seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

major and trace element abundances and strontium isotopes in the nyong basin rivers cameroon constraints on chemical weathering processes and elements transport mechanisms in humid tropical environments this paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments we studied the nyong river basin 27 800 km2 located on the northwestern part ntem complex of the congo craton central africa the dissolved concentrations i e 0 20 m of major and trace elements dissolved organic carbon doc and the 87sr 86sr ratios have been measured in more than 20 rivers draining watersheds with various surface areas 1 to 28 000 km2 all these rivers exhibit low major cations concentrations i e na mg k ca but high concentrations of some trace elements al fe th zr y ree silica and doc the total dissolved load tds is low 20 mg l 1 and dominated by silica and organic matter the comparison of different watersheds shows us that chemical weathering is more efficient in the small unit watersheds all the sampled rivers exhibit a wide range of 87sr 86sr ratios and high ca na ratio that can be explained by the heterogeneity of the crystalline rock constituting the cratonic basement four selected rivers mengong awout soo and nyong having different drainage areas and hydrological parameters were sampled over a 2 year period collected data show that all these rivers present the same monthly seasonal variations with higher concentrations during rainy season and lower concentrations during dry season this implies that the weathering and transport mechanisms of small watersheds can be extended to the whole nyong basin in the small unit watersheds chemical weathering mainly occurs in swamp zones where mineral dissolution is enhanced by humic substances these swamp zones constitute a pool of organic rich water which can be quickly mobilized during rain seasons in these waters doc and insoluble element concentrations e g al fe and th were strongly correlated which show the key role of organic colloidal matter in the transport of some insoluble elements some other relationships al vs fe ree vs al were also examined in order to get information about chemical weathering or element transport based on these data it has been concluded that the chemical composition of these river waters is controlled by geomorphic and historic factors e g thick cation poor soil in contrast the present day climatic parameters high rainfall and temperature play a minor role in water chemistry regulation even though they are likely to enhance mineral dissolution even if organic matter favors mineral dissolution chemical weathering in this area is low compared to other world regions which suggest on a global scale a relatively small effect of these environments on the co2 consumption

the genesis and recognition of anastomosed fluvial deposits data from the st mary river formation southwestern alberta canada abstract data from modern fluvial systems show that the term anastomosed is best reserved for suspended load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal peak the interchannel areas are topographic lows relative to the channel hanks and are places where crevasse splay marsh and lacustrine sedimentation occurs channel geometry is intrinsically a function of the rate of sediment partitioning the coarsest sediment is deposited in the channels on the levees and in proximal splay complexes whereas the silts and clays are deposited on the rest of the floodplain extrinsic controls on the formation of these systems are geological and climatic they include a source of suspended sediment and a highly seasonal water budget preservat on of anastomosed fluvial deposits requires a mechanism for maintaining very low depositional slopes over geologic time scales data from the st mary river formation in southwestern alberta show that the stratigraphic equivalents of these rivers are ribbon sandstone bodies encased in thin sandstones shales and siltstones the total distribution of facies rather than proof of simultaneous flow in multiple channels can be used to interpret correctly a deposit as anastomosed fluvial although anastomosed fluvial deposits form in a variety of tectonic settings they are apparently most common in foreland basins because these basins provide suitable source rocks and thrusting as a mechanism that can maintain the low depositional slopes over millions of years the mutual interaction among source rock climate and subsidence all of which play critical roles in forming and preserving these distinctive fluvial deposits allows inferences to be drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

distinct styles of fluvial deposition in a cambrian rift basin process based and facies models to account for the origin of pre vegetation i e pre silurian preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation in pre vegetation fluvial successions the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad tabular stacked sandstone sheets has been previously regarded as the dominant sedimentary mechanism this situation is closely analogous to modern day poorly vegetated systems developed in arid climatic settings however this study demonstrates the widespread presence of substantially more complex stratigraphic architectures the guarda velha formation of southern brazil is a 500 m thick synrift fluvial succession of cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions i an early rift stage system characterized by coarse grained channel elements indicative of a distributive pattern with flow transverse to the basin axis and two coeval systems from the early to climax rift stages that represent ii an axially directed trunk fluvial system characterized by large scale amalgamated sandy braid bar elements and iii a distributive fluvial system characterized by multi storey sandy braided channel elements that flowed transverse to the basin axis integration of facies and architectural element analysis with regional stratigraphic basin analysis palaeocurrent and pebble provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre vegetation rift basin setting identified major controls that influenced pre vegetation fluvial sedimentary style include i spatial and temporal variation in discharge regime ii the varying sedimentological characteristics of distinct catchment areas iii the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style whereby both the axial and transverse fluvial systems undertook a distinctive response to syn depositional movement on basin bounding faults detailed architectural analyses of these deposits reveal significant variations in geometry with characteristics considerably more complex than that of simple laterally extensive stacked sandstone sheets predicted by most existing depositional models for pre vegetation fluvial systems these results suggest that the sheet braided style actually encompasses a varied number of different pre vegetation fluvial styles moreover this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting

differentiated neogene bauxitization of volcanic rocks western cameroon morpho geological constraints on chemical erosion abstract lateritic weathering of miocene volcanic rocks from western cameroon highlands formed duricrusted bauxitic profiles two weathering profiles on ca 14 ma basalt and ca 16 ma trachyte were studied using geochemical mass balance functions less mobile elements ti and zr were used as references to quantify volumetric change strain e element transfer rate and geochemical mass transfers during the bauxitization process of basalt and trachyte conversion of parent rocks to kaolinite and goethite rich saprolites evolved to al fe rich bauxites mostly composed of gibbsite and iron oxy hydroxides goethite and hematite however formation of al fe bauxitic profiles required higher si leaching on trachyte than on basalt our results document that chemical weathering of a larger thickness of trachyte than basalt has been required to form a unit meter of weathering profile implying differential rates of rock chemical erosion and topographic decay of landscapes rates of chemical erosion and formation of lateritic weathering profiles in western cameroon have been mostly controlled by drainage conditions and volcanic rocks composition mostly sio2 content differences that also resulted in contrasted landscapes evolution during the neogene

sandstone body variability in the medial distal part of an ancient distributive fluvial system salt wash member of the morrison formation utah u s a abstract plan view exposures of the salt wash member of the morrison formation in central utah provide unique insights into the characteristics of fluvial sandstone bodies associated with ancient distributive fluvial systems dfs here we use field based observations and data from an unmanned aerial vehicle and structure from motion photogrammetry to document and interpret sandstone bodies across a 10 km2 area from the medial distal portion of the salt wash dfs we divide the fluvial sandstone bodies into four distinct groups very narrow narrow and medium width sandstone bodies as well as an additional classification of other sandstones very narrow sandstone bodies are 100 m and poorly exposed the sandstone bodies record a portion of the dfs that was occupied by fluvial channels with multiple orientations sizes and plan view morphologies similar to the variety of channels observed on modern dfs because the fluvial deposits are composed of distinct sandstone body types mean values from the combined data set tend to obscure important aspects of the depositional history cyclical deposits in the salt wash member suggest that autogenic processes were important in this region consistent with modern dfs overall dfs depositional models derived from regional scale studies provide an excellent framework for interpreting dfs strata however smaller scale details of these deposits particularly the variability in sandstone body orientations are not addressed by existing models

stratigraphic record of the early mesozoic breakup of pangea in the laurasia gondwana rift system rift basins of the central atlantic margins cam of north america and morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early mesozoic the best known is the newark basin of new jersey new york and pennsylvania where an astronomically calibrated magnetic polarity time scale is developed lacustrine cycles of milankovitch origin are commonly present in cam basins with the period changing from 10 ky paleoequator with coals to 20 ky 4 10n to perhaps 40 ky northward with evaporites cycles of100 ky 413 ky and2 my are also important four mostly unconformity bounded tectonostratigraphic sequences are present the anisian ts i is fluvial and eolian ts ii ts iv late triassic to early jurassic consist of tripartite lacustrine sequences caused by extension pulses the newark basin accumulation rate history allows comparison with quantitative rift basin models the north american plate s slow northward drift resulted in a relative shift of climate although the rapid humidification during the latest triassic and early jurassic is associated with a sea level rise the triassic jurassic mass extinction is of independent origin plausibly impact related

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

multifrequency geoacoustic imaging of fluid escape structures offshore costa rica implications for the quantification of seep processes quantification of fluid fluxes from cold seeps depends on accurate estimates of the spatial validity of flux measurements these estimates are strongly influenced by the choice of geoacoustic mapping tools multibeam bathymetry side scan sonar and chirp subbottom profiler data of several mound shaped cold seeps offshore central costa rica show great variety in morphology and structure although the features are only a few kilometers apart mound 11 a 35 m high and 1000 m in diameter structure situated in the se of the study area has an irregular morphology but a smooth surface on side scan sonar data while mound 12 30 m high 600 m across is a cone of more regular outline but with a rough surface and mound grillo 5 m high 500 m across shows the same rough surface as mound 12 but without relief video observations and sediment cores indicate that the structures are formed by the precipitation of authigenic carbonates and indications for extensive mud extrusion are absent except for one possible mudflow at mound 11 different sonar frequencies result in variable estimates of the extent of these mounds with low frequencies suggesting much wider cold seeps consequently overestimating fluid fluxes the absence of mud volcanism compared to accretionary prisms where mud volcanism occurs is related to different tectonic styles strong sediment overpressure and thrust faulting in typical accretionary prisms can generate mud volcanism while subduction erosion and normal faulting extension of the overriding plate at the costa rican margin result in fluid venting driven by only slight fluid overpressures

fluvial architecture and paleoenvironmental evolution of the los colorados formation norian postrift stage of the ischigualasto villa union basin nw argentina

variations in fluvial deposition on an alluvial plain an example from the tongue river member of the fort union formation paleocene southeastern powder river basin wyoming u s a abstract the tongue river member of the paleocene fort union formation is an important coal bearing sedimentary unit in the powder river basin of wyoming and montana we studied the depositional environments of a portion of this member at three sites 20 km apart in the southeastern part of the basin six lithofacies are recognized that we assign to five depositional facies categorized as either channel or interchannel wetlands environments 1 type a sandstone is cross stratified and occurs as lenticular bodies with concave upward basal surfaces these bodies are assigned to the channel facies interpreted to be the product of low sinuosity streams 2 type b sandstone occurs in parallel bedded units containing mudrock partings and fossil plant debris these units constitute the levee facies 3 type c sandstone typically lacks internal structure and occurs as tabular bodies separating finer grained deposits these bodies represent the crevasse splay facies 4 gray mudrock is generally nonlaminated and contains ironstone concretions these deposits constitute the floodplain facies 5 carbonaceous shale and coal are assigned to the swamp facies we recognize two styles of stream deposition in our study area laterally continuous complexes of single and multistoried channel bodies occur at our middle study site and we interpret these to be the deposits of sandy braided stream systems in the two adjacent study sites single and multistoried channel bodies are isolated in a matrix of finer grained interchannel sediment suggesting deposition by anastomosed streams a depositional model for our study area contains northwest trending braided stream systems avulsions of these systems created anastomosed streams that flowed into adjacent interchannel areas we propose that during late paleocene a broad alluvial plain existed on the southeastern flank of the powder river basin the braided streams that crossed this surface were tributaries to a northward flowing basin axis trunk stream that existed to the west

fluvial processes and cyclicity in terminal fan deposits an example from the late devonian of southwest ireland abstract the gun point formation is a sandstone dominated sequence approximately 1 5 2 5 km thick which forms part of a major 6 km late devonian alluvial basin fill succession in southwest ireland it is interpreted as the deposits of a southerly flowing terminal fan system which operated under a semi arid climate the proximal zone was characterized by low sinuosity gravelly feeder channels which passed downstream into a medial zone dominated by a sandy braidplain further south in the distal zone phases of high energy flood activity were centred on transient low sinuosity ephemeral channels and were separated by more quiescent periods of shallow sheetflooding subaerial exposure and non deposition sedimentary parameters such as set thickness coset thickness channel sand body density and maximum sand body thickness all exhibit regular cyclic fluctuations with a periodicity of 160 180 m the cycles are interpreted as evidence of regular discharge variations which may have been a response to the 412 ka orbital eccentricity cycle a second periodicity of approximately 700 m has also been detected and is thought to reflect long term variations in basin subsidence

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

alluvial sandstone composition and paleoclimate i framework mineralogy abstract systematic variations in compositional maturity of first cycle fluvial sandstones are found in the cutler formation permian and fountain formation permian pennsylvanian in colorado and in the gondwana supergroup permian triassic of peninsular india these variations reflect changing climate during deposition climate is considered to be the critical factor affecting maturity because source rocks granite granite gneiss and tectono environmental setting alluvial systems associated with basement cored block uplifts were similar and remained relatively unchanged throughout deposition of all three of the units the roughly 3 200 m thick gondwana succession consisting of five sandstone petrofacies is characterized by the following sequence of change oldest to youngest in compositional maturity expressed in q f r percentages 54 42 4 88 10 2 62 34 4 82 16 2 99 1 0 this sequence is a function of the changing paleoclimate glacial arid temperate humid warm semiarid warm semihumid warm humid associated with the overall global limatic change and the changing latitudinal location of india as it moved from a location close to the south pole northward during the interval of time represented by gondwana deposition lithologic and fossil evidence indicates that during deposition of the fountain formation climate gradually changed from relatively wet and warm to at least semiarid and warm this climatic change is the main factor responsible for the difference in compositional maturity of the older fountain sandstone q f r 65 26 9 relative to the younger q f r 54 36 10 cutler sandstone was deposited in an arid climate and has maximum compositional immaturity q f r 49 44 7 a log log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first cycle sandstones with differing climatic heritage bulk chemical composition data support interpretation of climate from framework mineralogy but by themselves are not sensitive enough to be unequivocal indicators the same is true for the ztr index and observations of degree and abundance of solution pits embayments on detrital quartz results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate tectonic settings such settings are characterized by coarse grained crystalline parent rocks short transport distances in low order streams deposition in nonroutine environments and shallow burial diagenesis however even in such optimum settings only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself

the effect of organic matter on chemical weathering study of a small tropical watershed nsimi zoetele site cameroon abstract the effect of organic matter during soil water interaction is still a debated issue on the controls of chemical weathering in a tropical environment in order to study this effect in detail we focused on the weathering processes occurring in a small tropical watershed nsimi zoetele south cameroon this site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry the lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones unsaturated slope soils on the hills and or elevated areas and water saturated soils in the swamp zone which represent 20 of the basin surface the study present chemical analysis performed on water samples collected monthly from different localities between 1994 1997 and on soil samples taken during a well drilling in december 1997 the results suggest the existence of chemical and spatial heterogeneities of waters in the basin colored waters flooding the swamp zone have much higher concentrations of both organic matter i e doc and inorganic ions e g ca mg al fe th zr than those from springs and groundwater from the hills nevertheless these organic rich waters present cation concentrations na ca mg k which are among the lowest compared to that of most world rivers the main minerals in the soils are secondary kaolinite iron oxi hydroxides quartz and accessory minerals e g zircon rutile we mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through sem observations the textural characterization of weathered minerals such as kaolinite zircon rutile and the secondary recrystallization of kaolinite microcrystals within the soil profile water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin thus in order to explain the increase of element concentration in the organic rich waters we suggest that organic acids enhance dissolution of minerals such as kaolinite goethite and zircon and also favors the transport of insoluble elements such as al fe ti zr and ree by chemical complexation sio2 aq concentrations in these waters are above saturation with respect to quartz dissolution of phytholithes amorphous silica may be responsible for this relatively high sio2 aq concentration al mg ratios obtained for the soil and the mengong river waters show that a significant amount of al does not leave the system due to kaolinite recrystallisation in the swamp zone soils geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone quantitatively we propose that the increasing amount in total dissolved solid tds due to organic matter and vegetation effect is about 35 in summary this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements thus the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

seismic tomography and earthquake locations in the nicaraguan and costa rican upper mantle the central american subduction zone exhibits large variations in geochemistry downgoing plate roughness and dip and volcano locations over a short distance along the arc results from joint inversions for vp vp vs and hypocenters from the tomography under costa rica and nicaragua tucan experiment give insight into its geometry and structure in both costa rica and nicaragua the intermediate depth seismic zone is a single layer no more than 10 to 20 km thick tomographic images show that throughout nicaragua and costa rica the slowest mantle p wave velocities appear below and behind the volcanic front indicating likely zones of highest temperature extending 80 to 120 km depth a sheet of high vp vs thought to be caused by melt is imaged directly beneath the nicaraguan volcanoes whereas a weaker broader anomaly is imaged beneath the costa rican volcanoes potentially indicating a greater extent of melting beneath nicaragua within the downgoing plate anomalously low velocities occur at least 20 30 km below wadati benioff zone seismicity to depths of 140 km beneath nicaragua and to 60 km depth beneath costa rica they indicate 10 20 serpentinized upper mantle of the downgoing plate beneath nicaragua similar to that inferred from refraction seaward of the trench but continuing to subarc depths this unusually hydrated lithosphere may introduce more water into the nicaraguan mantle initiating increased amount of melting and fluid flux to the arc

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

interface locking along the subduction megathrust from b value mapping near nicoya peninsula costa rica 1 to understand controlling factors for seismogenesis and strain accumulation in subduction megathrusts we examine seismicity patterns across nicoya costa rica to determine the overall and spatial variability in the earthquake frequency magnitude distribution along the interface the mean reduction in earthquake activity with magnitude b value is higher 1 06 than global subduction zone averages 0 6 0 8 suggesting the interface here is weakly coupled strong spatial variations in b are anticorrelated 0 53 with geodetic estimates of interface locking of e norabuena et al 2004 high b prevails in two regions including the subducted fisher seamount chain a broad zone of reduced b is observed at and offshore the central nicoya coast extending towards an imaged locked patch these results suggest b value studies may be useful in identifying regions of increased interface locking in subduction zones which may indicate regions capable of large slip in future large earthquakes

mineralogy of a soil chronosequence on andesitic lava in humid tropical costa rica abstract we studied a chronosequence of three soils developed on andesitic lavas in humid tropical costa rica in order to investigate mineralogical development and to evaluate the importance of time versus other soil forming factors morphological chemical and mineralogical methods were used to unravel evolution of the soils primary minerals in the

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

the march 25 1990 mw 7 0 ml 6 8 earthquake at the entrance of the nicoya gulf costa rica its prior activity foreshocks aftershocks and triggered seismicity on march 25 1990 a large earthquake mw 7 0 ml 6 8 occurred at the entrance of the nicoya gulf costa rica at 1322 55 6 utc producing considerable damage in central costa rica and generating much interest about whether or not the nicoya seismic gap nishenko 1989 had broken the local country wide seismographic network recorded 6 years of activity prior to this large earthquake 16 hours of foreshocks the mainshock and its aftershocks this network is operated jointly by the costa rica volcanological and seismological observatory at the national university ovsicori una and the charles f richter seismological laboratory at the university of california santa cruz cfrsl ucsc we obtained high resolution locations from this network and located the mainshock at 9 38 5 n 84 55 6 w depth is 20 0 km and the largest foreshock mw 6 0 march 25 1990 at 1316 05 8 utc at 9 36 4 n 84 57 1 w depth is 22 4 km we find that the aftershock zone abuts the southeast boundary of the nicoya seismic gap suggesting that the seismic gap did not rupture since the installation of the local network in april 1984 to march 24 1990 nearly 1900 earthquakes with magnitudes from 1 7 to 4 8 318 with magnitude 3 0 or larger have been located at the entrance of the nicoya gulf one of the most active regions in costa rica the march 25 earthquake occurred at the northwest edge of this region where a sequence of foreshocks began 16 hours prior to the mainshock the spatial temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture the mainshock ruptured an asperity of approximately 600 km2 of area with this area expanding to 4000 km2 after 7 days we present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount inversion of teleseismic broadband and long period p and sh waves yields a thrust faulting mechanism with the shallow plane striking 292 dipping 26 and with a rake of 88 in agreement with the subduction of the cocos plate under the caribbean plate local first motions for the largest foreshock and the mainshock agree with this solution we also present evidence suggesting that the march 25 1990 earthquake triggered and reactivated several seismic swarms in central costa rica and temporally decreased the activity in the epicentral area of the july 3 1983 ms 6 2 perez zeledon earthquake

seismic rupture associated with subduction of the cocos ridge the april 3 1983 costa rica earthquake mw 7 4 ms 7 3 occurred where the aseismic cocos ridge has been partially subducting beneath the caribbean plate the epicenter is at the eastern edge of the 70 110 km initial aftershock area which later shows considerable expansion p wave directivity analysis indicates that the rupture front propagated westward 63 km from the epicenter to where the major moment release was truncated the source time history of the 1941 event ms 7 5 which occurred in the same epicentral area appears to be similar to the 1983 event but with more complexity and a slower initiation of moment release the low stress drop of the 1983 event 2 3 bar and low ratio of seismic slip to tectonic slip 0 15 for the 1983 event suggest that the coupling between the descending slab and the overlying plate is weak yet a recurrence time for large events mag 7 0 in this zone of about 40 years is implied by the 1904 1941 and 1983 events suggesting that the cocos ridge causes increased partial coupling compared with that in the adjacent subduction zone the slow source rupture process may reflect the nature of the contact area between the costa rica arc and the subducting plate in the vicinity of the bouyant cocos ridge the normal faulting event of july 1 1979 mb 5 3 is consistent with present day uplift of the burica peninsula which previous studies suggest is related to the subduction of the cocos ridge the burica and osa peninsulas may behave as detached blocks which influence the distribution of partial coupling reflected in the aftershock expansion pattern

experimental evidence for mobility of zr and other trace elements in soils a soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the b horizon and then 1 cm of soil from the a horizon of a granitic podzol major and trace elements were leached from the column and accumulated in a reservoir at the base of the column total loss of elements from the soil over the course of the experiment ranged from 0 002 to 1 wt with major elements and the light and heavy rare earth elements ree showing the largest percentage losses zirconium 0 002 and then al 0 008 showed the lowest percentage loss the light ree were leached out of the soil preferentially to the mid ree all elements showed accumulation by a factor of 2 to 11 in the quartz layers at the base of the column particularly in the upper first 1 cm of the quartz major elements were leached from the column at a rate of 0 02 to 0 59 mol h 1 whereas zr nd sm gd dy rb and sr were leached at the rate of 0 5 to 30 10 6 mol h 1 concentrations of other ree in the reservoir increased over the duration of the experiment but they were poorly correlated with time so leaching rates were not calculated normalization of the major element leaching rates to take into account the constant flushing of water through the column the average annual rainfall in the allt a mharcaidh catchment in scotland from where the soil was sampled and the cross sectional area of the soil in the column together with the temperature of the soil in the column 70 c compared with the average annual temperature of the allt a mharcaidh catchment 5 7 c gave major element release rates from the soil of 0 002 to 0 97 meq m 2 yr 1 depending on the choice of ea the dissolution activation energy which are generally less than those measured in the field of 0 1 to 40 9 meq m 2 yr 1 calculations showed that despite the redistribution and loss of zr from the column assumptions of zr mobility would have had a negligible effect on calculated element release rates of na ca fe and mg however significant underestimates of the release of k 5 ti 57 al 5 and si 10 as well as some trace elements e g nd 23 rb 54 sr 24 would have occurred concentrations of ca and sr leached from the column correlated well rsq 0 93 p 0 01 supporting the idea of the use of sr release as a proxy for ca release in weathering rate calculations the release rates and percentage loss of ree from the soil varied between elements indicating that ree distribution patterns of rocks and soils may not be preserved in drainage waters

trough cross bedded conglomerate facies abstract gravel bed river deposits contain a range of facies that includes planar cross bedded gravel gp massive gravel gm and trough cross bedded gravel gt of these the gt facies is commonly interpreted to represent deposits of channel fills and or three dimensional 3d gravel dunes and or scours or pools in this study a calcrete conglomerate sheet built up of gt facies is found to consist of trough cross beds of 3d dune origin gtd and of bedload sheet origin gtb whereas gtd occurs as sets gtb is found to occur as isolated large troughs the gtd facies is characterised by high dips low width height ratios and foreset bed thickness of about 100 140 mm in contrast the gtb facies has low dips high width height ratios and foreset bed thickness usually less than 50 mm the bivariate plot of width height of troughs vs maximum dip effectively discriminates between these varieties of troughs a third variety of gt may be recognised by the presence of a scoop or spoon shaped lower bounding surface as seen in the depositional dip direction this facies has been demonstrated in other works to represent filling of scours or pools on the streambed and is designated as gtc for the preservation of 3d gravel dunes in the stratigraphic record special flow conditions are necessitated of prime importance is the rapid recession of flow subsequent to a short duration of peak discharge such discharge patterns are critical to the preservation of gravel dunes in the stratigraphic record and appear to have been met with in the case of glacial outburst floods or floods in the seasonal tropics

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

mineral weathering in trachydacitic derived soils and saprolites involving formation of embryonic halloysite and gibbsite at mt amiata central italy abstract mount amiata is a volcano that has been extinct for 200 ka it is located in southern tuscany central italy and is composed largely of trachydacitic rocks the soils of mt amiata show a dark carbon rich a horizon that overlies a slightly structured bw horizon or directly a very thick saprolite which at a depth of about 3 m still maintains some ellipsoidal cores of coherent material mineral weathering in two of these soils one under beech and the other under chestnut was studied in detail the mineralogy of the parent rock consists predominantly of volcanic glass with phenocrysts of feldspar principally k feldspar biotite and pyroxene the main weathering products are an embryonic form of halloysite and gibbsite while allophane is usually absent or occurs in minor amounts sem evidence indicates that the clay minerals form directly from the weathering of volcanic glass in an environment where si is preferentially depleted in relation to al in the saprolite an anomalous situation is observed where optically fresh ferromagnesian minerals are thickly coated by clay particles whereas the surfaces of the glass and feldspar grains are relatively much cleaner however the ferromagnesian minerals are also much more heavily colonised by fungal hyphae and it is proposed that this results in the preferential deposition or flocculation of colloidal suspensions which translocate from higher positions in the weathering profile

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

paleolimnology evolution in rift basins the ischigualasto villa union basin central western argentina during the triassic abstract extensional basins offer a high preservation potential for sedimentary and fossil records nonmarine rift basins contain diverse environments including fluvial and lacustrine lake deposits have long been recognized as significant archives of the paleoenvironmental record the ischigualasto villa union basin represents the only nearly continuous continental triassic record with up to 6000 m of alluvial fluvial and lacustrine deposits the deep lacustrine interval is known as the los rastros formation and consists of several lacustrine deltaic cycles a detailed facies analysis allowed the characterization of five facies associations and two types of cycles 1 a shoreline dominated progradational type i cycle containing facies association a offshore lacustrine deposits and facies association b marginal palustrine deposits along the shoreline of the lake or facies association c lacustrine siliciclastic coastal deposits and 2 a delta dominated progradational type ii cycle characterized by facies association a offshore lacustrine and prodelta deposits facies association d delta front deposits and facies association e delta plain deposits the sedimentary evolution of the chanares los rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts lowstand transgressive and highstand system tract biotic abiotic and sedimentologic data allowed the interpretation of the limnologic dynamics of the los rastros paleolake as a deep freshwater lake within a half graben exhibiting meromixis and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions the successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of bohacs et al to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin

focused fluid flow during subduction oxygen isotope data from high pressure ophiolites of the western alps abstract during subduction hydrothermally altered oceanic crust undergoes extensive dehydration the water released migrates upwards and may escape along the slab mantle interface or continue rising to hydrate the mantle overlying the subducting slab oxygen isotope ratios from eclogitic ophiolites of the western alps correlate closely with lithology samples with little altered basalt as a protolith have 18 o values of 5 1 6 0 material representing low temperature alteration zone rocks have 18 o values of 5 9 6 6 and rocks orginating in high temperature alteration zones have 18 o values of 4 6 5 3 these isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites these data suggest that the alpine rocks have not experienced significant fluid flow during metamorphism implying that the escape of fluid from subducted ocean crust is highly focused probably along structurally controlled pathways to the top of the slab the fluid may then continue largely along the slab mantle interface although a portion of it rises further to hydrate the overlying mantle

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

application of the telegraph model to coda q variations in southern california we examine waveforms and data used to construct coda magnitude in southern california to estimate the spatial variation of coda q and its dependence on frequency our analysis combined with independent borehole data suggests that coda is mainly generated by multiple scattering in the upper few kilometers of the crust where large impedance contrasts occur because of surface layering or fracturing the ubiquitous observation that coda q increases with frequency is explained as arising from multiple reverberations in the upper crust we suggest that the telegraph model that has been successfully used to describe reflection seismogram multiples in the exploration industry may also apply to earthquakes under this model the apparent increase of q with frequency is due to trapping of high frequency energy in the upper crust this behavior is expected if the associated reflector series has an exponential autocorrelation function a feature of the telegraph model at lower frequencies trapping is less efficient the combined effects give rise to an apparent absorption band that we suppose is superimposed on frequency independent intrinsic attenuation maximum apparent attenuation occurs at wavelengths equal to the dimensions of the regions of upper crust that contain the scattering layers at lower frequencies trapping is less effective and attenuation decreases as the longer wavelength waves sample the deeper crust and upper mantle where because of overburden pressures acoustic impedance contrasts are less extreme by taking spectral ratios of coda waves to direct s we estimate that intrinsic q is high 3000 and that coda may be modeled as multiple scattered s waves in a region of anisotropic scattering the exponential decay of the coda is a result of the perfectly reflecting surface of the earth with backscattering from random near surface layers causing progressive leakage and loss of energy downward into the more transparent lower crust and mantle

frequency dependent attenuation in the crust the shear wave attenuation q l in the crust beneath the northeastern us and scandinavia is determined as a function of depth and frequency using theoretical modeling of complete waveforms in comparison with observed data to determine attenuation in the upper crust of the northeastern us short period 0 25 t 2 sec rg waves recorded by linear arrays of seismometers are used for scandinavia rg and lg waves are combined to extend the investigation into the deeper crust the combination of rg waves and short period t 1 0 sec lg waves makes it possible to determine attenuation as a function of depth and frequency in the crust parameterized as q qo f to match the observations theoretical models for three mechanisms that contribute to attenuation in the crust frequency independent anelastic qa 1 and frequency dependent fluid flow qf 1 and scattering qs 1 are considered the best fitting models require low q q0 100 in the shallow crust that increases with increasing frequency as f0 5 above 1 hz in the middle crust between about 2 to 10 km depth q is moderate 100 q0 500 with a frequency dependence of f1 in the lower crust the attenuation is very low q 500 and its frequency dependence is difficult to establish the low intrinsic q and the frequency dependence in the uppermost crust can be explained by fluid flow in fractures in the middle crust the frequency dependence suggests scattering the increase of q in the lower crust is most likely due to annealing of microcracks under pressure and temperature as observed in the laboratory

distribution of duricrusted bauxites and laterites on the bamileke plateau west cameroon constraints from gis mapping and geochemistry estimation of the mineral resources potential is an important issue for most of developing countries the spatial distribution of bauxites and lateritic land surfaces on the bamileke plateau west cameroon has been investigated with a boolean modeling process into a gis environment on the basis of geological constraints such as elevation rock and soil types and landscape morphology field observation and srtm shuttle radar topographic mission data allowed the differentiation of two lateritic land surfaces separated by a minimum altitude difference of about 60 m these surfaces constrained by favorable rock types slope steepness and soil types provided a potential lateritic bauxitic area of 381 km 2 17 2 of the total study site field validation and the integration of legacy spatial data resulted in an area of 60 1 km 2 for potential bauxitic ores i e obviously duricrusted bauxitic surfaces with 47 8 km 2 in the upper surface and 12 3 km 2 in the lower surface alumina contents obtained from duricrust samples were analyzed by geostatistical methods and classical kriging interpolation to discriminate between bauxitic and ferruginous laterites this highlighted a geochemical trend from higher alumina values on the upper surface 40 66 wt to lower values on the lower surface 13 44 wt finally our study documents two duricrusted lateritic surfaces arranged in a staircase manner and having different geochemical characteristics the total bauxitic rich surface is distributed in five spots throughout the study area and covers 56 2 km 2 while ferruginous laterites occupy a spot of 3 9 km 2 gis mapping approach of lateritic land surfaces accounting for reliable constraints might be promising for larger scale investigations of mineral resources in cameroon

seismic evidence for fluids in fault zones on top of the subducting cocos plate beneath costa rica summary in the 2005 ticocava explosion seismology study in costa rica we observed crustal turning waves with a dominant frequency of 10 hz on a linear array of short period seismometers from the pacific ocean to the caribbean sea on one of the shot records from shot 21 in the backarc of the cordillera central we also observed two seismic phases with an unusually high dominant frequency 20 hz these two phases were recorded in the forearc region of central costa rica and arrived 7 s apart and 30 40 s after the detonation of shot 21 we considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active source seismic experiment such high frequency phases following shot 21 were not recorded after shots 22 23 and 24 all in the backarc of costa rica which might suggest that they were produced by some other source however earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude in addition we would have expected to see more arrivals from such an earthquake on other seismic stations in central costa rica we therefore investigate whether the high frequency arrivals may be the result of a deep seismic reflection from the subducting cocos plate the timing of these phases is consistent with a shear wave from shot 21 that was reflected as a compressional s p and a shear s s wave at the top of the subducting cocos slab between 35 and 55 km depth the shift in dominant frequency from 10 hz in the downgoing seismic wave to 20 hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency the spectral amplitude characteristics of the s p and s s phases from shot 21 are consistent with a very high v p v s ratio of 6 in 5m thick slab parallel layers this result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near lithostatic conditions the overpressured shear zone probably takes up fluids from the downgoing slab and it may control the lower limit of the seismogenic zone

tropical chemical weathering of hillslope deposits and bedrock source in the aburra valley northern colombian andes abstract tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles both processes are fundamental to landscape evolution and slope instability the aburra valley of the northern colombian andes is characterized by tropical conditions the valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid volcanic sedimentary ophiolitic and metamorphic sources respectively tropical chemical weathering of the aburra hillslope deposits and their respective bedrock were examined using x ray fluorescence and x ray diffraction analysis to document and quantify their chemical weathering profiles compositions and mineralogical properties the chemical index of alteration cia loss on ignition loi and the mobiles index imob were used to quantify the degree of weathering of hillslope deposits and bedrock source weathering trends were analyzed using a cn k and a cnk fm diagrams the material mantling the slopes in the aburra valley records an intense weathering history chemical weathering is characterized by increased development of clay minerals kaolinite halloysite and iron and aluminum sesquioxides lateritization characterizes the final stage of the weathering profiles concentrations of cao na2o k2o decrease markedly in the weathering products compared to the fresh bedrock source whereas concentrations of al2o3 fe2o3 and mgo increase significantly cia ratios of matrix slope deposits derived from all four sources near 100 whereas those of boulder slope deposits and saprolites are lower but exceed source rock values different a cn k weathering paths are evident for each lithotype validating the correlation established between the hillslope deposits and their various parents chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides as reflected by high loi anomalously low cia and varying enrichment trends on the a cnk fm diagrams consequently different chemical indices based on different criteria need to be combined to obtain best results as illustrated here by the combination of loi cia and imob the overall results suggest that tropical conditions have dominated for a long time in the northern colombian andes leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the quaternary however in addition to the influence of climatic factors tectonism has also undoubtedly influenced slope evolution in the aburra valley

volcanic soils of france abstract in france andic soils mainly occur in the massif central andic and other volcanic soils 5000 km 2 and on the vosges and the morvan hercynian massifs in the massif central silandosols on recent pyroclastics are distinguished from aluandosols on older mio pliocene lavas after a brief introduction to the parent materials soil characterisation survey genesis properties and land use are reviewed in two periods from 1960 to 1979 and from 1980 to 2000 soil distribution is related schematically to topo climatic sequences comparing recent pyroclastics to older volcanic lavas an udic or perudic soil moisture regime is the primary factor in andosol genesis in france silandosols occur only on recent pyroclastics and aluandosols on old lavas after reworking by glacial erosion from a mantle of loose and pre weathered materials the anthropic impacts include accelerated erosion and changes in carbon content

silicic magmas from the continental cameroon volcanic line oku bambouto and ngaoundere 40 ar 39 ar dates petrology sr nd o isotopes and their petrogenetic significance the intraplate cameroon volcanic line cvl straddles the african south atlantic continent ocean boundary and is composed mainly of alkaline basic volcanic rocks voluminous silicic volcanics characterize the continental sector of the cvl we present here new geochemical isotopic sr nd o and 40ar 39ar geochronological data on the main silicic volcanic centres of the western mt oku sabga and mt bambouto and eastern ngaoundere plateau cameroon highlands the silicic volcanism of mt oku sabga and mt bambouto occurred between 25 and 15 ma and is represented by voluminous quartz normative trachytes and minor rhyolitic ignimbrites at mt bambouto central volcano about 700 m of silicic volcanics erupted in less than 2 7 million years these silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites in general onset of the silicic volcanism migrated from ne oku 25 ma to sw sabga 23 ma bambouto 18 ma and mt manengouba 12 ma the silicic volcanism of the ngaoundere plateau eastern branch of the cvl is instead dominated by nepheline normative trachytes which are associated with strongly alkaline basalts and basanitic rocks these ne trachytes are younger 11 9 ma than the q trachytes of the western highlands the least differentiated silicic volcanics are isotopically similar 87sr 86sr 0 51278 to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials such interactions may however have played some role in the genesis of the most evolved silicic volcanics which have 87sr 86sr as high as 0 705 0 714 fractional crystallization is the preferred mechanism for genesis of the silicic melts of both western and eastern highlands as shown by modeling major and trace element variations the genesis of the least evolved q trachytes from the western highlands starting from slightly to moderately alkaline basalts is compatible with fractionation of dominantly plagioclase clinopyroxene and magnetite crystal fractionation may have occurred at low pressure and at qfm buffer f o2conditions parental magmas of the ngaoundere ne trachytes are likely instead to have been strongly alkaline basalts which evolved through crystal fractionation at higher p 6 2 kbar and f o2 qfm 2 the migration 25 to 12 ma of the silicic volcanism from ne to sw in the continental sector of the cvl is reminiscent of that 31 5 ma of the onset of the basic volcanism in the oceanic sector principe to pagalu islands of the cvl these ages and that 11 9 ma of the silicic volcanism of the ngaoundere plateau indicate that the cameroon volcanic line as a whole may not be easily interpreted as the surface expression of hot spot magmatism

controls on the stratigraphic development of the triassic fundy basin nova scotia implications for the tectonostratigraphic evolution of triassic atlantic rift basins abstract the late triassic synrift succession of the minas sub basin bay of fundy nova scotia includes a fluvio alluvial fan dominated unit lower and middle wolfville formation a transitional fluvio playa unit upper wolfville formation and an upper playa dominated package blomidon formation we describe the facies development in the 240 m thick upper wolfville formation examine vertical facies stacking patterns and develop a depositional model for this succession the unit comprises channelized and unconfined fluvial deposits interbedded with aeolian and playa sediments that grade upward into playa deposits the succession comprises progradational and retrogradational fluvial packages and thin aggradational playa claystones comparison with 11 other atlantic margin continental triassic rift basins allows an assessment of the controls that govern rift basin development the transition from fluvial to playa or lacustrine conditions occurs in all basins at different times the lack of synchronicity indicates that global climate change was not a controlling factor we suggest that the fluvial to lacustrine or playa transition was due to a decrease in source area relief related to a decline in regional tectonic activity and that the basin was hydrologically closed throughout sedimentation this tectonostratigraphic model may be appropriate for other triassic basins developed along the atlantic margin

paleosols as indicators of paleoenvironment and paleoclimate paleosols are ancient soils that have been incorporated into the geological record soils form in response to interactions among the lithosphere hydrosphere biosphere and atmosphere so paleosols potentially record physical biological and chemical information about past conditions near earth s surface as a result paleosols are an important resource for terrestrial environmental and climatic reconstructions long standing paleosol research topics include morphology classification and clay mineralogy all of which provide information about pedogenic processes and local paleoenvironments paleosols are also used to infer processes involved in the development of stratigraphic architecture and basin evolution recent paleosol research has introduced semiquantitative and quantitative measures for environmental and chronometric reconstructions that provide insight into major regional to global changes in temperature precipitation and atmospheric pco2 these new proxies focus on morphological and chemi

allogenic controls on the fluvial architecture and fossil preservation of the upper triassic ischigualasto formation nw argentina abstract the upper triassic ischigualasto formation in nw argentina was deposited in a fluvial system during the synrift filling of the extensional ischigualasto villa union basin the expansive exposures of the fluvial architecture and paleosols provide a framework to reconstruct the paleoenvironmental evolution of this basin during the upper triassic using continental sequence stratigraphy the ischigualasto formation deposition can be divided into seven sequential sedimentary stages the 1 bypass stage 2 confined low accommodation stage 3 confined high accommodation stage 4 unstable accommodation stage 5 unconfined high accommodation stage 6 unconfined low accommodation stage and finally 7 unconfined high accommodation stage the sedimentary evolution of the ischigualasto formation was driven by different allogenic controls such as rises and falls in lake levels local tectonism subsidence volcanism and climate which also produced modifications of the equilibrium profile of the fluvial systems all of these factors result in different accommodations in central and flank areas of the basin which led to different architectural configurations of channels and floodplains allogenic processes affected not only the sequence stratigraphy of the basin but also the vertebrate and plant taphocenosis therefore the sequence stratigraphy can be used not only as a predictive tool related to fossil occurrence but also to understand the taphonomic history of the basin at each temporal interval

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

organo metallic complexes in an andosol a comparative study with a cambisol and podzol current soil forming processes were studied in a nonallophanic andosol from the vosges mountains north eastern france for this purpose we conducted comparative studies on the organo metallic fraction dynamics of selected profiles of an andosol and other typical soils from this region cambisol and podzol these soils are derived from old volcanic rocks under mesic soil temperature and udic soil moisture regimes the andosol profile displayed the characteristics of nonallophanic andosols large accumulation of humus active al mostly complexed with humus and low ph special attention was given to physical and microbial stability of these organo metallic associations respectively by disaggregation tests and incubation experiments the results indicated that these associations showed a strong resistance against disaggregation by water and against biodegradation moreover the very low concentration of doc al and fe in soil solutions collected by zero tension lysimeters coupled with very low amounts of water dispersed clays obtained after dispersion tests suggested that no significant translocations of these components occurred in this andosol profile low mobility together with a strong resistance against mineralization may account for the large accumulation of carbon in this andosol

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

from seamount accretion to tectonic erosion formation of osa melange and the effects of cocos ridge subduction in southern costa rica 1 the costa rica portion of the middle america trench mat is characterized by active tectonic erosion a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward offshore studies demonstrate accelerated subduction erosion starting at the miocene pliocene boundary as the result of subduction of thickened galapagos related crust as presently represented by the cocos ridge the subduction of the cocos ridge also caused uplift and exposure of the outer forearc on the osa peninsula which offers a window to explore the tectonic evolution of the area the rocks outcropping on osa peninsula are a middle eocene middle miocene melange dominated by basalt chert and limestone resulting from accretion of seamounts the accretion dominated period of the mat evolution ended at the miocene pliocene boundary when thickened crust a paleo cocos ridge produced at the galapagos hot spot arrived at the trench the thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the osa melange the change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small kilometer to tens of kilometers size blocks that are going through differential vertical movements in response to the morphology of the subducting ridge subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin so that mass removal can develop on a previously growing margin and completely remove an accretionary prism

mineral and geochemical characterization of a leptic aluandic soil and a thapto aluandic ferralsol developed on trachytes in mount bambouto cameroon volcanic line abstract mineral and geochemical investigations were carried out on soil samples and fresh rock trachytes from two selected soil profiles tm profile on leptic aluandic soils and tl profile on thapto aluandic ferralsols from mount bambouto to better understand geochemical processes and mineral paragenesis involved in the development of soils in this environment in tm profile the hydrated halloysites and goethite occur in the weathered saprolite boulders of bc horizon while dehydrated halloysite gibbsite and goethite dominate the soils matrices of bc and a horizons in tl profile the dehydrated halloysites and goethite are the most abundant secondary minerals in the weathered saprolites of c and bc horizons while gibbsite hematite and kaolinite occur in the soil matrices of bc b and a horizons the highest gibbsite content is in the platy nodules of b horizon in both soil profiles organo metal complexes most likely of al and fe are present in the surface a horizon geochemically between the fresh rock and the weathered saprolites in both soils sio2 k2o cao na2o and mgo contents decrease strongly while fe2o3 and al2o3 tend to accumulate the molar ratio of sio2 al2o3 ki and the sum of ca mg k and na ions trb also decreases abruptly between fresh rocks and the weathered saprolites but increases significantly at the soil surface the tm profile shows intense al enrichment whereas the tl profile highlights enrichment in both al and fe as the weathering progresses upwards both soil profiles are enriched in ni cu ba and co and depleted in u th ta hf y sr pb zr and zn relative to fresh rock they also show a relatively low fractionation of the rare earth elements ree la nd sm eu tb yb and lu except for ce which tends to be enriched in soils compared to ci chondrite all these results give evidence of intense hydrolysis at soil deep in mount bambouto resulting in the formation of halloysite which progressively transforms into gibbsite and or dehydrated halloysite at the soil surface the prominent pedogenetic process refers to andosolization with formation of organo metal complexes in tl profile the presence of kaolinite in soil matrices bc and b horizons is consistent with ferralitization at soil deep in conclusion soil forming processes in mount bambouto are strongly influenced by local climate i in the upper mountain 2000 m the fresh misty and humid climate favors andosolization whereas ii in the middle lands 1700 2000 m with a relatively dry climate both andosolization at the soil surface and ferralitization at soil deep act together

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs