from a deformed peri tethyan carbonate platform to a fold and thrust belt an example from the central apennines italy abstract in the central apennines of italy the relationships between the foreland deformation of a peri tethyan carbonate platform lazio abruzzi domain and the subsequent contractional pattern have been clearly documented by means of stratigraphic and structural evidence stratigraphic and structural data point out the occurrence of pre thrusting normal fault systems and allow us to define their relationships with contractional structures miocene normal faults are particularly well documented by thickness and facies variation of foredeep deposits and by structural evidence these faults controlled sedimentation during tortonian early messinian times foredeep stage and produced most of the accommodation space for clastic deposits in their hanging walls on the contrary the subsequent positive inversion appears to be quite limited and generally the hanging wall sequences are not uplifted above the regional clear evidence that normal faults formed first and were later cut and or rotated during thrusting is supported by analyses carried out along the main fault surfaces the restoration of a balanced geological section across the study area based on the inference of an extended foreland affected by later contraction provided a total small shortening value this is lower than the previous estimated by several authors for the same foreland fold and thrust belt

application of thermochronology to hydrothermal ore deposits thermochronology finds many applications in economic geology utilizing temperature sensitive radiometric dating techniques to reveal low temperature upper crustal processes can elucidate many aspects of deposit genesis including timing and duration of mineralization processes rate of exhumation and erosion of intrusive ore deposits and comparative preservation potential the tools are utilized to best advantage when combined with other thermochronometry techniques that provide complementary information in addition when thermochronometers are combined with geochronometers e g zircon u pb over 800 c of thermal history is revealed from emplacement to erosion fig 1 with the advent of computational algorithms that provide more accurate and detailed models of thermochronology results the economic geologist has a powerful tool to use when assessing economic favorability of a region or prospect this chapter summarizes the various ways that low temperature thermochronometers have been utilized in studies of economic mineralization and primarily focuses on studies of porphyry ore deposits these deposits are well characterized and provide a good platform from which to demonstrate the extended understanding of ore formation processes that is provided by thermochronology studies in this section we review the fundamental application of u th he fission track and 40ar 39ar thermochronometry methods to mineralized systems we then present a synopsis of how these techniques can be used in thermal history analysis either alone or in combination with other chronometers in order to provide more information on deposit genesis the discussion is presented from low through to higher temperature thermochronometers corresponding to their presentation on figure 1 u th he thermochronology as documented earlier in the volume u th he thermochronology is based on measuring the accumulation of radiogenic 4he produced from u and th decay the daughter helium is retained until the mineral is heated to a temperature at which its structure and helium retentivity change this closure temperature varies from

the meandering gulf stream as seen by the geosat altimeter surface transport position and velocity variance from 73 to 46 w the surface geostrophic velocity field for the gulf stream region was analyzed for the position structure and surface transport of the gulf stream for 2 5 years of the geosat altimeter exact repeat mission synthetic data using a gaussian velocity profile were generated and fit to the sea surface residual heights to create a synthetic mean sea surface height field and profiles of absolute geostrophic currents most of cross track velocity variance was due to the meandering of the gulf stream rather than to rings or complicated velocity profiles which suggested that the simple model parameters are an efficient description of the gulf stream fluctuations an analysis of the model parameters and the actual geostrophic velocity profiles revealed two different flow regimes for the gulf stream connected by a narrow transition region coincident with the new england seamount chain the upstream region was characterized by relatively straight gulf stream paths long eulerian time scales and eastward propagating meanders the downstream region had more large meanders no consistent propagation direction and shorter eulerian time scales a 25 reduction in surface transport occurred in the transition region with a corresponding reduction in current speed and no change in gulf stream width a significant anomaly in surface transport near 69 w suggested the existence of small scale recirculation gyres there the highest variance empirical orthogonal functions of position and surface transport were significantly correlated with larger surface transports leading more northerly positions by a month

mean flow and variability in the kuroshio extension from geosat altimetry data using altimeter data from the geosat exact repeat mission erm we investigated the mean flow and temporal and spatial variations of the kuroshio extension in the region of 140 180 e and 30 40 n mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the kuroshio extension to be gaussian shaped and by successively fitting this synthetic current s height profile to the residual height data using the mean profiles from ascending and descending tracks we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2 5 years of the geosat erm both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the sst patterns from the noaa satellites the mean surface height difference across the kuroshio extension attains its maximum around 146 e between the two quasi stationary meanders and its decrease thereafter is mainly due to large scale recirculations on the southern side of the kuroshio extension the ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1 5 2 0 along the kuroshio extension path propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the kuroshio extension effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds in the upstream region of 141 e and 154 e annual variations in the surface height difference across the kuroshio extension h have a september maximum with an average amplitude of 0 2m for large scale interannual fluctuations anomalies in h are found to be significantly correlated with those of the current axis positions a larger surface height difference corresponds to a more northerly position of the kuroshio extension the interannual changes in h are possibly related to the 86 87 enso event in the low latitude pacific ocean

40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

mesozoic extension controlling the southern alps thrust front geometry under the po plain italy insights from sandbox models in the southern alps fold and thrust belt east west mesozoic rifting controlled the development of extensional faults the deposition of the sedimentary sequences and the thickness and distribution of the main detachment levels vertical and lateral heterogeneities strongly influenced the structural style of the subsequent north south contractional phase as revealed by surface mapping and by subsurface maps derived from seismic data along the strike of the chain lateral terminations of thrust faults occur at syn rift extensional faults trending normal with respect to the contractional structures from west to east the thrust sheet number decreases and the order of structures and their wavelength changes on a regional scale the effects produced by pre existing heterogeneities on the development of contractional alpine tectonic structures in the external area of the southern alps were evaluated by means of scaled sandbox analogue models modelled lateral heterogeneities produced different wavelength thrust sheets salients and recesses whereas vertical heterogeneities facilitated the development of structures of different orders first order structures detached at the base of the model and second order structures detached at the intermediate relatively weak layers comparison of analogue model results with nature showed good geometrical similarities 2006 elsevier ltd all rights reserved

evidence for an east west regional gravity trend in northern tunisia insight into the structural evolution of northern tunisian atlas abstract the atlas orogeny in northern algeria and tunisia led to the destruction of tethys oceanic lithosphere and cumulated in a collision of microplates rifted off the european margin with the north african continental margin the location of the boundary between african plate and kabylian microplate is expressed in northern algeria by a crustal wedge with double vergence of thrust sheets whereas in northern tunisia the geologic environment is more complex and the location of the plate boundary is ambiguous in this study we analyzed gravity data to constrain the crustal structure along the northern margin of tunisia the analysis includes a separation of regional and residual gravity anomalies and the application of gradient operators to locate density contrast boundaries the horizontal gradient magnitude and directional gradient highlight a prominent regional e w gravity gradient in the northern tunisian atlas interpreted as a deep fault active since at least the early mesozoic having a variable kinematic activity depending on the tectonic regime in the region the main e w gravity gradient separates two blocks having different gravitational and seismic responses the southern block has numerous gravity lineaments trending in different directions implying several density variations within the crust whereas the northern block shows a long wavelength negative gravity anomaly with a few lineaments taking into account the geologic context of the western mediterranean region we consider the e w prominent feature as the boundary between african plate and kabylian microplate in northern tunisia that rifted off europe this hypothesis fits most previous geological and geophysical studies and has an important impact on the petroleum and mineral resource prospection as these two blocks were separated by an ocean and they did not belong to the same margin

late miocene drying of central australia abstract a back to the future approach to climate change refines neogene records of paleoclimatic cooling and drying as future climate states for a warming world near alcoota station in central australia is a late miocene fossil mammal site for alcoota 10 ma and ongeva 8 ma local faunas the alcoota fauna includes the largest known land bird dromornis stirtoni and early diprotodontid kolopsis torus but the geologically younger ongeva fauna has a larger diprotodontid zygomaturus gilli these faunas are evolutionary predecessors of australian pleistocene megafauna calcareous paleosols recorded a marked turn toward aridity compared with underlying thick lateritic duricrusts which lack fossils laterites of likely middle miocene age are evidence of wet 1100 mm mean annual precipitation tropical 17 c mean annual temperature paleoclimates but these were deeply eroded and redeposited by the time of the late miocene mammals a succession of 19 paleosols of 5 pedotypes were recognized in the 19 m thick section with shallow calcareous nodule bk horizons as evidence of mean annual precipitation ranges from 100 to 400 mm and mean annual range of precipitation of 50 mm very similar to the modern climate of alcoota 296 mm precipitation with 56 mm difference between wet and dry month this was neither a rainforest nor monsoonal paleoclimate and vegetation inferred from paleosols was open woodland and gallery woodland similar to that of today central australia was not covered by monsoon rainforest during the late miocene and its paleoclimatic history of neogene desertification matches well that of other parts of the world

style of atlassic tectonic deformation and geodynamic evolution of the southern tethyan margin tunisia the structure of the southern atlas fold and thrust belt of tunisia was investigated using new geologic mapping seismic reflection data together with the construction of a balanced cross section the structural architecture of the tunisian foreland consists in a mixed tectonic style with deep seated basement faults shallower decollements within sedimentary cover and salt diapirism the restoration of the cross section shows a surface shortening of 8 1 km similar to 7 3 sequential restoration of the balanced cross section has been permitted to decipher the structure and evolution of the southern tethyan margin of tunisia structural geometry and orientation of the pre existing permian to cretaceous extensional structures controlled subsequent contractional deformation within the sedimentary cover during the shortening this inherited tectonic framework controlled the development of ene trending thrust related anticlines such as the orbata and chemsi structures nw trending lateral ramp such as the gafsa and fejej faults and diapir structures as the beidha anticline the inversion of the margin can be correlated with the onset of the convergence between africa and eurasia the first phase of the orbata thrusting growth might occur in late cretaceous continuous inversion of the margin occurred during the eocene and mainly during the late miocene to present day this study underlines the predominant role of inherited basement structures acquired during the evolution of the southern tethyan margin and their influence on the geometry of the atlassic thrust belt this structural analysis provides new perspectives for future hydrocarbon exploration in this poorly explored region c 2015 elsevier ltd all rights reserved

petroleum geology of tunisia recent discoveries and important oil shows have proven the existence of hydrocarbons in newly identified depocenters and reservoirs in general except for some areas around the producing fields tunisia is largely underdrilled the national company etap has decided to release data and to publish a synthesis on the petroleum geology of tunisia the geology of tunisia provides a fine example of the contrast between alpine folding which typifies northern tunisia and the african craton area of the saharan part eastern tunisia corresponds to an unstable platform forming plains or low hills and extending eastwards to the shallow pelagian sea there are a wide variety of basins central and northern tunisia represents a front basin the saharan ghadames basin or the chott trough are sag basins the gulf of gabes was formed as a distension margin the gulf of hammamet is a composite basin and several transversal grabens cut across the country including offshore and are rift type basins all these features are known to be oil prolific throughout the world two large fields and many modest size pools are known in tunisia oil and gas fields in the surrounding countries namely the saharan fields of algeria and libya the large bouri fieldmore offshore tripolitania and discoveries in the italian part of the straits of sicily suggest a corresponding potential in tunisia exposed paleogeographic and structural maps balanced sections and examples of fields and traps will support an optimistic evaluation of the future oil exploration in tunisia less

the southernmost margin of the tethys realm during the mesozoic and cenozoic initial geometry and timing of the inversion processes 1 along the northern border of africa pangea breakup has been diachronic during the jurassic the alpine tethys propagated northeastward from the atlantic to the alps during the permian the neo tethys propagated westward from oman to northwestern arabia then a secondary and late branch of neo tethys gave birth to the east mediterranean basin finally the two oceans connected at end of jurassic times achieving the development of africa northern plate boundary by the late cretaceous convergence between africa and eurasia led to the progressive closure of the tethys realm the continental collision is not completely achieved and the different segments of the confrontation zone maghreb central and east mediterranean zagros and oman expose different stages of the process however we emphasize the existence of synchronous geodynamic events from one end of the system to the other although they do not have the same meaning two of them are particularly important the campanian santonian c s event corresponds to 1 obduction and exhumation of high pressure low temperature metamorphic rocks around the arabian promontory 2 inversion along the margins of the east mediterranean basins and 3 lithosphere buckling in the atlas system maghreb and adjacent sahara platform the middle late eocene mle event corresponds to 1 the onset of collision at the northern corner of arabia 2 the onset of slab retreat in the mediterranean and 3 inversion along the margin of the east mediterranean as well as in the atlas the c s event coincides with a change in plate kinematics resulting in an abrupt increase of convergence velocity the mle event coincides with a period of strong coupling between the africa and eurasia plates and an abrupt decrease of convergence velocity in the middle of the system the central mediterranean seems to escape to the effects of convergence and is the site of quite permanent extensional movements since the triassic

halokinesis and structural evolution of the major features in eastern and southern tunisian atlas abstract the eastern and southern tunisian atlas is marked by the presence of three major faults the n s axis fault the nw se gafsa fault and the e w northern chott fault seismic lines crossing these structures show the existence of significant halokinetics that began in the early jurassic the aim of this paper is to study and better understand the triassic salt movements which are mainly linked to periods when the major basement faults were reactivated resulting in extension compression transtension or and transpression analysis of the seismic data allows us to conclude that the triassic salt migration started in the early jurassic and continued during the late jurassic and the early cretaceous although these periods are characterized by an extensional tectonic regime the synsedimentary halokinesis is accompanied by thickening and thinning of the overlying series salt remobilization was accentuated during the compressional deformation which occurred from the late tertiary to quaternary and resulted in local diapiric extrusions along the major structural features

the net transport of the antarctic circumpolar current through drake passage abstract estimates of the net transport through drake passage are made for three periods during which the year long drake 79 current meter array spanning the passage was in operation relative geostrophic shears from hydrographic surveys in january 1979 april 1979 and january 1980 were referenced to directed speed measurements to give profiles of net speed direct measurements were averaged in time to make them more compatible with the spatially averaged baroclinic shears the agreement between directly measured and baroclinic shears is generally good except in regions of large bathymetric relief and during periods when current cores were shifting past or between moorings the presence of cold core rings during two of the drake 79 hydrogaphic surveys resulted in intensified flow and increased transport within fronts but did not affect the net transport through the passage the three latest estimates of net transport 117 144 and 134 106 m3 s 1 are in close agreement with a previous estimate of 124

distribution of reynolds stress carried by mesoscale variability in the antarctic circumpolar current satellite altimeter data collected by the u s navy s geosat exact repeat mission were used to investigate turbulent stress resulting from the variability of surface geostrophic currents in the antarctic circumpolar current the altimeter measured sea level along the subsatellite track the variability of the along track slope of sea level is directly proportional to the variability of surface geostrophic currents in the cross track direction we used the two components of the slope at points where the satellite ground tracks cross to calculate orthogonal components of the current s variability because the grid of crossover points is dense at high latitudes the satellite data could be used for mapping the temporal and spatial variability of the current two and a half years of data were used to compute the statistical structure of the variability leading to a better understanding of the role of variability in the dynamics of the current the statistics included the probability distribution functions for each component of the current the time lagged auto correlation functions of the variability and the reynolds stress produced by the variability the results demonstrate that stress is correlated with bathymetry in some areas the distribution of negative stress indicate that eddies contribute to an acceleration of the mean flow strengthening the hypothesis that baroclinic instability makes important contributions to strong oceanic currents

tectonic evolution of the northern african margin in tunisia from paleostress data and sedimentary record abstract a reconstruction of the tectonic evolution of the northern african margin in tunisia since the late permian combining paleostress tectonic stratigraphic and sedimentary approaches allows the characterization of several major periods corresponding to consistent stress patterns the extension lasting from the late permian to the middle triassic is contemporaneous of the rifting related to the break up of pangea during liassic times regional extensional tectonics originated the dislocation of the initial continental platform in northern tunisia the evolution of the liassic ne sw rifting led during dogger times to the north african passive continental margin whereas in southern tunisia a n s extension associated with e w trending subsiding basins lasted from the jurassic until the early cretaceous after an upper aptian early albian transpressional event ne sw to ene wsw trending extensions prevailed during late cretaceous in relationship with the general tectonic evolution of the northeastern african plate the inversions started in the late maastrichtian paleocene in northern tunisia probably as a consequence of the africa eurasia convergence two major nw se trending compressions occurred in the late eocene and in the middle late miocene alternating with extensional periods in the eocene oligocene early middle miocene and pliocene the latter compressional event led to the complete inversion of the basins of the northwestern african plate originating the maghrebide chain such a study supported by a high density of paleostress data and including complementary structural and stratigraphic approaches provides a reliable way of determining the regional tectonic evolution

how to build an extensional basin in a contractional setting numerical and physical modelling applied to the mejerda basin at the front of the eastern tell of tunisia abstract in tunisia africa eurasia convergence resulted in the tell fold and thrust belt ftb part of the tell rif or maghrebides associated to the south to the atlas intra continental belt the mejerda rift basin is developed over the compressional boundary between the tell and the atlas the main objective of this study is to identify the mechanism responsible for such a basin development the abundance of triassic salt structures in the northern atlas compared to the tell suggests a friction change along the basal decollement formed by the salt level this frictional contrast is expected to produce a heterogeneous deformational response and complex structures on this basis numerical and analogue modelling were used to investigate the impact of a friction change on the deformation style the kinematic approach of the limit analysis theory was applied here via the software optum g2 the onset of faulting in all numerical tests reveals extensional faults appear at the friction boundary beyond the onset of faulting sandbox experiments show extensional features such as collapsing blocks and extensional faults overall the adopted numerical and physical approaches showed evidences of extension in particular the experiment using sandpaper and silicon putty pdms for the decollement transition shows viscous deformation mimicking diapiric movement these observations of numerical and physical model behaviors highlight that compression can produce an extensional basin at the top of a thrust wedge as expression of a strong basal friction change hence the mejerda basin is interpreted as a wedge top basin it developed during the growth of the tell and atlas ftbs the same configuration likely exists in other mountain belts the case of the molasse basin between the jura and the western alps is advocated

inversion of small basins effects on structural variations at the leading edge of the axial zone antiformal stack southern pyrenees spain the southern axial zone of the pyrenees consists of a foreland dipping system of thrusts and backthrusts forming the leading edge of a basement involved antiformal stack several small stephanian permian extensional basins were strongly inverted during the pyrenean cainozoic orogeny and controlled thrust geometry and kinematics causing lateral structural variation palinspastic restoration reveals three main grabens erill castell estac and gramos basins capped by undeformed triassic strata in the lower orri and erta thrust sheets the overlying nogueres thrust sheet partitioned into several slices contains smaller basins northern basins the erill castell half graben with a steeply dipping sedimentary infill was inverted along its border fault and extruded by bedding parallel backthrusts the narrow thick estac basin which was less deformed internally was a heterogeneity causing major lateral variation in the system the gramos basin originally wide and covered by a gentle unconformity angle was also inverted along its border fault but caused less lateral variation in the thrust system the northern basins appear only in the western part of the present day nogueres thrust sheet where minor units are strongly stacked and rotated in the east thrust slices are large and shallow dipping the study case is a good example of complex 3d thrust development influenced by the initial geometry of a heterogeneous basin system

the jarrafa trough pelagian sea structural evolution and tectonic significance abstract the structural and sedimentary characteristics of the jarrafa trough pelagian sea are presented buried ridges attributed to reefs of miocene age transgressive pliocene sedimentary layers and progradational quaternary series are well observed on the edges of the trough a distensive regime occurred along wnw ese and ene wsw tectonic axes throughout the miocene to the present intrusive bodies related to salt diapirism are recognized the jarrafa trough is interpreted as an element of the tertiary intracontinental belt of grabens running from europe to africa

integrated geophysical and geological study and petroleum appraisal of cretaceous plays in the western gulf of gabes tunisia abstract an integrated study of available seismic and calibrated wells has been conducted in order to ascertain the structural development and petroleum potential of the cretaceous formations of the western gulf of gabes this study has resulted in an understanding of the controls of deep seated tethyan tectonic lineaments by analysis of the cretaceous deposits distribution three main unconformities have been identified in this area unconformity u1 between the jurassic and cretaceous series unconformity u2 separating early from late cretaceous and known as the austrian unconformity and the major unconformity u3 separating cretaceous from tertiary series the seismic analysis and interpretation have confirmed the existence of several features dominated by an ne sw extensive tectonic regime evidenced by deep listric faults asymmetric horst and graben and tilted blocks structures indeed the structural mapping of these unconformities displays the presence of dominant nw se fault system n140 to n160 bounding a large number of moderate sized basins a strong inversion event related to the unconformity u3 can be demonstrated by the mapping of the unconformities consequence of the succession of several tectonic manifestations during the cretaceous and post cretaceous periods these tectonic events have resulted in the development of structural and stratigraphic traps further to the porosity and permeability enhancement of cretaceous reservoirs

geochronology of the baye mn oxide deposit southern yunnan plateau implications for the late miocene to pleistocene paleoclimatic conditions and topographic evolution 40ar 39ar laser incremental heating analyses of supergene k mn oxides from weathering profiles at the baye mn deposit southern yunnan plateau sw china were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution weathering profiles in the baye mn mine are dominated by 20 30 m thick saprolites which are locally covered by unconsolidated alluvial deposits we analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations in most cases different grains from the same hand sample and different samples from the same site have reproducible 40ar 39ar plateau ages the plateau ages of individual grains are also consistent with the respective isochron integrated and ideogram ages this age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles sixty four grains from 32 hand samples collected from weathering profiles at four sites a to d yield well defined plateau or pseudoplateau ages ranging from 2 98 0 07 to 0 16 ma 2 the age results show that mn oxides become progressively younger towards lower elevations samples collected from the highest elevation site 1855 m a s l have the oldest ages at 2 98 0 07 to 2 83 0 13 ma samples from intermediate elevations 1821 to 1815 m a s l yield ages ranging from 2 91 0 10 to 2 08 0 11 ma and those from the lowest sites 1768 to 1753 m a s l are younger than 1 25 0 08 ma the age versus elevation relationship indicates fast downward propagation of weathering front since the late pliocene and permits estimating the weathering rates at 3 3 3 8 6 6 1 2 and 11 1 1 9 m myr 1 during the time periods of 2 98 2 83 2 91 2 08 and 1 25 to 0 16 ma respectively the average river incision rates estimated from the surface exposed ages range from 235 10 to 416 19 m myr 1 during the past 3 million years the incision rate calculated from minimum exposed ages at sites b d is 33 2 m myr 1 for the time period of 2 91 1 25 ma which is consistent with the slow incision of streams on the ridges over the yunnan plateau since the early pleistocene three mn oxide pebbles contained in the alluvial deposits overlying the saprolite dominated weathering profiles at the two highest sites provided additional information on the timing of weathering four grains from two pebbles yield plateau ages of 6 32 0 19 to 5 27 0 10 ma whereas the other two grains from the third pebble indicate the minimum formation ages of 8 2 0 4 and 9 3 0 3 ma these ages confirm the existence of older weathering profiles now dismantled in the region manganese oxide 40ar 39ar ages of the baye deposit when combined with results from other localities indicate that lateritic weathering and supergene mn enrichment and by inference warm and humid climates conducive to intense weathering have prevailed over the plateau since the middle to late miocene the climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records confirming that supergene mn oxides can be used as a useful proxy for past climate age clusters of mn oxides at 2 9 2 4 1 2 0 8 and 0 6 0 4 ma are broadly coincident with and thus likely reflect intensification events of the indian summer monsoon that brings moisture and abundant precipitation to the yunnan plateau these clusters also coincide with the periods of significant surface uplift in the yunnan plateau demonstrating a causal link between topographic evolution plateau uplift and intensification of the monsoonal climate

40ar 39ar geochronological constraints on the evolution of lateritic iron deposits in the quadrilatero ferrifero minas gerais brazil weathering profiles overlying the sapecado pico and andaime iron ore deposits quadrilatero ferrifero qf minas gerais brazil reach depths of 150 400 m and host world class supergene iron orebodies in addition to hosting supergene ore bodies of global economic significance weathered banded iron formations at the quadrilatero ferrifero and elsewhere e g carajas hamersley are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth laser incremental heating 40ar 39ar results for 69 grains of hollandite group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the sapecado pico and andaime deposits reveal ages ranging from ca 62 to 14 ma older mn oxides occur near the surface while younger mn oxides occur at depth however many samples collected at the weathering bedrock interface yield ages in the 51 41 ma range suggesting that the weathering profiles in the quadrilatero ferrifero had already reached their present depth in the paleogene the antiquity of the weathering profiles in the quadrilatero ferrifero is comparable to the antiquity of dated weathering profiles on banded iron formations in the carajas region brazil and the hamersley province western australia the age versus depth distributions obtained in this study but not available for other regions containing similar supergene iron deposits suggest that little further advance of the weathering front has occurred in the quadrilatero ferrifero lateritic profiles during the neogene the results suggest that weathering in some of these ancient landscapes is not controlled by the steady state advance of weathering fronts through time but may reflect climatic and geomorphological conditions prevailing in a remote past the geochronological results also confirm that the ancient landsurfaces in the quadrilatero ferrifero probably remained immune to erosion for tens of millions of years deep weathering mostly in the paleogene combined with low erosion rates account for the abundance and widespread distribution of supergene iron manganese and aluminum orebodies in this region

chronology of exotic mineralization at el salvador chile by 40ar 39ar dating of copper wad and supergene alunite exotic copper mineralization is a complex hydrochemical process linking supergene enrichment lateral copper transport and precipitation of copper oxide minerals in the drainage network of a porphyry copper deposit at the el salvador porphyry copper deposit in northern chile the majority of the exotic ore comprised a mixture of copper bearing manganese oxyhydrates termed copper wad x ray diffraction scanning electron microscopy and electron probe microanalysis show that the copper wad is composed of copper bearing cryptomelane k1 2 mn3 mn4 8o16 x h2o and birnessite k0 33 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mn 7 3 9 end document o14 7h2o structures these natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40ar 39ar geochronology of supergene k mn oxides formed by weathering a suite of copper bearing cryptomelane and birnessite samples from exotic deposits within the el salvador district were characterized and dated by 40ar 39ar laser step heating supergene alunite kal3 so4 2 oh 6 found in paleospring feeder systems leading from the source zones of copper outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation although the ar retentivity of the layered birnessite structure has been questioned by others the ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground water interaction in the hyperarid atacama desert the 40ar 39ar analytical results show that ar and or k losses after crystallization excess 40ar and 39ar recoil do not pose significant problems therefore in the context of exotic copper deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within copper wad the 40ar 39ar dating of exotic mineralization at el salvador indicates that supergene and exotic mineralization processes were active at 35 ma about 5 m y after the emplacement of hydrothermal mineralization and continued until the middle miocene the majority of exotic mineralization extends from the oligocene miocene 24 ma boundary through the middle miocene 11 ma and relates to supergene fluid emanating in multiple directions from the source of copper in indio muerto which represents a topographic high above the exotic deposits one reconnaissance date of exotic mineralization at the chuquicamata porphyry copper deposit yielded an age of 17 03 0 03 ma coincident with the known supergene alunite dates for this deposit reconnaissance dating at exotica mina sur and el abra proved problematic due to the presence of contaminant silicate minerals within the copper wad ages derived here for exotic mineralization are similar to the known supergene ages throughout northern chile the dates indicate that supergene exotic mineralization began at the eocene oligocene boundary and continued through the oligocene eventually ceasing in the middle miocene due to desiccation of the atacama desert a series of 10 cm thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth apparent growth rates of the veins vary from 71 to 100 mm m y in the horizontal direction to 24 mm m y in the vertical presumably caused by differences in pressure regimes due to their orientation

estimating mean dynamic topography in boundary currents and the use of argo trajectories a mean dynamic topography mdt is required to estimate mean transport in the ocean to combine with altimetry to derive instantaneous geostrophic surface velocities and to estimate transport from shipboard hydrography a number of mdts are now available globally but differ most markedly in boundary currents and the antarctic circumpolar current we evaluate several mdts in two boundary currents off new zealand in the subtropical western boundary current system east of the country and in the subantarctic front to the south using satellite hydrographic and argo observations near two altimeter tracks argo float trajectories are combined with estimates of shear to produce new mdts along both altimeter tracks sufficiently high numbers of argo floats travel in both boundary currents to allow a useful estimate of the mean flow at 1000 m depth and conservation of potential vorticity is used to account for more realistic flow paths in finding a mdt we show the uncertainties in the estimates of velocity differences between 1000 m and the surface from density climatologies while often not estimated need to be considered the mdt computed from the argo trajectories is generally consistent with the cls09 mdt in both boundary currents and in some locations distinctly different from the mdt using a level of no motion assumption the comparison suggests velocities from argo trajectories can be usefully combined with other observations to improve estimates of flows and mdt in boundary currents

recirculation and seasonal change of the kuroshio from altimetry observations surface height fluctuations across an eastward flowing boundary current such as the gulf stream and the kuroshio can be caused by changes both in the upstream inflow and in the neighboring recirculation gyres clarifying these causes is important in understanding the time dependent nature of the boundary currents and their relationship to the surface wind and buoyancy forcing using satellite altimetry data to identify these causes however requires mean surface height field information which is not readily available from the altimetry observation owing to lack of accurate geoid data the present study presents a method to estimate the mean surface height profile across a boundary current system including recirculation gyres by combining altimetrically measured residual height data and historical hydrographic data applying this method to the kuroshio revealed that the absolute surface height profiles thus estimated agree well with the results from quarterly in situ hydrographic observations by separating the signals of the surface height fluctuations into those of the eastward flowing jet and the recirculation gyres we found that the seasonal cycle in the surface transport of the kuroshio which has a maximum in july and august is primarily due to the seasonal change in the intensity of the recirculation gyre south of the kuroshio

the neogene structural evolution of the western margin of the pelagian platform central tunisia abstract the deformation front of the atlas mountains in central tunisia is a structure termed the north south axis to the west of kairouan the north south axis consists of ne sw to nne ssw trending folds and thrusts a main decollement thrust in triassic evaporites has been mapped with secondary detachment of out of sequence thrusts in tertiary rocks and backthrusts at the base of the tertiary and in upper cretaceous sediments the upper cretaceous and tertiary sediments are backfolded in the hangingwalls of the backthrusts which are in turn folded in the se verging anticlines the backthrusts are roof thrusts to passive roof duplexes and imply the extensive development of thin skinned blind se verging thrusts a similar structural style is seen further west in the tunisian atlas but major nw verging backthrusts are not exposed adjacent to the pelagian platform the thrust structures are affected by later strike slip faults the thin skinned thrusting at the deformation front is middle miocene in age and was synchronous with africa europe collision the north south axis is therefore interpreted as the frontal structure of a collision related thrust belt the thrust belt is underlain by a nw se to e w trending middle jurassic to early cretaceous rift basin with probable mid cretaceous to oligocene post rift fill the ne sw trending thrust belt is detached from nw se to e w trending syn rift basement faults but changes in thrust belt geometry across the syn rift faults indicate that basement structures have compartmentalized the thrust belt the later strike slip faulting resulted from the reactivation of the syn rift basement faults in the post collisional framework of the thrust belt

calcium carbonate preservation patterns in the central north atlantic during the last 150 000 years abstract the dissolution record of the canary and eastern north american basins was examined in 15 cores spanning the last 150 000 years stratigraphic control was based on fluctuations in percent carbonate and 18 0 16 0 dissolution changes were determined based on variations in percent planktonic foraminiferal fragments and benthonic foraminifera results indicate three main dissolution events coincident with 18 0 stages 2 4 and 6 the lysocline apparently shallowed about 300 m in the canary basin at these times there is no simple linear relationship between dissolution intensity and ice volume dissolution was more intense during stage 4 than during stage 2 even though 18 0 records indicate the reverse is true for ice volume also carbonate preservation during the lower part of stage 3 is comparable to stages 1 and 5e a comparison of atlantic pacific carbonate dissolution patterns indicates a tendency for out of phase preservation patterns in 18 0 stages 1 and 2 and an in phase dissolution event during stage 4 the out of phase pattern may partially reflect variations in the production rate of north atlantic deep water the in phase dissolution peak correlates with a major cooling regression transport of organic carbon from transient shallow reservoirs into the deep sea could have contributed to formation of such a pattern however additional evidence from longer atlantic pacific records is needed before the shelf basin model can be more fully evaluated

timing and duration of supergene mineralization at the xinrong manganese deposit western guangdong province south china cryptomelane 40 ar 39 ar dating supergene mn oxide deposits are widely distributed in guangxi guangdong yunnan and hunan provinces south china accounting for 18 of the total mn reserves in the country direct dating of supergene mn enrichment however is lacking in this paper we present high resolution 40ar 39ar ages of mn oxides from the xinrong mn deposit western guangdong to place numerical constraints on the timing and duration of supergene mn enrichment a total of ten cryptomelane samples spanning a vertical extent of 67 m were dated using the 40ar 39ar laser incremental heating technique with seven samples yielding well defined plateau or pseudo plateau ages ranging from 23 48 0 91 to 2 06 0 05 ma 2 one sample yields a staircase spectrum that does not reach a plateau the spectrum however indicates the presence of two or more generations of mn oxides in the sample whose ages are best estimated at 22 34 0 31 and 10 2 0 86 ma respectively the remaining two samples gave meaningless or uninterpretable results due to significant 39ar recoil and contamination by old phases the 40ar 39ar data thus reveal a protracted history of weathering and supergene mn enrichment that started at least in the end of the oligocene or beginning of miocene and extending into the latest pliocene staircase apparent age spectra resulting from banded or botryoidal samples yield an average growth rate of mn oxides at 0 6 0 7 10 3 mm kyr 1 the values indicate that a 1 mm grain of mn oxides may host minerals precipitated during a time span of ca 1 5 m y and accumulation of mn oxides to form economic deposits under weathering environments may take millions of years the distribution of weathering ages shows that the oldest mn oxides occur on the top of the profile whereas the youngest minerals are found at the bottom suggesting downward propagation of weathering fronts however two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations such a complexity of age distribution is interpreted in terms of preferential penetration of mn rich weathering solutions along more permeable fault zones or as a result of multi stages of dissolution and re precipitation of mn oxides a synthesis of geochronological and geological data suggests that formation of the xinrong deposit was a consequence of a combination of favorable lithological climatic and structural conditions because the climatic and structural conditions are similar among the provinces of south china during the cenozoic the geochronological results obtained at xinrong may also have implications for the timing of supergene mn enrichment throughout south china

isotopic dating of supergene manganese oxides from the groote eylandt deposit northern territory australia a total of 51 conventional k ar dates on 22 manganese oxide samples from the groote eylandt manganese deposit is presented because most of the samples contained detrital mica from the bedrock k ar ages and their standard errors were corrected using material balance equations following determination of the contribution of radiogenic argon and potassium from the silicate residue in this way the times of formation of cryptomelane ba todorokite and romanechite and probably also vernadite were estimated in addition high resolution 40 ar 39 ar dating was used to date growth bands of pure void filling cryptomelane corrected k ar ages show that cryptomelane and ba todorokite the latter previously thought to be of diagenetic origin were precipitated between 18 and 6 ma early to late miocene as a result of chemical weathering through widespread dissolution and in situ replacement of pisoliths and ooliths romanechite in the lower part of the profile in the g south quarry must be older than 43 7 or 1 2 ma in the upper part of the g south quarry profile romanechite formed at about 30 ma these ages together with the cryptomelane and ba todorokite ages appear to define several episodes of intense chemical weathering in this deposit during the tertiary a pre late eocene episode older than 43 7 or 1 2 ma an oligocene episode around 30 ma and a miocene weathering megacycle 6 18 ma it is suggested that these weathering related processes had a much more significant effect on development of the deposit than previously thought the present mineralogy was produced by replacement processes during the tertiary associated with intense chemical weathering and not by sedimentary or diagenetic processes isotopic ages on manganese oxides from groote eylandt are similar to those determined on manganese oxides from northwestern australia and brazil indicating that periods of intense weathering in the tertiary recording humid climatic conditions were relatively widespread

growth processes and melange formation in the southern apennines accretionary wedge abstract the southern apennines owe their present geometry to the subduction of an apulian continental lithospheric slab and are thus best interpreted as an accretionary wedge southward the apenninic thrust front branches on to the still active ionian wedge which is related to the present calabro sicilian subduction the interpretation of conventional seismic reflection profiles and exploration wells combined with geologic field studies biostratigraphy and structural geology provide constraints for a geodynamic model for the post messinian growth of the apenninic prism our model for the apennines is a good analogy for submerged oceanic accretionary prisms in both instances features such as out of sequence thrusts underplating and duplexes are viewed as processes to explain the thickening of accretionary wedges syntectonic terrigenous deposits have accumulated in the synclines developed above the flats of pliocene thrust faults these depocenters represent piggy back basins younger accretion at the eastern border of the accretionary wedge has led to a progressive uplifting and tilting of the already emplaced thrust sheets including a continous westward migration of the depocenters in the piggy back basins during pliocene and early quaternary times a contrary west to east migration of the depocenters locally occurred where preexisting flats of pliocene thrust faults have been refolded rather than only being rotated dewatering processes and effects of overpressures on undercompacted sediment deformations can also be studied in the southern apennines melanges and broken formations form an important component of the trench apparently restricted to the base of tectonic nappes listric normal faults are a major structural element which preserves in combination with the erosion of topographic highs and resedimentation in the piggy back basins a uniform geometric shape critical taper for the prism the age and location of these distensive or gravitational effects are also discussed

on the origin and consequences of crustal scale extension between africa and sicily since late miocene insights from the kaboudia area western pelagian sea abstract the eastern tunisian shelf is located between the gulf of hammamet and the gulf of gabes and belongs to the pelagian domain which underwent a continuous ne directed extension and crustal thinning since late miocene this geodynamic process induced the formation of nw striking normal faults and associated grabens both in the tunisian atlas and the pelagian domain including the sicily channel rifting zone scrz in this study we combine regional stratigraphy from seismic reflection profiles focal mechanisms space geodetic measurements gnss and thermal indicators from well data to propose a backward tectonic reconstitution of the pelagian domain the thinning of the continental crust is demonstrated by gravity inversion integrating published seismic refraction profiles by completing the interpretation of tectono stratigraphic records we highlight a northeastward propagation of extension prevailing within the pelagian shelf from messinian to plio quaternary times we suggest that extensional tectonics is explained by 1 eastward slab tearing between the african continental lithosphere and the tethyan oceanic slab allowing asthenospheric upwelling and 2 forces arising from relative motions between nubia and the hyblean malta block the decoupling of these domains since late miocene likely results from a combination of the nw directed plate convergence inherited rheological contrasts within the continental crust and weakening arising from mantle flow induced by the tethyan slab rollback and tearing

frontal structure water masses and circulation in the crozet basin due to topographic steering by the crozet and kerguelen plateaus the antarctic circumpolar current acc core in the crozet basin area is shifted to its northernmost position in the southern ocean along the southern limb of the south indian subtropical gyre here the jet like current is embedded within a narrow band 26 5 kg m3 in the western half of the basin and heavier mode 26 7 kg m3 in its eastern half these different varieties of crozet basin mode water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone the area north of the kerguelen plateau appears to be an important source of water mass modification antarctic intermediate water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher oxygen richer colder subantarctic water energetic eddy activities observed along the frontal zone likely enhance such cross frontal exchange of water masses equatorward spreading of the modified thermocline water masses mode water antarctic intermediate water occurs as part of the anticyclonic circulation in the south indian subtropical gyre which is centered within the basin strong injection of north indian deep water into the acc south of amsterdam island is clearly evidenced for the first time this deep oxygen minimum water centered at about 3000 m can be traced to the east of madagascar from where it spreads southward through the eastern part of the crozet basin due to the influence of this water mass north atlantic deep water characteristics of the circumpolar deep water are significantly altered in kerguelen amsterdam passage our results indicate a cyclonic deep level circulation pattern in the crozet basin with a northward flow 9 sv of circumpolar deep water antarctic bottom water as a deep western boundary current and a partially compensating southward flow 3 sv of north indian deep water through the eastern part of the basin it is concluded that the crozet basin constitutes an important passage for the exchange of water masses between the southern ocean and the indian ocean

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

timing of supergene enrichment of low grade sedimentary manganese ores in the kalahari manganese field south africa abstract low grade carbonate rich manganese ore of sedimentary origin in the giant kalahari manganese field south africa is upgraded to high grade todorokite manganomelane manganese ore by supergene alteration below the unconformity at the base of the cenozoic kalahari formation incremental laser heating 40ar 39ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the kalahari unconformity peaked at around 27 8 ma 10 1 ma and 5 2 ma ago older ages are dominant in the upper part of the weathering profile while younger ages are characteristic of the deeper part of the profile younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years the oldest age obtained in the weathering profile namely 42 ma is considered a minimum estimate for the onset of the post african i cycle of weathering and erosion that followed the break up of gondwanaland and formation of the cretaceous to early cenozoic african land surface the youngest ages recorded at around 5 ma in turn correspond well to the pliocene transition from humid to arid climatic conditions in southern africa

basin analysis in the southern tethyan margin facies sequences stratal pattern and subsidence history highlight extension to inversion processes in the cretaceous panormide carbonate platform nw sicily abstract in the mediterranean the south tethys paleomargin experienced polyphased tectonic episodes and paleoenvironmental perturbations during mesozoic time the cretaceous shallow water carbonate successions of the panormide platform outcropping in the northern edge of the palermo mountains nw sicily were studied by integrating facies and stratal pattern with backstripping analysis to recognize the tectonics vs carbonate sedimentation interaction the features of the requienid limestone including geometric configuration facies sequence lithological changes and significance of the top unconformity highlight that at the end of the lower cretaceous the carbonate platform was tectonically dismembered in various rotating fault blocks the variable trends of the subsidence curves testify to different responses both uplift and downthrow of various platform blocks impacted by extensional tectonics physical stratigraphic and facies analysis of the rudistid limestone highlight that during the upper cretaceous the previously carbonate platform faulted blocks were subjected to vertical movements in the direction opposite to the displacement produced by the extensional tectonics indicating a positive tectonic inversion comparisons with other sectors of the southern tethyan and adria paleomargins indicate that during the cretaceous these areas underwent the same extensional and compressional stages occurring in the panormide carbonate platform suggesting a regional scale significance in time and kinematics for these tectonic events

eddy momentum flux and its contribution to the southern ocean momentum balance a large amount of momentum is transferred to the southern ocean by strong westerly winds analytical and numerical models have suggested that transient eddies may be important in transporting this momentum away from the region of wind forcing either horizontally1or vertically downwards where it is balanced by bottom topographic drag2 5 there are however few long term in situ observations of horizontal eddy momentum flux6 8 and no large scale measurements of vertical eddy fluxes to test these models as a result the momentum balance of the antarctic circumpolar current acc remains uncertain and the role of eddies controversial here we use geosat satellite altimeter data to resolve directional eddy kinetic energy and horizontal eddy momentum flux in the acc on fine spatial and temporal scales the complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography the horizontal eddy momentum flux tends generally to concentrate the mean flow although some regions of divergence are observed our results show that the zonally averaged horizontal eddy momentum flux from transient eddies is an order of magnitude too small and in the wrong direction to directly balance the eastward momentum input from wind1

mechanism and timing of tectonic inversion in cyrenaica libya integration in the geodynamics of the east mediterranean abstract in the eastern mediterranean the south tethys paleo margin experienced poly phased rifting episodes during paleozoic and mesozoic times this domain has been subsequently inverted by discontinuous events occurring since the late cretaceous as a consequence of the africa eurasia convergence the cyrenaica promontory northeast libya including the jabal al akhdar antiformal ridge is not yet involved in the africa eurasia collision zone it thus gives an opportunity to analyze tectonic inversions occurring early in the development of the system and their possible causal links with events occurring along the plate boundary i e within the hellenic subduction for this purpose we present new geological cross sections supported by offshore industrial 2d seismic profiles imaging the northeast prolongation of the antiformal ridge indenting the mediterranean ridge accretionary prism the onshore part of this ene wsw trending structure exposes upper cretaceous to recent sedimentary rocks well calibrations allow us to be precise about the geometry of the structure inherited from a lower cretaceous rift basin and the timing of the main tectonic events oligocene sediments seal unconformably the main episodes of contractional deformation just below growth strata of late ypresian to priabonian age indicate a protracted folding episode during the middle late eocene post dating an older late cretaceous inversion this evolution contrasts with what is observed in the adjacent sirt basin characterized at the same time by extensional deformation and subsidence finally we appraise how far these tectonic scenarios match the geodynamic evolution of the hellenic domain

dating of chemical weathering processes by in situ measurement of u series disequilibria in supergene fe oxy hydroxides using la mc icpms abstract constraints on the timing of weathering processes in the northern australian regolith have been obtained by in situ measurement of u series disequilibria in u rich supergene fe oxy hydroxides using a laser ablation mc icpms technique this approach has permitted the measurement of 234 u 238 u and 230 th 238 u activity ratios in finely crystalline fe oxy hydroxides from the ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes 230 th 238 u systematics in the finely crystalline fe oxy hydroxides yield 230 th ages ranging from 60 to 350 ka the most reliable 230 th ages cluster between 122 and 216 ka suggesting fe oxy hydroxides formation and associated weathering peaked during the previous 2 interglacial periods this is supported by the 230 th ages and isotope composition of pisolith fe oxy hydroxide pedogenic nodules cores which are demonstrated to behave as closed systems the u isotopic composition of the fe oxy hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite secondary overprinting is evident in some samples as a large range in 234 u 238 u our results suggest that weathering intensity varies with global climate cycles and that together with weathering events dated by 40 ar 39 ar of mn oxides elsewhere in northern australia feng y x and vasconcelos p 2001 quaternary continental weathering qeochronology by laser heating 40 ar 39 ar analysis of supergene cryptomelane geology 29 7 635 638 weathering rates in this region are orbitally forced

gulf stream surface transport and statistics at 69 w from the geosat altimeter geosat altimeter height measurements for the gulf stream for three subtracks from the exact repeat mission were processed to remove the geoid and mean gulf stream height a gaussian model for the gulf stream velocity profile was used to predict the form of the residual height profiles seen by geosat and the mean height profile using estimates of the width and position derived from the height residuals estimates of the maximum surface velocity and the surface transport were obtained by a least squares fit of the synthetic height profiles to the data location statistics from the geosat data were in good agreement with values obtained from infrared images of the gulf stream maximum surface velocities were typically between 1 2 and 2 0 m s 1 the time series showed that surface transport is correlated primarily with the magnitude of the surface velocities although an anomalously large increase in surface transport in the winter of 1987 1988 was caused by an increase in the width of the gulf stream an analysis of the average annual signal in transport showed maximum values in the late fall and minimum transport in the late spring maximum minimum surface transport generally occurred when the gulf stream was north south of its mean position

subsurface geodynamics and petroleum geology of transform margin basins in the sahel of mahdia and el jem eastern tunisia 1 geophysical structural and seismic stratigraphic analysis of the mahdia sahel of tunisia based on seismic data and sedimentologic and microtectonic field measurements reveals a dynamic basin formation from the aptian to the quaternary seismic structural and isopach mapping of seismic horizons of cretaceous and cenozoic megasequences shows the partition of the mobile graben depocenter and carbonate and siliciclastic platform basins these basins are limited by synsedimentary restrictions along strike slip faults oriented east west and north south the interplay of these wrench faults in regional transtension and transpression during the cretaceous and cenozoic induced the synchroneous opening and closing of the graben and platform basins producing large accumulatio s of black shales and turbidites and reef oolite and carbonate platform deposits this was associated with the dragging rotation and blocking of dekakilometer size blocks assimilated to lithospheric microplates these movements were accompanied by volcanism and triassic salt intrusions along the bordering active faults in the east west trending and north south trending corridors in these basins sequential inversion of basin subsidence and the superposition of rhombic basins and wrench depocenters created migrating depocenters and provided the conditions in rapidly subsiding areas for source rock maturation and later hydrocarbon migration into stratigraphic and tectonic traps the tunisian sahel domain at the northern border of the african plate is characterized by tectonic mecha isms which provide a model for tectono sedimentary basin evolution in a wrench context

quaternary continental weathering geochronology by laser heating 40ar 39ar analysis of supergene cryptomelane incremental laser heating analyses of supergene cryptomelane clusters extracted from three distinct weathering profiles from the mary valley region southeast queensland australia yield reproducible and well defined plateau ages ranging from 346 15 to 291 14 ka 2 a precipitation of supergene cryptomelane in this period implies that relative humid climate prevailed in southeast queensland from 340 to 290 ha a result consistent with oxygen isotope analyses of marine sediments from ocean drilling program site 820 and with regional pollen and spore records these results the first report on the precise ar 40 ar 39 dating of quaternary supergene cryptomelane indicate that ar 40 ar 39 analysis of pedogenic minerals provides a reliable geochronometer for the study of quaternary surficial processes useful in the study of soil formation rates continental paleoclimates and archaeological sites devoid of datable volcanic minerals

geochronological evidence for pervasive miocene weathering minas gerais brazil ar 40 ar 39 laser incremental heating analyses of 22 individual grains of supergene cryptomelane from three weathering profiles up to 400 km apart in the rio doce valley and barbacena regions at minas gerais brazil show that the formation of weathering profiles in these regions is contemporaneous suggesting a strong weathering event in the middle to late miocene 10 8 ma the preservation of these miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the miocene or that a much thicker weathering mantle was present in the region originally assuming a constant thickness of weathering profiles in the region throughout the tertiary we may calculate weathering front propagation rates of 4 8 m myr 1 during the past 10 ma copyright c 2004 john wiley sons ltd

structure paleogeographic inheritance and deformation history of the southern atlas foreland fold and thrust belt of tunisia 1 structural analysis of the southern tunisian atlas was carried out using field observation seismic interpretation and cross section balancing it shows a mix of thick skinned and thin skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by nw se oblique ramps and tear faults it confirms the role of the late triassic early jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern tunisian atlas in particular in the development of nw se oblique structures such as the gafsa fault the late triassic early jurassic structural pattern is characterized by a family of first order nw se trending normal faults dipping to the east and by second order e w trending normal faults limiting a complex system of grabens and horsts these faults have been inverted during two contractional tectonic events the first event occurred between the middle turonian and the late maastrichtian and can be correlated with the onset of the convergence between africa and eurasia the second event corresponding to the principal shortening tectonic event in the southern atlas started in the serravalian tortonian and is still active during the neogene the southern atlas foreland fold and thrust belt propagated on the evaporitic decollement level infilling the late triassic early jurassic rift the major eocene atlas event described in hinterland domains and in eastern tunisia did not deform significantly the southern tunisian atlas which corresponded in this period to a backbulge broad depozone

platform collapse and pelagic seamount facies jurassic development of central tunisia abstract in central tunisia the lower liassic facies consists of peritidal carbonates deposited on a wide platform which has been broken into irregular tilted blocks in the early pliensbachian black shale was deposited in some of the down faulted blocks during the early toarcian oceanic anoxic event the bajocian bathonian deposits are represented by ironstones and hemipelagic ammonite and zoophycos rich marl limestones the upper jurassic is composed by pelagic limestones and debris flow deposits the middle jurassic deep marine sedimentation was frequently associated with condensed sections submarine erosion leading to stratigraphic hiatuses the latter results from the interaction between global sea level fluctuations and regional tectonic activity generating an uplifted block in central tunisia in relation with early mesozoic rifting and associated distensional drifting of the north african tethyan margin this horst block has acted as a starved pelagic shoal on which sedimentation has been recorded especially during retrogradational and the aggradational phases

active fault segmentation in northern tunisia abstract active shortening structures in northern tunisia have developed by tectonic inversion since the pliocene after late miocene extensional collapse of the whole region restored plio quaternary deformation observed on reflection seismic lines indicates deformation rates around 0 6 0 8 mm yr in the studied segments and larger amounts of shortening to the west of northern tunisia 16 than to the east 7 which suggests tectonic inversion started earlier to the west and later propagated eastwards reaching northeastern tunisia in the late pliocene this shortening is registered on striated pebbles in quaternary alluvial terraces and fault slip data giving two populations of strain ellipsoids with n s and wnw ese maximum shortening morphometric analysis in combination with field fault segmentation mapping show that topographic uplift and drainage rejuvenation occurs in relation to 20 30 km long ene wsw reverse fault segments and related antiforms that are offset and linked by e w to wnw ese dextral and ne sw oriented sinistral faults the largest fully linked fault system is the alia thibar fault this 130 km long fault zone shows an helicoidal geometry with five different fault segments including reverse dextral sinistral and oblique faults due to the young age of tectonic inversion after late miocene extensional collapse of the region the present relief of northern tunisia is characteristic of a young thrust and fold belt with dominating axial valleys along synforms and an incipient transverse drainage development propagating from west to east

an overview of the late cretaceous eocene positive inversions and oligo miocene subsidence events in the foreland of the tunisian atlas structural style and implications for the tectonic agenda of the maghrebian atlas system abstract structural interpretation of surface and subsurface data in the eastern foreland basins of tunisia allows us to recognize positive inversion structures i e related to compressional events expressed and recorded in the paleogene sedimentary pile of the atlas domain these episodes are followed by a period of relative tectonic quiescence during oligocene early miocene with development of extensional structures with slightly tilted panels grabens and locally the development of listric normal faults branched at depth on decollement levels comparison of the seismic interpretations and field data collected along the south atlas front as well as in the atlas belt allows us to propose a tectonic scenario at the scale of eastern tunisia atlas in particular we emphasise the role of the so called atlas event middle late eocene which was initially defined in algeria but poorly outlined in tunisia we will show that the tectonic agenda defined in eastern tunisia is consistent with the one proposed elsewhere in the maghreb allowing us to propose a unified view of the geodynamic evolution of the whole atlas system during this period the negative inversion recorded during oligocene early miocene along with the occurrence of synsedimentary normal faults are related to an increase of the subsidence rate in the frame of continuous shortening coeval to basin formation in the front of the atlas the whole oligo miocene evolution results from two different mechanisms flexuration in the front of the atlas and the onset of the extension of the pantelleria linosa malta rift system of central mediterranean which attained a climax stage later during pliocene quaternary

improved gravity anomaly fields from retracked multimission satellite radar altimetry observations over the persian gulf and the caspian sea satellite radar altimetry observations are used to derive short wavelength gravity anomaly fields over the persian gulf and the caspian sea where in situ and ship borne gravity measurements have limited spatial coverage in this study the retracking algorithm extrema retracking extr was employed to improve sea surface height ssh measurements that are highly biased in the study regions due to land contaminations in the footprints of the satellite altimetry observations extr was applied to the waveforms sampled by the five satellite radar altimetry missions topex poseidon jason 1 jason 2 gfo and ers 1 along track slopes have been estimated from the improved ssh measurements and used in an iterative process to estimate deflections of the vertical and subsequently the desired gravity anomalies the main steps of the gravity anomaly computations involve estimating improved ssh using the extr technique computing deflections of the vertical from interpolated sshs on a regular grid using a biharmonic spline interpolation and finally estimating gridded gravity anomalies a remove compute restore algorithm based on the fast fourier transform has been applied to convert deflections of the vertical into gravity anomalies finally spline interpolation has been used to estimate regular gravity anomaly grids over the two study regions results were evaluated by comparing the estimated altimetry derived gravity anomalies with and without implementing the extr algorithm with ship borne free air gravity anomaly observations and free air gravity anomalies from the earth gravitational model 2008 egm2008 the comparison indicates a range of 3 5 mgal in the residuals which were computed by taking the differences between the retracked altimetry derived gravity anomaly and the ship borne data the comparison of retracked data with ship borne data indicates a range in the root mean square error rmse between approximately 1 8 and 4 4 mgal and a bias between 0 4062 and 2 1413 mgal over different areas also a maximum rmse of 4 4069 mgal with a mean value of 0 7615 mgal was obtained in the residuals an average improvement of 5 2746 mgal in the rmse of the altimetry derived gravity anomalies corresponding to 89 9 per cent was obtained after applying the extr post processing

submarine salt glacier of northern tunisia a case of triassic salt mobility in north african cretaceous passive margin stratigraphical sedimentological and structural data and a bouguer gravity map of medjez el bab meb in northern tunisia are used to illustrate a cretaceous example of salt extrusion on a passive continental margin located just south of the teboursouk thrust front a preferential decollement surface used by the continuous tertiary shortening in this area the meb structure is a simple n40 e box anticline removing the two tertiary foldings eocene and miocene leads to the exposure of the original feature of a simple submarine salt glacier the triassic salt rocks appear as an albian interstratified body between two cretaceous series with stratigraphic normal polarity suggesting a bedding parallel extrusion at the sediment water interface of the triassic salt in cretaceous times the formation of such salt extrusions are associated with extensional faulting probably both in the cover and basement the presence of a slope and basinwards salt flow this scenario is similar to the allochthonous salt described in other salt provinces characterizing passive margins

petroleum assessment of berkine ghadames basin southern tunisia hydrocarbon exploration in the berkine ghadames basin in southern tunisia has generally followed global economic trends in recent years improvements in seismic data acquisition combined with experience gained in log interpretation in low resistivity reservoirs have resulted in oil and gas discoveries in the upper silurian acacus formation enriching the hydrocarbon potential of the berkine ghadames basin in southern tunisia presently the tunisian daily oil production is approximately 43 000 bbl about half of which comes from the fields located in southern tunisia and is produced from the acacus formation the berkine ghadames basin is an intracratonic basin formed during the pan african orogeny it covers an area of approximately 350 000 km2 135 135 mi2 and extends into algeria libya and tunisia the sedimentary section within the basin ranges from cambrian to present and is approximately 7000 m 23 000 ft thick in the depocenter the basin has experienced several tectonic events which have modified its architecture and affected the petroleum systems and hydrocarbon pathways in this study the main elements of the petroleum geology systems are described with the geochemical modeling results the petroleum potential hydrocarbon generation expulsion time and quantity of hydrocarbon are assessed the main petroleum systems are also defined they are represented by the silurian tannezuft hot shale source rock with ordovician djeffara and silurian acacus reservoirs by silurian tannezuft hot shale with kirchaou reservoirs and by the devonian aouinette ouinine formation member iii source rock with kirchaou reservoirs the hydrocarbon traps in the area are mainly structural types the study describes and emphasizes the hydrocarbon migration pathway mechanism from source rocks to traps to predict and derisk future drilling locations fairway maps are generated for the three plays ordovician silurian and triassic although the paper focuses on southern tunisia an attempt is made to introduce libyan and algerian knowledge to evaluate the basin in a regional context

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

chronology of pleistocene weathering processes southeast queensland australia 40ar 39ar dating of mn oxides from the upper kandanga uka mn deposit southeast queensland australia shows that the weathering profile overlying the deposit is younger than 1 ma seventy plateau ages obtained for 81 grains from 17 samples fall into three groups i 1000 800 ka n 2 ii 630 510 ka n 2 and iii 400 50 ka n 66 the last age group can be further resolved using the mixture modelling solution by sambridge and compston sambridge m s compston w 1994 mixture modeling of multi component data sets with application to ion probe zircon ages earth planet sci lett 128 373 390 into two major age clusters sub groups iiia and iiib with peak values at 313 4 ka n 43 with an outlier and 213 7 ka n 22 respectively the results suggest that a supergene blanket in a mn deposit may develop relatively fast if weathering and erosional conditions are appropriate the age peaks measured in this study correspond to warm periods identified in specmap oxygen isotope records of deep sea sediments and local paleoclimatic data the correlation of weathering age peaks with oceanic climatic indicators suggests a climatic control on continental chemical weathering where warm and humid conditions are conducive to increasing weathering rates the clustering of supergene mn oxide precipitation ages suggests that weathering rates vary with time supporting an episodic weathering history for the profiles investigated

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

first 40ar 39ar dating of intense late palaeogene lateritic weathering in peninsular india lateritic surface processes have shaped large platform and cratons of the tropical belt constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long term climatic change and lithospheric deformation weathering histories have been documented for south america africa and australia but precise time constraints of the lateritic weathering processes in south india are still lacking we present 40ar 39ar ages of supergene cryptomelane k mn oxide formed in the sandur mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the mysore plateau and the adjacent deccan traps significant 40ar 39ar ages are estimated between 36 and 26 ma from well defined plateaus in step heating 39ar release spectra and from best fitted inverse isochrones these ages constitute firm time constraints that document intense late eocene oligocene lateritic weathering over peninsular india under the influence of warm and wet climate comparable to that prevailing in tropical humid forests these results imply that southern india was weathered between 36 and 26 ma and may have been dissected mostly in the neogene

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

subsurface mesozoic basins in the central atlas of tunisia tectonics sequence deposit distribution and hydrocarbon potential the mesozoic and cenozoic outcrops of the central and meridional atlas of tunisia have been studied by structural biostratigraphic and sedimentologic approaches we extend these studies to the subsurface employing seismic and well data addressing structures sequence stratigraphy and hydrocarbon potential the basin structures are related to strike slip fault systems associated with triassic evaporites mesozoic sedimentary sections in the areas of the gafsa sidi ali ben oun sidi aich and majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls we identified tectonic corridors where upper triassic evaporites intruded younger mesozoic rocks beginning in the jurassic the organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms triassic intrusions induced by the strike slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms mesozoic sequence deposits are distributed along the flanks of the basin platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits the time space distribution of the systems tract deposits indicates the possibility of formation of triassic jurassic and cretaceous carbonate and sandstone prograding turbidites reefs structural and stratigraphic pinch outs and unconformities in the flanks of salt domes and basins the geometry and distribution of reservoir and seal systems tracts permit the consideration of new triassic jurassic and cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms this begin page 886 is a new tectonic model of the central atlas of tunisia that shows new potential objectives and traps

autochthonous versus allochthonous upper triassic evaporites in the sbiba graben central tunisia abstract two dimensional seismic data and well data from the sbiba graben in central tunisia north africa reveal its asymmetrical structural geometry affected by e w n s nw se and ne sw master strike slip faults these faults have been intruded by upper triassic evaporites which can be classified into the following three types 1 sealed evaporite anticlines and domes along folded structures 2 extrusional wrinkles of salt walls at the borders of the graben related to along strike displacement of the master fault zone and 3 evaporite outpouring at the center of the graben interpreted as the result of fault and diapir movement detailed seismic stratigraphic analysis highlights a complex interplay between fault growth and evaporite movement which strongly controlled the evolution of the graben during the mesozoic tectonic stresses induced multidirectional faulting along inherited faults the vertical rising of the upper triassic evaporites was initiated locally at the intersection of master faults during regional extensional and transtensional events in the sub evaporite basement the interaction of normal faulting and evaporite diapirism caused lateral outpouring of surface piercing evaporites this phenomenon is well expressed in upper cretaceous upper cenomanian to lower turonian strata the master faults which cut the diapir at the border and the crest have caused the extrusion of evaporites the downward gravity driven flow of these evaporites across the slope surface and subsequent accumulation and preservation in mesozoic sediments is related to the fault escarpment and the adjacent fault induced depression there is an apparent relation between tectonic inheritance and inversions of the graben where interconnected deep faults and diapirs have been reactivated during extensional and contractional tectonic episodes causing evolution of the structures with the associated sedimentary lapouts several autochthonous triassic evaporites and associated fault escarpments have induced salt canopy structures

late archean manganese mineralization and younger supergene manganese ores in the anmod bisgod region western dharwar craton southern india geological characterization palaeoenvironmental history and geomorphological setting abstract the field setting mineralogical geochemical genetic and depositional features of late archean metasedimentary manganese as well as cenozoic supergene manganese and ferromanganese ores along the 75 km long linear tract of the anmod bisgod region in the western dharwar craton southern india are described the 2700 ma metasedimentary manganiferous formation is underlain by stromatolitic limestone dolomite and quartzite arenite and also occurs within the former the older quartzite arenite which contains redox sensitive detrital pyrite and uraninite as well as thucolite indicates sedimentation under anoxic to oxic transitional palaeoenvironmental condition whereas the younger stromatolitic limestone dolomite indicates development of oxygen in the depositional environment the manganese ore types have been classified into three categories one of which is a mn rich duricrust once capping the late archean manganiferous formation it formed during a cenozoic lateritic weathering event and was eroded in response to crustal uplift perhaps uniquely clastic deposits of the eroded mn duricrust occur in mineable quantities in the form of fluvial palaeochannel fills these cryptomelane rich supergene ores are also suitable for radiometric dating knowing their age would help to constrain the timing of mn duricrust formation and indirectly the regional weathering event responsible for the extensive laterite occurrences of southern india

the hammamet gabes and chotts basins tunisia a review of the subsidence history abstract we modeled the subsidence of three mesozoic basins in tunisia by using well data seismic lines and field data the results are presented here with an attempt to relate the succession of events deduced from the subsidence evolution analysis to the geodynamic evolution of the northern african margin the mesozoic and cenozoic geological history of tunisia has been broken down into two major periods separated by a transition period the first period from trias to aptian was characterized by a constant subsidence of variable intensity spread over most of tunisia and over all the studied basins this period was dominated by extension accompanying the tethyan rifting during trias lias the transition period during the upper cretaceous was characterized by extension and by the weak effects of far seated compressional events it is coeval with the first appearance of salt mobilization the last period in tertiary marked the appearance of the major collisional events mostly during neogene during this last period basins were characterized by very different behaviour depending on their location east or west of the north south axis and close or not to the tellian nappes among the three basins we studied we paid special attention to the southernmost gabes basin located south of the present eastern margin of tunisia the first order evolution of its subsidence is similar to that we infer for most of the tunisian basins we emphasize the role of the structuration of the gabes basin in the partition of the subsidence through time

mean sea surface height of the antarctic circumpolar current from geosat data method and application the mean sea surface height across the antarctic circumpolar current has been reconstructed from height variability measured by the geosat altimeter without assuming prior knowledge of the geoid for this study an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering gaussian jet model and errors have been estimated using monte carlo simulation the results are objectively mapped to produce a picture of the mean subantarctic and polar fronts which together comprise the major components of the antarctic circumpolar current the meandering jet model explains between 40 and 70 of the height variance along the jet axes the results show that the fronts are substantially steered by topography and that the jets have an average gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations the average height difference across the subantarctic front saf is 0 7 m and across the polar front pf 0 6 m the mean widths of the fronts are correlated with the size of the baroclinic rossby radius

monitoring the transport of the antarctic circumpolar current at drake passage abstract data from an extensive moored instrument array and three hydrographic cruises are used to calculate a time series of the net transport through the upper 2500 m of the drake passage the baroclinic geostrophic transport was monitored with heavily instrumented moorings at each side of the passage pressure gauge measurements at the northern and southern ends of the passage provided a reference speed at 500 m to adjust the relative transport the net transport of the upper 2500 m for the 370 day record ending in january 1980 averaged 121 106 m3 s 1 the total transport through the entire cross sectional area of drake passage is probably between 118 and 146 106 m3 s 1 fluctuations in the relative transport are related to changes in latitude of the northernmost front in the passage the subantarctic front variability of the net transport is coherent with the zonally averaged eastward wind stress

circumpolar response of southern ocean eddy activity to a change in the southern annular mode 1 analysis of satellite altimeter data reveals anomalously high eddy kinetic energy eke in the antarctic circumpolar current acc during the period 2000 2002 around 2 3 years earlier 1998 the circumpolar eastward wind stress as quantified by the southern annular mode sam showed a significant positive peak and we have shown previously that the acc peaked around 1998 in response an eddy resolving ocean model is used to investigate the delay between wind forcing and the eddy response and demonstrates that the lag is due to the time taken to influence the deep circulation of the acc winds over the southern ocean have shown a strong climatic increase over the past few decades if this increase in winds is also reflected as an increase in eddy activity as our analysis suggests it might then the increased poleward heat flux may have played a significant role in the observed warming of the southern ocean citation meredith m p and a m hogg 2006 circumpolar response of southern ocean eddy activity to a change in the southern annular mode geophys res lett 33 l16608 doi 10 1029 2006gl026499

cyrenaican shock absorber and associated inversion strain shadow in the collision zone of northeast africa far field compressional stresses resulting from arc collisions with the northeast coast of africa arabia propagated across the african plate in the santonian ca 84 ma and later similar pulses occurred through much of the cenozoic waves of inversion related features including folds thrusts and igneous activity developed during the compressional events structures generated were uneven in intensity and distribution and were commonly concentrated in existing rifts because of the irregular shapes of the north african continental margin and associated inboard basins cyrenaica underwent the most severe shortening although deformation extended eastward from libya to the levant areas south and southeast of cyrenaica the sirt basin and the far western desert of egypt underwent very little shortening because they occupied a regional strain shadow the eastern region of the western desert and sinai which were not sheltered by the stress absorbing cyrenaica inversion recorded strong compressional deformation synchronous with that in cyrenaica as closure of the mediterranean continues cyrenaica will become an indenter comparable to those of other orogenic belts the detailed tectonic evolution of individual northeast african basins has depended on their positions in relation to the cyrenaican strain shadow no single subsidence and paleostress history reflects the complete regional development of the converging mediterranean north african margin

geometry and kinematic evolution of inversion structures positive inversion structures form by the compressional reactivation of preexisting extensional structures experimental models and observations of natural structures are used to develop quantitative models for the geometry and kinematic evolution of inversion structures in this paper i analyze two main formation mechanisms of inversion structures 1 fault propagation folding on planar faults and 2 fault bend folding on listric faults inversion structures formed by fault propagation folding occur in the southern north sea the central montana platform and the kangean basin during extension a broad fault propagation or drape fold develops above the master fault with the fault subsequently breaking through the fold synextensional growth units deposited in the hanging wall typically thicken into the basin compressional reactivation results in slip reversal on the master and secondary faults their rotation to shallower dips and the development of a compressional fault propagation fold inversion structures formed by fault bend folding on listric faults occur in the taranaki basin and possibly in the southern north sea rollover folding in the hanging wall occurs during extension possibly accompanied by a small component of fault propagation folding in the vicinity of the fault tip deformation is primarily localized along a system of antithetic faults synextensional growth sediments typically thicken into the fault but also show thinning in the immediate vicinity of the fault during compression the extensional fold is first unfolded and then folded into a compressional fault bend fold the characteristic variations in bed geometry and thickness provide predictive models for interpreting the subsurface geometries of these two classes of inversion structures in areas with poor seismic data these models are particularly useful in exploring for structural traps in the complex and relatively unexplored synextensional growth units

structure and transport of the antarctic circumpolar current and agulhas return current at 40 e hydrographic data are presented from a near meridional section across the agulhas return current the subtropical front and the antarctic circumpolar current acc in the southwest indian sector of the southern ocean the location and characteristics of the fronts are examined and the water masses are defined the agulhas return current emerges as a strong current as far east as 40 e which forms a marked front that is separate from the subtropical front and can be distinguished by a separate water mass comparisons are made with a similar hydrographic section at the greenwich meridian and several notable differences are found the subantarctic front at the greenwich meridian is a pronounced feature carrying a large proportion of the geostrophic transport of the acc on the southwest indian section however the subantarctic front is merged with the polar front at 48 s where the flow associated with the acc is concentrated geostrophic calculations show a large volume transport 84 to 103 sverdrup 1 sv 106 m3 s 1 associated with the agulhas return current at 40 s this is quite separate from the transport of the acc which totals 138 sv south of the subantarctic zone and the crozet plateau or 150 sv including flow in the subantarctic zone the transport of the acc out of the south atlantic sector appears to be higher than the transport into the south atlantic sector at drake passage with the extra transport most probably carried by upper layer and intermediate waters eddies are seen to be an important feature of the subantarctic zone where they help to modify the water masses antarctic intermediate water appears in two forms a saline 34 4 indian ocean variety and a fresh 34 2 atlantic ocean variety the atlantic variety is freshest at the polar front and occupies the southern half of the subantarctic zone within the eddies in the central subantarctic zone there is interleaving between these two forms of antarctic intermediate water north atlantic deep water is unable to cross the southwest indian ridge but can pass south of the crozet plateau in the form of circumpolar deep water which is slightly more saline 34 75 than at drake passage 34 73

petroleum geology of east central tunisia the tectonic features of tunisia are complex and include folds all types of faults evaporite diapirs and the saharan flexure which separates a stable paleozoic province on the south from a subsidence zone of mesozoic and cenozoic strata of the offshore ashtart and tripolitania basins the remainder of the offshore region is mostly stable shelf of the pelagian craton which also extends onshore the present study focused on this shelf where two carbonate intervals contain proven hydrocarbon reservoirs the metlaoui formation of earliest eocene ypresian age and the zebbag formation of late cretaceous turonian age regionally well defined belts of metlaoui carbonates trend northwest on the northeast are open marine deep water micrites and marls with abundant planktonic foraminifers thick bars of nummulitid packstone grainstone were deposited in shallow water at an angle to the paleoshelf the reservoir is confined largely to the bars and visible effective porosity is best developed in those areas among the foraminifers filled with sand size nummulitid debris where secondary solution enlargement has occurred this lithology tested oil in two recent wildcat wells and is a commercial reservoir at sidi el itayem and ashtart fields distribution of zebbag carbonates is more complex a northwest trending platform was bounded on three sides by deep water where shale and micrite with planktonic foraminifers were deposited predominately back reef and lagoonal foraminifer rudist wackestones and packstones occur in narrow belts apparently controlled at least locally by block faulting they tested oil in two recent discoveries the most significant porosity is interparticle generally enlarged by solution in foraminifer packstones but intraparticle voids in foraminifers and rudists commonly contribute to the porosity analyses of surface and subsurface samples identified the bahloul basal turonian and bou dabbous ypresian formations as source rocks fluorescence spectra of several oils were compared with extracts from these samples and indicate the bahloul to be the source of oils in recent metlaoui and zebbag discoveries

cenozoic sedimentary dynamics of the ouarzazate foreland basin central high atlas mountains morocco cenozoic continental sedimentary deposits of the southern atlas named imerhane group crop out a in the ouarzazate foreland basin between the precambrian basement of the anti atlas and the uplifted limestone dominated high atlas and b in the ait kandoula and ait seddrat nappes where jurassic strata detached from the basement have been thrust southwards over the ouarzazate basin new biostratigraphic and geochronological data constraining the final eocene marine regression the characterization of the new ait ouglif detrital formation presumed to be of oligocene age and the new stratigraphic division proposed for the continental imerhane group clarify the major tectonogenetic alpidic movements of the central high atlas range four continental formations are identified at regional scale their emplacement was governed principally by tectonic but also by eustatic controls the hadida and ait arbi formations upper eocene record the major paleogene regression they are composed of margino littoral facies coastal sabkhas and fluviatile systems and reflect incipient erosion of the underlying strata and renewed fluvial drainage the ait ouglif formation presumed oligocene had not been characterized before it frequently overlies all earlier formations with an angular unconformity it includes siliciclastic alluvial deposits and is composed predominantly of numerous thin fining upward cycles the ait kandoula formation miocene pliocene is discordant extensive and represents a thick coarsening upward megasequence it is composed of palustro lacustrine deposits in a context of alluvial plain with localized sabkhas giving way to alluvial fans and fluviatile environments the upper conglomeratic formation quaternary is the trace of a vast conglomeratic pediment forming an alluvial plain and terraces the second and third formations correspond to two megasequences engendered by the uplift of the central high atlas in two major compressive phases during late oligocene and miocene pliocene times these two geodynamic events were separated by a tectonically calm phase materialized by palustro lacustrine sedimentation gorler et al 1988 tectono sedimentary analysis of the two megasequences shows that the basin structure and depositional processes were controlled by the compressive tectonic context generated by the collision of north africa and iberia in tertiary times jacobshagen et al 1988 the quaternary formation was apparently controlled by a tectonic continuum and by climatic variations

monitoring gulf stream transport by radar altimeter and inverted echo sounders abstract profiles of the gulf stream sea surface height ssh from the geosat altimeter were compared with thermocline depth measurements from inverted echo sounders ies for a period of about 19 months the correlation between ssh anomalies and thermocline depth anomalies at about 68 w was 0 92 to distinguish between agreement in the location of the gulf stream jet and agreement in the strength of the jet separate comparisons were made for jet location and the ssh difference across the gulf stream based on an error function fit to each geosat and ies profile the correlation between the jet centers from the two types of measurements was 0 97 and the correlation between the amplitudes of the error functions was 0 76 although the altimeter measures surface geostrophic velocity the high degree of vertical correlation in the gulf stream velocity profiles allows the altimeter to monitor transport fluctuations as well as changes in jet position based on this assessment of the accuracy of the altimeter

generation and evolution of a cyclonic ring at drake passage in early 1979 abstract an equatorward meander in the antarctic polar front at drake passage was observed to amplify and pinch off during january and early february 1979 forming a cold core cyclonic current fins with a radius of 50 km this ring appeared to move directly across the passage parallel to a submarine ridge before turning northeastward upon reaching a gap between the ridge and the south american continental rise its net motion was across the polar frontal zone and the ring was last observed to be pushing through the subantarctic front geostrophic surface speeds of up to 90 cm s 1 relative to 3500 m wore observed where the northern sector of the ring and the subantarctic front were in proximity a stability analysis of the banded flow regime across drake passage suggests that necessary conditions for baroclinic instability exist everywhere within the zonal current and those for barotropic instability exist adjacent to the fronts the beat and salt anomalies of this ring relative to the polar frontal zone

the mean sea surface height and geoid along the geosat subtrack from bermuda to cape cod measurements of near surface velocity and concurrent sea level along an ascending geosat subtrack were used to estimate the mean sea surface height and the earth s gravitational geoid velocity measurements were made on three traverses of a geosat subtrack within 10 days using an acoustic doppler current profiler adcp a small bias in the adcp velocity was removed by considering a mass balance for two pairs of triangles for which expendable bathythermograph measurements were also made because of the large curvature of the gulf stream the gradient wind balance was used to estimate the cross track component of geostrophic velocity from the adcp vectors this component was then integrated to obtain the sea surface height profile the mean sea surface height was estimated as the difference between the instantaneous sea surface height from adcp and the geosat residual sea level with mesoscale errors reduced by low pass filtering the error estimates were divided into a bias tilt and mesoscale residual the bias was ignored because profiles were only determined within a constant of integration the calculated mean sea surface height estimate agreed with an independent estimate of the mean sea surface height from geosat obtained by modeling the gulf stream as a gaussian jet within the expected errors in the estimates the tilt error was 0 10 m and the mesoscale error was 0 044 m to minimize mesoscale errors in the estimate the alongtrack geoid estimate was computed as the difference between the mean sea level from the geosat exact repeat mission and an estimate of the mean sea surface height rather than as the difference between instantaneous profiles of sea level and sea surface height in the critical region near the gulf stream the estimated error reduction using this method was about 0 07 m differences between the geoid estimate and a gravimetric geoid were not within the expected errors the rms mesoscale difference was 0 24 m rms

the subantarctic and polar fronts in relation to deep water masses through the southwestern atlantic the two principal fronts within the antarctic circumpolar current acc the subantarctic front saf and the polar front pf are investigated with respect to their spatial patterns and relations to deep water masses from the northern drake passage to the western and southern argentine basin observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters east of drake passage the width of the polar frontal zone pfz increases rapidly when the saf turns sharply northward as a part of the falkland malvinas current while the pf remains oriented mainly toward the northeast found within this region of the pfz are mesoscale eddies the upper layers of which consist of subantarctic mode water from the pacific the contrasts in upper layer water properties that identify the saf can be traced the entire length of the falkland current to the brazil falkland confluence zone and then southward to the southern argentine basin there the saf turns eastward and later but perhaps not always merges with the pf north of ewing bank to form a single intense current core strongly influenced by bottom topography at times this eastward current loops southward through a gap in the falkland ridge into the georgia basin thereby supplying relatively warm and salty subantarctic water to the antarctic zone east of the gap the two fronts become separated and are once again distinct features in the vicinity of the islas orcadas rise at depth within drake passage and the western scotia sea the saf and pf are not lateral boundaries between distinct water masses but are instead identified by enhanced vertical displacements of property isopleths with the northward turn of the saf east of drake passage a thick layer of circumpolar deep water cdw is advected over the falkland plateau into the argentine basin there it is joined by waters entering the argentine basin via a deep spreading route through the georgia basin denser cdw deep water from the weddell sea and episodically deep water from the southeastern pacific ocean together these waters form a northward flow off the patagonian shelf that extends from the sea surface to the bottom most of which turns back toward the south with the falkland current return also flowing south along the seaward side of the falkland current return well away from the western boundary is the main core of north atlantic deep water nadw detatched masses of nadw are observed within the cdw at the western boundary as well as within the acc south of the saf in the south central argentine basin

reply to comment on how to build an extensional basin in a contractional setting numerical and physical modeling applied to the mejerda basin at the front of the eastern tell of tunisia

metalliferous manganese oxide mineralization associated with the boleo cu co zn district mexico apart from its large reserves of cu co zn mineralization contained within stratiform ore beds or mantos the boleo district of central baja mexico also contains significant concentrations of mn oxides typically 2 3 wt mn interspersed with hydrothermal sulfides and both hydrothermal and weathering related metal oxide minerals within the mantos mn oxides occur as concentric nodules disk shaped concretions laminae granules or aggregates veins dendrites and various replacements the oxides are composed of varying amounts of hollandite cryptomelane todorkite romanechite pyrolusite and x ray amorphous mn oxide chemically the manto oxides are unique owing to their extraordinarily high abundances of co cu pb and zn typically 2 5 wt but up to 23 wt existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of mn oxides within the mantos a new mno 2 fe 2 o 3 t mgo diagram is proposed in order to discriminate among highly metalliferous mn oxides based on the new classification scheme manto mn oxides record a complex interaction between hydrothermal and diagenetic processes to better understand the mechanism of oxide deposition radiogenic isotopes sr and pb and ree geochemistry were used to delineate fluid and metal sources isotopic and ree constraints indicate that the boleo mn oxides are largely hydrothermal in origin however they show varying degrees of modification resulting from diagenetic processes hydrothermal mn oxides are syngenetic synsedimentary to syndiagenetic in origin and were precipitated in association with sulfides reflecting localized variations in redox conditions within the manto sequence due to the downward infiltration of metal bearing brines apart from morphological and stratigraphic evidence e g occurrence of nodular crusts along the upper contact of the manto a syngenetic origin is supported by a single k ar date of 7 0 0 2 ma which is within the estimated age range for deposition of boleo formation clastic sediments interaction between brine and seawater or brine and pore fluid as it infiltrates the sedimentary pile produced hydrothermal precipitates with certain hydrogenetic attributes i e positive ce anomaly lree enrichment on the other hand oxides with hydrogenetic and with mixed hydrothermal hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids by far the greatest degree of remobilization occurred in response to compactional dewatering of the manto where the dewatering pore waters enriched the uppermost manto strata in mn fe zn co and to lesser degrees cu later diagenetic modifications coincided with the development of a supergene overprint which typifies boleo formation strata at elevations above the present day water table

volume transport of the antarctic circumpolar current from bottom pressure measurements abstract meausrements from bottom pressure gauges located at the north and south sides of drake passage are used to extend the one year time series of volume transport of the antarctic circumpolar current acc given previously by whitworth a small error in that paper is corrected and a revised transport time series is presented which shows the importance of including the transport in the northern and southern margins of drake passage direct measurements of vertical shear averaged across the passage are in good agreement with geostrophic shear and suggest that the acc is in geostrophic balance although most of the acc transport is in the baroclinic field transport fluctuations are mainly barotropic transport estimates based on pressure difference alone differ from the estimates of whitworth by less than 10 106 m3 s 1 fluctuations in transport of almost half of the mean value occur over periods as short as two weeks

geometry and structural evolution of lorbeus diapir northwestern tunisia polyphase diapirism of the north african inverted passive margin detailed geologic mapping structural analysis field cross sections new dating based on planktonic foraminifera in addition to gravity signature of lorbeus diapir are used to characterize polyphase salt diapirism this study highlights the role of inherited faulting which controls and influences the piercement efficiency and the style and geometry of the diapir and also the localization of evaporite early ascent displaying diapiric growth during extension salt was extruded along the graben axis developed within extensional regional early cretaceous tectonic associated with the north african passive margin evolution geologic data highlight reactive diapirism during albian time most extreme extension period and passive diapirism during the late cretaceous post rift stage northeastern maghreb salt province gives evidences that contractional deformations are not associated with significant diapirism during shortening the initial major graben deforms as complex anticlines where diapirs are squeezed and pinched from their feeding