crustal thickness and vp vs ratio in nw namibia from receiver functions evidence for magmatic underplating due to mantle plume crust interaction a seismological network was operated at the junction of the aseismic walvis ridge with the northwestern namibian coast we mapped crustal thickness and bulk vp vs ratio by the h k analysis of receiver functions in the damara belt the crustal thickness is 35 km with a vp vs ratio of 1 75 the crust is 30 km thick at the coast in the kaoko belt strong variations in crustal thickness and vp vs ratios are found at the landfall of the walvis ridge here and at 150 km northeast of the coast the crustal thickness increases dramatically reaching 44 km and the vp vs ratios are extremely high 1 89 these anomalies are interpreted as magmatic underplating produced by the mantle plume during the breakup of gondwana the area affected by the plume is smaller than 300 km in diameter possibly ruling out the existence of a large plume head under the continent during the breakup

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

arcades products of stress controlled and discontinuity related weathering abstract weathering patterns on sandstone cliff faces and sloping surfaces were studied from the viewpoint of gravity induced stress combined with the presence of planar discontinuities in the rock critical evaluation of hundreds of photos taken in various regions around the world numerical modelling using finite element method including automatic stress controlled element removal technique of erosion modelling and physical in situ modelling of friable sandstone revealed the formation of a specific sub group of weathering forms related to planar discontinuities due to their striking similarity with elements known from architecture these pits are introduced as arcades or arcade like forms these pits and cavities vary in size but they are similar in shape coalesced into trains along the discontinuities and separated from each other by hourglass shaped pillars in their origin and geometry these forms differ from widely known cavernous weathering forms such as honeycombs and tafoni we have shown that arcade shape and size are controlled by the redistribution of gravity induced stress along planar discontinuities and the resulting stress shadows the stress shadow zones along the discontinuities are characterized by material loss owing to various weathering and erosional mechanisms although overlooked as yet stress controlled weathering along discontinuities has a strong effect on rock surface morphology and arcades can be taken as its principal example

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

an s receiver function analysis of the lithospheric structure in south america 1 we make use of s receiver function to investigate the structure and thickness of the crust and mantle lithosphere in the south american continent and adjacent areas the moho discontinuity has been detected at all stations and goes from 18 km beneath the coast of the continent to 80 km in the andean region the depth of the lithosphere asthenosphere boundary can be clearly detected below those stations that are located on stable areas from 50 km to 160 km the identification of this phase becomes more difficult when the stations are located near subduction zones we also observed the base of the subducted oceanic lithosphere down to depths of 220 km

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

interaction between a hotspot and a fracture zone the crustal structure of walvis ridge at 6 e highlights late stage volcanism covers old oceanic crust north of the florianopolis fracture zone no influence of fracture zone on formation of walvis ridge at 6 e walvis ridge at 6 e erupted in deep water environment abstract the walvis ridge is one of the major hotspot trails in the south atlantic and a classical example for volcanic island chains two models compete about the origin of the ridge it is either the result of a deep mantle plume or active fracture zones above mantle inhomogeneities among other things crustal information is needed to constrain the models here we provide such constraint with a 480 km long p wave velocity model of the deep crustal structure of the eastern walvis ridge at 6 e according to our data the walvis ridge stretches across the florianopolis fracture zone into the angola basin here we observe a basement high and thick basaltic layers covering the oceanic crust and the fracture zone we found two crustal roots along the profile one is located beneath the ridge crest the other one beneath the northern basement high in the angola basin the crustal thickness reaches 18 km and 12 km and the lower crustal velocities are 7 2 km s and 7 4 km s respectively the bathymetric expression of the ridge along the profile is less pronounced than closer to shore which is mainly attributable to the absence of a thick layer of volcanic debris rather than to reduced crustal thickness below the basement surface therefore this part of the ridge was never or only briefly subaerially exposed the crustal structure suggests that the ridge and the fracture zone formed independently of each other the oceanic crust north of the fracture zone which is buried underneath the basalt layer is younger than the reconstructed age of hotspot volcanism of the walvis ridge we interpret these structures north of the fracture zone to be at least partly a product of late stage volcanism

mobility and fractionation of rees during deep weathering of geochemically contrasting granites in a tropical setting malaysia in this study we describe the mobility and fractionation of rees in two deep up to 30 m tropical weathering profiles developed on two granites from the kuala lumpur pluton malaysia sampled at cheras and rawang on the basis of na2o and k2o both are s type granites but rawang has higher cao mgo and feo than cheras and lower sio2 with respect to al saturation rawang is i type and cheras is s type we compared the two profiles in terms of total rees magnitude and changes in ce and eu anomalies ree mobility and lree hree fractionation rawang profiles have higher ree contents display lower mobility for most except the heaviest rees and show higher lree hree fractionation than those from cheras these differences can be linked to differences in primary mineralogy and degree of weathering the latter controlling the type and volume of secondary minerals specifically bowl shaped parent rock normalised patterns in the cheras saprolites appear to be a result of apatite dissolution moreover moderate weathering evident in lower mineralogical indices of alteration mia at cheras has conserved parent rock ree patterns and fractionation factors in the saprolites by contrast more intense weathering observed in rawang profiles has produced abundant kaolinite group minerals that have preferentially retained lree which consequently display high lree hree fractionation this study provides important insights into the factors controlling ree mobility during tropical weathering and its potential as an indicator of weathering intensity

simulating the growth of tafoni throughout the world large caves in rocks tafoni are found which originate from salt weathering the mechanisms that control their development are poorly understood the growth of tafoni has been studied with a model that describes how a rock surface containing a small pit disintegrates by salt crystallization during wetting drying cycles in the model the rock is mapped on a grid the migration and crystallization of salts are simulated explicitly in the drying phase of a cycle at the end of each wetting drying cycle the amount of salt deposited at the grid nodes is evaluated and the shape of the rock surface is adjusted by removing nodes the length of the drying period in a single cycle proved to be the key parameter for short drying periods the amount of crystallized salt at the surface is proportional to the drying rate therefore for short drying periods most salts are deposited at the more exterior parts of the rock surface as a result most damage will develop at these parts of the surface which results in smooth surfaces due to the characteristics of the drying process for long drying periods most salts accumulate at regions with low evaporation rates which are the sheltered parts of the rock surface these parts are not exposed to the wind or the sun as a result the pit grows and a tafone develops copyright 2004 john wiley sons ltd

upper mantle structure at walvis ridge from pn tomography abstract passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up within the lispwal lithospheric structure of the namibian continental passive margin at the intersection with the walvis ridge from amphibious seismic investigations project combined on and offshore seismic experiments were designed to characterize the southern african passive margin at the walvis ridge in northern namibia in addition to extensive analysis of the crustal structures we carried out seismic investigations targeting the velocity structure of the upper mantle in the landfall region of the walvis ridge with the namibian coast upper mantle pn travel time tomography from controlled source amphibious seismic data was used to investigate the sub moho upper mantle seismic velocity we succeeded in imaging upper mantle structures potentially associated with continental break up and or the tristan da cunha hotspot track we found mostly coast parallel sub moho velocity anomalies interpreted as structures which were created during gondwana break up

origin and development of tafoni in tunnel spring tuff crystal peak utah usa bedding parallel tafoni are well developed over much of the surface of the tunnel spring tuff oligocene exposed in 300 m high crystal peak an inselberg the tunnel spring tuff is a crudely stratified non welded rhyolite ash flow tuff with 30 per cent porosity clasts of palaeozoic dolomite limestone and quartzite make up 10 per cent of the tuff the tafoni are remarkable because of their size up to 20 m wide but rarely wider than 4 m shape of the openings spherical arch like or crescent shaped and abundance up to 50 per cent of an outcrop face they are actively forming today calcite is abundant 10 to 40 per cent by weight in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls halite and gypsum generally make up less than 0 01 per cent of the efflorescence the absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni calcite aragonite halite and gypsum dust from modern salt pans less than 20 km from crystal peak are potential sources of salt in the tuff but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg calcite is the only evaporite mineral present in the tafoni in more than trace amounts and this mineral is readily available within the tuff itself as a result of rock weathering we propose that meteoric water containing carbonic acid infiltrates the tuff dissolves carbonate clasts and migrates to the steep flanks 20 of the peak through abundant megapores and micropores there it evaporates and precipitates calcite crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering the quasi uniform spacing of tafoni suggests that a self organization process is active in the water flow copyright 2000 john wiley sons ltd

complex deep structure of the african low velocity zone abstract we use recently deployed seismological arrays in africa to sample a 2d cross section through the mantle down to the core mantle boundary cmb by making use of travel time residuals of s scs and sks phases a new shear velocity model of the african low velocity zone alvz is derived our model suggests between 1 2 shear velocity reduction at the top and 5 at the bottom with respect to 1d reference models the average reduction over the whole low velocity zone lvz amounts to 2 in the presented model and is therefore about twice as strong as values found in global tomographic models the top of the lvz reaches up to 1200 km depth and its lateral extent at the cmb is about 35 we propose the existence of a gap of 300 km splitting the structure into two blocks our results are based on remarkable differences in sk k s travel time residuals over a few degrees distance the complexity of the structure could provide a key to an improved understanding of the deep mantle lvz dynamics and composition by comparison to geodynamic models the gap in the model might suggest that the 2d cross section is cutting through a 3d indentation in the boundary of the alvz but may also be interpreted as a sign of two individual plumes rather than one large homogeneous upwelling electronic supplement figures of preprocessed original waveforms aligned with the synthetics for the corresponding event and tables of applied corrections

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

seismic velocities poisson s ratios and potential auxetic behavior of volcanic rocks abstract the present compilation of p and s wave velocities vp and vs for 395 dry and 664 wet basalt 231 dry and 63 wet rhyolite samples provide a new database for evaluating the porosity dependence of vp vs and poisson s ratio and the potential auxetic behavior i e negative poisson s ratio of volcanic rocks the correlation is always positive for the water saturated rocks but can be either positive rhyolite or negative basalt for dry rocks depending on the poisson s ratio of the non porous solid matrix 0 auxetic behavior cannot occur in wet volcanic rocks but may appear in dry basalts with extremely high porosity 70 for basalt such that a re entrant foam structure forms the auxeticity of highly porous volcanic rocks such as pumice and reticulite will be suppressed by infiltration of water into the rocks hence volcanic rocks cannot be auxetic in nature if their vesicles are filled with water under the laboratory conditions however a transition from conventional to auxetic behaviors in pumice and reticulite is expected to occur by changing the degree of water saturation these results provide a set of basic parameters for the modeling of ground deformation and temporal and spatial variations in vp vs vp vs ratio as precursors of volcanic eruption or volcanic edifice collapse

subsidence history of the tethys himalaya abstract this article reconstructs the evolution of the passive northern margin of peninsular india facing the neotethys from the earliest rifting stages in the carboniferous to final collision with the active southern margin of asia in the early eocene classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base facies analysis biostratigraphic dating and palaeoenvironmental interpretation of an over 10 km thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record in each of the investigated composite sections three major stratigraphic gaps are consistently recognised in the rift sequence the rift unconformity mississippian the carboniferous permian hiatus largely kasimovian to asselian and the break up unconformity mid sakmarian two or even all three gaps may merge into a single hiatus spanning up to 70 ma overall gaps are associated with rift shoulder uplift which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections thermal subsidence and low sedimentation rates are documented by the mid permian to middle triassic drift sequence deposited while the neotethys was expanding between northern gondwana and the detached peri gondwanan blocks a marked increase in accumulation rates is recorded in the carnian norian but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of gondwana the tethys himalayan margin was uplifted around the jurassic cretaceous boundary at the onset of a major volcanic episode finally uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column deposited during the earliest stages of collision between india and asia in the latest paleocene to early eocene

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

forearc hyperextension dismembered the south tibetan ophiolites suprasubduction zone ophiolites are relics of oceanic upper plate forearcs and are typically preserved as discontinuous belts with discrete massifs along suture zones ophiolites usually contain an incomplete condensed section compared to average modern oceanic lithosphere the incompleteness and discontinuity of ophiolites are frequently attributed to dismemberment but tectonic causes remain poorly constrained here we show new paleomagnetic and field geological evidence for the preservation of extensional detachment faults that thinned and dismembered the south tibetan ophiolite belt during the early cretaceous similar to those documented in modern slow and ultraslow spreading ridges these detachments exhumed lithospheric mantle and subophiolitic melange to the seafloor which became unconformably covered by asia derived forearc strata we call this mechanism forearc hyperextension whereby widespread detachment faults accommodate upper plate extension above a subduction zone we propose that hyperextension is the key mechanism responsible for dismemberment of the south tibetan ophiolitic belt shortly after its magmatic accretion

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

rates of tafoni weathering on uplifted shore platforms in nojima zaki boso peninsula japan many tafoni occur on the faces of marine cliffs on three uplifted shore platforms with different altitudes and with known ages of emergence the mean value of the ten largest depths of tafoni d cm was obtained and the period of their formation t years was estimated the relationship between the two variables was found to be d 20 3 1 e 0 005t

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

salt as an agent in the development of cavernous weathering a well recognized spatial association exists between cavernous weathering and the presence of soluble salts disaggregation of rock in caverns is attributed to mechanical pressures exerted by crystallization and or hydration of salts however scanning electron micrographic analysis demonstrates that solutional etching is the major weathering process in caverns in near coastal locations in western australia and new south wales australia it is suggested here that the breakdown of the caverned sandstone is caused by sodium chloride which increases the rate of quartz dissolution

the origin of honeycomb weathering honeycomb weathering occurs throughout the world but the origin remains a matter of controversy wind erosion exfoliation frost shattering and salt weathering have been proposed as explanations although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near bellingham washington results from evaporation of salt water deposited by wave splash microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface cavity walls are not reinforced by precipitation of elements released by weathering as has often been suggested at other locations honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago

a pictorial study of tafoni development from the 2nd millennium bc a wall painting from the prehistoric settlement of akrotiri santorini greece which is known under the name of lilies fresco or spring fresco displays a rocky landscape with different stages of tafoni development this painting could be the oldest preserved picture of tafoni its significance is discussed relative to geomorphological concepts and prehistorical landscape evolution d 2004 elsevier b v all rights reserved

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

geophysical constraints on a modified crust below the rayn anticlines eastern saudi arabia abstract the gravity anomaly signature for a regional trans rayn geophysical traverse trgt across rayn anticlines ra illustrates alternating bouguer anomaly b a highs and lows highs over the anticline axes and lows in flanking sub basins we demonstrate for the first time that the bipolar gravity signatures actually camouflage a more extensive b a low of amplitude 63 mgal and wavelength 750 km developed between the qatar and amar arches 2d gravity modeling for trgt infers a 3 layer crustal model for ra i a regionally extensive upper crustal layer 10 15 km thick of lighter density by 0 02 g cm3 compared to normal upper continental crust ii a mid crustal 10 km thick transition layer of intermediate density and iii the lower mafic crust of higher than normal density the moho dips westward from 37 3 below ghawar anticline ga to 47 and 47 6 km below the kbs amar arches with a lateral density change in the lower crust following the same direction crustal thinning by as much as 22 occurred below ga the altered crust directly correlating to the basement uplift on its top and mantle uplift at its base an inverse correlation between crustal thicknesses and vp vs is observed for the entire ra displaying highest vp vs 1 93 and poisson s ratio 0 443 for the ga crust

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

seismic structure of the lithosphere beneath nw namibia impact of the tristan da cunha mantle plume northwestern namibia at the landfall of the walvis ridge was affected by the tristan da cunha mantle plume during continental rupture between africa and south america as evidenced by the presence of the etendeka continental flood basalts here we use data from a passive source seismological network to investigate the upper mantle structure and to elucidate the cretaceous mantle plume lithosphere interaction receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km we interpret this interface as the relic of the lithosphereasthenosphere boundary lab formed during the mesozoic by interaction of the tristan da cunha plume head with the pre existing lithosphere the velocity contrast might be explained by stagnated and frozen melts beneath an intensively depleted and dehydrated peridotitic mantle the present day lab is poorly visible with converted waves indicating a gradual impedance contrast beneath much of the study area converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1 5 s earlier than in the landward plume unaffected continental interior suggesting high velocities in the upper mantle caused by a thick lithosphere this indicates that after lithospheric thinning during continental breakup the lithosphere has increased in thickness during the last 132 myr thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here we propose that the remnant plume material which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration significantly contributed to the thickening of the mantle lithosphere

geophysical images and a crustal model of intrusive structures beneath the messum ring complex namibia the early cretaceous messum complex in namibia is an excellent example of the type of composite mafic and alkaline igneous ring complexes associated with continental rifts worldwide this study integrates reflection seismic gravity and aeromagnetic data from a profile across messum and reveals a heavily intruded root zone extending vertically to moho depths 40 km the crustal section to either side of the messum complex is characterized by a seismically transparent upper crust to 20 km depth and a laminar reflective lower crust below the crust below the messum complex is seismically transparent at all depths but it shows a combination of high seismic reflection strength and weak coherency relative to the unintruded crust and p velocities about 0 2 0 3 km s higher at all depths the seismic attributes are best explained by a network of mafic intrusions and this is supported by steep sided positive magnetic and gravity anomalies centered on the complex modelling indicates that about 30 gabbroic material in the crustal column can explain the velocity and potential field data an increase in reflective strength below 20 km suggests the degree of intrusion is greater in the lower crust but the difference is not resolved in the velocity or gravity models the new geometric and compositional constraints on the crust below the messum complex contradict the hypothesis that messum was the sole eruptive center for an estimated 8600 km3 of crustally derived rhyodacitic magmas that now form part of the regional parana etendeka volcanic sequences to our knowledge this is the first study to provide a seismic image of the whole crust below a typical igneous ring complex and our conclusion that the cross sectional area of the complex is the same in the lower as in the upper crust could not have been predicted from surface studies the data resolution is insufficient to define intrusion geometry in detail but the cylindrical 3 d shape throughout the crust and the lack of major breaks in the vertical distribution of intrusions indicate depth independent controls on the nature of the intruded zone the suggestion is that the magma transport and emplacement took place in a brittle fashion at all levels of the crust in response to conditions of high fluid magma pressure and a high magma flux

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

tafoni on coastal slopes south devon u k cavernous weathering tafoni development occurs on coastal slopes in greenschist bedrock at elevations up to 40 m above sea level the freshly weathered surfaces of the cavern interiors are irregular in morphology discordant to major rock structure formed by substantially weakened rock and associated with granular weathering debris the weathering debris contains soluble elements in proportions similar to those present in seawater and the penetration of elements associated with sea salts into the weathering surface to estimated depths of at least 0 1 0 2 m is indicated by the presence of chlorine scanning electron microscopy and microprobe analyses suggest that rock breakdown occurs principally through limited chemical weathering at grain boundaries the mechanism for the emplacement of marine salts within sheltered rock surfaces in the tafoni is postulated to be a combination of dry deposition under turbulent atmospheric conditions and wetting by coastal fog

microclimatic chemical and mineralogical evidence for tafoni weathering processes on the miaowan island south china abstract tafoni were widely distributed around the world however their processes of development remain unclear in this study the roles of microclimatic geochemical and mineralogical processes on tafoni development along the subtropical coastline of the miaowan island south china are investigated field observations were carried out during three visits to the island over a four year period 2011 2015 the orientation of 184 tafoni openings were measured and micrometeorological changes of three tafoni on opposite sides of the island were monitored by pocket weather trackers kestrel 4500 in two periods samples of residual debris inside three tafoni hosted in a large boulder the parent rock of the tafoni and from the weathering profile of a nearby bedrock outcrop were collected for x ray fluorescence xrf and x ray diffraction xrd analyses the field observations showed that tafoni were of different sizes and constantly produced flakes and debris inside the tafoni caves indicating their on going active development an increase in na in residual debris in tafoni caves on the miaowan island is the most obvious evidence of salt weathering salt weathering inside tafoni caves is not intense and does not match the salt rich environment outside the caves indicating that the influence of salt is not strong the loss of k ca and mg in the residue samples and the appearance of the clay mineral montmorillonite are caused by chemical weathering most of the tafoni openings face mountains demonstrating the effect of humidity in tafoni weathering tafoni cave shapes are related to the distribution of humid water vapour which tends to collect at the top of the cave and leads to more intensive development here than in other parts drastic daily changes in relative humidity inside tafoni caves accelerate mechanical weathering owing to swelling and shrinking of salt and clay minerals the miaowan island tafoni are formed by weathering but they cannot be simply interpreted as the product of a single weathering process

the s receiver functions synthetics and data example summary recently the s receiver function method has been successfully developed to identify upper mantle interfaces s receiver functions have the advantage of being free of s wave multiple reflections and can be more suitable than p receiver functions for studying mantle lithosphere however because of specific ray geometry and interference of diverse phases the s receiver function method has some technical difficulties and limitations we use synthetic seismograms to demonstrate the feasibility and limitations of s receiver functions for studying mantle structures full wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic s receiver functions for s sks and scs waves results show that s receiver functions can be obtained from waveforms of s sks and scs waves the synthetic s receiver functions for these incident waves show s to p converted phases at all discontinuities in the crust and upper mantle useful ranges of epicentral distances for calculation of s receiver functions are 55 85 for s 85 for sks and 50 75 for scs waves we apply both the s and p receiver function methods to data recorded at broadband station ykw3 in northwest canada the study shows that there is significant agreement among different receiver function methods and demonstrates the usefulness of s receiver functions for imaging the mantle lithosphere

3 d magnetotelluric image of offshore magmatism at the walvis ridge and rift basin highlights we report on marine 3d magnetotelluric study on walvis ridge derived 3d electrical resistivity model shows a large scale resistive zone which we link to crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting smaller scale resistive region is attributed to magma ascent during rifting rift basin is identified by low resistivity region the namibian continental margin marks the starting point of the tristan da cunha hotspot trail the walvis ridge this section of the volcanic southwestern african margin is therefore ideal to study the interaction of hotspot volcanism and rifting which occurred in the late jurassic early cretaceous offshore magnetotelluric data image electromagnetically the landfall of walvis ridge two large scale high resistivity anomalies in the 3 d resistivity model indicate old magmatic intrusions related to hot spot volcanism and rifting the large scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2 d offshore seismic profiles one of the high resistivity anomalies above 500 m has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust one of the arms stretches underneath walvis ridge the shape is suggestive of crustal extension due to local uplift it might indicate the location where the hot spot impinged on the crust prior to rifting a second smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting we attribute a low resistivity anomaly east of the continent ocean boundary and south of walvis ridge to the presence of a rift basin that formed prior to the rifting

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

geomorphology s role in the study of weathering of cultural stone abstract great monumental places petra giza angkor stonehenge tikal macchu picchu rapa nui to name a few are links to our cultural past they evoke a sense of wonderment for their aesthetic fascination if not for their seeming permanence over both cultural and physical landscapes however as with natural landforms human constructs are subject to weathering and erosion indeed many of our cultural resources suffer from serious deterioration some natural some enhanced by human impact groups from the united nations to local civic and tourism assemblies are deeply interested in maintaining and preserving such cultural resources from simple rock art to great temples geomorphologists trained in interacting systems process and response to thresholds rates of change over time and spatial variation of weathering processes and effects are able to offer insight into how deterioration occurs and what can be done to ameliorate the impact review of recent literature and case studies presented here demonstrate methodological and theoretical advances that have resulted from the study of cultural stone weathering because the stone was carved at a known date to a baseline or zero datum level some of the simplest methods e g assessing surface weathering features or measuring surface recession in the field provide useful data on weathering rates and processes such data are difficult or impossible to obtain in natural settings cultural stone weathering studies demonstrate the importance of biotic and saline weathering agents and the significance of weathering factors such as exposure microclimate and human impact more sophisticated methods confirm these observations but also reveal discrepancies between field and laboratory studies this brings up two important caveats for conservators and geomorphologists for the conservator are laboratory and natural setting studies really analogous and useful for assessing stone damage for the geomorphologist does cultural stone data have any real relevance to the natural environment these are questions for future research and debate in any event cultural stone weathering studies have been productive for both geomorphologists and conservators continued collaboration and communication between the geomorphic historic preservation archaeological and engineering research communities are encouraged

rates of development of tafoni in the moenkopi and kaibab formations in meteor crater and on the colorado plateau northeastern arizona tafoni are pits formed by non uniform weathering in otherwise uniform rock two equations have been proposed for the rate of development of tafoni both based on 2000 year old outcrops from the coast of japan we have taken tafoni measurements from the meteor crater arizona and vicinity that extend the equations back at least 50 000 years as reported in earlier studies we found pit depth to be the best tafone parameter to measure the size of the pit decreases significantly with increasing inclination of the rock surface however the size of the pit can vary greatly for other reasons in some cases the measurements are statistically significantly different between two stations taken from contiguous areas of similar inclination and aspect in an apparently homogeneous bed it is clear however that over tens of thousands of years tafoni enlarge significantly our data are generally log normal and all are markedly heteroscedastic the 1991 equation proposed by matsukura and matsuoka does not fit our data the 1996 equation proposed by sunamura provides a better fit we propose a sigmoidal equation d b1 e b2 b3 t where d is the depth t is the age and b1 b2 and b3 vary with lithology this new equation fits our data far better than the earlier published equations copyright 2002 john wiley sons ltd

tectonostratigraphy and provenance of an accretionary complex within the yarlung zangpo suture zone southern tibet insights into subduction accretion processes in the neo tethys abstract accretionary complexes record critical information about the history of subduction and accretion along the southern margin of asia prior to the india asia collision this paper presents detailed field mapping petrographic and detrital zircon u pb data from an accretionary complex within the yalung zangpo suture zone southern tibet from structurally higher to lower levels the accretionary complex consists of a serpentinite melange the chert matrix tangga melange the mud matrix pomunong melange and the coherent rongmawa formation the tangga melange consists of late triassic early cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra mafic rocks the tangga melange was accreted beneath ophiolitic rocks during the aptian the pomunong melange consists of a late jurassic early cretaceous hemipelagic siliceous shale and chert matrix with blocks from early permian seamounts and from late cretaceous trench fill sandstones it was accreted beneath the tangga melange after the aptian but prior to 71 ma structurally beneath these melanges to the south the uppermost cretaceous rongmawa formation consists of turbiditic sandstone pelagic chert and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench detrital zircons from sandstone blocks of the pomunong melange and the coherent rongmawa formation display similar u pb age spectra and are dominated by peaks at 71 231 ma 481 693 ma and 701 1372 ma these age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the lhasa terrane our data indicate that the serpentinite melange tangga melange pomunong melange and rongmawa formation comprise a southward younging accretionary complex that developed during the northward subduction of neo tethyan oceanic lithosphere beneath the south margin of the lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the asian margin prior to the india asia continental collision

a preliminary scanning electron microscope study of honeycomb weathering of sandstone in a coastal environment honeycomb weathering has been observed in a carboniferous sandstone at a coastal location near ballycastle on the north coast of northern ireland specimens of this sandstone have been analysed by x ray diffraction scanning electron microscopy and energy dispersive spectrometry results reveal that calcium sulphate gypsum is the only salt present and is found only at and immediately below the rock surface sem observations suggest that crystallization of salts in pores could easily dislodge quartz grains to promote granular disintegration whilst etching of quartz grain surfaces attests to chemical weathering activity within the rock however the reason for the development of the honeycomb pattern is not known

full waveform tomography of the upper mantle in the south atlantic region imaging a westward fluxing shallow asthenosphere abstract a prominent feature of the south atlantic region is its strongly asymmetric residual bathymetry across the ocean basin it has been suggested that the residual bathymetry is dynamic in nature arising from the large slow velocity seismic anomaly located in the lower mantle beneath the african plate unfortunately the pattern of mantle heterogeneity particularly in the upper mantle is not well known owing to the sparsity of seismic stations and the existence of large aseismic regions on the african and south american plates here we present a new seismic tomographic study of the south atlantic upper mantle our model is based on a full seismic waveform inversion of 4000 high quality seismograms for isotropic 3 d seismic structure using a powerful adjoint methodology capable of extracting maximum information from each seismogram the theory requires simulation of seismic wave propagation in 3 d heterogeneous earth models computed with a spectral element method where the differences between observed and synthetic seismograms are quantified using phase misfits obtained through a time frequency transform the model images a continuous channel of pronounced slow seismic velocity in the shallow sublithospheric mantle between 150 and 300 km depth that branches in between the cratonic roots under the african and south american continents at greater depth below 300 350 km the slow anomalies are less pronounced and a change in the convective planform is indicated by isolated round shaped patches in an overall faster mantle it is possible that the depthwise change of the convective planform from vertical to horizontal advection of hot buoyant material in a low viscosity asthenosphere can reconcile the anomalous residual bathymetry in the region and we present a simple fluid dynamic model of pressure driven flow to assess the feasibility of this scenario

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

identification of early carboniferous granitoids from southern tibet and implications for terrane assembly related to the paleo tethyan evolution abstractthis article presents zircon u pb and hf isotope data together with the whole rock major and trace element composition of early carboniferous granitoids newly identified from the jiacha and langxian areas in the southern lhasa terrane southern tibet the jiacha rocks are monzogranites that yield zircon u pb ages of 347 345 ma and ehf t values from 5 4 to 4 9 the langxian rocks are granodiorites with slightly older zircon u pb ages of 355 352 ma and lower ehf t values from 6 8 to 6 5 our data suggest that these granitoids were generated largely by reworking of paleoproterozoic ga basement materials in conjunction with literature data it is further argued that the southern and central parts of the lhasa terrane separated by the sumdo eclogite belt should have been an integrated block before the late paleozoic our study supports the notion that the lhasa terrane was derived from the northern margin of gondwanaland in association with formation of at least two stages of tethyan oc

deposystem architectures and lithofacies of a submarine fan dominated deep sea succession in an orogen a case study from the upper triassic langjiexue group of southern tibet abstract over thirty stratigraphic sections of the himalaya orogen upper triassic langjiexue group in southern tibet china were studied to interpret the environments and lithofacies the facies associations channel a lobe b levee interchannel c and basin plain d with nine facies a1 3 b1 3 and c1 3 were distinguished they form six architectural elements channel interchannel overbank levee crevasse splay outer fan lobe fan fringe and basin plain taking into account the facies analysis sub deposystem correlation paleocurrent dispersal pattern and restoration of primary stratal width the langjiexue group displays the architecture of a coalescing submarine fan dominated deep sea deposystem measuring about 400 500 km 600 700 km in size or even more one of the largest pre cenozoic submarine fans ever reported subdivisionally four fans lacking inner fans could have coalesced laterally within the submarine fan deposystem and at least six submarine fan developments were vertically succeeded by mid to outer fan deposits with progradational to retrogradational successions according to the range of 30 70 of sandstone content the fan deposystem is mud and sand rich suggesting a medium far over 400 600 km transport of sediment from the source area

tafoni development in a cryotic environment an example from northern victoria land antarctica tafoni are a type of cavernous weathering widespread around the world despite the extensive distribution of the tafoni their genesis is not clear and is still a matter of debate also because they occur in such different climatic conditions and on so many different types of substrate geomorphological characterization of more than 60 tafoni in three different antarctic sites two coastal and one inland between 74 and 76 s with sampling of weathering products and salt occurrences are described together with thermal data on different surfaces and wind speed recorded in different periods of the year in a selected tafone close to the italian antarctic station the aim of this present study is to provide further information to help understand the processes involved in the growth of tafoni in a cryotic environment and the relationship of these processes to climate with particular attention to the thermal regime and the role of wind the new data presented in this paper suggest that there is no single key factor that drives the tafoni development although thermal stress seems the most efficient process particularly if we consider the short term fluctuations the data also confirm that other thermal processes such as freezing thawing cycles and thermal shock are not really effective for the development of tafoni in this area the wind speed measured within the tafoni is half that recorded outside thus favouring snow accumulation within the tafoni and therefore promoting salt crystallization on the other hand the wind effect on the thermal regime within the tafoni seems negligible while both salt weathering and thermal stress appear active in this cryotic environment these are azonal processes and are therefore active in other climatic areas where tafoni are widespread such as the mediterranean region copyright 2007 john wiley sons ltd

south atlantic opening a plume induced breakup upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup the formation of the walvis ridge during the opening of the south atlantic is related to a putative plume induced breakup we investigated the crustal structure of the walvis ridge southeast atlantic ocean at its intersection with the continental margin and searched for anomalies related to the possible plume head the overall structure we identify suggests that no broad plume head existed during opening of the south atlantic and anomalous mantle melting occurred only locally we therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting leaving a track of kimberlites in the african craton

late triassic paleogeographic reconstruction along the neo tethyan ocean margins southern tibet abstract sandstone petrographic and u pb detrital zircon analyses of upper triassic sedimentary rocks from the northern margin of india tethyan himalaya sequence and southern margin of eurasia lhasa terrane provide new constraints on the mesozoic paleogeography of neo tethyan ocean basins the upper triassic nieru formation of the tethyan himalaya sequence ths near lazi city 29 n 87 5 e is dominated by indian affinity precambrian detrital zircons which are typical of the majority of the ths however the upper triassic langjiexue formation of the ths exposed to the east at 90 93 e longitude includes significant populations of permian to early jurassic 291 184 ma detrital zircons for which there is no known indian source in addition the upper triassic nieru formation near kangma town 28 5 n 90 e located 200 km to the southeast of lazi city yielded detrital zircon age spectra that are similar to those of langjiexue formation based on detrital zircon age spectra comparisons we propose that both the langjiexue and nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of australia furthermore we suggest that these ths units and age equivalent strata in northwest australia west sulawesi timor and west papua comprised a late triassic submarine fan along the northern australian shelf the upper triassic mailonggang formation in the southern lhasa terrane 35 km northeast of lhasa city 30 n 91 5 e is dominated by permian detrital zircons which were likely derived from proximal lhasa terrane sources the mailonggang formation differs from all age equivalent strata in the tethyan himalaya therefore we interpret that it was separated from greater india by the neo tethyan ocean

tafoni in the el chorro area andalucia southern spain using a combination of field laboratory and micromorphological evidence this study examines tafoni singular tafone in the el chorro area of andalucia southern spain and makes inferences concerning the processes responsible for their formation twenty five tafoni were randomly selected for field examination the morphology of these cavernous rock domes is characterized by a helmet shaped outer roof and an arched shaped cavern often with a partially overhanging visor measurements of height width and depth of the caverns revealed marked variations in size the presence or absence of lichen cover surface varnish overhanging visor cavern backwall stripes rock flaking weathering pits and cavern floor sediments was also noted surface hardness values obtained using a schmidt hammer are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures alkaline ph values and some base cation enrichment micromorphological analysis of thin sections prepared from undisturbed blocks reveals large quantities of pore filling cement consisting mainly of calcite mineral grains affected by weathering and pseudomorphic replacement and dark rounded nodules with a metallic appearance in terms of their formation different processes appear to act on different parts of the landform on the outer roof surfaces case hardening resulting from near surface cementation and surface varnish development is dominant on the inner cavern surfaces however core softening resulting from granular disintegration and flaking dominates exfoliation weathering running water and wind deflation also appear to play an important role in tafone formation a phased model of tafone evolution is proposed whereby the features pass through four phases of development initiation enlargement amalgamation and degradation in the study area there are examples of tafoni in each of these phases much of the evidence suggests that the tafoni are actively developing under current environmental conditions

microclimate and weathering in the central namib desert namibia data on rock surface microclimates at four sites kleinberg vogelfederberg gobabeb and ganab along a transect across the central namib desert over a 3 year period have been collected alongside some shorter term data sets rock surface temperature rst air temperature wind relative humidity and surface wetness data are presented here weathering of exposure blocks of local marble and granite has also been monitored over the 3 year period with change observed visually and with scanning electron microscopy complex temporal patterns of rock temperature fluctuations are observed over the 3 years although the four sites show very similar overall trends suggesting spatial homogeneity in the rock surface temperature regime relative humidity and surface wetness data show clear differences between the four sites related to the frequency of fogs granite blocks show no visible changes after 2 years of weathering whereas marble blocks especially those at ganab show sem evidence of structural weakening after 1 year

chemical weathering associated with tafoni at papago park central arizona papago park arizona is a pediment inselberg complex that hosts a variety of well developed tafoni and alveolar weathering forms the purpose of this paper is to analyse the nature of chemical weathering associated with the tafoni using backscatter electron microscopy bse and quantitative wavelength dispersive x ray analysis wds calcium rich and iron rich coatings occur on the outer shells of the tafoni calcium carbonate precipitation within mineral microfractures occurs on the underside of the tafoni chemical weathering of primary mineral grains provides a source of material found in the coatings the wds analyses show a near complete lack of salt forming elements copyright 1999 john wiley sons ltd

the onset of walvis ridge plume influence at the continental margin highlights crustal structure of walvis ridge reveals high seismic velocities in the lower crust intruding the african continent this modified crust is localized to approx 100 100 km within the continent no indication for a large plume head observed the opening of the south atlantic is a classical example for a plume related continental breakup flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at tristan da cunha to determine the effect of the proposed plume head on the continental crust we acquired wide angle seismic data at the junction of the walvis ridge with the african continent and modelled the p wave velocity structure in a forward approach the profile extends 430 km along the ridge and continues onshore to a length of 720 km crustal velocities beneath the walvis ridge vary between 5 5 km s and 7 0 km s a typical range for oceanic crust the crustal thickness of 22 km however is approximately three times larger than of normal oceanic crust the continent ocean transition is characterized by 30 km thick crust with strong lateral velocity variations in the upper crust and a high velocity lower crust hvlc where velocities reach up to 7 5 km s the hvlc is 100 to 130 km wider at the walvis ridge than it is farther south and impinges onto the continental crust of the kaoko fold belt such high seismic velocities indicate mg rich igneous material intruded into the continental crust during the initial rifting stage however the remaining continental crust seems unaffected by intrusions and the root of the 40 km thick crust of the kaoko belt is not thermally abraded we conclude that the plume head did not modify the continental crust on a large scale but caused rather local effects thus it seems unlikely that a plume drove or initiated the breakup process we further propose that the plume already existed underneath the continent prior to the breakup and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts