rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

do swarms of migrating barchan dunes record paleoenvironmental changes a case study spanning the middle to late holocene in the pampa de jaguay southern peru swarms of barchan dunes are common in arid environments unlike immobile or slowly moving dunes whose stratigraphy can be used to reconstruct paleoenvironmental changes the high migration rate of barchans seems to prohibit their use as geoarchives in this paper a possible use of barchan swarms for paleoenvironmental reconstructions is presented the approach is based on dune migration in a case study based on 500 dunes in the pampa de jaguay aeolian transport corridor in coastal southern peru middle to late holocene changes in the supply of aeolian material are inferred from calculating the time of dune initiation at the coastline limitations posed by the assumptions underlying this approach are discussed

incipient subduction of the ontong java plateau along the north solomon trench abstract the ontong java plateau ojp in the western central pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc solomon island arc sia to better understand the evolution of the north solomon trench nst active oblique convergence between the ojp and sia and late neogene development of malaita accretionary prism map we present 850 km of multichannel seismic reflection data integrated with 7832 km 2 of izanagi side scan sonar coverage we have focussed the study at the transition area between the well defined northwestern end of the north solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction the deeper structure beneath the survey area is discussed by phinney et al oceanic plateau accretion in the malaita accretionary prism inferred from multi channel seismic reflection data this issue using deeper penetration multichannel seismic reflection lines the serial cross sections provided by multichannel seismic profiling combined with the izanagi backscattering imagery provides a time series evolution for the development of the north solomon trench the main evolutionary stages include 1 the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust deeper penetration profiles by phinney et al show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust 2 the formation of a continuous thrust front in the central area deeper penetration profiles by phinney et al show this thrust front is surface expression of the same decollement present at depth to the north the boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical high angle tear fault with left lateral offset 3 the formation of a deep elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin the areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the map areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the map

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

inversion of field data in fault tectonics to obtain the regional stress i single phase fault populations a new method of computing the stress tensor summary we attempt a general definition of the inverse problem of computing the components of the regional stress tensor from a set of field data including the measurements of the strike and dip of several faults and the directions and senses of relative motion along these faults as indicated by slickenslides in previous treatments of this problem no experimental errors could be taken into account except those in measuring the pitch of slickenslides thus errors in the orientation of the fault strike and dip which have considerable practical importance were neglected in our work all experimental errors in the field measurements are taken into account so that the agreement between the computed stress tensor and the set of field measurements can be rigorously checked

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

high mg andesites from the southern termination of the new hebrides island arc sw pacific abstract in the southern new hebrides arc magmatic and tectonic processes are closely linked between 21 s and 22 s a normal broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc whereas south of 22 s at the southern termination of the arc a high mg andesite suite appears with the more mafic endmembers having mineralogical and compositional affinities to high ca boninites during the last 2 ma this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the n s spreading axis of the north fiji basin in this region which has been dominated for several million years by fast transform motions the low magma production rates in this section of the arc and the unusual boninite related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones abnormally small depth to slab distances beneath volcanoes and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back arc spreading axis the on going collision between the loyalty ridge and the arc may also contribute to these unusual magmatic characteristics as presently only a very small amount of convergence occurs along the southernmost segment of the trench in such a complex tectonic setting with a spreading axis propagating into an intra oceanic arc and major transform plate motions generation of high ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the hunter ridge the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis the high mg andesite suite probably results via a combination of fractional crystallization from such high ca boninitic parental magmas and subsequent magma mixing processes operative all along the southern termination of the arc

two mantle domains and the time scales of fluid transfer beneath the vanuatu arc u th isotope disequilibria can provide constraints on the time elapsed since fluid addition to the mantle wedge beneath island arcs the vanuatu arc offers new insights into these processes because pb isotopes there are not dominated by components from the subducting plate and so preserve the signatures of the mantle wedge the pb isotope data document the presence of separate pacific and indian mantle domains beneath the arc volcanoes the indian mantle was brought beneath the central part of the arc from the backarc by collision with the d entrecasteaux ridge resulting in a slowing of subduction there the distinction in the mantle wedge composition is also uniquely apparent in u th isotope data which define two subparallel arrays on the u th equiline diagram one anchored to high u th pacific matle and the other to lower u th indian mantle these data provide clrear evidence of the effects of variaible mantle composition on u th isotope disequilibria we argue that such arrays faithfully record the time elapsed since fluid release from the subducting plate the data indicate that this occurred ca 16 ka in the area of collision and slow subduction but ca 60 ka where the rate of subduction is substantially faster this suggests a link between the rate of subduction and the time elapsed since fluid release

effectiveness of natural zeroing of the thermoluminescence in sediments the last exposure to sunlight of detrital feldspars and quartz grains in quaternary sediments can be dated directly by thermoluminescence tl methods but the effectiveness of zeroing of the light sensitive tl signal by daylight varies widely depending upon details of the sedimentation processes to document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures i have collected and analyzed several eolian fluvial marine deltaic and lacustrine samples from less than 100 year old deposits as well as suspensions from sediment plumes also underwater spectral irradiance was measured during recovery of suspensions tl properties of polyrnineralic feldspar dominated 2 4 m and 4 11 m size fractions were measured in the laboratory unexpectedly some light sensitive tl was found to remain in all samples however the light sensitive tl was more effectively zeroed in some samples a proximal loess a sandy braided channel deposit tidal flat and upper delta front muds and thin clayey lacustrine laminae than in others a silty braided channel deposit submarine fan muds and silt bearing lacustrine laminae for the first group the minimum relict paleodose obtained with the partial bleach technique is about 0 4 0 3 gy corresponding to a relict tl age estimate of about 200 years for these sediments thus similar sediments excluding fluvial deposits as young as about 2 ka can be accurately and routinely dated without significant correction for a zero point error if one uses the partial bleach technique coupled with integrated optical bleaching energy fluences of less than 1 j cm 2 and wavelengths only above about 550 nm also some simple optical bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments finally a simple mathematical model is presented that helps account for the gross features of the observed bleaching response curves

the shape of the barchan dunes of southern morocco we present detailed shape measurements of several barchan dunes in southern morocco near la youne using these data we disprove the concept of shape invariance of barchan dunes of different sizes nevertheless some parts of the barchan dune scale and we try to distinguish these from non scaling ones furthermore we point out the importance of the exact position of the brink for the overall dune shape finally we propose a simple model for dune shape using our data to determine the model parameters despite its simplicity this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

origin of a complex and spatially diverse dune field pattern algodones southeastern california the algodones dune field of southeastern california shows a complex and spatially diverse dune field pattern that is superimposed upon a series of topographic lineations analysis of dune field pattern parameters dune crest length crest orientation dune spacing and defect density derived from aerial images indicates that the dune field pattern represents two constructional generations prominent compound crescentic dunes formed during the first constructional generation a younger generation consists of a variety of simple crescentic dunes linear dunes and zibars statistical differences in the pattern parameters between the dune groups within the second generation are resolved through consideration of the boundary conditions under which the dune pattern evolved and provide explanations for 1 diversity of dune types 2 range in implied constructional times 3 range in crest orientations and 4 the anomalous nature of the population of linear dunes the boundary conditions that have modified pattern development include orographic effects grain size vegetation areal extent and antecedent conditions topographic lineations in the algodones range from the western ramp which defines the field margin to subtle features masked by the pattern of dunes imaging of the western ramp using ground penetrating radar shows high angle cross strata migrating perpendicular to the lineation trend the most plausible hypothesis for the origin of the lineations is as dune ridges sequentially shed from adjacent lake cahuilla which is the source of algodones sands the overall geomorphic complexity of the algodones originates from the emplacement of the dune ridges during stages of lake cahuilla the two generations of dune field construction and the controls exerted by boundary conditions

luminescence dating applications in geomorphological research abstract chronological control remains critical to the interpretation of environmental change and to establishing long term rates of geomorphological processes radiocarbon dating will continue to be the central technique for age assessment of carbon bearing sediments deposited in the last ca 40 ka but alternative absolute dating strategies are required for older or non carbon bearing sediments they include uranium series dating of precipitated sediments k ar and ar ar dating of primary volcanic materials and cosmogenic isotopic dating of surface exposure ages an additional family of methods collectively termed luminescence techniques is increasingly providing a basis for the age assessment of detrital sedimentary deposits these methods directly date the deposition of detrital mineral grains typically quartz and feldspar and have been demonstrated to provide accurate deposition chronologies for up to 800 ka although 150 ka is more typical in the past decade a subset of luminescence techniques which employ optical sample stimulation methods called optical dating has made it possible to date samples from a much wider range of depositional systems given that the methods directly date sedimentation they allow direct assessment of depositional dynamism in numerous terrestrial and subaqueous including marine sedimentary systems and have demonstrated particular potential in the elucidation of chronologies from aeolian dune and loess fluvial colluvial coastal and volcaniclastic environments this paper provides a summary of the principles of the luminescence especially optical dating methods and shows how recent developments specifically single aliquot dating procedures have dramatically enhanced the scope and potential of the method the luminescence techniques are increasingly being incorporated into a wide range of geomorphological studies which seek to establish the nature of landscape changes over the past interglacial glacial cycle and beyond

local lithospheric stress distribution deduced by means of the total inversion algorithm and an objective classification method classification of populations of focal mechanisms are commonly used as a step in retrieving states of stress in the lithosphere although such classifications are usually made according to subjective criteria an objective one based on a classification algorithm is presented the procedure is based on two steps first each mechanism is characterized by the components of its principal tensional and compressional axes and then objectively classified accordingly second each mechanism is characterized by its geographic coordinates latitude and longitude after which each group obtained in the first step is divided into new groups according to geographic coordinates the classification algorithm together with a standard inversion algorithm is applied to data from aftershocks belonging to the irpinia southern italy earthquake of november 23 1980 a few incompatible groups of focal mechanisms obtained by the classification algorithm show a spatial overlap nevertheless the new methodology allows us to present a more accurate description of a local state of stress for an aftershock area than the stress pattern obtained from subjective classifications the new stress pattern is compared with principal stress axes deduced from the main event and two local states of stress deduced previously from the aftershock mechanisms

subducted slabs beneath the eastern indonesia tonga region insights from tomography tomographic images of mantle structure beneath the region north and northeast of australia show a number of anomalously fast regions these are interpreted using a recent plate tectonic reconstruction in terms of current and former subduction systems several strong anomalies are related to current subduction the inferred slab lengths and positions are consistent with neogene subduction beneath the new britain and halmahera arcs and at the tonga and the new hebrides trenches where there has been rapid rollback of subduction hinges since about 10 ma there are several deeper flat lying anomalies which are not related to present subduction and we interpret them as former subduction zones overridden by australia since 25ma beneath the bird s head and arafura sea is an anomaly interpreted to be due to north dipping subduction beneath the philippines halmahera arc between 45and 25ma a very large anomaly extending from the papuan peninsula to the new hebrides and from the solomon islands to the east australian margin is interpreted to be the remnant of south dipping subduction beneath the melanesian arc between 45and 25ma this interpretation implies that a flat lying slab can survive for many tens of millions of years at the bottom of the upper mantle in the lower mantle there is a huge anomaly beneath the gulf of carpentaria and east papua new guinea this is located above the position where the tectonic model interprets a change in polarity of subduction from north dipping to south dipping between 45and 25ma we suggest this deep anomaly may be a slab subducted beneath eastern australian during the cretaceous or subducted north of australia during the cenozoic before 45ma the tomography also supports the tectonic interpretation which suggests little neogene subduction beneath western new guinea since no slab is imaged south of the new guinea trench however one subduction zone in the tectonic model and many others that associated with the trobriand trough east of papua new guinea and the miocene maramuni arc is not seen in the tomographic images and may require reconsideration of currently accepted tectonic interpretations 4 2002 elsevier science b v all rights reserved

improvements in the knowledge of the anelastic structure of the iberian peninsula from seismic data of the iliha project summary attenuation coefficients of rayleigh waves propagating across the iberian peninsula and quality factor models were obtained for different two station great circle paths the data used were long period wave trains contained in the seismograms provided by the iliha array installed in the peninsula in the 1980s more than 300 seismograms were analysed to obtain 14 sets of similar paths consisting of either paths totally included in one of the four pure tectonic regions or mixed paths crossing several regions we used the two station method to determine attenuation parameters r and then an inversion method to derive q 1 models the attenuation coefficients of seismic energy in the 10 120 s range are in the range 2 8 10 3 0 1 10 3 km 1 and correlate well with both previous results obtained from analogue wwssn and most seismogeotectonic characteristics of iberia the q models obtained also agree with a few models for iberia determined earlier and they show significant variations from one region to another as a general feature the values are higher in the active seismotectonic regions typical maximum of 2 6 10 3 km 1 than in the more stable ones typical maximum of 1 4 10 3 moreover the maximum of q 1 in the active regions 145 10 3 120 10 3 is higher than in the stable ones 40 10 3 100 10 3 in general both the regionalized attenuation coefficients of rayleigh waves and the quality factor models exhibit a good agreement with previously published lg wave attenuation and coda q distribution patterns in iberia

objective regionalization of rayleigh wave dispersion data by clustering algorithms summary an objective regionalization of a restricted domain of the earth s surface in terms of phase and group velocity data without previous tectonic information is presented we apply our working scheme to local rayleigh wave velocities which were determined from the study of teleseismic events recorded at the broad band stations of the network of autonomously recording seismographs nars installed in the iberian peninsula during the iliha project according to the phase and group velocity maps computed at different wave periods each point of the domain studied is characterized by two dispersion curves corresponding respectively to phase and group velocities the procedure that we propose consists of the application of principal component analysis pca to those velocities defining the local dispersion characteristics of the medium and then an average linkage al algorithm it is worthy of mention that the pca procedure enhances the relationship between local phase and group velocities in this way we obtain a classification of the domain into 10 homogeneous regions each homogeneous area is almost coincident from a seismotectonic point of view with either neogene alpine or hercynian domains after the classification we are able to deduce by stochastic inversion eight different shear velocity structures for eight of the 10 homogeneous regions all the structures deduced suggest a well developed lowvelocity channel from 80 to 180 km in depth for the whole iberian peninsula but with a clearly distinct evolution of their shear velocities with depth some of the models reach shear velocities close to 4 75 km s at depths beneath this channel the highest shear velocities close to 5 0 km s l are obtained at a depth of 50 km and down to 200 km for region 3 neogene and two possible low velocity channels down to a depth of 225 km appear for models 7 and 8 hercynian domains the existence of low velocity channels in some models at depths of less than 30 km is discussed in terms of the resolving kernels and the generation of compatible models in short the main realistic differences between the models come from the shear velocities modelling each low velocity channel as an example the shear velocities for depths ranging from 80 to 180 km vary according to the region from 3 82 km s l to 4 42 km s

plate motions and continental extension at the rifting to spreading transition in woodlark basin papua new guinea can oceanic plate kinematics be extended into continental rifts abstract woodlark basin provides a unique opportunity to compare brittle extension subsidence and extension predicted from long term plate motions at the rifting to spreading transition of a non volcanic margin detailed analysis of reflection seismic data near the current rifting to spreading transition yields 111 km 23 km of brittle extension subsidence along the same transect predicts 115 km 47 km of extension long term plate motions derived from seafloor spreading and transform azimuths in the eastern portion of woodlark basin predict 220 km of extension in this area these results imply no large discrepancy between brittle extension and extension predicted from subsidence contrary to observations at ancient rifted margins however there remains a pronounced two fold discrepancy between post breakup plate motions and extension during rifting this implies that continental extension rates do not match seafloor spreading rates and requires that the extension within the rift can only be modeled using euler pole kinematics after the initiation of seafloor spreading this is hypothesized to result from a decoupling of the lithosphere in which the upper crust moves more slowly than the mantle lithosphere pre breakup plate motions are accommodated primarily by extension of the mantle lithosphere as a result of rollback of the solomon sea plate after the initiation of seafloor spreading spreading rates are controlled by the motion of the mantle lithosphere and the upper crust moves at the same rate as the rest of the lithosphere

arc lavas on both sides of a trench slab window effects at the solomon islands triple junction sw pacific abstract the woodlark spreading center wsc is subducted at the san cristobal trench forming a triple junction at the new georgia group ngg arc in the solomon islands wsc lavas are n morb at 100 km from the trench but with decreasing distance they have increasingly arc like sr nd pb isotopic ratios enrichments in rb k pb sr and depletions in hfse and y within 50 km of the trench on the simbo and ghizo ridges many recovered samples are island arc tholeiites to medium k calc alkaline andesites and dacites and many have the same or similar major and trace element and isotopic characteristics as true arc lavas in the ngg on the other side of the trench previous investigations have concluded that these wsc lavas are the result of relic back arc mantle enrichments resulting from subduction of the pacific plate prior to the late miocene at the north solomon trench 200 km to the north however the high silica wsc lavas are more arc like than those recovered from other distal back arcs and are more voluminous forming large submarine ridges and stratovolcanoes we suggest that true arc mantle migrates across the plate boundary from the adjacent ngg arc through slab windows created by the subduction of the wsc this leads to variable mixing between ngg arc and wsc n morb end members forming the transitional lavas recovered from the wsc lavas with similar arc like characteristics have previously been recovered on the chile rise near where it is subducted at the chile trench raising the possibility that such mantle transfer is a common phenomenon where active spreading centers are subducted the presence of slab windows may also be responsible for the unusual forearc volcanism in the ngg and melting of slab window margins may account for the presence of high silica adakite like lavas on the wsc

barchan ripples under unidirectional water flows in the laboratory formation and planar morphology the purpose of the present study is to investigate the formation and planar shape of barchan ripples generated by laboratory unidirectional water currents and to compare the morphology with that of barchans formed by laboratory waves and flows in natural environments a thin veneer of sand as a sediment source was placed initially at the upstream part of a recirculating flume and the change in bed configurations by the flow was recorded by a video camera two types of formative process were observed one was that barchan ripples grew from barchanoid bedforms and the other was that they developed from a small aggregate of sand particles the barchan ripples in the present test had the same characteristics in the height width relation and in the migration speed as previously reported from the desert environment and wave flume studies an examination of the planar shape of the barchans led to the result that the barchan ripples had larger values of body length width compared with those of barchans formed by water waves in the laboratory and by airflow in natural deserts the horn length width of the barchan ripples was smaller than that of barchan dunes in deserts but larger than that of barchans in oscillatory laboratory flows copyright 2005 john wiley sons ltd

morphology and composition of submarine barchan dunes on the scotian shelf canadian atlantic margin abstract submarine barchan dunes have been mapped on browns bank scotian shelf using multibeam sonar bathymetry and backscatter strength the morphology of subaerial barchans has been studied for almost a century but the advent of multibeam sonar technology now enables a quantitative investigation of their submarine counterparts the browns bank submarine barchans occur at a depth of 60 70 m and are crescentic in planform reaching almost 700 m in horn width and 5 m in height the barchans are convex to the se with steep lee faces to the nw indicating a dominant nw flowing current the barchans overlie a widespread gravel lag covered elsewhere with little or no sand obstacle marks emanate from the lee faces of the barchans and represent a lack of sand deposition and exposure of gravel lag on the sea floor the browns bank submarine barchan sediment texture is gravelly sand or sandy gravel and is primarily composed of subrounded to well rounded quartz grains the allometric relationship between submarine barchan slip face height and distance between horns is markedly different from that of subaerial barchans for the same dune height barchan horn width is about ten times greater in the marine environment the superimposition of megaripples on the stoss slopes of the submarine barchans suggests that the barchans are active and therefore represent an engineering risk to sea floor infrastructure current observations and models indicate that seasonal mean current strength is less than the critical velocity required for barchan migration however the barchans may be active under higher velocity storm induced currents repetitive multibeam sonar mapping is required to detect if barchan migration is occurring over longer time scales

aftershocks of the campania lucania italy earthquake of 23 november 1980 abstract the result of a study of 1130 aftershocks of the 1980 campania lucania earthquake are presented the data were collected primarily with a dense array of portable smoked drum seismographs deployed in the epicentral region a few days after the main shock tests performed on subsets of the data to establish the location accuracy are described in detail and epicentral maps and cross sections are presented and discussed one hundred and sixty well constrained fault plane solutions using upward traveling rays are presented the distribution of aftershocks forms an elongate cloud which does not define a plane this is not because the events are poorly located a subset of events located with epicentral errors of less than 1 km and depth errors less than 2 km forms a similar cloud to the complete aftershock set the aftershock mechanisms are very varied some are similar to the main event but many differ profoundly these include events on shallow or vertical planes and thrust events the latter include some events which require contraction in the direction in which the main event indicates extension the aftershocks of all mechanisms are intermixed in space in such a way that no simple system of faults can explain them a model to explain the aftershocks and the lack of surface ruptures in the main event is offered it is proposed that the main shock cuts between two aseismic detachment surfaces and that the aftershocks are a consequence of the complicated deformation that such a geometry demands whatever interpretation is adopted the results are a challenge to the commonly accepted view that aftershocks even approximately define simple fault planes or that aftershock mechanisms replicate the mechanism of the main shock defects in interpretations of the main shock and aftershocks process arise largely from a lack of information about the geometry of structures at depth in the epicentral region suitable deep reflection profiles could help to resolve this problem

spatial variability and hydrologic evolution of australian lake basins analogue for pleistocene hydrologic change and evaporite formation abstract processes evident in basins across hydrologic gradients from humid to arid regions today provide analogues for systems that may have operated in some individual basins during periods of past hydrologic change thus lakes in humid regions today with low salinity sedimentary facies and with morphology controlled largely by wave action may closely resemble past lacustral pluvial phases experienced in zones which today are relatively arid and saline conversely playas controlled today by groundwater and in some cases subject to defletion may resemble those arid and dune building stages experienced by basins in sub humid regions today as they underwent glacial age desiccation in any such sequence of spatial or temporal gradients from humid to more arid conditions the frequency of basin filling and drying modulates the rate of evaporitive loss of interstitial waters from the exposed basin surface this in turn controls the salinity evolution of the interstitial and local groundwaters the notion of closed systems for terminal lakes while satisfactory for analysis of surface water budgets is quite inapplicable to the evolution of brines where surfacewater groundwater exchange during the high salinity drying stages becomes a controlling factor affecting all salt budgets in these systems evaporites may only be preserved when the sub surface waters have evolved to salinites near saturation for the salts concerned if sub surface waters are frequently diluted by surfacewater groundwater exchange evaporites will not be preserved in the record even though the surface system appears hydrologically closed the process of interstitial water evolution which controls efflorescence deflation and salt weathering exerts a dominating influence on basin geomorphic expression as well as on hydrochemical fractionation

subduction of the woodlark basin at new britain trench solomon islands region abstract the woodlark basin located south of the solomon islands arc region is a young 5 ma oceanic basin that subducts beneath the new britain trench this region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several ma to obtain the image of the subducting slab at the western side of the woodlark basin a 40 day ocean bottom seismometer obs survey was conducted in 1998 to detect the micro seismic activity it was the first time such a survey had been performed in this location and over 600 hypocenters were located the seismic activity is concentrated at the 10 60 km depth range along the plate boundary the upper limit just about coincides with the leading edge of the accretionary wedge the upper limit boundary was identified as the up dip limit of the seismogenic zone whereas the down dip limit of the seismogenic zone was difficult to define the dip angle of the plate at the high seismicity zone was found to average about 30 using the cascadia subduction zone for comparison which is a typical example of a young plate subduction suggests that the subduction of the woodlark basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis 30 km landward from the trench axis furthermore as pointed out by previous researchers the convergent margin of the solomon islands region is imposed with a high stress state probably due to the collision of the ontong java plateau and a rather rapid convergence rate 10 cm year the results of the high angle plate subduction and inner crust earthquakes beneath the shortland basin strongly support the high stress state the collision of the ontong java plateau the relatively rapid convergence rate and moderately cold slab as evidenced by low heat flow rather than the plate age may be dominantly responsible for the geometry of the seismogenic zone in the western part of the woodlark basin subduction zone

an example of stress tensor distribution deduced from the aftershocks of the november 23 1980 southern italy earthquake abstract the total inversion algorithm tarantola and valette 1982 is applied to a set of focal mechanisms corresponding to the aftershocks of the southern italy november 23 1980 earthquake m s 6 9 and the elements of several stress tensors are computed the obtained complex distribution of stress for this region is analyzed and two zones around the main event corresponding to areas of tension and compression distributed at different depths are delimited a stress tensor was sought for a third region corresponding to events which occurred along the strike of the main event and with magnitudes greater than 2 2 but with ambiguous results

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

late quaternary paleoenvironments in the southwestern kalahari abstract landforms of the southwestern kalahari in southern africa include linear dune systems pans and associated lunette dunes and the valleys of ephemeral rivers present day climates in the region are transitional from arid to semi arid making the area sensitive to climatic change the linear dunes are part of an extensive system of fixed dunes developed in the surface kalahari sands the dunes were in existence prior to 32 000 yr b p but have been reactivated on a number of occasions since then in conditions of increased wind velocities more nw winds and reduced rainfall especially in northern and eastern parts of the region periods of dune activity probably occurred at 19 000 17 000 10 000 6000 and 4000 3000 yr b p the development of pans in the region has been strongly influenced by changes in groundwater conditions deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today such periods probably occurred 25 000 20 000 yr b p around 10 000 yr b p and just prior to 1000 yr b p still higher groundwater discharges promoted by increased regional rainfall resulted in the formation of spring tufas and seasonal or permanent lakes in the pans which were also fed by surface runoff during the intervals between 33 000 28 000 23 000 21 000 17 000 15 000 and 12 000 11 000 yr b p the headwaters of the major ephemeral streams lie outside the region but within the same climatic zone increased flow in the molopo river occurred prior to 19 000 and between 15 000 and 13 000 yr b p

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

bedform transport rates for the lowermost mississippi river 1 new methods of data collection and processing are developed to provide quantitative reach scale measurements of bedform transport mass within the tidally influenced mississippi river a multibeam swath profiler was used to collect daily bathymetry over a range of water discharges and bed elevation changes induced by dune migration are measured these values are coupled with bulk physical properties of the bed sediment to constrain mass flux and annual bedform transport is estimated at 2 2 x 10 6 metric tons mt the total annual sand flux from the mississippi river calculated by combining measured bedform transport rates and suspended sediment flux is estimated to be 20 x 10 6 mt survey data also provide information about the spatial distribution of dunes across the channel bottom straight reach segments are commonly mantled by dunes for the entire cross section while bends are typically areas of focused scour devoid of bedforms presumably any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach movement via suspension is therefore an important component for the downriver transport of bed materials in the lower mississippi river

deflation in the dustiest place on earth the bodele depression chad the bodele depression in chad is the largest single source of dust on earth because of a combination of strong near surface winds and a large reservoir of readily eroded and easily entrained low density sediment during the holocene and late pleistocene the bodele depression was host to a great lake palaeolake megachad that is now completely desiccated leaving the lake bed sediments exposed to deflation deflation in the bodele depression is revealed in erosional remnants of lake bed deposits exposed as yardangs on the lake floor erosional remnants indicate that locally up to 4 m of sediment has been removed by deflation optically stimulated luminescence dating shows that this occurred in less than 2400 years giving a rate of deflation of at least 1 6 mm yr 1 a range of other geomorphological features indicate that in some areas rates of deflation may be greater than or equal to 10 mm yr 1 deflation appears to be greatest in an arc around the northeastern margins of the lake basin where the diatomite deposits have been removed and revealed an underlying network of distributary channels preserved as gravel capped ridges the ridges which are part of an inverted topography are interpreted as deltaic distributary channels from an inland delta similar to the okavango delta examples of barchan dunes exist that were flooded by the palaeolake and which have now been exhumed from beneath layers of diatomite the distribution of extensive deflation around the northeasterly upwind end of the bodele depression probably results from a combination of large wind velocities an upwind source of abrasive sand and thinner diatomite deposits around the lake margin satellite image analysis shows that outcropping diatomite covers 24 049 km2 of the bodele depression furthermore the area from which diatomite has been removed by deflation is around 14 000 km2 combining this information with the ages and rates of deflation mentioned above indicates that some 61 000 km3 of diatomite has been eroded from the bodele depression during the past 1000 years

fossil spring deposits in the southern great basin and their implications for changes in water table levels near yucca mountain nevada during quaternary time fossil spring deposits are common in the southern great basin and their distribution provides important constraints on the hydrologic response of the regional water table to climate change this information is crucial because the proposed high level nuclear waste repository at yucca mountain will be located 200 400 m above the modern water table water tables will rise in response to a future return to glacial climates but the magnitude of the change and the consequences for radionuclide travel times and overall repository integrity are key uncertainties increased recharge during past pluvial periods in the spring mountains and sheep range caused water tables to rise and ground water to discharge over broad expanses of the las vegas valley system and in nearby pahrump sandy and coyote springs valleys in contrast other valleys in the region contain only small areas of pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium in these instances which include the valley wells area and piute and northern coyote springs valleys smaller ranges such as the clark and new york mountains supplied the moisture the change in water table levels since the last full glacial period varies between and within valleys from as little as 10 m in several areas to 95 m in the coyote springs valley at yucca mountain the water table has probably changed by 115 m in response to climate change the spring deposits and the mollusk faunas found with them often misinterpreted as lacustrine in origin share many essential features with active spring systems in northeast nevada deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high pale green mud containing a mix of aquatic semiaquatic and moist terrestrial mollusks accumulates in wet meadows and marshes associated directly with spring discharge the record in subbasins of the las vegas valley system is dominated by late wisconsin age sediments although pond sediments and alluvium belonging to at least one older pre wisconsin pluvial period are also locally exposed deposits from two even earlier episodes of spring discharge both of which also occurred during rancholabrean time 10 to 450 ka are exposed in the pahrump chicago piute and coyote springs valleys and in the valley wells the records from the pahrump and coyote springs valleys are especially well exposed and likely extend back to at least early pleistocene time

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

accelerating late quaternary uplift of the new georgia island group solomon island arc in response to subduction of the recently active woodlark spreading center and coleman seamount abstract the new georgia island group of the solomon islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc we have used coral reef terraces paleobathymetry of neogene sedimentary rocks and existing marine geophysical data to constrain patterns of regional quaternary deformation related to subduction of the recently active woodlark spreading center and its overlying coleman seamount these combined data indicate the following vertical tectonic history for the central part of the new georgia island group 1 subsidence of the forearc region tetepare and rendova islands to water depths of 1500 m and deposition of marine turbidites until after 270 ka 2 late quaternary uplift of the forearc to sea level and erosion of an unconformity 3 subsidence of the forearc to 500 m bsl and deposition of bathyal sediments and 4 uplift of the forearc above sea level with holocene uplift rates up to at least 7 5 mm yr on tetepare and 5 mm yr on rendova in the northeastern part of the new georgia island group our combined data indicate a slightly different tectonic history characterized by lower amplitude vertical motions and a more recent change from subsidence to uplift barrier reefs formed around new georgia and vangunu islands as they subsided 300 m by 50 100 ka subsidence was replaced by uplift that accelerated to holocene rates of 1 mm yr on the volcanic arc compared with rates up to 7 5 mm yr in the forearc area of tetepare and rendova uplift mechanisms such as thermal effects due to subduction of spreading ridges tectonic erosion or underplating of deeply subducted bathymetric features are not likely to function on the 270 ka period that these uplift events have occurred in the new georgia island group a more likely uplift mechanism for the post 270 ka accelerating uplift of the forearc and volcanic arc of the new georgia island group is progressive impingement of the coleman seamount or other topographically prominent features on the subducting plate regional effects we relate to this ongoing subduction related process include 1 late quaternary post 270 ka accelerating uplift of the rendova tetepare forearc area in response to initial impingement of the coleman seamount followed by exponentially increasing collisional contact between the forearc and seamount 2 later quaternary propagation of uplift arcward to include the volcanic arc as the area of collisional contact between the forearc and seamount increased and 3 large wavelength folding that has produced regional variations in late holocene uplift rates observed in both forearc southern rendova tetepare and volcanic arc new georgia island areas we propose that the dominant tectonic effect of coleman seamount impingement is horizontal shortening of the forearc and arc crust that is produced by strong coupling between the subducting seamount and the unsedimented crystalline forearc of the new georgia island group the horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topography may have terminated spreading of the woodlark spreading center entering the trench ghizo ridge and converted it to a presently active strike slip fault zone

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

rheologic controls on the dynamics of slab detachment abstract the detachment of subducted lithosphere slabs has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution for example along strike progression of uplift in the adriatic sea and west to east progression of volcanism in the trans mexican volcanic belt may be caused by propagating detachment of subducted slabs an understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations we demonstrate using two dimensional 2d numerical models that the timing occurrence and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness i e stress supported viscously of the lithosphere and rheology of the upper mantle in particular in models with a newtonian upper mantle where plastic yielding is the only weakening process within the slab detachment does not occur models accounting for non newtonian upper mantle deformation demonstrate two modes of detachment 1 slow and deep detachment of strong slabs 500 mpa or greater maximum yield strength controlled by heating of the slab and 2 fast and shallow detachment of weaker slabs 300 mpa controlled by yielding within the slab interior for slabs with a maximum yield strength of 500 mpa the time to detachment increases with slab age for ages ranging from 40 to 120 my indicating that detachment time is controlled by the integrated stiffness of the slab which limits the sinking rate however the detachment time for weaker slabs 300 mpa is independent of age because weaker slabs sink rapidly and the time to detachment is limited by the rate of flow of the surrounding mantle

late quaternary hydrological dynamics in the middle kalahari forcing and feedbacks the kalahari region has become a major source of quaternary palaeoenvironmental data derived primarily from the analysis of geomorphological proxies of environmental change one suite of data from palaeolacustrine landforms has recently provided a new record of major hydrological changes in the last 150 ka burrough s l thomas d s g bailey r m 2009 mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system quaternary science reviews in press here we present an improved analysis of the drivers and feedbacks of lake level change utilising information from three main sources data from the lake system itself from analyses of other late quaternary records within the region and from climate modelling simulations using the hadley centre coupled climate model hadcm3 suggest that once triggered the lake body was large enough to potentially affect both local and regional climates surface waters and their interactions with the climate are therefore an important component of environmental dynamics during the late quaternary through its capacity to couple middle kalahari environments to distant forcing mechanisms and to itself force environmental change we demonstrate that the existence or absence of megalake makgadikgadi adds a new level of complexity to the interpretations of environmental proxy records in southern africa s summer rainfall zone

interpreting geoproxies of late quaternary climate change in african drylands implications for understanding environmental change and early human behaviour abstract climate and environmental change in the late quaternary are widely regarded as key drivers of early human development and dispersal in africa robust records of change are required to assess the nature of potential impacts today s late quaternary palaeoenvironmental palaeoclimate reconstructions from the interior southern african basin kalahari are more spatially detailed have a stronger chronometric underpinning and span a longer time range than those of 10 20 years ago but are no less controversial reconstructions for the last 150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid latitude westerlies with more data than ever before why then is the record not yet delivering a more coherent picture of regional climate change this paper deliberately addresses current challenges and controversies including issues often ignored or neglected with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future sources of palaeo data from the interior are predominantly geoproxies reconstructions not based on highly resolved organic or isotope records but on interpretation of spatially extensive geomorphological proxies of past environments despite being increasingly datable using osl their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources consequently they are sometimes removed from regional syntheses of climate change these omissions are often replaced by spatially exaggerated interpretations of higher quality local records allowing more spatially coherent timeslice interpretations to be presented yet this is counter intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies or in climatic and environmental systems today in addition the quest to link regional records to master records from ice and ocean cores can force interpretation within a temporal scale that may be inappropriate for their analysis underplaying the reality of complexity within the environmental system there are not necessarily good or bad records of terrestrial palaeoenvironments in interior southern africa rather there are records that contain different suites of information about past land surface hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable consideration of these factors may resolve apparent conflicts between datasets resulting in a better understanding of past environmental dynamics and revealing the nature of environmental variability which may be a key facet to which human development has responded

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

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

barchan shaped ripple marks in a wave flume barchans isolated crescent shaped bedforms are believed to be formed under almost unidirectional wind or water ows and limited sand supply the formation of barchan morphologies under the action of purely oscillatory wave motion has not yet been fully investigated the present study attempted to form barchan topography in a wave ume and to compare this with barchans in the eld barchan morphologies of ripple size called the barchan ripples were generated from a at bed by the action of waves the horn width the distance between horn tips of the barchan ripples increased linearly with an increase in the total length the overall length projected on the centre line of the barchan with a coef cient common to barchan dunes in deserts the ratio of horn length to horn width of the barchan ripples was smaller than that of barchan dunes but similar to that of subaqueous barchans in the eld the longer the wave period was the larger the ratio of the body length to horn width became most subaqueous barchans formed under waves in the laboratory and unidirectional ows in the eld had blunter horns than subaerial barchans the shape of the barchan ripples changed with wave period the outer rim became rounder with increasing wave period the relationship between the base area and the height of barchan morphologies seems to be linear with a constant coef cient for the scale from ripples to dunes the barchan ripples showed a linear relationship between the height and the horn width similar to that for barchan dunes the migration speed of the barchan ripples was proportional to the cube of the ow velocity and was inversely proportional to height the same relation with a different value of the coef cient was obtained for barchan dunes copyright 2004 john wiley sons ltd

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

complex holocene lunette dune development south africa implications for paleoclimate and models of pan development in arid regions closed depressions pan or playa basins are very common landscape features in arid or formerly arid regions many have associated downwind dune assemblages lunettes that have been explained in terms of deflation of sediments from the basin these widespread features have also been used as proxies of late quaternary rainfall and atmospheric circulation change we use a detailed program of optically stimulated luminescence dating at witpan south africa to show that lunette construction may be rapid and spatially complex this study presents 33 ages from 9 sample sites on the dune that show how accumulation of the present dune largely occurred over the past 2 k y but with marked sectoral variability in accumulation rates and timing variability is attributed largely to local sediment supply factors geochemical analysis shows that primary deflation from the pan has not contributed significantly to the construction of much of the current lunette most sand is derived from recycling of older lunette sediments preserved today at only a few locations and from neighboring linear dunes these findings if representative of lunettes in other localities have marked implications for the types of paleoclimatic information that these features may yield

recharge to the inter dune lakes and holocene climatic changes in the badain jaran desert western china we present new estimates on evaporation and groundwater recharge in the badain jaran desert western inner mongolia of northwestern china based on a modified penman equation suitable for lakes in china geochemical data and water balance calculations suggest that local rainfall makes a significant contribution to groundwater recharge and that past lake level variations in this desert environment should reflect palaeoclimatic changes the chronology of lake level change established by radiocarbon and u series disequilibrium dating methods indicates high lake levels and a wetter climate beginning at ca 10 ka and lasting until the late mid holocene in the badain jaran desert the greatest extension of lakes in the inter dune depressions indicates that the water availability was greatest during the mid holocene relicts of neolithic tools and pottery of qijia culture 2400 1900 bc suggest relatively intensive human activity in the badain jaran desert during the early and middle holocene supporting our interpretation of a less harsh environment wetter climates during the holocene were likely triggered by an intensified east asian summer monsoon associated with strong insolation

implications of principal stress axes and eigenvalue ratios on critical orientation of fractures application to two tectonic regions in alaska the principal stress axes and eigenvalue ratios of the stress tensors from two active seismic regions in alaska eastern aleutian arc and eastern gulf of alaska are computed these results are obtained using focal mechanisms of earthquakes with magnitudes ranging from 5 5 to 8 2 and show for both regions slightly different compressional states with an orientation of the pressure axis of n26 w for the eastern gulf of alaska and n27 w for the east aleutian arc the eigenvalue ratios for the eastern gulf of alaska and east aleutian arc are 0 84 and 0 99 respectively the algorithm used in computations allows us to detect some focal mechanisms which are incoherent with respect to the obtained stress tensors these incoherences are explained in terms of some tectonic features of the region the orientations of possible pre existing fractures which would need an unrealistically high maximum shear stress to start a slip on their fault planes are also investigated these critical orientations depend on a constitutive frictional law as well as on eigenvalue ratios and eigenvectors of the stress tensors the orientation which is given in our case by the angle between the normal vector to a fracture and the regional tensional axis is approximately 59 for the eastern aleutian arc this angle ranges from 49 to 59 for the eastern gulf of alaska it must be pointed out that fractures which need unrealistically high shear stress to start slip on their fault planes are defined by a very narrow band of possible angles between tensional direction and the normal vector to fault plane

the eruptive history and volcanic hazards of savo solomon islands savo island is the 6 km diameter emergent summit of an andesitic dacitic stratovolcano rising from the iron bottom sound 35 km nw of honiara solomon islands savo has erupted at least three times within recorded history and the 3 000 inhabitants maintain extensive oral traditions of past events through description and interpretation of the volcaniclastic sequences on the island in conjunction with historical accounts and oral traditions we reconstruct the eruptive processes on savo block and ash flow baf deposits are volumetrically dominant on the island within three main depositional environments near vent sequences thick medial channel sequences and distal fan sequences the deposits comprise universally non vesicular and highly porphyritic 40 70 phenocrysts high silica andesite and dacite clasts these appear to have been derived from collapsing lava domes during an 1560 1570 a d eruption however eyewitness descriptions and crater morphology suggest that similar deposits formed from dome explosions or collapses of eruption columns during later eruptions 1830 1840 a d the high sodium magmas ca 5 7 wt na2o apparently crystallised and strongly degassed prior to eruption shallow explosions were possibly caused by entrapment of magmatic gases beneath a dome or conduit plug of highly crystalline near solid magma repeated sealing of the vent may have been due to inward collapse of the highly altered rocks of the surrounding hydrothermal system these rocks probably were saturated due to contemporaneous high intensity rainfall events bafs were hot enough to char vegetation and attain aligned clast trm thermal remnant magnetism up to 3 km from the vent many being accompanied by ash cloud surges changes with distance in the baf deposits appear mostly dependent on flow confinement and are limited to an overall decrease in thickness and maximum clast size and an increased definition of weak planar fabrics in distal fan sequences there is strong evidence for syn and post eruptive redeposition of primary deposits since the savo population is concentrated on coastal volcaniclastic fans we consider the greatest volcanic risk to life is from bafs associated ash cloud surges and lahars hence the main channels and fans are designated as the highest of three relative hazard zones on a simple map prepared to aid local education and planning initiatives on savo

pan floor barchan mounds ntwetwe pan makgadikgadi botswana their origin and palaeoclimatic implications abstract the variety of forms assumed to be dunes and palaeolakes of northern botswana have long been perceived as geoproxies for the elucidation of quaternary palaeoclimates mounds on the floor of ntwetwe pan have been regarded as modified barchans pertaining to a time in the past when conditions were drier than at present the morphology hydrogeology and biology of these placed in the wider context of the palaeolacustrine hinterland indicate that they are not dunes but subaqueous features spring mounds the morphology is inconsistent with barchans conspicuous features are the elevated eyes within the enclosing arms of the crescentic forms that occur furthest from the pan edge seepages discharge from their flanks commonly about 7 m higher than the dry saline pan floor on the top of forms nearer the pan margins permanent pools of potable water occur similarly elevated above the floor hydraulic pressure to sustain the upward discharge of groundwater is provided by the excellent recharge through silicate karst depressions in the hinterland particularly those of the 936 m palaeolake floor groundwater discharge still operating today was formerly much stronger and associated with lacustrine conditions a chronological framework is provided by archaeological remains on the palaeolake floors of the hinterland these indicate that the oldest palaeolake floor pertaining to the quaternary is the 920 m level periods of spring mound formation are associated with both this and the 912 m palaeolake the implications for future luminescence dating targeting palaeoenvironmental reconstruction are discussed in the light of this new model of mound formation

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

the southern italy earthquake of 23 november 1980 an unusual pattern of faulting abstract a complex rupture history for the 23 november 1980 magnitude 6 9 italian earthquake resulted in an unusual pattern of two high angle subperpendicular normal faults forming the corner of a down dropped crustal block evidence for this interpretation is from aftershocks the pattern of strong ground motion focal mechanism of the mainshock and two critically placed leveling profiles quantitative modeling of leveling profiles using smoothly varying dislocation segments provides confirmation of the fault pattern and specific verification of the existence of a secondary fault which is suborthogonal to the primary fault rupture began on the nw trending main fault with downward motion of the northeast block about 40 sec after initial motion began rupture initiated on an orthogonal ne trending subsidiary fault propagating away from the main fault this fault pattern is consistent with tectonic extension in a northeasterly direction

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

anelastic structure of the iberian peninsula obtained from an automated regionalization algorithm and stochastic inversion abstract an automated regionalization of a reduced domain of the earth s surface without a priori seismotectonic information and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of rayleigh waves our database is formed by path averaged attenuation coefficients derived from fundamental mode rayleigh waves crossing the iberian peninsula the wavetrains were recorded at the broad band stations installed some years ago in the iberian peninsula for the iliha project before the sequential application of the algorithms the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10 120 s period range all these maps were obtained by applying yanovskaya s formulation for laterally heterogeneous media to the set of path averaged attenuation coefficients mentioned above after that we divided the iberian peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm we then obtained by stochastic inversion the respective anelastic structure down to a depth of 225 km for the six homogeneous regions the coefficients vary from 1 0 to 2 0 10 3 km 1 and the q 1 structures ranging from 110 10 3 to 40 10 3 suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth respectively a shallow beginning of the asthenosphere is detected for the sixth homogeneous region this range is very similar to the depths deduced from previous tomographic studies of the iberian peninsula based on rayleigh wave phase and group velocities finally correlations obtained between the regions and some seismotectonic characteristics of the peninsula are discussed it is noteworthy that we cannot associate each of the six homogeneous regions with a single hercynian alpine or neogene domain

geochemistry of basalts from the western woodlark lau and manus basins implications for their petrogenesis and source rock compositions abstract geochemical compositions are reported for back arc and marginal basin basalts sampled during the 1990 cruise of r v akademik mstislav keldysh in the lau basin north lau ln king s triple junction lk central lau lc east lau le manus basin red star region mrs and western woodlark basin dobu seamount region wd continental rifting segment east basin region we the boundary between continental and spreading segments and franklin seamount region wf spreading rifting segment two main geochemical types of basalts have been distinguished within the lau and manus back arc marginal basins i depleted basalts comparable with n morb lavas lc le mrs and ii enriched basalt lavas ln lk vesicular pillow basalts lk vpb correspond to typical babb the sources of melts were depleted spinel or plagioclase lherzolite n morb type for lc le mrs basalts the n morb source was metasomatized by adding lile and volatiles arc source components for enriched lavas direct observations from mir submersibles in the area of the king s triple junction lau basin established that lile depleted massive sheet flow basalts overlie vesicular pillow basalts the western woodlark basin is a tectonic transition zone between typical oceanic spreading in the central part of the basin and continental rifting in easternmost papua new guinea the wd basalts show strong lile lree p and zr enrichment and are spatially associated with alkaline rhyolites the we and wf lavas demonstrate moderate enrichment in these elements the western woodlark lavas formed from multiple sources including n morb mantle arc source components and enriched mantle like oib

paleolandscape and paleoenvironmental interpretation of spring deposited sediments in dakhleh oasis western desert of egypt abstract iron rich sediments in dakhleh oasis western desert of egypt have been recognized as spring mounds and as archaeological sites where paleolithic materials have been recovered detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron rich nubian aquifer waters along the southern margin of the oasis controlled largely by localized faulting and the variable paleotopography of the basin floor the spring sediments represent peri lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during pleistocene pluvial activity principally resulted in tufas and lacustrine marls spring mound formation incorporated sediment through the ponding of surface water aeolian entrapment by local vegetation and the formation of iron precipitates in a low energy oxidative and acidic environment that was not dependent upon surface water inputs into the dakhleh basin the potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin the dakhleh spring mounds record a unique groundwater controlled paleoenvironment providing the first evidence of a bog iron in egypt and one of the few occurrences of iron rich wetland remnants in the modern sahara

width height and slope relationships and aerodynamic maintenance of barchans the allometric relationship between the height of barchan dunes and horn to horn widths is re examined the addition of new data from barchans in the namib desert and elsewhere confirms the relationship that the height of a barchan is approximately one tenth of the width of the dune height and width of the dune are closely linked to the three dimensional morphology of the dune in particular the relatively constant mean angles of around 11 at which the side slopes of the dune are maintained we suggest that the 3 d barchan form is directly controlled by aerodynamic processes constant feedback between three dimensional stoss face windward shape of the dune slipface height and horn to horn width and shear stress produces an equilibrium dune form

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

deep slab subduction and dehydration and their geodynamic consequences evidence from seismology and mineral physics abstract we present new pieces of evidence from seismology and mineral physics for the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the pacific and philippine sea slabs under western pacific and east asia the pacific slab is subducting beneath the japan islands and japan sea with intermediate depth and deep earthquakes down to 600 km depth under the east asia margin and the slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant pacific slab is roughly coincident with the ne sw daxing anling taihangshan gravity lineament located west of beijing approximately 2000 km away from the japan trench the upper mantle above the stagnant slab under east asia forms a big mantle wedge bmw corner flow in the bmw and deep slab dehydration may have caused asthenospheric upwelling lithospheric thinning continental rift systems and intraplate volcanism in northeast asia the philippine sea slab has subducted down to the mantle transition zone depth under western japan and ryukyu back arc though the seismicity within the slab occurs only down to 200 300 km depths combining with the corner flow in the mantle wedge deep dehydration of the subducting pacific slab has affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan slow anomalies are also found in the mantle under the subducting pacific slab which may represent small mantle plumes or hot upwelling associated with the deep slab subduction slab dehydration may also take place after a continental plate subducts into the mantle

subduction zone dynamics in the sw pacific plate boundary region constrained from high precision pb isotope data this study presents high precision pb isotope data obtained by mc icp ms for lavas from the solomon island arc that covers ca 1000 km of the sw pacific plate boundary following a reversal of subduction polarity the presence of the old subducted pacific oceanic crust and the presently subducting indian australian plate beneath the entire island arc is confirmed by geophysical and geochemical evidence new high precision pb isotope data now permit to distinguish more clearly between indian and pacific type subduction components in this complex plate tectonic constellation the values range from 206pb 204pb 18 345 to 18 845 207pb 204pb 15 498 to 15 545 and 208pb 204pb 38 120 to 38 372 indicating the absence of pelagic sediments in the sub arc mantle these data reveal a predominance of subduction components derived from ancient subduction of the pacific plate in contrast lavas with indian type pb isotope signatures are confined to the central new georgia group of the solomon islands where the young woodlark ridge spreading centre is currently subducting all other solomon arc segments with a lower geothermal gradient have apparently not yet been overprinted by younger subduction components over the past 6 myrs moreover trace element and pb isotope data for woodlark ridge basalts show that subduction components from the fossil pacific slab have locally modified the source of woodlark basin lavas altogether our study highlights the potential of high resolution pb isotope measurements to distinguish between indian and pacific type subduction components along the sw pacific plate boundary

multiphase quaternary highstands at lake ngami kalahari northern botswana large lake basins now almost entirely devoid of water in the kalahari s semi arid environment can be clearly identified from landsat imagery of northern botswana these basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that at its largest extent encompassed 60 000 km 2 in the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the palaeo makgadikgadi system consisting of the ngami mababe and makgadikgadi basins thepaucityofwellpreservedorganicmaterialintheregionhasledtothese 14 cagesbeingderivedfrominorganiccarbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method the application of optically stimulated luminescence osl dating to beach ridge sediments from lake ngami shaw p a bateman m d thomas d s g davies f 2003 holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change quaternary international 111 1 23 35 improved chronological resolution but was limited by the use of hand dug pits the new application of osl to samples derived from deep coringofbeachridgeshasallowedaccesstoamuchlonger threedimensional geomorphicrecordthanpreviouslyavailable seventyfour ages new osl ages from shorelines are presented allowing for the first time the opportunity to unravel the complex history of lake stages in the ngami basin multiple lake highstands are identified in the period 0 3 ka to 140 ka with several causes of these events includingtropicalandlocalclimatesandupstream channelavulsion identified tounravelthefullpaleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the okavango system 2007 elsevier b v all rights reserved

the morphology of barchan shaped sand banks from western torres strait northern australia abstract large barchan shaped sand deposits have been observed in the north west of torres strait these deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks a study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns 10 15 m west per year and that sediment is shed from their horns features that are characteristic of barchan dunes however the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually evasive channels and circulation of sediment around the sand bank a characteristic of subaqueous sand banks the presence of mutually evasive channels is the criteria used to categorise the deposits as sand banks barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes in the torres strait both these criteria are met previous work has demonstrated the presence of a net westward current through the torres strait that is driven by the southeast trade winds the relatively high displacement of the wind driven currents during the trade wind season relative to the monsoon appears to provide the necessary unidirectional regime to form barchans the low and typically eastwards displacement of the residual monsoon season current appears to have a negligible affect on the barchan form while seasonal wind driven currents appear to maintain the barchan shape of the sand banks tidal currents actively maintain mutually evasive channels observed by variations in dune orientation on the sand banks a sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form

geophysical and morpho tectonic study of the transition between seafloor spreading and continental rifting western woodlark basin papua new guinea abstract two major morpho tectonic domains separated by a major transfer zone are described at the transition between seafloor spreading and continental rifting in the western woodlark basin off shore eastern papua new guinea the oceanic domain comprises new oceanic crust formed during the bruhnes epoch older transitional crust and the rifted continental margins two rift branches are recognized within the continental domain the southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crustal accretion based on magnetic data seafloor spreading in the western part of the woodlark basin commenced between 3 5 and 2 5 ma along the northern margin of the basin and maintained its position adjacent to this margin up to 0 8 ma jaramillo event just prior to the bruhnes epoch the seafloor spreading axis jumped southward to its current position frequent jumps of seafloor spreading centers and relocation of paleoaxes indicate the instability of the woodlark extensional system the transition between the two domains is characterized by differential localization of extensional strain variations in the localization can result from progressive change of extensional mode when the initial rifting evolves into seafloor spreading and or from differences in lithospheric rheology as the rift propagates into the continental margin a consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced across the transfer zone by broadly distributed deformation within continental lithosphere

a great earthquake rupture across a rapidly evolving three plate boundary on 1 april 2007 a great tsunamigenic earthquake moment magnitude 8 1 ruptured the solomon islands subduction zone at the triple junction where the australia and solomon sea woodlark basin plates simultaneously underthrust the pacific plate with different slip directions the associated abrupt change in slip direction during the great earthquake drove convergent anelastic deformation of the upper pacific plate which generated localized uplift in the forearc above the subducting simbo fault potentially amplifying local tsunami amplitude elastic deformation during the seismic cycle appears to be primarily accommodated by the overriding pacific forearc this earthquake demonstrates the seismogenic potential of extremely young subducting oceanic lithosphere the ability of ruptures to traverse substantial geologic boundaries and the consequences of complex coseismic slip for uplift and tsunamigenesis

elastic anelastic regional structures for the iberian peninsula obtained from a rayleigh wave tomography and a causal uncoupled inversion the elastic and anelastic structure of the lithosphere and asthenosphere of the iberian peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of rayleigh wave fundamental mode the database consists of surface wavetrains recorded at the broadband stations located in the iberian peninsula on the occasion of the iliha project path averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and subsequently local dispersion curves were computed according to the yanovskaya ditmar formulation first a principal component analysis pca and the average linkage al clustering algorithm are applied to these local values in order to classify the iberian peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves without previous seismotectonic constraints second averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere this purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients the main features of the homogeneous regions are discussed by taking as reference the hercynic alpine and neogene domains of the iberian peninsula and two questions affecting the reliability of the elastic anelastic models are revised first the coherence of the shear velocity and q 1 models obtained by causal uncoupled inversion for each region is analysed second the influence of the causal phase and group velocities on the shear velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non causal inversion

vegetation induced subsurface precipitation of carbonate as an aggradational process in the permanent swamps of the okavango delta fan botswana abstract islands constitute an important geomorphological component of the permanent swamps of the okavango alluvial fan studies of the topography soil chemistry groundwater chemistry and vegetation cover across several islands indicate that many islands form as a result of the subsurface precipitation of calcite and probably amorphous silica which produces vertical expansion creating topographic relief the main agent responsible for this precipitation is transpiration especially by large deep rooted trees which increases the salinity of the groundwater leading to saturation in calcite and silica the process ultimately leads to the development of highly saline groundwater and results in marked zonation of vegetation across the islands numerical modelling of the process suggests that salinization of the groundwater beneath an island takes between 100 and 200 yr high calcite contents in island soils appear to be the product of several cycles of salinization which are interspersed with periods of swamp abandonment during which the more soluble alkali salts are leached from the islands island formation represents an important process in the overall aggradation of the alluvial fan

the campania lucania southern italy earthquake of 23 november 1980 abstract we present the main seismological results of our study of the campania lucania earthquake of 23 november 1980 a complete set of far field and local data has been analysed from long period body waves data we determine the fault plane solution 1 140 1 60 2 75 2 54 a depth of 15 km and calculate a seismic moment of 6 1025 dyne cm and a source duration of 6 s from data of a local network deployed immediately after the event we determine aftershock locations they are aligned in a direction nw se that fit extremely well with the focal solution determined above we can choose as fault plane the plane striking 140 and dipping at 60 and the event is a normal event with a large component of left lateral strike slip the source area evaluated from this aftershock distribution 14 km 40 km is quite suitable for an earthquake of a seismic moment of 6 1025 dyne cm

the ion chemistry of lakes and late holocene desiccation in the badain jaran desert inner mongolia china abstract in the markedly arid badain jaran desert one of the least studied areas in the world over a hundred permanent lakes exist among the dunes the geochemistry of water samples from nine lakes and from the groundwater in the desert and on its periphery was investigated ion chemistry of the water from these desert lakes changes considerably on a regional scale we argue that there is a positive correlation between the ages of these lakes and their totally dissolved solids tds varying between 1 2 and 398 2 g l tritium analysis of groundwater in wells gives an age generally older than four decades and younger than 100 years the lacustrine deposits freshwater fauna and shorelines indicate the existence of large water bodies with low salinities in the inter dune hollows of this desert during the early and middle holocene the precipitation in this desert during the early and middle holocene was estimated to be ca 200 mm year on average at least twice as high as at present on the basis of paleoecology tl and radiocarbon ages it was concluded that the huge enrichment of ions in the lakes and decrease of water availability originated from climatic desiccation during the last 4000 years the paleoclimate of the badain jaran desert reported here was remarkably consistent with the late quaternary climatic histories of the regions to the immediate north west east and south of the study area

controls on magmatism in an island arc environment study of lavas and sub arc xenoliths from the tabar lihir tanga feni island chain papua new guinea the tabar lihir tanga feni tltf islands of papua new guinea mainly comprise high k calc alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction related magmatism numerous sedimentary mafic and ultramafic xenoliths recovered from tubaf seamount located on the flank of lihir island provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region the sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with pacific affinity a majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the tltf arc leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50 or more of elements such as cu zn rb u pb and light ree rare earth elements most likely introduced in the xenoliths via hydrous fluids released from a subducted slab the leaching experiments demonstrated that the light ree enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit ree patterns that range from flat to light ree depleted sr nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87sr 86sr with decreasing 143nd 144nd ratios the labile phases in the ultramafic xenoliths represented by the leachates show decoupling between sr and nd with distinctly more radiogenic 87sr 86sr than the residues both leachates and residues exhibit very wide range in their pb isotopic compositions indicating the involvement of three components in the mantle wedge under the tltf islands two of the components can be identified as pacific oceanic mantle and pacific sediments some of the ultramafic samples and clinopyroxene separates however exhibit relatively low 206pb 204pb at elevated 207pb 204pb suggesting that the third component is either indian ocean type mantle or australian subcontinental lithospheric mantle geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized it did not significantly contribute to the isotopic and incompatible trace element compositions of tltf lavas compared to the mantle samples the tltf lavas have very restricted pb isotopic compositions that lie within the pacific morb range indicating that magma compositions were dominated by melts released from a stalled subducted slab with pacific morb affinity interaction of slab melts with depleted peridotitic component in the mantle wedge followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

from landform to process morphology and formation of lake bed barchan dunes makgadikgadi botswana abstract a suite of crescentic landforms is visible from remotely sensed imagery within the ntwetwe panpan in the makgadikgadi basin botswana we investigate the most distinct of these landforms using morphometric measurements sedimentary data and optically stimulated luminescence osl signal analysis comparative analysis with previously published barchan morphological data sets suggest the ntwetwe features fall within the spectrum of morphometric parameters found in a range of barchan dunefields from around the world there is currently insufficient comparative morphometric data from sub aqueous dunefields to be able to distinguish the particular formative environment of the dune osl signal analyses however support the hypothesis of grove 1969 grove a t 1969 landforms and climatic change in the kalahari and ngamiland geographical journal 135 191 212 that the last deposition of the sediments within the ntwetwe forms was most likely aeolian in origin luminescence signal analysis is employed to investigate potential transport and bleaching environments of the sediments forming the features but results in this case do not shed further light on the formative conditions of these enigmatic landforms

duricrust development and valley evolution process landform links in the kalahari duricrusts are an important landscape component of the kalahari region of central southern africa their exposures within the dry valleys mekgacha of the kalahari provide some of the most widespread surface outcrops of the terrestrial jurassic to holocene kalahari group sediments exposures have been extensively used in the construction of lithostrati graphic sequences on the assumption that valley systems have incised their courses through a pre existing duricrust sequence recent work however has identified the role of groundwater erosion processes in valley development which may have influenced duricrust formation studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the presence of a valley analyses of calcretes and silcretes in a series of profiles and thin sections from the letlhakeng area of botswana also indicate extensive alteration and diagenesis in association with former higher water tables sedimentary sequences within duricrust host materials can be identified but there is no evidence for correlation of duricrust cements between exposures profile studies from the auob valley in namibia however suggest that this valley has incised through a sequence of duricrusts caution is advised in future attempts to correlate duricrust types on the basis of valley exposures with the recommendation that where such exposures are used in a lithostratigraphic context only duricrust host material characteristics and not cementing materials should be considered

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

aeolian processes and sand encroachment problems in kuwait abstract the surface of kuwait is carved in a calcretized clastic sequence of miocene pleistocene age mostly covered by a thin blanket of recent aeolian deposits fast rate of development and increase of human activities contributed to denudation of the vegetative cover and exposure of dry loose sediments to wind action at present kuwait is intensively subjected to aeolian actions manifested by frequent occurrences of sand and dust storms this dictated the need for regional understanding of the magnitude of the aeolian processes their behavior with man made establishments and their long and short term impacts a comparison between the nature and distribution of recent surface sediments in kuwait in 1979 and 1990 shows significant changes that reflect the work of aeolian processes during this period the direct impact of the enhancement of aeolian action is represented by the increase in the rate of sediment transport transportation of fine grained particles in suspension frequently occurs during dust storms usually initiated in southern iraq the particles migrate over kuwait as thick dust clouds and finally settle in the northern part of the arabian gulf bedload transport of sand or saltation frequently occurs during summer whenever wind speed reaches 5 4 m s the average annual sand drift rate in kuwait is about 20 m 3 m width 1 yr 1 the majority of the drift occurring during the period may august to the southeast bedform transport or sand dune movement is also recognized in kuwait sand dunes mostly occur as small barchans of about 3 m height the measured rate of movement of these barchans varies with height a barchan of average height usually moves about 20 m in nine monthes january august the remarkable high rate of sand transport and the increase in development activities in the desert areas are responsible for the enhancement of the sand encroachment problems in kuwait almost all installations roads and farms are significantly affected by the accumulation of considerable amounts of aeolian sand these problems have pronounced adverse environmental and economic impacts

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change abstract lake ngami a 3000 km 2 basin at the distal end of the okavango delta botswana has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl linked to topographic thresholds and inflows current and fossil in the okavango system the lake has been ephemeral throughout the 20th century and completely dry since 1982 yet held a substantial body of water up to a level of 930 m asl when visited by european travellers in the mid nineteenth century the historical decline in the lake has been well documented and used as evidence for regional desiccation but the holocene record based on archaeological studies and radiocarbon dating of inorganic carbonates is more speculative a study of a 30 km 2 diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the holocene linked to a now abandoned inflow of the kunyere river a programme of 18 luminescence age determinations of the peripheral dautsa and magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90 100 ka or earlier and have been modified by holocene fluctuations in lake level and subsequent aeolian reworking the water level peaked at 936 m asl between 4 and 3 ka returned to 934 m at 2 8 2 ka ponding water into the phatane gap and fell rapidly thereafter to 932 m and thence to historical levels the 936 m level can be correlated with increased throughflow in the okavango delta hence increased rainfall in the catchment the subsequent decline can be attributed to episodic closure and rerouting of okavango distributaries in particular the thaoghe perhaps aided by anthropogenic activities as such the desiccation of the lake is not indicative of regional climatic change

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

extraterrestrial dunes an introduction to the special issue on planetary dune systems abstract aeolian dune fields have been described on earth mars venus and titan the plethora of data returned from recent planetary missions has enabled a new era in planetary geomorphic studies much of our understanding of planetary dune systems comes from the application of earth analogs wind tunnel experiments and modeling studies despite the range of atmospheric pressures composition and gravity many of the dune forms on extraterrestrial surfaces are similar to those on earth although some have notable differences in bedform scale and composition as an introduction to the special issue on planetary dune systems this paper summarizes the current state of knowledge of planetary dune studies and highlights outstanding questions that require further investigation

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

degradation of linear dunes in northwest ngamiland botswana and the implications for luminescence dating of periods of aridity the long morphodynamic history of the linear dune system of northwest ngamiland comprises both construction and degradation processes here we describe features resulting from the latter crest lowering during wet periods is associated with lateral transfer of sand into the straats weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred damaran saprolite from below the dunes to the surface flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated the implications for luminescence dating of dune sands to identify periods of aridity are discussed the dunes cannot simply be regarded as providing a stacked record of such periods they are very old and much of the record is missing as a result of crest lowering the chronostratigraphy includes degradational components which must be distinguished from those of construction bioturbation cannot be disregarded as a complication which overprints the luminescence signals