metallogenesis at paleo spreading centers lead isotopes in sulfides rocks and sediments from the troodos ophiolite cyprus abstract new pb isotopic data for metalliferous deposits and pelagic sediments from the troodos ophiolite complex cyprus are presented we use these data to re evaluate the genetic models for these deposits in the light of recent results obtained for different types of hydrothermal sites presently active on spreading ridges in contrast to previous conclusions of earlier studies we find that the troodos hydrothermal deposits sulfides as well as umbers and mn crusts have isotopic compositions very similar to that of most of the rocks from this ophiolite complex they display a 207 pb 204 pb ratio higher than present day typical ridges this result has been interpreted as evidence for a subduction related origin of the troodos ophiolite this different geodynamic setting of the ophiolite is also the reason for the isotopic difference between troodos and semail oman sulfides the latter being typical of a normal oceanic crust when examined in more detail the troodos sulfide results are very homogeneous within one site but show significant differences from one site to another at least three different isotopic end members are required to interpret these variations one of these end members is found in the pelagic sediments data analyzed here which have a 207 pb 204 pb typical of continental crust signature higher than that of the rocks and sulfides on present ridges the involvement of such a component has been similarly described for the guaymas basin and for the gorda rise deposits however some of the isotopic variations observed between sulfides cannot be interpreted only by this end member one possibility is that a second sedimentary component as yet unidentified has been involved in the hydrothermal circulation as recently demonstrated for red sea brines finally the determination of the exact proportion of pb leached out of the ophiolite basaltic crust itself would require a more complete knowledge of the initial isotopic heterogeneities within the massif indeed several rock results are significantly less radiogenic than those of the sulfides which however show themselves initial compositions this could result either from a sediment contribution to the sulfide pb or from a biased sampling of the massif heterogeneities this question remains open in the present state of the data

a high temperature hydrothermal deposit on the east pacific rise near 7 n abstract a seabeam survey of the east pacific rise epr led to the selection of several sites having structural characteristics favorable for hydrothermal activity dredging of such an area located at 7 n on the epr resulted in the recovery of sulfides oxides and fresh basalt chemical analyses and isotopic compositions showed that the recovered pyrites were probably precipitated directly from hot vent hydrothermal waters chemical analyses and isotopic composition of manganese iron oxides indicated that they too were of hydrothermal origin 210 pb pb measurements yielded ages of 90 10years for the deposits this site may still be undergoing hydrothermal activity

fine scale isotopic heterogeneity in the sub atlantic mantle the isotopic composition of sr and nd in oceanic basalts reflects the character and chemical evolution of their source regions with respect to rb sr and sm nd combined studies of the sr nd and pb isotopic systems in mid oceanic ridge basalts morb 1 2 have confirmed previously recognized heterogeneities on a relatively large scale for the oceanic mantle these heterogeneities have been ascribed to both long and short lived mantle processes and to mixing between enriched and depleted mantle sources for example see refs 1 4 however data for the mid atlantic ridge mar remote from known hot spots are available for only 14 samples of which only 3 are basaltic glass the most reliable sample type thus attempts to explain morb mantle heterogeneities are founded on limited data and remain largely speculative the extent of variation in isotopic composition of normal morb over a limited area of the mar is critical to any isotopic model for the oceanic mantle using high resolution seafloor mapping and sampling techniques we can evaluate isotopic composition on a fine scale both within a ridge segment and across the fracture zones terminating that segment we report here studies of a series of basaltic glass samples from the mar intersection with the kane fracture zone located at 22 24 n fig 1 which have implications for the dynamics of mantle melt segregation and the role of a major transform in mantle convection

u pb and th pb whole rock isochrons from metavolcanic rocks of the carolina slate belt felsic volcaniclastic rocks and massive pyrite of a part of the carolina slate belt in south carolina referred to as the haile brewer block seem to have remained closed systems with respect to u th and pb since 465 ma u pb and th pb whole rock isochrons indicate ages of 466 40 2 and 462 53 ma respectively the pb pb secondary isochron age of the volcaniclastic rocks is 455 ma because of the divergent ways in which u th and pb have behaved during the various alteration events that have affected the haile brewer block metamorphism post tectonic intrusion weathering the isochronous relationships most likely represent the emplacement age of the rocks the preservation of u pb and th pb isochrons is thought to be due to the originally glassy nature of the volcaniclastic rocks and to the relatively impermeable character of the rocks which may have been aided by the sealing of pore space by infiltration of penecon temporaneous silica rich hydrothermal fluids the u pb and th pb whole rock ages are considerably younger than the 550 ma age inferred from u pb zircon ages and ediacaran fossil assemblages for the slate belt in north carolina the younger ages agree however with the presence of sponge spicules no older than middle cambrian in the haile brewer block the younger age of the haile brewer block with respect to other parts of the carolina slate belt indicates that the belt is more complex than has been thought

basement reactivation in the development of rift basins an example of reactivated caledonide structures in the west orkney basin the west orkney basin is a poorly studied palaeozoic and mesozoic rift basin that overlies a highly seismic reflective caledonian orogenic basement terrane this study presents a novel approach of using 2d seismic data to map single intra basement reflection packages and their relationship to overlying normal faults in the west orkney basin it is demonstrated that the rift architecture of the west orkney basin is more linear with a prominence of relay zones relative to previous interpretations and that rift structures form complex discordant and concordant relationships with pre rift caledonide basement structures restoration of basement fabrics to their pre extensional geometry indicates that the reactivation of basement structures as normal faults has occurred only where the pre extensional dip of basement structures is greater than 30 the relatively high density of relay zones mapped in the west orkney basin is proposed to be the result of the rift system forming a partially exploitative relationship with basement structure where extension has been accommodated between segments that have or have not reactivated basement fabrics

origin of highly elongate calcite cemented concretions in some italian coastal beach and dune sands abstract highly elongate vertical and horizontal calcite cemented concretions in pleistocene shallow marine and coastal eolian sandstones are mixtures of terrigenous sand and carbonate biogenic grains cemented to various degrees by low mg calcite petrographic and stable carbon and oxygen isotopic data support the earlier hypothesis of mazzanti and parea 1979 that vertically oriented concretions formed by the precipitation of vadose meteoric water percolating downward toward the water table and that horizontal concretions were precipitated from phreatic groundwater flowing toward the coast many vertical concretions started growth as rhizocretions composed of micrite but enlarged and coalesced when equant sparry calcite was precipitated from percolating vadose water horizontal concretions cemented entirely by subequant calcite spar have coalesced into nearly completely cemented beds decimeter scale domains of curved horizontal concretion elements reflect the influence of adjacent phreatic flow domains each with their own particular flow paths aragonite and high mg calcite shells in the sands were the main source of carbon in calcite cement isotopically depleted 13c values of bulk samples 6 to 8 5o oo v pdb however require the input of a light carbon source probably soil co2 18o values of bulk samples are compatible with cementation chiefly by meteoric water at temperatures between 7 and 22 c the values are typical of air and inferentially groundwater temperatures today

metallogenesis at spreading centers lead isotope systematics for sulfides manganese rich crusts basalts and sediments from the cyamex and alvin areas east pacific rise

facies related diagenesis and multiphase siderite cementation and dissolution in the reservoir sandstones of the khatatba formation egypt s western desert in the fluvio deltaic quartz arenitic sandstones of the jurassic khatatba formation in the salam field egypt9s western desert diagenesis and sedimentary facies control reservoir quality fluvial channel sandstones have the highest porosities 10 15 and permeabilities 100 600 md in part because of siderite cementation which 1 inhibited compaction and quartz cement and 2 was later dissolved creating intergranular secondary porosity 1 4 of total porosity fluvial crevasse splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive shallow burial fe dolomite cement respectively siderite precipitation was multiphase and separated by distinct dissolution events the earliest siderite precipitated near surface within suboxic tropical coastal swamps containing predominantly meteoric waters some influence of marine waters is indicated by local enrichments in mg and ca the next major siderite generation shows a trend to decreasing mn and ca contents and is of shallow burial origin the last major siderite phase is mg rich and interpreted as deeper burial in origin some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities the most important dissolution however occurred during deep burial resulting in 1 a major corrosion surface predating the last mg rich zone 2 selective dissolution of some earlier zones and 3 secondary porosity this burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults other diagenetic phases observed include early diagenetic pyrite kaolin quartz bitumen and late diagenetic barite and illite kaolinite precipitated at shallow depths 130 c during and or after rapid late cretaceous burial quartz developed in two phases separated by oil migration this paper illustrates that in fluvio deltaic quartz arenites deposited under the influence of humid tropical climate reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies this paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages including early shallow burial and deep burial diagenesis the present study also shows that in multiphase siderite cements the earlier growth zones can be selectively dissolved and replaced by later siderite zones i e recrystallized during burial diagenesis these findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier formed siderites are essentially stable during diagenesis

the british caledonides interpretations from cenozoic analogues recent interpretations of cenozoic arc systems and collision belts facilitate reinterpretation of some aspects of british caledonide evolution end cambrian grampian collision of the passive dalradian foreland following southeastwards subduction beneath an island arc was accompanied by initiation of the highland boundary fault as a high angle south directed oblique slip thrust mid ordovician to early devonian northwestward oblique subduction of the iapetus ocean beneath the grampian orogen resulted in a continental margin magmatic arc back arc thrusting and development of an accretionary prism while southeastward subduction led to arc magmatism and back arc extension followed by initiation of the rheic ocean as a back arc marginal basin this syn subduction n s asymmetry of the iapetus ocean margins was analogous to the e w asymmetry of the modern pacific closure of iapetus was diachronous earlier in the northeast during end silurian collision the southern caledonides behaved as a passive foreland post collision foreland thrusting resulted in deposition and deformation of lower old red sandstone foreland basin deposits in wales and probably in northwest directed back thrusting in the region of the longford down accretionary prism subsequent dextral movement in the suture zone juxtaposed the southern caledonides with scotland and northern ireland beneath which northwestward subduction had continued into the early devonian

in situ stable isotopic evidence for protracted and complex carbonate cementation in a petroleum reservoir north coles levee san joaquin basin california u s a abstract knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation the stevens sands is a sequence of marine shales and deep sea fan sands that was deposited within the miocene monterey formation in the south central part of the san joaquin basin california during the upper miocene 10 6 ma rapid high precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi collection mode in conjunction with electron microprobe analyses indicate that carbonate cement zones within the stevens sands at north coles levee ncl have had a complex and protracted fluid history three main generations of carbonate cement were identified the relative timing of carbonate cement precipitation within the stevens sands at ncl was estimated using the thermal and burial history of the san joaquin basin in situ oxygen isotope data and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite water and dolomite water precipitation of these cement zones began soon after sediment deposition 7 ma and is ongoing early dolomite was precipitated at a temperature of 10 c near the sediment water interface and soon after sediment deposition calcite cements which are the most abundant variety precipitated semicontinuously between 4 ma and 5 ma at temperatures between 50 c and 65 c and depths of 800 m to 1300 m fe dolomite which is paragenetically late appears to have precipitated at temperatures near 100 c in response to pore pressure reduction which accompanied exploitation of the gas cap within the last 35 years carbon in these cements was likely derived from several sources including marine maturing hydrocarbons and a zone of methanogenesis

geometry styles and quantification of inversion structures in the jiyang depression bohai bay basin eastern china abstract inversion structures of jiyang depression evolved during mesozoic and cenozoic are classified into positive and negative inversion geometry styles the geometry styles of the reactivated faults in the study area are attributed to their multi phase history which includes at least four phases of compressive deformation and subsequent subsidence caused by extensional reactivation of faults since mesozoic in the case of negative inversion style the negative inversion ratios cannot be calculated by the same method of calculating the degree of positive inversion thus a new method of quantifying the negative inversion ratio is presented in this paper negative inversion ratio is defined as the ratio of extensional displacement to contractional displacement measured along the fault this method provides a useful complement to conventional estimation of the degree of inversion and adapts to any negative inversion faults based on this method we obtained the negative inversion ratios of some faults in the jiyang depression which indicates a decrease in deformational degree from east to west in the whole region

strike slip orogens an explanation for the hercynides the nature of oblique plate interactions is not yet well understood the oblique interaction of pacific and n american plates has resulted in large scale migration rotation and accretion of microplates intervening basins are ephemeral individually filled and deformed oblique motion results in zones of both compression and extension the latter may be sufficient to generate new oceanic crust and the former to obduct it both subduction and collision are restricted in time and space the resulting orogen is extraordinarily complex in detail the far more complicated hercynides are the result of dextral interaction between continental plates europe and africa the lateral variability of the orogen the major and long lived strike slip faults the bimodal volcanism and related ore deposits of the european trough the lizard ophiolite and its melanges the nw spanish ophiolites and the iberian pyrite belt are all explained using an oblique collision model in particular the generation deformation and obduction of ophiolites in oblique slip mobile zones is discussed

lead isotopic analysis within a multiproxy approach to trace pottery sources the example of white slip ii sherds from late bronze age sites in cyprus and syria lead isotope analyses were carried out on fragments of white slip ii ware a late bronze age cypriote pottery ware and on raw materials possibly used for their production sherds originate from three late bronze age sites hala sultan tekke and sanidha in cyprus and minet el beida in syria and clays come from the surroundings of sanidha a production site for white slip ware x ray fluorescence xrf and a principal component analysis pca are combined with pb isotope analyses to further investigate the effectiveness of the latter method within a multiproxy approach for pottery provenance study the pottery sherds from the three sites are compared between themselves and with potential raw material additional x ray diffraction xrd and analyses using a scanning electron microscope sem equipped with an energy dispersive x ray detection edx facility were performed on selected sherds and clays this work confirms that the clay source used for pottery production in sanidha derives from local weathered gabbro it also shows that different origins can be proposed for white slip ii ware sherds from hala sultan tekke and minet el beida and that clays were prepared prior to white slip ii ware production it finally confirms the effectiveness of pb isotopes in tracing pottery provenance not only by comparing sherd assemblages but also by comparing sherds to potential raw materials

pb isotopic composition of ophiolitic volcanogenic sulphide deposits troodos complex cyprus the volcanogenic sulphide ore deposits in the ophiolitic rocks of the troodos massif cyprus are thought to be 80 myr old1 analogues of the metal sulphide accumulations actively being deposited from 350 c water at a depth of 2 6 km on the east pacific rise 21 n 2 4 in both cases mineralization is thought to be a consequence of heat transfer by convective seawater circulation within the oceanic crust5 6 evidence includes fluid inclusions in quartz from ore deposits in cyprus with modal trapping temperatures of 298 370 c which have a bulk composition of 3 2 0 5 equiv wt nacl 1 n 273 this salinity is identical to seawater with 3 5 wt total dissolved solids4 7 we have examined the isotopic composition of pb as a geochemical tracer to clarify the origin of one of the metals contained in the sulphide concentrations in the troodos complex it was anticipated that the bulk of the lead would have been derived by leaching of the mafic ophiolitic rocks and indeed such a component is found to be present however the isotope ratios suggest that the lead is actually a mixture and also contains an important component derived from sea water

a multiple sulfur isotope study through the volcanic section of the troodos ophiolite abstract multiple s isotope systematics 34s and 33s and high resolution in situ s isotope measurements offer new perspectives on the study of biological and abiotic cycling of sulfur in hydrothermal systems we applied these techniques to the tethyian troodos ophiolite 91 ma from cyprus one of the best preserved remnants of oceanic crust in the world using materials from deep drill cores and surface sampling we focused on the volcanic section of the ophiolite including the hydrothermal massive sulfide deposit at agrokipia which represents a fossil zone of high temperature fluid upwelling and the akaki river section which displays a range of lower temperature alteration types the 34s and 33s values of bulk and sims secondary ion mass spectrometry analyses from the agrokipia sulfide deposits show that the sulfide minerals are largely derived from thermochemical reduction of entrained seawater sulfate and leached h2s from the root zone of hydrothermal upwelling the contributions of these two sources can vary substantially within individual sulfide grains indicating a very dynamic mixing between these sulfur sources microbial reworking of the sulfide mound is recorded in a sample with very elevated 33s values 0 22 the akaki and agrokipia volcanics experienced low temperature sulfur loss and removal of heavier sulfur isotopes due to partial oxidation by microbes while some intervals gained sulfur and have 34s and 33s values indicative of microbial sulfate reduction ree data of vein quartz containing pyrite with 34s 21 implies local ephemeral hydrothermal upwelling in the lower akaki volcanics possibly associated with the late stage boninitic magmatic activity in the troodos ophiolite suggesting that microbial sulfate reduction in oceanic crust may continue for 10 15 ma in crustal sections with prolonged igneous activity such as troodos

benefits and limitations of different 2d algorithms used in cross section restoration of inverted extensional faults application to physical experiments abstract in recent years 2d restoration techniques have been systematically used to restore cross sections through inverted basins the accuracy of these techniques and in particular which method better restores the inverted extensional faults to previous stages is uncertain and difficult to assess in natural examples to address this drawback the applicability of flexural slip and vertical oblique slip restoration techniques executed with section restoration software is tested through restoration of physical experiments of inverted extensional faults to their pre inversion stage the experiments chosen consist of simple listric and planar faults in which 1 the original state and the kinematic path followed by the rocks to reach the final state is known 2 the boundary conditions are known 3 erosion is absent and 4 the orientation of extension and compression vectors is equal comparing the restored sections with their corresponding actual pre inversion stage reveals that flexural slip is the best restoration method whereas the combination of different slip angles method gives the worst results the accuracy of these techniques depends to a great extent on the master fault geometry the coefficient of friction along it and the amount of inversion the best results are obtained for a physical model that consists of a listric fault with 60 dip at the top of the rigid footwall a shallow detachment low coefficient of friction along the fault and mild amount of inversion since the deformation mechanisms and the geometry of the inverted structures are non identical in physical experiments and in natural examples the results obtained in our study should be cautiously applied to cross sections across natural inverted basins

k ar and rb sr ages of celadonites from the troodos ophiolite cyprus k ar dating and rb sr isochrons of two celadonite veins from the troodos ophiolite cyprus consistently yield precipitation ages of 77 80 and 87 92 ma respectively their k ar bulk rock age data 84 7 2 3 ma combined with the oldest celadonite age and recently published radiolarian ages suggest that the troodos ophiolite was formed between 85 and 92 ma providing an estimate for the duration of celadonite formation of at least 7 m y but possibly up to 15 m y these data are consistent with data from deep sea drilling project holes suggesting that precipitation of the most significant portion of vein minerals in the oceanic crust occurs within about 12 m y but possibly up to 19 m y after crustal formation leaching experiments suggest that k ar rb and sr in exchangeable and nonexchangeable sites of the structure of stoichiometric celadonite maintain their isotope characteristics and are resistant to change from diagenetic processes or surface alteration

metal sources in the middle valley massive sulphide deposit northern juan de fuca ridge pb isotope constraints abstract massive sulphides drilled from the bent hill deposit during ocean drilling program leg 139 at middle valley northern juan de fuca ridge have 206 pb 204 pb 18 43 18 86 207 pb 204 pb 15 43 15 63 208 pb 204 pb 38 08 38 60 these span 35 of the total pb isotope range measured in hydrothermal sulphides from the east pacific ocean 206 pb 204 pb ratios overlap the range of local basalts but 208 pb 204 pb and 207 pb 204 pb require a contribution from a more radiogenic source that is similar to the overlying sedimentary sequence three samples have lower 207 pb 204 pb ratios than the mean basalt value which may reflect the preservation of heterogeneities in the local basalts the pb isotopic compositions of primary pyrrhotites are indistinguishable from the secondary pyrite dominated assemblage which recrystallised from low temperature hydrothermal fluids this may be explained by the importance of the sulphosalts as a host for pb in the deposit clastic sulphides from sediments at bent hill are at the radiogenic end of the massive sulphide range suggesting that surficial sulphides may not provide an unbiased sample of the past hydrothermal circulation responsible for massive sulphide deposition if the pb content of sediment is 100 times greater than the basalts the sulphide pb isotopes imply that the hydrothermal fluids have extracted pb from 60 to 900 times more basalt than sediment lead in the massive sulphide deposit has been extracted from 2 10 8 3 10 8 m 3 of oceanic crust demonstrating the local nature of hydrothermal circulation at mid ocean ridges the 3 he 4 he of hydrothermal fluids in the sulphides 6 2 10 6 7 8 10 6 show only half the variation between the sediment and basalt end members displayed by pb isotopes this is consistent with volatile acquisition from magma degassing while the pb is leached during hydrothermal fluid interaction with crystallised oceanic crust

a 6000 year geochemical record of human activities from alexandria egypt abstract recent multidisciplinary investigations of sediment cores from the ancient marine bay of alexandria egypt have documented local human activities during the iron age circa 900 1000 b c prior to alexander the great s arrival in 331 b c goiran 2001 veron et al 2006 stanley et al 2007 2010 corroborating the existence of the so called rakhotis as evoked in previous archaeological literature jondet 1916 weill 1919 chauveau 1999 baines 2003 lead pb isotopic analyses lia from alexandria bay indicate a possible anthropogenic imprint as early as circa 2300 2650 200 b c and to a lesser extent 3500 3800 170 b c veron et al 2006 here we demonstrate that lia in sediments from the nearby maryut lagoon display isotopic anomalies resulting from the release of contaminant pb into the lagoon during the egyptian early dynastic at 2897 187 b c and predynastic at 3520 145 b c periods that corroborate geochemical data from alexandria bay pb concentrations in maryut sediments show enrichments that mirror isotopic findings the absence of contaminant pb imprints within the sediments from the nearby nile canopic branch confirms that isotopic anomalies found in alexandria bay and maryut lagoon sediments are local and do not originate from long distance transport of contaminant pb associated with nile suspended material and or wind derived aerosols lia in alexandria sediment cores correspond to commonly mined pb and cu copper ores from turkey black sea region cyprus crete and the oman gulf lia substantiate the use of copper based relics in the alexandria region during the pre and early dynastic periods and provide insights into metal trading within the eastern mediterranean during the chalcolithic early bronze age transition 6000 years ago

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

timing and temperature of decollement on hydrocarbon source rock beds in cyclic lacustrine successions abstract the generation of hydrocarbons causes lubrication of source rock bedding planes which can enhance deformation during basin subsidence and subsequent tectonic events deformation includes decollement i e gravitational sliding of rocks above the lubricated horizon in cyclic sequences such as occur in many lacustrine basins decollement is generally restricted to the organic rich beds of the cycle case studies in lacustrine successions in the devonian orcadian basin scotland and the triassic jurassic hartford deerfield basin usa show that movement of material associated with the decollement surface occurred in the current down dip direction these extensional basins experienced rapid subsidence and a high heat flow such that hydrocarbon generation and therefore decollement occurred early in the basin histories during extension half graben rotation ensured that the beds were dipping during hydrocarbon generation facilitating decollement inversion of the orcadian basin reactivated decollement surfaces involving thrust motion in some cases minerals precipitated in tension cavities associated with decollement yield fluid inclusion homogenization temperatures in the ranges 96 139 c and 85 96 c for the orcadian and hartford deerfield basins respectively these data are consistent with burial into the oil window at depths of between 1 5 and 2 0 km

sr nd and pb isotope constraints on the formation of the metalliferous sediments in the nereus deep red sea previous chemical analyses of sediments from nereus deep core 412 located in the northern part of the red sea at about 23 20 n have demonstrated the occurrence of v rich authigenic magnetites associated with fe zn and cu sulphides mostly located at the bottom of the core the evolutionary stage of nereus deep is intermediate between that of the immature shaban deep and the more evolved atlantis ii deep previous studies have proposed either morb basalts by analogy with east pacific rise sulphide deposits or sediments as possible sources of the metals in the red sea metalliferous sediments sr nd and pb isotope systematics observed at ten levels in nereus core 412 lead us to distinguish three types of sediments 1 with predominantly basaltic fragments associated with loose titanomagnetite crystals 0 70413 87sr86sr 0 70622 0 51271 143nd144nd 0 51300 18 618 206pb204pb 18 673 2 detrital or biogenic sediments with a continental isotopic imprint 0 70938 87sr86sr 0 71323 0 51242 143nd144nd 0 51262 18 848 206pb204pb 18 890 and 3 v rich metalliferous sediments 0 70872 87sr86sr 0 70895 0 51260 143nd144nd 0 51274 18 717 206pb204pb 18 851 on sr nd and pb pb isotope diagrams this last group of sediments defines a field located between the fields of the first two groups our data suggest that the sr nd and pb in the studied metalliferous sediments were derived from overlying sedimentary beds in which the two main constituent detrital components show very distinct origins red sea morb and continental arabo nubian shield these data clearly support a model of convective circulation within the nereus deep sedimentary pile this model involves leaching two main components from the upper sedimentary layers of the core and a downward circulation of a hot fluid promoting the growth of v rich magnetites and fe zn and cu sulphides when the fluid cooled by mixing with the pore water of sediments from the lower part of the core a driving force represented by magmatic activity located near the 412 core in the nereus deep is required to explain the presence of hot convective fluids these data also suggest that for sedimented ridges the isotopic imprint of a basaltic source in metalliferous sediments or a hydrothermal ore deposit may result by interaction of a hot fluid with detrital sediments that are enriched in basaltic clasts and loose magmatic crystals

the geometry of listric growth faults in the devonian basins of sunnfjord w norway thick devonian clastic sequences accumulated in fault bounded basins in w norway at the close of the caledonian orogeny and mark the beginning of a phase of crustal extension traditionally the eastern boundary fault to the devonian basins has been regarded as a thrust but cross sections based on the use of a branch line reveal it to be an extension fault there is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging and foot walls of the boundary fault over 40 km of extensional dip slip can be measured at the n end of the fault the boundary fault is a flat lying listric fault with rollover anticlines in the hanging wall the fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction these ramps appear as wrench faults in outcrop the ramp flat geometry of the fault may have been inherited from a precursor caledonian thrust fault which reversed its sense of slip in early middle devonian times to form the basins over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 ma with an average slip rate on the fault of 3 mm yr 1

pb isotopes in sulfides from mid ocean ridge hydrothermal sites the authors report pb isotope ratios of sulfides deposited at seven recently active mid ocean ridge mor hydrothermal vents sulfides from three sediment starved sites on the juan de fuca ridge contain pb with isotope ratios identical to their local basaltic sources lead in two deposits from the sediment covered escanaba trough gorda ridge is derived from the sediments and does not appear to contain any basaltic component there is a range of isotope ratios in a guaymas basin deposit consistent with a mixture of sediment and mor basalt pb lead in a galapagos deposit differs slightly from known galapagos basalt pb isotope values the faithful record of pb isotope signatures of local sources in mor sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite hosted deposits and continental crustal averages in sediment hosted deposits the preservation of primary ophiolitic or continental crustal pb isotope signatures in ancient mor sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes

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

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

northern scotland as an atlantic north sea divide president9s anniversary address 1984 from the end of the caledonian orogeny until the present northern scotland has acted as a structural high separating the north sea and atlantic basins or their forerunners the geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre existing basement structure from devonian to early palaeocene times the massif was generally low lying and its behaviour with respect to vertical movements was essentially neutral the land surface carved in the crystalline basement during the devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins tectonic events of major importance in the basins such as the formation of the north sea and hebridean rift systems produced little change of conditions in the intervening massif it is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching the uneventful history of the massif was interrupted by two phases of mafic igneous activity in late carboniferous and palaeocene times which record responses to regional plate motions the formation of the palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the north sea the style of these events and the anomalous character of the continental margin off nw britain may have been determined by the refractory nature of the precambrian basement which inhibited stretching prior to sea floor spreading

multiple fluid migration events along the moine thrust zone scotland a palaeomagnetic study of different lithologies exposed along the moine thrust zone from skye to durness scotland has identified three chemical remanent magnetizations crms a potential fourth crm and one possible primary component that all reside in hematite local and regional fold tests suggest that the crms are post folding and post thrusting a contact test in the torridon group sandstone indicates that a permian crm is localized in and near the fault zone the permian crm which is also found in the durness group and associated alteration are direct evidence for post orogenic activity in which the thrusts vented excess heat during regional crustal extension on the isle of skye sandstones sleat torridon and eriboll groups in and around the fault zone contain a tertiary crm interpreted to be related to hydrothermal fluids associated with tertiary intrusions a possible mesozoic crm is also present but it could represent the vector addition of the permian and tertiary crms a devonian crm is interpreted to be related to hydrothermal fluids associated with devonian volcanic rocks or to fluids triggered by late stages of the caledonian orogeny geochemical and petrographical studies provide supporting evidence for multiple fluid flow events along the moine thrust zone these results show that palaeomagnetism can be used to date multiple fluid migration events and that dormant fault zones can act as conduits for flow

a simplified model of stress slip relationships application to the froy field northern north sea abstract we apply an analytical method sortan to determine fault slip as induced by far field stress the sortan method is based on the wallace bott hypothesis the method is applied to all major faults of the froy oil field located at the eastern margin of the southern viking graben north sea the purpose was to quantify the most likely senses of motion experienced by the faults during the late jurassic early cretaceous rifting event because strike slip displacement is generally difficult to determine based on classical exploration methods particular attention was paid to the simulated strike slip component along the major faults depth converted structural maps were used to determine fault attitudes the presence at the northern edge of the field of a major syn rift dextral strike slip fault the north froy fault allowed us to constrain the most likely characteristics of the prevailing stress field during rifting according to different possible stress states different models were run the results from the sortan modelling indicate that the strike slip component of motion was in general minor on the major faults within the oil field the inferred stress state is found consistent with rifting involving fault related displacement strongly dependent on 3 orientation this stress state prevailed at least during the latest stages of rifting latest jurassic early cretaceous of the northern north sea this case study shows that where paleostress inversion methods cannot be applied in the absence of direct observation the sortan method has the potential to constrain the slip behaviour of the brittle structure it also enables one to rapidly check the overall mechanical consistency of tectonic interpretations

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

diagenesis and sedimentary environment of miocene series in eboliang iii area this paper presents petrology mineralogy and elemental geochemistry research on diagenesis formation environment and source material of the upper ganchaigou formation of clastic rocks in the eboliang iii structural belt on the northern margin of the qaidam basin point line contacts of particles and directional alignment of feldspars show relatively intense compaction clays have high content with mean up to 35 03 of total cement content mainly consisting of illite chlorite and mixed layer illite smectite carbonate cements contain large amounts of calcite and little dolomite carbon and oxygen isotope composition of carbonate cement present 13c values between 6 8 and 4 0 mean 5 0 and 18o values between 11 1 and 5 4 mean 8 7 according to 13c and 18o calculations ancient salinity z values are distributed between 108 34 and 114 89 mean 112 77 and the formation temperature of carbonate cement is between 43 36 and 77 84 c mean 62 30 c from the analysis of major elements trace elements and organic carbon the diagenetic stage of the upper ganchaigou formation is regarded as the b period of the early diagenesis stage or the a period of the middle diagenesis stage the sedimentary environment is evolved from dry and cold freshwater and brackish water to warm and wet freshwater during deposition of the upper ganchaigou formation

new structural and re os geochronological evidence constraining the age of faulting and associated mineralization in the devonian orcadian basin scotland the devonian orcadian basin in northern scotland belongs to a regionally linked system of post caledonian continental basins extending northwards to western norway and eastern greenland extensional fault systems that cut the orcadian basin sequences are commonly assumed to be devonian with some limited inversion and reactivation proposed during the carboniferous and later times we present a detailed structural study of the regionally recognized fault systems exposed in the dounreay area of caithness which host significant amounts of authigenic mineralization carbonate base metal sulphides bitumen structural and microstructural analyses combined with re os geochronology have been used to date syndeformational fault infills pyrite suggesting that faulting brecciation and fluid flow events are likely to have occurred during the permian 267 5 3 4 3 5 ma stress inversion of fault slickenline data associated with mineralization suggest nw se regional rifting an episode also recognized farther west in sutherland thus a dominant set of permian age brittle faults is now recognized along the entire north coast of scotland forming part of the regional scale north coast transfer zone located on the southern margin of the offshore west orkney basin supplementary material onshore and offshore fault and fracture lineament data are available at https doi org 10 6084 m9 figshare c 2182433

pb isotope systematics of a fossil hydrothermal system from the troodos ophiolite cyprus evidence for a polyphased alteration history pb isotopic ratios have been determined on massive sulphides from the troodos ophiolite and their inferred source rocks epidosites and greenschist altered dikes the latter have distinctly higher pb isotope ratios than the sulphides the high 207 pb 204 pb ratios for the whole rocks indicate the involvement of an old sedimentary pb component extensive leaching of three epidosite samples shows that the radiogenic pb can be removed and the resulting trend for the leached samples is consistent with a pb pb age of 95 ma for these rocks inferred initial pb isotope ratios are similar to those of the sulphides this study and fresh glasses from the akaki canyon in cyprus therefore the pb incorporated in the massive sulphides may originate from these greenschist facies hydrothermally altered rocks the isotopic composition of the leachate pb points toward a sedimentary origin calculation of the pb isotopic ratios for 90 ma old average troodos sediment shows that the foreign pb in the epidosites may very well be derived from this source this pb probably does not reside within the mineral lattices of these metamorphic rocks but rather is adsorbed on mineral surfaces or is present in microscopic particulate matter that may have been filtered from the circulating seawater unmodified seawater is considered an unlikely pb source because of the extremely small dissolved pb contents of seawater furthermore no seawater or sedimentary signature is evident in the sulphides several models of pb transport from different sources into these rocks may be envisaged such as dissolution of miocene evaporites and brine formation equilibration of pore waters to the pb isotope ratios of host sediments and direct infiltration of deep sea particulate matter from the nepheloid layer with the limited amount of data presently available it is impossible to critically determine the most likely processes of pb transport copyright 2000 elsevier science ltd high temperature convectively driven submarine hydrother mal systems at sea floor spreading centres are believed to be of major importance to the heat transport from the newly formed oceanic crust these seawater fed hydrothermal systems are also believed to have major impacts on the chemical mineral ogical and physical properties of the oceanic lithosphere one of the most eminent processes is the formation of sulphide deposits by precipitation of sulphides from upwelling hot hy drothermal fluids upon mixing with cold circulating seawater in the more permeable portions in the upper parts of the oceanic lithosphere present day knowledge of these submarine hydrothermal systems comes partly from their most accessible component the hot springs that emanate hydrothermal fluids at or near the ridge axis but also from seawater basalt and seawater diabase interaction experiments and petrologic and geochemical studies on both in situ oceanic crust and ophiolites because the in ferred source rocks from these hydrothermal systems are lo

effects of interaction between ultramafic tectonite and mafic magma on nd pb sr isotopic systems in the neoproterozoic chaya massif baikal muya ophiolite belt sm nd rb sr and u pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the chaya massif baikal muya ophiolite belt eastern siberia in order to determine the relationship between mantle ultramafic and crustal mafic sections geological relations in the chaya massif indicate that the mafic magmas were emplaced into and interacted with older solid peridotite hand picked acid leached primary rock forming and accessory minerals olivine orthopyroxene clinopyroxene and plagioclase from the two harzburgite samples show coherent behavior and yield147sm 144nd 143nd 144nd and238u 204pb 206pb 204pb mineral isochrons corresponding to ages of 640 58 ma 95 confidence level and 620 71 ma respectively these values are indistinguishable from the crystallization age of the chaya mafic units of 627 25 ma a weighted average of internal isochron sm nd ages of four mafic cumulates the rb sr and sm nd isotopic systems in the harzburgite whole rock samples were disturbed by hydrothermal alteration these alteration related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution on the basis of initial sr and nd values ultramafic and mafic rocks of the chaya massif can be divided into two groups 1 harzburgites and the lower mafic unit gabbronorites withend 6 6 to 7 1 andesr 11 to 16 and 2 websterite of the lower unit and gabbronorites of the upper mafic unit end 4 6 to 6 1 andesr 8 to 9 initial pb isotopic ratios are identical in all rocks studied with mean values of206pb 204pb 16 994 0 023 and207pb 204pb 15 363 0 015 the similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence the isotopic data agree with a hypothesized formation of the chaya massif in a suprasubduction zone environment

devonian extensional tectonics versus carboniferous inversion in the northern orcadian basin the old red sandstone middle devonian orcadian basin was formed as a consequence of extensional collapse of the caledonian orogen onshore study of these collapse basins in orkney and shetland provides directions of extension during basin development the origin of folding of old red sandstone sediments that has generally been related to a carboniferous inversion phase is discussed syndepositional deformation supports a devonian age and consequently some of the folds are related to basin formation large scale folding of devonian strata results from extensional and left lateral transcurrent faulting of the underlying basement spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left lateral megashear in nw europe during late caledonian extensional collapse later inversion probably during the upper carboniferous is characterized by e w to ne sw contraction it induced reactivation of extensional faults as thrusts development of small scale folds and thrusts and right lateral transcurrent movement of the major faults such as the great glen and walls boundary faults

lead and sulfur isotopes and a model for the origin of the ducktown deposit tennessee strata bound sulfide deposits at ducktown tennessee have many characteristics of volcanogenie exhalative deposits but occur in an overwhelmingly sedimentary environment lead isotope ratios of galenas and massive pyritic and pyrrhotitic ores fall into three distinct groups that are not related to wall rock type type of ore or to orebody a positive correlation between the lead and sulfur isotope ratios of galena indicates a relation between sulfur and lead that does not seem to be an effect of metamorphism but is rather an original feature of the ore the groupings of lead isotope ratios and a possible bimodality of sulfur isotope compositions as well as the above mentioned positive correlation suggest that ducktown lead and sulfur represent the mixing of two sources the dominant source of lead was probably u pb enriched clastic sediments seawater sulfate may have dominated sulfur contributions from this source the second source could have been a deep seated mafic igneous source tapped by a deep fault associated with late precambrian rift basin development the deep source could have provided heat to drive the hydrothermal cell in the sediments a relatively nonradiogenic component of lead sulfur with delta 34 s sub sigma asymp 0 per mil a considerable amount of cu and zn as well as the small amphibolite pods at ducktown and along strike lead isotope studies of contemporaneous rift volcanics the mount rogers formation indicate that lead in ducktown ores could not have been derived from a similar source mount rogers felsic volcanics contain lead derived from a relatively u pb poor th pb enriched source that appears to have been granulitized grenville age basement lead isotope ratios of galenas from the gossan lead are similar to those at ducktown other blue ridge massive sulfide deposits have less radiogenic 206 pb 204 pb values but none contain any discernable basement component similar to that of mount rogers reconnaissance sulfur isotope ratios from other blue ridge massive sulfides suggest a tendency toward heavier values than those at ducktown

nd sr isotopic study of primitive lavas from the troodos ophiolite cyprus evidence for a subduction related setting primitive lavas from the troodos ophiolite have a large variation in initial 143 nd 144 nd ratios from nd 7 9 to 0 8 which precludes a mid ocean ridge origin initial 87 sr 86 sr ratios vary from 0 70415 to 0 70705 the range of nd values may be accounted for by a two component mixing model whereby variable amounts of a light rare earth element ree enriched component with nd between 5 and 7 has interacted with a depleted iherzolite source with nd 8 this is consistent with an arc related setting where fluids derived from a subducting slab induce further melting in an overlying iherzolite source

displacement of the moho by the outer isles thrust shown by seismic modelling deep seismic data fig 1 from the caledonides off the north west coast of scotland collected 1981 82 by the british institutions reflection profiling syndicate birps show evidence that caledonian thrusts have been reactivated as mesozoic normal faults1 2 the best example is the outer isles oi thrust which is mapped on land as a compressional fault and is revealed by these data to be overlain offshore by a thick wedge shaped mesozoic rift basin1 known as the north lewis nl basin here we have used synthetic seismic sections to identify distortions of the oi thrust reflection due to the nl basin models which compensate for the effects of this distortion show that the oi thrust has a steeper angle of dip than has been predicted previously assuming no distortion causing the oi thrust to project into a 2 3 km offset of the moho and hence to penetrate the entire crust our data suggesting that shear strains affecting the lower crust may be localized in character have adverse implications for rheological models which predict to the contrary that the lower crust is a region where strain is distributed

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

paleomagnetic dating of fracturing using breccia veins in durness group carbonates nw scotland abstract a paleomagnetic study of red fault related breccia veins in the cambro ordovician durness group in nw scotland was conducted to determine the time of brecciation the origin of the veins and the nature and timing of associated fluid related diagenetic alteration the veins contain brecciated fragments of the host durness group and strike either east west or north south clasts of breccia cemented by calcite suggest multiple brecciation events the host durness limestone is a gray dolomite and contains a devonian chemical remanent magnetization crm that resides in magnetite the veins contain magnetizations that reside in hematite and are interpreted as crms the breccias in north south veins contain a triassic crm whereas the veins with east west strikes contain a jurassic crm authigenic hematite is common in the breccias along growth planes in the calcite cements the two crms within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite the brecciation and fluid flow events are interpreted to be related to extension in the mesozoic which is consistent with the extensional history of the northern atlantic margins

cenozoic contractional reactivation of mesozoic extensional structures in the eastern cordillera of colombia 1 the eastern cordillera of colombia is key to understanding the role of inherited basement anisotropies in the evolution of active noncollisional mountain belts in particular the rio blanco guatiquia region of the eastern cordillera is exemplary in displaying a variety of phenomena that document the importance of the orientation geometry and segmentation of preorogenic anisotropies we document the first unambiguous evidence that extensional basement structures played an important role in determining the locus of deformation during contractional reactivation in the eastern cordillera detailed structural field mapping and analysis of industry seismic reflection profiles have helped to identify the inherited san juanito naranjal and servita normal faults and associated transfer faults as important structures that were inverted during the cenozoic andean orogeny apparently the more internal faults in the former rift basin were not properly oriented for an efficient reactivation in contraction however these faults have a fundamental role as strain risers as folding is concentrated west of them in contrast reactivated normal faults such as the more external servita fault are responsible for uplifting the eastern flank of the eastern cordillera in addition these structures are adjacent and intimately linked to the development of thin skinned faults farther east in part the superimposed compression in this prestrained extensional region is compensated by lateral escape the dominant presence of basement involved buckling and thrusting and the restricted development of thin skinned thrusting in this inversion orogen makes the eastern cordillera a close analog to the intraplate atlas mountains of morocco and other inverted sectors of the andean orogen farther south

crustal thinning and nature of extension in the northern north sea from deep seismic reflection profiling a regional network of deep seismic reflection profiles recorded in the northern north sea has been used to map mesozoic and cenozoic basin thickness and crustal thickness in the viking graben and adjacent platform areas depth to the reflection moho varies from about 20 km in parts of the viking graben to about 32 km beneath the shetlands and the norwegian margin the shallowing of the reflection moho beneath the viking graben implies crustal stretching factors for mesozoic extension greater than 2 in the center of the north sea basin the axis of crustal thinning is located directly beneath the viking graben the axis of the sedimentary basin steeply dipping basin bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle but no continuous reflective feature extending from the sedimentary basin to beneath the moho is observed on any of the deep profiles thus these data do not support the existence of lithosphere penetrating low angle detachments zones of simple shear as the cause of extension in the northern north sea some of the mantle reflectors dip to the west and some to the east suggesting that simple shear if it occurs in the upper mantle is not of uniform sense rather these data suggest a complex depth dependent pattern of brittle extensional deformation in the upper crust pervasive ductile extension bulk pure shear in the lower crust which is decoupled from deformation in the mantle and extension accommodated by discrete dipping shear zones in the lithospheric mantle

mesozoic rocks from the hebrides shelf and implications for hydrocarbon prospectivity in the northern rockall trough abstract shallow boreholes drilled in the west lewis and west flannan basins outer hebrides shelf have proved rocks of probable latest bajocian bathonian to barremian age where previously a mainly permo triassic infill has been assumed the full geological succession of the basins remains unproved although the permo triassic history of the basins may be similar the rocks encountered suggest that the basins may have dissimilar geological histories during the jurassic and early cretaceous lithologies are mainly fine grained poorly sorted sandstones or organic rich mudstones some correlation can be made with the isle of skye and the west shetland area the sandstones are poor hydrocarbon reservoirs but the middle jurassic and ryazanian mudstones although immature where drilled have excellent oil source rock potential and may have generated large amounts of hydrocarbons if they are present at depth in the adjacent northern rockall trough

the thrust structures of southern assynt moine thrust zone the moine thrust zone of southern assynt forms part of the northwest margin of the caledonide belt and has aroused controversy concerning amounts and timing of thrust displacement and sequence of thrust development recent mapping shows it to have been a foreland propagating thrust sequence the uppermost ductile moine thrust formed first followed by sequences of imbricates a major thrust the ben more thrust and then several lower duplex zones this sequence is clear from new observations that many of the earlier thrusts were folded and or breached during the development of the underlying structures a displacement of over 54 km has been estimated for the zone as a whole an alkaline igneous complex including the large borrolan syenite was intruded during the development of the thrust zone and much of it was carried some 30 km to the west northwest onto the foreland late extensional structures in southern assynt are an integral part of the caledonide thrust sequence and probably developed from the collapse of the thrust wedge as it climbed from stronger basement rocks on to a weaker cover sequence on the foreland

a novel combined approach for compositional and pb isotope data of leaded copper based alloys bronze coinage in magna graecia and rome 5th to 2nd centuries bce abstract lead is a ubiquitous and ambiguous component of ancient copper based artefacts as it can be a residue of the ore or could have been deliberately added in this study we present compositional and pb isotope data of a set of roman syracusan and south italian indigenous bronze coins from the 5th to 2nd centuries bce we describe an approach that considers the pb content and its potential implications for provenance and technology as a variable alongside pb isotope source characteristics by combining elemental with isotopic data of lead and in a subsequent step with trace element signatures this method allows to identify endmembers of natural and or anthropogenic mixing and to outline their possible provenance in a wider context our results show how the existing bronze currencies transform parallel to the changing political situation and how rome s bronze monetary instruments develop from a combination of local tradition and greek practices into a coinage in its own right

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

a review of deformation pattern templates in foreland basin systems and fold and thrust belts implications for the state of stress in the frontal regions of thrust wedges aesthetically appealing thrust systems and related large scale anticlines in both active and fossil foreland fold and thrust belts and the economic potential associated with the m have captured the interest of structural geologists for many decades as a consequence a large amount of data on sub seismic deformation patterns from thrust related an ticlines is available in the literature we provide a review of deformation pattern templates from fi eld data in foreland fold and thrust belts and show that the most frequent tre nds of sub seismic syn orogenic deformation structures hosted in km scale thrust related folds frequently and pa radoxically indicate a syn th rusting strike slip stress fi eld con fi guration with a near vertical 2 and a sub horizontal 3 rather than a contractional one where the latter is expected to be the vertical principal axis of the stress ellipsoid this a pparent inconsistency between sub seismic syn orogenic deformation structures and stress fi eld orientation is here named the 2 paradox field data support a possible expla nation of the paradox provided by the major role played by i nherited early orogenic extensional deformation struc tures on thrust fault nucleation nucleation of major thru sts and their propagation is facilitated and driven by the positive inversion and linkage of the early orogenic sub s eismic extensional inheritance developed in the foreland basin this process eventually leads to the development of l argereversefaultzonesandcanoccurbothincontractional and strike slip stress fi eld con fi gurations

processes of late cretaceous to late miocene episodic thrust sheet translation in the lycian taurides sw turkey the lycian taurides of sw turkey consist of an allochthonous mesozoic passive margin succession that was episodically detached from its autochthon and translated southeastwards between latest cretaceous to late miocene times a combination of structural analysis sedimentology palaeontology and geochemistry allows subdivision of this orogenic event into 1 latest cretaceous trench passive margin collision causing ophiolite obduction and detachment of the koycegyiz thrust sheet 2 mid late eocene continent continent collision renewing southeastwards thrusting and causing detachment of the teke dere thrust sheet and yavus thrust sheets 3 miocene extensional collapse of the orogen with southeastwards translation of the allochthon coeval with rift basin formation in the hinterland

metals and isotopes in juan de fuca ridge hydrothermal fluids and their associated solid materials the 87sr 86sr ratio of the hydrothermal solution hts 0 7034 is larger than that of basalt 0 7025 at the southern vent field of the juan de fuca ridge sjfr both the sr isotopic ratio for hts and the water rock interaction ratio lie between those at two sites farther south on the east pacific rise 13 n and 21 n these parameters may be closely related to subsurface temperatures and rates of magma ascent and to extent of faulting and surface areas of the frameworks of the hydrothermal systems for these three pacific ocean sites there is no steady geographical progression of these measured parameters nor of reported spreading rate with increasing latitude northward pb and nd isotopic measurements are uniform for all samples from the sjfr ranging only from 18 43 to 18 58 for 206pb 204pb fluids and associated solids and centering near 0 5131 for 143nd 144nd only fluids measured values for basalts and sulfides from the site have similar values relatively high 206pb 204pb values at the sjfr suggest the potential for the existence of an anomalous radiogenic heat source in the underlying mantle material a nd sm mass ratio of about 5 suggests that the system of the sjfr like those at 13 n and 21 n may be light rare earth element enriched the availability of three types of solid material associated with each fluid sample has provided an opportunity to study chemical distribution between fluids and solids sr isotopic compositions provide strong evidence that at least two of the types of solids are precipitates from their individually associated fluids for most metals measured the masses in the fluids were greater than the masses in solid materials however some metals or the solids that contain them may not have been conserved fractions possibly being lost during sampling or earlier a sr isotopic method of determining extent of seawater dilution of the fluid samples yields different results from the traditionally used method based on mg concentrations perhaps due to differing degrees of precipitation of the two metals

sedimentary basin inversion and intra plate shortening abstract sedimentary basin inversion the shortening of formerly extensional basins is accommodated mainly by compressional reactivation of extant faults and fractures across a wide range of scales as such inversion is a large scale manifestation of byerlee friction the dynamic criterion for fault reactivation governing the effective shear strength of the shallow crust basin inversion generates distinctive deformational architecture and it is implicated strongly in sedimentary basin exhumation as a principal source of horizontal stress inversion drives significant sedimentary porosity reduction and resultant fluid flow upper crustal deformation is critically dependent on fluid overpressure i e pore fluid pressures greater than would be calculated from a hydrostatic gradient and perhaps more than in any other tectonic setting overpressures are potentially large during sedimentary basin inversion this review therefore includes discussion of the role of fluid overpressure in inversion and the evidence for it collectively inversion has profound implications good and bad for the prospectivity of many petroliferous sedimentary basins thus recognizing the evidence for basin inversion quantifying its magnitude and understanding the mechanisms that accommodate inversion and other phenomena affected by it have become essential components of the basin analyst s remit

u th and pb isotopes in hot springs on the juan de fuca ridge the concentrations and isotopic compositions of u th and pb in three hydrothermal fluids from the juan de fuca ridge were determined the samples consisted of 10 2 57 6 of the pure hydrothermal end members based on mg contents the pb contents of the samples ranged from 34 to 87 ng g u from 1 3 to 3 0 ng g and th from 0 2 to 7 7 pg g these samples showed large enrichments of pb and th relative to deep sea water and some depletion of u they did not show coherent relationships with mg however indicating nonideal mixings between the hot hydrothermal fluids and cold ambient seawater particles filtered from these hydrothermal fluids contained significant amounts of th and pb which may effectively increase the concentration of these elements in the fluids when acidified the sup 234 u sup 238 u values in all samples show a sup 234 u enrichment relative to the equilibrium value and have a seawater signature the pb isotopic composition of the juan de fuca hydrothermal fluids resembles that of 21 sup 0 n east pacific rise and has a uniform mid ocean ridge basalt signature the hydrothermal systems at oceanic spreading ridges have circulated through a large volume of basalts therefore pb in these fluids may representmore the best average value of the local oceanic crust from the effects of u deposition from seawater to the crust and pb extraction from rock to the ocean the u pb ratio in the hydrothermally altered oceanic crust may be increased significantly copyright american geophysical union 1987 less

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

upper carboniferous and lower permian tectonostratigraphy on the southern margin of the central north sea hydrocarbon exploration in uk licence blocks 31 26 and 39 2 on the southern margin of the central north sea in 1996 and 1997 proved the presence of a palaeozoic sedimentary succession previously undescribed in the area biostratigraphic data from the sediments and radiometric dating of interbedded basaltic lavas indicate that the succession spans the upper carboniferous westphalian b to lower permian asselian interval this has been informally split into three sedimentary units the lower flora sandstone is of westphalian b c age this clastic interval is overlain by the first of two main lavas which is in turn overlain by the westphalian c d to stephanian upper flora sandstone the flora sandstones are interpreted to be lithostratigraphic equivalents to the schooner formation of the southern north sea the flora sandstones are separated from the grensen formation by an angular unconformity possibly related to variscan asturic inversion the grensen formation is in turn overlain by a second basaltic lava flow both basaltic units are assigned to the inge volcanics formation rotliegend group petrographic data indicate that the grensen formation also forms part of the rotliegend group a major extensional event of permian age post dates all of these strata extensional faults are planed by a regional unconformity saalian unconformity that is overstepped by permian to cretaceous strata

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

the lead and strontium isotope geochemistry of metalliferous sediments associated with upper cretaceous ophiolitic rocks in cyprus syria and the sultanate of oman metalliferous sediments consisting dominantly of fine grained iron and manganese oxides and hydroxyoxides have been widely recorded from the crests of sea floor spreading ridges and as a basal faci

the crustal scale architecture of the eastern succession mount isa the influence of inversion the three dimensional crustal architecture of the eastern part of the mount isa inlier is investigated from serial cross sections constructed using geological map data revised chronostratigraphy gravity magnetics worms multiscale wavelet edges of potential field data and seismic data the top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 ma these rocks constitute the mount isa eastern succession and they were intruded by mafic felsic plutons dykes and sills of various ages before and during the isan orogeny ca 1 6 1 5 ga the eastern succession overlies a felsic metamorphic basement which in turn sits on a tonalitic gabbroic lower crust the depositional basin architecture for the eastern succession was controlled by major n s trending structures that penetrated the lower crust and accommodated e w extension these structures also underlie major upper crustal structures such as the mitakoodi culmination and snake creek anticline that were formed by contraction in the isan orogeny positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier and governs its architecture at the crustal scale inversion involved reactivation of basement penetrating structures which localised contractional structures in the cover sequences above as well as influencing pluton emplacement the felsic metamorphic basement may have been penetratively deformed during inversion the spatial association between the basin controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction or that much of the eastern succession has remained essentially parauthochthonous relative to the basement

facies analysis of a back tilted lacustrine basin in a strike slip zone lower devonian scotland abstract lower devonian lower old red sandstone basins developed along active faults in northern scotland the dingwall strathpeffer basin is a half graben which thickens into the helmsdale polinturk fault a splay off the major strike slip great glen fault zone facies analysis allows the distinction of facies associations which represent alluvial fan perennial lake and playa mudflat ephemeral stream environments syn sedimentary faulting at the margin was the major control on alluvial fan sedimentation the lacustrine facies were fringed by alluvial facies but at times of high water level extended to the active basin margin sedimentary structures and pseudomorphs after gypsum suggest an arid to semiarid environment in which the alluvial fan stream and floodplain sedimentation was largely ephemeral during arid periods the basin centre lake environment was constricted and the surrounding mudflats experienced intense evaporation while after flooding episodes the lake increased in size the thickest lacustrine sequences occur close to the basin margin alluvial fans due to back tilting of the fault blocks fault motion and or climatic fluctuations caused significant interdigitation of the facies the mixture of alluvial fan and organic rich lacustrine facies close to the bounding fault offers some potential for trapping of hydrocarbons

investigating the crustal structure of a strike slip step over zone along the great glen fault strike slip step overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the earth s crust experimental reprocessing of a marine deep seismic reflection profile cutting across an 35 km wide right stepping step over developed on the great glen fault system north of the inner moray firth basin east of northwest highlands scotland reveals a restricted zone of prominent reflections and diffractions beginning at 7 8 km depth continuing as deep as the interpreted moho discontinuity at 26 27 km geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a 12 km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step over region the known kinematic history of the great glen fault system together with the observed direction of the step over implies that a zone of contraction would have formed within the step over during early caledonian sinistral strike slip the seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near vertical zone of strike slip which passes deeper into a layer of low angle dipping thrusts or shear zones the results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike slip faults

boundary conditions in sandbox models of crustal extension an analysis using distinct elements abstract the geometries of faults that form in the brittle hanging wall above large low angle detachments has been an active topic of research in petroleum geology and geophysics for the last decade in order to understand the progressive development of the geologic structures and to identify possible petroleum traps experimentalists have used sandbox models to try to gain insight into the kinematics and mechanics of the deformation of overburden above detachments although the structures observed in some of the models resemble those observed in nature boundary conditions and constitutive properties of these models have rarely been measured precisely so the experiments are often difficult to interpret we use an entirely numerical approach based on the distinct element method dem in which the overburden is represented by hundreds of individual elements that interact mechanically through contact forces the faults that form in an assemblage of elements resemble those observed in nature the boundary conditions are prescribed as are the material reactions of individual elements so the numerical experiments are more easily interpreted we suggest that dem models complement sandbox models and can be used to interpret the progressive development of geologic structures