brittle tectonic evolution along the western margin of south africa more than 500 myr of continued reactivation abstract the brittle structural history of western south africa has been investigated by remote sensing and field studies to build a conceptual scheme for its 500 ma long evolution paleostress tensors were computed from a significant fault slip dataset and a relative geochronological succession of brittle deformation events was established this was aided by separating in time faulting events through the usage of cretaceous weathering horizons silicified fluvial deposits paleosols and 77 54 ma olivine melilitite plugs as time markers the oldest features recognized formed during four compressional episodes assigned to the neoproterozoic pan african evolution this history is expressed by sub vertical conjugate fracture sets and fits well the inferences derived from remote sensing the greatest compressive direction rotated from nw se to nne ssw and finally to almost e w a subsequent ene wsw oriented extensional episode is associated with the local effects of the opening of the atlantic ocean and was followed by a second ca e w extensional episode linked to the well acknowledged mid cretaceous 115 90 ma event of margin uplift a late santonian 85 83 ma nw se compressive paleostress deformed the late cretaceous sequences and was in turn followed firstly by a renewed episode of ne sw extension and later by ca nne ssw late maastrichtian 69 65 ma shortening the latter is broadly coeval with the emplacement of the gamoep magmatic suite a phase of wnw ese cenozoic extension is assigned to the extensional phase recorded in the okawango delta interpreted as reflecting propagation of the east african rift system into southern africa no stress tensor was computed for the present day wegener anomaly stress field oriented nw se however in situ stress measurements were used to perform slip tendency analysis which indicates that under the currently existing stress conditions wnw ese and nnw sse striking faults are critically stressed and are the most likely reactivated in agreement with the present seismicity

controls on the transition from meandering to straight channels in the wetlands of the okavango delta botswana tooth s mccarthy t s 2004 controls on the transition from meandering to straight channels in the wetlands of the okavango delta botswana earth surface processes and landforms 29 13 1627 1649 rae2008

rift kinematics during the incipient stages of continental extension evidence from the nascent okavango rift basin northwest botswana high resolution aeromagnetic data from the nascent okavango rift in northwest botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension crosscutting relationships between west northwest trending 180 ma karoo dikes and reactivated northeast trending proterozoic basement faults are used to document the kinematics of cenozoic faulting during the initial stages of rifting depth estimates to the top of the dikes using three dimensional euler deconvolution solutions have produced the following interpretations 1 the okavango rift is a half graben with a downthrow of 200 300 m 2 the width of the okavango rift 100 20 km is similar to that of more mature continental rifts such as the tanganyika and baikal rifts this suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin nor the amount of extension it is suggested that the cumulative downthrow sediment infill included and subsidence may be a better indicator of the relative maturity of rift basins 3 preexisting basement faults exert a major control during rifting and reactivation processes do not occur synchronously along the entire length of preexisting faults 4 the okavango rift is defined by normal faults there is no evidence of major strike slip faults thus excluding a pull apart tectonic model for this nascent continental rift stage 5 the preexisting sekaka shear zone terminates the okavango rift to the south suggesting that such shear zones represent major barriers during longitudinal propagation of rifts this probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts

source characteristics of the 2017 mw6 4 moijabana botswana earthquake a rare lower crustal event within an ancient zone of weakness abstract the 2017 moijabana earthquake in central botswana mw6 4 was a large and deep event for a continental interior and occurred in a region with little historical seismicity based on insar measurements of surface deformation spanning the event and teleseismic observations we determine the ruptured fault plane and finite rupture model of the earthquake although this oblique normal faulting earthquake is too deep to uniquely determine the rupture plane geometry from insar alone the best fitting fault plane constrained by the joint inversion of teleseismic waveforms and insar data has a southwest dip and a strike of 126 roughly consistent with the geologically mapped strike of the kaapvaal craton s northern edge our results indicate that the earthquake had a total duration of 10 s characterized by two major asperities the first asperity nucleated in the lower crust and then the rupture propagated up dip the lower crustal asperity shows a much shorter rise time compared with the shallower asperity indicating that contrasts in stress or material properties may have played an important role in the rupture process the earthquake appears to have occurred in the limpopo belt a proterozoic orogenic belt that represents an ancient zone of weakness between the archean zimbabwe and kaapvaal cratons in the present day this zone of weakness may be responding to the stress field imposed by the east african rift system

diagenesis in okavango fan and adjacent dune deposits with implications for the record of palaeo environmental change in makgadikgadi okavango zambezi basin northern botswana abstract this work considers the spatial distribution and ages of western moz basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands previously published and new tl osl ages imply that extensive over washing of dune sands took place at least 100 ka ago while ages on okavango floodplains imply that the fan was formed ca 40 ka and has since undergone periods of higher and lower flood regimes sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay enhanced amorphous silica ceas which bonds weakly formed aggregates the time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using osl tl techniques hence earlier dune over washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation 100 000 years ago the relatively abundant ceas matrix in floodplain sands implies more recent semi continuous flood events again of an acidic near neutral nature leading to the formation of smectite in this case the floodplain sediments are dated as having been deposited around 40 and 11 ka when porewater content may have accelerated clay formation and silica dissolution the dual nature of the ceas in the islands reflects a changing environment from smectite dominated flooding events to sepiolite dominated desiccation events flooding may also correspond to tl osl ages over the past 40 000 years which contributed to accelerated ceas formation the sepiolite is associated with a ca rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation this work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events the implications of different stages of sand diagenesis may be significant in enhancing palaeo environmental interpretations in semi arid fluvial environments

terrestrial heat flow in botswana and namibia we report terrestrial heat flow measurements from 25 new sites in botswana and namibia twelve sites from the archean kaapvaal zimbabwe craton and the limpopo belt have a mean of 47 4 mw m 2 standard error of the mean but show a trend of increasing heat flow from less than 40 mw m 2 within the cratonic nucleus to about 60 mw m 2 at its edge thirteen sites in the proterozoic and pan african mobile belts that surround the craton have a mean of 66 3 mw m 2 increasing from about 60 mw m 2 at the cratonic margin to more than 70 mw m 2 several hundred kilometers away radiogenic heat production in near surface basement rocks was measured for four sites on the craton and seven sites in the mobile belts the limited heat flow heat production data from the craton are insufficient to establish a correlation heat flow and heat production data from the mobile belts when combined with published data from six sites in zambia exhibit a linear trend with an intercept of 50 3 mw m 2 and a slope of 5 5 1 1 km some but not all of the observed difference in surface heat flow between the mobile belts and the cratonic nucleus may be due to differences in crustal heat production that part of the regional variation in surface heat flow which originates at greater depths implies a fundamental difference in thermal structure of the lithosphere between the craton and the mobile belts in southern africa

massive sulfide deposition and trace element remobilization in the middle valley sediment hosted hydrothermal system northern juan de fuca rdge the bent hill massive sulfide deposit and odp mound deposit in middle valley at the northernmost end of the juan de fuca ridge are two of the largest modern seafloor hydrothermal deposits yet explored trace metal concentrations of sulfide minerals determined by laser ablation icp ms were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid mineral reactions during sulfide deposition detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits trace elements mn co ni as se ag cd sb pb and bi analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins enrichments of some trace metals pb sb cd ag in sphalerite at the margins of the deposits are best explained by dissolution reprecipitation processes consistent with secondary remineralization results of reaction path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater model results are consistent with patterns of observed mineral paragenesis and help to identify conditions ph redox temperature that may be responsible for variations in trace metal concentrations in primary and secondary minerals differences in trace metal distributions throughout a single deposit and between nearby deposits at middle valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems

210pb 230th and 10be in central indian basin seamount sediments signatures of degassing and hydrothermal alteration of recent origin 1 isotopic 210pb 238u 230th 10be major and trace elements and micromorphological and microchemical data were used to identify recent 100 yrs hydrothermal alteration of a 200 kyr sedimentary record from the flank of a seamount in the central indian basin located at the edge of the 75 30 e fracture zone alteration effects are also reflected in 1 the depleted sedimentary organic carbon 2 dissolution features of radiolarian skeletons 3 the presence of altered minerals such as smectite and zeolites and 4 distinctly different magnetic properties in the altered sediments we interpret a predominant influence of neutral chloride type hydrothermal fluids this is the first report of recently occurring sediment alteration by shallow circulating sub surface fluids along the indian ocean intra plate seamount environment

sedimentological and geochemical evidence for palaeo environmental change in the makgadikgadi subbasin in relation to the moz rift depression botswana abstract this work considers new evidence for palaeo environmental change taking place during the pleistocene in northern botswana duricrusted strandlines along the northeastern margin of sua pan provide palaeo environmental data pertaining to the makgadikgadi subbasin msb with inferences regarding the larger makgadikgadi okavango zambezi moz rift depression field xrd and geochemical data show that msb strandlines comprise calcretes lu1 type mgo rich calcretes with silica lu2 type sil calcrete lu3 type and silcrete lu4 type early freshwater episodes appear to have been followed by calcrete dominated drying phases interspersed with repeated silcretisation calcretisation through pan littoral sediments may have been both biogenically and environmentally induced calcite precipitation was in part controlled by the mg ca ratio of pore water in the pan littoral zone suggesting closed basin type evaporative conditions which were followed by a major desiccation interval phases of silcrete precipitation appear to be related to periods when the geochemistry of the lake littoral more closely resembled present day na co 3 so 4 cl type brines silica saturated acidic moderately saline groundwater preceded si precipitation which took place as the ph reduced si mobilisation occurred inter alia as a result of quartz grain dissolution enhanced by diatoms bacteria and algal growth in the moist pan littoral sio 2 rich pore waters migrated through cracked and desiccated calcrete into areas of lower salinity and lower ph resulting in preferential calcite removal and silcrete precipitation approximate tl dates imply that exposed littoral sand underwent calcretisation during the drying phases of extensive palaeo lakes which occurred prior to 110 ka 80 90 ka and 41 43 ka these wet periods compare fairly well with vostok core chronologies for southern africa

new chronology for the southern kalahari group sediments with implications for sediment cycle dynamics and early hominin occupation abstract kalahari group sediments accumulated in the kalahari basin which started forming during the breakup of gondwana in the early cretaceous these sediments cover an extensive part of southern africa and form a low relief landscape current models assume that the kalahari group accumulated throughout the entire cenozoic however chronology has been restricted to early middle cenozoic biostratigraphic correlations and to osl dating of only the past 300 ka we present a new chronological framework that reveals a dynamic nature of sedimentation in the southern kalahari cosmogenic burial ages obtained from a 55 m section of kalahari group sediments from the mamatwan mine southern kalahari indicate that the majority of deposition at this location occurred rapidly at 1 1 2 ma this pleistocene sequence overlies the archaean basement forming a significant hiatus that permits the possibility of many phanerozoic cycles of deposition and erosion no longer preserved in the sedimentary record our data also establish the existence of a shallow early middle pleistocene water body that persisted for 450 ka prior to this rapid period of deposition evidence from neighboring archeological excavations in southern africa suggests an association of high density hominin occupation with this water body

basement fabric controls rift nucleation and postrift basin inversion in the continental margin of ne brazil abstract in passive continental margins the brittle reactivation of shear zones and their role in the deformation and deposition of sedimentary basins are still a matter of debate in this research we investigated the role of the brittle reactivation of precambrian shear zones in the nucleation of rift and postrift faults in the onshore portion of the sergipe alagoas and pernambuco basins in northeastern brazil we combine and interpret a dataset of aeromagnetic and topographic data associated with reflection seismic and borehole data to analyze the evolution of a portion of the atlantic continental margin of brazil our results indicate that in the crystalline basement the magnetic lineaments are correlated with ductile structures as shear zones and the continuity of these lineaments in the sergipe alagoas and pernambuco basins is interpreted as the shear zones below the sedimentary cover of these basins we document the following phases of the brittle reactivation of basement shear zones 1 the opening of the south atlantic ocean in the early cretaceous under an extensional stress regime and 2 tectonic inversion induced by the mid atlantic ridge push and the andean cordillera rise in the neogene quaternary under a predominantly strike slip stress regime during the rift phase the brittle reactivation of the shear zones controlled the locations and architectures of the rifts these structures acted as zones of weakness and were reactivated as normal faults the brittle reactivation of shear zones was still active during the postrift phase and was responsible for the development of compressional structures the reverse faulting and related folding pattern indicate tectonic inversion in the late cretaceous cenozoic the structures formed during the postrift phase under a strike slip regime are consistent with the present day stress field indicating that tectonic inversion is an active phase of the brazilian margin

crustal and upper mantle shear wave velocity structure of botswana the 3 april 2017 central botswana earthquake linked to the east african rift system rayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3 d crustal and upper mantle shear wave velocity model of botswana the model shows low crustal velocities associated with the passarge and nosop sedimentary basins whereas the kaapvaal zimbabwe maltahohe and congo cratons are recognized by high mantle velocities the lowest upper mantle shear wave velocity beneath northeastern botswana is associated with the southwestern branch of the east african rift system this low velocity mantle anomaly appears to be linked to the crust of the okavango rift zone and the location of the 3 april 2017 mw 6 5 earthquake in central botswana we suggest that fluids or melt at the base of the crust from the southward continuation of the east african rift zone triggered the intraplate earthquake in an extensional tectonic setting

upper mantle p and s wave velocity structure of the kalahari craton and surrounding proterozoic terranes southern africa new broadband seismic data from botswana and south africa have been combined with existing data from the region to develop improved p and s wave velocity models for investigating the upper mantle structure of southern africa higher craton like velocities are imaged beneath the rehoboth province and parts of the northern okwa terrane and the magondi belt indicating that the northern edge of the greater kalahari craton lithosphere lies along the northern boundary of these terranes lower off craton velocities are imaged beneath the damara ghanzi chobe belt and may in part reflect thinning of the lithosphere beneath the incipient okavango rift lower velocities are also imaged to the north and northwest of the bushveld complex beneath parts of the okwa terrane magondi belt and limpopo belt indicating that cratonic upper mantle in some areas beneath these terranes may have been modified by the 2 05 ga bushveld and or 1 1 ga umkondo magmatic events

centrifuge modelling of the influence of crustal fabrics on the development of transfer zones insights into the mechanics of continental rifting architecture abstract centrifuge analogue experiments are used to model the reactivation of pre existing crustal fabrics during extension the models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones the typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement the transfer zone was also characterised by cross basin faults with both trend and strike slip component of movement opposite to that displayed by the master faults typically different structural patterns were obtained by changing the offset angle between the rift segments supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre existing discontinuities with respect to the stretching vector in the models the aspect ratio ratio of length vs width of the transfer zone shows a positive correlation with the offset angle i e the more the inherited fabric is parallel to the extension direction the longer and narrower the transfer zones in case of staircase offset of the rift segments 90 the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped prominent rise of the ductile lower crust was also observed at the transfer zone many of these geometrical features display striking similarities with natural rift systems the results of the current experiments provide useful insights into the mechanics of continental rift architecture supporting that rift propagation width and along axis segmentation may be strongly controlled by the reactivation of pre existing pervasive crustal fabrics

multiple ash layers in late quaternary sediments from the central indian basin abstract we have investigated three sediment cores collected from water depths 5000 m along the transect 76 30 e in close proximity to a fracture zone in the central indian basin cib the cores yielded five volcanic horizons of which four have visual and dispersed shards rhyolitic glass shards of bubble wall platy angular and blocky types were retrieved from various stratigraphic horizons in the cores the abundance of glass shards composition of bulk sediments and 230thexcess ages of the host sediments were used to distinguish the volcanic horizons of the four volcanic horizons three are now newly reported and correspond to ages of 85 107 109 and 142 146 ka while the fourth horizon is of 70 75 ka by using trace element ratios and cr and nb based normative calculations cryptotephra has been identified for the first time from the cib sediment the cryptotephra forms the fifth ash horizon and is of 34 ka a comparison with the published data on volcanic tephra in and around the indian ocean indicate the shard rich horizon srh of 70 75 ka to resemble the younger toba tuffs ytt while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records these tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the cib due to the lack of terrestrial equivalents of these tephra it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300 m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the indian ocean than previously reported

chemical characteristics of newly discovered black smoker fluids and associated hydrothermal plumes at the rodriguez triple junction central indian ridge abstract the chemical characteristics of hydrothermal fluids in the indian ocean have been revealed for the first time a hydrothermal field called the kairei field including active black smoker chimneys was discovered by rov kaiko dives at 25 19 17 s 70 02 40 e 2450 m depth on the southwestern flank of an off axis knoll located 15 miles north of the rodriguez triple junction central indian ridge after detailed tow yo surveys of the hydrothermal plume distribution the temperature of the fluid expelled from the most active chimney was almost stable 359 360 c throughout three successive days of fluid sampling the endmember fluid has a chemical composition similar to the hydrothermal fluids sampled from sediment starved mid ocean ridges in the pacific and the atlantic oceans suggesting typical interactions between hot fluid and mid ocean ridge basalts in the subseafloor reaction zone

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

native nickel in the tag hydrothermal field sediments mid atlantic ridge 26 n space trotter guest from mantle or a widespread mineral connected with serpentinization 1 tiny metallic particles have been found in the sediments of the trans atlantic geotraverse tag hydrothermal field mid atlantic ridge mar the metallic grains are almost pure ni and have compositional and structural characteristics similar to those of other terrestrial metallic ni0 occurrences native nickel grains documented here for the first time in oceanic sediments appear in calcareous oozes intercalated with metalliferous layers layers with bedrock rubble and sporadically scattered cosmic microspherules there are three possibilities for the metallic ni0 genesis 1 cosmic 2 lower mantle at the core mantle boundary or 3 upper mantle lower crust the flake like appearance of the ni0 grains rules out cosmic ablation origin no unequivocal evidence was found that the ni0 particles could have formed under high pressure conditions similar to those at the core mantle boundary ni0 formation through seawater serpentinization of the upper mantle lower crust is the most plausible mechanism at the slow spreading mar seawater penetrates down through highly fractured crest zone and serpentinizes the olivine and pyroxene of the upper mantle lower crust ultramafics mafics serpentinization results in the generation of highly reduced volatile species h2 and hydrocarbons in such an extremely reduced environment fe and other transition metals co ni cu zn released from olivines and pyroxenes tend to appear as native metals if fs2 approaches 0 the lightened serpentinized blocks uplift and expose the serpentinized ultramafics mafics at the rift valley s walls i speculate that tectonic movements and weathering could liberate the native metallic grains scattered in the serpentinized rocks which may then have been dispersed in the proximate sediments by near bottom currents

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

seismicity within a propagating ice shelf rift the relationship between icequake locations and ice shelf structure iceberg calving is a dominant mass loss mechanism for antarctic ice shelves second only to basal melting an important process involved in calving is the initiation and propagation of through penetrating fractures called rifts however the mechanisms controlling rift propagation remain poorly understood to investigate the mechanics of ice shelf rifting we analyzed seismicity associated with a propagating rift tip on the amery ice shelf using data collected during the austral summers of 2004 2007 we apply a suite of passive seismological techniques including icequake locations back projection and moment tensor inversion we confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h even during periods of quiescence we find significant deformation around the rift tip moment tensors calculated for a subset of the largest icequakes mw 2 0 located near the rift tip show steeply dipping fault planes horizontal or shallowly plunging stress orientations and often have a significant volumetric component they also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf this suggests a complex system of deformation that involves the propagating rift the region behind the rift tip and a system of rift transverse crevasses small scale variations in the mechanical structure of the ice shelf especially rift transverse crevasses and accreted marine ice play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts

joint inversions of three types of electromagnetic data explicitly constrained by seismic observations results from the central okavango delta botswana the okavango delta of northern botswana is one of the world s largest inland deltas or megafans to obtain information on the character of sediments and basement depths audio magnetotelluric amt

fault growth and propagation during incipient continental rifting insights from a combined aeromagnetic and shuttle radar topography mission digital elevation model investigation of the okavango rift zone northwest botswana 1 digital elevation models dem extracted from the shuttle radar topography mission srtm data and high resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the okavango rift zone orz northwest botswana significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive or have large throws and small scarp heights indicating waning activity faults on the outer margins of the rift exhibit either 1 large throws or significant scarp heights and are considered older and active or 2 throws and scarp heights that are in closer agreement and are considered young and active fault linkages between major fault systems through a process of fault piracy have combined to establish an immature border fault for the orz thus in addition to growing in length by along axis linkage of segments the rift is also growing in width by transferring motion to younger faults along the outer margins while abandoning older faults in the middle finally utilization of preexisting zones of weakness allowed the development of very long faults 100 km at a very early stage of continental rifting explaining the apparent paradox between the fault length versus throw for this young rift this study clearly demonstrates that the integration of the srtm dem and aeromagnetic data provides a 3 d view of the faults and fault systems providing new insight into fault growth and propagation during the nascent stages of continental rifting

composition and origin of hydrothermal ironstones from central pacific seamounts abstract ironstones recovered from five late cretaceous seamounts in the central pacific region probably formed during late stage edifice building volcanism ironstones are dense and compact with the appearance of brown chert the ironstones are characterized by a goethite mineralogy with feooh contents up to 88 extreme fractionation of fe and mn low trace element and rare earth element abundances low co zn ratios and isotopic equilibration temperatures of about 20 45 c these characteristics indicate that the ironstones formed from hydrothermal fluids ironstones probably formed below the seawater seafloor interface as indicated by their occurrence as a proximal hydrothermal deposit presence of primary goethite cement pervasive replacement of rocks by goethite and absence of interbedded pyro clastic beds

characteristics and possible origin of native aluminum in cold seep sediments from the northeastern south china sea abstract although native aluminum al has been reported to occur in various geological settings for more than 20 locations but its mechanism of formation still remains to be elucidated we report the occurrence and characterization of al particles recovered from the surface sediment cf4 and a short core sediment rov g 37 cm length obtained at cold seeps in the northeastern continental slope ncs of the south china sea scs x ray diffraction analysis shows that the collected particles are metallic aluminum with the unit cell edge a of 4 059 0 005 a cf4 2 and 4 029 0 004 a t26 28 2 rov g the al particles occur as spherules irregular plates and elongated forms with typical lamellar structures their chemical compositions are 95 07 99 84 al the average values is 98 42 with very small amounts of si fe ti s zn mg ca k na cu co and p and are similar to al particles from the east pacific rise and the central indian basin but differ markedly to those from other locations after ruling out several possibilities of al sources and mechanism of formation an alternative cold seep mechanism is proposed for the origin of al in the scs during the processes of anaerobic oxidation of methane aom and pyrite formation high h2s and h2 partial pressures result in strong reducing micro environments under which al oh 4 is reduced to its metallic state by the microbial bacterial processes

hydrothermal oxide and nontronite deposits on seamounts in the eastern pacific deposits of fe oxide mud nontronite and fe mn crusts were sampled from the summits of two seamounts in the eastern pacific where low temperature 0 15 c hydrothermal fluids are issuing the deposits consist of x ray amorphous fe oxyhydroxide and are rich in fe 43 wt contain minor si and p 4 and 3 5 respectively and have very low mn 0 01 and other trace element contents other deposits where no current hydrothermal activity was observed consist of mud and crusts composed of amorphous material and poorly crystalline hematite goethite and smectite these deposits have slightly higher mn up to 2 and other trace element contents the mineralogical and chemical differences between the active and inactive deposits and crusts are interpreted to be due to recrystallization and contamination with detrital biogenic hydrogenous and possibly hydrothermal sulfide material nontronite deposits capped by fe mn oxides were sampled from one of the seamounts these deposits have compositions similar to other seafloor nontronite deposits and formed at low temperatures 30 c during mixing of low temperature hydrothermal fluids with seawater the oxide deposits consist almost entirely of long delicate filaments remarkably similar in morphology to genera of fe oxidizing bacteria such bacteria were probably important in catalyzing the oxidation and precipitation of iron from hydrothermal fluids on the seamounts the occurrence of nearly identical filament rich oxide deposits on two widely separated seamounts suggests that this type of deposit may be common on seamounts but previously unrecognized

regional aeromagnetic and stratigraphic correlations of the kalahari copperbelt in namibia and botswana abstract the late mesoproterozoic to neoproterozoic kalahari copperbelt kcb in namibia and botswana is widely covered by kalahari sand which precludes direct correlations between known stratabound sediment hosted cu ag districts we use a combination of review of literature data and newly processed and interpreted high resolution aeromagnetic maps in both countries to provide a new correlative cross border interpretation lithostratigraphic control on the aeromagnetic response allows detailed indirect mapping of the kalahari copperbelt lithotectonic domains below the sand cover this enabled us to redefine the width and lateral extent of the kcb as two continuous magnetic domains the rehoboth and ghanzi chobe domains extending from central namibia to northern botswana and helped in resolving problems of stratigraphic correlations across the international border the rehoboth magnetic domain in the western part of the kcb in namibia records continental arc magmatism at 1200 ma during orogenic events along the northwestern edge of the kalahari craton this was followed at 1110 1090 ma by widespread magmatism identified within the entire kcb and related to the 1112 1106 ma umkondo large igneous province the basal parts of the tsumis group in namibia and ghanzi group in botswana were deposited in shallow water environments after a period of erosion and peneplanation subsequently and prior to the sturtian glaciation the host rocks of the cu ag deposits formed by the deposition of chemically reduced shales and siltstones that formed in deeper water and overlie chemically oxidised shallow water sandstones this regional interface which is both a permeability barrier and redox boundary played a critical role in the formation of the stratabound sediment hosted cu ag deposits of the kalahari copperbelt and the interface with its strong magnetic contrast can be followed through the entire ghanzi chobe magnetic domain of the copperbelt the whole kcb was affected by the damara orogeny during early cambrian times which resulted in the formation of a ne sw trending 250 km wide fold and thrust belt

late quaternary lake level fluctuations in the mababe depression middle kalahari palaeolakes and the role of zambezi inflows abstract a systematic drilling and optical dating programme on middle kalahari beach ridge relict shoreline sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the okavango delta northern botswana this paper presents 23 ages from the mababe depression and establishes four shoreline construction phases in the late quaternary coeval with other sub basin lake high stands lake ngami these synchronous lake phases result from a coalescence of the sub basins into a unified palaeolake lake thamalakane covering an area of 32 000 km2 six additional ages are also presented from the chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as lake thamalakane phases this suggests that increased flow in the chobe and zambezi system significantly contributed to the middle kalahari lake phases in both the post glacial and holocene periods the integration of these new data and their compatibility with other regional and tropical palaeo archives is discussed in the light of understanding quaternary climate drivers within the kalahari

geochemical evidence of terrigenous influence in deep sea sediments up to 8 s in the central indian basin abstract thirty two representative deep sea sediment samples have been studied from three manganese nodule bearing environments in the central indian basin diatomaceous radiolarian ooze clay pelagic brown clay and calcareous ooze they have been analysed for major and minor elements such as si al ca mg na k fe mn p ti ba cu ni co v zn and cr r mode factor analysis reveals five important sources for these elements 1 detrital loaded with fe ti al mg mn p and k 2 combined hydrogenetic diagenetic mn ni cu co and fe 3 biogenic si 4 seasalts na and mg and 5 dissolution residue ba factor scores distributional trends of detrital elements al ti and fe concentrations and al al fe mn ratios provide new geochemical evidence for the effect of the terrigenous influx from the ganges brahmaputra rivers into the northern part of the basin up to 8 s which inhibits the growth of manganese nodules in this area in contrast incorporation of transitional metals into sediments and manganese nodules in the other two areas of the basin is governed by hydrogenetic and diagenetic processes

native aluminium spherules and particles in the central indian basin sediments implications on the occurrence of hydrothermal events abstract we have sampled particles of native aluminium al in two sediment cores from the central indian basin cib the cores are geographically separated but are located at the base of two seamounts the native al particles occurring as grains and spherules have an average al content of 95 and are associated with volcanogenic hydrothermal material morphologically and compositionally the specimens are similar to those reported from the east pacific rise after ruling out several processes for the presence of the native al we hypothesize that during progressive melting of magma a basaltic magma is produced which has high contents of reductants such as methane and hydrogen and a low oxygen fugacity during the upward migration of such magma reduction to metallic aluminium and the formation of native al particles takes place

sediment in the black smoker area of the east pacific rise 18 5 s abstract a sediment profile perpendicular to the east pacific rise epr was investigated in an area in which massive sulfides have been observed along the axis of the epr a holocene to late pleistocene age is concluded for all of the up to 3 m long cores leading to sedimentation rates of 0 3 2 6 cm 1000 a the sediments are composed of hydrothermal precipitate calcareous tests and in traces hyaloclastic material the proportion of carbonate increases with increasing distance from the spreading center the hyaloclastic material is more or less weathered and is concentrated near the spreading center the hydrothermal component is composed mainly of iron and manganese hydroxides the proportion of hydrothermal matter in the sediment is higher near the axis of the epr also near the axis the elements b p cr and v are enriched in the hydrothermal component whereas mn zn cu fe and as are depleted the core profiles show a fluctuation in the proportion of hydrothermal material periods of higher hydrothermal input correlate with periods of the best calcite preservation diagenetic alteration of the sediment increases with increasing distance from the spreading center i e with decreasing sedimentation rate mo zr co ni rb th y ba la and ree are enriched in the sediment by diagenetic processes

present stability of the larsen c ice shelf antarctic peninsula we modelled the flow of the larsen c and northernmost larsen d ice shelves antarctic peninsula using a model of continuum mechanics of ice flow and applied a fracture criterion to the simulated velocities to investigate the ice shelf s present day stability constraints come from satellite data and geophysical measurements from the 2008 09 austral summer ice shelf thickness was derived from bedmap and icesat data and the density depth relationship was inferred from our in situ seismic reflection data we obtained excellent agreements between modelled and measured ice flow velocities and inferred and observed distributions of rifts and crevasses residual discrepancies between regions of predicted fracture and observed crevasses are concentrated in zones where we assume a significant amount of marine ice and therefore altered mechanical properties in the ice column this emphasizes the importance of these zones and shows that more data are needed to understand their influence on ice shelf stability modelled flow velocities and the corresponding stress distribution indicate that the larsen c ice shelf is stable at the moment however weakening of the elongated marine ice zones could lead to acceleration of the ice shelf due to decoupling from the slower parts in the northern inlets and south of kenyon peninsula leading to a velocity distribution similar to that in the larsen b ice shelf prior to its disintegration

native aluminium in metalliferous sediments from the east pacific rise axial zone 21 s abstract an endogenous model of the formation of small amounts of 100 m metallic aluminium particles from the metalliferous sediments of the east pacific rise 21 s is proposed on the basis of a detailed study of their morphology chemical composition x ray data spatial and temporal distribution according to this model native aluminium is formed as an accessory mineral during the process of the pre chamber evolution of basic and ultrabasic systems in the spreading zones under high p t and low o 2 conditions as well as during the later serpentinization of basites and ultrabasites from the second and third oceanic layer as a result of the active tectono magmatic processes in the rift zones the native aluminium particles are carried upwards into the seafloor water layer by the upwelling vent fluids of the hydrothermal circulation cells during periods of high hydrothermal activity the number of al particles carried upwards increases the spatial distribution of the particles depends on the energy of the hydrothermal system and the direction and energy of the seafloor currents the thin oxyhydroxide silicate and chloride crusts formed on the surface of the al particles as well as a monoatomic oxide film prevent further alteration of the particles

morphology and long term landscape evolution of the south african plateau in south namibia the south namibian plateau which belongs to the south african plateau is situated between 25 29 s and 15 20 e this study examines the relationships between this portion of the plateau and the development of planation surfaces and scarps bounding the plateau geomorphological analysis through morphometric indices extracted from a dem 90 m and its derived maps as well as field observations supported by the literature data permit the demonstration that tectonics climate and base level are the main factors that control the formation and evolution of the planation surfaces and scarps that separate them from west to east the south namibian plateau consists of four main morphological provinces the coastal province and the intermediate upper and kalahari provinces in the interior plateau these provinces include five cenozoic planation surfaces that are differentiated by their elevations regional slopes relative reliefs and erosion processes these surfaces have been formed by etchplanation and pediplanation the drainage pattern and river length profiles show that the coastal province evolved differently than the interior plateau the development of the coastal province is controlled by eustasy and tectonics whereas climate related processes and tectonics control that of the interior plateau the presence of several scarps separating these surfaces the most degraded and older of which are inside the plateau reflects a polyphase evolution this evolution can be highlighted by two models 1 an eastward migration mantle dynamics causing a slight uplift and rejuvenation of the system by destabilizing the coupled weathering erosion equilibrium and 2 an internal propagation of isostatic rebound induced by the retreat of the western scarp leading to a significant flattening in the central part of the plateau

use of the geochemical and biological sedimentary record in establishing palaeo environments and climate change in the lake ngami basin nw botswana sediment samples from a continuous 4 6 m profile in the dry bed of lake ngami in nw botswana were analysed for geochemistry and dated using both 14c and tl methods certain units in the profile were found to be diatom rich and these with the geochemical results were used as indicators of high and low lake levels within the basin the lake ngami sediments contain a high proportion of sio2 51 92 5 wt avg 72 4 wt and variable levels of al2o3 2 04 17 2 wt avg 8 88 wt based on elevated al2o3 and organic matter loiorgc results lacustrine conditions occurred at ca 42 ka until 40 ka and diatom results suggest that relatively deep but brackish conditions prevailed at 40 ka the lacustrine sedimentary record was terminated abruptly possibly by tectonic activity at ca 19 ka shallow aerobic turbulent conditions were prevalent but lake levels were at this time increasing to deeper water conditions up until ca 17 ka this period coincides with the late glacial maximum a period of increased aridity in the central southern africa region generally increasing sr ca ratios and decreasing loiorgc and al2o3 from ca 16 to 5 ka suggest decreasing inflow into the basin and declining lake levels based on the enrichment of lree results slightly alkaline conditions prevailed at ca 12 ka diatom results also support shallow alkaline conditions around this time these lake conditions were maintained primarily by local rainfall input as the region experienced a warmer wetter phase between 16 and 11 ka lake levels rose rapidly by 4 ka probably in response to enhanced rainfall in the angolan catchment these results indicate that lake levels in the lake ngami basin are responding to rainfall changes in the angolan catchment area and local rainfall the results confirm that the present day anti phase rainfall relationship between southern africa and regions of equatorial africa was extant during the late quaternary over the angolan highlands and nw botswana

sulfide impregnated and pure silica precipitates of hydrothermal origin from the central indian ocean abstract this is the first report about silica rich hydrothermal precipitates which were sampled together with hydrothermal sulfides chimney fragments in an extinct vent field in the central indian ocean there are two kinds of silica rich rocks a jasper which is impregnated and replaced to various degrees mainly by sphalerite and to a lesser extent by barite pyrite and traces of chalcopyrite and an opalite which is an almost pure silica phase without any sulfide or sulfate impregnations but which is sometimes covered by manganese crusts no internal concentric zoning indicating typical chimney structures can be recognized in the jasper and or opalite samples the textures rather suggest a sedimentary silica and or iron deposition from diffuse low temperature 60 c vent fluids partly with still visible indications of former bacterial mats and synsedimentary deformation structures the sphalerite and barite impregnations within the jasper however are considered to have precipitated from white smoker type fluids since they were deposited under intermediate temperatures between 155 and 265 c according to fluid inclusion studies the sulfur isotopic composition 34 s of our sulfide samples has mean values of 6 1 for sphalerite and 5 7 for pyrite indicating a mixture of predominantly basaltic sulfur with subordinate amounts of reduced seawater sulfur the oxygen isotope signals of some pure jasper concentrate samples indicate that the mean formation temperature calculated from these values lies at 63 2 c the relationship between the massive pyrite and chalcopyrite ores from the extinct chimney structures and the silica rich precipitates can be explained by different cycles of hydrothermal activity one high temperature above 300 c cycle dominated by pyrite and chalcopyrite formation and one later epithermal below 300 c cycle which resulted in sphalerite and silica dominated precipitates furthermore zonation and zone refining processes are part of the evolution of the mineralized field

evolution of the indian ocean triple junction between 65 and 49 ma anomalies 28 to 21 reinterpretation of newly published geophysical data kamesh raju and ramprasad 1989 and older profiles of the central indian basin associated with similar studies of the madagascar and crozet basins shows that the indian ocean triple junction trace on the indian plate corresponds at anomalies 23 and 22 to a n38 e offset of the magnetic lineations oblique to both the southeast indian ridge seir and central indian ridge cir spreading directions the conjugate triple junction trace on the african plate identified in the madagascar basin is associated with a roughly north south offset parallel to the southwest indian ridge swir fracture zones in order to account for these observations and the velocity triangle of the indian african and antarctic plates close to the triple junction a ridge fault fault mode is proposed with a propagatorlike seir cir offset the triple junction jumped between anomalies 24 and 23 and between anomalies 22 and 21 restoring a ridge ridge ridge configuration which immediately turned to a pseudo ridge ridge fault and later to a true ridge fault fault configuration after the triple junction jump at anomaly 21 the former seir cir offset was accommodated by a new cir fracture zone the lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo ridge ridge fault or an unstable ridge ridge ridge configuration prevailed before anomaly 24 in agreement with the velocity triangles which predict more unstable triple junction modes both modes support the creation of numerous swir fracture zones presently observed between 52 30 e and 59 30 e as a consequence of the triple junction evolution between anomalies 29 and 24 this result suggests that the physiography of the swir records the history of the triple junction

volcanogenic hydrothermal iron rich materials from the southern part of the central indian ocean basin abstract we report the occurrences of magnetite rich spherules ti rich particles and possibly nontronite recovered at 19 s and 76 e from the pelagic clay in the central indian ocean basin ciob it is proposed that the magnetite spherules were formed as a result of volcanic activity in this part of the ciob while the presence of nontronite suggests localized hydrothermal activity the magnetic spherules and the associated materials are compared with similar occurrences 555 km to the north in the vicinity of the 76 30 e fracture zone considering radiolarian age dating of the sediment the age of the volcano hydrothermal episode responsible for the formation of these materials seems to be between 425 and 650 ka it is inferred that there could have been widespread local volcanism in the ciob

east african rift system offset age and tectonic significance of the tanganyika rukwa malawi intracontinental transcurrent fault zone abstract the 1000 200 km tanganyika rukwa malawi fractures area is shown to be a right lateral intracontinental transform zone belonging to the east african rift system using the results carried out from complementary techniques remote sensing field structural analysis and k ar isotopic dating it appears that tectonics in this area have acted at different times the oldest being at 2000 1400 m y the youngest being still active today and are mainly due to the reactivation of nw se structures the area consists mainly of large nw se en echelon dextral transcurrent faults along which the southern tanganyika rukwa and northern malawi troughs are opening ne sw transverse faults showing a curved horizontal shape end these troughs axially eocene to recent volcanic activity is associated with the intersection of the northeastern main nw se transcurrent fracture the livingstone fault

is the okavango delta the terminus of the east african rift system towards a new geodynamic model geodetic study and geophysical review the okavango graben og has been considered as the terminus of the southwestern branch of the east african rift system ears since the 1970 s based on fault morphology and early seismic and geophysical data thus it is assumed to be an incipient rifting zone analogous to the early stage of mature rifts in the ears recent geodetic data and geophysical studies in the area bring new insights on the local crust and lithosphere mantle activity and fault activity in this study we computed the velocities for three permanent gps stations surrounding the graben and undertook a review of the new geophysical data available for the area the northern and southern blocks of the graben show exclusively a low strike slip displacement rate of about 1 mm yr revealing the transtensional nature of this basin the seismic record of central and southern africa is revealed to be instrumentally biased for the events recorded before 2004 and the og may not represent the most seismically active area in botswana anymore moreover no significant lithosphere and crustal thinning is found in the tectonic structure as well as no strong negative bouguer anomaly and surface heat flux thus the og does not match the classical model for a rifting zone we propose a new geodynamic model for the deformation observed west of the ears based on accommodation of far field deformation due to the differential extension rates of the ears and the displacement of the kalahari craton relative to the nubian plate

a joint receiver function and gravity study of crustal structure beneath the incipient okavango rift botswana rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins in order to decipher the characteristics of rifting at its earliest stage we conduct the first teleseismic crustal study of one of the world s youngest continental rifts the okavango rift zone orz where the magma has not yet breached the surface results from receiver function stacking and gravity modeling indicate that the crust mantle boundary beneath the orz is uplifted by 4 5 km and the initiation of the orz is closely related to lithospheric stretching possible decompression melting of the subcrustal lithosphere occurs beneath the orz as evidenced by a relatively low upper mantle density based on the gravity modeling

structural and environmental controls on antarctic ice shelf rift propagation inferred from satellite monitoring 1 iceberg calving from ice shelves accounts for nearly half of the mass loss from the antarctic ice sheet yet our understanding of this process is limited the precursor to iceberg calving is large through cutting fractures called rifts that can propagate for decades after they have initiated until they become iceberg detachment boundaries to improve our knowledge of rift propagation we monitored the lengths of 78 rifts in 13 antarctic ice shelves using satellite imagery from the moderate resolution imaging spectroradiometer and multiangle imaging spectroradiometer between 2002 and 2012 this data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings we found that 43 of the 78 rifts were dormant i e propagated less than 500 m over the observational interval we found only seven rifts propagated continuously throughout the decade an additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy twelve of the fifteen active rifts were initiated at the ice shelf fronts suggesting that front initiated rifts are more active than across flow rifts although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with we did find a statistically significant correlation between the arrival of tsunamis and propagation of front initiated rifts in eight ice shelves this suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood

geophysical evidence for marine prolongation of the palghat cauvery shear system into the offshore cauvery basin eastern continental margin of india abstract the precambrian basement shear zones played a major role in developing offshore structures that are zones of weakness along the eastern continental margin of india a compilation and analysis of various geophysical datasets particularly magnetic gravity seismicity and seismic reflection data support a clear geometric connection between the shear zones and offshore structures the presence of shear zones within the crystalline basement in the continental domain and oceanic domain is demarcated primarily by prominent magnetic lineation we identified three major magnetic lineaments in the offshore cauvery basin which correspond to the plaghat cauvery shear system pcss of southern granulite terrain sgt the faults interpreted in the seismic sections are correlated to the extension of pcss and indicate the location of reactivation of the shear zones which give rise to considerable intraplate seismicity the geophysical crustal model shows deep crustal scale faults associated with the marine prolongation of pcss the shear zone extended structures in the offshore might have formed at the breakup stage at the onset of rifting and might be extending up to the continent ocean transition zone cotz the pcss partly seems to accommodate the stress created by the northward push of indian plate against the asian plate it is also possible that the shear zone might have extended to the nearby land mass that marks it as one of the principal component in gondwana reconstruction

the sedimentary dynamics of active fluvial channels on the okavango fan botswana the 1800 km2 okavango alluvial fan of northern botswana represents an unusual depositional setting in which peat forming perennial swamps 6000 km2 occur in a region of aeolian and semi arid sedimentation within an incipient graben of the east african rift a channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water which spreads laterally from the channels through permeable channel margins sustaining the flanking swamps all sediment introduced is deposited on the fan a detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal bedload greatly exceeds suspended load at least by a factor of four vegetation and peat form permeable levees which confine the channels in the upper reaches two way exchange of water occurs between channel and swamp depending on the season but on the fan itself channels lose water to the swamp bedload measurements reveal that the channel system is in a state of grade disequilibrium with interspersed depositional and erosional reaches deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system but on the midfan deposition of bedload occurs by channel bed aggradation at a rate of up to 5 cm yr 1 further down slope the channel enters a large lake where all remaining bedload is deposited the presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system either by avulsion or neotectonics

paleo megalake and paleo megafan in southern africa the near juxtaposition of the makgadikgadi basin botswana the world s largest saltpan complex with the okavango delta one of the planet s largest inland deltas technically an alluvial megafan has intrigued explorers and scientists since the middle of the 19th century it was clear from early observations that the makgadikgadi basin once contained a huge lake paleo lake makgadikgadi several authors have since speculated that this lake also covered wide regions to the north and west of the makgadikgadi basin our interpretation of unusually high quality helicopter time domain electromagnetic htem data indicates that paleo lake makgadikgadi extended northwestward at least into the region presently occupied by the okavango delta the total area of paleo lake makgadikgadi exceeded 90 000 km2 larger than earth s most extensive freshwater body today lake superior north america our htem data constrained by ground based geophysical and borehole information also provide evidence for a paleo megafan underlying paleo lake makgadikgadi sediments

hydrovolcanic activity in the central indian ocean basin does nature mimic laboratory experiments abstract classical works in the 1960 s on spherules volcanic extraterrestrial cosmic etc were carried out in order to distinguish these morphologically and compositionally so as to understand their genesis yet in some cases their origin has remained enigmatic and indeterminate we present here the finding of volcanic magnetite spherules up to 475 m in diameter which were recovered from the base of a seamount basement age 50 ma in the central indian ocean basin ciob the spherules have quenched and crystalline textures analogous to cosmic ones but compositionally are fe enriched avg 74 together with other magmaphile elements a distinctive lack of a ni core is noted vis a vis si enriched areas the spherules are very similar to those experimentally produced by simulated hydrovolcanic activity involving interaction of a thermite melt with water saturated quartzo feldspathic sand our finding seems to indicate such a process to have occurred in nature at abyssal depth the plausible mechanism we conceive is the reaction of fe rich lavas or hydrothermal emanations with the abundant surrounding siliceous ooze which is the dominant sediment type in the basin we regard this is as the first report of an authentic proof for hydrovolcanism at oceanic depth of 5200 m which probably occurred 10 ka ago as suggested by the associated radiolarian species

hydrogeophysical investigations in the western and north central okavango delta botswana based on helicopter and ground based transient electromagnetic data and electrical resistance tomography abstractthe okavango delta is a huge alluvial megafan in northwestern botswana despite numerous geologic geochemical geophysical and hydrologic investigations over the past half century the sedimentary units underlying the delta are largely unknown to address this issue helicopter transient electromagnetic data htem have been collected across the entire delta and coincident ground based electrical resistance tomographic ert and transient electromagnetic tem data have been acquired at two locations one along the delta s western margin and one in its north central region inversions of the htem data have yielded three layer resistivity models in which a relatively homogeneous conductive layer is sandwiched between two resistive layers the three layer htem model is reproduced in models obtained from independently and jointly inverting the ground based data the conductive layer s low resistivities and depths to its upper and lower boundaries are practically equal in the htem and ground based m

hydrothermal alumina rich clays and boehmite on the gorda ridge abstract hydrothermal alteration of basalt on the northern gorda ridge produced an alteration crust up to 1mm thick that can be divided into two layers based on mineralogy an inner layer next to the basalt surface is an al rich interlayered smectite chlorite similar to high temperature al rich clays recovered from the east pacific rise 21 n with minor quartz zeolite phillipsite and relict plagioclase an exterior layer of boehmite alo oh and minor anatase results from severe leaching of the underlying clays the alteration crust is progressively enriched in al ti v and cr from interior to exterior by residual weathering the presence of boehmite and absence of a silica rich phase in the exterior of the crust indicates the hydrothermal fluids were silica undersaturated or silica was maintained in solution by extremely acidic fluids this constitutes the first reporting of boehmite as a major alteration mineral in a mid ocean ridge hydrothermal environment

ice shelf ocean interactions under the amery ice shelf seasonal variability and its effect on marine ice formation 1 marine ice is an important factor in ice shelf stability an extensive marine ice layer is present under the amery ice shelf ais east antarctica this paper documents observations on the seasonal variability of the ais ocean interaction beneath the marine ice layer we focus on data collected during 2002 through a borehole at am01 100 km from the ice shelf calving front and use additional data from two other boreholes to complement the study at am01 the top 20 m of the water column is super cooled almost year round protecting the marine ice layer and promoting frazil ice formation the mixed layer thickness varies from 50 m in february to at least 160 m by june as the water column cools and freshens high salinity shelf water hssw abruptly arrives at am01 in june august as an eddy like flow we suggest that the flow characteristics are a result of baroclinic instabilities in addition the inflow of hssw results in a steepening of the isopycnals that enhances the upwelling of ice shelf water this study documents for the first time a seasonal signal in the formation of marine ice under the ais our results highlight the vulnerability of the marine ice layer to ocean variability with potential consequences for the overall ice shelf mass balance

normal vs strike slip faulting during rift development in east africa the malawi rift kinematic analysis of neogene and quaternary faults demonstrates that the direction of extension in the malawi rift rotated from east northeast to southeast rift development commenced with the formation of half grabens bounded by northwest north and northeast striking normal faults owing to slightly oblique rifting the northwest striking faults in the northernmost rift segment show a small dextral oblique slip component whereas north and northeast oriented faults in the central part of the rift display a sinistral oblique slip component this first event resulted in block faulting and basin subsidence which is largely responsible for the present day basin morphology of lake malawi a major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike slip faulting assumed a major role the strike slip regime amplified uplift of basement ridges within the rift in regions of local transpression but it also created alluvial basins because of local transtension this new kinematic style is compatible with the recent seismicity older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift toward the center the faults depict an increase in strike slip components of movement suggesting deformation propagation toward the rift center which results in a narrowing of the active rift environments with time

aeromagnetic patterns in southern uruguay precambrian mesozoic dyke swarms and mesozoic rifting structural and tectonic evolution abstract new high resolution airborne magnetic data of uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms main faults and other magnetic bodies which compose the uruguayan shield we combined geophysical analyses vertical derivatives upward continuation euler deconvolution structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the uruguayan shield and contribute to a better understanding of its tectonic evolution the magnetic maps revealed several outstanding features in the uruguayan shield the paleoproterozoic dyke swarm is larger denser more widespread and complex than originally thought suggesting a possible plume origin in addition a new mesozoic dyke swarm as complex as the previous one was identified crosscutting the paleoproterozoic dyke swarm and the neoproterozoic orogenic structures moreover this swarm is connected to volcanic calderas in the merin basin and shows displacements along neoproterozoic shear zones in the magnetic maps revealing its brittle reactivation during mesozoic times the new observations clarify how proterozoic basement structures controlled the development of the mesozoic rift paleoproterozoic dyke swarms were reactivated as normal faults and neoproterozoic structures hindered the rift growth deflecting the deformation in transcurrent movements meanwhile the mesozoic dyke swarm was developed in a perpendicular direction to the neoproterozoic structures moreover these findings contradict the current rift model for uruguay and rise a new model in which the mesozoic rift developed as two rift basins connected by a central transfer zone generated by the reactivation of dom feliciano belt structures between the sierra ballena and sarandi del yi shear zones

influence of precambrian shear zones on the formation of oceanic fracture zones along the continental margin of brazil abstract structural inheritance control of oceanic fracture zone formation by a large basement shear zone during continental breakup is still a matter of debate in this research we investigated the reactivation of precambrian shear zones and their role in the formation of the ascension and fernando poo fracture zones during the evolution of the south atlantic ocean we used aeromagnetic and multichannel 2d seismic reflection data associated with previous geological and geophysical data to analyze how the precambrian shear zones may have influenced the formation of oceanic fracture zones our results suggest that the fracture zones are extensions of the shear zones in the transition between continental and oceanic crust the sao miguel do aleixo and pernambuco shear zones were reactivated during the pangea breakup and acted as zones of weakness controlling the location of the ascension and fernando poo fracture zones during the process of oceanic crust spreading our model suggests that the formation of these fracture zones influenced by the shear zones occurred in the early stages of ocean opening it indicates that the fracture zones that formed in the late opening stages were related to the thermal subsidence stage as suggested in previous studies

bacterial response to contrasting sediment geochemistry in the central indian basin in order to investigate whether geochemical physiographic and lithological differences in two end member sedimentary settings could evoke varied microbe sediment interactions two 25 cm long sediment cores from contrasting regions in the central indian basin have been examined site tvbc 26 in the northern siliceous realm 10 s 75 5 e is organic c rich with 0 3 0 09 total organic carbon site tvbc 08 in the southern pelagic red clay realm 16 s 75 5 e located on the flank of a seamount in a mid plate volcanic area with hydrothermal alterations of recent origin is organic c poor 0 1 0 07 significantly higher bacterial viability under anaerobic conditions generally lower microbial carbon uptake and higher numbers of aerobic sulphur oxidizers at the mottled zones characterize core tvbc 26 in the carbon poor environment of core tvbc 08 a doubling of the 14 c uptake a 250 times increase in the number of autotrophic nitrifiers a fourfold lowering in the number of aerobic sulphur oxidizers and an order higher of denitrifiers exist when compared to core tvbc 26 this suggests the prevalence of a potentially autotrophic microbial community in core tvbc 08 in response to hydrothermal activity microbial activity at the northern tvbc 26 is predominantly heterotrophic with enhanced chemosynthetic activity restricted to tangreen mottled zones the southern tvbc 08 is autotrophic with increased heterotrophic activity in the deepest layers notably the bacterial activity is generally dependent on the surface productivity in tvbc 26 the carbon rich core and mostly independent in tvbc 08 the carbon poor hydrothermally influenced core the northern sediment is more organic sink controlled and the southern more hydrothermal source controlled hydrothermal activity and associated rock alteration processes may be more relevant than organic matter delivery in these deep sea sediments thus this study highlights the relative importance of hydrothermal activity versus organic delivery in evoking different microbial response in the central indian basin sediments

degassing of metals and metalloids from erupting seamount and mid ocean ridge volcanoes observations and predictions recently it has been reported that the element polonium degasses from mid ocean ridge and seamount volcanoes during eruptions published and new observations on other volatile metal and metalloid elements can also be interpreted as indicating significant degassing of magmatic vapors during submarine eruptions this process potentially plays an important role in the net transfer of chemical elements from erupting volcanoes to seawater in addition to that arising from sea floor hydrothermal systems in this paper a framework is constructed for predicting and assessing semiquantitatively the potential magnitude and chemical fingerprints in the water column of metal and metalloid degassing using 1 predictions from a summary of element volatilities during mafic subaerial volcanism worldwide and 2 limited data from submarine volcanic effusives the latter include analyses of polonium and trace metals in near volcano water masses sampled following a submarine eruption at loihi seamount hawaii 1000 m bsl in 1996 the element volatility predictions and observations show good agreement considering the limited dataset some of the highest volatility main group and transition element enrichments in seawater over loihi are predicted by the degassing mass transfer model i present when expanded to cover all submarine volcanic activity it is predicted that exit fluxes of these elements are up to 102 103 greater by degassing than by normal mor hydrothermalism in contrast mor exit fluxes of low volatility alkali and alkaline earth elements are likely 102 106 greater from hydrothermal inputs degassing inputs to the ocean are probably highly episodic occurring almost entirely during eruptions these are times of enhanced and abnormal hydrothermalism as well although major hydrothermal and degassing events may not be chemically recognizable in real water masses as wholly distinct entities it is nevertheless possible to predict to what extent each process flavors the effluents of the other degassing at mid ocean ridges may explain a variety of observations previously ascribed to complexities occurring during hydrothermal venting and or fluid ascent in the buoyant hydrothermal plumes above ridges

constraining helicopter electromagnetic models of the okavango delta with seismic refraction and seismic reflection data abstractelectrical resistivity models derived from exceptionally high quality helicopter transient electromagnetic data recorded across the okavango delta in botswana one of the world s great inland deltas or megafans include three principal layers 1 an upper heterogeneous layer of dry and water saturated sand 2 an intermediate electrically conductive layer that likely comprises saline water saturated sand and clay and 3 a lower fan shaped electrically resistive layer of freshwater saturated sand gravel and or crystalline basement if part of the lower layer comprises a freshwater aquifer it would be evidence for a recently proposed paleo okavango megafan and a major new source of freshwater in an attempt to constrain the interpretation of the lower layer we acquired two high resolution seismic refraction and re ection data sets at each of two investigation sites one near the center of the delta and one along its western edge the interface between unconsolidated sediments and basement near

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

structure of the lithosphere beneath the barotse basin western zambia from magnetotelluric data author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in tectonics 38 2 2019 666 686 doi 10 1029 2018tc005246

native copper and α copper zinc in sediments from the tag hydrothermal field mid atlantic ridge 26 n nature and origin abstract native copper and copper zinc occur as strands and elongated grains up to 300 m in length within the sediments from the trans atlantic geotraverse tag hydrothermal field mid atlantic ridge 26 n they are remarkably similar in composition and crystal structure to copper and copper zinc occurrences found in other natural environments the results of mineralogical studies are discussed in terms of the possible mechanisms of native metal formation in the complex tag field with an asymmetric and highly fractured rift valley and mature active and relict sulfide mounds native copper and copper zinc grains disseminated in the tag sediments are either inherited from 1 primary magmatic or metamorphic crustal source 2 hydrothermal deposits and 3 the alteration of primary deposits or formed 1 authigenically or 2 biogenically within on the sediment cover native metallic particles could have been formed as accessory minerals disseminated in the ridge crest basic rocks and or massive sulfide mounds degradation of these rocks and mass wasting of the mounds have liberated the metallic grains which have in turn been deposited into adjacent sediments

nature source and composition of volcanic ash in sediments from a fracture zone trace of rodriguez triple junction in the central indian basin abstract volcanic glasses associated with pumice micronodules and palagonite like lithic fragments were recovered from a volcanic terrain in a fracture zone defined as rodriguez triple junction trace in the central indian basin morphologically the tephra consist predominantly of bubble wall shards with the co occurrence of platy and blocky types rarity of vesicles in the shards suggests that they are formed in depths deeper than 4000 m below the volatile fragmentation depth vfd energy dispersive spectrometer eds analyses have shown that the tephra have sio2 contents ranging from 74 to 77 suggesting that they are mainly rhyolitic and formed from melts of siliceous composition the depositional age of the tephra layers determined by radiolarian assemblages and excess 230th decay profile corresponds to an age older than 180 kyr the tephra are rare in sediments younger than 239 kyr the beginning of the volcanic eruption contributing this volcanic ash however cannot be estimated occurrence of large sized tephra suggests proximal source probably from a local volcanic source compositional and morphological contrasts age constraints occurrence of large sized tephra and tectonic setting of the cored site location suggest an intraplate volcanism triggered by reactivation of tectonic activity in the fracture zone trace of the indian ocean triple junction during the late pleistocene