

seismic reflectivity of detachment faults of the iberian and tethyan distal continental margins based on geological and petrophysical data abstract low angle detachment faults are key to our understanding of the tectonic evolution of magma poor rifted continental margins in seismic images of present day rifted margins the identification and interpretation of such features is however notoriously difficult and ambiguous we address this problem by studying the structure and seismic response of such faults through a synoptic interpretation of petrophysical data and geological evidence from the distal segments of the present day west iberian and the ancient tethyan margins on the basis of the geologically well constrained remnants of the tethyan margins which are spectacularly preserved and exposed in the alps of eastern switzerland vertical profiles at four key geological settings of a typical magma poor rifted margin are constructed and their synthetic seismic responses are compared to the observed seismic data from corresponding locations in the present day iberian margin the seismic structure of these profiles is considered as the sum of deterministic large scale and the stochastic small scale components both components are analyzed for all pertinent lithologies the large scale structures are derived from laboratory measurements on samples from both the west iberian and tethyan margins whereas the small scale fluctuations are constrained predominantly on the basis of well log data from the iberian margin different realizations of the simulated stochastic small scale velocity fluctuations illustrate the potential variability of impedance contrasts and its impact on the seismic response from lithological interfaces and fault structures our results indicate that the nature of the seismic response from low angle detachment faults is largely determined through the fracture healing behavior of the surrounding rocks geological evidence from the exposed fragments of the tethyan margins indicate that fracture healing is generally well developed in crustal lithologies but largely absent in mantle lithologies it is for this reason that low angle intra crustal detachment faults tend to be seismically undetectable conversely crust mantle detachments have a complex and variable seismic response depending on the nature of the damaged zone and on the frequency content of the seismic data these model based inferences are consistent with the available evidence from the present day iberian passive margin and thus open new perspectives for the interpretation of the corresponding seismic images

velocity analysis from common offset gpr data inversion theory and application to synthetic and real data we implemented a procedure to estimate the electromagnetic em velocity using common offset ground penetrating radar gpr data the technique is based on the inversion of reflection amplitudes to compute the series of reflection coefficients used to estimate the velocity in each interpreted layer the proposed method recursively calculates the incident angles at any interface taking into account the offset between antennas and needs as input in addition to the picked amplitudes values a reference amplitude for each analysed gpr trace and a velocity value for the first shallowest layer the latter two parameters can be estimated directly from the available data or can be better constrained by further dedicated gpr acquisitions or by additional direct measurements we critically evaluated the performances for both synthetic and real data acquired with different antenna frequencies and we demonstrated that the new method can be applied in several real situations despite the necessary approximations and simplifying hypotheses the velocity values calculated for each layer are consistent with direct information and with cross validations obtained considering profiles acquired using different antennas and various path directions tests of the method on synthetic and real data sets show that the errors in the calculated velocity fields are quite low and comparable with more demanding velocity analysis techniques the obtained em velocity field is crucial in many processing steps such as for example true amplitude recovery depth conversion and imaging and provide essential information to characterize the subsurface materials

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

the depositional characteristics and oil potential of paleo pearl river delta systems in the pearl river mouth basin south china sea abstract delta systems in the zhuhai zhujiang and hanjiang formations are interpreted as having formed during periods of sea level rise from the late oligocene to mid miocene 30 10 5 my b p deltas in the zhuhai formation were developed in a shallow water environment on a sandy wave or fluvial dominated coasts thick and blanket like sandstones are common but mudstones are infrequent oil bearing zones have been found only in the upper part of the systems overlain by mudstone of the zhujiang formation deltas in the zhujiang formation are interpreted as having formed in a deeper water environment on sandy wave or fluvial dominated coasts carbonate platforms started growing on the dongsha massif as the sea level slowly rose the moderate sandstone mudstone ratio in the delta systems and a zone of secondary porosity in the carbonate rocks provide an excellent reservoir rock and seal rock for the basin deltas in the lower hanjiang formation are similar to those of the zhujiang formation but the upper part of the formation is interpreted as having formed in a shelf environment the thick shelf mudstone interbedded with offshore bar sandstones would be one of the better regional seal rocks in the study area no carbonate rock developed on the dongsha massif due to the rapid rise in sea level all delta systems from the zhuhai to the zhujiang formation are stacked in an onlap pattern indicating a more and more expanding marine transgression which was caused by eustatic sea level rise this marine transgression is almost unique and contrasts to the progradational offlap pattern of cenozoic delta systems in the gulf of mexico it resulted in thick sandstone deposits immature or poorly mature source rock and no ductile mudstones with no growth faults or rollover structures hydrocarbons generated from eocene oligocene lacustrine source rocks are trapped by drape structures related to local basement highs or by carbonate rock with secondary porosity most of the remaining undiscovered reserves in the basin will be found in non structural subtle traps especially stratigraphic traps

different expressions of rifting on the south china sea margins abstract rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin being opened quite recently in the tertiary in a scissor shape manner the south china sea scs offers an image of the rifting structures which varies along strike the basin margins the scs has a long history of extension which dates back from the late cretaceous and allows us to observe an early stretching on the northern margin onshore and offshore south china with large low angle faults which detach the mesozoic sediments either over triassic to early cretaceous granites or along the short limbs of broad folds affecting palaeozoic to early cretaceous series these early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone because these low angle faults reactivate older trends they vary in geometry according to the direction of the folds or the granite boundaries a later set of faults characterized by generally e w low and high angle normal faults was dominant during the eocene associated half graben basement deepened as the basins were filling with continental or very shallow marine sediments this subsequent direction is well expressed both in the north and the sw of the south china sea and often reactivated earlier detachments at places the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the phu khanh basin east of the vietnam fault a third direction of faults which rarely reactivates the detachments is ne sw and well developed near the oceanic crust in the southern and southwestern part of the basin this direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part and ended by the late miocene after the onset of the regional mid miocene unconformity known as mmu and dated around 15 5 ma latest miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast the se margin of the south china sea does not show the extensional features as well as the northern margin detachments are common in the dangerous grounds and reed bank area and may occasionally lead to mantle exhumation the sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the late miocene when it suddenly deepened this period also corresponds to the cessation of the shortening of the nw borneo wedge in palawan sabah and sarawak we correlate the variation of margin structure and composition of the margin mainly the occurrence of granitic batholiths and mesozoic broad folds with the location of the detachments and major normal faults which condition the style of rifting the crustal boudinage and therefore the crustal thickness

to see or not to see rifted margin extension a characteristic of rifted margins is the extension discrepancy i e the amount of extension estimated from fault geometries on seismic images is far too little to explain the observed crustal thinning and subsidence either the crust has been thinned in some other way or the amount of extension has been severely underestimated to investigate the latter we create a model structural section across a rifted margin by focusing extension in the center of a rift producing successive phases of crosscutting faults from one side of this section a synthetic seismic image is generated and interpreted as if it were a real profile just as for real margins apparent listric faults and eroded fault block crests are seen but these are not present in the model and instead represent intersecting fault surfaces and are thus diagnostic of polyphase faulting just as for real margins the amount of extension measured from the seismic is only a fraction of the true extension just as for real margins this extension discrepancy increases markedly oceanward demonstrably for the synthetic margin and by implication for real margins the extension discrepancy is the failure of the seismic method to image unambiguously the polyphase faulting required to accommodate increasing extension combined with a general lack of awareness of the features outlined here diagnostic of such faulting

seismic stratigraphy and tectonic structure from a composite multi channel seismic profile across the entire dangerous grounds south china sea abstract we interpret a more than 500 km long composite multi channel seismic reflection profile across the entire dangerous grounds the south china sea five tectonostratigraphic units are determined together with seven sequence boundaries detailed analysis of extensional features based on measurements of fault heaves revealed two major phases of extension separated by a distinct unconformity which likely correspond with the beginning of sea floor spreading in the south china sea early extension occurred during continental rifting late cretaceous early oligocene and resulted in formation of half grabens and rotated blocks controlled by a deeply rooted detachment system extension continued in our study area during the drifting phase of the east subbasin of the south china sea late oligocene early miocene until the subsequent opening of the southwest subbasin at about 25 ma but its intensity decreased markedly thereafter deeply rooted detachment systems evolved possibly during this second phase of extension at the continent ocean transition area from a coincidence of the lowermost reflections and moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust mantle boundary it is suggested that the dangerous grounds reflect a magma poor rift system at the initial stage of mantle unroofing we suggest that a widespread carbonate platform developed across the dangerous grounds concurrent with the period of seafloor spreading in the southwest subbasin of the south china sea fault related stretching factors f along the line show discrepancies with the whole crustal stretching factors c the continental crust of the dangerous grounds may have experienced nonuniform thinning

continuous profiles of electromagnetic wave velocity and water content in glaciers an example from bench glacier alaska usa we conducted two dimensional continuous multi offset georadar surveys on bench glacier south central alaska usa to measure the distribution of englacial water we acquired data with a multichannel 25 mhz radar system using transmitter receiver offsets ranging from 5 to 150 m we towed the radar system at 5 10 km h 1 with a snow machine with transmitter receiver positions established by geodetic grade kinematic differentially corrected gps nominal 0 5 m trace spacing for radar velocity analyses we employed reflection tomography in the pre stack depth migrated domain to attain an estimated 2 velocity uncertainty when averaged over three to five wavelengths we estimated water content from the velocity structure using the complex refractive index method equation and use a three phase model ice water air that accounts for compression of air bubbles as a function of depth our analysis produced laterally continuous profiles of glacier water content over several kilometers these profiles show a laterally variable stratified velocity structure with a low water content 0 0 5 shallow layer 20 30 m underlain by high water content 1 2 5 ice

the breakup sequence and associated lithospheric breakup surface their significance in the context of rifted continental margins west iberia and newfoundland margins north atlantic regional 2d seismic reflection profiles and borehole data are used to characterise the syn to post rift transition in the shallow offshore porto basin and in deep offshore regions of west iberia and newfoundland east canada the interpreted data highlight the development of a regional stratigraphic surface at the time of complete lithospheric breakup between west iberia and newfoundland this surface usually called breakup unconformity is renamed in this work as lithospheric breakup surface lbs on the basis that 1 it is not always developed as an unconformity and 2 all lithosphere is involved on the breakup process not only the continental crust depositional changes occur across the lbs in association with late aptian lithospheric breakup which is marked by the deposition of a breakup sequence bs rather than a single stratigraphic surface stratigraphic correlations between strata in shallow and deeper parts of the two margins lead us to propose the breakup sequence bs as representing the transitional period between lithospheric breakup and the establishment of thermal relaxation as the main process controlling subsidence on divergent continental margins the results in this work are important for other continental margins as they demonstrate that during lithospheric breakup significant quantities of sediment bypassed the inner proximal margins of west iberia and newfoundland on their way to the outer proximal margin in addition the interpreted data show that complete lithospheric breakup between conjugate margins is recorded by similar tectono stratigraphic events in iberia and newfoundland these events are associated with reservoir successions in sediment overfilled basins and with carbon rich strata black shales in sediment starved basins

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

examples of reverse time migration of single channel ground penetrating radar profiles a single channel ground penetrating radar gpr profile portrays a distorted unfocused image of subsurface structure due to apparent position shifts associated with dipping reflectors and to diffractions from corners and edges a focused image may be produced from such data by using any of the migration algorithms previously developed for seismic data we use reverse time migration based on the scalar wave equation field work was performed over a simple stratigraphic soil sequence and a complicated fluvial environment in the migrated images reflector continuity is enhanced and the level of detail available for high resolution interpretation is significantly increased

major unconformities termination of extension events and associated surfaces in the south china seas review and implications for tectonic development abstract the distribution of unconformities and end of cenozoic rifting events in the south china seas scs reflects both the modes of rift development and the effects of driving mechanisms continental rifting began in the eastern basins during the paleocene and propagated westwards to the vietnam basin margin in the late eocene continental breakup around 32 28 ma caused a regional reduction or cessation in extensional activity particularly affecting basins furthest from the spreading centre basins in the slope and deepwater area north of the spreading centre exhibit reduced fault activity until 21 20 ma propagation of oceanic crust westwards between 25 and 23 ma and termination of seafloor spreading sometime between 20 5 and 16 ma affected fault activity in the qiongdongnan and nam con song basins in the phu khanh basin and south in the dangerous grounds area extension continued until about 16 ma ending at the red unconformity the end of seafloor spreading around 20 5 ma reflects loss of extensional driving force as thinned continental crust entered the nw borneo subduction zone controversially a key component of the driving force maybe attributed to slab pull a transitional period of about 5 7 my between the onset of subduction of continental crust and final jamming of the subduction zone deep regional unconformity dru is inferred the last pulse of extension was focussed in the western scs and terminated around 10 5 ma detailed understanding of proto south china seas development remains uncertain and controversial

the effect of coastal processes on phytoplankton biomass and primary production within the near shore subtropical frontal zone this study evaluated drivers of phytoplankton net primary production npp rates and chlorophyll a chla concentrations within the coastally oriented subtropical frontal zone stfz off the south island of new zealand time series measurements of hydrographic parameters macronutrients size fractionated npp and chl a were conducted on a bi monthly basis from july 2009 to november 2010 this study found that nutrient limitation in these waters is controlled by the dual influx of silicate inputs from riverine sources in coastal neritic water nw and oceanic inputs of nitrate from the high nutrient low chlorophyll hnlc region of the offshore sub antarctic surface waters sasw total chl a concentrations and primary production rates were perennially higher in near shore nw and modified subtropical waters stw than in the sasw with highest indicators of biological production observed in the austral spring and summer seasons october to march these periods of peak production and biomass were dominated in both parameters by microphytoplankton 20 mm size fractions the coupled dominance by these large phytoplankton and the near depletion of silicate in all characterised waters within the frontal system indicate the importance of silicic diatoms as drivers of bloom production the influence of coastal waters on the stfz system is most pronounced with the intrusion of neritic water beyond the shelf boundary during periods of surface water thermal stratification and riverine dilution through flooding events these two events were notably observed during the spring 2009 sampling cruise in december 2009 and in the flood event in may 2010

the peridotite ridge province in the southern iberia abyssal plain seismic constraints revisited the west iberia margin has become the type example of a magma starved margin in which extension during breakup led to the exhumation of mantle rocks apparently over a broad area much of our understanding of this process is based on geophysical and drilling data from the southern iberia abyssal plain where the seaward part of the region of exhumed mantle is comprised of a series of margin parallel basement ridges here we analyse wide angle and normal incidence seismic data from a series of intersecting profiles across this peridotite ridge province and develop a three dimensional picture of its structure using these seismic data we infer a location and age magnetic anomaly m1 125 127 ma of first formed oceanic crust in this region that are consistent with ocean drilling program data and data from the conjugate margin we infer that the peridotite ridge province is about 70 km wide with strong serpentinization 75 at the top of basement reducing to much lower degrees 25 around 2 km below we map the geometry of some prominent reflectors on the landward margin of the peridotite ridge province that likely represent detachment faults and show that one of these has a domal structure similar to that of detachment faults at slow to ultra slow spreading mid ocean ridges

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

how can asymmetric detachment faults generate symmetric ocean continent transitions asymmetry or symmetry of magma poor rifted margins refers commonly to the crustal architecture and the occurrence or absence of large scale extensional detachment faults while distal parts of magma poor rifted margins are often considered to be asymmetric the observation of downlapping sedimentary sequences over exhumed mantle domains at conjugate margins suggests a symmetric evolution during mantle exhumation on the basis of seismic observations along the iberia newfoundland and australia antarctica margins we propose that their most distal parts show evidence for the development of multiple out of sequence asymmetric detachment faults we present evidence for cyclic delocalization and re localization of deformation resulting in an apparent symmetry of the exhumed mantle domain the interaction between out of sequence detachment systems and the successive rise of the asthenosphere may explain the observed transition from fault controlled to magma controlled strain accommodation and the transition to more symmetric and localized accretion associated with the formation of a stable spreading center

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

density muon radiography of la soufriere of guadeloupe volcano comparison with geological electrical resistivity and gravity data density muon radiography is a new method to determine the average density of geological bodies by measuring the attenuation produced by rocks on the flux of cosmic muons we present such density radiographies obtained for the soufri ere of guadeloupe lava dome both in the north south and east west planes these radiographies reveal the highly heterogeneous density structure of the volcano with low density regions corresponding to recognized hydrothermally altered areas the main structures observed in the density radiographies correlate with anomalies in electrical resistivity cross sections and a density model obtained from gravity data

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

inversion for the density depth profile of polar firn using a stepped frequency radar 1 translating satellite measurements of ice sheet volume change into sea level contribution requires knowledge of the profile of density as a function of depth within the ice sheet and how this profile changes over time this paper describes an interferometric method of inverting ground penetrating radar returns for the profile of firn density as a function of depth the method is an interferometric implementation of the common midpoint approach performed using a stepped frequency phase sensitive ground penetrating radar by recording the phase difference of returns with a range of antenna separations the different path lengths through the firn allow recovery of a smoothed representation of the density profile this density model is characterized by three parameters surface density and two decay lengths for porosity each operating over a different density range our results suggest that the stepped frequency radar used here can accurately recover differences in two way traveltime and produce useful estimates of the density profile in a test of the method performed at summit station in greenland the recovered density depth profile agreed with independent density measurements from an ice core and a neutron probe to within 6 root mean square error

polyphase faulting during the development of the west galicia rifted margin rifted margins are commonly characterised by an extension discrepancy the amount of extension measurable from the observed faulting is far less than that required to explain the crustal thinning it is shown here that polyphase faulting may provide a simple explanation for this paradox but can be very hard to recognise on seismic sections however at the west galicia rifted margin the galicia interior basin between the galicia bank and the mainland and the deep galicia margin to the west of the galicia bank a combination of high quality depth images seismic velocity information and stratigraphic control through odp drilling and submersible sampling provide complementary evidence for polyphase faulting berriasian hauterivian rifting in the galicia interior basin occurred along two sets of faults the first unroofed deep crustal rocks evidenced by high seismic velocities close to top basement the second cut and dismembered these early faults further rifting up until the aptian then focussed west of the galicia bank where two further phases of faulting can be inferred from the diachronous nature of seismostratigraphic units tilted within fault blocks removal of the latest phase of faulting aligns discontinuous reflections within the fault blocks into an anastomosing network of earlier faults restoration along these brings the crust back to its early hauterivian state similar to the present structure of the interior basin

successive detachment faults and mantle unroofing at magma poor rifted margins seismic depth images show that magma poor rifted margins between iberia and newfoundland are characterized by two different types of detachment that led to the unroofing of a broad expanse of mantle low angle detachments develop in serpentinites at the base of pre thinned crust and control further crustal thinning these detachments are cut by large offset faults rooting at a steep angle but with an exhumed slip surface and footwall flexurally rotated to a low angle during unroofing successive generations of this second type of detachment lead to the roughly symmetric unroofing of a broad expanse of mantle as new detachments repeatedly cut through the footwall of the preceding detachment leaving the abandoned root zone as landward dipping reflectors within exhumed mantle on both sides of the developing rift

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

the need to adapt the exploration model from the oil patch to contaminated site characterization a case from hill afb utah usa for decades the oil industry has employed a working model for hydrocarbon exploration in which large scale geophysical surveys are undertaken prior to a second phase of intensive targeted drilling this latter phase may be conducted in conjunction with further focused geophysical studies the geophysical surveys provide lateral coverage and continuity that are used to drive placement of drilling locations the reason for this approach is simple wells are expensive relative to geophysical surveys also practical limits on lateral coverage preclude optimization of exploration targets based on well information alone

applying reflection tomography in the postmigration domain to multifold ground penetrating radar data acquisition and processing of multifold ground penetrating radar gpr data enable detailed measurements of lateral velocity variability the velocities constrain interpretation of subsurface materials and lead to significant improvement in image accuracy when coupled with prestack depth migration psdm reflection tomography in the postmigration domain was introduced in the early 1990s for velocity estimation in seismic reflection this robust accurate method is directly applicable in multifold gpr imaging at a contaminated waste facility within the u s department of energy s hanford site in washington the method is used to identify significant lateral and vertical velocity heterogeneity associated with infilled waste pits using both the psdm images and velocity models in interpretation a paleochannel system that underlies the site and likely forms contaminant migration pathways is identified

the continent ocean transition at the southeastern margin of the south china sea abstract the south china sea formed by magma poor or intermediate volcanic rifting in the paleogene we investigate the structure of the continent ocean transition cot at its southern margin off nw palawan between the continental blocks of reed bank and the islands of palawan and calamian several surveys recorded by the bgr from 1979 to 2008 established a comprehensive database of regional seismic lines accompanied with magnetic and gravity profiles we interpret two major rifted basins extending in the ne direction across the shelf and slope separated by a structural high of non volcanic origin the continent ocean transition is interpreted at the seaward limit of the continental crust when magnetic spreading anomalies terminate some 80 100 km farther north the area in between displays extensive volcanism as manifest by extrusions that occasionally reach and cut the seafloor by dykes and by presumed basaltic lava flows occurring after break up the cot is highly variable along the nw palawan slope one type shows a distinct outer ridge at the cot with a steep modern seafloor relief the other type is characterised by rotated fault blocks bounded by listric normal faults ramping down to a common detachment surface half grabens developed above a strongly eroded pre rift basement the seafloor relief is smooth across this other type of cot we suggest the pre rift lithospheric configuration had major influence on the formation of the cot besides transfer zones volcanic domains confined to the north of competent crustal blocks correlate with the style of the cot gravity modelling revealed an extremely thinned crust across the shelf we propose a depth dependent extension model with crust being decoupled from mantle lithosphere explaining the discrepancy of subsidence observed across the south china sea region

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

structure and sediment budget of yinggehai song hong basin south china sea implications for cenozoic tectonics and river basin reorganization in southeast asia the temporal link between offshore stratigraphy and onshore topography is of key importance for understanding the long term surface evolution of continental margins here we present a grid of regional high quality reflection seismic and well data to characterize the basin structure we identify fast subsidence of the basin basement and a lack of brittle faulting of the offshore red river fault in the yinggehai song hong basin since 5 5 ma despite dextral strike slip movement on the onshore red river fault we calculate the upper crustal whole crustal and whole lithospheric stretching factors for the yinggehai song hong basin which show that the overall extension observed in the upper crust is substantially less than that observed for the whole crust or whole lithosphere we suggest that fast basement subsidence after 5 5 ma may arise from crustal to lithospheric stretching by the regional dynamic lower crustal mantle flow originated by collision between india eurasia and indian oceanic subduction below the eurasian margin in addition we present a basin wide sediment budget in the yinggehai song hong basin to reconstruct the sedimentary flux from the red river drainage constrained by high resolution age and seismic stratigraphic data the sediment accumulation rates show a sharp increase at 5 5 ma which suggests enhanced onshore erosion rates despite a slowing of tectonic processes this high sediment supply filled the accommodation space produced by the fast subsidence since 5 5 ma our data further highlight two prominent sharp decreases of the sediment accumulation at 23 3 ma and 12 5 ma which could reflect a loss of drainage area following headwater capture from the paleo red river however the low accumulation rate at 12 5 ma also correlates with drier and therefore less erosive climatic conditions

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

complex rifted continental margins explained by dynamical models of depth dependent lithospheric extension observations from a number of rifted margins e g south atlantic salt basin mid norwegian margin exmouth plateau reveal wide regions of extremely attenuated crust and depositional environments that indicate depth dependent lithospheric extension although the one dimensional thermal kinematic consequences of depth dependent extension are understood no comprehensive process based explanation for the complex style of these margins exists here we present self consistent numerical models of passive margin formation that explain the depth dependent extension the width of the margin its characteristic tripartite nature and why such margins are prone to deposition of evaporites under appropriate climatic conditions some features that are important to reproducing the observed characteristics include decoupling between upper and lower parts of the lithosphere during stretching contrasting wide and narrow extensional styles above and below the decoupling level and progressive focusing of crustal extension toward the rift axis

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

movement along a low angle normal fault the s reflector west of spain 1 the existence of normal faults that moved at low angles less than 20 has long been debated one possible low angle fault is the s detachment at the west galicia spain margin and thought to occur at the top of serpentinized mantle it is unlikely that s was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts it appears to have rooted beneath the conjugate flemish cap margin and it is similar to structures elsewhere that also appear to be rooted detachments here we analyze depth images to identify synrift sediment packages above s and use the geometry of these synrift packages to constrain the angle at which s both formed and remained active we find that s must have remained active at angles below 15 too low to be explained simply by the low friction coefficient of partially serpentinized peridotites instead we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed enabling moderately high fluid pressures to develop

subsurface radar propagation deconvolution the frequency dependence of attenuation is typically far greater for subsurface radar waves than for seismic waves since this frequency dependence causes the subsurface radar pulse shape to change as it propagates through the earth conventional wavelet deconvolution techniques are often inadequate to reconstruct the earth s reflectivity series the application of a time variant filter that undoes the effect of frequency dependent attenuation can substantially improve the resolution of subsurface radar data this filter is minimum phase with amplitude characteristics equal to the inverse of the attenuation function of the medium investigated preliminary tests on data from surveys over a water tank an underground mine stope and fractured granite suggest that good results can also be achieved by assuming attenuation is linearly proportional to frequency similar to a constant q model

velocity variations and water content estimated from multi offset ground penetrating radar the common midpoint cmp processing technique has been shown to be effective in improving the results of ground penetrating radar gpr profiling when radar data are collected with the cmp multioffset geometry stacking increases the signal to noise ratio of subsurface radar reflections and results in an improved subsurface image an important aspect of cmp processing is normal moveout velocity analysis our objectives are to show the effect of multiple velocity analyses on the stacked radar image and particularly to demonstrate that this velocity information can also be used to determine subsurface water content most gpr surveys are very limited in spatial extent and assume that within the survey range radar velocity structure in the shallow subsurface can be adequately approximated by a single velocity function in data processing in this study we show that variation in radar velocity can be quite significant and that the stacked profile improves as the number of velocity analysis locations is increased interval velocities can be calculated from the normal moveout velocities derived in the cmp velocity analysis with some reasonable assumptions about subsurface conditions necessary for radar propagation interval velocity can be converted to an estimate of volumetric water content therefore by collecting gpr data in the multioffset cmp geometry not only is the radar profile improved but it also allows for an interpretation of subsurface variation in water content we show the application of these techniques to multioffset gpr data from the chalk river test area operated by atomic energy of canada limited