scratching technique for the study and analysis of soil surface abrasion mechanism aeolian abrasion is the most fundamental and active surface process that takes place in arid and semi arid environments its nature is a wear process for wind blown grains impinging on a soil or sediment surface which causes particles and aggregates to fracture from the soil surface through a series of plastic and brittle cracking deformation such as cutting ploughing and brittle fracturing using a universal micro tribometer umt a scratching test was carried out on six soil surfaces sandy soil sand loam silt loam loam silt clay loam and silt clay the results indicate that traces of normal and tangential force vs time show a jagged curve which can reflect the plastic deformation and brittle fracturing of aggregates and particles of various sizes fractured from the soil surfaces the jagged curve peaks and the area enclosed underneath may represent the bonding forces and bonding energies of some aggregates and grains on the soil surface respectively connecting the scratching test with an impact abrasion experiment furthermore demonstrates that soil surface abrasion rates are proportional to the square of speeds of impacting particles and to the 2 6 power of mean soil grain size and inversely proportional to the 1 5 power of specific surface abrasive energy or to the 1 7 power of specific surface hardness

geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

a numerical model of deformation and fluid flow in an evolving thrust wedge abstract to investigate deformation and fluid flow in an actively deforming tectonic wedge we model the evolution of a large two dimensional 100 km long 5 km thick mechanically and hydrologically homogeneous and isotropic pile of sedimentary strata that is deformed to become a thrust wedge we compare both dry and wet cases in order to illustrate 1 the relative importance of fluids on wedge evolution and 2 the effect of brittle deformation on fluid flow we use an elastic plastic constitutive relation including a mohr coulomb failure criterion and a non associated flow rule and coupled fluid flow with bulk rock properties that approximate typical foreland sedimentary strata simulations are made both with and without dilation the model is fully dynamic but inertial forces remain small results show that deformation within the wedge is accommodated by reverse slip movement on shear bands which migrate in both directions through the wedge as both fore and back thrusts the model has features predicted by the critical taper theory 1 overall wedge geometry 2 crudely self similar growth during evolution 3 more intense deformation toward the rear of the wedge the models show strong overall in sequence faulting behavior with major thrusts isolating relatively undeformed packages which are moved in a piggyback manner upon the active thrusts intermittent out of sequence faulting does however occur in order to maintain the wedge taper fluid flow in the deforming wedge is dominated by topography but is also strongly affected by dilational plastic deformation in all simulations there is focused fluid flow within fault zones when mechanical time stepping is shut off uncoupled flow systems evolve to steady state where inflow equals outflow by subtracting the two states we isolate the mechanical fluid response from the total coupled system response the mechanical fluid response is manifest as contours of head and pressure difference and focusing of fluid flow in areas of active deformation in non dilational simulations these zones are quite localized and are a result of the contrasting stresses in and adjacent to a brittle shear zone in simulations with dilation the zones of head and pressure difference are broad and regional in extent they are ten times greater in magnitude than in non dilation simulations and they track a rock damage front that moves through the wedge

origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

the factors influencing the abrasion efficiency of saltating grains on a clay crusted playa the entrainment and subsequent transport of pm10 particulate matter 10 m has become an important and challenging focus of research for both scientific and practical applications arid and semi arid environments are important sources for the atmospheric loading of pm10 although the emission of this material is often limited by surface crusts it has been suggested that the primary mechanisms through which pm10 is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities to examine the importance of saltation abrasion in the emission of pm10 a series of field wind tunnel tests were conducted on a clay crusted surface near desert wells arizona in a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate although there can be considerable variation in this relationship this paper more closely examines the source of the variability in the abrasion efficiency the amount of pm10 emitted by a given quantity of saltating grains the abrasion efficiency was found to vary with the susceptibility of the surface to abrasion the ability of the sand to abrade that surface and the availability of material with a caliper size 10 m within the crust specifically the results of the study show that the abrasion efficiency is related to the crust strength the amount of surface disturbance and the velocity of the saltating grains it is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models and can make theoretical models too complex and impractical to be of use copyright 2001 john wiley sons ltd

effects of subducted seamounts on megathrust earthquake nucleation and rupture propagation 1 subducted seamounts have been linked to interplate earthquakes but their specific effects on earthquake mechanism remain controversial a key question is under what conditions a subducted seamount will generate or stop megathrust earthquakes here we show results from numerical experiments in the framework of rate and state dependent friction law in which a seamount is characterized as a patch of elevated effective normal stress on the thrust interface we find that whether subducted seamounts generate or impede megathrust earthquakes depends critically on their relative locations to the earthquake nucleation zone defined by depth variable friction parameters a seamount may act as a rupture barrier and such barrier effect is most prominent when the seamount sits at an intermediate range of the seamount to trench distances 20 100 of the nucleation zone to trench distance moreover we observe that seamount induced barriers can turn into asperities on which megathrust earthquakes can nucleate at shallow depths and rupture the entire seismogenic zone these results suggest that a strong barrier patch may not necessarily reduce the maximum size of earthquakes instead the barrier could experience large coseismic slip when it is ruptured

transformation of an archean craton margin during proterozoic basin formation and magmatism the albany fraser orogen western australia abstract the albany fraser orogen is a well preserved example of proterozoic modification of an archean craton margin the formation of two successive basin systems accompanied by magmatism along the southern and southeastern yilgarn craton margin in western australia reflect distinct changes in tectonic regimes resulting in significant transformations of the archean craton crust provenance analysis from detrital zircons indicates that the first basin system the c 1815 1600 ma barren basin was dominantly filled with neoarchean detritus derived from the yilgarn craton and paleoproterozoic detritus derived from coeval voluminous mostly felsic magmatism the abundance of locally derived sediment deposited onto a reworked archean yilgarn craton substrate coupled with the progressively more juvenile isotopic signature of magmas indicates a largely extensional tectonic setting consistent with either a continental rift basin or a long lived backarc basin system this extensional regime led to the formation of a passive margin and oceanic basin along the craton edge the c 1600 1305 ma arid basin and formation of an ocean continent transition in the most heavily modified outboard region of the craton margin the eastern nornalup zone a change in tectonic regime is denoted by the formation of an oceanic arc the c 1410 ma loongana arc preserved in the adjoining madura province oceanic subduction led to accretion of the loongana arc onto the craton edge recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin uppermost arid basin this accretion event triggered crustal thickening and the earliest phase of magmatism of stage i of the albany fraser orogeny at c 1330 ma a change to west dipping subduction beneath the accreted portion of the loongana arc placed the orogen into a backarc setting into which the remainder of the recherche supersuite mafic and felsic magmas were intruded thus the albany fraser orogen is regarded as an archean craton margin that preserves a long history of proterozoic transformation dominated by extensional processes that resulted in the formation of orogen wide basin systems accompanied by magmatism the recognition that the albany fraser orogen has always been part of the west australia craton and the tectonic regimes that have affected it provide important constraints on reconstructions of the nuna and rodinia supercontinents

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

an experimental study on the susceptibility of crusted surfaces to wind erosion a comparison of the strength properties of biotic and salt crusts abstract penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems despite the early success of this procedure questions remain concerning the effect of the angle of penetration the role of fatigue and the relative importance of the ultimate strength as compared to the elasticity of the crust this paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts the results demonstrate that the angle of penetration is not important but the density of penetrations is the decreased strength observed with high densities of penetration repeated loading and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events the elasticity of the surface appears to be at least as important to consider as the ultimate strength although much stronger the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies the notable elasticity of biotic crusts appears to afford them some protection against abrasion

heterogeneous strain regime at the west of the ogasawara plateau in the western pacific ocean from inversion of earthquake focal mechanisms abstract the ogasawara plateau has a major control on the geology of the region between the izu bonin and mariana subduction zones this region is important for understanding active collisional zones we attempt to illustrate the stress field in the wadati benioff zone of this region through seismic estimations 49 events m 3 5 are chosen for waveform inversion by the isola software to distinguish the stress field according to available focal mechanism solutions the heterogeneous stress field parameters are determined for several depth ranges based on the epicentral distribution and seismic stress field the subduction system at the west of the ogasawara plateau can be divided into three segments separated by 26 5 n and 24 n at depths of 0 50 km in the outer trench slope the slab is characterized by a varying stress filed the best fit stress model with horizontal 3 and vertical 1 is related to the plate bending and preexisting faults for the central segment the solutions present a compressional stress regime in the outer trench slope without preexisting faults induced by the resistance of the buoyant plateau the solutions for the depth range of 0 50 km in the forearc region 1 coincides with the plate convergence and 3 dips steeply to the west in the northern and central segments controlled by the friction effect and convergence of the plates however this depth range in the southern segment shows an extensional stress range associated with subduction of older slab segment relatively low rate of plate convergence and the morphological adjustment of the slab between different segments at depths of 50 200 km the inhomogeneous stress field exhibits a complicated extensional stress system with strike slip focal mechanisms at depths of 200 400 km the seismicity presents a pronounced seismic gap and the stress field cannot be determined the seismic gap may be attributed to the missing or tear of the subducting slab which is caused by the subduction of the buoyant ogasawara plateau the mechanism solutions are characterized by an inhomogeneous strain regime below 400 km driven by gravity and strong contortion of the slab and resistance to penetration of the 660 km discontinuity in contrast there are little earthquakes below 400 km in the southern region of the ogasawara plateau

effects of rheology and mantle temperature structure on edge driven convection implications for partial melting and dynamic topography abstract edge driven convection which affects partial melting intraplate volcanism and dynamic topography is small scale convection that occurs along a lithospheric keel with a sharp contrast in lithospheric thickness various factors including rayleigh number lateral mantle temperature heterogeneity and geometry of the keel influence the edge driven convection and the correlation between edge driven convection and surface expressions dynamic topography and volcanism is complicated we performed a finite element study to quantify the effects of these factors on dynamic topography and partial melting we found that the dynamic topography is more prominent when a strong edge driven convection cell develops which corresponds to homogeneous mantle temperatures and the absence of mantle wind in contrast the development of edge driven convection cells and dynamic topography near the lithospheric keel are hindered when the mantle temperature is strongly heterogeneous laterally varying 280 k this indicates that a large lateral contrast in mantle temperature results in a strong mantle wind that may prevent the development of edge driven convection cells an increase in the rayleigh number results in more vigorous convection and enhances partial melting our study shows that the location of volcanic activity at craton edges and passive margins can be reproduced in models with weakly heterogeneous mantle temperature for given mantle viscosity the existence of a strong mantle wind e g related to subducting slabs or mantle plumes may inhibit the formation of an edge driven convection cell and its related partial melt near a lithospheric keel however mantle conditions with weak temperature heterogeneity 17 1019 pa s which corresponds to the rayleigh number of 1 8 106 do not induce partial melts despite the development of edge driven convection cells our model parametrized the condition and location of edge driven convection cells and partial melts which can contribute to understanding anomalous intraplate volcanisms such as in jeju island south of the korean peninsula and the tanzania craton near the east african rift

control of seafloor roughness on earthquake rupture behavior earthquake rupture complexity is described for three recent large underthrusting earthquakes along the costa rican subduction zone the 1983 osa 1990 nicoya gulf and 1999 quepos events these earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate the 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities in contrast the 1983 event occurred along the subducting cocos ridge and had a complex rupture history comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process

pore pressure distribution of a mega splay fault system in the nankai trough subduction zone insight into up dip extent of the seismogenic zone we use the pore pressure distribution predicted from a waveform tomography wt velocity model to interpret the evolution of the mega splay fault system in the nankai trough off kumano japan to map pore pressure around the mega splay fault and plate boundary decollement we integrate the high resolution wt velocities with laboratory data and borehole well log data using rock physics theory the predicted pore pressure distribution shows that high pore pressures close to lithostatic pressure along the footwall of the mega splay fault extend seaward to the trough region and the normalized pore pressure ratio is nearly constant over that extent this continuity of the overpressured zone indicates that a coseismic rupture can potentially propagate nearly to the trough axis we interpret a high pressure belt within an accretionary wedge on the landward side of the present mega splay fault as evidence of the ancient mega splay fault because the ancient mega splay fault soles into the active mega splay fault the active mega splay fault may function as a basal detachment fault and is directly connected to the seaward plate boundary decollement

deep slab fluids fuel extremophilic archaea on a mariana forearc serpentinite mud volcano ocean drilling program leg 195 1 as the pacific plate subducts beneath the mariana forearc it releases water that hydrates the overlying mantle wedge converting it to serpentinite that protrudes to form mud volcanoes at the seafloor excess h2o ascends through these mud volcanoes and exits as cold springs at their summits the composition of this deep slab derived water has been determined by drilling on two of these seamounts it has a ph of 12 5 and relative to seawater is enriched in sulfate alkalinity na cl k rb b light hydrocarbons ammonia 18o and deuterium and depleted in chloride mg ca sr li si phosphate and 87sr within the upper 20 m below seafloor at south chamorro seamount a microbial community operating at ph 12 5 made up overwhelmingly of archaea is oxidizing methane from the ascending fluid to carbonate ion and organic carbon while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

deformation of a seamount subducting beneath an accretionary prism constraints from numerical simulation we examined the process of seamount subduction via a numerical simulation using the finite element method applying a frictional force on the plate interface that is proportional to the normal stress we calculate the incremental stress due to infinitesimal deformation of the seamount associated with subduction and consider the implications for stress buildup and fracturing of the seamount itself our results show that the maximum shear stress concentrates at both flanks of the seamount which suggests that fracturing will start there we can surmise that eventually the seaward flank may be more apt to break than the landward flank at shallow depth if the confining pressure there is sufficiently low we consider this to be a possible scenario for the generation of a thrust fault imaged at the seaward flank of the muroto seamount which is subducting under the nankai trough accretionary prism

translating the mineral systems approach into an effective exploration targeting system abstract holistic mineral systems approaches are being increasingly adopted in the study of ore deposits but application in industry has been suboptimal because of the lack of a conceptual framework to help translate the mineral system understanding to practical application a four step process is proposed for linking the conceptual mineral system with data available to support exploration targeting these steps include translation from 1 critical processes of the mineral system to 2 constituent processes of the mineral system to 3 targeting elements reflected in geology and 4 targeting criteria used to detect the targeting elements directly or by proxy this translation process is illustrated using examples from komatiite hosted ni cu pge and orogenic au mineral systems ranking of targets must be at the critical process level rather than the current practice of mixing processes elements and proxies the significance of targeting elements and their mappable criteria changes as a function of the scale at which the critical processes operate and the availability of relevant geoscience datasets this scale dependence is rarely recognised in current practice sources of uncertainty when translating the mineral system understanding into an effective exploration targeting system include 1 resolution of a mappable criterion in a dataset 2 how well the targeting criterion reflects the targeting element 3 how well the targeting element reflects the critical process and 4 whether the process being mapped is critical for the genesis of a mineral deposit the first three sources of uncertainty collectively are termed the representativeness of the targeting criteria i e how representative is a mappable criterion of the constituent process for which it is a proxy the last source of uncertainty reflects our lack of understanding of the processes that control mineralisation and highlights areas for further research

global plate motion and the geoid a physical model summary the effect of lateral variations in the sub lithospheric viscosity i e in the lithosphere asthenosphere coupling is incorporated in dynamic earth models driven by plate velocities if the coupling is stronger under fast moving continents it is shown that a geoid low will develop in their wake and a high in front of them thus the well known low in the indian ocean could be explained at least in part in terms of induced upper mantle dynamics similarly if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs whereas ridges could correspond to much weaker geoid lows this also seems to agree with some features of the observed geoid at very long wavelengths the mathematical framework of such dynamic earth models is developed extensively here these models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth their internal flow patterns are only driven by imposed surface velocities not by internal loads

prediction of faulting from the theories of elasticity and plasticity what are the limits elasticity rigid plasticity and elasto plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting however in practice the limits of their application are not always clear in this paper we test the behaviour of these different models using as examples tectonic problems of indentation of a die compression with basal shear bending of a plate and normal faulting around a dike by comparing the results of these tests we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting the theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains the theory of rigid plasticity is more appropriate for large deformations where the geometry of faults can be found by the method of characteristics this method works well for zones of failure that are not severely constrained by elastic material outside e g when faults are connected to the free surface a viscous substratum or a zone of weakness non associated elasto plasticity is the most complete theory among those considered in this paper it describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

an experimental study of multiple grain size ejecta produced by collisions of saltating grains with a flat bed collision data are presented from coloured high speed films of three size fractions of sand grains saltating over a bed of the total grain population each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded the results confirm earlier findings that v3 v1 0 5 0 6 vn v1 08 and the rebound angle increases with decreasing grain size ejected grains are examined in relation to their size the impactor size ejection speed and angle and the number of ejecta per collision in addition changes in grain parameters are observed with time for fine impactors ejection speeds generally increase with a decrease in ejecta size but the fine fraction does not follow this trend for the coarse and medium impactors ejection angles are typically between 40 and 60 with coarse grains having shallower mean angles than fine ejecta the number of ejections per collision increases with a decrease in particle size for each impactor size the general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes there is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains much of the forward momentum of the saltating grains is transfered to creeping grains the composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud

crustal structure of a proterozoic craton boundary east albany fraser orogen western australia imaged with passive seismic and gravity anomaly data abstract we use passive seismic and gravity data to characterize the crustal structure and the crust mantle boundary of the east albany fraser orogen in western australia a proterozoic orogen that reworked the southern and southeastern margin of the archean yilgarn craton the crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen but narrows to the southwest common conversion point profiles show a clear transition from a wide symmetric moho trough in the northeast to a one sided northwestern moho dip in the southwest where the moho appears to underthrust the craton towards its interior this change appears to coincide with the inferred trace of the ida fault a major terrane boundary within the yilgarn craton forward modelling of gravity anomaly data using the retrieved moho geometry as a geometric constraint shows that a conspicuous elongated gravity low on the northwestern side of the east albany fraser orogen is almost certainly caused by thickened archean crust to obtain a model that resembles the regional gravity pattern the following assumptions are necessary high density rocks occur in the upper crust of the fraser zone at depth inside the moho trough and in parts of the eastern nornalup zone east of the moho trough although our gravity models do not constrain at which crustal level these high density rocks occur active deep seismic surveys suggest that large extents of the east albany fraser orogen s lower crust include a mesoproterozoic magmatic underplate known as the gunnadorrah seismic province the simplest interpretation of the imaged crustal structure is that the gunnadorrah seismic province is underthrust beneath the yilgarn craton likely as a consequence of crustal shortening during accretion further east the imaged geometry overall appears to show a wedge of archean lower crust that was driven between the exhumed fraser zone and the gunnadorrah seismic province effectively splitting the paleo to mesoproterozoic crust of the east albany fraser orogen the vertical splitting of proterozoic crust by a cratonic crustal wedge comparable to what we image in this study may be a process that contributed to forming many craton margins around the world

the affinity of archean crust on the yilgarn albany fraser orogen boundary implications for gold mineralisation in the tropicana zone abstract craton margins can be subject to a wide array of gold genesis and redistribution processes although high grade terrains on craton margins are frequently viewed as less prospective than lower grade counterparts in contrast to this the high grade tropicana zone a newly defined archean crustal component on the eastern margin of the yilgarn craton within the albany fraser orogen afo contains a significant proterozoic gold deposit this deposit and zone comprise mid amphibolite to granulite facies gneissic rocks with evidence of partial melting and granite injection the tropicana zone contains significant low si lile enriched granites classed as sanukitoids along with the distinctive compositions the rarity of these rocks within any archean craton suggests that the granitoid protoliths represent a single suite emplaced during one event due to the intense granulite facies overprinting of the tropicana zone rocks determination of the magmatic protolith age for these sanukitoids is challenging nonetheless the best age estimate for magmatism is 2692 16 ma based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt this age is older than compositionally similar magmatism found within the yilgarn craton although a sanukitoid in the northern foreland of the afo has a similar age furthermore the granulite facies metamorphic zircon growth in the tropicana zone at 2718 2554 ma was prolonged compared to that in the yilgarn craton nonetheless the hf isotopic signature of the tropicana zone zircon shares strong similarity to that from the eastern goldfields superterrane of the yilgarn craton this implies that the tropicana zone reflects a deeper crustal level of the yilgarn craton exhumed and thrust nw an unknown distance over the craton edge in addition we observe that the granulite facies zircons have a less radiogenic hf isotope signature than the preserved pre metamorphic zircon cores based on correlations with alpha dose u and th content and 176hf 177hf we suggest this reflects the preferential destruction and release of unradiogenic hf from inherited zircon whereas the protolith sanukitoid zircon with lower u and th content was more resistant to mobilisation during high grade metamorphism we note this situation may be a more general response of the hf isotopic system in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic hf reservoir than its felsic counterpart juvenile granitic veins dated at c 1780 ma intruded into the tropicana zone indicate that the tropicana zone was structurally emplaced at or before c 1780 ma given similar proterozoic magmatic events are well documented from para autochthonous adjacent units re os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c 2100 ma supportive of a palaeoproterozoic age of mineralisation this mineralisation event is distinct from major proterozoic tectonothermal events elsewhere in the afo sanukitoid magmas are well known for gold fertility and were likely the original source of gold in the tropicana zone which was subsequently concentrated into brittle structures during several episodes gold mineralisation post dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining yilgarn craton

tensile test of natural microbiotic crust abstract the mechanical properties of microbiotic crust are important to stabilize sandy surfaces in arid and semi arid regions we developed a new experimental approach to perform the tensile test of natural microbiotic crust the modulus of elasticity ultimate stress and one dimensional stress strain relation was obtained the microbiotic crust we measured exhibits linear elastic behavior firstly and then breaks this mechanical character was applied to the study of the rupture of microbiotic crust under the impact of one saltating grain a very simple dimensionless variable was suggested to quantitatively assess surface stability

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

lateral ramps as an aid to the unroofing of deep crustal rocks seismic evidence from the grenville province we suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust our suggestion is based on a study of the distribution of high pressure metamorphosed continental rocks in the mesoproterozoic grenvillian orogen in eastern quebec these rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed they are currently exposed in a 100 km 60 km lobate embayment north of the manicouagan reservoir using a combination of surface structural data and seismic reflection data from lithoprobe line 55 we reconstruct the three dimensional form of the high pressure rocks our reconstruct ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile they make extensive use of the fact that much of the seismic reflectivity on line 55 comes from outside the vertical plane of the seismic line because of the sinuous nature of the line and its obliquity to tectonic strike the validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high pressure rocks at the surface the lateral ramp probably provided a favorable pathway for the extrusion of deep seated rocks from the deep crust and their transport onto the foreland

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

pacific plate subduction beneath the central mariana and izu bonin fore arcs new insights from an old margin 1 multichannel seismic mcs profiles and bathymetric data from the central mariana and izu bonin subduction systems image the subducting pacific plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc subducting oceanic crust varies along the mariana margin from 5 3 to 7 km thick and is covered by 0 5 2 km thick sediments and numerous seamounts oceanic crustal thickness east of the izu bonin trench is 6 km faulting resulting from flexure of the incoming pacific plate begins up to 100 km east of the trench axis near the 6 km depth contour the plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes 25 incoming crust breaks along new faults with a trench parallel strike the mariana trench axis is commonly a graben that accommodates an abrupt change within 8 beneath the outer fore arc we infer that the plate fails there rather than simply bends under the applied loads along portions of the mariana margin subducting seamounts displace the trench axis westward and uplift the toe of the slope surprisingly west of the toe there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction nor of significant subduction erosion or sediment underplating mcs profiles across the base of the mariana inner trench slope provide evidence for both complete subduction and small scale accretion of pacific plate sediments however we found no evidence for long term sediment accretion the subducting plate dips 9 12 beneath serpentinite seamounts on the izu bonin and mariana fore arcs along the mariana margin the majority of these seamounts are located 50 70 km west of the trench where the mantle wedge is 3 7 km thick between 8 10 km thick fore arc crust and the top of the subducting plate the apparent lack of significant deformation of the mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben

a wind tunnel study of particle kinematics during crust rupture and erosion abstract sediments with protective crusts of varying type were subjected to particles fed from an upwind source during wind tunnel experiments carried out to compare their ability to resist erosion and alter the kinematics of the saltation cloud a laser doppler anemometer measured the distribution of particle velocity for saltators impacting each crusted surface and for particles ejected from this same surface inclusive of ricochets biotic crusts grown on sand in an environmental chamber were able to withstand erosion over several hours of continuous particle impact as compared to brittle salt crusts which regardless of wind speed or sodium chloride concentration eroded fully within one half hour despite the appearance of deep pits and grooves on the ruptured crusts loss of mass dominated over particle trapping on these rough surfaces while decreasing salt concentration between 320 g kg 1 and 80 g kg 1 generally was found to be associated with an increase in the mass flux of particles ejected from the surface little to no correlation with either wind speed or particle impact speed was observed the range over which varying salt concentration affects the momentum of ejected particles is rather narrow within 160 g kg 1 temporal changes in the velocity distribution of ejected particles with crust rupture and deflation are complicated by variations in crust strength with depth this is especially true of biotic crusts and weak salt crusts fast moving particles associated with the upper 40 of the cumulative velocity distribution generally demonstrate little variation from the control surfaces and probably represent ricochets in comparison those within the lower 60 of the distribution are significantly affected by crust rupture and erosion and may include low energy rebounds as well as the entrainment of new particles loosened from within the crust while such measurements are exceedingly rare they are needed for the validation of physically based models of crust erosion

tomographic evidence for a subducted seamount beneath the gulf of nicoya costa rica the cause of the 1990 mw 7 0 gulf of nicoya earthquake 1 tomographic images constrain the existence of a subducted seamount beneath the gulf of nicoya costa rica the subducted seamount is found at a depth of 30 km within the rupture area of the march 25 1990 mw 7 0 gulf of nicoya earthquake the gulf of nicoya earthquake was a typical thrust type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting cocos plate and the overriding plate precise relocation of the mainshock and its aftershocks in a 3 d p wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate we interpret the subducted seamount as an asperity whose rupture caused the 1990 gulf of nicoya earthquake

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

major crustal boundaries of australia and their significance in mineral systems targeting abstract for over 35 years deep seismic reflection profiles have been acquired routinely across australia to better understand the crustal architecture and geodynamic evolution of key geological provinces and basins major crustal scale breaks have been interpreted in some of the profiles and are often inferred to be relict sutures between different crustal blocks as well as sometimes being important conduits for mineralising fluids to reach the upper crust the widespread coverage of the seismic profiles now allows the construction of a new map of major crustal boundaries across australia which will better define the architecture of the crustal blocks in three dimensions it also enables a better understanding of how the australian continent was constructed from the mesoarchean through to the phanerozoic and how this evolution and these boundaries have controlled metallogenesis starting with the locations in 3d of the crustal breaks identified in the seismic profiles geological e g outcrop mapping drill hole geochronology isotope and geophysical e g gravity aeromagnetic magnetotelluric data are used to map the crustal boundaries in plan view away from the seismic profiles some of the boundaries mapped are subsurface boundaries and in many cases occur several kilometres below the surface hence they will not match directly with structures mapped at the surface for some of these boundaries a high level of confidence can be placed on the location whereas the location of other boundaries can only be considered to have medium or low confidence in other areas especially in regions covered by thick sedimentary successions the locations of some crustal boundaries are essentially unconstrained unless they have been imaged by a seismic profile from the mesoarchean to the phanerozoic the continent formed by the amalgamation of many smaller crustal blocks over a period of nearly 3 billion years the identification of crustal boundaries in australia and the construction of an australia wide gis dataset and map will help to constrain tectonic models and plate reconstructions for the geological evolution of australia and will provide constraints on the three dimensional architecture of australia deep crustal penetrating structures particularly major crustal boundaries are important conduits to transport mineralising fluids from the mantle and lower crust into the upper crust there are several greenfields regions across australia where deep crustal penetrating structures have been imaged in seismic sections and have potential as possible areas for future mineral systems exploration

a geological assessment of airborne electromagnetics for mineral exploration through deeply weathered profiles in the southeast yilgarn cratonic margin western australia abstract mineral exploration in regolith dominated environments is challenging requiring the development of new technical tools and approaches when airborne electromagnetics aem is combined with information on stratigraphy mineralogy geochemistry drilling and landscape observations in a geological context it becomes a powerful approach to describe the architecture of the regolith cover this has significant implications for mineral exploration in any regolith dominated terrain rdt this research presents two case studies of aem data integrated in a geological context for mineral exploration in the yilgarn craton margin albany fraser orogen afo in one of the study sites presented study site 1 neale tenement the availability of aem data allowed for lateral and vertical extrapolation of the information contained in datasets at specific locations thereby creating a 2d architectural model for the regolith cover in addition it was determined 1 the total thickness of the regolith cover and its variability between 2 m and 65 m 2 that low conductivity transported overburden and silcrete units with a total thickness between 5 and 45 m is widely distributed capping the upper saprolite and 3 that the silcrete unit varies laterally from being completely cemented to permeable and that these permeable areas windows coincide vertically with mineralogical textural moisture salt content changes in the underlying saprolite resulting in increased conductivity this has been interpreted as resulting from more intense vertical weathering and consequently a higher vertical geochemical dispersion of the basement signature towards surface aem has been used to assist in identifying and describing the lateral continuity of these windows in areas with no direct field observations surface geochemical sampling above these permeable areas may deliver more reliable geochemical basement signatures in the second study site silver lake tenement the aem data was strongly influenced by the high conductivity of the hypersaline groundwater this had a significant effect on the aem response resulting in reduced depth penetration and reduced resolution of subtle conductivity contrasts between cover units despite this the aem data set combined with geological observations in the area was able to map the presence and extent of a buried palaeochannel network the most significant architectural sedimentary feature in the cover this interpretation allowed for a more efficient drilling campaign to be designed to sample the fresh basement rock suites in the area by avoiding drilling into palaeochannels integrated and constrained by the geological context the application of aem conductivity models by geologists is envisioned as one of the most promising tools within the exploration geologist toolbox to understand the architecture of the cover

vertical motions of the puerto rico trench and puerto rico and their cause 1 the puerto rico trench exhibits great water depth an extremely low gravity anomaly and a tilted carbonate platform between reconstructed elevations of 1300 m and 4000 m i argue that these features are manifestations of large vertical movements of a segment of the puerto rico trench its forearc and the island of puerto rico that took place 3 3 m y ago over a time period as short as 14 40 kyr i explain these vertical movements by a sudden increase in the slab s descent angle that caused the trench to subside and the island to rise the increased dip could have been caused by shearing or even by a complete tear of the descending north american slab although the exact nature of this deformation is unknown the rapid 14 40 kyr and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending the proposed shear zone or tear is inferred from seismic morphological and gravity observations to start at the trench at 64 5 w and trend southwestwardly toward eastern puerto rico the tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount

tropicana translated a foreland thrust system imbricate fan setting for c 2520 ma orogenic gold mineralization at the northern margin of the albany fraser orogen western australia

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

role of grain size in phyllonitisation insights from mineralogy microstructures strain analyses and numerical modeling abstract brittle y and p planes exist in an exposure of greywacke in the garhwal lesser himalaya india although y planes are well developed throughout the p planes are prominent only in some parts domain a and not elsewhere domain b to investigate why the p planes developed selectively the following studies were undertaken 1 clay separated xrd analyses clinochlore and illite are present in both the domains 2 strain analyses by rf method it deduces strain magnitudes of 1 8 for the ductile deformed quartz grains from both the domains a and b 3 grain size analyses of quartz clasts domain a is mostly composed of finer grains area up to 40 000 m2 whereas domain b consists of a population of coarser grains area 45 000 m2 a 2d finite element modeling of linear elastic material was performed using comsol software to investigate the control of grain size variation on the generation brittle shear planes the results of numerical modeling corroborate the known fact that an increase in grain size reduces the elastic strain energy density a broader grain size distribution increases the effects of diffusion creep and resists the onset of dislocation creep thus rocks with coarser grain population domain b tend to resist the generation of shear fractures unlike their fine grained counterpart domain a

collisions of quartz grains with a sand bed the influence of incident angle the velocities with which grains were observed to emerge from a sand bed after an intersaltation collision at u 40 cm s 1 are reported for four bed attitudes from horizontal bed to adverse bed slope 15 the principal effect of bed angle is to alter the magnitude and direction of the ricochet velocity however emergent velocities of dislodged grains are consistent with reptation path lengths comparable to the length of the upwind face of ripples in the corresponding wind calculations of the loss of forward momentum at collision using the data for the range of bed attitudes studied suggest that creep is most vigorous on the sloping upwind face of the ripple and diminishes at the crest as a result the crest would be expected to accumulate the coarse material which moves predominantly by creep the saltations originating in ricochet from the sloping back of the ripple are more vigorous and more concentrated in plan than are those originating at the crest however the saltation path length is at least an order of magnitude greater than the ripple wavelength and the probability distribution of path lengths is quite dispersed consequently it is very unlikely that these spatial patterns of ricochet are preserved sufficiently distinctly in the saltation cloud and subsequent collision distribution to be the agent of ripple development this study therefore supports a view of moving grain interaction with the bed in which saltation provides the power to mobilize grains but ripple growth is associated with reptation and particularly with a pattern of impact which develops with the bed relief creep is more active on upwind facing slopes than at the crest which therefore is a zone of net creep grain deposition

a seismic stratigraphic analysis of mariana forearc basin evolution new seismic data collected in the 14 5 18 5 n mariana segment of the izu bonin mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc these sediments were most likely deposited from 35 ma to the present the oldest stratigraphic units 1 2 and 3 are syn rift volcaniclastic deposits unit 4 deposits accumulated during a period of mild structural inversion which resulted in several isolated reverse faulted anticlines within the forearc sedimentary basin a late period of extensional deformation began near the end of unit 5 deposition and continued through unit 6 sedimentation to the present seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with ne nw and nne trends fault offset calculations show that basement faults with dips between 45 and 50 account for only 4 total extension in the forearc south of 16 3 n normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin from correlations with the known geologic history we hypothesize that extension during deposition of units 1 through 3 corresponds to rifting of the eo oligocene arc between 35 ma and 29 ma older deposits being too thin to be seismically resolvable localized compression during unit 4 accumulation occurred some time after eo oligocene rifting in the early miocene late stage normal faulting near the end of deposition of unit 5 and throughout unit 6 accumulation may be associated with the opening of the mariana trough backarc basin from 8 ma years to the present there is a higher density of these later faults in the inner forearc between 15 5 and 17 n and in the outer forearc between 14 n and 18 n recent extension is at least partially accommodated by reactivation of older basement faults with the same ne nw and nne trends stratigraphic relationships indicate that the inner forearc south of 16 3 n has differentially subsided and tilted trenchward possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc

fluid budgets along the northern hikurangi subduction margin new zealand the effect of a subducting seamount on fluid pressure we estimate fluid sources around a subducted seamount along the northern hikurangi subduction margin of new zealand using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data calculated fluid sources are input into an independent fluid flow model to explore the key controls on overpressure generation to depths of 12 km in the thermomechanical models sediment transport through and beneath the wedge is calculated assuming a pressure sensitive frictional rheology the change in porosity pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration our calculations yield more precise information about source locations in time and space than previous averaged estimates for the hikurangi margin the volume of fluid release in the wedge is smaller than previously estimated from margin averaged calculations 14 m3 yr 1 m 1 and is exceeded by fluid release from underlying subducting sediment 16 m3 yr 1 m 1 clay dehydration contributes only a small quantity of fluid by volume 2 m3 yr 1 m 1 from subducted sediment but the integrated effect is still significant landward of the seamount fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid flow models using subducted sediment permeability derived from porosity and testing two end members for decollement permeability models in which the decollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment however if the subduction interface becomes impermeable with depth significant fluid overpressure develops in subducting sediment landward of the seamount the location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration shallow slow slip events sses landward of the seamount are caused by anomalous fluid pressures alternatively it may result from frictional effects of changing clay content along the subduction interface

erosion profiles due to particles entrained by wind application of an eolian sediment transport model erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains a recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry the modeling of vertical erosion profiles in turn provides an exacting test of the transport model realistic distributions of liftoff velocities for saltating grains give rise to kinetic energy flux profiles characterized by a strong maximum as much as 1 4 m above the bed during strong winds kinetic energy flux due to suspended grains also peaks above the bed the height and strength of the maxima depend very strongly on the grain size distribution as grain size diminishes increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface for both saltating and suspended grains kinetic energy flux scales with the fifth power of the wind shear velocity erosion profiles in man made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces lending considerable support for the model the modeling also elucidates several aspects of ventifaction as small ventifacts 1 2 m in diameter and the lower portions of larger ones are predominantly eroded by saltating grains upwind surface irregularities are quickly damped leading to facets that dip upwind and sharply truncate lee surfaces sandblasting by suspended grains dominates along the upper portions of large ventifacts where grain paths are partially deflected by the obstacle and may lead to radially symmetrical flutes and grooves

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

subducted seamounts and recent earthquakes beneath the central cascadia forearc bathymetry and magnetic anomalies indicate that a seamount on the juan de fuca plate has been subducted beneath the central cascadia accretionary complex and is now located 45 km landward of the deformation front passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology gas hydrate stability and sedimentation based on potential field data and a new three dimensional seismic velocity model we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of cascadia more deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along strike variations in the thickness of the subduction channel which may affect coupling across the plate boundary

threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air desert soils having clay crusts mostly from the mojave desert were tested for threshold friction velocity the friction velocity at which soil erosion begins with an open bottomed wind tunnel the soils were also tested for content of clay water soluble material calcium carbonate organic material mineralogy of clay and of salts soil moisture modulus of rupture and crust thickness if no loose material existed on the soil surface crusts having modulus of rupture greater than 0 7 bar and crust thickness of 0 7 cm to 0 3 cm were effective in protecting against wind erosion disturbed clay crusts having modulus of rupture before disturbance greater than 2 bar with thickness less than 1 9 cm did not experience significant wind erosion modulus of rupture was related to composition of soil but was shown to depend mostly on clay content soil composition is related to modulus of rupture in an empirical equation

fault plane geometry in the source region of the 1994 sanriku oki earthquake abstract we present the results of a seismic refraction experiment conducted off eastern aomori northeastern japan forearc region where the detailed crustal structure was poorly known an objective analysis using high quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth and in particular to obtain a well constrained geometry of the plate boundary in the source region of the 1994 sanriku oki earthquake our results show that the subducting pacific oceanic plate increases its dip from about 5 to approximately 15 around the longitude of 143 5 e 80 km landward of the trench axis near the latitude of 40 n the location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution a cluster of aftershocks of the 1994 sanriku oki earthquake occurred around the bending point this is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary furthermore the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies

geodynamic effects of subducted seamount at the manila trench insights from numerical modeling abstract we used numerical modeling to investigate the geodynamic effects of subducted seamounts at the manila trench a series of numerical modeling experiments were conducted with variable parameters including the activation volume vact and cohesion c which influence lithospheric rheology the plate convergence velocity and the age of subducting slab modeling results indicate that varying the vact and c within an appropriate range have limited effects on the geodynamic process of subduction a lower vact allows the slab to sink more easily and results in a steeper dip angle a slab break off is more likely to occur under subduction at depths of 100 300 km while the existence of a seamount further promotes the break off process the convergence rate is a key parameter affecting the break off timing and depth in contrast under subduction where subducted oceanic plate move faster upper plate the model results exhibit non break off steady subduction slab age is another factor controlling break off where break off time extends with slab age a subduction without seamount will cause a 2 myr delay in break off timing we suggest that the low velocity zone under the manila trench at 17o n is the result of a break off event due to subduction of the zhenbei huangyan seamount chain further to the north such as the location at 19o n the absence of seamount and an older oceanic crust would favor a delay in break off timing during subduction

an analysis of soil crust strength in relation to potential abrasion by saltating particles wind erosion depends on the ease with which particles can be detached from the soil surface but suitable tests to characterize this property are not available two possible methods to determine surface soil strength in the field were therefore compared on a range of artificially crusted surfaces these were made by spraying or tension wetting aggregates 10 2 2 0 5 and 0 5 mm from a structurally unstable sandy loam followed by drying each test involved measuring the force exerted on a probe driven at a steady rate into the surface using either a flat tipped 0 6 mm diameter penetrometer or a flat ended cylindrical punch with inner and outer diameters of 5 and 6 mm respectively both probes showed that crusts could be produced reproducibly depending on the probe and aggregate size penetration mainly occurred either as a result of aggregates being deflected out of the pathway of the probe or by genuine rupture of aggregates or of the crusted surface the penetrometer because it was comparable to the size of sand grains gave results that can be used to characterize surface erodibility to saltating particles the punch gave results that would be unsuitable for this purpose as would other strength tests that are on too large a scale penetrometer results were analyzed to calculate the energy required for penetration it was thus possible to demonstrate that only the spray wetted fine aggregates had a surface that could undergo large scale rupture by saltating sand grains for all other surfaces saltating particles would be unable to supply sufficient energy to rupture aggregates or the crusted surface erosion could only occur by a slower process of abrasion in which smaller particles or aggregates are chipped away from the surface however it is shown that saltating particles could rupture the interaggregate bonding in the 2 0 5 mm aggregate surfaces thus permitting creep an alternative and potentially simpler way of characterizing surface erodibility by using a surface modulus of elasticity is also discussed our results demonstrate that the small diameter penetrometer is a promising technique for characterizing erodibility of aggregated and crusted surfaces 1997 by john wiley sons ltd

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

observations of collisions of saltating grains with a granular bed from high speed cine film high speed photography was used to record saltating sand grains colliding with a horizontal noncohesive bed of similarly sized grains impacting grain bed interaction is discussed in general the process as observed from the films is then described in terms of the apparent bed contact length abcl and various parameters of the impacting grains and any ejected grains examples are given of typical behaviour of bed grains in response to impacting grains of different sizes saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments so that previously buried grains become available for entrainment this process is discussed in relation to the potential release of dust particles into the airflow

measurement of sand creep on a flat sand bed using a high speed digital camera in this study a novel method for measuring sand creep along a flat bed was developed specifically a high speed digital camera was employed to record the sand creep movement with an unsigned image subtraction a new image queue could be formed from the original experimental images the particle tracking was then calculated after which the velocity and trajectory of the creep of sand grains was obtained in addition the creep fluxes of the different sand samples could also be calculated based on the velocities of the creep grains

effect of soil crusting on the emission and transport of wind eroded sediment field measurements on loamy sandy soil field data are reported for the horizontal and vertical flux of wind eroded sediment on an agricultural field in northern germany measurements were made during a windstorm that hit the region on 18 may 1999 the magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field measuring the physical crust strength at 45 locations with a torvane the relationships between crust strength and the horizontal fh and vertical fv sediment fluxes were investigated both fluxes decreased as the surface crust became stronger the decay behaved as an exponential function for both types of flux the horizontal sediment flux over a crusted surface can be accurately predicted by completing marticorena and bergametti s journal of geophysical research 100 1995 16415 erosion model with a crust function the vertical particle flux over crusted soil can be calculated by adding a similar function to alfaro and gomes s journal of geophysical research 106d 2001 18075 dust production model the study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust s protective effect provided that the crust has a minimum thickness however once the crust becomes perforated its protective effect disappears very quickly leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

mechanical and hydrological effects of seamount subduction on megathrust stress and slip subduction of rough seafloor and seamounts is thought to impact a broad range of geodynamic processes including megathrust slip behaviour forearc fluid flow and long term structural evolution in the overriding plate although there are many conceptual models describing the effects of seamount subduction our quantitative and mechanistic understanding of the underlying deformation and fluid processes remains incomplete here we investigate the interplay between sediment consolidation faulting and the evolution of stress and pore fluid pressure in response to seamount subduction using a numerical model that couples mechanical and hydrological processes and is constrained by laboratory and field observations our results show that subducting topography drives marked spatial variations in tectonic loading sediment consolidation and megathrust stress state downdip of a subducting seamount on its leading flank enhanced compression and drainage lead to large fault normal stress and overconsolidated wall rocks a stress shadow in the seamount s wake leads to anomalously high sediment porosity these variations help explain observed patterns of megathrust slip with earthquakes and microseismicity favoured at the downdip edge of seamounts and aseismic or slow slip in the updip stress shadow changing stresses and pore fluid pressures during subduction of seamounts as simulated with a numerical model that couples mechanical and hydrological processes help explain observed patterns of megathrust slip

wind erosion of crusted soil sediments saltating particles increase the rate of dust release from sediments in arid and semi arid areas they also break interparticle bonds in aggregated and crusted soils thereby increasing the number of particles available for entrainment this pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths dislodgement of surface particles decreases with increasing crust strength as measured by a cylindrical flat ended penetrometer in addition initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator the volume of material removed depends linearly on the kinetic energy of the abraders

granulite hosted gold tectonic setting and lithogeochemistry of the tropicana deposit western australia the tropicana gold deposit 7 9 moz is hosted by the neoarchean amphibolite to granulite grade tropicana gneiss these rocks form part of the plumridge terrane a tectonic unit that separates the in situ eastern margin of the yilgarn craton from proterozoic elements of the albany fraser orogen western australia the gneissic host package to the orebody originally constituted a suite of medium to high k calc alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc amphibolitized mafic intrusions have back arc basin basalt like compositions consistent with their having formed during extension and incipient rifting of the arc gold mineralization occurs mainly within a sheet like syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone coarse perthite textures in microcline ruffle exsolution in brown biotite and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism relative to the unmineralized host gneisses the mineralized syenitic gneisses show significant enrichments in s and the ore elements mo te tl ag au w and occasionally bi and sb but not as cu or zn in the k group elements and also in p light rare earth elements lree th u zr and hf depletions in most major elements si ti fe mg and ca as well as v cr ni and co are considered to reflect dissolution of relatively low volume mafic phases hornblende augite fe ti oxides and plagioclase in the protoliths some of the distinctive enrichments p lree zr hf are linked to volume loss in excess of 50 associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit but the ore elements and strongly enriched similar to x10 th and u were added from the hydrothermal fluids involved exhumation of the tropicana gneiss into the upper crust was accompanied by fluid ingress and retrogression this produced mm to cm scale wavy biotite pyrite shears solution seams and disseminations throughout the syenitic gneiss which were further retrogressed to sericite chlorite assemblages in places both of which host most of the gold in the deposit thus although the present deportment of gold is unambiguously associated with the postpeak metamorphic greenschist grade retrogression at ca 2520 ma there is compelling evidence that the paucity of quartz and carbonate veining atypical element enrichments and sheet like nature of the tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic relatively deep shear zone at present we have no direct evidence for the timing of introduction of gold and a protracted history of gold precipitation redistribution and enrichment is implied

aeolian shear stress ratio measurements within mesquite dominated landscapes of the chihuahuan desert new mexico usa a field study was conducted to ascertain the amount of protection that mesquite dominated communities provide to the surface from wind erosion the dynamics of the locally accelerated evolution of a mesquite coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements although shear stress partition models exist that can describe this their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements this study combines the use of meteorological towers and surface shear stress measurements with irwin sensors to measure the partition of shear stress in situ the surface shear stress within preferentially aligned vegetation within coppice dune development exhibited highly skewed distributions while a more homogenous surface stress was recorded at a site with less developed coppice dunes above the vegetation the logarithmic velocity profile deduced roughness length based on 10 min averages exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned while the site with more homogenously distributed vegetation showed very little variation in the roughness length this distribution in roughness length within an area defines a distribution of a resolved shear stress partitioning model based on these measurements ultimately providing potential closure to a previously uncorrelated model parameter

earthquake rupture stalled by a subducting fracture zone we showed that the rupture produced by the great peru earthquake moment magnitude 8 4 on 23 june 2001 propagated for 70 kilometers before encountering a 6000 square kilometer area of fault that acted as a barrier the rupture continued around this barrier which remained unbroken for 30 seconds and then began to break when the main rupture front was 200 kilometers from the epicenter the barrier had relatively low rupture speed slip and aftershock density as compared to its surroundings and the time of the main energy release in the earthquake coincided with the barrier s rupture we associate this barrier with a fracture zone feature on the subducting oceanic plate

deformation and faulting of subduction overriding plate caused by a subducted seamount we conducted numerical experiments to simulate elastoplastic deformation of the overriding plate caused by a subducted seamount calculations revealed development of a distinct pair of fault like shear zones including a landward dipping forethrust fault initiated from the seamount top and a seaward dipping backthrust fault from the landward base of the seamount significant dome shaped surface uplift was predicted above the thrust faults lesser developed seaward dipping backthrust faults were calculated to develop under certain conditions the overriding plate was calculated to deform in two stages in stage i elastic deformation leads to the formation of fault like shear zones after major faults have cut through the entire plate plastic deformation on faults dominates stage ii on the subduction interface compressional normal stress was calculated to increase on the landward leading flank of the seamount and decrease on the seaward trailing flank these changes together with associated stress singularities at seamount edges could affect earthquake processes

global distribution of seamounts inferred from gridded geosat ers 1 altimetry using a forward modeling approach based on an axisymmetric gaussian seamount i characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the geosat and ers 1 satellite missions the global seamount distribution is long tailed and resembles a power law distribution for seamounts in the height range 2 7 km smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data several factors are likely to influence the height of volcanic seamounts such as melt availability magma driving pressure and plate thickness the observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights whether a seamount will reach that height depends most likely on supply side factors such as melt availability and magma driving pressure but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate specifically compressional stresses directly beneath the seamount as a consequence of the lithosphere s flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface thus limiting the growth of the seamount because oceanic plate strength primarily is controlled by plate age the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement using analytical solutions i present a simple flexural model that predicts the observed global height age relationship

tectonic and geological framework for gas hydrates and cold seeps on the hikurangi subduction margin new zealand the imbricated frontal wedge of the central hikurangi subduction margin is characteristic of wide ca 150 km poorly drained and over pressured low taper not vert similar 4 thrust systems associated with a relatively smooth subducting plate a thick trench sedimentary sequence not vert similar 3 4 km weak basal decollement and moderate convergence rate not vert similar 40 mm yr new seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures and to establish the geological framework for gas hydrates and fluid seeps we discuss the stratigraphy of the subducting and accreting sequences characterize stratigraphically the location of the interplate decollement and describe the deformation of the upper plate thrust wedge together with its cover sequence of miocene to recent shelf and slope basin sediments we identify approximately the contact between an inner foundation of deforming late cretaceous and paleogene rocks in which widespread out of sequence thrusting occurs and a 65 70 km wide outer wedge of late cenozoic accreted turbidites although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1 2 myr this differs from the offshore hawkes bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin resulting in uplift and complex deformation of the lower slope and a narrow 10 20 km active frontal wedge five areas with multiple fluid seep sites referred to informally as wairarapa uruti ridge omakere ridge rock garden and builders pencil typically lie in 700 1200 m water depth on the crests of thrust faulted anticlinal ridges along the mid slope uruti ridge sites also lie in close proximity to the eastern end of a major strike slip fault rock garden sites lie directly above a subducting seamount structural permeability is inferred to be important at all levels of the thrust system there is a clear relationship between the seeps and major seaward vergent thrust faults near the outer edge of the deforming cretaceous and paleogene inner foundation rocks this indicates that thrust faults are primary fluid conduits and that poor permeability of the cretaceous and paleogene inner foundation focuses fluid flow to its outer edge the sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the decollement within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector bsr and commonly a seismically resolvable shallow fault network through which fluids and gas percolate to the seafloor no active fluid venting has yet been recognized over the frontal accretionary wedge but the presence of a widespread bsr an extensive protothrust zone 200 km by 20 km in the hikurangi trough and two unconfirmed sites of possible previous fluid expulsion suggest that the frontal wedge could be actively dewatering there are presently no constraints on the relative fluid flux between the frontal wedge and the active mid slope fluid seeps article outline

crust strength a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces a wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength the sediments consisted of varying proportions of large sand sized particles with a fine particle cement the energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface it is important to consider distributions of parameters rather than mean values only since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities at the impact velocities used in this study mean velocity 4 4 m s 1 with standard deviation of 0 51 surfaces containing less than 12 per cent fine material were easily eroded but insignificant erosion occurred when the fine particle content exceeded 60 per cent small amounts of cementing material were easily ruptured allowing the large sand grains to be moved largely in creep by the bombarding particles a significant amount of energy was lost to the bed as the percentage of fine material increased the surface became more difficult to break up and less energy was lost to the bed the probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles copyright 2001 john wiley sons ltd

structural controls on gold mineralization on the margin of the yilgarn craton albany fraser orogen the tropicana deposit western australia the tropicana gold deposit is located adjacent to the margin of the yilgarn craton in the albany fraser orogen western australia the deposit is hosted in granulite facies quartzo feldspathic gneisses of the archean tropicana gneiss ore bodies comprise biotite pyrite alteration concentrated in shear zones that formed during ne sw shortening in the late archean and clearly postdate the formation and deformation of high grade gneiss fabrics d1 and d2 the orientation of the ore bodies is controlled by the shear zones that are in turn localised by the gneissic banding mineralization also involved solution and coeval microfracturing and veining of more competent pegmatitic units the mineralizing event d3 was followed by at least two further deformations which reactivated and overprinted the biotite fabrics with sericite and chlorite created new shear zones and affected gold distribution d5 consisted of dextral shear on e w shear zones which subdivide the deposit into five major structural domains the importance of structurally controlled permeability at tropicana is similar in cratonic lode gold deposits as is the protracted deformation fluid flow history like renco mine in zimbabwe tropicana gold deposit was formed by hydrothermal fluid flow peripheral to the craton economic gold mineralization was clearly post peak metamorphism

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

seismic reflection character of the hikurangi subduction interface new zealand in the region of repeated gisborne slow slip events summary we use seismic reflection data to map the geometry and character of the subduction interface in the gisborne area of the hikurangi subduction margin new zealand which experiences repeated shallow 15 km slow slip events the reflection character and geometry in this area is highly variable which we interpret to be related to the subduction of seamounts and underthrust sediments three zones of high amplitude interface reflectivity hrz 1 2 and 3 are interpreted to be the result of fluid rich sediments that have been entrained with subducting seamounts the interface above the hrz zones is shallower than the surrounding areas by 2 4 km due to the warping of the interface to accommodate seamount subduction these zones of high amplitude reflectivity and shallower interface geometry correlate broadly with locations of recorded slow slip events from 2002 to 2008 we hypothesize that effective stresses on the interface may be lower along the northeast margin in areas of high amplitude reflectivity due to 1 the enhanced underthrusting of fluid rich sediment 2 reduced overburden stresses where the interface has been warped to shallower depths to accommodate seamount subduction and 3 potential fluid flow concentration effects leading to overpressure along these shallower interface corrugations from our observations we propose localized reductions in effective stress caused by interface structural relief may be a potential factor in promoting shallow slow slip events

reequilibration processes in magnetite from iron skarn deposits textural and compositional data for magnetite from nine iron skarn deposits in canada romania and china show that most samples have reequilibrated by dissolution and reprecipitation oxy exsolution and or recrystallization the dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined whereas the oxy exsolution occurs only in high ti magnetite forming exsolution lamellae of fe ti al oxides electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite notably si mg ca al mn and ti whereas oxy exsolution is effective in decreasing the ti content of high ti magnetite many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations bif iron oxide copper gold iocg or porphyry cu fields using the ti v versus ca al mn discrimination diagram this pattern suggests that trace element data for magnetite that has unusual composition and or reequilibrated cannot be reliably used as a petrogenetic indicator mixing of externally derived saline fluids with fe rich magmatic hydrothermal solutions an increase in temperature and local decreasing pressure and f o 2 are considered the most important causes for the dissolution and reprecipitation or recrystallization of the magnetite increasing f o 2 and decreasing temperature may facilitate oxy exsolution of fe ti al oxides in high ti magnetite results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

geometrical effects of a subducted seamount on stopping megathrust ruptures 1 we have numerically simulated dynamic ruptures along a slip weakening megathrust fault with a subducted seamount of realistic geometry demonstrating that seamounts can act as a barrier to earthquake ruptures such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level for larger seamount height to width ratio and for shorter seamount to nucleation distance as the seamount height increases from 0 to 40 of its basal width the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as 20 we further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level these results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions including the case of the thrust fault being lubricated by seamount top fluid rich sediments as suggested from observations in the japan and sunda trenches

the grain bed collision in sand transport by wind in the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain bed collision has been identified as having a significant role the grain bed collision has been studied by both physical experiments and numerical simulation a principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system thus the study of the exact physical mechanism of the collision has to some extent been neglected the transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension thus the precise physics of the grain bed collision become highly significant with regard to the physics of dust release from a surface this paper re examines previous collision data and compares them with a simple collision model this model proposed that the impinging grain strikes a single surface grain and rebounds an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding the collision data are shown to support the model first because the duration of the collisions observed on the high speed film supports a short contact time with the surface and second because the data fit well with the model

a conceptual model of wind erosion of soil surfaces by saltating particles a conceptual model is described for the prediction of wind erosion rates dependent on the distribution of impact energy delivered to the surface by saltating grains p ei and the distribution of local surface strength p es methods are presented for the measurement of both distributions and consequent loss of material from the bed it is concluded that saltating sand grains can rupture weak crusts under even moderate wind conditions and that the rate of erosion will depend on the shape of the distribution tails copyright 1999 john wiley sons ltd