hydrogeologic role of geologic structures part 1 the paradigm abstract grouting to reduce fracture permeability is one option for minimizing ground water inflow to a large acid producing lead zinc mine in fractured metamorphic rock in north idaho for grouting to reduce mine water inflow effectively the hydrogeologic characteristics of the various scales of structurally controlled fracturing must be identified and a conceptual model of the ground water flow system must be developed this paper is the first of two papers which use fracture mapping geologic structural mapping and a series of underground hydraulic stress tests to develop a conceptual model of structurally controlled ground water flow in the vicinity of the mine these data were collected in an effort to identify order within the structurally controlled spatial permeability distribution geologic structural discontinuities ranging from joints to faults that extend for several miles form a geologic structural hierarchy in the rock surrounding the mine the tiers of the hierarchy control ground water flow into the mine at different scales the most prominent faults control ground water inflow on the scale of the entire mine various levels of hydraulic continuity are evident within the rock mass bounded by two of the most prominent faults the highest level of hydraulic continuity appears to be associated with a set of sub parallel steeply dipping faults minor faults joints and relict bedding planes to a limited extent connect the fractures of this set and form a lower level of hydraulic continuity the next lower level of hydraulic continuity within the hierarchy is related to a major fault that is characterized in drill core by abundant gouge the hydraulic continuity of the matrix within the unfractured quartzite is the lowest level within the hierarchy these levels constitute the components of the order within the spatial permeability distribution that we have interpreted from the structural and hydraulic stress test data

factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

scotland s denudational history an integrated view of erosion and sedimentation at an uplifted passive margin denudational history is commonly reconstructed from basin sediments derived from the denuded source area and less frequently from the source area itself northern britain is an important source area for the surrounding sedimentary basins and this paper reviews the erosional history of scotland from devonian time to the present using evidence both from onshore geology and geomorphology and from patterns of sedimentation in surrounding basins cover rocks were extensive in scotland during late palaeozoic time but the persistence of sediment source areas within the upland areas of scotland makes it unlikely that basement highs were ever completely buried and depths of post devonian erosion of basement have been correspondingly modest 1 2 km during mesozoic time scotland experienced several major erosional cycles beginning with uplift reactivation of relief and stripping of cover rocks followed by progressive reduction of relief through etchplanation and culminating in extensive marine transgressions in late triassic late jurassic and late cretaceous time mid paleocene pulses of coarse sediment to the moray firth basin coincided with major uplift this uplift was associated with major differential tectonics within the highlands with warping and faulting along the margins of the minch and the inner moray firth basins tectonic activity was renewed on a lesser scale in late oligocene time and continued into late neogene time differential weathering and erosion under the warm to temperature humid climates of neogene time created the major elements of the preglacial relief with formation of valleys basins scarps and inselbergs features often closely adjusted to lithostructural controls and in some cases with precursors that can be traced back to devonian time the history that can be read from the onshore region complements the source area history interpreted from sedimentary basins derived from these areas

crustal taper and topography at passive continental margins terra nova 23 349 361 2011 abstract the long term evolution of kilometre s high seaward facing escarpments at passive margins is linked directly to the crustal thinning gradient the development of post rift faults in the onshore parts of the margin and the associated distribution of drainage patterns landscape types and sediment dispersal patterns reflect this linkage for the seismically well imaged norwegian margin as well as for a number of passive margins worldwide we identify a scaling relationship that correlates escarpment height with the distance to the highly extended distal margin or more specifically the taper break the highest escarpments and the most asymmetric margin topography are found inboard of sharply tapering crystalline crust independent of margin age conversely escarpments are lower where the taper is gentle thus the topography of passive margins does not primarily reflect age magmatism climate or mantle convection but more probably the response to loading of the laterally variable fault controlled architecture of the crystalline crust

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

extensional inheritance and surface processes as controlling factors of mountain belt structure surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts the first order effects of these parameters have been studied extensively throughout the last decades but their relative importance remains notoriously difficult to assess and document we use lithospheric scale plane strain thermomechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion surface processes are modeled through the combination of a simple sedimentation algorithm where all negative topography is filled up to a prescribed reference level and an elevation dependent erosion model our results show that 1 extensional inheritance facilitates the propagation of basement deformation in the retro wedge and 2 increases the width of the orogen 3 sedimentation increases the length scale of both thin skinned and thick skinned thrust sheets and 4 results in a wider orogen 5 erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro wedge a comparison of the modeled behaviors to the high atlas the pyrenees and the central alps three extensively studied natural examples characterized by different degrees of inversion is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure

drainage basin sensitivity to tectonic and climatic forcing implications of a stochastic model for the role of entrainment and erosion thresholds long term average rates of channel erosion and sediment transport depend on the frequency magnitude characteristics of ood ows that exceed an erosion threshold using a poisson model for rainfall and runoff analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold solutions are derived and numerically tested for three erosion transport formulas the howard kerby shear stress incision model the bridge dominic sediment transport model and a generic shear stress sediment transport model results imply that non linearity resulting from threshold effects can have a rst order impact on topography and patterns of dynamic response to tectonic and climate forcing this non linearity becomes signi cant when fewer than about half of ood events are capable of detaching rock or sediment predicted morphology and uplift gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope linear or shear linear erosion laws these results imply that particle detachment thresholds are not details that can be conveniently ignored in long term landscape evolution models copyright 2004 john wiley sons ltd

hydromechanical behavior of a deformable rock fracture subject to normal stress lbl 12297 preprint to be published in the journal of geophysical research hydromechanical behavior of a deformable rock fracture subject to normal stress y w tsang and p a witherspoon january 1981 two week loan copy this is a library circulating copy which may be borrowed for two weeks for a personal retention copy call tech info diu is ion ext 6782 prepared for the u s department of energy under contract w 7405 eng 48

borehole dilution tests in the vicinity of an extraction well abstract a mathematical solution is derived for a borehole dilution test conducted in the proximity of an extraction well under conditions where the theis assumptions are applicable the solution is similar to the steady state velocity case except for a factor that accounts for the transient response of the aquifer to pumping curves calculated from the derived solution show a delay in response that increases with the specific storage of the aquifer and with the radius from the extraction well the delay decreases with increasing values of hydraulic conductivity and aquifer thickness deviations of the response curves from the straight line steady state velocity case can be used to calculate the hydraulic diffusivity while the limiting slope at late time can be used to calculate groundwater flux when borehole dilution tests are conducted at several intervals over the thickness of the aquifer under steady state velocity conditions k z can in principle be calculated using several equations depending on the type of information available two existing methods to account for the distortion of the flow lines near the open test interval are related to well construction and development reasonable estimates of borehole factors can only be obtained for minimally developed wells

tidally induced cross frontal mean circulation analytical study an analytical model is developed to study the tidally induced mean circulation in the frontal zone four distinct forcing mechanisms are identified which result in the generation of the counterclockwise bernoulli cell the clockwise ekman cell the clockwise frontal cell and the stokes drift facing in the direction with the shallow water to the left the decomposition of the cross frontal circulation provides a dynamical framework for interpreting and understanding its complex structure to illustrate the underlying physics three model configurations are considered pertaining to a homogenous ocean and winter and summer fronts for a homogeneous ocean the circulation is dominated by three cells for the winter front the offshore bernoulli cell is strengthened and for the summer front two counterrotating cells are found in the vertical direction associated with the two branches of the front the dependence of the cell structure on the ekman burger and other dimensionless numbers is examined

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

relating cenozoic north sea sediments to topography in southern norway the interplay between tectonics and climate abstract about 482 000 km 3 of sediment ca 24 m ma accumulated in the north sea during the cenozoic early cenozoic sedimentation was likely due to uplift of the circum north atlantic landmasses related to continental break up kilometre scale transient uplift and in some areas permanent uplift generated sources for progradational influx of clastic sediments from scotland the shetland platform and to a lesser degree southwestern norway the eocene sedimentation pattern was similar to the palaeocene with lower rates of accumulation associated with flooding and tectonic quiescence sediment influx from the shetland platform continued throughout the cenozoic while supply from southern norway increased markedly around the eocene oligocene coeval with the greenhouse icehouse transition mass balance calculations of sediment and eroded rock volumes suggest that while some topography along the western margin of norway may be pre cenozoic significant uplift of the main paleic surface in southern norway occurred around the early oligocene sedimentation rates were almost ten fold higher than the cenozoic average in the plio pleistocene slightly higher than the global average mass balance calculations indicate that plio pleistocene erosion over deepened a pre existing topography

progress on upwelling studies in the china seas east asian marginal seas surrounding china exhibit rich ocean upwelling mostly in response to the southwesterly summer monsoon upwelling in the china seas namely the south china sea the taiwan strait the east china sea the yellow sea and the bohai sea has become increasingly important because the potential changes in the upwelling may have dramatic ecosystem socioeconomic and climate impacts this paper reviews the progress of upwelling studies in the china seas since the year 2000 by presenting the principal characteristics and new understanding of 12 major upwelling regions in the china seas upwelling exhibits long term variability at intraseasonal to multi decadal scales as well as short term variability frequently caused by tropical cyclones it is also associated with the el nino southern oscillation local environmental variation and biogeochemical factors the coastal upwelling around hainan island and the upwelling or cold dome northeast of taiwan island are specifically highlighted because they have attracted great interest for decades this paper summarizes upwelling mechanisms in terms of wind topography tide stratification and background flow with applications mostly to the china seas finally we propose some topics for future upwelling research i e potential intensification of coastal upwelling under global climate change downwelling intrusion of upwelling into coastal embayments and the influence of upwelling on fishery and biogeochemical processes

upwelling off yangtze river estuary in summer 1 the mechanisms of upwelling off the yangtze river estuary yre and in the adjacent waters in boreal summer are studied using numerical modeling first the persistent feature of this phenomenon is confirmed using cruise observations satellite sea surface temperature sst and sst climatologic data then the masnum marine science and numerical modeling wave tide circulation coupled numerical model is employed to simulate the upwelling patterns on the basis of the simulation a set of numerical experiments are designed to explore the main mechanisms inducing the upwelling the results suggest that tidal mixing plays a predominant role in inducing the upwelling in offshore waters strong tidal mixing results in considerable horizontal density gradient across tidal fronts upwelling is induced as a branch of the secondary circulation which is stimulated by the cross frontal density gradient topography also exerts profound influences on upwelling by steering bottom currents to ascend upward and regulating tidal fronts in both location and intensity besides the tides and topography other dynamical factors also alter the strength of upwelling locally the yangtze river discharge yrd and taiwan warm current twc account partly for the upwelling off the yre and near zhoushan islands respectively the influence of wind on upwelling is small in the coastal waters near zhoushan islands the wind forcing exerts negative influences on upwelling by weakening the encroachment of twc onto the continental shelf which may exceed the positive effects of ekman pumping

twin lake tracer tests setting methodology and hydraulic conductivity distribution following evaluation of three potential sites a fluvial sand aquifer with high 0 7 1 6 m d 1 linear average groundwater flow velocities was instrumented with a fully penetrating injection well and 82 monitoring installations over a 40 m flow path length two natural gradient tracer migration experiments have employed pulse injections of a nonreactive radioisotope 131i in a study of hydraulic conductivity structure and dispersive characteristics of the flow system the use of a gamma emitting radionuclide in these experiments allowed measurement of tracer distribution by scanning fully penetrating boreholes with a scintillation detector this technique provided many advantages in data collection over conventional tracers including unprecedented resolution of the vertical distribution of tracer measurements without pumping and the attendant concerns for flow system perturbation and minimal manpower for sample analysis tracer breakthroughs at monitors along the axis of flow revealed the presence of heterogeneities in aquifer hydraulic conductivity at two scales over the 40 m path length six regions with hydraulic conductivities substantially different from adjacent strata were identified thicknesses of these regions varied from 0 2 to more than 3 m their lateral extents in the direction of groundwater flow ranged from 15 m to more than 40 m smaller strata with an average thickness of 0 34 m and lengths of less than 10 m were detected within all six of the hydraulic conductivity regions but there was no evidence of increases in local dispersivity with increased transport distance as a result of these variations the six hydraulic conductivity regions do however have measurable effects on whole aquifer dispersion stratification characteristics of the tested aquifer believed to have been deposited in a sandy braided river are consistent with those observed in similar modern environments estimation of the magnitude and spatial structure of hydraulic conductivity variations in aquifers of concern would be greatly aided by studies of the stratification characteristics in well exposed modern or lithified sedimentary analogues

why intracontinental basins subside longer 3 d feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

erosion on a line abstract an erosion model is proposed to calculate erosion rates for plane strain models in which the earth s surface is represented on a line the fundamentals of river erosion networks are captured by two principles hack s law which describes the drainage area structure of river network and a stream power erosion law which describes the rate of incision of a river for a simple morphology of parallel transverse rivers with rectangular drainage basins this allows the earth s surface to be parameterized by two heights the trunk stream channel height and the interfluvial ridge height the resulting expressions are solved for the simple cases of constant uplift rate and a constant mean slope as occurs in critical wedge problems in the latter case the uplift rate is variable and changes in space so that the trunk channel elevation and the interfluvial ridge elevation average to maintain a constant mean slope a general numerical solution is presented for application to any numerical model with arbitrary surface velocity variable rock erodibility and precipitation this algorithm is coupled to a plane strain plastic deformation model to demonstrate the utility of the model

analysis and interpretation of single well tracer tests in stratified aquifers this study deals with the definition and measurement of the dispersive properties of a stratified aquifer based on the single well tracer test knowledge of such dispersive properties are of fundamental importance to the evaluation analysis and simulation of contaminant migration in groundwater a subject of great interest in recent years in the single well test tracer is pumped into the formation for a period of time and then pumped out concentration data are obtained from the injection withdrawal well and from one or more sampling observation wells which may be multilevel our analysis of such a test is based on a lagrangian eulerian numerical model which considers the depth dependent advection in the radial direction and local hydrodynamic dispersion in the vertical and radial directions results show that the movement of an injected tracer in a stratified aquifer may be accurately simulated without resorting to the use of a scale dependent full aquifer dispersivity if the flow field is known in sufficient detail when the advection process is simulated accurately the values of local dispersivity will be small constant and on the order of those measured in the field or laboratory at individual levels in the aquifer the full aquifer breakthrough curves measured in observation wells in a single well test in a stratified aquifer are determined by the hydraulic conductivity profile in the region between the injection withdrawal well and the observation well if the travel distance between these wells is typical of most test geometries however the relative concentration versus time data recorded at the injection withdrawal well during the withdrawal phase is primarily a measure of mixing in the aquifer due to local dispersion which has taken place during the experiment the amount of mixing will depend on both the hydraulic conductivity distribution in the aquifer and the size of the experiment as the experiment scale increases the effects of local vertical dispersion will become larger compared to the effects of local radial dispersion local vertical dispersion will cause a solute traveling in a high conductivity layer in an aquifer to migrate into adjacent low conductivity layers where its movement will be relatively slow in comparison in the initial design of a tracer test it is important to have some idea of the type of nonhomogeneity with which one is dealing more information of a broad nature concerning the types or classifications of nonhomogeneities that exist in natural aquifers would be very useful future research in this area is needed

comment on double porosity models for a fissured groundwater reservoir with fracture skin by allen f moench this paper constitutes a commendable effort to summarize and expand the theoretical basis for the analysis of drawdown data for fractured terrane that can be characterized by matrix blocks the method applies porous media theory to fractured rocks by defining saturated hydraulic conductivity of the fracture system as the product of the hydraulic conductivity by the ratio of the total volume of the openings to the bulk volume of the rock block or fracture according to the method described by gringarten 1982 moench 1984 incorporated the effect of a mineralized film skin on the wall of the fractures where the skin has its own separate hydraulic conductivity and its own thickness the geometries of the block are considered in the controlling equation for the fissured network via the control of the geometry on permeability distribution and on boundary conditions in several respects this approach is similar to the standard approach for leaky aquifers when leakage is derived from storage in the confining layer except for the addition of the hydraulic properties and thickness of the mineralized layer on the walls of the fractures by varying the values of the hydraulic properties of the fracture system and the mineralized layer on the walls of the fractures virtually any drawdown curve can be simulated by the theory not surprisingly the drawdown curves produced by the method type curves are similar in shape to standard leaky aquifer curves within the influence of boundaries or with the influence of partial penetration the method could in fact produce curves identical to those of hantush 1960 see lohman 1979 given appropriate geometry and the absence of a skin on the fractures

how sediment promotes narrow rifting application to the gulf of california 1 if the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension then sedimentation should also affect that mode we propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning allowing a rift to extend more easily in a narrow rift mode we examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two dimensional regional scale numerical experiments of extending thick continental crust with varied initial thermal conditions geometries rheological parameters and sedimentation properties depending on initial model conditions the onset of rifting in thick continental crust occurs in the narrow rifting wide rifting or core complex mode with continued extension all cases eventually transition to a narrow rifting mode for a system in wide rift mode moderate to fast sedimentation shortens the time needed to transition to a narrow rift in the gulf of california changes in extensional style correlate with sediment thickness with an earlier transition to narrow rifting in the north versus the south we compare our model results to the extensional history of the gulf of california where sediments may have caused the northern and north central domain of the gulf delfin tiburon and guaymas segments to transition to narrow rifting before the south central domains alarcon segment

coastal upwelling in the east china sea in winter 1 the dynamic mechanisms of the upwelling off the east china sea ecs coast in wintertime are studied first the upwelling signals off the ecs coast are identified by the observed temperature salinity nutrients and dissolved oxygen data obtained during the cruises in january 1999 the masnum wave tide circulation coupled model is then employed to simulate the hydrography of the ecs comparisons between the simulations and observations show that the model performance is satisfactory on the basis of successful simulation four numerical experiments are conducted to investigate the upwelling mechanisms the results suggest that the density or salinity front which separates the inshore low salinity coastal water and the offshore taiwan warm current twc is the primary inducement for the upwelling owing to strong density gradient the baroclinic pressure gradient force pgf is quite large near the frontal zone and this pgf elicits an upwelling branch along the topography slope wind twc and tide affect the density front in extension and intensity thus exerting subsidiary influences on the upwelling according to ekman s theory the northerly monsoon is downwelling favorable however the net effects of wind on the upwelling off the ecs coast in winter are positive because it drives the changjiang river diluted water cdw flowing southward and forms the density front similarly the resultant effects of twc on the upwelling are negative for obstructing the pathway of cdw tide contributes to the upwelling because tidal mixing facilitates the expansion of cdw

upwelling induced by the frictional stress curl and vertical squeezing of the vortex tube over a submerged valley in the east china sea we conducted a process oriented modeling study to investigate the characteristics and dynamics of the prominent upwelling over a vast submerged valley in the east china sea ecs the valley is inversely funnel shaped with the west bank and the east bank oriented in the north south direction a cross bank upward transport occurred along the west bank it intensified northward and peaked around the head of the valley an along bank southward pressure gradient force pgf formed the cross bank geostrophic transport for the upwelling over the valley the pgf reached its maxima at the head of the valley our momentum and vorticity dynamic analyses revealed that a bottom stress curl mainly contributed the pgf along the west bank at the same time both the bottom stress curl and the nonlinear vorticity advection contributed to the pgf around the head the bottom stress curl was due to the bottom shear vorticity of the coastal current and the curvature vorticity around the head the nonlinear vorticity advection formed because of the vertical squeezing of vortex tube as the current flowed over the valley the nonlinearity mainly affected the pgf around the head whereas the bottom stress curl contributed to the pgf over the entire valley the ratio of the nonlinear to frictional contributions to the pgf increased as the coastal current intensified our study demonstrates that the pgf that drives the upwelling over the valley is the combined result of the nonlinearity due to vertical squeezing of vortex tube and bottom frictional effects

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

an analysis of dispersion in a stratified aquifer the dispersion of a conservative solute produced as a result of vertical variations of hydraulic conductivity in a horizontal stratified aquifer of finite thickness is analyzed by applying the moment method of aris to solve the governing advection dispersion equation describing mass transport in the analysis it is assumed that the aquifer is of constant thickness and of infinite lateral extent the hydraulic conductivity is a known function of the vertical coordinate only and the flow is unidirectional parallel to the stratification the applicable aris moment equations are developed in a suitable nondimensional form analytical solutions are obtained for the zeroth and first moments and for the time derivative of the second moment of the longitudinal concentration distribution for the case of an instantaneous plane source for several idealized hydraulic conductivity profiles parabolic linear step function and cosine profiles and their even periodic extensions the analysis gives the time dependent variation of the longitudinal macrodispersivity for these idealized cases throughout the transient development of the dispersion process the results of the analysis are applied to a field measured hydraulic conductivity profile and predicted values of the longitudinal macrodispersivity are compared with field results an important conclusion from the analyses is that nonuniformities in the hydraulic conductivity profile which persist over long distances may produce rather large values of longitudinal macrodispersivity which are comparable to those observed in some aquifers and which are much larger than those predicted by some previous stochastic analyses implications of the analytical results for field dispersion problems are discussed

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

three dimensional numerical simulations of crustal systems undergoing orogeny and subjected to surface processes as several modeling studies indicate the structural expression and dynamic behavior of orogenic mountain belts are dictated not only by their rheological properties or by far field tectonic motion but also by the efficiency of erosion and sedimentation acting on its surface until recently numerical investigations have been mainly limited to 2 d studies because of the high computational cost required by 3 d models here we have efficiently coupled the landscape evolution model cascade with the 3 d thermomechanically coupled tectonics code fantom details of the coupling algorithms between both codes are given we present results of numerical experiments designed to study the response of viscous plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation in particular we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen we show that increasing the efficiency of fluvial erosion increases the frontal thrust angle which in turn decreases the width of the orogen in addition the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not this illustrates the strong coupling between tectonics and surface processes we also demonstrate that an along strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen in both the across strike and along strike directions overall our results suggest that surface processes by enhancing localization of deformation can act as a positive forcing to topographic building

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

factors controlling the distribution and transport of trichloroethene in a sand aquifer hydrogeology and results of an in situ transport experiment groundwater underlying the perth metropolitan area was found to be contaminated by extensive plumes of trichloroethene tce ammonium and sulphate and regions of high dissolved organic carbon concentration at the study site approximately 600 m downgradient of the presumed source area depth specific sampling in the vertical plane across the plumes showed tce concentrations up to 1300 g l 1 and ammonium and sulphate concentrations up to 90 and 470 mg l 1 respectively the plumes were approximately 100 150 m wide and between 3 9 m thick no significant degradation of tce was observed also depth specific sampling suggests that separate tce plumes are present within the shallow aquifer which may be indicative of multiple tce sources depth specific sampling enabled a better definition of the plumes which is important for possible risk analysis or remediation to determine the transport characteristics of the tce plume a natural gradient field transport experiment using deuterium labelled tce tcedl and bromide was carried out over a displacement distance of 13 m tcedl was stable and non retarded no in situ transport experiment making use of deuterated tce has been reported previously also no previous field experiment has shown tce to move conservatively modelling of the field transport experiment indicated a longitudinal dispersivity of 0 005 m which is the lowest longitudinal dispersivity ever reported for the scale of the experiment i e for a vertical interval of 0 8 m and a horizontal distance of 13 m although not measured the vertical transverse dispersivity is expected to be less than the longitudinal dispersivity the aquifer at the site therefore is characterised by exceptionally little heterogeneity no aquifer however is homogeneous and the tracer experiment revealed the aquifer to be hydraulic conductivity stratified the data suggest that the longitudinal dispersivity estimated from depth integrated concentrations will increase with both the vertical and the horizontal scale and is not expected to approach a constant value hydrogeological investigations indicated that horizontal transverse spreading of the plume was negligible since annual variations in the flow direction 3 were minor therefore any significant transverse spreading of the plumes must result from disturbances of the mean advective flow e g due to localised recharge or extensive pumping these effects are site specific preventing the use of generalised dispersion coefficients together with the results of the transport experiment these data indicate that tce within the acquifer moves as a conservative compound is highly mobile and that high concentrations will be preserved to substantial distances downgradient of the source areas

performance analysis and simulation of a two well tracer test at the mobile site during the late summer of 1984 a two well tracer test was performed which involved travel distances of 38 3 m to 90 m a bromide tracer slug was injected at one well while groundwater was simultaneously pumped from the other well the two well test which required 32 days to complete was then simulated by geotrans inc using a three dimensional advection dispersion model with a longitudinal dispersivity of 0 15 m a transverse dispersivity of 0 05 m and the permeability distribution being measured independently in a single well test single well test results which involved a travel distance of 5 5 m indicated that the hydraulic conductivity varied by about a factor of 4 throughout the aquifer thickness the two well test simulation made without knowledge of the test results and without any calibration was remarkably good local hydrodynamic dispersion was negligible so that the concentration versus time breakthrough at the withdrawal well could be simulated just as well with a quasi three dimensional advection model having zero hydrodynamic dispersion this implies that over the travel distances applicable to the two well test the spreading of the slug in the aquifer depended mainly on macroscopic velocity variations that were quantifiable in terms of the inferred permeability distribution this result which pertains to an aquifer in a fluvial terrace deposit of quaternary age is similar to that observed by pickens and grisak in a sandy glaciofluvial aquifer near the chalk river nuclear laboratories in canada such flow systems can result in fully three dimensional plumes with high concentrations of contaminant moving over large horizontal distances in the higher permeability zones

holocene sedimentary evolution and hypoxia development in the subaqueous yangtze changjiang delta china abstract this study investigates holocene sedimentary evolution and hypoxia development using borehole cores cjk06 and cjk09 in combination with other published core data based on lithology and microfossil benthic foraminifera characteristics seven types of sedimentary facies were identified from the base upward river tidal flat tide influenced river transgressive lag estuary inner shelf and prodelta isochronous correlation among the cores was established relying on accelerator mass spectrometry 14c dates facies succession indicates that a weak progradation occurred in coastal environments 12 10 ka bp due to the younger dryas induced deceleration of sea level rise rapid deposition locally occurred in the southern marginal area of tide dominated estuary environments 10 8 ka bp likely due to the junction of the yangtze and qiantangjiang river currents and marine current induced fluid mudflows prevailed in the shallow marine environments 8 1 ka bp with the cooling climates of 5 1 ka bp furthermore prodelta and shallow marine environments co occurred with an uneven progradation of the delta within the last 1 ka while deposition occurred just inside the more southern delta influenced area the occurrence of euryhaline benthic foraminifera suggests that an enhanced freshwater discharge of the mid holocene 8 5 ka bp was followed by a sharp decline in the late holocene 5 1 ka bp with climate change the occurrence of cold water benthic foraminifera indicates a severe cold water condition during the mid holocene due to the intrusion of upwelling currents triggered by the propelling force of warm currents and summer monsoon winds in addition the occurrence of low oxygen foraminiferal assemblages reveals that hypoxia has developed since 10 ka bp in stages consistent with the sedimentary evolution localized hypoxia formation coincident with the southern depocenter during the early holocene 10 8 ka bp severe hypoxia with enhanced freshwater discharge and upwelling current intrusion during the mid holocene and weakened hypoxia of the late holocene mainly due to the sharp decline in freshwater discharge within 1 ka bp freshwater discharge from the approaching river mouth and increased nutrient inputs from enhanced human activities on land have contributed to the prevalence of hypoxia especially in the southern deltaic area overall it was revealed that the freshwater discharge modulated with climate variations and delta progradation plays a primary role in controlling the sedimentary evolution and hypoxia development during the mid late holocene

the influence of rifting on escarpment migration on high elevation passive continental margins 1 using numerical models that couple surface processes flexural isostasy faulting and the thermal effects of rifting we show that fault bounded escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to survive as retreating escarpments if the lower crust under the rift flank is substantially stretched in this configuration a drainage divide that persists through time appears landward of the initial escarpment in a position close to a secondary bulge that is created during the rifting event at a distance that depends on the flexural rigidity of the upper crust moreover the migration of the escarpment to the secondary bulge occurs when the pre rift topography dips landward otherwise the evolution of the escarpment is guided by the pre existing inland drainage divide to illustrate this new mechanism for the evolution of passive margins we study the examples of southeastern australia and southeastern brazil we propose that a pre existing inland drainage divide with rift related flank uplift can produce the double drainage divide observed in southeastern australia on the other hand we conclude that it is possible that the serra do mar escarpments on the southeastern brazilian margin originated as a secondary flexural bulge during rifting that persisted through time in both cases the retreating escarpment scenario is unlikely and the present day margin morphology can be explained as resulting from rift related vertical motions alone without requiring significant post rift rejuvenation

the effect of relay ramps on sediment routes and deposition a review abstract a better understanding of the interplay between rift basin evolution and sediment transport paths can improve the success rate of locating hydrocarbon reservoirs at passive rift margins this paper reviews the current knowledge and suggests future research directions relay ramps at extensional basin margins can form drainage entry points if tectonic activity exceeds sedimentation and incision rates leading to a diversion of sedimentary flow paths towards the ramp during base level lowstands channels and canyons may incise into the relay ramp and provide flow paths from the basin margin into the basin their orientation and geometry mainly develops as a response to faulting and fracturing and their activity is influenced by base level fluctuations flow constraints such as channels parallel to the ramp axis direct flows to the foot of the relay ramp where sediment accumulates in response to the basin topography in subaqueous settings however turbidity currents are likely to spill at least partly over channel levees and flow down the fault slope into the basin depositing its load adjacent to the en echelon boundary faults channels and canyons with oblique and perpendicular orientations to the boundary faults can funnel flows down the hanging wall fault onto the basin floor by passing the relay ramp the prevalent basinward tilt of relay ramps can direct unconstrained subaqueous gravity flows also directly into the basin in subaerial settings the duration of channel activity in relation to relay ramp evolution strongly depends on the ratio between flow incision rates and tectonic uplift drainage direction on the footwall may revert if footwall uplift exceeds incision rates and the feeding of former depocentres terminates in the course of rift margin development relay ramp bounding faults may link causing the breaching of relay ramps and eventually their burial the effect of continued rifting on ramp remnants and associated syn rift deposits however is not sufficiently known and needs further investigation

some observations on the scale dependence of permeability by pumping tests 1 the dependence of scale in estimates of permeability or transmissivity in pumping tests is addressed in terms of internal architecture the need for using geologic models that go beyond treating heterogeneity as a multinormal logarithmic field is noted by using stratification as a vehicle and simulating flow the role of the internal architecture of the aquifer is demonstrated specifically transient data generated with the help of a stratified model are evaluated without consideration of internal architecture or geological imprint and the consequences on estimates of aquifer properties are examined particularly in terms of scale evidence of scale effects should suggest the need for evaluating pumping tests with models that incorporate geologic considerations

global sediment thickness data set updated for the australian antarctic southern ocean we present a new 5 min sediment thickness grid for the australian antarctic region 60 e 155 e 30 s 70 s new seismic reflection and refraction data have been used to add detail to the conjugate australian and antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously on the margins sediment thickness estimates were computed from velocity depth functions from sonobuoy refraction velocity solutions ground truthed against seismic reflection data for the southeast indian ridge abyssal plains sediment thickness contours from geli et al 2007 were used the new regional minimum sediment thickness grid was combined with the global national geophysical data center ngdc sediment grid to create an updated global grid even using the minimum estimates sediment accumulations on the extended australian and antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the ceduna basin compared to the global ngdc compilation of sediment thickness data

sedimentary signals of the upwelling along the zhejiang coast china abstract the muddy deposits of the zhe min coastal area have attracted much recent research interest however the sedimentary processes that dominate along the zhejiang coast remain controversial determining sedimentation rates is important for understanding sedimentary processes and depositional patterns in the present study 23 vibrocores were collected from the muddy area along the zhejiang coast to analyse their sediment accumulation rates over the past 100 years using 210pb geochronology three types of typical excess 210pb profiles were identified in the sediment vibrocores the spatial distribution of the sedimentation rates derived from the 23 vibrocores as well as previously published data demonstrated that the middle part of the study area around 29 n experienced relatively high sedimentation rates and that the rate of deposition gradually decreased landwards and seawards the location of high sedimentation rates is approximately consistent with that of the holocene maximum thickness deposition finest surface sediments and high concentration of chlorophyll resulting from the existence of upwelling along the zhejiang coast besides the fluvial inputs

a very efficient o n implicit and parallel method to solve the stream power equation governing fluvial incision and landscape evolution abstract we present a new algorithm to solve the basic stream power equation which governs channel incision and landscape evolution in many geomorphic settings the algorithm is highly efficient because computation time increases linearly with the number of points used to discretize the landscape and is ideally suited to parallelization it is also unconditionally stable because it uses an implicit scheme for the time integration of the landscape evolution equation which means that large time steps can be used without sacrificing accuracy in this paper we describe the algorithm and present results that demonstrate its efficiency and accuracy

modes of extensional faulting controlled by surface processes we investigate the feedbacks between surface processes and tectonics in an extensional setting by coupling a 2 d geodynamical model with a landscape evolution law focusing on the evolution of a single normal fault we show that surface processes significantly enhance the amount of horizontal extension a fault can accommodate before being abandoned in favor of a new fault in simulations with very slow erosion rates a 15 km thick brittle layer extends via a succession of crosscutting short lived faults heave 10 km using simple scaling arguments we quantify the effect of surface mass removal on the force balance acting on a growing normal fault this leads us to propose that the major range bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes

tectonic and climatic controls on rift escarpments erosion and flexural rebound of the dhofar passive margin gulf of aden oman we investigate the respective roles of climatic parameters and the flexural rigidity of the lithosphere in the erosion history and behavior of two adjacent rift escarpments along the northern coast of the gulf of aden in oman at this 25 myr old passive margin we define a type 1 scarp which is high sharp crested and has retreated 25 30 km inland from its master fault and a type 2 scarp which exhibits a more rounded profile lower relief and still coincides with its mapped normal fault trace since about 15 ma the margin has been seasonally affected by monsoon precipitation but with contrasting effects at the type 1 and type 2 escarpments depending on the position of the intertropical convergence zone in the geologic past during peak monsoon conditions both scarps experienced heavy rainfall and runoff whereas during monsoon starved conditions such as today the type 2 scarp experienced a foggy moist climate while the type 1 scarp remained much drier in order to assess the relative effects of climate and flexural parameters on the present day morphology of the dhofar margin we present onedimensional numerical models of erosion and flexure along two profiles representative of the type 1 and type 2 scarps unlike most surface process models previously published where present day topography is the only criterion by which to evaluate the quality of model outputs model behavior here is additionally constrained by independent estimates of denudation provided by geological cross sections well defined fault traces and other stratigraphic markers the best fitting models indicate that the type 1 escarpment formed under relatively arid climatic conditions and was affected by significant erosion recession and flexural uplift due to a low 7 km effective elastic thickness in contrast the morphology of the type 2 fault scarp was smoothed by a more humid climate but a high effective elastic thickness 15 km prevented it from uplifting or receding in addition we show that the sedimentary load acting at the foot of the escarpments exerts significant influence on their morphological evolution though this parameter is often neglected in other scarp evolution models

slip acceleration on normal faults due to erosion and sedimentation results from a new three dimensional numerical model coupling tectonics and landscape evolution abstract although it is well accepted that mass redistribution on earth s surface due to erosion and sedimentation affects the rate of crustal deformation the influence of surface processes on the slip behaviour of individual faults remains poorly understood here we use the new tool casqus which combines landscape evolution modelling with three dimensional tectonic models to investigate how erosion and sediment deposition affect fault behaviour in extensional tectonic regimes a systematic set of experiments with models consisting of an isostatically responding elastic upper crust containing a single normal fault shows that the slip accumulation on the fault is accelerated by erosion and sedimentation depending on fault geometry extension rate and parameters controlling erosion and sediment deposition the slip rate of the normal fault is up to 15 higher when surface processes are taken into account the slip acceleration can be explained by an increase in the differential stress caused by erosion of the fault footwall and sedimentation in the hanging wall which alter the faulting induced flexure of the crust we observe a similar slip rate increase in a more complex model of a growing horst bounded by three faults of different length our results show that a positive feedback exists between surface processes and tectonics and imply that geologically derived fault slip rates that integrate over several earthquake cycles may contain a contribution from surface processes

a study of the dynamic factors of the summer time upwelling in the tsushima warm current region the dynamic factors governing the summer time upwelling along the japanese shelf area and its structures are examined using an eddy resolving ocean general circulation model a sensitivity experiment suggests that the dominant factor is the topographic irregularities that enhance the upwelling along the shelf slope but the influences of the seasonal variation of the tsushima warm current twc inflow and wind stress are insignificant the dynamic analysis shows that the upwelling is a part of the secondary overturning circulation that appears along the shelf from april to october the dynamic structure shows the dominant term balance in essence i e a triad of the term of pressure gradient coriolis force and horizontal advection the local pattern of the upwelling is closely correlated to the horizontal variations of the shelf slope and the horizontal advection term the horizontal variation in the shelf topography is a key factor to determine the local pattern of the summer time upwelling

wave induced mixing in the upper ocean distribution and application to a global ocean circulation model 1 from the reynolds stress expression the wave induced vertical viscosity or diffusivity bv is defined which can be used as a parameter to estimate the strength of wave induced mixing in addition a parameter d5 is introduced to represent a wave induced mixing penetration depth the global distribution of bv averaged over the upper 20 m is calculated and its latitudinal transects in boreal summer and winter is discussed the results show that in summer the wave induced mixing is strong in the southern oceans south of 30 s and in winter it is strong in the north pacific and the north atlantic north of 30 n as well as in the southern oceans south of 40 s adding bv to the vertical diffusivity in a global ocean circulation model yields a temperature structure in the upper 100 m that is closer to the observed climatology than a model without the wave induced mixing

a tracer test in a shallow heterogeneous aquifer monitored via time lapse surface electrical resistivity tomography we illustrate a case study of a saline tracer test in a shallow highly heterogeneous aquifer monitored by means of surface time lapse ert the test was aimed at identifying the system s hydraulic properties some of the expected limitations of the method particularly caused by the strong decrease in ert resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location the vast heterogeneity of the system ranging in lithology from clay to gravel at a scale of meters to tens of meters reflects itself in the tracer migration and distribution over time the tracer is trapped in the low permeability regions and from these it is slowly released over time high resolution surface ert proves effective at picturing this system behavior over time the extreme heterogeneity is also a challenge in the attempt to translate bulk electrical

variations in temperature and salinity of the surface water above the middle okinawa trough during the past 37 kyr abstract east china sea ecs is an important climate modulator of east asia in the last glacial period the global sea level the path and strength of the kuroshio current experienced great changes combined with the variable volume of fresh run off input they made the hydrographic situation in the ecs quite different from nowadays based on high resolution alkenone sea surface temperature sst and oxygen isotope composition of planktonic foraminifera globigerinoides sacculifer we reconstructed paleo sea surface salinity sss of a long piston core dgks9604 retrieved from the middle okinawa trough of the eastern ecs the 18o and sst records display significant variations with global ice volume synchrony of the millennial scale climate events like yd and heinrich events of core dgks9604 to the ice core from the northern high latitudes and the synchroneity of deglacial warming with the bolling allerod warming suggests a strong coupling of the sst variations in the marginal pacific ocean to the climate of the north atlantic most likely through the asian monsoon atmospheric circulation the ecs documents lowest sst 22 c at 26 cal kyr bp and 3 c sst difference between the full glaciation 26 to 19 cal kyr bp and mid to late holocene 6 cal kyr bp present the overall long term hydrographic variations in the middle okinawa trough are controlled by temporal and spatial variations in i the intensity and position of the kuroshio current ii intensity of the asian summer monsoon and iii sea level fluctuations coupled with ecs topography saline surface water dominated over the middle okinawa trough during early pre glaciation 37 to 31 cal kyr bp last deglaciation 19 to 11 6 cal kyr bp and mid to late holocene 6 cal kyr bp present whilst freshened surface water prevailed during the late pre glaciation 31 to 26 cal kyr bp full glaciation 26 to 19 cal kyr bp and early holocene 11 6 to 6 cal kyr bp

field study of dispersion in a heterogeneous aquifer 3 geostatistical analysis of hydraulic conductivity observations of the spatial variability of hydraulic conductivity at a tracer test site located at columbus air force base in mississippi are presented direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging slug tests and a laboratory permeameter to test undisturbed soil cores indirect methods estimating hydraulic conductivity were also evaluated including soil grain size analyses surface geophysical surveys and mapping of sediment facies the spatial covariance of the 2187 hydraulic conductivity values obtained with the borehole flowmeter method was examined the log hydraulic conductivity variance ln2k and the horizontal and vertical correlation scales h and v of 4 5 12 8 m and 1 6 m respectively were estimated assuming second order stationarity of the conductivity field the covariance parameters are uncertain with bounding values that are 24 76 above or below the estimate covariance parameters estimated with more limited nonflowmeter data were within the same magnitude as those obtained using the extensive flowmeter data suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters nonstationarity of the ln k field was examined by removing three dimensional polynomial trend surfaces and calculating variograms of the residuals significantly lower estimates for ln2k h and v of 2 7 4 8 m and 0 8 m respectively were obtained from the third order log conductivity residuals after trend removal the bounding parameter values differ 15 44 from the estimated values accounting for unsteady flow and the uncertainty in the covariance parameters of the third order log conductivity residuals the calculated longitudinal and horizontal transverse macrodispersivities ranged from 1 5 m to 1 8 m and 0 3 m to 0 6 m respectively

is unique scaling of aquifer macrodispersivity supported by field data spreading of conservative solutes in groundwater due to aquifer heterogeneity is quantified by the macrodispersivity which was found to be scale dependent it increases with travel distance stabilizing eventually at a constant value however the question of its asymptotic behavior at very large scale is still a matter of debate it was surmised in the literature that macrodispersivity scales up following a unique scaling law attempts to define such a law were made by fitting a regression line in the log log representation of an ensemble of macrodispersivities from multiple experiments the functional relationships differ among the authors based on the choice of data our study revisits the data basis used for inferring unique scaling through a detailed analysis of literature marcodispersivities in addition values were collected from the most recent tracer tests reported in the literature we specified a system of criteria for reliability and reevaluated the reliability of the reported values the final collection of reliable estimates of macrodispersivity does not support a unique scaling law relationship on the contrary our results indicate that the field data can be explained as a collection of macrodispersivities of aquifers with varying degree of heterogeneity where each exhibits its own constant asymptotic value our investigation concludes that transport and particularly the macrodispersivity is formation specific and that modeling of transport cannot be relegated to a unique scaling law instead transport requires characterization of aquifer properties e g spatial distribution of hydraulic conductivity and the use of adequate models

the impeller meter for measuring aquifer permeability variations evaluation and comparison with other tests a knowledge of the variation of horizontal hydraulic conductivity with vertical position k z is important in understanding the transport and dispersive properties of aquifers using an impeller meter to measure the discharge distribution in a screened well while pumping at a constant rate is a promising technique for obtaining the k z function such an application is described herein and the resulting k z functions are compared with those obtained previously using tracer tests and multilevel slug tests impeller meter data were the most convenient to obtain and tracer data the most difficult the k z functions obtained by the three methods were not identical but quite similar overall this similarity between both borehole tests and the larger scale tracer test showed that nonstationary hydraulic conductivity trends in a stochastic hydrologic sense exist in the test aquifer the impeller meter method was better able to detect the higher k layers than was the multilevel slug approach overall the results suggest that a practical strategy for fitting impeller meter tracer or multilevel slug test data to a given aquifer is to use the selected testing procedure to obtain a dimensionless distribution and then a standard pumping test to measure combining both types of information enables dimensional values for k z to be calculated in low permeability aquifers or near the bottom of a test well the fluid velocity due to pumping may be below the stall velocity of an impeller thus there is a definite need for the commercial development of more sensitive flow measuring devices such as heat pulse flowmeters hess 1986 which will extend the resolution of this field method

thermomechanical implications of sediment transport for the architecture and evolution of continental rifts and margins erosion and deposition redistribute mass as a continental rift evolves which modifies crustal loads and influences subsequent deformation surface processes therefore impact both the architecture and the evolution of passive margins here we use coupled numerical models to explore the interactions between the surface crust and lithosphere this interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink if transport is efficient there are two possible outcomes 1 faulting within the zone of extension is longer lived and has larger offsets this implies a reduction of the number of faults and the width of the proximal domain 2 efficient transport of sediment leads to significant deposition and hence thermal blanketing this will induce a switch from brittle to ductile deformation of the upper crust in the distal domains the feedbacks between these two outcomes depend on the extension history the underlying lithospheric rheology and the influence of submarine deposition on sediment transport high erosion sedimentation during early faulting leads to abrupt crustal necking while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains in models where rheological conditions favor the formation of asymmetric conjugate margins only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization these models suggest that passive margin architecture can be strongly shaped by the solid earth structure sea level and climatic conditions during breakup

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

simulations of two well tracer tests in stratified aquifers at the chalk river and the mobile sites a simulation study of two well injection withdrawal tracer tests in stratified granular aquifers at two widely separated sites is presented the first site is located near the chalk river nuclear laboratories in canada and the second site is located in mobile alabama field data and test conditions at these sites are substantially different in terms of vertical distributions of hydraulic conductivity well spacings flow rates test durations and tracer travel distances furthermore the test at the chalk river site was conducted in a recirculating mode whereas the test at the mobile site was conducted in a nonrecirculating mode simulations of these tests were performed in three dimensions using the curvilinear finite element model developed in the previous paper of this series the simulations incorporated measured vertical variations in relative hydraulic conductivity and local dispersivity values that are small fractions between 1 1000 and 1 100 of the spacing between the injection and the withdrawal wells the local dispersivities are used to account for local hydrodynamic dispersion and are chosen independently so that they are not affected by the scales of the tests simulation results obtained from the model are presented interpretation of these results is made in conjunction with measured breakthrough curves at the withdrawal well and multilevel observation wells for the chalk river site predicted and measured breakthrough curves at the withdrawal well are in good agreement over the earlier part of the test duration deviation of the field data from the model prediction occurs over the second part where the predicted breakthrough curves show a declining trend but the field data plot does not for the mobile site predicted and measured breakthrough curves at the withdrawal well show similar trends throughout the entire test duration and are in good agreement overall model predictions of the effect of hydraulic conductivity stratification on temporal distributions of concentration are generally consistent with data from multilevel observation wells at both sites the model results also show interesting effects of stratification on spatial distributions of concentration and on the evolution and spreading pattern of tracer plumes these effects have not been fully assessed in the field measurement and should be investigated further

discharge discharge variability and the bedrock channel profile long term bedrock incision is driven by daily discharge events of variable magnitude and frequency with ineffective events below an incision threshold we explore theoretically how this short term stochastic behavior controls long term steady state incision rates and bedrock channel profiles combining a realistic frequency magnitude distribution of discharge with a deterministic detachment limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress our model predicts a power law relationship between steady state slope and drainage area consistent with observations the exponent of this power law is independent of discharge mean and variability while the amplitude factor which controls mountain belt relief is a power law function of mean runoff with an exponent of 0 5 and a complex function of runoff variability in accordance with evidence that incision occurs between 6 and 20 of time in rapidly incising rivers 1 mm yr our model predicts that channel steepness is virtually insensitive to runoff variability runoff variability can only decrease channel steepness for very slow incision rates and or weak lithologies the relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross sectional geometry and runoff variability this contradicts models neglecting discharge stochasticity in which the steepness incision scaling is set by the incision law exponent our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the late cenozoic within the context of a detachment limited model

on conducting the modified slug test in tight formations the method introduced by bredehoeft and papadopulos 1980 for conducting a modified slug test in tight formations does not assure the condition of approximate equilibrium necessary at the start of the test in addition compressibility in the shut in well can be significantly larger than the compressibility of water which bredehoeft and papadopulos indicate should be used in the calculations errors that may result from using the original test procedure are discussed and a modified procedure and testing arrangement are presented

the onshore intrusion of kuroshio subsurface water from february to july and a mechanism for the intrusion variation abstract to examine the variation of kuroshio subsurface water kssw off the coast of zhejiang province china we carried out six monthly cruises from february to july 2015 observations show that the kssw appears in a frequently reported algal bloom area near the coast as early as in april and persists until july however the kssw was absent in february we set up a fine resolution 4 km model which can reproduce most of the observed features model results and cruise data suggest that a nearshore kuroshio branch current nkbc originated from the kssw northeast of taiwan can emerge as early as in april which is responsible for the observed kssw from april to july however in february nkbc turns seaward before reaching this algal bloom area and thus the kssw is absent in addition to the traditional mechanisms of wind bottom stress and cooling we suggest a new mechanism of this nkbc variation in the origin area of the nkbc the kssw is more shoreward squeezed and flows more westward flowing northward in february while northwestward in april in april than february the change of flow direction caused different meeting point between the shelf break and the kuroshio current according to topographic beta spiral different meeting points will induce different upwelling velocity and then contribute to the different intrusion pattern of nkbc this direction change is one of the essential ingredients which enable nkbc to reach as far as the algal bloom area hence algal blooms in april may be potentially influenced by this nkbc whose water properties are significantly different to the coastal water

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

seismic structure gravity anomalies and flexure of the amazon continental margin ne brazil 1 seismic and gravity data have been used to determine the structure of the sediments crust and upper mantle that underlie the amazon continental margin offshore ne brazil seismic reflection profile data reveal a major unconformity at 7 s two way travel time twtt which we interpret as marking the onset of the transcontinental amazon river and the formation of the amazon deep sea fan system in the late miocene seismic refraction data show mean sediment velocities that decrease by 1 5 km s 1 in a seaward direction we attribute this decrease to facies changes associated with sediment progradation and the development of topset foreset and bottomset beds seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the atlantic ocean but is unusually thin 4 2 km we attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of south america and africa in the early cretaceous the seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data suggests the amazon fan loaded lithosphere with an unusually high flexural strength while a high strength lithosphere explains the overall depth of the seismic moho there are discrepancies of up to 2 km beneath the upper fan where the modeled flexed moho is shallower than the seismic moho and beneath the middle fan where it is deeper gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan we attribute these lateral density differences to proximity to the ceara rise which is believed to have formed during the late cretaceous in a mid ocean ridge setting fan loading of a relatively strong dense and hence cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the gurupe arch onshore despite its proximity to leaky transform faults the margin that underlies the amazon fan appears to be of nonvolcanic origin the main differences with other nonvolcanic margins such as west iberia and newfoundland are a greater sediment accumulation a narrower zone of transitional crust and a lack of any evidence for extreme extension and mantle serpentinization

double porosity models for a fissured groundwater reservoir with fracture skin theories of flow to a well in a double porosity groundwater reservoir are modified to incorporate effects of a thin layer of low permeability material or fracture skin that may be present at fracture block interfaces as a result of mineral deposition or alteration the commonly used theory for flow in double porosity formations that is based upon the assumption of pseudo steady state block to fissure flow is shown to be a special case of the theory presented in this paper the latter is based on the assumption of transient block to fissure flow with fracture skin under conditions where fracture skin has a hydraulic conductivity that is less than that of the matrix rock it may be assumed to impede the interchange of fluid between the fissures and blocks resistance to flow at fracture block interfaces tends to reduce spatial variation of hydraulic head gradients within the blocks this provides theoretical justification for neglecting the divergence of flow in the blocks as required by the pseudo steady state flow model coupled boundary value problems for flow to a well discharging at a constant rate were solved in the laplace domain both slab shaped and sphere shaped blocks were considered as were effects of well bore storage and well bore skin resultsmore obtained by numerical inversion were used to construct dimensionless type curves that were applied to well test data for a pumped well and for an observation well from the fractured volcanic rock terrane of the nevada test site less

intrusion of the kuroshio subsurface water in the southern east china sea and its variation in 2014 and 2015 traced by dissolved inorganic iodine species abstract the circulation systems and ecological environment of the east china sea ecs are greatly influenced by the kuroshio the nutrient rich kuroshio subsurface water kssw has been subject to considerable research attention but the intrusion pathway extent and variation of the kssw over the entire southern ecs shelf are still debated in june 2015 the dissolved inorganic iodine species diis were determined and hydrographic data were collected and analyzed in the southern ecs analysis of the collected data suggests that the kssw mainly intrudes onto the ecs shelf from northeast of taiwan and bifurcates into a nearshore branch and an offshore branch the nearshore branch ultimately appears in the subsurface layer within the 50 m isobath off the zhejiang coast north of 30 n and the offshore branch flows along the 100 m isobath and covers the area outside the 80 m isobath another kssw intrusion along the shelf edge is located at 28 n this additional kssw intrusion is weaker and the offshore area in the southern ecs is mainly influenced by the kssw intrusion northeast of taiwan compared with the kssw intrusion on the ecs continental shelf depth

thermal and mechanical evolution of u s atlantic continental margin abstract the postrifting tectonic subsidence of the u s atlantic continental margin appears to be due to thermal contraction following continental breakup this subsidence is greatly increased by the effect of sediment loading models for the response of the lithosphere to loads show that the rigidity of the lithosphere is a strong function of temperature hence the flexural rigidity at the u s margin should vary in both space and time the thermal structure and subsidence of the margin has been modelled using a two dimensional extension model where the amount of extension has been allowed to vary across the margin and lateral conduction of heat has been included the equivalent elastic thickness of the lithosphere has been approximated as the depth to an isotherm thermoelastic effects of the cooling lithosphere as well as the flexural loading of the sediments have been included the lateral flow of heat and flexure have contrasting effects the lateral heat flow tends to cause uplift of the coastal plain region landward of the hinge zone and to increase the subsidence rate in the regions of the transitional crust in addition the effects of lateral heat flow modify the thermal history of the sediments and heat flow flexure on the other hand causes subsidence of the coastal plain and decreases the sediment accumulation beneath the transition zone the stratigraphy of the u s margin is seen to be the result of the complex interaction of these effects as their contributions vary through the history of the margin end of article last page 1671

observation and three dimensional simulation of chloride plumes in a sandy aquifer under forced gradient conditions two well forced gradient tracer experiments over a distance of 5 m were carried out in a coastal sandy aquifer at georgetown south carolina the evolution of three dimensional chloride plumes during two tracer experiments was observed a three dimensional finite element model for flow and transport was used with extensive hydraulic conductivity data obtained from slug tests to assess our ability to predict solute transport in the aquifer results showed that our predictive ability is limited to the bulk behavior of the plumes which is mainly controlled by some significant heterogeneities in addition hydraulic conductivity values estimated by the hvorslev method for analysis of slug tests best represented the hydraulic conductivity distribution of the sandy aquifer finally the results of a long term experiment illustrate the importance of temporal variability in boundary conditions in the prediction of solute transport in aquifers

flow through fractures flow through fractures is often idealized as flow between two parallel plates plane poiseuille flow the opening or aperture between parallel plates is unambiguous and its relation to flowrate is well known however fractures in rock have uneven walls and a variable aperture a model for flow in a fracture is proposed wherein the fracture is represented by a set of parallel plate openings with different apertures the model leads to a modified poiseuille equation for flow which includes an aperture frequency distribution for the fracture any arbitrary aperture distribution can be used in order to simplify computation and demonstrate the properties of the model a log normal form of distribution is assumed even when an analytical form of the distribution is assumed two parameters rather than a single value representing aperture size are required to determine flowrate models of aperture change for a fracture undergoing compression fracture walls deforming and extension fracture walls separating are developed which constrain the additional parameter and allow calculation of flowrate as a function of mean aperture the theoretical relationships developed between mean aperture and flowrate can be used to interpret published laboratory data for single fractures

geostatistical analysis of centimeter scale hydraulic conductivity variations at the made site 1 spatial variations in hydraulic conductivity k provide critical controls on solute transport in the subsurface recently new direct push tools were developed for high resolution characterization of k variations in unconsolidated settings these tools were applied to obtain 58 profiles vertical resolution of 1 5 cm from the heavily studied macrodispersion experiment made site we compare the data from these 58 profiles with those from the 67 flowmeter profiles that have served as the primary basis for characterizing the heterogeneous aquifer at the site overall the patterns of variation displayed by the two data sets are quite similar in terms of both large scale structure and autocorrelation characteristics the direct push k values are on average roughly a factor of 5 lower than the flowmeter values this discrepancy appears to be attributable at least in part to opposite biases between the two methods with the current versions of the direct push tools underestimating k in the highly permeable upper portions of the aquifer and the flowmeter overestimating k in the less permeable lower portions the vertically averaged k values from a series of direct push profiles in the vicinity of two pumping tests at the site are consistent with the k estimates from those tests providing evidence that the direct push estimates are of a reasonable magnitude the results of this field demonstration show that direct push profiling has the potential to characterize highly heterogeneous aquifers with a speed and resolution that has not previously been possible

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

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

the role of elasticity in simulating long term tectonic extension author posting oxford university press 2016 this article is posted here by permission of oxford university press for personal use not for redistribution the definitive version was published in geophysical journal international 205 2016 728 743 doi 10 1093 gji ggw044

kuroshio subsurface water feeds the wintertime taiwan warm current on the inner east china sea shelf the taiwan warm current twc has an overwhelming influence on the heat salt and nutrients balance on one of the broadest shelf in the world the east china sea shelf in winter the twc flows in an unusual upwind direction and reaches the changjiang yangtze river estuary but its origin and pathway are intensely debated here combined evidences from current measurement hydrographic and stable isotopic data all suggest that the wintertime twc intrusion off the changjiang estuary mainly originates from the kuroshio subsurface water northeast of taiwan rather than from the taiwan strait warm water the kuroshio branched water northeast of taiwan can intrude into the inner shelf near the zhe min coast via bottom layer manifesting by a pronounced boundary at 50 m isobath around 28 n and thereby feeds the twc intrusion into the changjiang estuary the intrusion complicates the hydrological process in the estuary and shelf sea and its impact on marine environment deserves more research attentions

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

circulation hydrographic structure and mixing at tidal fronts the view from georges bank and discussion the steep slope on the northern side of georges bank and its location in the fundy maine tidal system result in a persistent summertime frontal system comprising a tidal mixing front and a stratified tide topography interaction at the bank edge recent field studies have provided a high resolution description of the circulation hydrographic structure and mixing in the region frontal features include an along front residual jet a surface convergence zone regular variations in frontal structure and position over the tidal period and tidal modulation cycle large amplitude internal waves and strong spatial and temporal variations in small scale turbulence the observations suggest that the magnitude of cross front and vertical exchange in frontal regions can be site specific depending on the relative importance of the underlying physical processes

a numerical study on the changjiang diluted water in the yellow and east china seas 1 a numerical simulation is performed using the princeton ocean model pom to investigate the behavior of the changjiang diluted water cdw and to evaluate the freshwater budget in the yellow and east china seas in addition the major process controlling the behavior of the cdw is discussed the simulated cdw distributions are fairly consistent with the observed distributions showing a remarkable seasonal variation the cdw extends to the northeast toward the tsushima korea straits during summer while it hugs the chinese coast to the southwest within a narrow band in winter tracer experiment is carried out to track the freshwater originating only from the changjiang and to evaluate the freshwater budget in the yellow and east china seas the result shows that about 68 percent of the changjiang discharge is transported through the tsushima korea straits and that about 27 percent is transported through the taiwan strait in autumn and winter to investigate the main process controlling the behavior of the cdw mentioned above another experiment is carried out without wind forcing it is found that the eastward extension of the cdw is strongly constrained by the taiwan tsushima warm current system over the shelf regime the wind is however responsible for the northeastward extension of the cdw in summer in addition the southwestward extension at the taiwan strait during winter is done by strong northeasterly monsoon the experiment without wind forcing shows that freshwater transport through the tsushima korea straits increases to 90 percent of the total changjiang discharge