petrogenesis of the tertiary anorogenic volcanic series of southern queensland australia in the light of trace element geochemistry and o sr and pb isotopes the magmas of the tertiary volcanic province of s queensland are chemically bimodal and occur in numerous volcanic centres at least three representing original shield volcanoes the mafic lavas are dominantly hawaiites and tholeiitic andesites whereas the silicic magmas comprise mainly trachytes rhyolites and comendites the silicic rocks exhibit variable trace element abundance patterns there is a progressive depletion of sr ba v mg ni cr mn and p through the trachytes to the rhyolites and comendites while the behaviour of zr nb lree y and zn is very variable rb th and to a lesser extent pb exhibit a more regular behaviour becoming most generally concentrated in the comendites and rhyolites these trace element patterns are modelled by application of the rayleigh distillation model using partition coefficients based on analysed phenocrysts from the s queensland silicic lavas trace mineral phases namely zircon chevkinite and allanite are shown to be important in the probable control of lree zr and th abundances while nb and zn are probably controlled during fractionation by magnetite trace element data for the hawaiites and tholeiitic andesites also indicate extensive although variable levels of fractional crystallization of these magmas the sr and o isotopic compositions of the mafic lavas trachytes comendites and rhyolites are as follows initial 87sr 86sr ratios 0 70357 0 70456 0 70432 0 70589 0 70495 0 70917 and 0 70708 0 70863 respectively 18o range between 5 6 7 0 mafic lavas 4 9 8 7 trachytes 5 0 7 6 comendites and 8 1 10 4 per mil rhyolites pb isotopic compositions are variable showing a variation of 6 7 per cent for 206pb 204pb ratios through the range of volcanic compositions the rhyolites exhibit a much greater divergence in their o sr and pb isotopic compositions compared with those of associated mafic lavas than is found in the trachytes and comendites within the silicic volcanics positive correlations exist between 18o and initial sr ratios and between pb isotopic compositions and initial sr ratios with one group of trachytes providing a noteworthy exception these correlations are not so clearly defined for the mafic lavas although these do exhibit positive correlations between differentiation index 18o and initial sr isotope ratios the development of the silicic magmas excepting two groups is interpreted in terms of a model in which assimilation and fractional crystallization occur concurrently involving a basalt or hawaiite magma component and a crustal component modelled on the analysed carboniferous basement greywackes outeropping in the region the data indicate however that differentiation continued in isotopically closed systems i e isolated from the wallrocks the highly depleted sr and ba abundances of the rhyolites and comendites suggest that contamination did not occur after differentiation had ceased the rhyolites have the highest isotopic input of the crustal components and are interpreted as crustal anatectic melts produced locally within the crust in response to basalt hawaiite magma intrusion whereas most of the trachytes and comendites are interpreted as primarily the differentiated products from original mafic parental magmas with variable assimilation of crustal wallrock components the isotopic data suggest that only the minerva hills trachytic lavas and a glass house comendite have not been significantly modified by wallrock assimilation processes the erpted mafic magmas were also evidently modified by isotopic crustal wallrock interactions which independent petrological data suggest has occurred at intermediate to lower crustal depths

musings over sedimentary basin evolution field geologists and explorationists are of necessity immersed in numerous very detailed surface and subsurface observations they are often perplexed by the choice of relatively simple geophysical models that so elegantly explain the origin and evolution of sedimentary basins geophysicists on the other hand search for a simple theme to explain the origin of sedimentary basins and much like managers are often impatient with lengthy detailed geological discourse that often uses fancy jargon to hide the very real difficulty that geologists have in separating important evidence from mere encyclopaedic description the following musings address the quality and limitations of geologic and geophysical evidence that may be used to evaluate the relative roles of stress thermal effects and gravity loading which have been so lucidly summarized by m h p bott in the preceding summary the fine papers presented during this meeting of course have led to significant modifications of some of my earlier thoughts because these have been previously published elsewhere bally snelson 1980 bally 1980 they are summarized here only for the convenience of the reader

geochemical hypothesis for hydrated magnesium borate deposit in salt lake nw china chloropinnoite 2mgo 2b2o3 mgcl2 14h2o was a new borate obtained from the natural concentrated salt lake brine in qinghai xizang plateau p r china the phase transitions from chloropinnoite dissolved in water and boric acid solution would strongly consist of magnesium borate minerals deposited in a salt lake of china the obtained results from phase relations and kinetic mechanism of chloropinnoite water boric acid system would further propose a new geochemical hypothesis for hydrated magnesium borate deposition the chloropinnoite was diluted by rich boric acid or solution and it would accelerate the phase transition of chloropinnoite into other borates which would explain well the geochemical formation of hydrated borate minerals deposited in qinghai xizang plateau china

discussion of a simple method to determine breaker height and depth for different deepwater wave height length ratios and sea floor slopes by j p le roux coastal engineering 54 2007 271 277 abstract in the recent paper by j p le roux coastal engineering 54 2007 271 277 the author provides a simplified approach to calculating the depth length and height of waves at the onset of depth induced breaking i e at the breaker line however the proposed methodology and the comparisons to other methods suffer from a large number of inconsistencies and basic calculation errors in addition there are a number of erroneous physical interpretations and many of the conclusions are based on erroneous data the remaining conclusions are either not new or based on circular logic such as to render them moot in the following we will not attempt to point out all the errors or inconsistencies that we found instead we focus on major points of contention

sulfate borate association glauberite probertite in the emet basin implications for evaporite sedimentology middle miocene turkey abstract probertite and glauberite are the main rock forming minerals in the lacustrine doganlar succession of the emet basin the present paper seeks to characterize and interpret the facies and depositional settings of this mineral association probertite is made up largely of microfibers arranged in microbundles and displays a variety of lithofacies many lithofacies show deformation and flowage structures which can be assigned to multistage growth of probertite under synsedimentary conditions glauberite displays laminated lithofacies formed by fine grained clear crystals and thin to thick bedded lithofacies composed of coarser crystals and variable proportions of siliciclastic matrix depositional pairs of probertite and glauberite occur at millimeter laminites centimeter alternations and meter cycles scales probertite and glauberite are primary products grown subaqueously in a relatively shallow saline lake either on the bottom probertite and glauberite or as cumulate crystals glauberite and interstitially below and close to the bottom the laminites are made up of microcycles composed of several types of laminae the microcycles are interpreted as predominantly pluriannual precipitates in the proposed lake model the brine body is not deep as indicated by frequent episodes of subaerial exposure recorded in the succession this study provides new insight into the genesis of glauberite and probertite the interpretation of nodular lithofacies the type of microfiber evaporites the evaporite units made up of sulfate borate alternations and the petrographic description of glauberite laminites and their paleoenvironmental interpretation

exceptionally preserved ostracodes from a middle miocene palaeolake california usa abstract exceptionally well preserved specimens of a new cypridid ostracode crustacea raepula ira sp nov are described from palaeolake sediments of the middle miocene barstow formation of the mud hills southern california this is only the second occurrence of exceptionally preserved ostracodes from the miocene based on ages obtained from associated volcanic tuffs the palaeolake sediments were deposited between 16 3 and 15 8 ma the ostracodes form one element of a diverse lake community dominated by fairy shrimps copepods diatoms larvae of diving beetles flies and mosquitoes and body fossils and ephippia of branchiopods and anomopods the ostracodes are preserved three dimensionally with their soft anatomy replicated in microcrystalline silica submicron scale details such as sensory setae are preserved surpassing the resolution of most other ostracode bearing lagerstatten and allowing their biology to be compared with extant taxa supplementary material red blue anaglyphs of the exceptionally preserved ostracodes are available at http www geolsoc org uk sup18407

origin of the deccan trap flows at mahabaleshwar inferred from nd and sr isotopic and chemical evidence abstract the deccan flows at mahabaleshwar are divisible into a lower and an upper group based on nd and sr isotopic ratios which define two correlated trends this distinction is supported by incompatible element ratios and bulk compositions the data reflect contamination in a dynamic system of magmas from an lil depleted e juv 8 mantle by two different negative e juv endmembers one undoubtedly continental crust the other either continental crust or enriched mantle the depleted mantle source anomalously high in 87 sr 86 sr may have been in the subcontinental lithosphere or a region of rising indian ocean morb mantle

mineralogy diagenesis and hydrochemical evolution in a probertite glauberite halite saline lake miocene emet basin turkey abstract the emet basin is one of the neogene basins in western turkey containing significant amounts of borate minerals mainly colemanite the petrologic study of core samples from two exploratory wells in the doganlar sector under optic and electron microscopy reveals a complex mineral association in which probertite glauberite and halite constitute the major primary phases without mineral precursors precipitated in a saline lake placed in a volcano sedimentary context other sulfates anhydrite gypsum thenardite celestite and kalistrontite borates colemanite ulexite hydroboracite tunellite kaliborite and aristarainite and sulfides arsenopyrite realgar and orpiment are attributed to early diagenesis so far the doganlar deposit is the most important deposit of probertite known up to now chemical changes in the groundwater inflow led to the precipitation of ca bearing borates colemanite in the tuff flat environment surrounding the lake while na ca sulfates and borates glauberite and probertite precipitated in the center of the lake fluid inclusion compositions in halite indicate that the advanced brines correspond to the na k cl so 4 type during restricted stages of the saline lake the residual brines seeped through the tuff flat sediments leading to transformations of previous precipitates that resulted in the formation of k bearing minerals the abundance of coccoid like biogenic dolomite colloidal arsenopyrite and the isotopic composition of sulfates are indicative of bacterial sulfate reduction in contrast arsenic sulfides are attributed to acidophilic micro organisms in oxidizing conditions fluctuations of redox conditions in both free and interstitial brines control the biological influence in some diagenetic transformations

deep seismic reflection profiling and the growth of the continental crust abstract deep seismic reflection profiling provides evidence for the style and magnitude of phanerozoic crustal growth by both lateral and vertical accretion profiles across convergent zones have been interpreted as showing tectonic underplating of the continental crust by fragmented oceanic lithosphere profiling across extensional zones provides evidence for magmatic underplating of the crust in addition to studies over active areas deep reflection profiling over large areas of western europe north america and australia shows that the lower crust is strongly reflective and seismically distinct from the upper crust and upper mantle the layering is detected beneath many different terranes of varying crustal type and basement age from archaean to cenozoic and is often apparently younger than the tectonic or metamorphic age of the exposed upper crust thus continental growth curves estimated from upper crustal measurements alone may underestimate phanerozoic crustal additions and mean crustal ages similarly estimated may be too high

emplacement and tectonic implications of the mid proterozoic peralkaline flowers river igneous suite north central labrador abstract the flowers river igneous suite ca 1270 ma is composed of several discrete plutons of peralkaline granite eccentrically surrounding a circular area of felsic subaerial volcanic rocks dikes of peralkaline granite are oriented tangentially and radially to the volcanic center individual plutons which vary from ring shaped or arcuate to irregular in plan view were emplaced at or near the surface during and or following cogenetic volcanism these features coupled with a remarkable bulls eye aeromagnetic anomaly pattern imply that the flowers river igneous suite is a rudimentary ring complex individual ring dikes were substantially enlarged and modified by piecemeal stoping during emplacement preservation of the volcanic terrain from subsequent erosion appears to reflect cauldron subsidence of the roof of the largest granite pluton the flowers river igneous suite was emplaced in a stable intracratonic environment immediately following a period of substantial uplift and erosion this uplift was permanent induced isostatically by the addition of large volumes of mantle derived magmas during the mid proterozoic heat from the mantle derived magmas together with decompression during uplift and erosion caused small degrees of partial melting in the lower crust and the formation of independent pockets of a type magma these melts intruded the extended crest of the uplifted region forming isolated felsic volcanic centers including the flowers river igneous suite

the kerguelen isles southern indian ocean new results on deep structure from refraction profiles abstract additional seismic refraction profiles shot across the kerguelen archipelago terres australes et antarctiques francaises known as taaf located in the southern indian ocean substantiate the oceanic type crust of the archipelago and the kerguelen plateau as well the crust mantle boundary is better modelled by a transition than by a discrete interface seismic velocities of 6 60 6 80 km s are in the range of those within oceanic layer 3 velocities of 7 2 7 5 km s observed at long ranges may be interpreted according to at least two distinct hypotheses 1 1 notwithstanding their rather low values they represent upper mantle material and might be assigned to peridotites serpentinised by hydrothermal circulations as reported for various oceanic and continental features 2 2 these velocities are assigned to the lower crust intruded by melt produced by adiabatic decompression of upper mantle material during rifting underplating a small increase in mantle temperature caused by the presence of heat sources i e the southeast indian ridge and the kerguelen mantle plume nearby may have produced huge quantities of melt facilited by the slow movement of the antarctic plate then building up the aseismic kerguelen plateau

a function to determine wavelength from deep into shallow water based on the length of the cnoidal wave at breaking reply to discussion by t s hedges abstract the equations of hedges hedges t s 2009 discussion of a function to determine wavelength from deep into shallow water based on the length of the cnoidal wave at breaking by j p le roux coastal eng although yielding similar wavelengths are not consistent with the fact that the horizontal water particle velocity in the wave crest should equal the wave celerity at breaking over a nearly horizontal bottom

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

low temperature thermal history and landscape development of the eastern adirondack mountains new york constraints from apatite fission track thermochronology and apatite u th he dating the adirondack mountains in northern new york state form an elongate domal exposure of mainly high grade metamor phic tectonites in a mountainous setting with topographic relief of 1 km the origin of the adirondack mountains and this relief has long been enigmatic since the adirondacks presently lie within an intracratonic setting inboard of the north american passive mar gin and far from any active plate boundaries through the application of apatite fi ssion track aft thermochronology and apatite u th he ahe dating within the eastern adirondack mountains this study provides constraints on both the thermal and ero sional effects of mesozoic passage near a hot spot and the timing of relief development aft thermochronology and ahe dating record relatively stable thermal conditions within the eastern adirondacks from the middle jurassic into the early cretaceous during the early cretaceous ca 130 120 ma the region underwent heating as sociated with progressive movement near the great meteor hotspot resulting in the temporary establishment of an elevated geo thermal gradient following regional heat ing cooling rates increased considerably ca 105 95 ma likely due to both thermal doming producing an increase in erosion rate and the relaxation of isotherms after passage near the hotspot the regional distribution of aft ages across the eastern adirondacks reveals no systematic age gradient from core to periph ery which would be expected under con ditions of persistent high relief during the decay of a crustal root over many tens to hun dreds of millions of years instead thermo chronological data suggest that the present relief developed during the late cretaceous cenozoic through plateau dissection during periodic base level changes

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

crustal structure of the british isles and its epeirogenic consequences crustal receiver functions have been calculated for a network of 51 three component broadband seismometers distributed across the british isles and nw europe over 3200 receiver functions were assembled for 1055 events for each station preliminary estimates of crustal thickness and vp vs ratio were obtained from h plots stacked receiver functions were then inverted to determine shear wave velocity as a function of depth each result was checked by guided forward modelling and by monte carlo error analysis in this way the robustness of our final calculated velocity profiles was carefully tested a set of depth migrated profiles was also constructed using an average of 50 events for each station over a range of backazimuths these profiles agree well with legacy wide angle crustal models our results show that crustal thickness varies between 24 and 36 km across the british isles thicker crust is found beneath north wales and beneath central scotland thinner crust occurs beneath northwest scotland and northwest ireland by combining our database with the results of controlled source wide angle experiments and with depth converted reflection profiles we have produced a detailed crustal thickness map for a region encompassing the british isles our synthesis of crustal thickness and structure has important implications for the tectonic and magmatic histories of this region complex moho structure with lower crustal p wave velocities of 7 km s 1 occurs beneath regions of cenozoic magmatism which may be consistent with magmatic underplating thin crust beneath northern britain suggests that present day long wavelength topography is maintained by regional dynamic support originating beneath the lithospheric plate

a coupled modeling scheme for longshore sediment transport of wave dominated coasts a case study from the southern baltic sea abstract a modeling scheme based on dynamic coupling of a high resolution 1d cross shore model to a 2dh area model is developed to calculate the total longshore sediment transport lst rate in wave dominated coasts the purpose of this coupling strategy aims at resolving the lst with a high resolution both temporally and spatially inside the surf zone and with a coarser spatial resolution seaward of the surf zone the 2dh area model operates on a fixed pre designed regional grid parent grid and the 1d cross shore model is dynamically coupled to the boundary of the parent grid with a time varying domain starting from the first wave breaking point and ending at the maximum wave set up point the time varying domain is generated in the 1d model by resolving the landward wave propagation from the offshore conditions provided by the 2dh area model at every time step with a high resolution cell size the 1d model resolves the wave propagation processes and resulting lst along the profile the coupled model is applied to study the lst in the pomeranian bight at the southern baltic sea simulation results are compared with three other different hierarchical modeling methods from empirical formulas such as cerc and kamphuis to a 2dh area simulation the comparative study indicates that the dynamically coupled model can be a reliable tool in practical applications especially for the areas where hydrodynamics is controlled by complex bathymetry e g multiple longshore bars or morphologically induced circulation patterns

mineralogy and sedimentology of the miocene gocenoluk borate deposit kirka district western anatolia turkey the miocene boratiferous district of kirka in western anatolia turkey is the most important na borate borax resource in the world two separate deposits in the kirka district are located near the villages of sarikaya and gocenoluk eskisehir province borax is intensively exploited in open pit mines in the sarikaya deposit while only small quarries of colemanite are known in the gocenoluk deposit recent exploratory drilling in the gocenoluk area intersected a thick succession of dolostones tuffs and three borate bearing units lower intermediate and upper borate units in them the most abundant borate mineral is ulexite ca na borate passing at depth to probertite borax na borate is only present in the intermediate borate unit minor amounts of colemanite ca borate and hydroboracite ca mg borate occur at the base and or top of each mineralized unit pyroclastic layers within the borate units show intense alteration by alkaline boron bearing waters and formation of diagenetic clay minerals smectites zeolites analcime and borosilicates searlesite the gocenoluk succession is interpreted as a shallow ephemeral alkaline lake deposit in which carbonates formed as stromatolites and travertines borate precipitation in the gocenoluk area took place interstitially within muddy and carbonate sediments in a lateral progression from marginal ca borates towards na ca borates and rarely to na borates in the center of the lake authigenic silicate mineral distribution shows parallel changes toward the center of the lake that reflect increasing ph gradient

contrasting types of boron bearing deposits in magnesian skarns from romania abstract two types of boron bearing deposits displaying contrasting properties were identified in five magnesian skarns from romania at b i a bihor pietroasa cacova ierii masca b isoara and ocna de fier these should be defined as fe poor mg b and fe rich mg fe b respectively the skarns are developed at the contact of igneous bodies of upper cretaceous age with dolomite sequences they occur as metasomatic columns since a roughly consistent zoning was found from core to rim in the skarn bodies borates suanite kotoite ludwigite szaibelyite rarely fluoborite occur in the outer skarn zones whereas the inner zones typically contain magnesian silicates e g forsterite humite group minerals diopside and phlogopite and spinel the mg b type of skarn deposit is typified by the abundance of suanite szaibelyite and kotoite and by the scarcity of ludwigite and magnetite the b bearing portions of the fe rich skarns consist dominantly of bands of ludwigite alternating with magnetite small amounts of kotoite forsterite and clinohumite are also present the differentiation of the two types of deposits is further emphasized by the development of f rich parageneses i e fluoborite fluorite f rich humites only in the fe poor skarns all b bearing skarns in romania experienced generally similar peak contact metamorphic conditions of 400 650 c and pressures up to 3 kbar borates have similar geochemical signatures with respect to minor elements as the contemporaneous silicates

kurnakovite deposits on the qinghai tibet plateau ii an investigation from chemical kinetics of chloropinnoite dissolution lakes on the qinghai tibet plateau qtp are of particular interest to researchers because of their unusual high concentrations of lithium li boron b and potassium k kurnakovite a member of the inderite group is well distributed on the qtp however the geochemical mechanisms of kurnakovite transformation requires further identification and clarification this study aims to elucidate the geochemical mechanisms of kurnakovite deposits on the qtp from chloropinnoite dissolution using chemical kinetic and spectroscopic techniques the new borate chloropinnoite 2mgo 2b2o3 mgcl2 14h2o was obtained from the natural concentrated salt lake brine on the qtp the kinetics of chloropinnoite dissolved in 4 5 wt boric acid solution at 303 313 and 323 k were investigated the characterization of the phase transitions and the kinetics were carried out by chemical titration analysis x ray powder diffraction spectrometry fourier transform infrared ft ir and raman spectrometry the results demonstrate that similar kinetic processes occur at all three temperatures and can be divided into dissolution supersaturation and precipitation according to three distinct kinetic curves kurnakovite was the final phase transition terminal secondary mineral within the chloropinnoite boric acid solution the dissolution rate of chloropinnoite the dissolution stage could be described by second order pseudo homogeneous reaction model according to the spectroscopy data geochemical mechanism of kurnakovite was identified the new geochemical hypothesis well explains the geochemical mechanism of kurnakovite minerals on the qtp

tourmaline at diagenetic to low grade metamorphic conditions its petrologic applicability article i nfo tourmaline developed under low temperature conditions b 300 c typically has distinctive morphological and chemical characteristics that reveal significant information about the environments in which they crys tallize under diagenetic conditions tourmaline can develop as overgrowths on detrital tourmaline cores or as new authigenic crystals tourmaline as monopolar crystal overgrowths is found in sandstones shales conglomerates dolostones and limestones nucleating exclusively at the positive end of the c axis cpole on the detrital grain a feature related to the mineral s polar nature these overgrowths tend to have compo sitions dominated by x site vacant tourmaline species e g foitite and can be linked to growth in aqueous environments low in na with other chemical characteristics related to the host rocks as temperature in creases tourmaline overgrowths develop at both the cpole andc pole but continue to exhibit a signif icant degree of morphological and chemical asymmetries these differences at either pole diminish as a function of increasing temperature such that it can be used as an intramineral geothermometer authigenic tourmalines without detrital tourmaline nuclei are found in many sedimentary or weakly meta morphosed metasedimentary rocks as well as fluid dominated geothermal systems here tourmalines are commonly foititic but have mg fe contents influenced by the host rock composition low temperature tour malines developed in hypersaline environments e g salt domes have distinct chemical characteristics high na fe 3 and w o and follow a trend between oxy dravite and povondraite o p trend this composi tional trend can be used as a guide to indicate tourmaline development within low temperature hypersaline environments and appears to be diagnostic of an oxidizing environment with high na in the fluid and high sa linity with the resulting reduction of the activity of h2o the o p trend is retained in some meta evaporite de posits that are relatively high grade and is found in tourmaline from some types of economically important metal deposits although tourmalines in low temperature environments are largely overlooked they have po tential to be a useful indicator of the chemical and thermal environments in a regime largely absent of such indicators

is there an archean crust beneath chesapeake bay a crescent shaped terrane of continental crust centered on chesapeake bay is interpreted as being a collisional relic of archean crust that was probably accreted during late hercynian times arcuate gravity and magnetic anomalies which represent the possible suture separate avalonian carolinian terranes from what seems to be african like rocks seen in boreholes in the east chesapeake bay region seismic and heat flow data also suggest an important lithological variation in basement type across the suture

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

a regional crustal velocity model for the southeastern united states arrival time data from a southeastern united states earthquake are analyzed to formulate an average regional crustal velocity model the event studied is the magnitude 3 7 mb shallow earthquake 2 km depth that occurred on 26 august 1979 near the lake jocassee reservoir in northwestern south carolina over 90 seismograms were collected and read by at least two independent interpreters some 250 phases have been timed including 60 pn 80 pg 50 sn and 70 sg lg arrival times the arrival time data can be interpreted in terms of a single layer crust with average velocities of 6 25 0 04 km sec for compressional waves and 3 65 0 11 km sec for shear waves the average upper mantle velocities are 8 11 0 19 km sec p waves and 4 59 0 13 km sec s waves the mohorovic ic discontinuity is found to lie at varying average depths throughout the region 31 km coastal plain 33 km piedmont 40 km valley and ridge central united states a regional velocity model derived from the above results has the aforementioned average velocities a crustal thickness of 40 km and a set of time corrections for mantle refraction phases coastal plain pn 1 68 sec sn 2 74 sec piedmont pn 1 40 sec sn 2 28 sec valley and ridge and central united states no corrections this new velocity model is tested for precision by relocating several regional earthquakes and comparing the resulting travel time residuals with those from the earlier locations the tests demonstrate that the new model can be used to develop good results

an extension of the airy theory for linear waves into shallow water as a fully developed airy wave propagates from deep into shallow water its crest becomes more peaked while the trough flattens out the median crest diameter mcd defined as the distance between the wave flanks under the crest at a level halfway between the crest and trough therefore decreases relative to the similarly defined median trough diameter mtd which remains constant up to the breaking point the mcd is directly related to other wave characteristics which enables water particle velocities to be calculated for any water depth without having to recur to more complex higher order stokes cnoidal or fenton theories over a nearly horizontal bottom most fully developed wave characteristics can be expressed as functions of the wave period tw it is shown that the horizontal particle velocity at the bottom under the breaker crest is at least 9 times faster than under the breaker trough which explains why sediment is transported landward under fair weather conditions the proposed equations also shed new light on the formation of spilling plunging and surging collapsing breakers

insight into diagenetic processes from authigenic tourmaline an example from carboniferous and permian siliciclastic rocks of western poland abstract authigenic tourmaline is a rare and minor component of siliciclastic rocks however once it occurs it can be used to interpret a number of subsurface parameters and processes such as fluid composition redox state salinity ph and fluid migration paths notwithstanding the conditions of tourmaline authigenesis still need to be recognize with greater clarity carboniferous and permian siliciclastic rocks in the subsurface of western poland in the vicinity of the dolsk fault show unusually high boron contents at places surpassing 1000 g g which are associated with the presence of authigenic tourmaline the mineral forms fine elongated crystals of nano to micrometer width grouped in radiating or semi parallel aggregates which are dispersed in the clay matrix of shales and graywackes coat grain surfaces and line secondary pores in porous sandstones replace some grains in volcaniclastic breccia and overgrow detrital tourmalines in quartzarenites regardless of host rock compositions the tourmaline is si deficient moderately al rich and shows significant x site vacancies and highly variable mg mg fetot ratios in rare zoned tourmaline the mg mg fetot ratios distinctly decrease and vacancies slightly increase towards the growth direction the results suggest that the tourmaline nucleated rapidly under a strong driving force at high boron concentration and high temperature the fluids which enabled the process were acidic reducing b and mg bearing and of low na and ca contents the tourmaline grew primarily at the expense of kaolinite possibly of other clay minerals and k feldspar in the temperature range 150 200 c its crystallization was a short episode in the diagenetic history of the host rocks tourmaline authigenesis across the carboniferous permian unconformity in the proximity of a prominent fault zone is evidence for fluid and heat migration along faults and for temporarily open geochemical system during late diagenesis the source of boron has been unproven yet the circulation of bitterns originated from upper permian evaporites was highly probable

seismicity seismic reflection studies gravity and geology of the central virginia seismic zone part i seismicity the central virginia seismic zone is an area of persistent low level seismicity in the piedmont province of that state its north south dimension is 120 km and it extends some 150 km in an east west direction from richmond to lynchburg the results of instrumental monitoring there by a seismic network during the past 5 yr are reported herein to provide a useful constraint on geologic structural models considerable care must be exercised in the location of earthquakes and in the specification of the accuracy of those locations our procedures for accomplishing those tasks were as follows first a velocity model specific to the network locale was developed from seismic refraction and reflection data second evaluation of the detection capability of the network indicated that the detection of seismic events should be virtually complete down to a magnitude of 1 5 third the locational capability of the network was assessed by locating blasts originating from three different quarries operating within the network area as if they were earthquakes of unknown location the average calculated locations when compared to the known blast sites indicated average differences of 2 km horizontally and 4 km vertically except for the northeastern portion there the location capability degrades to fair for epicenters 4 km and to poor for estimation of focal depths 8 km the resultant decrease in locational capability is probably due to the presence of low velocity coastal plain sediments fourth the location of all of the earthquakes relative to each other was tested by use of special joint hypocenter calculations the recalculated locations differed from the original locations in a random manner on the average that difference was 3 km and it indicates the absence of serious bias introduced by combining earthquake locations derived from different subnetworks of seismographs ninety six pre network pre 1978 earthquakes for the most part felt events with noninstrumental locations and 34 network determined hypocenters 1978 1982 for mostly smaller not felt shocks form the basic data set of this study the spatial distribution exhibited by the 130 hypocenters is diffuse both vertically and horizontally such a pattern favors multiple rather than singular seismogenic structures seventy five percent of the focal depths are in the upper one third of the crust at depths of 11 km or less these multiple shallow sources will be shown in part 4 of this sequence of papers to be along and above a master detachment fault

faulting and graben formation in western and central europe rifts and rift related basins play a pre eminent role among the sedimentary basins of western and central europe through time grabens developed in a number of different megatectonic settings whereby the principal mechanisms governing their subsidence was crustal stretching and subcrustal erosion the level of volcanic activity associated with rifting is highly variable and can change significantly during the development history of a rift some rifts are totally non volcanic the development of rift domes is generally associated with volcanic activity uplifting of a rift dome can induce a reversal in the subsidence pattern of a rift intracontinental rifts with their thinned crust are prone to inversion when the respective craton is subjected to tangential stresses in the process of inversion the crust of rifts is mechanically thickened again

a seismic refraction and reflexion study of the continent ocean transition beneath the north biscay margin the structure of the northern margin of the bay of biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent ocean boundary these blocks formed by rifting in late jurassic early cretaceous time and are now thinly covered by post rift sediments of aptian to recent age seismic refraction profiles were occupied on the shelf on either side of and across the continent ocean transition to the shelf using p ubs and o bs with explosives and a 4 x 1000 in 3 4 x 16400 cm 3 airgun array two ship expanding spread multichannel 48 trace seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition a two ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf conventional 48 trace single ship multichannel profiles were located along all the refraction and two ship reflexion lines interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms conventional seismic processing techniques have been used to prepare the two ship multichannel seismic data for interpretation the survey is believed to be the first attempt to apply two ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust the results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide the seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone however extension values calculated in the upper crust from the rotation of fault blocks are much less 1 1 1 4 and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust

magma underplating of continental crust experiments conducted over the past decade have shown that common mantle melting products have densities at moho pressures that exceed the density of average continental crust in a situation of perfect gravitational equilibrium they would not rise to or near the surface the exact behaviour reflects the coupling of crust to the mantle strong at early periods of magmatic cycle but weak at more evolved stages where crustal melting and degassing commences spreading of underplate magmas in rift zones above subduction zones or even hot spots may perturb the state of the crust over very large areas where dense magmas occur beneath normal 30 km continental crust they may crystallize to form gabbro peridotiteanorthosite complexes dense crustal components may founder with melting assimilation and degassing producing very high temperature fluids rich in h2o co2 co and s halogen species depending on compositions the process acts as density filter cleaning the crust and lowering its mean density this process must have been particularly efficient during the archean when komatiite magmas were common opportunities for magma contamination and mixing processes must abound in situations of large scale underplating when crust is overthickened underplate magmas may crystallize to products with eclogitic affinity with the possibility of their foundering in normal mantle

gypsum hydroboracite association in the sijes formation miocene nw argentina implications for the genesis of mg bearing borates abstract this paper deals with sedimentologic and diagenetic aspects of the evaporitic facies of the sijes formation miocene central andes nw argentina which contains the largest known hydroboracite reserves in the world in outcrop the sulfate minerals are secondary gypsum and minor anhydrite and the borate minerals are hydroboracite with subordinate inyoite and colemanite and some ulexite in the monte amarillo member of the sijes formation it is possible to distinguish two coeval shallow lacustrine subbasins in which the gypsum accumulated in the margins and the hydroboracite in the centers the intermediate zones being characterized by mixed gypsum hydroboracite layers in the depositional sequence primary gypsum gypsarenite and syndepositional anhydrite in association with limited amounts of calcium borates colemanite inyoite precipitated first followed by hydroboracite calcium magnesium borate alternations of gypsum and hydroboracite layers also formed hydroboracite is mainly a primary mineral although it replaced some gypsum under synsedimentary conditions the formation of colemanite which occurred during early diagenesis is linked to the precipitation of calcium sulfates gypsum and anhydrite whereas inyoite coexists with both calcium sulfates and magnesium bearing borates transformations among the various borate minerals during burial diagenesis were not detected primary gypsum was transformed into anhydrite from early diagenesis to moderate burial diagenesis the boron source of these deposits seems to be related to the volcanic hydrothermal activity in the central andes during the miocene

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

a simple method to determine breaker height and depth for different deepwater wave height length ratios and sea floor slopes existing easily applicable methods to calculate the depth and height of breaking waves are hampered by two obstacles first the breaker depth is usually required to compute its height and vice versa second the equations take into account either the deepwater height to wavelength ratio or the sea floor slope but not both a simple iterative procedure is therefore proposed which incorporates both elements for fully developed waves breaking over a nearly horizontal bottom the breaker height and depth are also direct functions of the deepwater wavelength

paleozoic and grenvillian structures in the southern appalachians extended interpretation of seismic reflection data interpretive reprocessing of seismic reflection data has elucidated paleozoic and grenvillian structures in the southern appalachians the seismic data include a 7500 km grid of adcoh seisdata and cocorp reflection profiles that traverse the blue ridge and inner picdmont geologic provinces of north carolina south carolina and georgia surface geology and potential field data were used to constrain the interpretation the reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline blue ridge inner piedmont allochthon including the locations of the blue ridge master decollement haycsville fault and brevard fault zone on the basis of the reprocessed data all of the major faults within the allochthonous upper crust sole in the blue ridge master decollement reflections extending to the southeast from beneath the surface location of the hayesville fault to the blue ridge thrust might be the seismic signature of a high strain zone this implies that internal deformation of the blue ridge allochthon associated with the alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies relative amplitude seismic data enabled the discrimination between blue ridge inner piedmont crystalline rocks and underlying lower paleozoic shelf strata thereby delineating the blue ridge thrust the interpreted geometry constrains the top of the shelf sequence beneath the blue ridge to depths of less than 3 km this relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the blue ridge midcrustal reflections from within the upper to iower crust are interpreted to originate from preserved grenvillian structures that were reactivated at the basement surface during late proterozoic early cambrian extension reflection continuity is occasionally disrupted by interpreted post grenvillian pre early cambrian low density intrusions topography at the basement surface possibly caused by the intrusions is interpreted to have controlled the formation of some of the structures within the overlying allochthon including blue ridge and brevard fault zone ramps correlation of seismic time structure contour maps with available gravity data and two dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low density sources within the autochthonous crust discontinuous reflection packages from depths of 36 42 km are interpreted to originate from the mohorovi i discontinuity the reflectors trend about n15 e with a true dip of approximately 15 nw

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

magmatic underplating and crustal growth in the emeishan large igneous province sw china revealed by a passive seismic experiment abstract in an attempt to characterize the subsurface structure that is related to fossil mantle plume activity a comprehensive geophysical investigation was conducted in the emeishan large igneous province elip the nature and geometry of the crust were examined within the scheme of the domal structure of elip which comprises the inner intermediate and outer zones which are defined on the basis of the biostratigraphy of pre volcanic sediments the bulk crustal properties within the inner zone are characterized by high density high p wave velocity high vp vs ratios and large crustal thickness a visible continuous seismic converter is present in the upper part of the crust in the whole intermediate zone and the eastern part of the inner zone but it is absent in the inner zone where another seismic converter is observed in the lower part of the crust the geometric configuration of these converters is attributable to the addition of mantle derived melts to the pre existing crust and subsequent interaction between them the crustal geometry which is delineated by the migrated image of receiver functions from the passive seismic experiment and the crustal properties collectively suggest that a mafic layer of 15 20 km thickness and 150 180 km width exists at the base of the crust in the inner zone such a mafic layer reflects a vertical crustal growth through magmatic underplating at the base of the crust and intraplating within the upper crust the salient spatial correlation between the deep crustal structure and the dome strongly supports a genetic link between crustal thickening and plume activity if the pre volcanic domal uplift is generated by the permian emeishan mantle plume this arrangement is further supported by the consistency of the extent of crustal uplift estimated by isostatic equilibrium modeling and sedimentary data this study therefore characterizes and provides evidence for a plume modified crust in a large igneous province

a simple method to determine breaker height and depth for different wave height length ratios and sea floor slopes reply to discussion by m c haller and p c catalan

charnockite geobarometers based on coexisting garnet pyroxene plagioclase quartz numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr in contrast few continuous reaction geobarometers exist which have had wide application one based on the reaction of cordierite to garnet sillimanite and quartz is limited by a relatively narrow pressure range of application and also by conflicting calibrations1 2 because the mixing relations of mg and fe and the role of h2o in cordierite3 are not understood another successful barometer uses the assemblage garnet plagioclase al2sio5 and quartz4 5 continuous geobarometers based on the reaction of orthofer rosilite to fayalite plus quartz6 and on the aluminium content of enstatite in equilibrium with garnet7 have been used to a lesser extent the former barometer is limited by rarity of the requisite assemblage and the latter by uncertain experimental calibration for crustal rocks we calibrate here two mineralogical geobarometers from measured thermodynamic data the participating minerals pyroxenes garnet plagioclase and quartz are frequently associated in high grade metamorphic rocks calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures

borate formation in a perennial lacustrine setting miocene pliocene furnace creek formation death valley california usa abstract the miocene pliocene furnace creek formation was deposited in marginal lacustrine and lacustrine environments in an extensional setting laminated facies comprising marlstone limestone and mudstone marlstone laminites lack features of surficial or shallow water deposition indicating that sediment accumulated on the floor of a deep perennial lake meter scale mounds of porous limestone with a clotted micritic fabric are spring apron tufas that formed where groundwater discharged into more saline lake bottom waters the bedded borate facies comprises meter scale bodies of borate mainly colemanite interbedded with laminated mudstone the massive borate facies comprises thick nonbedded borate layers locally containing host rock clasts with sharp to diffuse boundaries borate formation took place at or below the sediment water interface at the bottom of a permanently stratified carbonate lake the composition of the borate deposits varies from mainly colemanite to zoned with colemanite surrounding a core of probertite and ulexite high ca2 na ratios favored primary deposition of hydrated ca borates which subsequently transformed by diagenetic dehydration to colemanite gradual depletion of ca2 and decreasing ca2 na ratios resulted in naca borate precipitation finally renewed onset of elevated ca2 na ratios again caused ca borate deposition

the magnetic quiet zone in the eastern gulf of aden implications for the early development of the continental margin summary the magnetic quiet zone in the eastern gulf of aden is located between the oceanic crust of sheba ridge and the continental crust of arabia and somalia and is separated from both by important structural boundaries the seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity the landward boundary is marked by escarpments made up of a series of normal faults these escarpments extend from 2 3 km below sea level to 1500 m above sea level and are equivalent of the hinge zone found at mature continental margins the magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography the basement lacks the topographic slope characteristic of mid ocean ridge flanks and is characterized by moderately rough relief the crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined they are slightly greater than those of oceanic crust new heat flow measurements show high values 95 7 123 3 mw m 2 in the quiet zone with values decreasing from sheba ridge toward the coast the unusual structure of the quiet zone and the observations that more opening appears to have occurred between arabia and somalia than can be accounted for by the oceanic crust of sheba ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational listric faulting and dyke injection this possibility is investigated using both a stretching or lithospheric attenuation model and a model in which a portion of the extension occurs through dyke injection it is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension 4 6 are required in the deep seaward portion of the quiet zone this results in more extension than is compatible with the documented motion between arabia and africa however formation of the magnetic quiet zone occurred over a period of 10 15 myr rather than instantaneously as assumed in the simple models when the effects of a finite length rifting episode are considered less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone

apatite fission track results from ireland and the porcupine basin and their significance for the evolution of the north atlantic abstract apatite fission track analysis was applied to samples from the irish mainland and exploration boreholes 13 3 1 26 26 1 and 26 28 1 in the northern porcupine and donegal basins to study the regional thermal and tectonic history of the irish continental margin samples from the irish mainland have apatite ages ranging from 301 25 to 56 5 ma with mean track lengths in the range 13 89 0 25 to 11 19 0 22 m and indicate higher temperatures during the middle jurassic about 160 180 ma and during the late cretaceous to early tertiary about 50 80 ma these two periods of increased palaeotemperatures correlate with major reorganizations in plate movements and suggest that uplift and erosion after the maximum palaeotemperatures was achieved as a consequence of a variation in the regional pattern of crustal stress the results from borehole 26 28 1 indicate that within the resolution of the data all samples are presently at their maximum post depositional burial temperatures to produce the mean track length results in the range 11 24 0 49 to 11 81 0 20 m from borehole 26 26 1 it is necessary to invoke post palaeocene geothermal gradients in excess of 50 c km increased relative to the present day gradient 28 c km similar increased geothermal gradients are recognized during the early tertiary in adjacent sedimentary basins and may be explained as a result of heat advection by gravity driven fluid flow established in response to tectonic uplift of the basin margins

geotectonic remobilisation of the lower crustal segment of southern peninsular india abstract deep crustal exposures on various continental platforms have recently attracted much attention with regard to their evolution and relevance to present day tectonics the granulite terrain of the southern part of peninsular india exhibits unique characteristics the small dimensions bounded by two rifted margins high relief as a result of recent uplift and anomalous gravity magnetic and electromagnetic signatures the geophysical anomalies over this region have been cohered to produce a tentative geodynamic model in which the break up of gondwana 150 my bp subsequent rifting of the western margin of india 70 my bp and the collision with asia and post collision stresses 50 my bp play a role in the remobilisation of the crust mantle regime of south india the geophysical signatures over this deep crustal cross section differ markedly from those of its counterparts in other parts of the world and thus may reflect the consequences of recent remobilisation rather than features frozen in since proterozoic time in this sense deep crustal imaging of southern peninsular india may differ from that in other continental platforms

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

entrainment threshold of sand to granule sized sediments under waves abstract an improved method is presented to determine the threshold boundary velocity required to entrain sediments under waves using the non dimensional group settling velocity of sediments ranging from very fine sand to granules 0 1 3 3 mm together with a dimensionless boundary velocity in combination with a more accurate method to calculate the actual boundary velocity under linear as well as non linear waves this allows sediment entrainment to be predicted from deep water up to the breaker zone

sediment entrainment under fully developed waves as a function of water depth boundary layer thickness bottom slope and roughness abstract many sediment entrainment equations for oscillatory waves are based on the linear airy theory for deep water but at the depth where such waves begin to transport sediments they commonly have trochoidal or cnoidal non linear forms these changes in the wave profile together with the fact that it is displaced upward with respect to the still water level swl have a profound influence on the hydrodynamics a method is presented to determine the thickness of the boundary layer from the wave profile which can be used to calculate the boundary velocity under the wave crest and trough respectively in any water depth the critical boundary velocity can be determined from a published procedure based on laboratory experiments that takes account of the sediment and water properties as well as the wave period an adjustment is made for the bottom slope and roughness so that differential land or seaward sediment entrainment can be predicted for any defined wave cycle the results explain why sediments are normally transported landward under fair weather conditions and seaward during storms

evolution of the pannonian basin system 2 subsidence and thermal history analysis of sedimentation and thermal data indicates that for most parts of the pannonian basin observed heat flow thermal gradients rates of thermal subsidence and vitrinite reflectance are about twice those predicted by assuming a uniform extension model for basin formation in late miocene time a modified extension model whereby large amounts of heat are added to the uppermost mantle during extension roughly equivalent to replacing the entire sub crustal lithosphere by asthenosphere produced results in good agreement with observation and could also account for periods of uplift observed during extension crustal extension beneath most parts of the basin is estimated to have been about 80 to 170 1 8 to 2 7 although in the deepest parts of the basin deeper than 4 km crustal extension could have been much greater analysis of extension within the danube and zala basins indicates that the late miocene extension in these basins involved the entire lithosphere but subsidence and heat flow data are not well enough constrained to determine if extra heating is required during extension on the basis of tectonic and structural data the vienna basin has been interpreted as the result of thin skinned extensional tectonics above a shallow detachment surface within the upper crust royden et al this issue analysis of thermal and subsidence data from the northern vienna basin suggests that little or no heating occurred during basin extension these data are consistent with a thin skinned extensional origin for the vienna basin and are in better agreement with this interpretation than with the assumption of a uniformly extended lithosphere beneath the basin this study emphasizes the need for good thermal subsidence and structural data in making reconstructions of extensional and thermal histories of sedimentary basins when such data are available it is found that a simple uniform extension model does not explain very well the evolution of some sedimentary basins such as the vienna and pannonian basins in the first instance the basin is thought to be the result of upper crustal extension only due to its special tectonic setting in the second instance the extra heating during extension is due to some unknown processes in the lower lithosphere that may be a general feature of lithospheric extension thus lithospheric extension and associated heating may be distributed in a variety of ways not all extensional sedimentary basins form in the same way application of a uniform subsidence model to basins for which good thermal subsidence and structural data are not available or not properly analysed can lead to reconstructions of thermal histories in error by a factor of 2 or more

episodic cenozoic tectonism and the development of the nw european passive continental margin the north atlantic margins are archetypally passive yet they have experienced post rift vertical movements of up to kilometre scale the cenozoic history of such movements along the nw european margin from ireland to mid norway is examined by integrating published analyses of uplift and subsidence with higher resolution tectono stratigraphic indicators of relative movements including results from the stratagem project three episodes of epeirogenic movement are identified in the early mid and late cenozoic distinct from at least one phase of compressive tectonism two forms of epeirogenic movement are recognised referred to as tilting coeval subsidence and uplift rotations 1 over distances of 100s of kilometres and sagging strongly differential subsidence rotations up to 4 over distances 100 km each epeirogenic episode involved relatively rapid 10 ma km scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity bounded stratigraphic units early cenozoic tilting late paleocene to early eocene c 60 50 ma caused the basinward progradation of shelf slope wedges from elongate uplifts along the inner continental margin and from offshore highs mid cenozoic sagging late eocene to early oligocene c 35 25 ma ended wedge progradation and caused the onset of contourite deposition in deep water basins late cenozoic tilting early pliocene to present 4 0 5 ma again caused the basinward progradation of shelf slope wedges from uplifts along the inner margin including broad dome like features and from offshore highs the early mid and late cenozoic epeirogenic episodes coincided with atlantic plate reorganisations but the observed km scale tectonic movements are too large to be accounted for as flexural deflections due to intra plate stress variations mantle lithosphere interactions are implied but the succession of epeirogenic episodes of differing form are difficult to reconcile with the various syn to post rift mechanisms of permanent and or transient movements proposed in the hypothetical context of a plume beneath iceland the epeirogenic movements can be explained as dynamic topographic responses to changing forms of small scale convective flow in the upper mantle tilting as coeval upwelling and downwelling above an edge driven convection cell sagging as a loss of dynamic support above a former upwelling the inferred cenozoic succession of epeirogenic tilting sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening a process that may also be the underlying cause of plate reorganisations the postulated episodes of flow reorganisation in the ne atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin as well as on other atlantic type passive margins

comparative planetology and the origin of continental crust abstract this paper summarizes crustal evolution in silicate planets the moon mercury mars venus and meteorite parent bodies and outlines a general theory for the origin of continental crust based on comparative planetology and terrestrial evidence silicate planets and some meteorite parent bodies have undergone early global differentiation nonplate tectonic followed by a late basin forming bombardment and a long basaltic second differentiation evidence for analogous stages except a late bombardment can be found in terrestrial geology modified by conditions of hydrous magma generation and persistent tectonism continental crust is inferred to have largely been formed in the early archean by andesitic volcanism accompanying catastrophic early degassing this global crust was disrupted by basin forming impacts that initiated mantle upwelling and sea floor spreading basaltic overplating and underplating from mantle plumes continued on the continents producing greenstone belts mafic dike swarms and by partial melting of underplated basalt and sialic crust the granitoids of archean granite greenstone terrane this process produced most of the present continental crust by 2 5 ga ago when the global tectonic style evolved into the present one of tectonism marginal to and within continents subsequent crustal evolution has largely been deformation metamorphism anatexis and rearrangement of previously formed crustal segments not separation of new sialic crust from the mantle the crustal growth that has occurred since the archean has primarily been crustal thickening resulting directly or indirectly from generation of basalt in the subcontinental tectosphere viewed in the light of evidence from comparative planetology the continents are interpreted as the greatly altered remnants of an originally global primordial crust the result of an evolutionary sequence shared by all the silicate planets to some degree depending on their mass volatile content and location in the solar system this theory can be tested by deep drilling in archean terranes to locate remnants of the original crust by determination of the composition of the highland crust of mars and by investigations of the crust of venus

lower crustal reflections in central virginia usa summary vibroseis reflection data across the blue ridge and piedmont provinces in virginia acquired in 1981 by the united states geological survey have been reprocessed at virginia tech the 12 fold 14 56 hz vibrator data were originally 8 s in length but were extended to 14 s during correlation interpretation of the data was further improved by using a well defined crustal velocity model derived by previous workers from earthquake and blast analyses in the area low reflectivity areas on the seismic section are interpreted to represent grenville aged crust on the section in contrast to highly reflective allochthonous units these regions of reflectivity allow for mapping of the gross crustal structure strong subhorizontal arrivals at 9 to 12 s in the reflection data are interpreted to be from lower crustal layering just above the mohorovicic discontinuity moho defined by earlier refraction work this layering may be reflective only beneath regions of deformed crust and not at the base of the undisturbed craton the depth to these reflectors is approximately 35 km beneath richmond virginia and increases gently westward until the reflections disappear beneath the blue ridge mountains about 70 km west of richmond these thicknesses are in agreement with earlier refraction work which also indicated greater moho depths to the west a rethickening of the crust near the atlantic coast is also interpreted from the refraction data but is not evident on the reflection line probably due to an acquisition or energy penetration problem the deep reflections consist of a subhorizontally layered package about 5 km in thickness thinning slightly toward the craton their base coincides with the crust mantle boundary determined from refraction data indicating that the reflections lie within the lower crust lower crustal reflections were not recorded west of the piedmont on the multichannel data pmp arrivals recorded by earlier workers from beneath the blue ridge however are consistent with a moho reflector about 8 km shallower than indicated by the refraction data if the crust mantle boundary is a second order discontinuity smooth transition zone there the pmp arrivals may have been reflected from near the upper boundary of the transition zone and the refracted waves may have travelled along the base the discrepancy in depths may therefore be a measure of the transition zone thickness

a comparison of velocity profiles in unidirectional currents and the wave boundary layer implications for sediment entrainment abstract although current velocity profiles are commonly used to predict sediment entrainment under unidirectional currents it has not been possible to use this method for waves because the nature of velocity profiles within the wave boundary layer was unknown based upon theoretical wave modelling that allows the thickness of the boundary layer and the boundary velocity to be determined in any water depth combined with an empirical method to determine the critical boundary velocity the different forms of the law of the wall for smooth transitional and rough boundary conditions are evaluated for the wave boundary layer the results show that velocity profiles in the wave boundary layer are of the same form as in unidirectional currents but that the critical shear velocity differs from that of the latter for smooth boundaries the shear velocity necessary to entrain grains of a specific size and density is more than that required under unidirectional currents but the inverse is true for fully rough conditions this is attributed to additional forces under waves related to the balance between the static pressure and static pressure gradient

the origins of the mengye potash deposit in the lanping simao basin yunnan province western china abstract the lanping simao basin lsb is a mesozoic cenozoic continental margin rift basin in western china it formed during the opening and closing of the tethys ocean this basin is also known as a metal belt as it hosts several metal deposits besides the mengye potash deposit however the exact dates of the formation either in the paleocene or the cretaceous and thus the origins of the marine continental or mixed origins of the mengye deposits remain disputed based on the basin s evolution materials of marine origin and or remnant seawater should be present but instead the salt layers of the mengye potash deposit present typically continental lithological features this study examines and reviews evaporative minerals br cl and i cl molar ratios and isotopes of s b and sr i and i cl data for this area has not been previously reported the basin s evaporative minerals are dominated by halite and sylvite the amounts of anhydrite chlorocalcite langbeinite glaserite tachyhydrite and glauberite are small all of these form in both marine and continental environments the values of i and the i cl molar ratios of halite and sylvite are from 0 07 to 0 27 ppm and from 0 03 to 0 11 10 6 respectively dependent on organic substances br and molar br cl values are from 89 08 to 555 45 ppm and from 0 06 10 3 to 0 38 10 3 respectively all of the br cl molar ratios are lower than those of seawater and most of them are 34 s 11 b and 87 sr 86 sr values for evaporative minerals indicate a continental origin for the mengye potash deposit however materials of hydrothermal origin are widely distributed in the basin and may have played an active role for the formation of the potash deposit thus the mengye potash deposit could be of continental origin with a remnant seawater trace