a seismic refraction study of the crustal structure associated with the adamawa plateau and garoua rift cameroon west africa summary recordings of quarry blasts along a 300 km profile across the adamawa plateau at the eastern end of the cameroon volcanic line permitted the determination of the crustal structure of the region to the north of the plateau in the area of the garoua rift an eastern arm of benue trough the crust is about 23 km thick and is underlain by upper mantle with a p wave velocity of 7 8 kms 1 the southern part of the plateau by comparison is associated with a more normal crustal thickness of about 33 km and underlain by upper mantle with a velocity of 8 0 kms 1 between these two areas the seismological experiment was not able to delineate how the crust changes however if the topography and gravity field reflect crustal thickness then there is a rapid change in crustal thickness at the northern edge of the adamawa plateau the study thus provides the first seismic evidence for major crustal thinning beneath the garoua arm of the west african rift system as well as providing evidence for an alternative model for the formation of the cameroon volcanic line

field evidence for flank instability basal spreading and volcano tectonic interactions at mt cameroon west africa the mt cameroon volcano is the highest and most active volcano of the cameroon volcanic line little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics after reviewing the tectonic evolution of the region the analysis of a digital elevation model and results from a field campaign dedicated to mapping geological structures in the summit area and at the se base of mt cameroon are presented mt cameroon is a lava dominated volcano with long steep over 30 flanks it is elongate parallel to its well defined rift zone the summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults geological profiles were measured along rivers cutting through a topographic step at the se base of mt cameroon this step is associated with deformed miocene sediments from the douala basin that are overlain by volcanic products weak sediments of this area are deformed by 050 060 and 020 030 trending asymmetrical folds verging toward the se and thrusts faults related to the spreading of the volcano over its mechanically weak substratum combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice repeated dyke intrusions the weak sedimentary substratum and tectonic structures

intracontinental movements in western gondwanaland a palaeomagnetic test abstract early cretaceous intracontinental movements within africa and or south america to account for misfit problem in the pre drift reconstruction of western gondwana have been tested using palaeomagnetic poles from both continents each continent has been considered as comprising separate subplates according to the boundaries proposed by pindell and dewey 1982 and curie 1983 i e northern africa naf southern africa saf northern south america nsa and southern south america ssa visual and statistical distribution of late permian to jurassic poles from the african subplates indicate two distinct groups before rotating saf relative to naf while after the rotation they become indistinguishable the distribution and sparsity of data from south america limit the study in this continent as the data are statistically indistinguishable before and after rotation of ssa with respect to nsa when the test is extended to the reconstruction of western gondwana the tightest grouping of the poles occurs when an intracontinental movement is assumed within africa this suggests that the misfit between the two continents is probably due to intracontinental deformation within africa and not in south america if confirmed this may have profound implications in the kinematics of the processes related to the opening of the south and central atlantic oceans and also the origin of the west african rift system

surface fitting in geomorphology examples for regular shaped volcanic landforms abstract in nature several types of landforms have simple shapes as they evolve they tend to take on an ideal simple geometric form such as a cone an ellipsoid or a paraboloid volcanic landforms are possibly the best examples of this ideal geometry since they develop as regular surface features due to the point like circular or fissure like linear manifestation of volcanic activity in this paper we present a geomorphometric method of fitting the ideal surface onto the real surface of regular shaped volcanoes through a number of case studies mt mayon mt somma mt semeru and mt cameroon volcanoes with circular as well as elliptical symmetry are addressed for the best surface fit we use the minimization library minuit which is made freely available by the cern european organization for nuclear research this library enables us to handle all the available surface data every point of the digital elevation model in a one step half automated way regardless of the size of the dataset and to consider simultaneously all the relevant parameters of the selected problem such as the position of the center of the edifice apex height and cone slope thanks to the highly performing adopted procedure fitting the geometric surface along with calculating the related error demonstrates the twofold advantage of the method firstly we can determine quantitatively to what extent a given volcanic landform is regular i e how much it follows an expected regular shape deviations from the ideal shape due to degradation e g sector collapse and normal erosion can be used in erosion rate calculations secondly if we have a degraded volcanic landform whose geometry is not clear this method of surface fitting reconstructs the original shape with the maximum precision obviously in addition to volcanic landforms this method is also capable of constraining the shapes of other regular surface features such as aeolian glacial or periglacial landforms

compressibility of natural soils subjected to long term acidic contamination this paper presents the results of a systematic analysis aimed at establishing whether acidic pore fluids can affect the properties of natural soils in particular their compressibility marine deposits with different mineral compositions and undisturbed soil structure were collected for this research from three coastal areas in japan pleistocene clays from the osaka and ariake bays were obtained from boreholes at a depth of more than 10 m whereas the kawasaki mud a relatively young deposit of holocene was dredged from the bed of the tokyo bay soil specimens were placed in special containers which were designed to reproduce the process of long term soil water chemical interaction and leached with solutions of sulfuric acid for different periods of time ranging from 1 to 9 months at the end of each time interval standard compression tests were performed to study the behavior of soil in an acidic environment it was found that clay mineralogy and soil structure had a significant effect on the compressibility of clays at low ph in the case of the osaka and ariake clays the compressibility significantly increased with a decrease in ph values a finding that was primarily attributed to changes in the soil s structure in contrast the effect of acidic leaching on the properties of kawasaki mud was observed to be the opposite laboratory data showed that in acidic medium the compressibility of soil decreased presumably due to the collapse of the diffuse double layer

active versus passive continental rifting evidence from the west african rift system fitton j g 1983 active versus passive continental rifting evidence from the west african rift system in p morgan and b h baker processes of continental rifting tectonophysics 94 473 481 the west african rift system comprises the cretaceous benue trough and the tertiary to recent volcanic cameroon line the two features are remarkably similar in shape and size and may be superimposed perfectly by rotating one with respect to the other by 7 about a pole in sudan three stages in the geological history of the system are postulated 1 the benue trough was produced by lithosphere stretching as one arm of an rrr triple junction during the early stages of the opening of the south atlantic 2 at about 80 ma santonian a short lived period of clockwise rotation interrupted the otherwise anticlockwise rotation of africa and decoupled the lithosphere from the asthenosphere the hot zone in the asthenosphere beneath the benue trough thus became displaced relative to the lithosphere and moved to a new position beneath cameroon and the gulf of guinea 3 anticlockwise rotation and asthenosphere lithosphere coupling were restored allowing the hot zone to manifest itself as the cameroon line the cameroon line is possibly a unique example of what a rift system produced actively by a thermal anomaly in the asthenosphere would look like it is significant that despite a long history of volcanism and uplift the cameroon line has not developed a graben structure the implication is that rift systems with graben structures are produced passively by lithosphere stretching

direct evidence of ancient shock metamorphism at the site of the 1908 tunguska event abstract shock metamorphism is rarely found at the surface of the earth the most used structures to identify shock metamorphism are true planar deformation features pdfs in quartz now accepted as diagnostic indicators of a meteorite impact here we present several lines of evidence for shock metamorphism and pdfs developed in quartz occurring on samples centered on a circular geological structure on mount stojkovic 60 54 06 n 101 55 40 e which lies within southern surface exposures of the siberian traps the shock event appears to have occurred during the eruption of the surface siberian traps basalts that cover this region curiously mount stojkovic lies within 3 km of the tree fall epicenter of the 1908 tunguska event based on current estimates of the phanerozoic impact distribution there is at most a 1 in 17 000 chance that the 1908 bolide would randomly fall on the site of a previous impact structure capable of creating shocked quartz just as improbable would be an airbust event incapable of creating a small crater that could have produced shock metamorphism our preferred least implausible hypothesis is that the shock metamorphism here was associated with a terrestrial event a hyperexplosive volcanic gas eruption called verneshot

the relationship between calc alkaline volcanism and within plate continental rift volcanism evidence from scottish palaeozoic lavas lower carboniferous lavas from the midland valley and adjacent regions of scotland are mildly alkaline and intraplate in nature the sequence is dominated by basalt and hawaiite although mugearite benmoreite trachyte and rhyolite are also present basic volcanic rocks display the lil element and lree enrichment typical of intraplate alkali basalt terrains low initial87sr 86sr 0 7029 0 7046 high end 0 4 to 5 6 and moderately radiogenic206pb 204pb 17 77 18 89 ratios are also comparable with alkali basalts from other continental rifts and oceanic islands when the carboniferous lavas are compared with subduction related lavas of old red sandstone age erupted in and around the midland valley ca 50 ma earlier at 410 ma remarkable similarities are apparent significant overlap occurs in nd and pb isotopic compositions sr isotopic compositions are however more radiogenic in the older subduction related lavas this combined with high k and rb concentrations in ors lavas may be explained by the incorporation of a sediment component derived from the subducted slab which by lower carboniferous times had been lost from the mantle source region by convection a pronounced negative nb anomaly in the ors subduction related lavas may be explained by the retention of a nb bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone allowing for the effects of the added component from the subducted slab there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas both may have been derived from chemically similar portions of mantle if volcanic arc lavas are derived from the mantle wedge the implication is that such a source lies at relatively shallow depth within the upper mantle the same may therefore apply to the carboniferous continental rift basalts this evidence combined with the fact that there is no evident hot spot trail across the midland valley despite a long period of within plate volcanism and rapid plate movements during the carboniferous suggests that the alkali basalt magmatism is not the product of a deep seated mantle plume rather the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle

a lithospheric instability origin for the cameroon volcanic line the cameroon volcanic line cvl is an enigmatic structure that defies common dynamic models of melt generation and volcanic activity on earth there magma generation and intrusion has been sustained for more than 70 myr over a 1600 km long chain straddling the ocean continent boundary with no detectable spatial age progression the chain is nearly perpendicular to the coastline and terminates in a y shaped structure that has not been affected by absolute plate motions implying that the mantle upwelling that feeds magmatic activity is attached to the continent we propose that this form of volcanism is due to a new type of instability that may develop within the subcontinental lithospheric mantle at the edge of a continent laboratory experiments document how lithosphere beneath a continental block of finite size can become unstable due to cooling from above the instability pattern is made of linear upwellings and downwellings that converge radially towards the center of the continent in an outer region and an array of polygonal cells in a central region the pattern is characterized by branching structures that are reminiscent of the strike and y shaped outline of the cvl the instability develops over long timescales with small rates of upwelling and melting and is attached to the continent by construction downwellings adjacent to upwellings induce compression in the crust which may account for deformation in the benue trough just before the onset of cvl magmatism

a teleseismic delay time study across the central african shear zone in the adamawa region of cameroon west africa summary p wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the adamawa plateau and the central african shear zone of central cameroon to determine the variation in crust and upper mantle velocity associated with these structures a plot of the mean relative residuals for the stations shows a long wavelength 300 km variation of amplitude 0 45 s the slowest arrivals are located over and just to the north of the faulted northern margin of the adamawa plateau the residuals do not correlate with topography surface geology or the previously determined crustal structure in any simple way the aki inversion technique has been used to invert the relative residuals into a 3 d model of velocity perturbations from a mean earth model the results show the region is divided roughly into three blocks by two subvertical boundaries striking ene and traversing both the crust and upper mantle down to depths greater than 190km the central block which is 2 per cent slower than the adjacent blocks roughly corresponds to the central african shear zone the adamawa plateau as an individual uplifted area is explained by the interaction of a regional anomalous upper mantle associated with the west african rift system and the central african shear zone which provided a conduit for heat flow to the surface

applying terzaghi s method of slope characterization to the recognition of holocene land slippage abstract during site investigations engineering geologists are expected to recognize landslides both active and dormant those without substantial experience evaluating the evolution of geomorphic features associated with localized mass wasting often mischaracterize site conditions karl terzaghi suggested seven steps in characterizing natural hillslopes including 1 a literature review 2 site reconnaissance 3 a program of subsurface exploration 4 preparation of sketches and diagrams illustrating the essential features 5 preparation of a preliminary geology report 6 observation and mapping of geologic structures exposed during excavation for construction and 7 development of contingency plans for anticipated variations in old landslide features based on our interpretation of terzaghi s original principles we review some of the most common interpretive problems with nomenclature of landslide features describe common mistakes in characterizing old landslides and present technical approaches that often aid in the recognition of old landslide features a wide variance in geological terms exists for describing landslides and various terms have been applied to the same features by workers in different areas which often leads to confusion some of these terms include nomenclature such as slip surfaces alluvial fans colluvium and slope creep active surficial landslides are relatively easy to recognize when they are well exposed although these features tend to become obscured with time anomalous hummocky topography often provide key recognition features of a slope s geomorphic evolution deeper seated bedrock landslides can be more difficult to recognize because their features become increasingly mollified with time these features can often be identified by anomalous geomorphic features such as isolated saddles or anomalous discontinuous benches these features are some of the best locations to excavate borings or exploratory trenches well placed trenches across headscarp grabens can provide more detailed structure of old landslides and are usually a cost effective approach additional subsurface exploration can often be employed to characterize landslides small diameter borings are usually employed for geotechnical investigations but can easily be applied to landslides depending on the mean particle size diameter d50 downhole logging of large diameter holes is the best method to evaluate complex subsurface conditions such as dormant bedrock landslides

petrology of the bimodal cenozoic volcanism of the kapsiki plateau northernmost cameroon central africa the kapsiki plateau is the northernmost volcanic zone of the cameroon line the volcanism 27 35 ma is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites trachytes and rhyolites as felsic lavas mg rich olivine phenocrysts occur only in basalts the hawaiite contains andesine olivine and ca rich pyroxene phenocrysts and sanidine and quartz xenocrysts the phonolites contain alkali feldspar and na rich clinopyroxene phenocrysts two types of trachytes occur peralkaline trachytes with ti na and f rich aegirine augite richterite arfvedsonite phenocrysts and non peralkaline trachytes with an aenigmatite type undetermined mineral ti rich biotite and zircon phenocrysts similarly two types of rhyolites occur peralkaline with quartz and arfvedsonite phenocrysts and non peralkaline with quartz and biotite phenocrysts differentiation indices thornton and tuttle 1960 of the lavas range from 22 to 97 with a large gap in the range 34 82 some basalts are primitive 530 ppm ni 1100 ppm cr in basaltic lavas phonolites and non peralkaline trachytes and rhyolites zr and nb covary with approximately constant ratios o3 1 zr nb 5 3u however peralkaline trachytes and rhyolites have high concentrations in zr up to 2180 ppm and nb up to 780 ppm with correlative higher zr nb ratios 6 3 8 3 some rhyolites have abnormal ree patterns with kinks depleted in light ree probably resulting from stability of na ree f complexes under hydrothermal conditions despite a large gap between basaltic and felsic lavas major and trace element distributions indicate co magmatism for both the basaltic and felsic lavas the differentiation of the lava series is dominated by crystal fractionation the role of fluids in rhyolite genesis being of minor effect as evidenced by constant values of y ho and zr hf throughout the series the kapsiki plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the cameroon line the continental crust appears to have no significant role in their genesis the hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic phonolitic magmas the kapsiki plateau basaltic magmas may have originated from an infra asthenospheric reservoir similarly to other basaltic magmas generated throughout the cameroon line the sr isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials while the nd isotopic composition is only slightly affected q 2000 elsevier science b v all rights reserved

the formation and impact of landslide dams state of the art abstract the blocking of river courses by mass movements is very common in mountainous areas with deep and narrow valleys landslide dams may pose serious threats to people and their livelihoods downstream in the case of abrupt dam failure since the publication of benchmark reviews of costa and schuster 1988 and korup 2002 there is a growing number of studies focusing on the formation stability and short term impacts of landslide dams this review combines the insights of all these studies builds on current concepts of landslide dams and suggests ways to unify terminologies and classifications we furthermore present a new worldwide database compiled from literature data it contains 410 landslide dams 1 million m3 in volume that were formed since 1900 since these have the most complete data entries these data show that dam longevity is among other factors correlated with the type of landslide forming the dam those formed by rock debris avalanches and rockslides have longest lifespans however the influence of landslide type or material on dam longevity decreases with time after dam formation to ensure consistency in the next database generation we suggest guidelines for data collection to provide a solid basis for evaluating dam stability and governing factors a preliminary classification matrix for landslide dam stability that combines topographic setting and the internal structure of the dam body is another outcome of our review furthermore an evaluation of the various geomorphic stability indices proposed in the literature regarding their suitability and limitations in assessing dam formation and stability shows that they predict the probability of dam formation reasonably well but that their application to longevity estimates requires further assessment the geomorphic impacts of landslide dams in the short medium and long term are summarized and illustrated with key examples finally for a better understanding of the factors controlling dam stability we recommend to 1 include dam composition and sedimentary structures in future case studies 2 maintain and update the worldwide database for sound statistical analyses 3 refine landslide dam stability indices and test them for different landslide types and 4 study hazard cascades related to multiple dams in one watershed for long term landscape evolution studies we suggest to 5 quantify terrestrial sediment flux related to landslide dams 6 detect ancient landslide dams in river profiles and 7 further exploit the sediment archives in former impoundment areas

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

landslide response to climate change in permafrost regions abstract rapid permafrost thaw in the high latitude and high elevation areas increases hillslope susceptibility to landsliding by altering geotechnical properties of hillslope materials including reduced cohesion and increased hydraulic connectivity this review synthesizes the fundamental processes that will increase landslide frequency and magnitude in permafrost regions in the coming decades with observational and analytical studies that document landslide regimes at high latitudes and elevations we synthesize the available literature to address five questions of practical importance which can be used to evaluate fundamental knowledge of landslide process and inform land management decisions to mitigate geohazards and environmental impacts after permafrost thaws we predict that landslides will be driven primarily by atmospheric input of moisture and freeze thaw fracturing rather than responding to disconnected and perched groundwater melting permafrost ice and a plane of weakness between ground ice and the active layer transition between equilibrium states is likely to increase landslide frequency and magnitude alter dominant failure styles and mobilize carbon over timescales ranging from seasons to centuries we also evaluate potential implications of increased landslide activity on local nutrient and sediment connectivity atmospheric carbon feedbacks and hazards to people and infrastructure last we suggest three key areas for future research to produce primary data and analysis that will fill gaps in the existing understanding of landslide regimes in permafrost regions these suggestions include 1 expand the geographic extent of english language research on landslides in permafrost 2 maintain or initiate long term monitoring projects and aerial data collection and 3 quantify the net effect on the terrestrial carbon budget

is climate change responsible for changing landslide activity in high mountains climate change manifested by an increase in mean minimum and maximum temperatures and by more intense rainstorms is becoming more evident in many regions an important consequence of these changes may be an increase in landslides in high mountains more research however is necessary to detect changes in landslide magnitude and frequency related to contemporary climate particularly in alpine regions hosting glaciers permafrost and snow these regions not only are sensitive to changes in both temperature and precipitation but are also areas in which landslides are ubiquitous even under a stable climate we analyze a series of catastrophic slope failures that occurred in the mountains of europe the americas and the caucasus since the end of the 1990s we distinguish between rock and ice avalanches debris flows from de glaciated areas and landslides that involve dynamic interactions with glacial and river processes analysis of these events indicates several important controls on slope stability in high mountains including the non linear response of firn and ice to warming three dimensional warming of subsurface bedrock and its relation to site geology de glaciation accompanied by exposure of new sediment and combined short term effects of precipitation and temperature based on several case studies we propose that the following mechanisms can significantly alter landslide magnitude and frequency and thus hazard under warming conditions 1 positive feedbacks acting on mass movement processes that after an initial climatic stimulus may evolve independently of climate change 2 threshold behavior and tipping points in geomorphic systems 3 storage of sediment and ice involving important lag time effects

the lake nyos gas disaster chemical and isotopic evidence in waters and dissolved gases from three cameroonian crater lakes nyos monoun and wum abstract to better understand the cause of the nyos gas disaster of august 21 1986 we conducted geochemical and limnological surveys in october 1986 of three lakes nyos monoun and wum which are located in the cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids lake wum was studied as a non active control co2 is dissolved in significant concentrations about 1 5 of saturation in gas active lakes nyos and monoun but is virtually absent in lake wum stable isotopic ratios of total dissolved carbon 13c 3 for nyos and 5 5 for monoun and of helium 5 7 ratm for nyos and 3 6 ratm for monoun indicate a mantle origin of these gases however so42 and cl concentrations are found to be very low concentrations of dissolved chemical species like fe2 mg2 ca2 and hco3 are high in the two gas active lakes whereas they are very low in the gas inactive lake high salinities in the gas active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic co2 rich warm water the gas active lakes are characterized by increasing temperature and salinity with increasing depth indicating an active influx of heat and dissolved materials at the bottom density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes on account of dissolved chemical species the concentrations of dissolved carbonate species co2 aq and hco3 are positively correlated with those of ionic dissolved species indicating their common occurrence in the bottom water the august 1986 gas bursts from lake nyos were most likely caused by rapid exsolution of dissolved co2 within the lake an explosive process such as a phreatic eruption or a co2 gas jetting from beneath the bottom is unlikely because of low concentrations of cl and so42 no oxygen isotopic shift low turbidity and no reported perturbation of the bottom sediments exsolution of co2 bubbles could occur if co2 saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season exsolution of co2 at the upper layers was possibly accelerated by upwelling of a two phase fluid co2 bubbles and solution a mechanism known as a pneumatic lift pump resulting in discharge of a large amount of co2 gas the h2s concentration in the gas cloud must have been kept far below the lethal level because of a high fe2 concentration of the lake water

seismicity of the cameroon volcanic line 1982 1990 abstract this paper is dedicated to soba djallo director of irgm who died in 1991 results of seismic monitoring of the continental segment of the cameroon volcanic line for the period 1982 to 1990 are reported mount cameroon continues to be seismically the most active area with a well defined seismicity pattern characterized by single and swarm events with duration magnitudes between 2 and 3 occurring at depths down to 55 km on average the earthquakes here occur at the rate of about 2 events every 3 days further north the seismicity is low except for isolated felt events located along the foumban shear zone which probably controls the location of the earthquakes as well as the volcanic centres making up the volcanic line during this eight year period two fatal carbon dioxide gas emissions from crater lakes occurred in 1984 l monoun and 1986 l nyos causing 37 and 1700 deaths respectively seismic monitoring indicates that no seismic activity was associated with these lakes thus favouring an aseismic cause to the disasters these gas emissions the increased number of felt earthquakes and a recent natural explosion on mount cameroon suggest either a more active phase of the cameroon volcanic line and or a greater awareness by the local population of the phenomena associated with this volcanic province

geophysical evidence of cretaceous volcanics in logone birni basin northern cameroon central africa and consequences for the west and central african rift system abstract detailed analyses and interpretation realized by combining existing 2d reflection seismic and gravity magnetic data of the logone birni basin lbb in the west and central african rift system wcas have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin the volcanic activity in the lbb is restricted to the cretaceous period three main volcanic episodes are identified and are associated to the neocomian late albian and cenomanian turonian rifting phases respectively the volcanic bodies within the lower cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the upper cretaceous bodies are morphologically expressed in the forms of dykes and sills the volcanic activity was more intense in the western region of the central lbb zina sub basin along the cameroon nigeria border whereas it was scanty and scattered in the other parts of the basin the main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults although wcas is associated with large amount of crustal extension and minor volcanism the intense volcanic activity observed in lbb during the cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north south and southwest by the lake chad poli and chum triple junctions tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere thus providing conduits for magma migration

the influence of debris flows on channels and valley floors in the oregon coast range u s a debris flows are one of the most important processes which influence the morphology of channels and valley floors in the oregon coast range debris flows that initiate in bedrock hollows at heads of first order basins erode the long accumulated sediment and organic debris from the floors of headwater first and second order channels this material is deposited on valley floors in the form of fans levees and terraces in channels deposits of debris flows control the distribution of boulders the stochastic nature of sediment supply to alluvial channels by debris flows promotes cycling between channel aggradation which results in a gravel bed morphology and channel degradation which results in a mixed bedrock and boulder bed morphology temporal and spatial variability of channel bed morphology is expected in other landscapes where debris flows are an important process

a new tectonic model for the cameroon line central africa abstract the cameroon line a major geological feature in central africa has been considered successively as a series of horsts and grabens a continental rift and a mega shear zone it is marked out by about 60 anorogenic complexes and a dozen volcanic centres all of which have alkaline affinity remote sensing allows us recognition of the main lineament trends n70 n s n135 and e w while autocorrelation analysis reveals a major fault zone striking n30 in western cameroon and n15 in the northern region a mega left lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes syntectonic leucogranites marking out older shear zones and vein dykes the n70 adamawa fault zone a pan african fracture reworked during albian aptian times is the only shear zone of continental scale that could have initiated en echelon mega tension gashes within the cameroon line during a cainozoic left lateral transcurrent movement