determining vertical leakage from the great artesian basin australia through up scaling field estimates of phreatic evapotranspiration summary understanding the water balance of large groundwater systems is fundamental for the sustainable management of the resource the vertical leakage i e discharge to upper aquifers or the unconfined water table component of the great artesian basin gab is an example of a poorly constrained but large component of the water balance of australia s largest groundwater resource field estimates of phreatic evapotranspiration et were made at discharge zones along the southwestern margin of the gab using eddy covariance station and micro lysimeter measurements and inversion of chloride isotope soil profile measurements the field estimates were assigned to three major landforms associated with areas of increasingly higher evaporative discharge and progressively decreasing depths to the water table these landforms were mapped using remote sensing and digital elevation data with characteristically higher soil moisture salt precipitation and lower surface temperature compared to areas distal to discharge zones based on the field measurements broad ranges of phreatic et 0 5 10 10 100 and 100 300 mm y 1 were assigned to the major land types the higher phreatic et discharge zones mapped by supervised classification of satellite data are 8 28 of the total regional vertical leakage component estimated by numerical modelling of the gab in comparison the higher discharge zones estimated by landform mapping are 73 251 of the total vertical leakage component estimated by modelling the mapped distribution of the high discharge areas has important implications for modelling of the gab in the western sub basin most of the estimated recharge can be accounted for by phreatic et in the high discharge zones located around the basin margins implying that vertical leakage rates distal to the margins are very low and that discharge may exceed current recharge in contrast the results for the eastern sub basin suggest that vertical leakage rates around the south australian portion of the basin margin are low and that more of the vertical leakage component in the eastern sub basin is occurring distal to the basin margins consequently the pathways for vertical leakage in the eastern sub basin are likely to be more complex than for the western sub basin

origin of high mountains in the continents the southern sierra nevada active and passive seismic experiments show that the southern sierra despite standing 1 8 to 2 8 kilometers above its surroundings is underlain by crust of similar seismic thickness about 30 to 40 kilometers thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range but little subcrustal lithosphere is present beneath the eastern high sierra and adjacent basin and range these and other data imply the crust of both the high sierra and basin and range thinned by a factor of 2 since 20 million years ago at odds with purported late cenozoic regional uplift of some 2 kilometers

lithium saltworks of the salar de atacama a model for mgso 4 free ancient potash deposits abstract potash basins similar to those in the geological past no longer exist at present there are only hypersaline lakes such as those in the tibetan plateau where carnallite kcl mgcl2 6h2o precipitates given that the purpose of common saltworks is to obtain nacl the halitic stage is not exceeded by contrast saltworks extracting lithium by solar evaporation reach more advanced stages of brine evolution and saline precipitation in the salar de atacama saltworks with a relative humidity commonly below 30 monthly average salts precipitate following a sequence of halite nacl sylvite kcl carnallite and bischofite mgcl2 6h2o and display a useful model of evolution similar to that which gave rise to mgso4 free potash deposits in the past prismatic acicular gypsum caso4 2h2o the main accessory mineral is transformed into a less hydrated calcium sulfate probably anhydrite caso4 during carnallite precipitation mineral crystal size and annual sedimentary thickness of each paragenesis are compared with those in ancient potash deposits the behavior of sylvite and carnallite as cements is observed at the onset of their precipitation in ponds where both minerals coexist with previous halite millimeter cyclicity due to day night thermal variations is observed in these saltworks whereas cm and mm cyclicities have been interpreted in ancient sites as annual regular and intra annual irregular respectively conditions similar to those existing at the salar de atacama were necessary to preserve carnallite which is commonly the most evolved mineral in the ancient deposits the development of potash deposits in the south pyrenean foreland basin and the rhine graben near the eocene oligocene boundary is attributed to global aridity and cooling and to the basin restriction associated with a decrease in the ocean level caused by the spread of the antarctic ice cap

mantle transition zone beneath the caribbean south american plate boundary and its tectonic implications abstract we analyzed receiver function data recorded by a temporary broadband array deployed as part of the bolivar project and the permanent seismic network of venezuela to study the mantle transition zone structure beneath the caribbean south american plate boundary and venezuela significant topography on both the 410 km and the 660 km discontinuities was clearly imaged in the ccp common conversion point stacked images beneath the southeastern caribbean the 410 km is featured by a narrow 200 km ew 25 km uplift extending in the ns direction around 63 west while the 660 km is depressed by 20 km in a narrow region slightly west to the uplift a scenario that is more consistent with westward descent of the oceanic south american plate rather than a break off of nnw dipping proto caribbean oceanic lithosphere along the el pilar fault we also found a thick transition zone beneath the falcon region in northwestern venezuela possibly associated with the subducted nazca plate a flat 410 km was observed beneath the guayana shield suggesting that the shield has a stable and moderately deep keel which has little effect on the underlying transition zone structure

estimation of evaporation from the normally dry lake frome in south australia depth profiles of deuterium composition expressed as delta values have been used to make point estimates of evaporation from the floor of the normally dry lake frome a salt lake in northern south australia values from five sites varying between 90 and 230 mm yr 1 have been used to obtain a relatively crude approximation for mean evaporation rate from the lake of 170 mm yr 1 under the assumption that this represents discharge of water from the regional groundwater system the groundwater inflow into the lake is calculated as 5 108 m3 yr 1 two possible models which describe groundwater inflow into the lake are discussed

lithospheric thinning in the eastern indian craton evidence for lithospheric delamination below the archean singhbhum craton abstract we herein present shear velocity structure extending down to 300 km depth below the archean singhbhum odisha craton soc and proterozoic chotanagpur granitic gneissic terrain cggt which has been obtained through the inversion modeling of p receiver functions we use three component broadband recordings of 200 teleseismic earthquakes 30 90 from a 15 station seismic network that has been operational in the eastern indian shield since february 2013 we obtain the thinnest crust of 35 km overlying a thin lithosphere of 78 km below the region near south singhbhum shear zone which could be attributed to the 1 6 ga plume activity associated with dalma volcanic however the thickest crust of 47 km overlying a thin lithosphere of 81 km is noticed below the region near the singhbhum granite of 3 6 ga this thinning of lithosphere could be attributed to the delamination of lithospheric root due to the himalayan orogeny with a shortening rate of 2 cm year this delamination model in soc gets further support from the densification of the lower crust which could result from repeated episodes of basaltic underplating associated with episodes related to dalma 1 6 ga and rajmahal 117 ma volcanisms this led to relatively more mafic heterogeneous and deformed crustal structure in soc as well as egmb with an average crustal vs of 4 0 km s in comparison to that in cggt with an average crustal vs of 3 9 km s as seen through our modeling results the thickest lithosphere of 100 km is observed in the southwestern soc as well as northeastern cggt we also notice that a sharp and flat moho in cggt which could be attributed to thermal reactivation and large volume melting of the mafic cratonic crust during the late archean subduction process and associated volcanism episodes this model gets further support from the estimated 169 km thick lower vs zone in the upper mantle below cggt our modeling results also support a northward subduction of archean soc under the proterozoic cggt during 1 0 1 6 ga as earlier proposed by the geological age data from the region

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

slabs hotspots cratons and mantle convection revealed from residual seismic tomography in the upper mantle to further investigate the fate of slabs the structure of hotspots and the style of mantle convection we calculate residual tomographic maps in the upper mantle by excluding from the tomography the first order effects of conductive cooling of oceanic plates deep cratonic roots and cooling or partial melting associated with subducted lithosphere no correlation is found between residual tomography in the upper mantle and the 30 130 ma subduction the good correlations between residual tomography in the transition zone 400 650 km and 0 30 ma subduction at spherical harmonic degree l 2 can be explained either by slab accumulation in the transition zone beneath some subduction zones or by the poor lateral resolution of seismic tomography hotspots correlate with the residual tomography in the shallow 100 400 km and the lowermost mantle at degree l 2 however correlations of hotspots and seismic velocities in the middle mantle and correlations between residual tomography at shallow depths and lowermost mantle tomography are poor therefore the connection between residual tomography at shallow depths and seismic tomography in the lowermost mantle is not clear craton distribution correlates with distribution of hotspots and residual topography very well at degree l 6 this might indicate that cratonic roots may affect the locations of upwellings and thus modulate upper mantle convection or that the degree l 6 could be intrinsic in upper mantle convection with cratons and and other near surface features establishing the pattern

unusual lithospheric structure beneath the hyderabad granitic region eastern dharwar craton south india abstract using multiparametric geological and geophysical data the evolutionary history of the lithosphere beneath the late archean early proterozoic hyderabad granitic region hgr of the eastern dharwar craton of south indian shield has been attempted our study reveals that the entire granitic region and the surroundings 200 km 200 km is being neotectonically uplifted possibly due to a major intrusive body situated at sub crustal depth leading to changes in river courses and also erosion of several kilometers 10 km from its upper crustal column beneath this region the moho is elevated 32 33 km the reduced heat flow is of the order of 28 mw m2 and the asthenosphere is located at shallow depths of about 124 km i e far less than 200 400 km depth found in precambrian shield areas of the globe the cratonic mantle lithosphere beneath hgr contains a highly conductive 15 m hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km where the estimated temperatures could be in the range of 900 1000 c the surface granitic layer containing unusually high radioactivity 5 25 w m3 appears very thin probably a kilometer or so beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies rocks of acid to intermediate composition persistent episodic thermal reactivations during the last 2 6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual

modeling of craton stability using a viscoelastic rheology archean cratons belong to the most remarkable features of our planet since they represent continental crust that has avoided reworking for several billions of years even more it has become evident from both geophysical and petrological studies that cratons exhibit deep lithospheric keels which equally remained stable ever since the formation of the cratons in the archean dating of inclusions in diamonds from kimberlite pipes gives archean ages suggesting that the archean lithosphere must have been cold soon after its formation in the archean in order to allow for the existence of diamonds and must have stayed in that state ever since yet although strong evidence for the thermal stability of archean cratonic lithosphere for billions of years is provided by diamond dating the long term thermal stability of cratonic keels was questioned on the basis of numerical modeling results we devised a viscoelastic mantle convection model for exploring cratonic stability in the stagnant lid regime our modeling results indicate that within the limitations of the stagnant lid approach the application of a sufficiently high temperature dependent viscosity ratio can provide for thermal craton stability for billions of years the comparison between simulations with viscous and viscoelastic rheology indicates no significant influence of elasticity on craton stability yet a viscoelastic rheology provides a physical transition from viscously to elastically dominated regimes within the keel thus rendering introduction of arbitrary viscosity cutoffs as employed in viscous models unnecessary

a thinned lithospheric image of the tanlu fault zone eastern china constructed from wave equation based receiver function migration 1 we apply the newly proposed wave equation based receiver function poststack migration method to the northern china interior structure project broadband data to image the lithospheric structure of the tanlu fault zone area in eastern china our migration result reveals a 60 to 80 km thick present day lithosphere beneath the study region significantly thinned from the paleozoic lithosphere of 180 km the lithosphere asthenosphere boundary lab is coherently imaged along the 300 km east west profile displaying an arc like shape with its apex roughly coincident with the transverse location of the tanlu fault zone on the surface an obvious uplift from 36 km to 32 km of the moho is also clearly detected right below this fault zone the coincidence of the imaged moho uplift and the lab apex with the surface location of the tanlu fault zone provides seismological evidence for the steep geometry and deep penetration of the fault system and indicates that the tanlu fault zone might have acted as a major channel for anthenosphere upwelling during the mesozoic cenozoic continental extension and lithospheric thinning in eastern china frequency analysis and synthetic modeling suggest that both the moho and the lab are sharp and strong the latter in particular is constrained to have a 3 7 drop in s wave velocity over a depth range of 10 km or less such a rapid velocity change at the base of the lithosphere in the study region cannot be solely explained by thermal variation but likely reflects the presence of volatiles or melt in the asthenosphere or is partially attributed to the compositional contrast between the preserved depleted and dehydrated cratonic lithospheric veneer and the uplifted hydrated and fertile asthenospheric materials

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

fine structure of the 410 km discontinuity the april 14 1995 earthquake in western texas m w 5 7 produced a strong topside reflection off the 410 km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern united states data from 394 vertical short period and 24 broadband instruments provide dense coverage of this event from distances of 11 to 19 and provide a detailed look at the subcontinental 410 km structure the salient features of this data set are 1 the strong dependence on wavelength of the 410 km triplication range 2 the uniform amplitude ratio of the direct p and reflected p 410 phases on both short period and broadband recordings throughout the triplication and 3 the abrupt termination of the short period p 410 phase at 13 3 these features are best modeled by a composite discontinuity in which a sharp velocity jump of 3 is overlain by a linear velocity jump of 3 5 spread over 14 km the interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long and short period triplication range and the step like behavior of the p 410 short period amplitude which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity this composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency additionally this is the structure expected from mineralogical arguments for the to olivine phase transition

anomalous crustal and lithospheric mantle structure of southern part of the vindhyan basin and its geodynamic implications abstract tectonically active vindhyan intracratonic basin situated in central india forms one of the largest proterozoic sedimentary basins of the world possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin has led to surge in geological and geophysical investigations by various agencies an attempt to synthesize such multiparametric data in an integrated manner has provided a new understanding to the prevailing crustal configuration thermal regime and nature of its geodynamic evolution apparently this region has been subjected to sustained uplift erosion and magmatism followed by crustal extension rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle almost 5 6 km thick sedimentation took place in the deep faulted jabera basin either directly over the bijawar mahakoshal group of mafic rocks or high velocity high density exhumed middle part of the crust detailed gravity observations indicate further extension of the basin probably beyond nsl rift in the south a high heat flow of about 78 mw m2 has also been estimated for this basin which is characterized by extremely high moho temperatures exceeding 1000 c and mantle heat flow 56 mw m2 besides a very thin lithospheric lid of only about 50 km many areas of this terrain are thickly underplated by infused magmas and from some segments granitic gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks a 5 8 km thick retrogressed metasomatized zone with significantly reduced velocities has also been identified around mid to lower crustal transition

local displacement of diapir contacts and its importance to pluton emplacement study abstract the extent to which mid crustal plutons have grown by diapirism cannot be deduced from the horizontal shortening of ductilely deformed host rocks and associated displacement of lateral intrusive contacts by contrast the vertical shortening above plutons and upward displacement of their roof contacts may provide a good measure of the diapiric component of pluton emplacement this is related to the vertical bulk translation of buoyant material in immature diapirs and associated escape flow of overlying rocks the length change of a vertical material line situated above the crest of an immature upright diapir corresponds to the upward displacement of the local contact the length change of horizonal material lines situated at lateral contacts of model diapirs however differs markedly from the contact displacement this is related to the absence of horizontal symmetry planes in the finite strain field about upright diapirs of any shape and the importance of vertical shear at the lateral contacts the host rock strain held of a hypothetical pluton grown in situ by spherical expansion ballooning has an infinite number of symmetry planes the strain held is characterized by large volume losses but the length change of all radial lines corresponds to the local magnitude of contact displacement this precludes the possibility of differentiating in ductile host rocks above plutons between vertical diapirism and spherical expansion unless gauges of volume change are available

the nature and thickness of lithosphere beneath the archean dharwar craton southern india a magnetotelluric model abstract the dharwar craton in the southern indian shield region occupies a key position in reconstructions of the earliest supercontinent ur as well as in the global neoarchean configurations of continental assembly here we synthesize the available geophysical data that include heat flow seismic tomography temperature profile derived from xenoliths and magnetotellurics mt from the dharwar craton a thickness of more than 200 km is identified for the lithosphere beneath this craton by magnetotelluric model our inference lends support to the models from recent seismological investigations that argue for the presence of a thick lithosphere as well as the upper mantle structure derived from the analysis of mantle xenoliths a synthesis of recent geophysical and geochemical data together with our magnetotelluric model indicate the existence of a thick lithospheric root beneath the dharwar craton and suggest that not much of the lithosphere has been eroded since precambrian in this region leading to the conclusion that the dharwar craton preserves an archean tectosphere an increase in the lithospheric thickness from east to west in the dharwar craton and the lithosphere asthenosphere boundary lab migration to shallower levels observed from geophysical data towards the east is interpreted as a probable reflection of the mesoproterozoic subduction erosion of the archean cratonic keel

group velocity dispersion characteristics and one dimensional regional shear velocity structure of the eastern indian craton abstract in the past three years a semi permanent network of fifteen 3 component broadband seismographs has become operational in the eastern indian shield region occupying the archean 2 5 3 6 ga singhbhum odisha craton soc and the proterozoic 1 0 2 5 ga chotanagpur granitic gneissic terrane cggt the reliable and accurate broadband data for the recent 2015 nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one dimensional regional shear velocity structure of the region first we measure fundamental mode rayleigh and love wave group velocity dispersion curves in the period range of 7 70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern indian craton eic we observe that group velocities of rayleigh and love waves in soc are relatively high in comparison to those of cggt this could be attributed to a relatively mafic rich crust mantle structure in soc resulting from two episodes of magmatism associated with the 1 6 ga dalma and 117 ma rajmahal volcanisms the best model for the eic from the present study is found to be a two layered crust with a 14 km thick upper crust uc of average shear velocity vs of 3 0 km s and a 26 km thick lower crust lc of average vs of 3 6 km s the present study detects a sharp drop in vs 2 to 3 at 120 260 km depths underlying the eic representing the probable seismic lithosphere asthenosphere boundary lab at 120 km depth such sharp fall in vs below the lab indicates a partially molten layer further a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88 103 km depths suggesting a 88 103 km thick thermal lithosphere below the eic this could also signal the presence of small amounts of partial melts thus this 2 3 drop in vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz back arc volcanism associated with the archean proterozoic subduction 1 6 ga dalma volcanism and 117 ma rajmahal volcanism the main result of our modeling provides evidences for the absence of keel or thick lithosphere below the eic

variations in the crustal structure of the lutzow holm bay region east antarctica using shear wave velocity abstract crustal structure in the lutzow holm bay region lhb east antarctica is studied by analyzing seismic waveforms derived from records in the vicinity of syowa station syo 69 0 s 39 6 e one dimensional crustal models for shear waves in particular are obtained in order to investigate crustal heterogeneity radial receiver functions developed from teleseismic p waveforms of broadband seismographs at syo in 1990 1993 are used in the time domain inversion as the starting model in the receiver function inversion a crustal attenuation model qs for shear waves from local earthquakes of telemetry networks in 1987 1989 is derived by shifting the lapse time of the coda part of s waves lateral heterogeneity is investigated by analyzing several backazimuth groups namely the 50 100 120 160 210 250 and 300 360 sectors are investigated velocity changes are recognized as a sharp moho at 36 38 km depth in the continental areas in the 50 100 and the 120 160 sectors in contrast smooth variation of crustal velocities is revealed together with transitional moho in the bay backazimuths in the 210 250 and the 300 360 sectors in the continental backazimuths a difference in crustal velocities is recognized between metamorphic terrain of granulite facies and the granulite amphibolite transitional zone shear velocity models are compared with crustal reflectivity derived from the analysis of refraction data on the mizuho plateau the reflective layers are found at depths of 24 45 km and appear to be related to the bouguer gravity anomalies by correlating the seismic models of crustal structure with other geophysical and geological evidence the crustal evolution in lhb can be explained in relation to the regional metamorphism at 500 ma and the last break up of gondwana at 150 ma

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

a historical approach to continental flood basalt volcanism insights into pre volcanic rifting sedimentation and early alkaline magmatism abstract continental flood basalts are widely thought to be produced from mantle plume heads however plume theories do not observe any role for lithospheric rifting before flood basalt events and consider irrelevant the fact that most if not all continental flood basalts have erupted through deep rifts containing thick sedimentary sequences at best plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads however the fact that cfbs of the world erupted through deep ancient rift zones and alternative dynamical considerations of flood basalt genesis directly lead towards a new historical approach to flood basalt emplacement this approach takes cognizance of the basic unity of geological history and processes satisfactorily explains pre volcanic rifting sedimentation mantle metasomatism and early pre tholeiite enriched alkaline magmatism for tens of millions of years both incubating and impacting plume heads ought to lead to pre volcanic lithospheric doming which is usually not observed in flood basalts continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull apart splitting along pre existing lithospheric discontinuities such as deep rifts or fracture zones

the geomorphometry of endorheic drainage basins implications for interpreting and modelling their evolution endorheic basins are catchments with no hydrological connection with marine environments they cover 20 of the earth s surface and are mostly located in arid regions their drainage networks converge to lakes salt flats or alluvial plains whose dynamics are strongly driven by precipitation evapotranspiration and groundwater discharge among other factors integrated surface drainage and the creation of whole drainage systems typical of open basins are commonly restricted in these regions interestingly the fluvial basin morphology of endorheic basins has not been extensively studied and a variety of quantitative morphological descriptors used in open basins have not been utilized in the geomorphic analysis of endorheic basins the objective of this study is to better understand the basin morphology of endorheic river basins by using well known geomorphological properties and their variations across scales for three basins in northern chile we computed the following descriptors and the corresponding relevant scales the cumulative distribution of contributing area the horizontal shape of the basins i e hack s law normalized euclidean length and sinuosity of the streams slope area relationship horton s ratios and drainage density we detected several properties typically found in open basins but certain features which seem to be unique to closed basins were also identified in particular we found that horizontal and vertical geomorphic features seem to be linked which suggests that an independent treatment of these features may not be appropriate for closed basins similar results were found regardless of the basin area which illustrates the relevant effects of features that are specific to these particular regions it is expected that our findings will improve both the geomorphic assessment of these basins and hydrological modelling of surface water and groundwater copyright c 2013 john wiley sons ltd

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

groundwater discharge processes at a central australian playa abstract water balance studies have been undertaken at a groundwater discharge playa spring lake near curtin springs in arid central australia hydraulic calculations indicate that the groundwater inflow to the playa is about 2000 m 3 d 1 this is equivalent to evaporative discharge integrated over the playa surface area 15 km 2 of about 49 mm yr 1 as the mean annual rainfall is 226 mm the total evaporative discharge is about 275 mm yr 1 measured water losses from the playa surface during a 3 month dry period july september 1985 ranged from 0 28 to 0 45 mm d 1 this is of the expected order of magnitude for the cooler half of the year the rate of water loss is variable over the playa surface the playa is surrounded by sand dunes monitoring of water level fluctuations indicates that annual rainfall events induce groundwater recharge in the playa marginal zone where the water table is within 4 m of the ground surface evaporation from the playa has resulted in the development of highly saline brines with density 1 20 or more and containing more than 250 g 1 2 total dissolved solids the brines are sodium chloride waters with appreciable magnesium and sulphate at the playa margin a transition zone has developed between saline regional groundwaters which underlie sand dunes and the playa hypersaline brines salt may be diffusing downwards beneath the regional grounwaters gypsum is precipitating in the playa the effects of seasonal and longer term drought are shown by the development of gypsum fields on the playa bed and playa marginal gypsum terraces which are later colonised by halophytic plants long term reduction in groundwater head results in the vegetation of playas which no longer actively discharging groundwater become susceptible to alluviation or aeolian sand drift thus playas migrate by a process playa capture analogous to river capture but induced by groundwater head decay

continental scale rheological heterogeneities and complex intraplate tectono metamorphic patterns insights from a case study and numerical models abstract continental plates are built over long periods of time through successive extensional and compressional cycles they are therefore rheologically heterogeneous this heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes the study of the neoproterozoic borborema shear zone system of northeast brazil highlights a systematic link between marked changes in its tectono metamorphic pattern and the pre existing structure of the plate that is characterized by juxtaposition of continental domains either comprising an old basement palaeo to eoproterozoic or accreted during an extensional event between 1 0 and 0 7 ga in neoproterozoic time when the shear zone system was developed these domains displayed different geotherms and lithospheric thicknesses and therefore contrasted rheological behaviours we use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate from the very beginning of the deformation weak and stiff heterogeneities induce strain localization due to a lower initial effective viscosity or to stress concentrations at their tips respectively shear zones propagate from the heterogeneities and finally coalesce forming a network of high strain zones bounding almost underformed blocks within this network shear zones transfer strain between the different heterogeneities and model boundaries the evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere the resulting finite strain field is heterogeneous and displays rapid lateral variations in vertical and or rotational deformation such a heterogeneous strain distribution may induce contrasted magmatic metamorphic and uplift evolutions within an orogenic belt as observed in the borborema shear zone system and other collisional belts

a theory of the earth hutton and humpty dumpty and holmes paradoxes can be viewed as a bother or as a rich source of information paradoxes fallacies and logical inconsistencies along with the unstated assumptions behind old worldviews often form the foundation for a new worldview this is where science differs from myth and just so stories mantle dynamicists and geochemists have developed a worldview that in many respects is incompatible with modem theories of planetary accretion geodynamics and geophysics and with the geological context of volcanoes a primordial undegassed mantle as both the furnace and the fuel of midplate volcanoes occupies a central role in the geochemical models although modem data alone are sufficient to overturn this view it is interesting to address it in terms of its own assumptions fallacies and inconsistencies and the history of ideas this is a vast field so in honour of james hutton i focus on volcanoes their products their causes and their sources

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

depth extent of cratons as inferred from tomographic studies we address the question of the depth extent of mantle high velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies we divided old continents into two age provinces 800 1700 ma middle proterozoic and older than 1700 ma archean and early proterozoic the areas included in these age provinces are cross correlated with the tomography at different depths to quantify the global occurrence of high velocity anomaly hva extensions beneath cratons statistically significant hvas underlie the oldest cratons to a depth of 250 km there is no significant consistent correlation between tomography and younger cratons the older cratons have correlated hvas extending to depths varying from 100 to 450 km there is no significant difference in the average velocity anomaly beneath the early proterozoic part and the archean part of the canadian shield whereas the middle proterozoic part does not contain any hva the mantle under early precambrian crust is distinctly different from mantle elsewhere we propose two different hypotheses to explain these results 1 cratons of early proterozoic age or older contain ancient and deep physically attached roots or 2 these cratons contain a small permanent physical root that induces cold downwellings beneath it a root thickness of 200 km can be explained by an feo poor olivine rich mineralogy whereas a requirement of high strength probably demands in addition a volatile poor composition different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved and to their subsequent deformational history or they may represent different snapshots in time of transient roots such as thermal boundary layers

anisotropy of the flexural response of the lithosphere in the canadian shield 1 we show that the two dimensional coherence between topography and bouguer gravity is anisotropic throughout most of the canadian shield excepted hudson bay basin we interpret the axis of maximum coherence as the direction where the lithosphere is weakest in the southeastern shield and the appalachians the azimuthally averaged coherence cannot be fitted by the thin elastic plate model in the other regions it yields the effective elastic thickness the average coherence increases in the direction perpendicular to the main tectonic boundaries the axis of high electrical conductivity and the seismic fast axis this consistency suggests that the same stress field affected the entire lithosphere during the last tectonic event

upper mantle tomography and dynamics beneath the north china craton 1 a high resolution tomographic model of the upper mantle beneath the north china craton ncc is determined using a large number of precisely hand picked teleseismic p wave arrival times the results are generally consistent with previous results but high quality arrivals provide new insights into the dynamics beneath the ncc obviously north south trending low velocity low v zones are revealed down to 300 400 km depth under the shanxi rift and tanlu fault zone while a north south trending high velocity high v zone representing the remainder of detached lithosphere is visible down to 200 km depth under the western portion of eastern ncc high v anomalies representing the detached lithosphere are detected at 200 400 km depth under central and eastern ncc under the ordos block high v anomalies are visible above 400 km depth indicating intact lithosphere broad high v anomalies representing the stagnant pacific slab are imaged with a low v anomaly from datong volcano to the edge of bohai sea in the mantle transition zone beneath eastern and central ncc suggesting that the pacific slab has subducted to central ncc but with a gap a continuously y shaped low v structure is clearly imaged under datong volcano and bohai sea from the lower mantle through this gap in the mantle transition zone to the upper mantle indicating the existence of a lower mantle plume these results suggest that in addition to the subduction of the pacific plate the plume has also played an important role in lithospheric destruction by thermal erosion of the asthenosphere and detachment of the lithosphere beneath the ncc

evaporation and land surface energy budget at the salar de atacama northern chile playa systems are driven by evaporation processes yet the mechanisms by which evaporation occurs through playa salt crusts are still poorly understood in this study we examine playa evaporation as it relates to land surface energy fluxes salt crust characteristics groundwater and climate at the salar de atacama a 3000 km 2 playa in northern chile containing a uniquely broad range of salt crust types land surface energy budget measurements were taken at eight representative sites on this playa during winter august 2001 and summer january 2002 seasons measured values of net all wave radiation were highest at vegetated and rough halite crust sites and lowest over smooth highly reflective salt crusts over most of the salar de atacama net radiation was dissipated by means of soil and sensible heat fluxes dry salt crusts tended to heat and cool very quickly whereas soil heating and cooling occurred more gradually at wetter vegetated sites sensible heating was strongly linked to wind patterns with highest sensible heat fluxes occurring on summer days with strong afternoon winds very little energy available at the land surface was used to evaporate water eddy covariance measurements could only constrain evaporation rates to within 0 1 mm d k1 and some measured evaporation rates were less than this margin of uncertainty evaporation rates ranged from 0 1 to 1 1 mm d k1 in smooth salt crusts around the margin of the salar and from 0 4 to 2 8 mm d k1 in vegetated areas no evaporation was detected from the rugged halite salt crust that covers the interior of the salar though the depth to groundwater is less than 1 m in this area these crusts therefore represent a previously unrecorded end member condition in which the salt crusts form a practically impermeable barrier to evaporation q 2005 elsevier b v all rights reserved

a new class of equilibrium geotherms in the deep thermal lithosphere of continents abstract equilibrium lithospheric geotherms are generally assumed to have a self similar shape with a nearly linear thermal gradient in the upper portions of the thermal lithosphere and a smoothly decreasing gradient in its lowermost portions large thermal gradient increases are thought to be unsustainable because they would seem to imply a local heat flux imbalance that would be smoothed away over time simulations of mantle convection allowing for continents lead us to suggest that this may not be the case and that kinked geotherms may exist in thermal equilibrium within specific continental regions this new class of geotherms arises from the coexistence of two heat loss modes within the continental lithosphere oceanic heat loss is dominated by convection i e plate creation and subduction continental heat transfer on the other hand is of a conjugate nature with conduction dominating in stable chemically buoyant portions of continental lithosphere and convection operating in its weaker lower portions at the ocean continent boundary horizontal heat transfer occurs within a zone of accommodation where oceanic and continental heat loss are reconciled if continents are underlain by mantle downwellings then this horizontal heat transfer can involve a convective pattern that allows weak lower portions of the thermal lithosphere to flow differentially below a passive continental margin horizontal advection can thus be low in the near surface regions of the margin leading to mild thermal gradients but can outweigh vertical conduction at depth leading to steep gradients this allows inflected i e kinked geotherms to be generated downstream of the margin the thermal structure of the lower lithosphere adjusts to that of the continental interior and inflections decay however within a finite downstream region inflected geotherms can be maintained indefinitely by continued sub continental flow a similar situation can occur when weak lower portions of the continental thermal lithosphere flow from thinner to thicker regions of chemically stabilized upper lithosphere

viscoelastic mantle convection and lithospheric stresses summary accurate predictions of stress distribution in the lithosphere are of major importance for approaching more realistic numerical models of the mantle lithosphere system since stress fields in the lithosphere computed in convection models differ substantially between viscous and viscoelastic rheologies it is essential to employ a viscoelastic rheology when accurate stresses are to be predicted in mantle convection models involving the lithosphere this difference in stress distribution and magnitude has important implications for accurate modelling of stress dependent processes like power law creep shear heating and plasticity a further requirement for computation of accurate stress fields is to ensure numerically divergence free solutions in the boussinesq approximation we present the technical background required for implementation of numerically incompressible solutions and for implementation of a maxwell viscoelastic rheology in the frame of the finite element method fem we employ the jaumann invariant stress derivative in our implementation and demonstrate that the choice of the invariant stress derivative is irrelevant for geodynamic simulations we discuss potential numerical advantages of a viscoelastic rheology when large viscosity variations are applied in thermal convection models due to the physical transition from effectively viscous to elastic behaviour in a viscoelastic model the introduction of viscosity cut offs generally applied in viscous models can be avoided

hydrogeochemical fluxes and processes contributing to the formation of lithium enriched brines in a hyper arid continental basin abstract the accumulation of solutes delivered via shallow groundwater and surface water in arid to hyper arid closed basins and the complex hydrogeochemical processes associated with their transport are key to deciphering mechanisms responsible for the formation of brines and salts in these environments a rigorous investigation of the hydrogeochemical fluxes and the formation of the li enriched brine in the hyper arid basin of the salar de atacama chile is presented water fluxes within the salar de atacama basin range from 0 003 m3 s to 1 50 m3 s with a poor correlation between discharge and drainage basin area the inflowing waters have major elemental and li concentrations that are within the same order of magnitude over a period of several decades the element fluxes indicate for example that 58 of the li in the halite nucleus and brines is derived from the south and southeast parts of the basin the average 18ovsmow and dvsmow of inflow waters indicate that there are varied sources of recharge corresponding to the defined sub watershed regions the average water flux weighted 87sr 86sr of inflow waters allows further definition of where in the regional watershed the solutes are derived the predicted water flux weighted 87sr 86sr of the brines 0 70813 compared to the measured average 87sr 86sr 0 70801 indicates that there may be unaccounted sources of water and solutes to the basin brines elemental accumulation time scales for the volume of halite and brine in the nucleus balance on time frames of 0 5 ma mg 1 0 ma ca 1 9 ma li and 2 2 ma k as compared to 47 ma na and 53 ma cl further evidence that na and cl require additional sources not accounted for in the modern inflows to the topographic watershed dissolved noble gas concentrations of marginal and nucleus brines specify that the brines formed in an atmospheric equilibrated state and have since been isolated below the salt crust a process based model is presented to explain the extremely high concentrations of li in brines which incorporates the 7li isotope signatures of waters in the basin to further decipher on a first order the contributions of processes such as low temperature weathering and secondary phase formation

present day plate motion constraint on mantle rheology and convection large scale mantle convection to first order is a system driven by interior density anomalies modulated by variable plate thickness and extreme rheology variations at the top of the mantle the rheological difference between oceanic and continental regions significantly influences the surface velocity we apply a three dimensional newtonian viscous flow model to explain the large scale present day plate motions the density anomalies are derived from seismic tomography and a slab model with a viscosity difference of a factor of 30 60 between continents and oceans in the upper 90 km of the earth we are able to explain both the observed large scale poloidal and toroidal plate motions the viscosity difference between continental and oceanic regions has major control on both the poloidal toroidal kinematic energy partitioning and the pattern of toroidal motion nonlinear rheology can help establish toroidal motion plate motions can be explained by assuming either layered or whole mantle flow in order to match the amplitude of observed plate motions the value of the reference viscosity corresponding to that of between 400 to 670 km depth is 1 6 10 21 pa s for layered mantle flow and 3 2 10 21 pa s for whole mantle flow however the predicted net rotation of the lithosphere from both layered and whole mantle flow models is very small and cannot account for the amplitude of the net rotation obtained from the plate tectonic models assuming a fixed hotspot reference frame

anomalous lithosphere beneath the proterozoic of western and central australia a record of continental collision and intraplate deformation a new surface wave seismic tomography model of australia is presented which provides a means of investigating the lithospheric structure beneath the proterozoic regions in the west north and centre of the continent with improved resolution and reliability the dominant feature of the model is a region of low seismic wavespeeds in the uppermost mantle at approximately 75 km depth beneath central australia the zone of slow wavespeeds is underlain by a region of fast wavespeeds more typical of continental lithosphere this layered velocity structure and strong positive wavespeed gradient makes the shallow anomaly hard to explain in terms of high mantle temperatures and typical steady state continental geotherms a possible thermal explanation requires the impact of the redistribution of high heat producing elements within the crust alternatively a mineral or minerals with low seismic velocities such as amphibole in the shallowest part of the lithosphere with a more conventional lithology in the deep continental root below could explain the seismic wavespeeds the anomaly is located directly beneath the zone where the australian cratons amalgamated in the proterozoic and where subsequently there have been periods of intraplate tectonic activity suggesting a correlation between the prolonged history of deformation and a highly unusual lithospheric structure in western australia the capricorn orogen and pilbara craton have a similar lithospheric thickness whereas a thicker lithosphere is observed beneath the yilgarn to the south in northern australia large regions appear to be underlain by fast wavespeeds similar to those observed beneath the yilgarn craton variations in the shear wavespeeds beneath the arunta also indicate that there is not always an obvious correlation between the overlying surface geology and seismic structures observed in the upper mantle

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

dynamic interaction of cold anomalies with the mid ocean ridge flow field and its implications for the australian antarctic discordance abstract negative thermal anomalies beneath a mid ocean ridge are dynamically isolated from the ambient upwelling and diverging flow field in the asthenosphere whose viscosity is on the order of 5 10 19 pa s or less this study examines on what condition a near ridge cold anomaly ascends with the upwelling passive flow and spread off axis three dimensional numerical modeling demonstrates that for a given magnitude of the cold anomaly the viscosity of the asthenosphere the spreading rate and the interference from continental rifting are the dominant controlling factors to the ascent descent of the anomaly to overcome the weight of such an anomaly and couple it with the upwelling either the spreading rate or the asthenospheric viscosity has to be high in a low viscosity asthenosphere the cold anomaly also ascends during the early stage of continental rifting due to the enhanced upwelling induced by the thick continental lithosphere the dynamic interaction between the cold anomaly and the ambient flow renders a transient nature of the subsidence of the seafloor which may lead to exaggerated temperature variation estimated by using a conduction model alone the scenarios examined are employed to place a constraint on dynamic models recently proposed for the australian antarctic discordance in which the source of the negative anomaly is hypothesized to be deeply rooted in the upper mantle with the asthenospheric viscosity less than 10 20 pa s the upwelling of the cooler material from great depths which causes a significant topographic low at the discordance is made possible only by rifting of the australian continent off the gondwanaland

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

isotopic evidence for chaotic imprint in upper mantle heterogeneity 1 the intrinsic structure of the isotope data set of samples from the mid atlantic ridge and east pacific rise believed to represent the isotopic composition of their mantle source reveals a close relationship between sample spatial distribution and their geochemical features the spatial distribution of their isotopic heterogeneity is self similar on a scale between 5000 and 6000 km about 1 6 of earth s circumference suggesting a self organized structure for the underlying mantle this implies the imprint of chaotic mantle processes induced by mantle flow and likely related to an early phase of highly dynamic behavior of the earth s mantle the size of the identified self organized region reflects the large length scale of upper mantle chemical variability and it is likely frozen since the proterozoic the geochemical heterogeneity of the asthenosphere along the ridges is believed to record a transition in the thermal conditions of the earth s mantle and to be reflected in the l 6 peak expansion of several geophysical observables

tracing the influence of the trans european suture zone into the mantle transition zone cratons with their thick lithospheric roots can influence the thermal structure and thus the convective flow in the surrounding mantle as mantle temperatures are hard to measure directly depth variations in the mantle transition zone mtz discontinuities are often employed as a proxy here we use a large new data set of p receiver functions to map the 410 km and 660 km discontinuities beneath the western edge of the east european craton and adjacent phanerozoic europe across the most fundamental lithospheric boundary in europe the trans european suture zone tesz we observe significantly shorter travel times for conversions from both mtz discontinuities within the craton caused by the high velocities of the cratonic root by contrast the differential travel time across the mtz is normal to only slightly raised this implies that any insulating effect of the cratonic keel does not reach the mtz in contrast to earlier observations in siberia we do not find any trace of a discontinuity at 520 km depth which indicates a rather dry mtz beneath the western edge of the craton within most of covered phanerozoic europe the mtz differential travel time is remarkably uniform and in agreement with standard earth models no widespread thermal effects of the various episodes of caledonian and variscan subduction that took place during the amalgamation of the continent remain only more recent tectonic events related to alpine subduction and quarternary volcanism in the eifel area can be traced while the east european craton shows no distinct imprint into the mtz we discover the signature of the tesz in the mtz in the form of a linear region of about 350 km width with a 1 5 s increase in differential travel time which could either be caused by high water content or decreased temperature taking into account results of recent s wave tomographies raised water content in the mtz cannot be the main cause for this observation accordingly we explain the increase equivalent to a 15 km thicker mtz by a temperature decrease of about 80 k we discuss two alternative models for this temperature reduction either a remnant of subduction or an indication of downwelling due to small scale edge driven convection caused by the contrast in lithospheric thickness across the tesz any subducted lithosphere found in the mtz at this location is unlikely to be related to variscan subduction along the tesz though as eurasia has moved significantly northward since the variscan orogeny

the scales of mantle convection seismic topographic and gravity data show that there are two important scales of mantle convection these are associated with spherical harmonic degrees l 2 and 6 the l 2 pattern corresponds to the pattern of subduction cooling since the breakup of pangea most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle the l 2 distribution of upwellings and downwellings is likely to be a long lived feature of the earth cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness an l 1 pattern of convection is probably related to supercontinents and their breakup degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid cratons with their deep cold keels control the l 6 pattern and may even cause it by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle subduction zones reinforce the craton pattern downwellings preferentially occur under cratons upwellings and hotspots occur at complementary locations supercontinents and their associated subduction zones l 1 constantly assemble and reassemble in the african atlantic hemisphere and the continental fragments in the dispersed state periodically settle into the polar band of geoid lows l 2 and 6 that now includes the americas antarctica australia and india if cratons control the l 6 pattern of convection and many patterns of l 6 are possible e g sectoral zonal checker board then upper mantle convection may reorganize roughly every 30 ma as cratons move about the hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings a third and smaller scale of convection order 400 1000 km in dimension is just below the resolution of global tomography but shows up in the gravity field geoid and topography this scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection and the thickness of the upper mantle low viscosity zone convective domains of this dimension are also implied by the scales of chemical homogeneity lengths of rifts ridges and seamount chains fracture zone spacing and mid ocean ridge segmentation the temperature geochemistry and fertility of these upper mantle scale domains is controlled by their previous history of subduction continental insulation or refrigeration and processing by ridges fertile or volatile rich or hot cells can be mistaken for plumes hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale geochemical domains of various sizes exist in the upper mantle they are broad scale features rather than point sources as in plume theories lithospheric dynamics and geometric focusing not mantle dynamics control the dimensions of so called hotspot eruptives

3d lithosphere density structure of southern indian shield from joint inversion of gravity geoid and topography data abstract we present the 3d crustal and lithospheric structure and crustal average density distribution of southern indian shield south of 18 n sri lanka and adjoining oceans the model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data the derived crustal thickness of 10 25 km in the oceanic region increases to 34 35 km along the coast a crustal thickness of 34 38 km is obtained beneath the eastern dharwar craton and 36 45 km beneath the western dharwar craton and the southern granulite terrain sri lanka has a thinner crust of 30 35 km the lithosphere asthenosphere boundary is located at depths of 70 120 km under oceanic regions and 150 180 km below the dharwar craton and the northern block of southern granulite terrain a notably thinned lithosphere of 130 km near bangalore in the eastern dharwar craton 140 km beneath the southern block of southern granulite terrain and 130 km in sri lanka is observed the thickness of the lithosphere 130 km near bangalore is inferred as the frozen in signature of a small fossil mantle plume and or tectono compositional effect of a rifted margin and a suture considerable stretching and or convective removal of pristine lithosphere in the southern block of southern granulite terrain and adjoining sri lanka before disappearing completely in the archaean northern block of southern granulite terrain and dharwar craton is suggested

imaging the lithosphere asthenosphere boundary of the indian plate using converted wave techniques 1 the lithospheric structure of the indian plate has been investigated using converted wave techniques p and s receiver functions and a novel stacking analysis technique without using deconvolution applied to a large seismological data set from permanent and temporary broadband seismic stations we observe coherent energy from at least two seismic discontinuities i e the crust mantle moho and lithosphere asthenosphere boundary lab in the uppermost mantle here we provide a novel seismic image of the indian lithosphere showing definitive evidence of its flexure which is interpreted to be primarily caused by the hard collision at 55 myr resulting in the world s highest mountain chain the himalayas and the tibetan plateau results from geoidal and gravity studies do suggest postcollisional flexuring of the indian plate however the flexure lacks observational constraints the observed wavelength of the flex is 1000 km with the thickness of the indian shield lithosphere varying from 70 km to 140 km such a low value for a continent implies that the indian plate has been reworked in the past the plate deepens in the himalayan region to a depth of 170 km further the converted phases are interpreted to be resulting from the bottom of the lithosphere we clearly demonstrate that these are distinct and different from the midlithospheric discontinuity for a large number of stations the midlithospheric discontinuity and lab are clearly separated in depth our observations suggest that the archaean lithosphere is no longer intact and is prone to deformation

climate surface pore water interactions on a salt crusted playa implications for crust pattern and surface roughness development measured using terrestrial laser scanning sodium accumulating playas also termed sodic or natric playas are typically covered by polygonal crusts with different pattern characteristics but little is known about the short term hours dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness it is important to understand these interactions because playa crust surface pore water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity here we present the first high resolution 10 3m hours co located measurements of changing moisture and salt crust topography using terrestrial laser scanning tls and infra red imagery for sua pan botswana maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity these peaks experienced non elastic expansion overnight of up to 30 mm and up to an average of 1 5 mm night during the 39 day measurement period continuous crusts on the other hand showed little nocturnal change in moisture or elevation the dynamic nature of salt crusts and the complex feedback patterns identified emphasise how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles

shear velocity structure of the laterally heterogeneous crust and uppermost mantle beneath the indian region abstract the shear velocity structure of the indian lithosphere is mapped by inverting regionalized rayleigh wave group velocities in time periods of 15 60 s the regionalized maps are used to subdivide the indian plate into several geologic units and determine the variation of velocity with depth in each unit the hedgehog monte carlo technique is used to obtain the shear wave velocity structure for each geologic unit revealing distinct velocity variations in the lower crust and uppermost mantle the indian shield has a high velocity 4 4 4 6 km s upper mantle which however is slower than other shields in the world the central indian platform comprised of proterozoic basins and cratons is marked by a distinct low velocity 4 0 4 2 km s upper mantle lower crustal velocities in the indian lithosphere generally range between 3 8 and 4 0 km s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities respectively a remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the indian plate the south bruma region along the eastern subduction front of the indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle high velocities 4 8 km s are also observed in the upper mantle beneath the ninetyeast ridge in the northeastern indian ocean

the rise of solid state diapirs when magma inside a diapir solidifies it increases in density and becomes a stiff power law fluid if the diapir is still positively or negatively buoyant after solidification it will continue to rise or start to sink the results presented here suggest that solidified diapirs may rise sink a distance of up to a few kilometres in geologically reasonable times rocks and bubble and or crystal rich magmas behave as power law fluids this paper compares numerical results of viscosity and strain rate profiles across power law and newtonian diapirs and wall rocks the comparison shows that the core of power law diapirs deforms more slowly and the margins much faster than in newtonian diapirs this pattern of strain rate distribution leads to the often observed isotropic or weakly deformed core surrounded by a strongly sheared margin furthermore the results suggest that the strain rate of the ambient fluid wall rock is imposed across the contact into a power law diapir resulting in similar strains on either side of the contact and that power law wall rocks in contrast to newtonian wall rocks deform into a rim synform as the diapir rises

testing the limits of virtual deep seismic sounding via new crustal thickness estimates of the australian continent we apply virtual deep seismic sounding vdss to data collected from both permanent and temporary seismic stations in australia with the goal of examining i the resilience of the method to the presence of complex lithospheric structure and ii the effectiveness of different approaches for estimating bulk crustal properties namely thickness and vp data from the permanent wrab in the northern territory station is ideal for benchmarking vdss large number and favourable distribution of recorded earthquakes with the results from several approaches agreeing on a thickness of 40 42 km application of vdss to data from the temporary bilby array a linear distribution of broadband stations that traverses central australia shows that strong moho reflections can be retrieved with as few as two earthquakes even at the transition between crustal blocks of different character and in the presence of thick sedimentary basins crustal thickness varies between 36 54 km and compare well with the reflectivity character of nearby deep seismic reflection lines furthermore we find that off line estimates of crustal thickness calculated by binning the source regions according to back azimuth produce estimates of crustal thickness that are consistent with the regional geology overall we find that vdss is a powerful technique for estimating crustal thickness and velocity due to its insensitivity to complex short wavelength structure and requirement of a small number earthquakes to produce a stable result however not all schemes tested for extracting bulk crustal properties appear to be robust and stringent data quality checking is still required during implementation

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

the evaluation of brine prospects and the requirement for modifications to filing standards the recent increase in demand for lithium has led to the development of new brine prospects particularly in the central andes the brines are hosted in closed basin aquifers of two types mature halite dominant and immature clastic dominant the estimate of elemental resources in these salars depends on a detailed knowledge of aquifer geometry porosity and brine grade the geometry of the aquifers can be evaluated by classical geophysical and drilling techniques but because the resource is a fluid with the attendant problems of in aquifer mixing and reorganization existing codes for filing resource and reserve estimates need modification total porosity is relatively straightforward to measure but effective porosity and specific yield which are required to estimate the resource are more difficult recovery factors are low compared with most metalliferous and industrial mineral deposits due to reliance on pumping of the brine from wells for extraction these and related issues lead us to believe that modifications to the existing standards for reporting mineral resources and reserves are required for these prospects

a 106ka paleoclimate record from drill core of the salar de atacama northern chile abstract a 100 m long salt core sqm 2005 from the salar de atacama northern chile 23 s 68 w a dry lake bed contains a 106 kyr paleoclimate record of hydrologic balances on the western slopes of the central andes of south america six u series disequilibrium dates range sequentially from 106 1 6 4 to 5 4 2 7 ka based on sedimentary structures and petrographic textures of salts and associated siliciclastic sediments interpretations of paleoenvironments and net hydrologic balance suggest that relatively wet periods saline lakes and expanded mudflats existed in the salar de atacama from 75 7 to 60 7 ka and from 53 4 to 15 3 ka with the wettest perennial lake interval from 26 7 to 16 5 ka short relatively wet periods also occurred in the holocene from 11 4 to 10 2 ka and from 6 2 to 3 5 ka these wet periods at salar de atacama correspond well with other late pleistocene climate records from the central andes that are a function of the net hydrologic budget the minchin tauca lake sequence from salar de uyuni bolivia is synchronous with the 53 4 15 3 ka saline lake wet period at salar de atacama the tauca phase coincided with the wettest perennial lake interval at salar de atacama from 26 7 to 16 5 ka the coipasa lake phase from uyuni was probably contemporaneous with the early holocene wet period at salar de atacama from 11 4 to 10 2 ka the early holocene wet interval at salar de atacama was also synchronous with the maximum holocene lake levels of the chilean altiplano lakes to the east and with grass rich rodent middens between 11 8 and 10 5 ka in age from the atacama basin the lake period at salar de atacama between 26 7 and 15 3 ka compares closely with low 18o values low anion concentrations and high accumulation rates from the sajama glacial ice core southwestern bolivia between 25 and 15 5 ka it is concluded that wet and dry phases were synchronous in the central andean plateau of southwestern bolivia and along the western flank of the andes of northern chile over the past 50 kyr this region which today receives its moisture from the amazon basin to the east probably also did so in the past the wet periods at salar de atacama are in phase with january insolation maxima at 15 s at 70 45 and 20 ka which suggests that past changes in summer insolation influenced precipitation patterns in the central andes however there are no major wet periods linked to the january insolation maximum at 93 ka and the present day extremely dry climate in the atacama desert is associated with a peak in summer insolation therefore there must be other important influences over precipitation in the central andes which modify the first order insolation controls

lithospheric structure beneath the archaean slave province and proterozoic wopmay orogen northwestern canada from a lithoprobe refraction wide angle reflection survey the slave geological province a relatively small craton in nw canada that includes the oldest rocks in the world is one of five cratons that form the archaean continental core of north america the palaeoproterozoic wopmay orogen lying between the craton and the phanerozoic northern canadian cordillera represents crustal growth through addition of a series of north south oriented magmatic arcs and accreted terranes as part of lithoprobe s multidisciplinary studies line 11 of the snore 97 seismic refraction wide angle reflection survey investigated the structure of the lithosphere in this region interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling results show that crustal thickness at 32 35 km remains relatively constant from the archaean regions in the east to proterozoic domains in the west contrary to average values of crustal thicknesses from global compilations that suggest thicker proterozoic crust at the westernmost end of the line where crustal extension and basin formation are inferred the crust thins to 30 km upper crustal velocities to about 12 km depth lie in the range of 5 8 6 2 km s 1 the middle crust of thickness 8 10 km has velocities between 6 2 and 6 6 km s 1 average velocities for the lower crust differentiated as a 10 km thick layer range between 6 6 and 6 9 km s 1 with a higher velocity region 7 1 km s 1 in the transition zone between the fort simpson magmatic arc and hottah accreted terrane in the wopmay orogen where a palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data in the same area anomalously low upper mantle velocities 7 5 km s 1 have been found indicating that the effects of collision subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure may have been retained for 1800 ma data from most record sections with offset distances greater than 250 km show continuous wide angle reflections from upper mantle depths of about 75 km and almost 100 km although poorly constrained velocities in this depth range vary from 8 4 to 8 8 km s 1 interpretation of these phases and mantle reflections from equivalent depths in coincident near vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the palaeoproterozoic orogen

tomography on pp p waves and its application for investigation of the upper mantle in central siberia abstract we develop the method of tomographic inversion called rr r scheme which is based on joint use of teleseismic p or s refracted rays and corresponding pp or ss rays with bounce points located within a study region this scheme allows imaging the deep seismic structure beneath blank areas where there are neither recording stations nor earthquakes utilization of differential travel times makes it possible to avoid the difficulty of source and station corrections which cause problems in teleseismic tomography the rr r scheme has been applied to more than 10 000 ray pairs from the isc database to investigate a large region from the north arctic ocean to the northern part of china and mongolia inversion was carried out in several separate windows covering the study area down to the depth of 530 km velocity anomalies were computed in grid nodes distributed in the study 3d area according to ray density the tomography results from different windows were combined into one general map of the lithospheric structure in central siberia principal positive p wave velocity anomalies 1 3 down to 400 km are associated with the precambrian siberian craton negative velocity anomalies are related to the thin lithosphere of the west siberian plate the altai mountains and the hangai plateau 3 1 in the depth range 30 to 430 km a strong negative anomaly beneath the precambrian hangai block is apparently associated with a mantle plume positive velocity anomalies in sedimentary basins at southern siberia are believed to have the crustal origin

u s mid atlantic margin structure and early thermal evolution marine seismic experiments have revealed that the u s east coast rifted margin crustal structure is dominated by voluminous volcanics emplaced during continental breakup we present results from seismic data recorded onshore during the 1990 edge mid atlantic onshore offshore seismic experiment that extend the margin crustal profile westward 180 km across the coastal plain and place the offshore results into a broader tectonic context the onshore offshore crustal model defines the crustal thinning profile across the margin enabling calculation of total extension based on direct measurements of crustal thickness we find that the crust beneath the coastal plain is largely unextended has a uniform thickness of 35 km shows no evidence for magmatic additions of sufficient volume to affect seismic velocity and that the total half extension of the crust is 30 to 40 km less than previous estimates by a factor of 3 to 4 we also present results of depth migration of the offshore vertical incidence and wide angle seismic data that yield the most complete and accurate image to date of reflectivity across the u s east coast margin we use this image to analyze margin subsidence by exploiting the stratigraphy of the subaerially extruded basalts beneath the sediments we find that 1 margin subsidence is consistent with a petrogenetic model for initial seafloor spreading magmatism involving high average melting pressures 3 5 4 0 gpa and potential temperatures 1500 c for a present day thermal lid thickness of 225 km 2 it is difficult to explain margin subsidence with a present day lid thickness of much less than 150 km and 3 rapid subsidence following breakup suggests the initially hot mantle underlying the rifting margin cooled very quickly implying a limited and finite initial distribution of hot mantle rather than a continuous upwelling or a ubiquitously warm upper mantle

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

rates and geochemical processes of soil and salt crust formation in salars of the atacama desert chile abstract the hyperarid atacama desert contains numerous local basins with surficial salt crusts known as salars where evaporation of shallow groundwater drives the major soil processes we examine chemical and isotopic profiles in two soils of differing ages from the salar llamara to determine the geochemical processes involved in their formation evaporation which provides salts to the soils through mineral precipitation decreases with increasing salt crust thickness and average 0 03 mm m 2 d 1 over geological time frames salt distribution varies predictably with depth and soil age with the most soluble compounds concentrated nearest to the land surface indicating the direction of fluid flow 34s values of mineral sulfate tend to decrease with decreasing soil depth following a pattern indicative of rayleigh like fractionation as solute rich waters migrate toward the land surface 13c and 18o values of carbonate suggest that the uppermost halite layers which contain very small amounts of carbonate have a strong biological signature in contrast carbonate rich layers deeper in the profiles consist of largely unmodified lacustrine carbonate that formed in highly evaporitic lake conditions the continuous upward evaporation of water and dissolved solutes creates a rugged and physically dynamic halite crust composed of rounded salt nodules the crust undergoes deliquescence as atmospheric relative humidity rises from marine air intrusions and we found that the halite nodules on the surface of the salar llamara are nearly always at or above deliquescence relative humidity the interiors of these nodules are therefore able to buffer the large diurnal changes in atmospheric relative humidity allowing for the survival of halophilic microbial communities in an otherwise very dry environment radiocarbon measurements of occluded organic c in the surface crusts indicate that c cycling occurs at differing rates depending on local micrometeorological conditions and that a given salt crust feature may persist for thousands of years once formed

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

interference between pluton expansion and non coaxial tectonic deformation three dimensional computer model and field implications abstract structures in orogenic belts are greatly controlled by interactions between pluton related and tectonic related strain fields however many aspects of the geometry and evolution of these structures are poorly known for example 1 what is the geometry of these structures if regional deformation is coaxial 2 how does this geometry change with progressive deformation this paper describes a three dimensional simulation of strain shape and magnitude as well as foliation and lineation patterns formed by interactions between pluton expansion and homogeneous coaxial deformation this simulation suggests that interactions between pluton expansion and regional uniaxial shortening generate patterns that are distinct from the patterns produced by regional uniaxial extension or regional non coaxial deformation for example shortening produces doughnut shaped constrictional rings about the pluton whereas extension produces apple shaped flattening regions around the pluton in addition shortening produces one continuous doughnut shaped low strain region around the pluton on the other hand extension creates two separate ellipsoidal low strain regions at the ends of the pluton furthermore shortening forms foliation rings that have triangular two dimensional sections and lineation is concentric around the axis of maximum shortening in contrast extension produces two separate foliation and lineation cones at the ends of the pluton all these structural and strain elements migrate away from the pluton with increasing expansion rates and towards the pluton with increasing tectonic deformation rates although structural studies around plutons should always integrate microstructural radiometric and field measurements simulation generated patterns can be used to evaluate 1 origin geometry and evolution of structures around plutons 2 mechanisms that explain inclusion trails within porphyroblasts 3 contribution of pluton related strains to strains measured in the country rock and 4 type and three dimensional orientation of regional kinematic schemes

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

thick crust beneath the ordos plateau implications for instability of the north china craton abstract surrounded by seismicity and other manifestations of active deformation the ordos plateau or the western portion of the north china craton ncc is a uniquely stable terrane in asia results from virtual deep seismic sounding and crustal receiver functions suggests that the crust under the eastern ordos is thicker at least 60 km than expected from previous studies and from its modest elevation 1500 m above sea level receiver functions also reveal a pronounced elastic impedance contrast within the crust at 40 km depth which we interpret as the conrad discontinuity the presence of a 20 km thick layer of mafic lower crust between the conrad and moho discontinuities would maintain crustal isostasy the 1000 km long seismic profile from the ordos plateau in the west to the north china basin in the east reveals that crustal thickness changes by almost a factor of two across the active shanxi rift in central ncc over a distance of only about 100 km insofar the current configuration of the lithosphere under the ordos plateau might serve as a proxy for the initial condition prior to reactivation of the eastern part of ncc where a cratonic keel no longer seems to exist our results support the hypothesis that lower crust foundering was due to transformation of a thick mafic lower crust to a garnet rich assemblage possibly caused by hydration associated with subduction during and or before mid mesozoic times

the long term strength of europe and its implications for plate forming processes field based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons this partitioning of deformation suggests that the cratons are stronger than surrounding younger phanerozoic provinces however although archaean and phanerozoic lithosphere differ in their thickness and composition their relative strength is a matter of much debate one proxy of strength is the effective elastic thickness of the lithosphere te unfortunately spatial variations in te are not well understood as different methods yield different results the differences are most apparent in cratons where the bouguer coherence method yields large te values 60 km whereas the free air admittance method yields low values or 1 5 gyr old is much larger mean te 60 km than that of younger lithosphere mean te 30 km we propose that this strength difference reflects changes in lithospheric plate structure thickness geothermal gradient and composition that result from mantle temperature and volatile content decrease through earth s history

modeling flow and reactive transport to explain mineral zoning in the atacama salt flat aquifer chile geochemical processes are typically neglected when modeling groundwater flow in highly saline systems in the atacama salt flat northern chile field data indicate that the main aquifer presents a mineralogical zoning consisting of carbonates sulfates and halite which is a result of precipitation and or dissolution reactions when precipitation and or dissolution of minerals are active processes in the aquifer physical properties that control groundwater flow such as porosity and permeability would also change until now groundwater flow in the atacama salt flat has been simulated with conventional constant or variable density flow models this study for the first time investigated the importance of incorporating geochemical processes in modeling the atacama salt flat aquifer in particular the impact of mineral precipitation dissolution on aquifer s physical properties and flow patterns was studied to this end the shemat code was utilized to develop a two dimensional numerical groundwater flow model in the eastern border of the atacama salt flat the shemat code solves the classic groundwater flow equation coupled to the reactive transport of ions and mineral precipitation with variable density viscosity porosity and permeability when geochemical processes were included in the simulations the model reproduced reasonably well the mineral zoning evidenced by field observations mineral precipitation was responsible of a strong permeability reduction in areas of the aquifer associated with halite and calcite formation permeability alterations were responsible of changes in the equivalent freshwater head that resulted in significant flow pattern variations compared to variable density flow models these results highlight the importance of considering geochemical processes that modify the aquifer s physical properties when modeling groundwater flow in highly saline aquifers such as the atacama salt flat aquifer

floating brine crusts reduction of evaporation and possible replacement of fresh water to control dust from owens lake bed california summary owens lake california a saline terminal lake desiccated after diversion of its water source was formerly the single largest anthropogenic source of fugitive dust in north america over 100 billion m 3 yr 1 of fresh water are projected to be used for mandated dust control in over 100 km2 of constructed basins required to be wetted to curtail emissions an extensive evaporite deposit is located at the lake s topographic low and adjacent to the dust control basins because this deposit is non dust emissive it was investigated as a potential replacement for the fresh water used in dust control the deposit consists of precipitated layers of sodium carbonate and sulfate bathed by and covered with brine dominated by sodium chloride perennially covered with floating salt crust evaporation e rates through this crust were measured using a static chamber during the period of highest evaporative demand late june and early july 2009 annualized total e from these measurements was significantly below average annual precipitation thus ensuring that such salt deposits naturally remain wet throughout the year despite the arid climate because it remains wetted the evaporite deposit may therefore have the potential to replace fresh water to achieve dust control at near zero water use

geophysical evidence for chemical variations in the australian continental mantle 1 the relative density to shear velocity scaling provides a diagnostic for the presence of compositional variations in the mantle we invert shear wave velocity from a recent 3 d model and gravity anomalies for radial profiles of of the uppermost mantle beneath australia we performed calculations for the three major tectonic provinces that constitute the continent and found significant differences between them the profile for the phanerozoic region can be explained by thermal variations alone in contrast negative values of suggest that variations in composition are important between 75 and 150 km depth in the proterozoic continental lithosphere central australia it is likely that chemical variations are also required to explain the inferences for the archean craton west australia but poor tomographic resolution precludes a definitive conclusion the scaling factors found are consistent with chemical depletion of deep precambrian lithosphere which supports a tectosphere model for the australian continental roots

anomalies of temperature and iron in the uppermost mantle inferred from gravity data and tomographic models we propose a method to interpret seismic tomography in terms of thermal and compositional anomalies in addition to the tomographic model we use gravity data which provide information on the density expressed as a relative density to shear wave velocity scaling factor ln ln vs the inferred values of are not consistent with the presence of thermal anomalies alone however simultaneous anomalies of temperature and composition explain the observations compositional anomalies can have several origins but we find the most relevant parameter to be the global volumic fraction of iron xfe fe fe mg we invert the tomographic model s16rlbm woodhouse and trampert 1995 and the density anomalies correlated to vs anomalies 0 vs v0 for anomalies of temperature t and iron fe the partial derivatives are provided by a numerical method that reconstructs density and seismic velocity for given temperatures and petrologic models vacher et al 1998 down to z 300 km depth the distribution of temperature and iron anomalies strongly depends on the surface tectonics the continental mantle below old cratons and stable platforms is colder than average and depleted in iron whereas the oceanic mantle is mostly homogeneous due to uncertainties on the reference state of the mantle error bars on t and fe reach 10 of the inverted values finally we apply these results to the stability of continental roots and test the hypothesis that the negative buoyancy induced by lower than average temperatures is balanced by the positive buoyancy induced by the depletion in iron we find that continental roots are stable only if the viscosity of the mantle is strongly temperature dependent however some uncertainties remain on the real effects and importance of rheology 2002 elsevier science b v all rights reserved

an assessment of stratigraphic completeness in climate sensitive closed basin lake sediments salar de atacama chile abstract dated subsurface cores 40 m 100 m and 200 m in length from the salar de atacama chile record changes in climate and tectonics over the past 325 ka these cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change the same basic facies efflorescent halite crust and cement subaerial halite and chevron halite saline lake occur in each core the dominant facies is efflorescent halite formed in a subaerial environment like that in the atacama basin today thick well preserved efflorescent crusts in the salar de atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines similar pedogenically formed evaporites may be pre served in the geologic record the n s trending salar fault system has downdropped the eastern block of the salar de atacama core 2002 relative to the western block cores 2005 and 2031 relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium series ages from core 2005 major faulting along the salar fault system occurred between 16 5 and 5 4 ka with offset of 26 5 m and faulting rates of 2 4 m kyr aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting although stratigraphic units in core 2002 shifted down 30 m over the past 60 ka the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores faulting along the salar fault system despite significant offset did not alter the basic stratigraphic record temporal completeness of the stratigraphic record within the limits of the age dates and their errors of greater than 3 ka was examined qualitatively by comparing long term sedimentation rates 0 6 0 9 m kyr and shorter term sedimentation rates 0 3 3 6 m kyr the similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of salar de atacama sediments at least at the 10 kyr scale although hiatuses long periods of subaerial exposure and dissolution of evaporites may have occurred the main climate fluctuations at salar de atacama over the last 100 ka are recorded in all three subsurface cores paleoclimate records obtained from such cores are therefore representative of basin wide climate change and are valid for regional paleoclimate reconstructions

evaporation rate from a salt pan estimates from chemical profiles in near surface groundwaters evaporation of water from saturated saline soils or sediments induces gradients of the solutes in the remaining interstitial solution these gradients lead to a systematic and predictable solute distribution in the soil solution which may be used in some cases to estimate the rate of evaporation simple one dimensional transport models for the distribution of cl and br indicate net evaporation rates from the salt covered surface of lake eyre south australia of between 0 9 and 2 8 cm yr 1 the former is a minimum estimate averaged over 20 yrs which includes the major 1974 filling event while the latter reflects the most recent summer evaporation period

3d mapping hydrodynamics and modelling of the freshwater brine mixing zone in salt flats similar to the salar de atacama chile abstract salt flat brines are a major source of minerals and especially lithium moreover valuable wetlands with delicate ecologies are also commonly present at the margins of salt flats therefore the efficient and sustainable exploitation of the brines they contain requires detailed knowledge about the hydrogeology of the system a critical issue is the freshwater brine mixing zone which develops as a result of the mass balance between the recharged freshwater and the evaporating brine the complex processes occurring in salt flats require a three dimensional 3d approach to assess the mixing zone geometry in this study a 3d map of the mixing zone in a salt flat is presented using the salar de atacama as an example this mapping procedure is proposed as the basis of computationally efficient three dimensional numerical models provided that the hydraulic heads of freshwater and mixed waters are corrected based on their density variations to convert them into brine heads after this correction the locations of lagoons and wetlands that are characteristic of the marginal zones of the salt flats coincide with the regional minimum water brine heads the different morphologies of the mixing zone resulting from this 3d mapping have been interpreted using a two dimensional 2d flow and transport numerical model of an idealized cross section of the mixing zone the result of the model shows a slope of the mixing zone that is similar to that obtained by 3d mapping and lower than in previous models to explain this geometry the 2d model was used to evaluate the effects of heterogeneity in the mixing zone geometry the higher the permeability of the upper aquifer is the lower the slope and the shallower the mixing zone become this occurs because most of the freshwater lateral recharge flows through the upper aquifer due to its much higher transmissivity thus reducing the freshwater head the presence of a few meters of highly permeable materials in the upper part of these hydrogeological systems such as alluvial fans or karstified evaporites that are frequently associated with the salt flats is enough to greatly modify the geometry of the saline interface