late quaternary arroyo formation and climate change in the american southwest arroyos entrenched ephemeral streams that form in desert environments first appeared in the arid and semiarid american southwest after 8000 14 c yr b p for at least 7 k y prior to that time climate vegetation and groundwater conditions were not conducive for arroyo formation along the floors of desert valleys after a hiatus in arroyo formation the frequency of arroyo cutting and filling increased dramatically after 4000 14 c yr b p the early holocene arroyos and increased frequency of arroyo incision after 4000 14 c yr b p are related to the establishment and changes in postglacial vegetation climate and groundwater conditions as a result arroyo sequences preserve a record of large scale climate change and small scale climatic perturbations that occurred during the holocene human modification of valley flood plains is an additional factor that contributed to mid nineteenth and early twentieth century arroyo cutting

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

gully annealing by aeolian sediment field and remote sensing investigation of aeolian hillslope fluvial interactions colorado river corridor arizona usa abstract processes contributing to development of ephemeral gully channels are of great importance to landscapes worldwide and particularly in dryland regions where soil loss and land degradation from gully erosion pose long term land management problems whereas gully formation has been relatively well studied much less is known of the processes that anneal gullies and impede their growth this study of gully annealing by aeolian sediment spanning 95 km along the colorado river corridor in glen marble and grand canyon arizona usa employed field and remote sensing observations including digital topographic modelling results indicate that aeolian sediment activity can be locally effective at counteracting gully erosion gullies are less prevalent in areas where surficial sediment undergoes active aeolian transport and have a greater tendency to terminate in active aeolian sand although not common examples exist in the record of historical imagery of gullies that underwent infilling by aeolian sediment in past decades and evidently were effectively annealed we thus provide new evidence for a potentially important interaction of aeolian hillslope fluvial processes which could affect dryland regions substantially in ways not widely recognized moreover because the biologic soil crust plays an important role in determining aeolian sand activity and so in turn the extent of gully development this study highlights a critical role of geomorphic ecologic interactions in determining arid landscape evolution

contrasting effects of microbiotic crusts on runoff in desert surfaces abstract microbiotic crusts mcs play an important role in surface hydrology by altering runoff yield in order to study the crust s role on water redistribution rainfall and runoff were measured during 1998 2000 at three sites within the northern chihuahuan desert new mexico usa the sevilleta national wildlife refuge sev the white sands national monument ws and the jornada experimental range jer whereas quartz and gypsum sand characterize the sev and ws sites respectively both of which have high infiltration rates silty alluvial deposits characterize the jer site runoff was measured in four pairs of 1 8 6 4 m 2 plots having mcs one of which was scalped in each pair no runoff was generated at ws whether on the crusted or the scalped plots runoff was however generated at sev and jer being higher on the crusted plots at sev and lower on the jer plots the results were explained by the combined effect of a parent material and b the crust properties such as species composition microrelief surface roughness and exopolysaccharide eps content reflected in the ratio of carbohydrates to chlorophyll whereas the effective rainfall the fines and the eps content were found to explain runoff initiation the effective rainfall and the crust microrelief were found to explain the amount of runoff at sev and jer where runoff generation took place the findings attest to the fundamental role of the parent material and the crust s species composition and properties on runoff and hence to the complex interactions and the variable effects that mcs have on dryland hydrology

chemical models of vitrinite reflectance evolution a global kinetic model of vitrinite reflectance evolution is developed to illustrate a new approach to vitrinite reflectance modelling the method utilizes quantitative pyrolysisgas chromatographic data from isolated vitrinite kerogens to derive the concentrations of structurally specific moieties alkylphenol precursors in vitrinite kerogens as a function of rank this data set is used to calibrate a gaussian distributed activation energy model for phenol precursor loss from vitrinites using published burial histories phenol yield is then correlated with vitrinite reflectance the predictive vitrinite reflectance model based on this phenol system is tested against a well from the us gulf coast area the results being compared with a lopatin model of the same well this type of vitrinite reflectance model is numerically compatible with current generation kinetic models of hydrocarbon production from source rock kerogens and has a better heating rate independence than lopatin s method

some pragmatic perspectives in source rock geochemistry abstract conceptual advances in petroleum source rock geochemistry made during the last decade are reviewed and some problem areas in analytical geochemistry are outlined areas of discussion include views of source rocks based on assessment of hydrocarbon expulsion kinetic models of kerogen maturation vitrinite reflectance evolution and a discussion of the data dilemma facing modern source rock geochemistry the increased application of petroleum geochemistry to the quantitative solution of geological problems is stressed

wind erodibility of soils at fort irwin california mojave desert usa before and after trampling disturbance implications for land management recently disturbed and control i e less recently disturbed soils in the mojave desert were compared for their vulnerability to wind erosion using a wind tunnel before and after being experimentally trampled before trampling control sites had greater cyanobacterial biomass soil surface stability threshold friction velocities tfv i e the wind speed required to move soil particles and sediment yield than sites that had been more recently disturbed by military manoeuvres after trampling all sites showed a large drop in tfvs and a concomitant increase in sediment yield simple correlation analyses showed that the decline in tfvs and the rise in sediment yield were significantly related to cyanobacterial biomass as indicated by soil chlorophyll a however chlorophyll a amounts were very low compared to chlorophyll a amounts found at cooler desert sites where chlorophyll a is often the most important factor in determining tfv and sediment yield multiple regression analyses showed that other factors at fort irwin were more important than cyanobacterial biomass in determining the overall site susceptibility to wind erosion these factors included soil texture especially the fine medium and coarse sand fractions rock cover and the inherent stability of the soil as indicated by subsurface soil stability tests thus our results indicate that there is a threshold of biomass below which cyanobacterial crusts are not the dominant factor in soil vulnerability to wind erosion most undisturbed soil surfaces in the mojave desert region produce very little sediment but even moderate disturbance increases soil loss from these sites because current weathering rates and dust inputs are very low soil formation rates are low as well therefore soil loss in this region is likely to have long term effects published in 2006 by john wiley sons ltd

geomorphic threshold conditions for ephemeral gully incision abstract ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies furthermore little is known about the factors controlling ephemeral gully erosion in this paper the available information on the initiation and location of ephemeral gullies is summarised focusing on the relationship between the upslope drainage area a and the critical slope gradient s cr for ephemeral gully initiation by plotting the non linear relationship between critical slope gradient measured immediately upstream of the incision head and drainage area at the incision head for gullied sites it was possible to draw a straight line on log log paper through the lower most points for each of the available datasets representing a critical slope area relationship for incision consequently below this line no incision occurs this line or critical relation can also be written as a power function between critical slope and area although many factors vary between the different datasets the exponent of the drainage area relationship 0 4 showed very little variation the observed critical slope area relationship can be related to a simple model of channel initiation by overland flow furthermore this relationship can be used to identify potentially unstable sites where control measures should be undertaken

do mosses serve as sink for rain in the negev desert a theoretical and experimental approach abstract regarded as highly developed crusts thick moss dominated crust mdc provides a relatively thick protective layer over sand dunes which together with cyanobacteria dominated crust cdc play an important role in their stabilization it was previously claimed that due to their thickness and high organic matter mdc may serve as sink for rain in the negev and consequently reduce water infiltration and cause shrub mortality due to the relatively short rainy period we hypothesized that mdc may not serve as a potent sink for rainwater and may therefore not be responsible for plant mortality toward this end the infiltration depth of sand underlying mdc and cdc was measured following sprinkling experiments and natural rain events whereas mdc was relatively effective in intercepting low depth rain events intercepting additional 0 8 mm in comparison to cdc no significant differences in the infiltration depth were found between mdc and cdc following high depth or consecutive rain events based on these measurements and long term calculations it is concluded that mdcs does not have a significant effect on rain infiltration in the negev and therefore cannot be held responsible for shrub mortality management practices that call for the reduction in mdc cover should therefore be reconsidered

rift propagation in rotational versus orthogonal extension insights from 4d analogue models abstract in rift settings extension rates often vary along strike due to rotation about a vertical axis or euler pole yet tectonic modelers traditionally apply constant along strike deformation rates here we compare rift development and propagation under traditional orthogonal extension versus rotational extension conditions the set ups involve brittle viscous layering and localize deformation through structural weaknesses seeds our models provide first order insights into the differences in rift development between both boundary conditions orthogonal extension produces a rift basin with constant synchronous along strike features whereas rotational extension induces along strike structural gradients diachronous rift development causing rift propagation and the development of v shaped basins we observe important viscous flow associated with differential pressure gradients in rotational extension we also describe the important effects of strain partitioning between rift axis and model boundaries the quantifying of which is crucial to avoid incorrect model interpretations although our model results are first order only they are in good agreement with various natural examples and previous modeling studies and highlight the importance of considering the third dimension when studying tectonic systems

interactions between propagating rotational rifts and linear rheological heterogeneities insights from three dimensional laboratory experiments the lateral propagation of rifts is a consequence of the relative divergence of lithospheric plates about a pole of rotation modern and ancient examples of rifts are known to overprint pre existing linear anisotropies in the crust and lithosphere such as lithospheric boundaries crustal sutures and thermal anomalies here we investigate how propagating rifts interact with pre existing structures by using three dimensional analogue experiments with rotational extensional boundary conditions and variably oriented linear weak zones in the lithospheric mantle when linear weaknesses are oriented at low angles to the rift axis early strain localization occurs in narrow domains which merge at later stages resulting in continental break up by unzipping strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis in these experiments early sub parallel v shaped basins propagate toward the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary the experimental results are characterized in terms of their evolution patterns of strain localization and surface topography as a function of the lithospheric heterogeneity obliquity angle comparison of the experiments to ancient and modern examples in nature may help to elucidate the common but still poorly understood process of propagating rift lithospheric heterogeneity interaction

the role of asthenospheric flow during rift propagation and breakup continental rifting precedes the breakup of continents leading to the formation of passive margins and oceanic lithosphere although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling or passive rifting caused by far field extensional stresses it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere three dimensional numerical experiments of rifting near an euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis parallel motion in the asthenosphere dome but also to promote along axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole we show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time dependent phases of compressional and transcurrent stress regimes despite an overall divergent plate setting our experiments predict overdeepened bathymetry at the tip of the propagating rift as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting we also explain the episodes of basin inversion documented in many rifted continental margins

continental break up of the south china sea stalled by far field compression the outcome of decades of two dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean however geological observations unequivocally show that continental break up propagates in the third dimension at rates that do not scale with the rate of opening here we perform three dimensional numerical simulations and compare them with observations from the south china sea to show that tectonic loading in the direction of propagation exerts a first order control on these propagation rates the simulations show that in the absence of compression in that direction continental break up propagates fast forming narrow continental margins independently of the coupling when compression is applied propagation stagnates forming v shaped oceanic basins and wide margins changes in out of plane loading therefore explain the alternation of fast propagation and relative stagnation these new dynamic constraints suggest that the west to east topographic gradient across the indochinese peninsula prevented continental break up propagation through the 1 000 km wide continental rift of the central and west basin of the south china sea until the direction of stretching changed 23 million years ago resulting in bypassing and acceleration of continental break up propagation tectonic loading in the direction of propagation exerts an important control on the propagation of continental break up according to three dimensional simulations of the south china sea

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

gullying and erosion control at archaeological sites in grand canyon arizona gully erosion of cultural sites in grand canyon national park is an urgent management problem that has intensified in recent decades potentially related to the effects of glen canyon dam we studied 25 gullies at nine sites in grand canyon over the 2002 monsoon erosion season to better understand the geomorphology of the gully erosion and the effectiveness of erosion control structures ecs installed by the park under the direction of the zuni conservation program field results indicate that hortonian overland flow leads to concentrated flow in gullies and erosion focused at knickpoints along channels as well as at gully heads though groundcover type soil shear strength and permeability vary systemat ically across catchments gradient and to a lesser degree contributing drainage area seem to be the first order controls on gully extent location of new knickpoints and ecs damage the installed ecs do reduce erosion relative to reaches without them and initial data suggest woody checkdams are preferable to rock linings but maintenance is essential because damaged structures can exacerbate erosion topographic data from intensive field surveys and detailed photogrammetry provide slope contributing area data for gully heads that have a trend consistent with previous empirical and theoretical formulations from a variety of landscapes the same scaling holds below gully heads for knickpoint and ecs topographic data with threshold coefficients the lowest for gully heads slightly higher for knickpoints and notably higher for damaged ecs these topographic thresholds were used with 10 cm digital elevation models to create simple predictive models for gully extent and structure damage the model predictions accounted for the observed gullies but there are also many false positives purely topographical models are probably inadequate at this scale and application but models that also parameterize the variable soil properties across sites would be useful for predicting erosion problems and ecs failure copyright 2006 john wiley sons ltd

effects of biological soil crusts on surface roughness and implications for runoff and erosion abstract soil surface roughness has a strong influence on runoff and erosion affecting surface storage capacity water flow routing and velocity and modifying runoff rates it also reduces soil detachment by raindrops and the shear effect of water flow on the soil surface in arid and semiarid ecosystems biological soil crusts bscs commonly appear in clearings between plants depending on the dominant component in the bsc community the roughness of the soil surface may vary considerably changing the hydrologic and erosive response of the soil the aim of this study is to determine the effect of physical and biological crusts on soil surface roughness and their influence on runoff and erosion for this purpose we set up open plots containing different bsc types and treatments and recorded runoff and total erosion from all the events for 18 months micro topographic indexes were calculated from high resolution digital surface models of the plots built from terrestrial laser scanner height data after comparing different spatial resolutions and indexes we concluded that only the local random roughness index in a 40 mm moving window provided a precise estimation of the roughness induced by bscs though it did not have a direct effect on runoff response the best relationship between microtopography and runoff on biologically crusted soils was found for surface storage capacity which appears as a powerful predictor of the runoff coefficient on long temporal scales sediment yield was not well predicted by any of the micro topographic indexes studied the only index that was significantly related to sediment yield was the local random roughness in a 40 mm moving window but even this explained only a third of the erosion variance

lithospheric strength and rift migration controls on synrift stratigraphy and breakup unconformities at rifted margins examples from numerical models the atlantic and south china sea margins synrift stratigraphy and the distribution of breakup related erosional unconformities vary vastly between passive margins and cannot be explained by classical rifting models here we use numerical modeling to predict their spatiotemporal distribution we show that synrift stratigraphy mimics rift architecture which is controlled by lithospheric strength basinward rift migration during extension produces 1 oceanward younging syntectonic and posttectonic sequences 2 rift migration unconformities rmus predating breakup and 3 a breakup unconformity bu that only extends over the outermost margins since breakup is not linked with a sudden stress drop with small synrift sedimentation the rmus and bu laterally merge to form a margin wide unconformity in symmetric wide conjugate margins which arise for weak lithospheres such as the south china sea a long phase of distributed deformation with little subsidence results in early synrift sediment over most of the margins rmus merge into a single event that marks the subsequent focusing of deformation into a narrow breakup area which experiences short lived intense thinning and subsidence in asymmetric conjugate margins lateral rift migration transports shallowly deposited early synrift sediments from the narrow to the wide hyperextended margin leading to a condensed syntectonic sequence and a single bu in the narrow margin and a series of rmus in the wide one for very weak lower crusts lateral rift migration generates large synrift sag basins in the wide margin as in angola and congo margins our models resemble the observed margins tectonic diversity and may be used as templates for interpreting their distal unexplored areas

micropiping processes and biancana evolution in southeast tuscany italy abstract biancane badlands consisting of small domes dissected by rills and micropipes with rough disordered microrehef can be found along the apennines in italy the dominant processes forming biancane differ from those of badlands formed on smectite rich mudrocks as micropipes associated with pseudokarstic enlargement of pores and cracks predominate and form the main routes for evacuation of eroded material biancana evolution is controlled by water infiltration into intact bedrock producing an erodible weathering rind which is more porous than intact rock this rind is easily removed by rill or micropipe flow and erosion is therefore weathering controlled depending on rind production by infiltrating water infiltration is initially slow and stepped due to slow water movement through very small capillary pores in intact rock alternating with rapid filling of macropores and cracks this occurs due to rapid matrix pore enlargement by dispersion and or dissolution the infiltration pattern is accurately reproduced by a model built on progressive development of weathering layers by moisture penetration model results are consistent with weathering rind depths and erosion observed in the field and show that a pipe network can be generated on newly exposed rock by the rainfall of one year propagation of the pipe network diverts a progressively larger proportion of runoff into micropipes expanding weathering rind production within the biancana as well as on the surface internal weathering and flow progressively dominate with few unweathered corestones and the biancana gradually collapses into a penultimate souffle like form

equations for the near surface mass flux density profile of wind blown sediments the erosion of sediment by wind and the resulting mass flux density profile is thought to be described by a mathematical function that bears information on the mechanisms responsible for the movement of individual particles by the wind and such functions have been studied extensively in this study several functions are evaluated that have been proposed to describe the variation in mass flux density with height of wind blown sediment with the flux containing a mixture of particles in suspension and saltation based on detailed field data at four land types in the minqin area of north western china where severe wind erosion occurs high resolution mass flux density measurements at 50 heights collected at 20 mm intervals to a height of 1 m above the surface were obtained using vertically segmented samplers three kinds of functions fit the measured flux density profiles reasonably well but a three parameter modified exponential function is preferred because it contains fewer coefficients to be defined and provides a reasonably good fit to the measured mass flux density profiles this and previous conclusions suggest that the decay with height of mass flux density of sediments dominated by saltation particles as in the present study tends to follow a modified exponential function law but a modified power function law replaces the modified exponential function law when the height extends to a level high enough to be dominated by suspension particles copyright 2011 john wiley sons ltd

lithological properties and weathering response on badland hillslopes rills and rill networks are important geomorphological features in clay rich badland areas rill network geometry varies with climate and topography but the lithological properties are also critical rill systems on mudrock and sandstone in alberta badlands were studied to examine progressive changes in material properties as smectite rich mudrocks and sandstones undergo weathering and the impact of seasonal changes in precipitation patterns three plots with well developed rill systems on each unit were examined in may 2001 and 2003 geometric properties were determined along a dense grid and surface and subsurface samples were analyzed for texture crust structure sem mineralogical composition xrd content of major elements xrf aggregate stability and moisture content the rill width depth ratio decreased from 2001 to 2003 on mudrock rill networks the popcorn crust found in 2001 changed to a thin dense crust in 2003 2001 materials had higher na and ca concentrations and the 2003 materials have higher al k concentrations unweathered mudrock shards were subjected to incremental weathering in controlled laboratory conditions depending on moisture input different crust types developed on materials after 10 wetting drying cycles material responded to different amounts of precipitation by alteration from swelling to dispersion

geodynamics of the south china sea abstract the beginning of seafloor spreading in the south china sea scs is now established from iodp drilling leg 349 at 33 ma chron 12 32 ma is the oldest chron identified in the scs the nature of the crust of the northeastern part of the scs located north of chron c12 where chrons 15 to 17 were previously identified is not oceanic but thinned continental crust intruded by volcanic elongated features emplaced 17 22 ma ago based on magnetic anomaly identifications the end of the scs spreading could be either 15 5 20 5 ma briais et al 1993 barckhausen et al 2014 or something else however as post spreading magmatic activity 13 3 5 ma largely masks the spreading fabric in particular near the axis of the east sub basin published locations of the axial magnetic anomaly extinct spreading axis and spreading rates are not reliable a contoured map of the extremely dense set of magnetic data shows that a few magnetic lineations belonging to the magnetic seafloor spreading fabric are still preserved and parallel to the n055 bathymetric seafloor spreading trends identified on swath bathymetric maps in the central part of the scs suggesting that the extinct ridge axis is n055 trending with potential n145 transform faults based on published swath bathymetric data oceanic domains with different seafloor spreading lineaments have been delimited n055 n075 and n085 and provide important constraints used to propose a kinematic sketch of the scs opening as a consequence the zhongnan faults zone located between the east and southeast sub basins acted as a major fracture zone system during the scs opening with horizontal offsets varying from 50 km to 140 km the flow line pattern defined from the seafloor spreading lineaments and the few identified fzs have been used to highlight conjugate segments of continental margins during the first phases of opening of the scs from the fit of continents to chron c10 30 ma the n175 extension observed in the xisha trough and eventually south of the macclesfield bank extends to the qui nhon ridge located along the eastern margin of vietnam in the southward prolongation of the red river fault system normal faults curve toward the south with a horsetail geometry interpreted as evidence for a few tens of kilometers of dextral motion along the qui nhon ridge since chron 10 and until the end of scs opening the plate boundary located between the southern south china sea sscs and eu plates jumped westward several times from the location of the ulugan fault near palawan to the western limit of the southwest basin explaining the progressive formation of the scs from east to west and giving the characteristic v shape of the scs the opening of the whole scs is linked and occurred simultaneously with the northward subduction of the proto scs whose suture is located south of palawan and extends westwards in north borneo

spatial and temporal evolution of rifting and continental breakup in the eastern black sea basin revealed by long offset seismic reflection data the age and distribution of the syn rift and early post rift infill records the spatial and temporal distribution of extension and breakup processes in a rift basin the eastern black sea basin ebsb is thought to have formed by back arc extension during cretaceous to early cenozoic time however a lack of direct constraints on its deep stratigraphy leaves uncertainties over the time duration and location for rifting and breakup processes in the basin here we use the enhanced imaging provided by 2d long offset seismic reflection profiles to analyze the deep structural and stratigraphic elements of the ebsb based on these elements we infer the presence of two distinct late cretaceous syn rift units recording initial extension rift stage 1 over the continental highs shatsky ridge and the mid black sea high followed by strain localization along the major basin bounding faults and rift migration towards the basin axis rift stage 2 overlying these units palaeocene eocene and oligocene units show a syn kinematic character in the nw with evidence for ongoing extension until oligocene time towards the se these sequences are instead post kinematic directly overlaying a basement emplaced during breakup we interpret the palaeocene oligocene units to record the time spanning from the initiation of breakup late cretaceous palaeocene in the se to the end of extension oligocene in the nw the first ubiquitously post rift infill is the lower miocene maykop formation our results highlight the along strike temporal variability of extension and breakup processes in the ebsb

deep crustal structure of the conjugate margins of the sw south china sea from wide angle refraction seismic data abstract the south china sea is the largest marginal basin of se asia yet its mechanism of formation is still debated a 1000 km long wide angle refraction seismic profile was recently acquired along the conjugate margins of the sw sub basin of the south china sea over the longest extended continental crust a joint reflection and refraction seismic travel time inversion is performed to derive a 2 d velocity model of the crustal structure and upper mantle based on this new tomographic model northern and southern margins are genetically linked since they share common structural characteristics most of the continental crust deforms in a brittle manner two scales of deformation are imaged and correlate well with seismic reflection observations small scale normal faults grabens horsts and rotated faults blocks are often associated with a tilt of the velocity isocontours affecting the upper crust the mid crust shows high lateral velocity variation defining low velocity bodies bounded by large scale normal faults recognized in seismic reflection profiles major sedimentary basins are located above low velocity bodies interpreted as hanging wall blocks along the northern margin spacing between these velocity bodies decreases from 90 to 45 km as the total crust thins toward the continent ocean transition the continent ocean transitions are narrow and slightly asymmetric 60 km on the northern side and no more than 30 km on the southern side indicating little space for significant hyper stretched crust although we have no direct indication for mantle exhumation shallow high velocities are observed at the continent ocean transition the moho interface remains rather flat over the extended domain and remains undisturbed by the large scale normal faults the main decollement is thus within the ductile lower crust

controls of inherited lithospheric heterogeneity on rift linkage numerical and analog models of interaction between the kenyan and ethiopian rifts across the turkana depression inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts the turkana depression in the east african rift connects the main ethiopian rift to the north with the kenya rift in the south this region is characterized by a nw se trending band of thinned crust inherited from a mesozoic rifting event which is cutting the present day n s rift trend at high angle in striking contrast to the narrow rifts in ethiopia and kenya extension in the turkana region is accommodated in sub parallel deformation domains that are laterally distributed over several hundred kilometers we present both analog experiments and numerical models that reproduce the along axis transition from narrow rifting in ethiopia and kenya to a distributed deformation within the turkana depression similarly to natural observations our models show that the ethiopian and kenyan rifts bend away from each other within the turkana region thus forming a right lateral stepover and avoiding a direct link to form a continuous n s depression the models reveal five potential types of rift linkage across the pre existing basin three types where rifts bend away from the inherited structure connecting via a 1 wide or 2 narrow rift or by 3 forming a rotating microplate 4 a type where rifts bend towards it and 5 straight rift linkage the fact that linkage type 1 is realized in the turkana region provides new insights on the rheological configuration of the mesozoic rift system at the onset of the recent rift episode

properties and spatial distribution of microbiotic crusts in the negev desert israel abstract playing a cardinal role in surface stabilization and in carbon and nitrogen fixation microbiotic crusts play a crucial role in arid regions where they may serve as useful biomarkers for wind power and wetness duration this is especially the case on relatively unstable and infertile sand dunes in the negev desert where high correlations between the crust chlorophyll content and the daytime wetness duration were found yet only scarce data are available as to the possible link between the chlorophyll content and other physical color thickness strength crack density surface roughness and infiltrability and biological protein carbohydrate organic matter and species composition factors which determine in turn the crust type and its effect upon geomorphological and ecological processes no data are available on crust type distribution these were the aims of the current research when a cluster analysis was performed five types of microbiotic crusts were defined four of which were cyanobacterial a d and one moss dominated crust e the crusts differed in their physical and biological properties they showed an increase in chlorophyll content protein carbohydrates and organic matter from a to e with concomitant increase in species diversity thickness roughness and strength but with some variables crack density and infiltrability showing a reversed trend at the moss dominated crust the increase in the biomass components of the crust and the gradual change of the physical properties are explained by the improved physical conditions primarily wetness duration which facilitates longer hours of photosynthetic activity and consequently the introduction of additional more mesic species such as green algae lichens and mosses extended wetness duration was found to shift the crust type from cyanobacterial to moss dominated crust the spatial distribution of the crusts as verified by crust mapping coincided with the daylight surface duration which in turn was controlled by topography aspect angle and slope position it implies that whereas initial physical conditions dictates species composition and thus crust type the crust type in turn is responsible for characterizing the physical properties of the surface which may largely affect ecological and geomorphological processes

cenozoic tectonic evolution of southeastern thailand derived from low temperature thermochronology low temperature thermochronological techniques specifically apatite u th he and apatite fission track dating were used to reconstruct the thermal history of southeastern thailand this area is intersected by vast and complex fault networks related to the cenozoic mae ping and three pagodas faults these were identified from satellite imagery and confirmed by field observations new apatite fission track and apatite u th he data were collected from crystalline basement blocks within these fault networks ages obtained range from 48 to 24 ma with most of the samples clustering between 36 and 24 ma thermal history modelling indicates late eocene oligocene exhumation of the exposed granitic and metamorphic basement rocks in southeastern thailand exhumation was regional and was contemporaneous with sinistral fault activity during the late eocene early oligocene along the mae ping fault and three pagodas fault moreover this exhumation occurred coevally with a synrift phase of intracontinental offshore rift basin and half graben basin development in the eastern gulf of thailand the phase of exhumation ended in the early miocene as a result of the changing plate tectonic forces along the complex plate boundaries of sundaland supplementary material aft data radial plots and the detailed procedure of thermal history modelling are available at https doi org 10 6084 m9 figshare c 4633064

continental interior and edge breakup at convergent margins induced by subduction direction reversal a numerical modeling study applied to the south china sea margin the dynamics of continental breakup at convergent margins have been described as the results of back arc opening caused by slab rollback or drag force induced by subduction direction reversal although the rollback hypothesis has been intensively studied our understanding of the consequence of subduction direction reversal remains limited using thermo mechanical modeling based on constraints from the south china sea scs region we investigate how subduction direction reversal controls the breakup of convergent margins the numerical results show that two distinct breakup modes namely continental interior and edge breakup edge refer to continent above the plate boundary interface may develop depending on the maturity of the convergent margin and the age of the oceanic lithosphere for a slab age of 15 45 ma increasing the duration of subduction promotes the continental interior breakup mode where a large block of the continental material is separated from the overriding plate in contrast the continental edge breakup mode develops when the subduction is a short duration event and in this mode a wide zone of less continuous continental fragments and tearing of the subducted slab occur these two modes are consistent with the interior relic late mesozoic arc and edge relic fore arc rifting characteristics in the western and eastern scs margin suggesting that variation in the northwest directed subduction duration of the proto scs might be a reason for the differential breakup locus along the strike of the scs margin besides a two segment trench associated with the northwest directed subduction is implied in the present day scs region

a 4500 year record of large floods on the colorado river in the grand canyon arizona a sequence of flood deposits left by the colorado river in the grand canyon arizona provides evidence of at least 15 floods with peak discharges greater than 5500 m 3 sec 1 over the last 4500 yr ten floods during the last 2000 2300 yr had discharges greater than 6800 m 3 sec 1 one flood 1600 1200 yr ago had a discharge exceeding 14 000 m 3 sec 1 a flow rate more than twice the largest gaged flood this record of flooding is one of the longest for a major u s river and combined with the gaged record of twentieth century floods allows determination of the frequency and history of large floods that have affected key aspects of colorado river geomorphology

post orogenic extension and metamorphic core complexes in a heterogeneous crust the role of crustal layering inherited from collision application to the cyclades aegean domain summary the development of metamorphic core complexes mcc corresponds to a mode of lithospheric continental stretching that follows collision in most of the models that explain the formation of the mcc high thermal gradients are necessary to weaken the lower crust and to induce its ascent such models fail to explain the exhumation of high pressure low temperature metamorphic rocks in metamorphic core complex structures as observed in the cycladic blueschists in the aegean domain besides account for the lithological crustal stratification induced from collision has never been tested in this paper we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post orogenic continental lithosphere the models are designed and validated by petrological structural and time data from the cyclades as a result high thermal gradients moho temperature higher than 800 c are neither necessary nor always sufficient to induce the development of a metamorphic core complex at the contrary the rheological layering of the crust inherited from collision is a first order parameter controlling the development of extensional structures in post orogenic settings cold mcc can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units as observed in the cyclades

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

lithospheric buoyancy forces in africa from a thin sheet approach many features in rift zones and passive margins have been successfully explained by lithospheric stretching models driven solely by tectonic forces however the origin of these forces and whether they are sufficient for amagmatic rifting of an initially thick lithosphere remain open questions we use a thin sheet approach and the crust 2 0 model to compute vertically averaged deviatoric stresses arising from horizontal gradients of gravitational potential energy computed deviatoric stress directions agree well with earthquake focal mechanisms and stress indicators from the world stress map project we compare the resulting force per unit length with the integrated strength of the lithosphere in the east african rift and show that buoyancy forces may account for a significant part of the force budget causing continental rifting but are likely insufficient to rupture an initially thick and cold continental lithosphere this suggests other processes contribute at a significant level to the force balance driving continental rifting in east africa

clay mineralogy as a crucial factor in badland hillslope processes abstract comprehensive and important work has been done in understanding badland processes on both the big scale and more recently on smaller scale in order to understand the subtle differences between sites however most studies lack important information regarding clay mineralogy and their importance for weathering processes type and rate of weathering depend on material physico chemical properties climatic regimes and slope characteristics as weathering progresses the surface conditions change influencing erosion processes usually badland lithologies are reported as sodium rich highly dispersive materials but that is not invariably true the primary objective of this study is to show that clay minerals have an important role in defining surface and subsurface processes on badland hillslopes field studies conducted in dinosaur park badlands alberta canada chinguacousy badlands ontario canada and sestino badlands tuscany italy have shown that there are two dominant clay rich lithologies smectite rich and smectite poor mudrocks different surface crust and weathering profiles were observed on smectite rich mudrocks during two field years with distinct precipitation inputs after a dry fall winter season the popcorn surface was very well developed 2 3 cm the subsurface layer 10 15 cm thick overlaid the layer of smaller shards followed by the unweathered material after a very wet fall winter season the surface layer was denser and thinner 0 5 cm with wider cracks and a thin subsurface layer attached to the crust weathering profiles on smectite poor lithologies did not show any differences with variable climatic conditions a thin surface layer was composed of soft thin and small 0 5 0 3 0 1 cm shards a layer of larger shards overlying unweathered material was beneath it in order to better understand the described occurrences badland materials were monitored in controlled laboratory conditions during an incremental weathering experiment six samples of each material were subjected to 10 cycles of simulated rainfall which duration ranged 10 60 min very short rainfalls 10 20 min can cause swelling of clay minerals in smectite rich materials and formation of popcorn surface during drying periods during subsequent wetting drying periods the crust becomes flatter and denser and desiccation cracks become wider and deeper samples subjected to 50 and 60 min of rainfall showed maximum swelling after only one cycle of rainfall after this swift swelling samples became unstable and dispersive during subsequent wetting dispersion became dominant resulting in flatter surfaces thinner crust and narrower desiccation cracks smectite poor mudrocks appeared to be resistant to variable wetting and drying large shards broke down into smaller due to differential swelling of illite and chlorite once they were reduced to tiny flaky shards the surface became compacted the surface cracks served as the initial pathways for sediment and water movement knowing if the material is smectite rich or smectite poor can help in prediction of processes and the erosion rates the amount intensity and duration of precipitation will determine not only the rate of weathering but also the type of the weathered surface on smectite rich surfaces smectite poor materials appear to be more resistant to change in precipitation

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

factors controlling the mode of rift interaction in brittle ductile coupled systems a 3d numerical study 1 the way individual faults and rift segments link up is a fundamental aspect of lithosphere extension and continental break up little is known however about the factors that control the selection of the different modes of rift interaction observed in nature here we use state of the art large deformation 3d numerical models to examine the controls on the style and geometry of rift linkage between rift segments during extension of crustal brittle ductile coupled systems we focus on the effect of viscosity of the lower layer the offset between the rift basins and the amount of strain weakening on the efficiency of rift linkage and rift propagation and the style of extension the models predict three main modes of rift interaction 1 oblique to transform linking graben systems for small to moderate rift offset and low lower layer viscosity 2 propagating but non linking and overlapping primary grabens for larger offset and intermediate lower layer viscosity and 3 formation of multiple graben systems with inefficient rift propagation for high lower layer viscosity the transition between the linking mode 1 and non linking mode mode 2 is controlled by the trade off between the rift offset the strength of brittle ductile coupling and the amount of strain weakening the mode transition from overlapping non connecting rift segments mode 2 to distributed deformation mode 3 is mainly controlled by the viscosity of the lower layer and can be understood from minimum energy dissipation analysis arguments

decollements detachments and rafts in the extended crust of dangerous ground south china sea the role of inherited contacts we investigate the crustal structure of the dangerous ground south china sea through processing and interpretation of coincident wide angle reflection and refraction seismic data continental crust of dangerous ground has been moderately thinned down to 15 km so that most of the structures accompanying the early opening of the south china sea from cretaceous to miocene have been preserved subbasement reflectors as well as refraction velocities image an interpreted dismantled mesozoic metamorphic unit in the southernmost section of our study area a rollover structure indicates that the reflective base of the unit was used as a decollement where low angle normal faults root and blocks rafted the metamorphic unit is discontinued in a nearby basin located immediately to the north where the refraction velocity model shows thinning of the crust from 20 to 15 km with the presence of a 5 km high mantle dome in this deeper basin mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an similar to 15 km offset crustal detachment surface that we link down to the mantle dome we infer that the detachment reactivated an inherited low angle contact most probably related to the yanshanian belt in map view the reactivated structure forms a half graben basin oriented nne ssw oblique to the generally accepted direction of extension this orientation follows the general trend of a granitic belt that spanned the south china margin prior to extension related to the subduction of the paleo pacific

insights into the effects of oblique extension on continental rift interaction from 3d analogue and numerical models abstract continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity these rift segments arise from rift initiation at non aligned crustal heterogeneities and need to interact to develop a full scale rift system here we test the effects of 1 oblique extension and 2 initial heterogeneity seed offset on continental rift interaction with the use of an improved analogue model set up x ray computer tomography ct techniques are used to analyse the 3d models through time and the results are compared with additional numerical models and natural examples the experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike slip dominated systems towards the strike slip domain we also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface initial soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal rather than linking at depth the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough orthogonal extension promotes the formation of intra rift horsts which may provide hydrocarbon traps in nature

crusted microtopography on badland slopes in southeast spain summary badland slopes developed in soft miocene marine marls of the guadix baza depression of southeast spain have four crust types developed on them the bare crusted surfaces are confined to slopes with southerly aspects the north facing slopes are sparsely vegetated and these crusts are generally absent the crusts and the weathered mantle on which they occur have been tested for soil ec aggregate stability and investigated in thin section the distribution of crust types in a representative badland gully has been mapped one of the crust types the pediment crust is a depositional feature found on low angled pediments flanking the steep gully side slopes the other three crust types domed tower and pinnacle represent a sequence of increasing age and increasing stability associated with progressive colonisation by the blue green algae microcoleus vaginatus and scytonema and the lichen dermatocarpon lachneum while this is accompanied by a progressive reduction in soil ec the evidence suggests that the algae and lichen play an important role in stabilising the soil surface the development sequence of these crust types is outlined

extension modes and breakup processes of the southeast china northwest palawan conjugate rifted margins abstract our understanding of continent ocean transition structures and magmatism in the absence of excessive magmatic additions has been guided by the observations and models developed at the magma poor iberia newfoundland conjugate margins recently these models have been challenged in the south china sea in light of new iodp expeditions 367 368 368x we have used an integrated analysis of high quality seismic reflection and gravity anomaly data calibrated against recent deep sea drilling results to investigate margin structure and tectono magmatic interplay during continental breakup and early seafloor spreading between the se china nw palawan conjugate margins the eocene oligocene south china sea rifting initiates in a heterogeneous and likely thermally un equilibrated lithosphere formed by the mesozoic yenshanian orogeny gravity and joint inversion methods confirm lateral variation of basement densities across the conjugate margins lithospheric and basement heterogeneities induced a rifting style characterized by a series of highly thinned rift basins associated with extensional faulting soling out at various crustal levels final rifting in late eocene triggered decompression melting forming mid ocean ridge type magmatism which emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks this initial magmatic activity was concomitant with continued deformation of continental crust by extensional faulting integrated analysis of seismic reflection profiles and gravity anomaly data combined with deep sea boreholes accurately locate the continent ocean boundary we show that the initial igneous crust continentward of oceanic magnetic anomaly c10n is asymmetric in width and in morphology for the conjugate margins the wider and faulted newly accreted domain on the se china side indicates that magmatic accretion is associated with tectonic faulting during the formation of initial oceanic lithosphere we suggest that deformation was not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically until the stabilisation of the spreading ocean ridge around c10n the analysis of the south china sea breakup reveals a transient interplay between faulting magmatic budget and extension rates during the formation of the continent ocean transition and initial emplacement of igneous crust

the south china sea margins implications for rifting contrasts abstract implications regarding spatially complex continental rifting crustal extension and the subsequent evolution to seafloor spreading are re examined for the northern and southern rifted margins of the south china sea previous seismic studies have shown dramatic differences in the present day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south china although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment this difference was accommodated by the early formation of oceanic crust creating the present day south china sea basin adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west using the observed cross sectional areas of extended continental crust derived from deep penetration seismics two end member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin the two models examined predict that the oceanic crust of the south china sea basin toward the west did not begin forming until sometime between 6 12 my after its initial formation 32 ma toward the east these results are compatible with crustal age interpretations of marine magnetic anomalies assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the south china sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two we suggest the most likely causes of the rifting differences were east west variations in the rheology of the pre rift crust and associated east west variations in the thermal structure of the pre rift lithosphere the calculated widths of rifted continental crust for the northern and southern margins when combined with the differential widths of seafloor generated during the seafloor spreading phase indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within 10 for all three eastern central western segments examined

nature and origin of the j magnetic anomaly offshore iberia newfoundland implications for plate reconstructions the nature and origin of the j magnetic anomaly along the iberia newfoundland margins are controversial and its validity for plate kinematic reconstructions questioned at present it is interpreted as either an oceanic isochron or an edge effect of oceanic crust corresponding to lithosphere breakup both interpretations result in restorations that are in conflict with the current knowledge from pyrenean and north atlantic geology we combine seismic interpretations and dating of magmatic additions with magnetic data to examine the nature and formation process of this anomaly and discuss its value for plate restorations we show that the j anomaly is the result of polygenic and multiple magmatic events occurring during and after the formation of the first oceanic crust therefore we conclude that the j anomaly cannot be used for plate kinematic studies and more generally we question the validity of using ill defined magnetic anomalies outside unequivocal oceanic domains for plate reconstructions this article is protected by copyright all rights reserved

comparative geochemistry of vitrinite kerogens from carboniferous and tertiary provinces abstract in this study vitrinite concentrations were prepared from vitrain bands from two carboniferous coal seam locations in eastern united states a tertiary age coal seam in alaska and a tertiary age coal seam in western canada these vitrinite samples were characterized using pyrolysis gas chromatography in addition to petrographic and bulk chemical analyses the purpose of this study is to apply quantitative pyrolysis gas chromatography to a set of vitrinite concentrates from different coal basins of different age which are at relatively similar levels of rank and determine whether compositional variation can be observed

microfacies and petroleum properties of coals as revealed by a study of north sea jurassic coals abstract the influence of the composition and microstructural aspects of coal microfacies on petroleum properties were investigated in light of petrographic and geochemical studies the first study concerns a collection of north sea jurassic coals of various maceral compositions the second study is a pyrolysis gas chromatography study on pure microlithotypes from a similar french jurassic coal the results show that hydrocarbon potential is logically increased by a high content in liptinite but more often in hydrogen rich vitrinite in the same way the hydrocarbon potential is reduced by a high inertinite content mainly due to pre sedimentary oxidation of the original biomass but also due to early diagenetic processes that lead especially to the formation of micrinite or to post sedimentary alterations these trends indicate that total hydrocarbon potential is increased by an autochthonous origin of the organic matter that was deposited in a relatively closed paleoenvironment and thus protected from detrital and organo detrital inputs finally the oil potential of humic coals as determined by open pyrolysis gas chromatographic studies as related to the weight of rock is similar to classical source rocks the structural aspects of the coal microfacies which probably play a role in the release of oil from coal seams in natural conditions are also discussed

disturbance stream incision and channel evolution the roles of excess transport capacity and boundary materials in controlling channel response abstract channel incision is part of denudation drainage network development and landscape evolution rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non incised surfaces large anthropogenic disturbances similar to large or catastrophic natural events greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power field examples of channel responses to antrhopogenic and natural disturbances are presented for fluvial systems in the mid continent and pacific northwest usa and central italy responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems similar disturbances are shown to result in varying relative magnitudes of vertical and lateral widening processes and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks this apparent contradiction is explained through an analysis of temporal adjustments to flow energy shear stress and stream power with time numerical simulations of sand bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half show identical adjustments in flow energy and the rate of energy dissipation the processes that dominate adjustment and the ultimate stable geometries however are vastly different depending on the cohesion of the channel banks and the supply of hydraulically controlled sediment sand provided by bank erosion the non linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems the sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape with erosion from the channel banks generally the dominant source disturbances that effect available force stream power or flow energy or change erosional resistance such that an excess of flow energy occurs can result in incision channel incision therefore can be considered a quintessential feature of dis equilibrated fluvial systems

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

rift segment interaction in orthogonal and rotational extension experiments implications for the large scale development of rift systems abstract during extension of the continental lithosphere rift basins develop these are often initially offset and must interact and connect in order to create a continuous rift system that may ultimately achieve break up when simulating extensional tectonics and rift interaction structures analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation resulting in rotational extension and associated rift propagation and structural gradients here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings our modelling efforts show that rotational extension and orthogonal extension produce significantly different large scale structures rotational extension can cause important variations in rift maturity between rift segments delay rift interaction zone development and make rift segments propagate in opposite directions still local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting furthermore we find that various degrees of rift underlap produce three basic modes of rift linkage structures low underlap distance high angle experiments develop rift pass structures with increasing underlap distance ca 40 transfer zone basins develop high degrees of underlap 30 tend to result in en echelon sub basins our results match with data from previous modelling efforts and natural examples we furthermore propose a large scale tectonic scenario for the east african rift system based on rotational extension and associated rift propagation these insights may also be applicable when studying other large scale rift systems

symmetry of the south china sea conjugate margins in a rifting drifting and collision context abstract the symmetry of conjugate rifted margins is a first order observable feature reflecting the geodynamic processes acting during and after continental rifting the symmetry of the south china sea scs rifted margins can be deduced from comprehensive geophysical datasets on a continental margin scale here we combine three key approaches 1 lateral fault distributions are delineated according to free air gravity anomalies 2 crustal stretching styles are mapped based on gravity inversion constrained by deep seismic profiles and 3 total extension of conjugate margins is estimated according to margin restorations and tectonic settings we infer a predominantly symmetric rifting style caused by pure shear extension with only narrow domains of asymmetric deformation in continent ocean transition cot regions that have undergone simple shear and where the lower crust of the highly thinned distal margin is embrittled before continental breakup however our analysis also suggests that this symmetry has been modified by post rift geodynamic processes southward lower crustal flow which occurred only on the southern margin due to the low viscosity of the lower crust and the slab pull induced by the subducting proto scs plate during seafloor spreading shifted the crustal stretching styles from symmetric to asymmetric the collision between the southern margin and borneo thickened the lower crust more than the upper crust at the southern end of the southern margin and shortened the southern margin this event had a large impact on the se margin but a small impact on the sw margin we conclude that 1 for the se and ne margins the crustal stretching styles shifted from asymmetric to approximately symmetric and the total extension shifted from symmetric to asymmetric 2 for the sw and nw margins the crustal stretching and total extension remained asymmetric and symmetric respectively

wind transport of sand surfaces crusted with photoautotrophic microorganisms abstract wind tunnel experiments show that while surfaces inoculated with photoautotrophic organisms are stable in winds up to 19 m s 1 only filamentous cyanobacteria especially nostoc commune afford considerable protection against abrasion from windborne particles crusts formed by the polysaccharide secreting alga chlamydomonas acidophila break down easily under impact the entrainment of large low density aggregates possibly contributing to higher transport rates than would otherwise be expected for untreated sediment surface disintegration encompasses a suite of complex scale dependent processes which vary with the structure thickness and strength of the crust independent measurements of crust strength based on flexure testing are in good agreement with wind tunnel evaluation of crust stability the accommodation of a relatively large amount of deformation without rupture is an important property of fibrous photoautotrophic mats perhaps outweighing consideration of their peak strength

a genetic link between transform and hyper extended margins abstract the similarity between the geometry of the west african and south american coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm however using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions using state of the art 3d coupled thermo mechanical numerical models we simulate for the first time crustal deformation at the onset of oceanisation along large offset oblique margins our models show that obliquity causes oceanic rift propagation to stall resulting in an apparent polyphased tectonic evolution and in some circumstances leads to the formation of hyper extended margins as a result conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading with their lengths differing by a factor of 5 to 10 before the final phase of break up occurs accounting for this discrepancy should ameliorate future palaeogeographic reconstructions

land degradation in drylands interactions among hydrologic aeolian erosion and vegetation dynamics article i nfo land degradation in drylands is one of the major environmental issues of the 21st century particularly due to its impact on world food security and environmental quality climate change shifts in vegetation composition accelerated soil erosion processes and disturbances have rendered these landscapes susceptible to rapid degradation that has important feedbacks on regional climate and desertification even though the role of hydrologic aeolian erosion and vegetation dynamic processes in accelerating land degradation is well recognized most studies have concentrated only on the role of one or two of these components and not on the interactions among all three drawing on relevant published studies here we review recent contributions to the study of biotic and abiotic drivers of dryland degradation and we propose a more holistic perspective of the interactions between wind and water erosion processes in dryland systems how these processes affect vegetation patterns and how vegetation patterns in turn affect these processes notably changing climate and land use have resulted in rapid vegetation shifts which alter the rates and patterns of soil erosion in dryland systems with the predicted increase in aridity and an increase in the frequency of droughts in drylands around the world there could be an increasing dominance of abiotic controls of land degradation in particular hydrologic and aeolian soil erosion processes further changes in climate may alter the relative importance of wind versus water erosion in dryland ecosystems therefore acquiring a more holistic perspective of the interactions among hydrologic aeolian erosion and vegetation dynamic processes is fundamental to quantifying and modeling land degradation processes in drylands in changing climate disturbance regimes and management scenarios

process interactions temporal scales and the development of hillslope gully systems howgill fells northwest england abstract hillslope gully development is often controlled by interactions between multiple processes rather than by individual processes such interactions may have both spatial and temporal characteristics a twenty year monitoring program of hillslope gullying in the howgill fells northwest england has identified the roles of both on slope interactions and slope stream coupling interactions which can be related to several temporal scales on slope interactions are primarily seasonal whereas those related to coupling produce a cyclicity over timescales of up to several years over the longer term decades or more progressive changes in gully morphology modify the nature of both sets of interactions and through negative feedback mechanisms may ultimately lead to gully stabilisation gully development is controlled by the rates of headwall recession and the basal incision stabilisation behaviour after basal stabilisation vegetation encroachment onto the eroding slopes takes place faster than headwall retreat resulting in a finite age and limiting size for gully development a simple model based on the current rates of development adequately describes the morphology of modern gullies but is inappropriate when applied to an older generation of stabilised gullies this suggests that there have been changes in sediment generation and removal rates during the late holocene

linking environmental regimes space and time interpretations of structural and functional connectivity connectivity as a concept has been increasingly part of discussions or explanations in hydrology geomorphology and ecology we address recent critiques of this approach by demonstrating how a refinement which distinguishes structural connectivity from functional connectivity can be used to explain patterns observed in very different environmental systems these systems are found in linkages between surface and subsurface flowpaths and the hyporheos in the river don a temperate river channel in yorkshire uk in surface and subsurface fluxes in agricultural land in the uk and in vegetation and surface conditions in a degrading environment at the sevilleta lter site in the semi arid southwest usa first we demonstrate long term geological and structural controls mediated by in channel processes second human organization of landscape elements is a significant control on runoff and erosion so that similar events can produce very different responses and vice versa third linkages between the removal of grass vegetation and runoff and erosion produce non linear and path dependent feedbacks which control the subsequent degradation of the landscape making the process difficult to reverse as a result of these studies we argue that even in cases where connectivity cannot be directly quantified at least at present this limitation does not prevent the concept from being a useful heuristic device for exploring responses of complex systems furthermore this result implies that an increasing need exists for disciplinary connectivity to investigate such systems

crustal structure and inferred rifting processes in the northeast south china sea abstract taiger project deep penetration seismic reflection profiles acquired in the northeastern south china sea scs provide a detailed view of the crustal structure of a very wide rifted continental margin these profiles document a failed rift zone proximal to the shelf a zone of thicker crust 150 km from the shelf and gradually thinning crust toward the cob spanning a total distance of 250 300 km such an expanse of extended continental crust is not unique but it is uncommon for continental margins we use the high quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and in turn what rheological variations are predicted that allow for protracted crustal extension upper crustal thinning is greatest at the failed rift uc 7 5 but is limited farther seaward uc 1 2 we interpret that the lower crust has discordantly thinned from an original 15 17 km to possibly less than 2 3 km thick beneath the central thick crust zone and more distal areas this extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere the observed upper crustal thickness variations and implied rheology lower crustal flow are consistent with large scale boudinage of continental crust during protracted extension

successive rifting events in marginal basins the example of the coral sea region papua new guinea reactivation of extensional structures is commonly inferred during rift evolution in that context we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the coral sea region papua new guinea the first event r 1 poorly documented occurred during the triassic along an older n s permian structural fabric during the jurassic extensional faults were reactivated through a second extensional episode r 2 which formed small 10 20 km basins bounded by n s ne sw and e w listric faults extension prolonged during the lower cretaceous with seafloor spreading in the owen stanley oceanic basin now incorporated in the papuan fold and thrust belts a third late cretaceous extensional phase r 3 gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent ocean transition seafloor spreading in the coral sea followed from danian to ypresian this extensional system is sealed by unequally preserved eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from oligocene onward this overall evolution suggests various extensional systems that are geographically and temporarily defined the one another the early rifting of the crust is controlled by preexisting continental features resulting in the local pangaea breakup in contrast the coral sea propagator cuts through the rifted margin and is controlled by a subduction complex in accordance with the tasman sea opening this evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around australia

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin