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

the junction between pan african mobile belts in namibia its structural history abstract in namibia there is a junction between a pan african age 650 450 m y orogenic system which follows the western rim of africa and an ene trending arm which branches off into eastern namibia and zambia the se verging structures of this eastern arm were superimposed on earlier westward verging folds of the northern coastal belt the eastern arm was a zone of high temperature metamorphism and granite intrusion early in its history and this zone of high heat flow affected the style intensity and orientation of structures of the coastal belt these change from open flexural folds in the north away from the zone of high heat flow to intense recumbent folds associated with high grade metamorphism in the south where they deform rocks of the eastern arm the change in deformation intensity was largely responsible for a change in trend of the folds to produce a pronounced arcuate fold belt at the junction between the coastal and eastern arms

the importance of overpressure timing and permeability evolution in fine grained sediments undergoing shear abstract we summarise new experimental data which clarify the hydrological behaviour of clayey sediments undergoing shear and which emphasise the importance of the relative timing of deformation and overpressure fluid flow in fine grained materials subjected to shear is strongly dependent on the previous stress history of the material underconsolidated sediments such as those that retain high porosities through retarded dewatering and consequent excess pore fluid pressure generation deform by bulk volume loss which reduces permeability the deformation fabrics are pervasive in style weakly developed and without a significant influence on hydrological properties in contrast sediment that has been consolidated and then subjected to excess pore fluid pressure deforms dilatively along discrete brittle shear zones which markedly affect the permeability moreover below a certain threshold of effective stress these localised zones prompt a markedly non linear increase in hydraulic conductivity a laboratory demonstration and quantification of the concept of fracture permeability

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

chemistry and mineralogy of outcrops at meridiani planum analyses of outcrops created by the impact craters endurance fram and eagle reveal the broad lateral continuity of chemical sediments at the meridiani planum exploration site on mars approximately ten mineralogical components are implied in these salt rich silicic sediments from measurements by instruments on the opportunity rover compositional trends in an apparently intact vertical stratigraphic sequence at the karatepe west ingress point at endurance crater are consistent with non uniform deposition or with subsequent migration of mobile salt components dominated by sulfates of magnesium striking variations in cl and enrichments of br combined with diversity in sulfate species provide further evidence of episodes during which temperatures ph and water to rock ratios underwent significant change to first order the sedimentary sequence examined to date is consistent with a uniform reference composition modified by movement of major sulfates upward and of minor chlorides downward this reference composition has similarities to martian soils supplemented by sulfate anion and the alteration products of mafic igneous minerals lesser cementation in lower stratigraphic units is reflected in decreased energies for grinding with the rock abrasion tool survival of soluble salts in exposed outcrop is most easily explained by absence of episodes of liquid h2o in this region since the time of crater formation

the okahandja lineament and its significance for damaran tectonics in namibia the okahandja lineament is considered to represent the southern margin of a magmatic arc produced by the northerly subduction of the damran ocean between 750 and 520 ma ago sediments of the orogen and granitoid intrusives produced by subduction and crustal melting underwent shear deformation between 675 and 575 ma in a low angle zone of sinistral sense shear

a damara orogen perspective on the assembly of southwestern gondwana the pan african damara orogenic system records gondwana amalgamation involving serial suturing of the congo sao francisco and ro o de la plata cratons north gondwana from 580 to 550 ma before amalgamation with the kalahari antarctic cratons south gondwana as part of the 530 ma kuunga damara orogeny closure of the adamastor ocean was diachronous from the aracuao belt southwards with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580 550 ma in the kaoko belt and at 545 530 ma in the gariep belt peak deformation metamorphism in the damara belt was at 530 500 ma with thrusting onto the kalahari craton from 495 ma through to 480 ma coupling of the congo and ro o de la plata cratons occurred before final closure of the mozambique and khomas damara belt oceans with the consequence that the kuunga suture extends into africa as the damara belt and the lufilian arc and zambezi belt of zambia palaeomagnetic data indicate that the gondwana cratonic components were in close proximity by c 550 ma so the last stages of the damara kuunga orogeny were intracratonic and led to eventual out stepping of deformation metamorphism to the ross delamerian orogen c 520 500 ma along the leading edge of the gondwana supercontinental margin

basalt weathering rates on earth and the duration of liquid water on the plains of gusev crater mars where martian rocks have been exposed to liquid water chemistry versus depth profi les could elucidate both martian climate history and potential for life the persistence of primary minerals in weathered profi les constrains the exposure time to liquid water on earth mineral persistence times range from 10 k y olivine to 250 k y glass to 1 m y pyroxene to 5 m y plagioclase such persistence times suggest mineral persistence minima on mars however martian solutions may have been more acidic than on earth relative mineral weathering rates observed for basalt in svalbard norway and costa rica demonstrate that laboratory ph trends can be used to estimate exposure to liquid water both qualitatively mineral absence or presence and quantitatively using reactive transport models qualitatively if the martian solution ph 2 glass should persist longer than olivine therefore persistence of glass may be a ph indicator with evidence for the ph of weathering the reactive transport code crunchflow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le for the profi le measured on the surface of the exposed martian rock known as humphrey in gusev crater the calculated exposure time is 22 k y which is a minimum due to physical erosion if correct this estimate is consistent with short term episodic alteration accompanied by ongoing surface erosion more of these depth profi les should be measured to illuminate the weathering history of mars

evidence in favor of small amounts of ephemeral and transient water during alteration at meridiani planum mars in light of the controversy surrounding the origin of sulfate rich rocks analyzed by the mars exploration rover opportunity thermodynamic and kinetic models explored the consequences of in situ alteration of basaltic sand by pristine sulfuric acid simulations at 273 k and current martian atmosphere show that it is possible to simultaneously account for both chemical and mineralogical observations at the meridiani landing site but only when the amounts of water are small water rock mass ratio 1 the aqueous solutions are highly acidic ph 3 and the lifetimes of liquid water are extremely short on the order of tens of years furthermore the best agreement between observations and models is obtained if evolved fluids are removed after alteration if this simple self consistent scenario is relevant to bedrock formation at meridiani it provides stringent constraints on the issues of where and when liquid water was present at the surface of mars

mineralogy of the light toned outcrop at meridiani planum as seen by the miniature thermal emission spectrometer and implications for its formation 1 analysis of miniature thermal emission spectrometer mini tes data has led to the recovery of a pure end member spectral shape related to the light toned outcrop observed at meridiani planum data from the mer mossbauer spectrometer apxs and previous mini tes measurements were used to constrain a spectral library used to determine the mineralogy of the outcrop from this spectral shape linear deconvolution of the outcrop spectral shape suggests that it is composed primarily of al rich opaline silica mg ca and fe bearing sulfates plagioclase feldspar nontronite and hematite conversion of modeled mineralogy to chemistry shows good agreement with the chemical composition of the outcrops determined by apxs details of the analysis procedure and implications for the formation of the outcrop are discussed along with terrestrial analogs of the ancient environment at meridiani

the neoproterozoic of southwestern africa with emphasis on platform stratigraphy and paleontology abstract neoproterozoic sediments in southwestern africa occur in the pan african kaoko khomas gariep vanrhynsdorp and saldania belts they were deposited in response to rifting break up of a supercontinent formed at 1000 ma and subduction and collisions of the congo kalahari rio de la plata and malvinas plates these collisions led to the formation of a neoproterozoic cambrian supercontinent this paper deals with the geology of the platform successions of the kaoko khomas gariep and vanrhynsdorp belts deposition of the sediments occurring in these belts commenced 900 to 1000 ma ago and probably ended between 500 and 520 ma the sediments record wilson cycles that ended with the deposition of red beds palaeontological and carbon isotope studies on the damara sequence gariep complex nama group and vanrhynsdorp group make intra and inter basinal correlations possible prior to deposition of the nama group two or perhaps three glaciations took place over an extensive area in southwestern africa these being the kaigas chuos 725 700 ma numees 650 600 ma or 610 590 ma and possibly the varianto 745 725 ma glaciations the numees tillite is overlain by a stratigraphic unit containing pinkish dolomite during deposition of the nama group the minor vingerbreek 600 545 ma glaciation or cold water period occurred the major kaigas chuos and numees and minor vingerbreek glaciations correspond with low 13c isotope ratios and appear to have taken place at relatively low palaeolatitudes the riphean sediments were deposited predominantly during rifting and break up of a supercontinent during the late vendian to early cambrian deposition of the nama group in southern namibia and the penecontemporaneous vanrhynsdorp group in the republic of south africa took place in the nama foreland basin detrital micas indicate that deposition of the nama group took place between 635 and 530 ma the nama foreland basin which was connected to a major ocean to the south formed as a result of collisions between the congo kalahari and rio de la plata plates with the exception of the nama group little is known about the palaeontology of the neoproterozoic in southwestern africa the lower nama group is latest proterozoic in age it contains an ediacaran fauna a depauperate assemblage of organic walled microfossils and a relatively well diversified ichnofauna with brooksella diplocraterion and nereites which are atypical of vendian strata many trace fossils are exceptionally small the occurrence of the shelly fossil cloudina and probable calcified metaphytes in the lower nama group suggest that skeletalization in animals and metaphytes occurred almost simultaneously towards the end of the proterozoic the upper nama group is early cambrian in age as indicated by the occurrence of the trace fossil phycodes pedum the upper nama group dominated by red beds contains a low diversity ichnofauna with trace fossils of normal size bio and chemostratigraphy combined with sequence stratigraphy will probably establish how the prominent unconformities which occur in the neoproterozoic of southwestern africa are related to the periods of formation of the wonoka canyons in south australia

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

in situ and experimental evidence for acidic weathering of rocks and soils on mars experimental data for alteration of synthetic martian basalts at ph 0 1 indicate that chemical fractionations at low ph are vastly different from those observed during terrestrial weathering rock surface analyses from gusev crater are well described by the relationships apparent from low ph experimental alteration data a model for rock surface alteration is developed which indicates that a leached alteration zone is present on rock surfaces at gusev this zone is not chemically fractionated to a large degree from the underlying rock interior indicating that the rock surface alteration process has occurred at low water to rock ratio the geochemistry of natural rock surfaces analyzed by apxs is consistent with a mixture between adhering soil dust and the leached alteration zone the chemistry of rock surfaces analyzed after brushing with the rat is largely representative of the leached alteration zone the chemistry of rock surfaces analyzed after grinding with the rat is largely representative of the interior of the rock relatively unaffected by the alteration process occurring at the rock surface elemental measurements from the spirit opportunity pathfinder and viking 1 landing sites indicate that soil chemistry from widely separated locations is consistent with the low ph low water to rock ratio alteration relationships developed for gusev rocks soils are affected principally by mobility of fe and mg consistent with alteration of olivine bearing basalt and subsequent precipitation of fe and mg bearing secondary minerals as the primary control on soil geochemistry copyright 2006 by the american geophysical union

formation of silica by low temperature acid alteration of martian rocks physical chemical constraints 1 theoretical geochemical modeling has been used to evaluate the formation conditions of amorphous silica during aqueous alteration of typical martian igneous rocks at 0 c the models show that some silica can form during low temperature alteration of mafic to ultramafic rocks over a large range of ph and water rock ratios silica dominated deposits like those found at the columbia hills in gusev crater on mars could form at solution ph below 2 and water rock ratios of 102 104 high water rock conditions could represent acid flow through rocks solution discharge from an acid spring and or surface flow of released solutions low ph favors dissolution of silicates and saturation of solution with respect to relatively insoluble silica which then precipitates partial evaporation or freezing of released solutions would also cause precipitation of amorphous silica modeling shows that ti oxides are also present in silica rich deposits more soluble minerals e g ferric oxides phyllosilicates could precipitate downstream from partially neutralized evaporated or frozen solutions temperatures above 0 c are not required to form abundant silica through acid alteration of martian rocks

a 3 5 ga record of water limited acidic weathering conditions on mars abstract the secondary mineral budget on earth is dominated by clay minerals al hydroxides and fe oxides which are formed under the moderate ph high water to rock ratio conditions typical of earth s near surface environment in contrast geochemical analyses of rocks and soils from landed missions to mars indicate that secondary mineralogy is dominated by mg fe ca sulfates and fe oxides this discrepancy can be explained as resulting from differences in the chemical weathering environment of earth and mars we suggest that chemical weathering processes on mars are dominated by 1 a low ph sulfuric acid rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite fe ti oxides is promoted and 2 relatively low water to rock ratio such that other silicate phases with slower dissolution rates e g plagioclase pyroxene do not contribute substantially to the secondary mineral budget at the martian surface under these conditions al mobilization is limited and the formation of significant al bearing secondary phases e g clays al hydroxides al sulfates is inhibited the antiquity of rock samples analyzed in situ on mars suggest that water limited acidic weathering conditions have more than likely been the defining characteristic of the martian aqueous environment for billions of years

overview of mars surface geochemical diversity through alpha particle x ray spectrometer data multidimensional analysis first attempt at modeling rock alteration 1 principal component analysis and a hierarchical clustering method have been employed to describe and quantify the compositional variability of martian rocks and soils measured by the alpha particle x ray spectrometers onboard the mars exploration rovers a robust classification of samples emerges which defines distinct rock classes and sheds light on the petrogenetic relationships between rocks this is particularly useful in the case of rocks from gusev crater where significant chemical diversity is observed this approach also highlights that compositional variability of rocks at meridiani is dominated by variations in sulfur content the relative proportions of other elements remaining approximately constant for soils variations in fe concentration dominate because of the presence of hematite rich berry bearing samples on the basis of this observation a simple geochemical model of acid fog alteration of martian basalts has been tested assuming either equivalent alteration of all phases or preferential alteration of certain phases thus taking into account kinetic considerations the results show that for certain ranges of so3 basalt many of the compositional and mineralogical features measured at both sites may be explained the secondary mineralogy and bulk rock compositions predicted by the model are broadly consistent with rock and soil compositions from gusev and meridiani especially if the role of brine circulation and evaporation are considered although agreement is not perfect comparison of observations and models argues in favor of variable interaction of the martian surface with sour gas explaining the high local abundance of sulfates for example

the structural evolution of the gariep arc in southern namibia south west africa abstract current research in sw namibia recognises the gariep arc as a southerly extension to the damaran orogenic front of central and northern namibia we consider the gariep arc to have evolved by large scale thrusting of cover sediments and basement gneisses as flat lying sheets emplaced to the south east basic rocks with ophiolitic affinities are confined to the highest thrust sheet however due to the gravitational instability caused by virtue of its high density this obducted mass collapsed onto a previously undeformed foreland centred in the schakalsberg mountains localised extensional flow beneath the thrust sheet initiated a series of imbricate thrust slices emplacing cover sediments and basement to the south east onto a new foreland now located east of rosh pinah a model of differential movement in a thrust regime probably due to localised spreading of an obducted mass is suggested for the origin of the observed arcuate structures

geology of the skorpion supergene zinc deposit southern namibia the skorpion zinc deposit in southwestern namibia comprises a significant nonsulfide orebody 24 6 mt at 10 6 zn and subordinate amounts of primary base metal sulfide mineralization which structurally underlies the nonsulfide ores at depth the host rocks of the blind nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano sedimentary sequence of neoproterozoic age that has formed in an initial continental rift system between the kalahari cratonic province and the rio de la plata cratonic province the metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within plate affinity and have been emplaced under an extensional crustal regime zircons from a rhyolitic vesicular flow extruded approximately contemporaneously with the volcanic hosted sulfide mineralization have been dated by shrimp u pb analysis and yield an age of 751 9 5 5 ma the basin that hosts the skorpion deposit has been shaped by horst and half graben tectonics which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments the rock sequence was affected by a major tectono metamorphic event the pan african brasiliano orogeny at approximately 550 to 545 ma metamorphism reached uppermost greenschist to lowermost amphibolite facies and the complex deformational style of the region is the result of intense faulting folding and thrusting the skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks it is irregularly shaped transgressive to sedimentary layering and has a relatively flat top which is covered by a blanket of unmineralized overburden consisting of calcrete boulder beds and recent sand dunes at depth the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north the nonsulfide ore minerals comprise predominantly sauconite zn smectite substantial amounts of hemimorphite and smithsonite and subordinate amounts of scholzite tarbuttite atacamite hydrozincite and hetearolite the ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids fractures and breccias and as replacements of feldspar and mica the metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides gossanous manganese and iron hydroxide rich siliciclastic metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite rich plagioclase and of minor orthoclase local alteration phenomena also include silicification and rare secondary barite both a result of the alteration and breakdown of feldspar minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks structurally below the nonsulfide orebody the primary sulfide minerals comprise pyrite pyrrhotite sphalerite minor chalcopyrite and galena as well as secondary chalcocite and colloidal sphalerite brunckite the volcanic hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin in contrast the skorpion nonsulfide zinc deposit is of postmetamorphic origin as shown by two hydrous low p low t ore assemblages the secondary assemblages occur at least partly as delicate euhedral and subhedral crystals in secondary pore space open fractures veins and breccias the nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals the skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water as suggested by textural evidence and by initial stable isotope data the timing of ore formation is still undefined but lies within the large age bracket between 545 ma peak metamorphism and subsequent partial exhumation and the deposition of the nonmineralized overburden of namib desert gravels 21 ma and dune sands

variation in metamorphic style along the northern margin of the damara orogen namibia the northern margin of the inland branch of the pan african damara orogen in namibia shows dramatic along strike variation in metamorphic character during convergence between the congo and kalahari cratons m3 metamorphic cycle low p contact metamorphism with anticlockwise p t paths dominates in the western domains ugab zone and western northern zone and high p barrovian metamorphism with a clockwise p t path is documented from the easternmost domain eastern northern zone the sequence of m3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite chlorite muscovite at the same time as an andalusite biotite muscovite transposed foliation was developed in the matrix the peak t metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite muscovite the kfmash p t pseudosection for metapelites in the ugab zone and western northern zone contact aureoles indicates tight anticlockwise p t loops through peak metamorphic conditions of 540 570 c and 2 5 3 2 kbar these semi quantitative p t loops are consistent with average p t calculations using thermocalc which give a pooled mean of 556 26 c and 3 2 0 6 kbar indicating a high average thermal gradient of 50 c km in contrast the eastern northern zone experienced deep burial high p moderate t barrovian m3 metamorphism with an average thermal gradient of 21 c km and peak metamorphic conditions of c 635 c and 8 7 kbar the calculated p t pseudosection and garnet compositional isopleths in kfmash appropriate for the metapelite sample from this region document a clockwisep t path early plagioclase kyanite biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing xfe xmg and xca and decreasing xmn compositions indicating steep burial with heating the developed kyanite garnet biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats indicating decompression which was followed by retrogressive cooling and chlorite muscovite growth the clockwise p t loop is consistent with the foreland vergent fold thrust belt geometry in this part of the northern margin earlier formed 580 570ma pervasive matrix foliations m2 were overprinted by contact metamorphic parageneses m3 in the aureoles of 530 3ma granites in the ugab zone and 553 514ma granites in the western northern zone available geochronological data suggest that convergence between the congo and kalahari cratons was essentially coeval in all parts of the northern margin with similar ages of 535 530ma for the main phase of deformation in the eastern northern zone and northern platform and 538 505ma highgrade metamorphism of the central zone immediately to the south consequently nne ssw directed convergent deformation and associated m3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the inland branch in the west heat transfer was dominated by conduction and externally driven by granites whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production the ultimate cause was along orogen variation in crustal architecture including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession

the nama foreland basin sedimentation major unconformity bounded sequences and multisided active margin advance abstract the nama foreland basin developed in neoproterozoic times as a peripheral foreland basin between the kalahari craton in the east and the partly contemporaneous damara and gariep orogenic belts in the north and west the foreland basin is subdivided into three basins from north to south the zaris witputs and vanrhynsdorp basins deposits of the nama group proper fill the first two basins whereas the third contains the vanrhynsdorp group which differs in many aspects from the nama group only during its latest phase of development did the foreland basin form one continuous depository the stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis flexural bulging and sea level changes that affected the basin throughout its history palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin as mountain belts emerged along the interactive northern and western orogenic margins transport directions switched to southerly and southeasterly foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens three major unconformity bounded sequences composite sequences probably of second order cyclicity can be recognised the uppermost composite sequence cs3 comprises the fish river subgroup whereas the lower composite sequence cs1 is composed of the flaminkberg formation and the kwanous kuibis and lower and middle schwarzrand subgroups cs2 comprises the knersvlakte brandkop and upper schwarzrand subgroups cs1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a type 1 unconformity at the base of the nomtsas formation cs2 contains a conformable lowstand flysch delta partly derived from cs1 deposits eroded on flexural bulges as well as an orogenic source in the northwest cs3 is entirely composed of orogen derived molasse onlapping a basal type 2 unconformity and overstepping older sequences in a cratonward direction a number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading the age of the nama and vanrhynsdorp groups is estimated at between 600 ma and 530 ma as determined by the age of the basement the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast which affected nama and vanrhynsdorp rocks along the active basin margins marginal tectonogenesis of the nama foreland basin resulted from northwesterly subduction of the kalahari plate underneath the congo and south american plates through a process of sinistral transpression along the western kalahari margin leading to sequential closure of the proto atlantic adamastor ocean from north to south

in situ evidence for an ancient aqueous environment at meridiani planum mars sedimentary rocks at eagle crater in meridiani planum are composed of fine grained siliciclastic materials derived from weathering of basaltic rocks sulfate minerals including magnesium sulfate and jarosite that constitute several tens of percent of the rock by weight and hematite cross stratification observed in rock outcrops indicates eolian and aqueous transport diagenetic features include hematite rich concretions and crystal mold vugs we interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history the environmental conditions that they record include episodic inundation by shallow surface water evaporation and desiccation the geologic record at meridiani planum suggests that conditions were suitable for biological activity for a period of time in martian history

stratigraphy of the port nolloth group of namibia and south africa and implications for the age of neoproterozoic iron formations uncertainties in the number and age of glacial deposits within the port nolloth group have hindered both structural and stratigraphic studies in the neoproterozoic gariep belt of namibia and south africa these uncertainties are compounded by major lateral facies changes that complicate correlations locally herein we report the results of integrated geological mapping chemo and litho stratigraphic and sedimentological studies that shed light on the age and stratigraphic architecture of the port nolloth group particularly we have distinguished an additional glacial deposit herein referred to as the namaskluft diamictite which is succeeded by a ca 635 ma basal ediacaran cap carbonate this interpretation indicates that the stratigraphically lower iron bearing numees diamictite is not marinoan or gaskiers in age as previously suggested but is instead a ca 716 5 ma sturtian glacial deposit a sturtian age for the numees formation is further supported by the discovery of microbial roll up structures in the dark limestone of the bloeddrif member that caps the diamictite a re evaluation of the age constraints indicates that all neoproterozoic iron formations may be of sturtian age and thus indicative of secular evolution of the redox state of the ocean