cenozoic lateritic weathering and erosion history of peninsular india from 40 ar 39 ar dating of supergene k mn oxides since deccan traps extrusion 65 ma ago thick weathering mantles have developed over peninsular india on both the western coastal lowland and adjacent plateau separated by the western ghats escarpment manganiferous lateritic profiles formed by supergene weathering of late archean manganiferous protores are exposed on paleolandsurface remnants on both sides of the escarpment petrological and geochemical characterizations of samples from those mn lateritic profiles allowed identifying cryptomelane k mn oxide dated by 40ar 39ar geochronology the ages obtained document major weathering periods ca 53 50 ma and ca 37 23 ma in the highland and ca 47 45 ma ca 24 19 ma and discrete weathering pulses at 9 ma and 2 5 ma in the lowland old ages of the highland 53 50 ma and the lowland 47 45 ma indicate synchronous lateritic weathering across the escarpment at a time the peninsula started to drift across the equatorial belt intense weathering periods at ca 53 45 and ca 37 23 ma are interpreted to reflect the early eocene climatic optimum and the onset of asian monsoon regimes respectively the ages further indicate that most of the dissection of the highland must have taken place after 23 ma whereas the lowland was weakly incised essentially after 19 ma our results also document divergent erosion and weathering histories of the lowland and the highland after the eocene suggesting installation of a dual climatic regime across the western ghats escarpment

relief inversion in the geomorphological evolution of sub saharan west africa abstract the geomorphology of much of sub saharan west africa is dominated by the presence of plateaux and plains with ferruginous and locally aluminous bauxitic duricrusts the plateaux occur at different elevations and have been correlated as two or more palaeosurfaces across much of the region the duricrusts have generally been considered to be residual formed by conformable erosion and chemical wasting of immediately underlying bedrock this concept has been central to interpretations as diverse as the formation and evolution of the landscape and the development of geochemical exploration models recent regolith landform mapping field observations and experience from mineral exploration in southern mali and burkina faso however demonstrate that the duricrusts are mainly ferricretes i e fe oxide cemented sediments these observations require a re interpretation of the geomorphological evolution of the region during the cenozoic the landscape has evolved by several cycles of weathering and erosion deposition triggered by climatic tectonic or other environmental changes it is proposed that an initial bauxitic lateritic regolith was partly eroded following uplift and or a change to a more arid climate and that the detritus rather than being removed was deposited on slopes and valleys during a subsequent humid period of lateritic weathering fe oxide cementation of this detritus formed ferricrete dehydration and hardening of the ferricrete after further uplift or aridity increased its resistance to erosion resulting in relief inversion with the detritus in turn being deposited downslope this too has been weathered and cemented to form a younger ferricrete the occurrence of ferricrete landforms in adjacent countries noted by field observation and inferred from satellite imagery demonstrates that relief inversion is a very widespread and important phenomenon in southern mali burkina faso and adjacent countries in semi arid west africa

40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

supergene ferromanganese wad deposits derived from permian karoo strata along the late cretaceous mid tertiary african land surface ryedale south africa a previously unknown type of ferromanganese wad deposit is described at the ryedale mine situated 110 km west of johannesburg in the northwestern province of south africa the wad was derived from supergene alteration of glossopteris bearing permian strata of the karoo supergroup that fill shallow karstic depressions in neoarchean malmani dolomite of the transvaal supergroup the depressions contain up to several million tons of friable and highly porous ferromanganese wad with an average mn fe ratio of about 0 3 and high grades of between 77 and 91 wt percent fe2o3 mno a detailed mineralogical petrographical and geochemical study of the orebody suggests that the wad is a saprolitic residue of manganiferous blackband iron ores known to be associated with permian coal measures of the karoo supergroup the protore was deposited in shallow lakes in preexisting karstic depressions in the malmani dolomite as finely laminated mud composed of biogenic detritus and mn fe oxyhydroxide precipitates siliciclastic detritus is conspicuously absent suggesting that the lakes were exclusively fed by reducing and acidic ground water that leached manganese and iron from the underlying malmani dolomite anaerobic early diagenesis led to the transformation of oxide precipitates into mn fe carbonates much later during the african cycles of erosion and weathering the blackband ores were exhumed partly eroded and altered by deep lateritic weathering to form the present ferromanganese wad

mineralogy geochemistry and genesis of the post gondwana supergene manganese deposit of the carletonville ventersdorp area north west province south africa abstract the post gondwana period is characterized by some of the unique geologic events which shaped the present geomorphological landscape in various parts of the world one of the most important and unique features associated with this geologic period is the formation of high valence state mn4 manganese oxides in weathering crusts of uplifted landscapes underlain by manganiferous protores the study of these manganese oxide minerals has proven not only to be industrially beneficial but they also provide insights into the evolution of the paleo surfaces especially in tropical regions here we present new results on mineralogy geochemistry and genetic model of the north west manganese deposit in the general nice lease area the general nice ore deposit represents a near surface accumulation of supergene manganese wad and nodule deposit in a weathering crust underlain by manganiferous dolostones of the transvaal malmani subgroup the ore formation is attributed to in situ dissolution leaching and surficial weathering of the underlying manganiferous dolostones of the malmani subgroup and carbonaceous protore of the waterval saprolite in a typically oxidic lacustrine setting the combined processes led to the release and mobilization of mn rich colloidal particles sols into the overlying water column of the depository which later precipitated as manganese nodules upon interaction with the alluvial sediments which acted as substrates and centers of accretion mineralogical studies conducted on both scanning electron microscope sem eds and x ray diffraction xrd indicated the predominance of the manganese oxides romanechite cryptomelane and galaxite as major manganese oxide phases and pyrolusite vernadite occurring in minute quantities diagenetic features indicate alteration of pyrolusite and its replacement by romanechite this is evident from the romanechite veinlets cross cutting the fractured groundmass of pyrolusite galaxite appears to represent the latest stage of authigenic mineral precipitation and occurs mostly around the pore spaces other mineral components include hematite goethite clays muscovite silica and inclusions of ilmenite and detrital zircon these accessory minerals occur either in the form of nuclei grains or inclusions in manganese oxide cement

40ar 39ar dating of west african lateritic cryptomelanes abstract dating weathering processes is an innovative approach to understanding climate dependant modifications of the earth s surface potassium bearing manganese oxides are a ubiquitous feature of weathering mantles developed from mn bearing parent rocks recent studies have demonstrated the possibility of dating k rich manganese oxides from the hollandite group such as cryptomelane k 1 2 mn 8 o 16 nh 2 o using 40 ar 39 ar argon ion laser probe method this work applies that approach to cryptomelanes formed in the lateritic manganese ore deposit of tambao northern burkina faso a present unexploited deposit among the cryptomelane richest in the world two samples of massive mn rich concretions 10mm and 4mm in thickness were collected from the massive 80 m high mn rich hill at tambao a detailed petrographical study shows that these cortical formations are composed of successive na rich cryptomelane layers with na na k ba up to 20 seven and five subsamples from these two concretions were studied with a 40 ar 39 ar laser probe the age spectra indicate ages that are coherent with the general organization of the concretions showing a regular decrease from cores towards rims within the two concretions ages range from 49 8 0 4 ma to 48 1 0 1 ma and from 47 5 0 3 ma to 44 5 0 4 ma respectively the youngest ages being obtained for the most external layers the integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm ma these ages clusters show that weathering at tambao was functioning during the early eocene and thus suggest that there is a relationship between the formation of the bauxites of the african surface and the mn fe laterites

the lithosphere asthenosphere boundary in the tien shan karakoram region from s receiver functions evidence for continental subduction 1 significant s to p converted wave energy from the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively has been observed in the tien shan and karakoram orogenic belts and used for the determination of the thickness of the crust and the mantle lithosphere the moho lies at depths of 45 65 km in the tien shan and adjacent regions and deepens to 70 km depth in pamir and karakoram the thickness of the lithosphere varies between 90 and 120 km underneath the tien shan and increases to a value of 160 km beneath the tarim basin to the south of the tarim basin the lower boundary of the asian lithosphere can be clearly followed to a depth of 270 km beneath central pamir and karakoram while the indian lithosphere is observed beneath the karakoram dipping from 130 km to 170 km towards the north the observations indicate a scenario of continental collision and subduction consistent with seismicity and the surface wave tomography results

the drainage of africa since the cretaceous much of the drainage of africa is relatively youthful many of its major rivers have shown substantial changes in their courses since the break up of gondwanaland in the cretaceous in addition many of the rivers have distinctive morphological characteristics such as inland deltas cataracts and elbows of capture tectonic and climatic changes including the development of the east african rift system and the aridification of the quaternary help to explain the nature of these rivers the history of the saharan rivers the niger the nile the congo the cunene the zambezi the limpopo and the orange is reviewed

supergene manganese ore records 75 myr long campanian to pleistocene geodynamic evolution and weathering history of the central african great lakes region tectonics drives climate assists abstract the southeastern part of the democratic republic of the congo locally hosts proterozoic manganese deposits the deposits of kisenge kamata are the most significant but manganese ores are also known to occur at kasekelesa former katanga province and mwene ditu former kasai province for the present study cryptomelane rich samples from these two localities were dated using the 40ar 39ar step heating method with a co2 laser probe the ages obtained are within a range of c 77 myr to c 2 myr cryptomelane formation took place at c 76 4 ma c 59 6 ma c 45 ma c 35 ma c 23 8 ma c 15 4 ma and c 13 3 ma at kasekelesa and it occurred at c 35 ma c 22 4 ma c 15 ma c 5 5 7 2 ma c 3 6 ma and c 2 1 2 3 ma at mwene ditu the campanian age c 76 4 ma recorded at kasekelesa is the oldest 40ar 39ar age that has up to now been recorded for mn ores from africa it documents the formation of oxidized ore along a campanian or older erosion surface which could be part of the african erosion surface the complete age record suggests that continent wide tectonics accounts for most of the recognized supergene ore formation episodes controlled by vertical lithospheric movements that are ultimately responsible for alternating stages of landscape stability and erosion tectonics is thus regarded as the first order control for secondary ore formation in central africa over the last 80 myr climate is a second order control because sufficient water supply is needed for supergene enrichment whereby climatic conditions are recognized to have been favourable during some relatively cold late mesozoic and paleogene periods as well as during some humid and warm neogene stages

crustal scale pure shear foreland deformation of western argentina 1 new analyses of teleseismic body waves from moderate earthquakes in western argentina demonstrate that active shortening of the andean foreland occurs on reverse faults extending to 40 50 km depth existing crustal scale models of foreland deformation invoke thin skinned fault geometries which root into an east dipping mid crustal decollement whereas thin skinned thrust sheets dominate shallow crustal structure seismological and geological data illustrate that planar reverse faults and pure shear deformation involving more than 75 of the crust characterizes this thick skinned structural province

spatial connectivity in a highly heterogeneous aquifer from cores to preferential flow paths 1 this study investigates connectivity in a small portion of the extremely heterogeneous aquifer at the macrodispersion experiment made site in columbus mississippi a total of 19 fully penetrating soil cores were collected from a rectangular grid of 4 m by 4 m detailed grain size analysis was performed on 5 cm segments of each core yielding 1740 hydraulic conductivity k estimates three different geostatistical simulation methods were used to generate 3 d conditional realizations of the k field for the sampled block particle tracking calculations showed that the fastest particles as represented by the first 5 to arrive converge along preferential flow paths and exit the model domain within preferred areas these 5 fastest flow paths accounted for about 40 of the flow the distribution of preferential flow paths and particle exit locations is clearly influenced by the occurrence of clusters formed by interconnected cells with k equal to or greater than the 0 9 decile of the data distribution 10 of the volume the fraction of particle paths within the high k clusters ranges from 43 to 69 in variogram based k fields some of the fastest paths are through media with lower k values suggesting that transport connectivity may not require fully connected zones of relatively homogenous k the high degree of flow and transport connectivity was confirmed by the values of two groups of connectivity indicators in particular the ratio between effective and geometric mean k on average about 2 and the ratio between the average arrival time and the arrival time of the fastest particles on average about 9 are consistent with flow and advective transport behavior characterized by channeling along preferential flow paths

modes tempo and spatial variability of cenozoic cratonic denudation the west african example 1 quantifying long term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of co 2 and sediments in the context of long term cenozoic climate change it is also a key to understanding long term landform evolution processes operating over most of the continental surface and their control onto the sediment routing system we study the surface evolution of west africa over three erosion periods 45 24 24 11 and 11 0 ma recorded by relicts of three subcontinental scale lateritic paleolandsurfaces whose age is bracketed by 39 ar 40 ar dating of lateritic k mn oxides denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early inherited reliefs the subregion underwent low and homogeneous denudation 2 20 m ma 1 over most of its surface whatever the considered time interval this homogeneity is further documented by a worldwide compilation of cratonic denudation rates over long term intermediate and modern cenozoic time scales 10 0 10 7 yr these results allow defining a steady state cratonic denudation regime that is weathering limited i e controlled by the thickness of the lateritic regolith available for stripping steady state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance under such regimes lowering of base levels and their fossilization are primarily imposed by long term eustatic sea level fall and climate rather than by epeirogeny the expression of steady state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated components 9 200 words 13 figures 2 tables

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

paleoproterozoic manganese and base metals deposits at kisenge kamata katanga d r congo the kisenge kamata manganese deposit katanga d r congo belongs to a group of paleoproterozoic sedimentary manganese ore occurrences that have been recognized for various parts of africa it is a relatively small deposit with total estimated reserves of similar to 12 mt ore but it is of major importance for understanding key aspects of the geology of the kasai block of the congo craton the available constraints indicate that the deposit formed in the 2 0 1 9 ga time interval in the orosirian period the manganese rich facies of the kisenge kamata deposits range in composition from manganese carbonate rocks to non calcareous deposits dominated by manganese rich garnet associated rocks are mainly grey graphitic shales the kisenge kamata sediments are significantly enriched in cu co ni zn and other accessory metals v mo ga field constraints and geochemical data suggest that the kisenge kamata sediments were probably deposited in a relatively small intracratonic basin graben within the kasai block with an inlet allowing periodical access for ocean water influx carrying mn ree and other metals primary carbonate ore has relatively low delta c 13 3 to 6 parts per thousand and high delta o 18 13 to 20 parts per thousand values confirming their derivation from marine fluids

the long term evolution of the congo deep sea fan a basin wide view of the interaction between a giant submarine fan and a mature passive margin zaiango project abstract we have integrated the relatively unknown distal domains of the lower congo basin where the main depocenters of the congo submarine fan are located with the better constrained successions on the shelf and upper slope through the analysis of thousands of km of 2d seismic reflection profiles off shore the congo angola passive margin the basin architecture is depicted by two ca 800 km long regional cross sections through the northern congo and southern angola margin a large unit deposited basinward of the aptian salt limit is likely to be the abyssal plain equivalent of the upper cretaceous carbonate shelf that characterized the first post rift deposits in west equatorial african margins a latest turonian shelf deepening event is recorded in the abyssal plain as a long period coniacian eocene of condensed sedimentation and basin starvation the onset of the giant tertiary congo deep sea fan in early oligocene following this event reactivates the abyssal plain as the main depocenter of the basin the time space partitioning of sedimentation within the deep sea fan results from the interplay among increasing sediment supply margin uplift rise of the angola salt ridge and canyon incision throughout the neogene oligocene early miocene turbidite sedimentation occurs mainly in nw se grabens and ponded inter diapir basins on the southern margin angola seaward tilting of the margin and downslope salt withdrawal activates the up building of the angola escarpment which leads to a northward congo shift of the transfer zones during late miocene around the miocene pliocene boundary the incision of the congo submarine canyon confines the turbidite flows and drives a general basinward progradation of the submarine fan into the abyssal plain the slope deposition is dominated by fine grained hemipelagic deposits ever since results from this work contribute to better understand the signature in the ultra deep deposits of processes acting on the continental margin as well as the basin wide sediment redistribution in areas of high river input

geodynamic and climate controls in the formation of mio pliocene world class oxidized cobalt and manganese ores in the katanga province dr congo the katanga province democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves they originated from sediment hosted stratiform copper and cobalt sulfide deposits within neoproterozoic metasedimentary rocks heterogenite the main oxidized cobalt mineral is concentrated as cobalt caps along the top of silicified dolomite inselbergs the supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world class manganese cap deposits in western katanga i e the black earths that are exploited by both industrial and artisanal mining here we provide constraints on the genesis and the timing of these deposits ar ar analyses of oxidized mn ore and in situ u pb sims measurements of heterogenite yield mio pliocene ages the ar ar ages suggest a multi phase process starting in the late miocene 10 5 ma when the metal rich substratum was exposed to the action of meteoric fluids due to major regional uplift further oxidation took place in the pliocene 3 7 2 3 ma and formed most of the observed deposits under humid conditions co and mn caps on metal rich substrata and coeval fe laterites on barren areas these deposits formed prior to the regional shift toward more arid conditions in central africa arid conditions still prevailed during the quaternary and resulted in erosion and valley incision which dismantled the metal bearing caps and led to ore accumulation in valleys and along foot slopes

field evaluation of methods for determining hydraulic conductivity from grain size data summary determination of hydraulic conductivity k and its variation in space is often a major objective of hydrogeological site investigations however measurement of k at a high spatial resolution in sedimentary aquifers is a challenge there are a number of field methods that can be used to determine k although they differ greatly in terms of their spatial resolution one commonly used approach is to estimate k from grain size analyses but the reliability of the resulting k estimates is unclear the aims of this study are to compare frequently used formulas for the determination of k from grain size data for a broad range of sediment types and to evaluate how well these methods predict k sonic sampling was used to obtain minimally disturbed cores in a highly heterogeneous sedimentary aquifer and k values of grain size analyses from 108 core samples were calculated despite the high correlation of calculated k derived from different formulas mean k values differed by several orders of magnitude between the formulas for the evaluation of the reliability of the k estimates high resolution direct push slug tests dpsts were also performed in the close vicinity of the cores a high correlation between ln k dpst and ln k grain size was found for most of the applied formulas nevertheless sample heterogeneity i e the presence of small clay layers in a generally highly permeable sample led to k grain size estimates that were significantly smaller than the k dpst estimates based on these results the applied formulas appear to be suitable for an initial assessment of aquifer k however considering the difference in calculated k mean values results are not sufficiently reliable for the high resolution analyses of k variations needed for flow or transport modeling

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

earthquake depth distributions in central asia and their relations with lithosphere thickness shortening and extension this paper examines the relationship between seismogenic thickness lithosphere structure and rheology in central and northeastern asia we accurately determine earthquake depth distributions which reveal important rheological variations in the lower crust these variations exert a fundamental control on the active tectonics and the morphological evolution of the continents we consider 323 earthquakes across the tibetan plateau the tien shan and their forelands as well as the baikal rift ne siberia and the laptev sea and present the source parameters of 94 of these here for the first time these parameters have been determined through body wave inversion the identification of depth phases or the modelling of regional waveforms lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening and to locations where rifting coincides with abrupt changes in lithosphere thickness such as the ne baikal rift and w laptev sea the lower crust in these areas is seismogenic at temperatures that may be as high as 600 c suggesting that it is anhydrous and is likely to have great long term strength lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity topography relations the variation in crustal rheology indicated by the distribution of lower crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts to the localization and orientation of rifting in combination these processes can also be responsible for the separation of the front of the thin skinned mountain belts from their hinterlands when continents separate

timing of supergene enrichment of low grade sedimentary manganese ores in the kalahari manganese field south africa abstract low grade carbonate rich manganese ore of sedimentary origin in the giant kalahari manganese field south africa is upgraded to high grade todorokite manganomelane manganese ore by supergene alteration below the unconformity at the base of the cenozoic kalahari formation incremental laser heating 40ar 39ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the kalahari unconformity peaked at around 27 8 ma 10 1 ma and 5 2 ma ago older ages are dominant in the upper part of the weathering profile while younger ages are characteristic of the deeper part of the profile younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years the oldest age obtained in the weathering profile namely 42 ma is considered a minimum estimate for the onset of the post african i cycle of weathering and erosion that followed the break up of gondwanaland and formation of the cretaceous to early cenozoic african land surface the youngest ages recorded at around 5 ma in turn correspond well to the pliocene transition from humid to arid climatic conditions in southern africa

a regional climate model study of the impact of tectonic and orbital forcing on african precipitation and vegetation abstract the development of the east african rift system ears caused by tectonic forcing during the past 20 million years is believed to influence the regional climate in africa however changes in the earth s orbital parameters also have an influence on climate on these timescales to analyze the influence of both tectonic and orbital forcing we applied the non hydrostatic regional climate model cosmo clm cosmo model in climate mode cclm the regional simulations are driven by different global simulations performed with the coupled ocean atmosphere general circulation model echo g to analyze the impact of tectonic forcing a different topography representing a possible past stage of the development of the ears is applied in the model the results indicate that tectonic forcing has a strong impact on precipitation in africa caused by circulation changes to analyze the impact of orbital forcing which means the impact of changing insolation patterns we chose configurations of the earth s orbit that result in significantly different patterns of insolation one example is the orbital configuration of the last interglacial at 125 000 years before present when the seasonality of insolation in the northern hemisphere was greatly enhanced whereas in the southern hemisphere it was substantially weakened compared to preindustrial conditions the simulation of this time slice shows a strong impact of orbital forcing on precipitation in large parts of africa caused by altered moisture transport the application of a biome model helps in analyzing the impact of the two forcing factors on vegetation for a more direct comparison with proxy data

chemical and physical erosion rhythms of the west african cenozoic morphogenesis the 39 ar 40 ar dating of supergene k mn oxides chemical weathering and mechanical erosion are first order processes of long term tropical morphogenesis which is still poorly deciphered for lack of time constraints we address this issue by laser probe 39ar 40ar dating of generations of cryptomelane k1 2mn8o16 nh2o from the manganese ore deposit of tambao in northern burkina faso this mn deposit results from the supergene weathering of carbonate and silicate mn protores underneath lateritic palaeolandsurfaces it consists of an upper cryptomelane rich domain and a lower domain where pyrolusite b mno2 is the dominant mn oxide the oldest 39ar 40ar ages 59 45 ma are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts 3 4 2 9 ma apparent correlations of 39ar 40ar age groups with d18o and eustatic curves allow definition of the different stages of morphogenesis paleocene eocene ages 59 45 ma bracket a greenhouse period propitious to bauxitic weathering the lack of significant ages between 45 and 29 ma characterizes a period dominated by mechanical erosion during which detrital sediments including lateritic materials were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface two major weathering periods separated by a second erosion episode 24 18 ma are also depicted at the end of oligocene 29 24 ma and lower to mid miocene 18 11 5 ma in the upper domain during which newly shaped land surfaces conspicuously weathered the shorter weathering and erosion episodes recorded in the lower domain from 18 to 2 9 ma led to the final geomorphic changes that were conducive to the formation of glacis the preservation of old cryptomelane 59 45 ma in the upper part of the ore deposit indicates a cenozoic denudation limited to the erosion of previous bauxites and partly of ferricretes

evidence of a large upper cretaceous depocentre across the continent ocean boundary of the congo angola basin implications for palaeo drainage and potential ultra deep source rocks abstract the analysis of 2d deep seismic reflection profiles across the slope and abyssal plain of the angola oceanic basin reveals the existence of a significant and formerly unknown depocentre beneath the giant cenozoic congo deep sea fan between 7000 m and 9000 m depth deposited directly onto the aptian oceanic crust the unit which is up to 2 5 km thick and extends for more than 200 km basinwards of the continent ocean boundary is probably aged albian turonian its radial fan shaped depocentre is centred on the present day congo river outlet and contains at least 0 2 mio km 3 of sediments these observations and the results from flexural modelling indicate that 1 the location of the congo river s outlet has remained fairly stable since the late cretaceous and 2 the basal unit was indeed sourced by a palaeo congo river probably located nearby the present day one thus the atlantic sedimentary system related to the exoreism of the congo river is much older than previously thought thermal modelling indicates that the maturation history of this upper cretaceous deposits is highly influenced by the interaction between the initial high heat flow of the young oceanic crust and further increase in sediment supply due to the progradation of the overlying tertiary deep sea fan during the miocene hence despite low present day heat flow values should the unit have source rock potential its basal section may be currently generating hydrocarbons all in all the results from our models also suggest that the interplay between an initially high heat flow and the further high sediment supply in areas of major river input may be a key factor for the thermal maturation of potential source rocks deposited onto a present day cold oceanic crust

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

enkf coupled with groundwater flow moment equations applied to lauswiesen aquifer germany summary we describe a field application of a new data assimilation method recently proposed by panzeri et al 2013 2014 the method couples a modified ensemble kalman filter enkf algorithm with stochastic moment equations mes governing space time variations of theoretical ensemble mean and covariance values of groundwater flow state variables hydraulic heads and fluxes whereas traditional enkf entails monte carlo mc simulations and suffers from inbreeding our approach obviates both synthetic case studies have shown the me based approach to be computationally efficient and accurate when compared to mc based results here we use our me based method to assimilate drawdown data recorded during cross hole pumping tests in the heterogeneous alluvial lauswiesen aquifer near tubingen germany our results include an estimate of log transmissivity distribution throughout the aquifer and corresponding measures of estimation error we validate our calibrated model by using it to predict drawdowns recorded during another pumping test at the site and compare its performance with that of standard mc based enkf

an intermediate depth earthquake beneath tibet source characteristics of the event of september 14 1976 this paper presents the results of a detailed study of an intermediate depth earthquake beneath southern tibet september 14 1976 0643 51 6 ut 29 8l n 89 57 e focal depth 90 10 km mb 5 4 from the differences in arrival times of p pp and sp and from rayleigh wave spectral densities the focus is estimated to be at depth of 90 10 km and therefore in the uppermost mantle from the polarities of the first motions of p waves inversion of the relative amplitudes of p pp and sp and synthesis of their waveforms the source mechanism is constrained to be predominantly normal faulting with the t axis approximately horizontal and trending east west this mechanism is similar to those of shallow earthquakes in tibet the occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 to 800 c in the mantle beneath the tibetan crust the seismic moment of an equivalent double couple point source is estimated to be about 9 1024 dyne cm with an uncertainty factor of 1 3 from the sp amplitudes in the period range near 10 s and 5 1024 dyne cm also with an uncertainty factor of 1 3 from the spectral density of rayleigh waves with periods of 50 s

differentiated hydrothermal and meteoric alterations of the lueshe carbonatite complex democratic republic of congo identified by a ree study combined with a sequential acid leaching experiment the behavior of rare earth elements ree during hydrothermal alteration and progressive weathering has been studied by acid leaching experiments on minerals and whole rock samples of the lueshe carbonatite complex ne of democratic republic of congo the fresh and hydrothermally altered samples were successively leached with acetic acid hac 1 n and hydrochloric acid hcl 1 n an additional leaching step with nitric acid hno3 1 n was applied on meteoric weathered material after this sequential extraction procedure the residues were dissolved in a fluorhydric nitric solution hf hno3 the lueshe carbonatite complex contains a suite of ree carriers identified as ca bearing minerals including carbonates apatite and pyrochlore as primary minerals that are accompanied by hydrothermal ancylite ce parisite ce synchysite ce and monazite ce the hydrothermal alteration induces an enrichment in the light ree but the authigenic minerals except monazite do not in fact play an important part in the pre lateritic ree stock because of their low amounts during the early stage of weathering the residual minerals retain the bulk of the ree the distribution being only slightly modified during intense weathering the leaching experiments indicate that the ree are redistributed among secondary ree phosphate minerals and bonded to fe hydroxides in the lateritic profile

multistage genesis of the late cretaceous manganese karst hosted tasdremt deposit high atlas morocco the eastern part of the souss basin morocco contains several mn deposits in the tasdremt district three mn orebodies occur within the cenomanian turonian dolostones and the senonian coniacian to maastrichtian detrital series the main orebody being located at the boundary between them the mn ores consist of coronadite group minerals mostly coronadite and hollandite in a karstified dolostone new field observations petrographic analyses and geochemical data define the tasdremt deposits as a karst hosted accumulation 11 60 wt mn particularly enriched in ba 1 5 8 2 wt and pb 1 0 5 0 wt with poor contaminations in al fe and p this study shows that the ore forming process is similar to that occurring in the imini c3 level located 100 km to the north east such similarities with the high grade pyrolusite bearing ore suggest that the tasdremt deposit is a lateral equivalent of the imini deposits however the scarcity of pyrolusite in tasdremt results in lower mn grades the tasdremt ores being considered an aborted incomplete system in comparison with the imini deposits 40ar 39ar geochronology of k bearing mn oxides yields late cretaceous ages defining three phases at 91 5 ma 77 5 82 ma and 65 67 ma although the source of metals remains hypothetical mineralizing fluids were carried by o2 free groundwater that mixed with o2 rich shallow meteoric waters at the tasdremt depositional site the dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of mn oxides causing the coeval increase of ph and eh respectively the early atlasic deformation during the late cretaceous is associated with mineralization events and was responsible for creation of low stand reliefs from tasdremt to imini this period enabled karstification and mineralization connecting the tasdremt deposits to other african mn deposits is difficult since the latter consist of laterite resting above paleoproterozoic mn protores and consequently formed under different conditions from karst hosted deposits it is likely that other mn occurrences formed along the atlas belt in similar settings

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

planation surfaces as a record of mantle dynamics the case example of africa abstract there are two types of emerged relief on the earth high elevation areas mountain belts and rift shoulders in active tectonic settings and low elevation domains anorogenic plateaus and plains characteristic of the interior of the continents i e 70 of the earth emerged relief both plateaus and plains are characterized by large erosional surfaces called planation surfaces that display undulations with middle several tens of kilometres to very long several thousands of kilometres wavelengths i e characteristic of lithospheric and mantle deformations respectively our objective is here 1 to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to central africa and 2 to reconstruct the growth of the very long wavelength relief since 40 ma as a record of past mantle dynamics below central africa i the african relief results from two major types of planation surfaces etchplains weathering surfaces by laterites and pediplains pediments these planation surfaces are stepped along plateaus with different elevations this stepping of landforms records a local base level fall due to a local tectonic uplift ii central africa is an extensive etchplain type weathering surface called the african surface from the uppermost cretaceous 70 ma to the middle eocene 45 ma with a paroxysm around the early eocene climatic optimum restoration of this surface in central africa suggests very low elevation planation surfaces adjusted to the atlantic ocean and indian ocean with a divide located around the present day eastern branch of the east african rift iii the present day topography of central africa is younger than 40 30 ma and records very long wavelength deformations 1000 2000 km with 1 the growth of the cameroon dome and east african dome since 34 ma 2 the angola mountains since 15 12 ma increasing up to pleistocene times and 3 the uplift of the low elevation 300 m congo basin since 10 3 ma some long wavelength deformations several 100 km also occurred with 1 the low elevation central african rise since 34 ma and 2 the atlantic bulge since 20 16 ma these very long wavelength deformations record mantle dynamics with a sharp increase of mantle upwelling around 34 ma and an increase of the wavelength of the deformation and then of mantle convection around 10 3 ma

extension on the tibetan plateau recent normal faulting measured by insar and body wave seismology we use insar and body wave modelling to determine the faulting parameters for a series of five mw 5 9 7 1 normal faulting earthquakes that occurred during 2008 including the march 20 yutian earthquake mw 7 1 one of the largest normal faulting events to have occurred recently on the continents we also study three earlier normal faulting earthquakes that occurred in southern tibet between 1992 and 2005 coseismic deformation for each of these eight events is measured with ascending and descending interferograms from envisat ers and alos sar data elastic dislocation modelling of the line of sight insar displacements and body wave seismological modelling of p and sh waves are used to estimate fault parameters and are found to be in good agreement for all the events studied the use of insar to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity only five of the eight events are associated with a clear surface topographic feature suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone the observations in all cases are consistent with slip on planar surfaces with dips in the range 40 50 that penetrates the uppermost crust to a depth of 10 15 km we find no evidence for active low angle dip less than 30 normal faulting the contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3 4 mm yr 1 or 15 20 per cent of the rates of extension measured across the plateau using gps 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km this observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

planation bauxites and epeirogeny one or two paleosurfaces on the west african margin mapping of lateritic bauxites over the west african rifted margin and analysis of the geomorphic properties of these bauxites combined with available geological data lead to a discussion of the presence of either two meso cenozoic planation surfaces or a single eocene surface to account for the morphotectonic and paleoclimatic evolution of the guinean landforms at large scale two stepped bauxitic levels are documented ongoing or episodic uplift following gondwana breakup and meso cenozoic climate change are proposed to have allowed the formation and abandonment of an early mid cretaceous surface today preserved as the higher bauxitic level and the setting of an eocene planation surface bearing a second generation of bauxites making the lower bauxitic level the single eocene surface hypothesis requires that paleogene bauxitization preserved large pre existing relief to explain two stepped bauxitic levels of the same age the two surface hypothesis is favored because it would explain rebauxitization of alluvial pebbles of bauxites under the lower lateritic level

malachite an indicator of major pliocene cu remobilization in a karstic environment katanga democratic republic of congo abstract malachite cu3 oh 2 co3 is a common mineral in the central african copperbelt where it constitutes a high grade 57 cu copper ore and is exploited industrially and by artisanal miners the genesis of this supergene ore in katanga has yet received little attention here we focus on the genetic processes giving rise to this regionally important mineral malachite is secondary to primary copper sulfides which indicates that it ultimately formed by oxidation of the latter in fine grained siliciclastic host rocks it is often partly impregnative commonly with variations in malachite content parallel to the bedding this type may form either by absolute enrichment or by in situ replacement of sulfides within voids of various types including dissolution cavities in carbonate host rocks it precipitates in distinctive forms such as thick laminated crusts and speleothemes up to several tens of centimeters in length ree geochemistry suggests that malachite formed in groundwater environments through in situ recombination of carbonate ions originating from dissolving host rock carbonates and downward percolating cu2 leached from copper sulfides thermodynamic modeling confirms this genetic environment and shows that chrysocolla cu al 2h2si2o5 oh 4 nh2o coexists with malachite when si content increases in the groundwater fluids the importance of karstic features in katanga is also re evaluated karstic features appear to be rather frequent and to constitute ideal site traps for the formation of secondary base metal ores

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

the age of supergene manganese deposits in katanga and its implications for the neogene evolution of the african great lakes region abstract supergene manganese deposits commonly contain k rich mn oxides with tunnel structure such as cryptomelane which are suitable for radiometric dating using the 39ar 40ar method in africa mn deposits have been dated by this method for localities in western and southern parts of the continent whereas only some preliminary data are available for central africa here we present new 39ar 40ar ages for mn oxide samples of the kisenge deposit in southwestern katanga democratic republic of the congo the samples represent supergene mn oxide deposits that formed at the expense of primary paleoproterozoic rhodochrosite dominated carbonate ores main phases of mn oxide formation are dated at c 10 5 ma 3 6 ma and 2 6 ma for a core that crosses a mineralized interval the latter shows a decrease in age with increasing depth recording downward penetration of a weathering front surface samples of the kisenge deposits also record a c 19 2 ma phase as well as c 15 7 ma 14 2 ma and 13 6 ma phases the obtained ages correspond to distinct periods of paleosurface development and stability during the mio pliocene in katanga because katanga is a key area bordered to the north by the congo basin and to the east by the east african rift system these ages also provide constraints for the geodynamic evolution of the entire region for the mio pliocene the kisenge deposits record ages that are not systematically found elsewhere in africa although the 10 5 11 ma event corresponds to a roughly simultaneous event in the kalahari manganese field south africa the rest of the katanga paleosurface record differs somewhat from records for other parts of africa for which older eocene ages have been obtained this difference is most probably related to the specific regional geodynamic context uplift of the east african plateau with associated erosion and the opening of the east african rift system at c 25 ma are events whose effects in the study area interfere with those of processes responsible for the development of continent wide paleosurfaces

modeling of the koyna india aftershock of 12 december 1967 short period teleseismic and near regional long period waveforms from a large aftershock mb 5 3 origin time 06 18 36 8 utc of the 10 december 1967 koyna main event were modeled to determine source depth and focal mechanism identification of pp and sp on the short period waveforms yielded a well constrained source depth of 3 5 to 4 0 km the focal mechanism was determined using a systematic trial and error grid testing technique in which seven p first motions two sh first motions two teleseismic pp p ratios one teleseismic sp p ratio and one regional sh p ratio were included this heterogeneous but sparse data set yielded a constrained normal fault mechanism at 100 20 strike 40 10 dip and 240 20 rake rupture directivity is inferred from consistently high surface reflection direct wave amplitude ratios a circular fault of 1 km radius with a rupture initiation point at the lower edge and an assumed rupture velocity of 0 9 vs explains the anomalously high amplitude ratios and yields a seismic moment of 3 8 1023 dyne cm the shallow source depth is similar to that found for the main event 4 5 km and the 13 september foreshocks 5 km in previous modeling studies and is consistent with the premise that the koyna earthquakes were triggered by the impoundment of the koyna reservoir however the normal fault mechanism differs substantially from the previous events which were left lateral strike slip faults trending nne the differing aftershock mechanism is probably due to the reactivation of old faults in the koyna area caused by stress readjustment after the koyna main event

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

first 40ar 39ar geochronology of lateritic manganiferous pisolites implications for the palaeogene history of a west african landscape abstract a pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in west africa using 40 ar 39 ar laser probe analysis on different generations of cryptomelane from fe mn pisolites and their embedding lateritic matrices prior to the 40 ar 39 ar geochronological analysis a detailed petrographical study has allowed the identification of successive assemblages from mn pisolites to fe mn pisolites embedded in mn rich and fe rich matrices respectively the age spectra obtained for pisolites including cores cortices and matrices enables the definition of three age clusters around 56 59 44 47 and 24 27 ma the first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced mn leaching and fe al accumulations due to warmer and wetter climatic conditions in the age interval 56 47 ma that characterizes the tertiary greenhouse effect period propitious to bauxite formation in west africa the absence of 40 ar 39 ar ages between 44 and 27 ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled but could still be favourable to fe and or al oxy hydroxides formation before drier climatic conditions became rather effective at the oligocene period to sustain mechanical erosion rather than chemical weathering processes the age cluster 24 27 ma indicates however a reactivation of the manganiferous lateritic weathering late oligocene the 40 ar 39 ar dating results are discussed in terms of climatic condition changes during the palaeogene that were favourable to the genesis of either al and fe or mn oxy hydroxides in the course of development and evolution of the pisolitic formation the 40 ar 39 ar dating also provides a new highlight to large scale geomorphological patterns of west africa

stochastic simulation of soil particle size curves in heterogeneous aquifer systems through a bayes space approach we address the problem of stochastic simulation of soil particle size curves pscs in heterogeneous aquifer systems unlike traditional approaches that focus solely on a few selected features of pscs e g selected quantiles our approach considers the entire particle size curves and can optionally include conditioning on available data we rely on our prior work to model pscs as cumulative distribution functions and interpret their density functions as functional compositions we thus approximate the latter through an expansion over an appropriate basis of functions this enables us to a effectively deal with the data dimensionality and constraints and b to develop a simulation method for pscs based upon a suitable and well defined projection procedure the new theoretical framework allows representing and reproducing the complete information content embedded in psc data as a first field application we demonstrate the quality of unconditional and conditional simulations obtained with our methodology by considering a set of particle size curves collected within a shallow alluvial aquifer in the neckar river valley germany

40ar 39ar ages of precambrian manganese ore minerals from sweden india and morocco in order to test their chronometric potential 40ar 39ar stepheating and 4he analyses have been carried out on five manganese ore minerals of the hollandite cryptomelane series from three precambrian manganese deposits ultevis sweden sitapar india bachkoun morocco samples from the metamorphic occurrences ultevis and sitapar yielded ar ages of 1 8 ga and 0 95 ga interpreted as the age of postmetamorphic cooling hollandites ultevis and of an early k introducing alteration process subsequent to amphibolite facies metamorphism cryptomelanes sitapar both data are consistent with known chronologies of the svecokarelian and satpura orogenic cycles a date of 670 ma obtained for a hollandite from a volcanogenic vein deposit bachkoun however contrasts with published extrusion ages of 580 560 ma for the volcanic host rocks ouarzazate series probably due to incorporation of excess argon the use of the 40ar 39ar technique together with multiple isotope systematics made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 ma measurement of fractional ar losses during vacuum step heating 500 1600 c although indicating good ar retentivities failed to define model diffusion parameters because of non linear arrhenius arrays helium diffusion results 200 1200 c indicated retention of radiogenic 4he by the samples corroborated by u he mineral dates between 0 96 and 0 31 ga potassium bearing manganese oxides are therefore able to retain argon possibly also 4he through geological times and may thus provide ages of ore forming processes and perhaps later cooling and alteration stages