reassessment of the rifting process in the western corinth rift from relocated seismicity the seismic activity in the western part of the corinth rift greece over the period 2000 2007 monitored by a dense network of three component stations is analysed in terms of multiplets and high precision relocation using double difference techniques this detailed analysis provides new insights into the geometry of faults at depth the nature and the structure of the active zone at 6 8 km depth previously interpreted as a possible detachment and more generally into the rifting process the seismicity exhibits a complex structure strongly varying along the rift axis the detailed picture of the seismic zone below the rift indicates that its shallower part at depths of 6 8 km is 1 1 5 km thick with a complex microstructure and that its deeper part at depths of 9 12 km gently dipping to the north 10 20 is 0 1 0 3 km thick with a microstructure consistent with the general slope of the structure although the nature of this seismic zone remains an open question the presence of seismicity beneath the main active area the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift this active zone more likely represents a layer of diffuse deformation the geometry of the mapped active faults is not well defined at depth as no seismicity is observed between 0 and 4 km except for the aigion fault rooting in the seismic layer at 6 km depth with a dip of 60 a distinct cloud of seismicity may be associated with the antithetic kalithea fault on which the 1909 fokis earthquake m s 6 3 may have occurred the link between the 1995 rupture m s 6 2 and the faults known at the surface has been better constrained as the relocated seismicity favours a rupture on an offshore blind fault dipping at 30 rather than on the deeper part of the east helike fault consequently the 1995 event is expected to have decreased the coulomb stress on the east helike fault to explain these seismic observations along with the geodetic observations a new mechanical model for the rifting process in this region is proposed involving non elastic mostly aseismic uniform ns opening below the rift axis coupled with the downward and northward growth of a yet immature detachment the reported gps rates would mainly result from this deep silent source and the seismicity would reveal the detachment position not yet connected to the ductile lower crust in such a model the strong fluctuations of microseismicity would result from small strain instabilities undetected by continuous gps and possibly related to pore pressure transients

a reappraisal of the 1894 atalanti earthquake surface ruptures central greece this article presents the results of new field and aerial photo surveys of the atalanti fault and of the mesoseismal area of the 20 and 27 april 1894 earth quakes coupled with a reanalysis of contemporary reports and previous investiga tions these are used to gain a better understanding of the faults responsible for these events and their seismic behavior the first shock was smaller and probably located inshore or offshore the malessina peninsula no resolving field evidence has been found to locate the seismogenic structure responsible for this shock on the basis of the limited information available we suggest the malessina escarpment a 12 km long ene trending nw dipping fault as a possible structure responsible for this event on the other hand the second and largest shock is definitely related to the atalanti fault sensu stricto a main wnw trending n dipping active fault extending between the platirema valley a few km nw of the town of atalanti and larymna the total length of the rupture recognized in the field is about 32 km but it can be extended further se up to 40 km no evidence for a longer rupture extending some other 20 km to the nw between the karagiozis river and ag kostantinos is found the complex geometry of the fault with bends and step overs appears to be controlled by preexisting transverse structures minimum coseismic vertical throws measured in the field after more than a century elapsed from the earthquake are 30 80 cm thus consistent with contemporary reports indicating 1 m average slip rates are not well constrained the available estimates fall in the range 0 1 0 5 mm yr confirming the smaller amount of crustal extension taking place in this area with respect to other nearby regions such as the corinth gulf no new data are available to define the average recurrence interval typical of the atalanti fault however a reconsideration of the existing information induced us to rule out the possibility that the famous 426 b c earthquake occurred on the atalanti fault on the basis of the extent and size of the rupture recognized in the field a m 6 8 is estimated for the second and largest shock

timing kinematics and cause of aegean extension a scenario based on a comparison with simple analogue experiments abstract since the early 1970s most geodynamic models proposed for the eastern mediterranean consider that the initiation of extension in the aegean during the late middle miocene or later was a direct consequence of the westward extrusion of anatolia away from the arabia eurasia collision front in the late 1990s a few models have taken into account the increasing evidence for an earlier initiation of aegean extension arising mostly from the analysis of deformation within the various alpine metamorphic complexes of the area based largely on our own investigations we first present a review of the data that reliably constrain the minimum age for the initiation of extension in these complexes it shows that regional scale extension with a pattern of stretching orientations similar to that of the pliocene pleistocene was already strongly active in the aegean before the onset of arabian indentation into eurasia lower miocene vs middle or upper miocene this implies that the initiation of aegean extension did not result from the lateral extrusion of anatolia instead extension may have started owing to a process of gravity spreading of the continental lithosphere that had previously been thickened during alpine collision the main arguments favouring this interpretation are presented and a preliminary scenario is proposed for the neogene evolution of the eastern mediterranean in which lateral extrusion of anatolia occurred lately during southward spreading of the aegean lithosphere in order to test this hypothesis further physical experiments simulating the horizontal spreading of a continental lithosphere toward a free boundary have been carried out the analysis of various elements of the deformation field displacements strain rotations fault pattern and kinematics both in the experiments and in the aegean domain reveals an excellent agreement between the two not only do early stages of aegean extension appear to fit the gravity spreading hypothesis but so does the recent to present internal deformation of the aegean transverse e w active shortening observed in the northern aegean is commonly taken as one of the most striking pieces of evidence that aegean extension results from the lateral extrusion of anatolia the experiments indicate that such transverse shortening may equally develop as a result of pure gravity spreading associated with a pattern of faults and rotations that compares well with that of the northern aegean pure dextral shearing along some strands of the north aegean trough fault system and the related development of an asymmetric pattern of faults in the northern aegean are eventually the only features that evidently arise from the extrusion of anatolia since less than 3 ma finally we argue that during the early stages of aegean extension since at least 21 ma and until ca 11 ma the distribution of extensional strain on a regional scale has been controlled by the distribution of earlier thickening post thickening thermal weakening through partial melting of deep levels of the crust probably accounts for the localization of extensional strain within the areas of thick crust restoring the aegean in its most likely configuration at the onset of bulk crustal extension suggests that 1 the extensional strain rate remained roughly constant or has eventually decreased since the onset of aegean extension and 2 the amounts of extension before and after 16 ma have been roughly equal

synorogenic hydrothermal origin for giant hamersley iron oxide ore bodies geologic mapping basin analysis and calculated fluid compositions indicate that giant orebodies of microplaty hematite and possibly martite goethite in the hamersley province of western australia were formed by heated fluids driven by early paleoproterozoic orogenesis detrital grains of microplaty hematite in the mcgrath trough a foreland basin in front of the northward advancing ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 c and locally up to 400 c were involved regional circulation of hydrothermal fluids including heated surface water through reduced banded iron formations occurred during or soon after the ophthalmian orogeny we speculate that martite goethite orebodies previously considered mesozoic cenozoic could also be related to heated paleoproterozoic meteoric fluids migrating northward away from the ophthalmian fold belt

seismicity normal faulting and the geomorphological development of the gulf of corinth greece the corinth earthquakes of february and march 1981 three major destructive earthquakes of m s 6 7 6 4 6 4 occurred in the eastern part of the gulf of corinth in february and march 1981 associated normal faulting was observed on both the north and the south sides of the gulf examination of teleseismic local seismic surface faulting and geomorpbological data suggests that the first and second of these shocks activated major north dipping normal faults these faults control the topography and bathymetry and are related to the recent uplift and subsidence of the coastline it is probable that the first earthquake occurred on a faultwhich outcrops underwater and the second on a fault which is visible on land it appears that the third shock activated an antithetic normal fault dipping southwards on the north side of the gulf and that the overall structure of the gulf of corinth graben is asymmetric the northern antithetic fault clearly demonstrates the geomorphological changes associated with developing young normal faulting in addition we suggest that observations of recent coastline movement provide a powerful tool for identifying major active fault systems and predicting their historical role in the paleogeography of an area

streamer tomography velocity models for the gulf of corinth and gulf of itea greece summary the gulf of corinth goc greece is a rapidly extending active continental rift with a record of large damaging earthquakes an extensive multichannel seismic mcs survey of the goc conducted in 2001 provided in addition to the processed mcs images the opportunity to constrain velocity structure using refracted arrivals recorded along a 6 km long streamer we use first arrival traveltimes to derive tomographic p wave velocity models for several profiles collected in the central portion of the goc eight of the profiles are closely spaced north south lines crossing the goc and extending into the gulf of itea goi a ninth profile is an east west oriented tie line the n s profiles image the relatively simple velocity structure of the deep corinth rift basin and more complicated structure of the northern margin of the currently active rift integration of the velocity models with migrated mcs sections shows that south of the goi the basement which comprises mesozoic nappes occurs at a velocity of 4 5 km s 1 in the velocity models although the actual velocity at or just below the top of basement is probably closer to 5 5 5 km s 1 the maximum sediment thickness in the corinth basin is 2 2 km the basement shallows to the north into a fault bounded terrace in the central region between the two gulfs sediment cover in this central region decreases in thickness from west to east beneath the goi low average velocities beneath the rift onset reflector indicate the presence of pre rift sediments the pre rift velocity structure in the goi is complex with significant lateral variation from west to east the e w line shows that high velocity basement is shallow 1 km depth and flat to the west of the goi but dips 20 east down to 1 5 km beneath the pre rift sediments of the goi

the origin of coexisting carbonates in banded iron formations a micro mineralogical study of the 2 4 ga itabira group brazil this is the first petrographic and chemical study on the origin of co existing calcite dolomite ankerite and sideroplesite in the 2 4 ga itabira group brazil the itabira group hosts silicic and dolomitic bifs known as quartz itabirites and carbonate itabirites which are composed of micritic quartz and dolomite with inclusions of nano and micro hematite nanometric si bearing ferrihydrite clusters are observed in the micritic dolomite mineralogical and textural features indicate a primary oxidizing depositional environment for the itabira group at 2 4 ga limestones of the itabira group include early diagenetic hematite nodules intergrown with dolomite silica in the ferrihydrite structure increases its stability up to 500 c and the nano and micro hematite included in dolomite and quartz were partially protected from later transformation by being enclosed within the dolomite and quartz crystals positive eu and y anomalies and heavy rare earth element enrichments of all lithofacies along with their low terrestrial element abundance excluding the shales indicate their primary precipitation from a mixed hydrothermal marine environment in the case of the carbonate itabirites fluids were co2 rich dolomite precipitated abiotically due to higher temperatures 100 c and high mg ca ratios the facies changes from carbonate rich to quartz rich are attributed to transgression and regression episodes and or a change in the upwelling hydrothermal fluid composition post depositional reductive dissolution affected the quartz itabirites facilitating the precipitation of ankerite sideroplesite and magnetite sulphides from a reducing diagenetic solution rich in mn mg and fe either as lenses or disseminated in the quartz itabirites and shales the sideroplesite richest samples have the most negative 13c values quartz itabirite 9 49 shale 5 76 indicating that at least part of the c is of organic origin either through the oxidation of organic matter or introduced via the diagenetic fluids specularite is replacive and likely related to a post depositional hydrothermal event this study shows that careful mineralogical investigations combined with ree y and trace element geochemistry is necessary in order to decipher the depositional environments which are oxidizing mixed marine hydrothermal post depositional reductive diagenesis masks in part these primary conditions

microstructures textures and deformation mechanisms in hematite abstract deformation and recrystallization of hematite in iron formations and high grade ore bodies of the caue formation from the quadrilatero ferrifero district minas gerais brazil resulted in several types of fabrics depending on the tectonic and metamorphic imprint the fabrics vary from randomly oriented granoblastic to strongly oriented lepidoblastic mylonitic with local development of cataclastic features complete pole figures of the crystallographic planes 003 basal plane 110 prism and 104 rhombohedron were measured on several samples from different areas by means of neutron diffraction revealing a typical crystallographic preferred orientation for the different fabric types the configuration and intensity of the maxima are directly related to the magnitude of the deformation and the type of response of the ore the analysis of the textures and microstructures of the iron ores and comparison with experimental deformation data lead us to the conclusion that three main mechanisms are active basal slip diffusion processes and anisotropic grain growth post tectonic recrystallization and secondary grain growth did not affect the textures developed during deformation

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

the structures stratigraphy and evolution of the gulf of corinth rift greece summary a multichannel seismic and bathymetry survey of the central and eastern gulf of corinth goc greece reveals the offshore fault geometry seismic stratigraphy and basin evolution of one of earths most active continental rift systems active right stepping en echelon north dipping border faults trend ese along the southern gulf margin significantly overlapping along strike the basement offsets of three akrata derveni sithas and xylocastro are linked the faults are biplanar to listric typically intermediate angle 35 in the centre and 45 48 in the east near the surface but decreasing in dip and or intersecting a low or shallow angle 15 20 in the centre and 19 30 in the east curvi planar reflector in the basement major s dipping border faults were active along the northern margin of the central gulf early in the rift history and remain active in the western gulf and in the subsidiary gulf of lechaio but unlike the southern border faults are without major footwall uplift much of the eastern rift has a classic half graben architecture whereas the central rift has a more symmetric w or u shape the narrower and shallower western gulf that transects the 40 km thick crust of the hellenides is associated with a wider distribution of overlapping high angle normal faults that were formerly active on the peloponnesus peninsula the easternmost sector includes the subsidiary gulfs of lechaio and alkyonides with major faults and basement structures trending ne e w and nw the basement faults that control the rift architecture formed early in the rift history with little evidence other than the vrachonisida fault along the northern margin in the marine data for plan view evolution by subsequent fault linkage several have maximum offsets near one end crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the derveni fault the dominant strikes of the corinth rift faults gradually rotate from 090 120 in the basement and early rift to 090 100 in the latest rift reflecting a 10 rotation of the opening direction to the 005 presently measured by gps the sediments include a locally 1 5 km thick early rift section and a late rift section also locally 1 5 km thick that we subdivide into three sequences and correlate with seven 100 ka glacio eustatic cycles the gulf depocentre has deepened through time currently 700 mbsl as subsidence has outpaced sedimentation we measure the minimum total horizontal extension across the central and eastern gulf as varying along strike between 4 and 10 km and estimate full values of 6 11 km the rift evolution is strongly influenced by the inherited basement fabric the regional nnw structural fabric of the hellenic nappes changes orientation to ese in the parnassos terrane facilitating the focused north south extension observed offshore there the basement penetrating faults lose seismic reflectivity above the 4 14 km deep seismogenic zone multiple generations and dips of normal faults some cross cutting accommodate extension beneath the goc including low angle 15 20 interfaces in the basement nappes the thermally cool forearc setting and cross orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation

40ar 39ar dating of west african lateritic cryptomelanes abstract dating weathering processes is an innovative approach to understanding climate dependant modifications of the earth s surface potassium bearing manganese oxides are a ubiquitous feature of weathering mantles developed from mn bearing parent rocks recent studies have demonstrated the possibility of dating k rich manganese oxides from the hollandite group such as cryptomelane k 1 2 mn 8 o 16 nh 2 o using 40 ar 39 ar argon ion laser probe method this work applies that approach to cryptomelanes formed in the lateritic manganese ore deposit of tambao northern burkina faso a present unexploited deposit among the cryptomelane richest in the world two samples of massive mn rich concretions 10mm and 4mm in thickness were collected from the massive 80 m high mn rich hill at tambao a detailed petrographical study shows that these cortical formations are composed of successive na rich cryptomelane layers with na na k ba up to 20 seven and five subsamples from these two concretions were studied with a 40 ar 39 ar laser probe the age spectra indicate ages that are coherent with the general organization of the concretions showing a regular decrease from cores towards rims within the two concretions ages range from 49 8 0 4 ma to 48 1 0 1 ma and from 47 5 0 3 ma to 44 5 0 4 ma respectively the youngest ages being obtained for the most external layers the integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm ma these ages clusters show that weathering at tambao was functioning during the early eocene and thus suggest that there is a relationship between the formation of the bauxites of the african surface and the mn fe laterites

tropical laterites life on land and the history of atmospheric oxygen in the paleoproterozoic the ca 2 2 ga hekpoort paleosol of the transvaal supergroup in southern africa has been considered a type example and the youngest iron depleted paleosol formed under a reducing atmosphere in the early precambrian however the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron depleted pallid lower zone and an iron enriched lateritic upper zone previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds the laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass revised stratigraphic correlation indicates that the hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of wolhaarkop in griqualand west this information indicates that the oxygen evolution curve based on loss or retention of iron in paleosols should be reexamined

the anatomy of shallow crustal transpressional structures insights from the archaean carajas fault zone amazon brazil abstract the carajas fault zone cfz is the most prominent structure in the e w trending carajas strike slip system casss an ancient upper crustal fault network of probable late archaean age that cuts across the precambrian amazonian craton in brazil the subvertical faults reactivated pre existing high grade basement shear zone fabrics itacaiunas shear zone and display a complex long lived history of movement 1 ga dominated by oblique and strike slip displacements a postulated early regional phase of dextral movement downfaulted units of low grade to unmetamorphosed basement rocks into dilational bends and offsets in the casss subsequent sinistral transpression ca 2 6 ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity

types and general characteristics of the bif related iron deposits in china abstract bifs banded iron formations in china which account for approximately 64 of the total identified resources in the country belong to two categories a predominant neoarchean algoma type and a few paleoproterozoic superior type a minor group of bifs developed in the neoproterozoic is also representing having possible correlation with the snowball earth scenario spatially the bifs in china are mainly distributed in the north china craton they experienced intense metamorphism and deformation which led to the majority of iron oxides changing into coarse grained magnetites though their ores generally contain only 30 tfe the bifs are potential targets of industrial exploitation by magnetic beneficiation on the basis of ore forming age host formations mineral assemblages and grade of the ores the bif related iron deposits in china are classified into seven types 1 anshan type neoarchean volcanic sedimentary formations with magnetite as the main ore mineral 20 tfe

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

improved modelling of the quaternary evolution of the gulf of corinth incorporating erosion and sedimentation coupled by lower crustal flow abstract numerical modelling incorporating coupling between surface processes and induced flow in the lower continental crust is used to address the quaternary evolution of the gulf of corinth region in central greece the post early pleistocene marine depocentre beneath this gulf overlies the northern margin of an older early pleistocene and earlier lacustrine basin the proto gulf of corinth basin or pgcb in the late early pleistocene relief in this region was minimal but subsequently dramatic relief has developed involving the creation of 900 m of bathymetry within the gulf and the uplift by many hundreds of metres of the part of the pgcb south of the modern gulf which forms the gulf s main sediment supply it is assumed that as a result of climate change around 0 9 ma erosion of this sediment source region and re deposition of this material within the gulf began both processes occurring at spatial average rates of 0 2 mm a 1 modelling of the resulting isostatic response indicates that the local effective viscosity of the lower crust is 4 10 19 pa s indicating a moho temperature of 560 c it predicts that the 10 mm a 1 of extension across this 70 km wide model region at an extensional strain rate of 0 15 ma 1 is partitioned with 3 mm a 1 across the sediment source 2 mm a 1 across the depocentre and 5 mm a 1 across the hinge zone in between the latter value being an estimate of the extension rate on normal faults forming the major topographic escarpment at the southern margin of the gulf this modelling confirms the view suggested previously that coupling between this depocentre and sediment source by lower crustal flow can explain the dramatic development in local relief since the late early pleistocene the effective viscosity of the lower crust in this region is not particularly low the strong coupling interpreted between the sediment source and depocentre results instead from their close proximity in detail the effective viscosity of the lower crust is expected to decrease northward across this model region due to the northward increase in exposure of the base of the continental lithosphere to the asthenosphere in the south the two are separated by the subducting hellenic slab the isostatic consequences of such a lateral variation in viscosity provide a natural explanation for why since 0 9 ma the modern gulf has developed asymmetrically over the northern part of the pgcb leaving the rest of the pgcb to act as its sediment source

kinematics of rock flow in a crustal scale shear zone implication for the orogenic evolution of the southwestern hellenides combined shear sense criteria finite strain data and vorticity analyses were used to study the deformation path in a curved crustal scale shear zone phyllite quartzite series of the southwestern hellenides the results are combined with data on the structural evolution of a cover nappe pindos thrust belt to provide new insights into the orogenic evolution of this region ductile deformation within the phyllite quartzite series was associated with a top to the west southwest shearing and was partitioned into two structural domains a root zone and a frontal domain the root zone is characterized by vertical coaxial stretching high strain and upward movement of the material while the frontal domain comprises simple shear deformation at the base and pure shear at the top this pattern suggests superposition of pure shear on simple shear deformation and implies tectonic extrusion of the material from the root zone the initiation of brittle deformation in the pindos thrust belt was associated with westward translation above the sub horizontal pindos thrust later as the mountain range elevated normal faulting at high altitudes and migration of thrusting to the west occurred while east directed folding and thrusting in the belt started to the east according to the proposed model crustal thickening was taking place throughout the oligocene and early miocene including the subduction of the apulian beneath the pelagonian microcontinent and the intracontinental subduction of the phyllite quartzite series during the lower miocene vertical buoyancy forces led to the successive steepening of the shear zone and the simultaneous duplexing of its basement facilitating tectonic extrusion of the material from its root zone finally an indentation process caused vertical expulsion of the orogenic wedge and gravity collapse in the brittle crust

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

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

carbonate alteration of the upper mount mcrae shale beneath the martite microplaty hematite ore deposit at mount whaleback western australia the formation of large martite microplaty hematite ore deposits in northwest australia remains a contentious topic in part because important evidence supporting a unifying genetic model has not been observed at all deposits carbonate replacement of silica has been found along normal faults below ore at the mount tom price and giles mini deposits which suggests an early hypogene process during ore formation however such rocks have not been identified at the largest martite microplaty hematite deposit mount whaleback in this study samples of the mount mcrae shale are examined for their chemistry mineralogy and petrography these samples were collected from several key locations including an area that immediately underlies ore along the mount whaleback fault at mount whaleback compared to unaltered black mount mcrae shale from wittenoom gorge in the north and altered black and red mount mcrae shale at mount whaleback reddish green mount mcrae shale along the mount whaleback fault is greatly enriched in mgo and cao and depleted in sio2 this chemistry arises from significant amounts of fine to medium grained ferroan dolomite and ankerite and cross cutting chlorite and carbonate veins the composition is distinct from that produced during regional metamorphism and most likely represents hydrothermal alteration after metamorphism the lack of carbonate rich silica poor rocks in the overlying dales gorge member at mount whaleback is consistent with pervasive oxidation of most rocks in the region during or after ore genesis a process that removed carbonates although several questions remain unanswered these results support models that invoke an early hypogene stage during the formation of the martite microplaty hematite deposits in the hamersley province

a comparison of geodetic and finite strain pattern in the aegean geodynamic implications abstract a comparison of the active strain pattern of the aegean constrained by global positioning system measurements and the oligo miocene finite strain field shows some striking similarities in terms of direction and rates of extension the geometry of extension at crustal scale is also very similar with north dipping normal faults connecting near the brittle ductile transition to shallow north dipping shear zones this pattern of extension is not significantly modified by the recent extrusion of the motion of anatolia and aegea the persistence over more than 25 myr of the same pattern of extension suggests that the cause for extension resides within the aegean lithosphere gravitational collapse allowed by extensional boundary conditions after 30 ma slab retreat is the primary cause for post orogenic extension in the aegean and western anatolia

crystalline lithology across the kapuskasing uplift determined using in situ poisson s ratio from seismic tomography seismic refraction tomography has been used to image lateral velocity variations both vp and vs within the shallow crystalline basement using travel times from a high resolution reflection profile across the kapuskasing uplift structure in northern ontario comparison of these in situ velocities and the outcrop geology indicates a correlation of velocity with the major lithologic units anorthosite vp 6150 6350 m s and vs 2700 3000 m s mafic gneiss vp 6000 6250 m s and vs 2775 3000 m s tonalite vp 5850 6000 m s and vs 2950 3025 m s vp measurements on core samples from the main lithologic units confirm that tonalite can be be clearly distinguished from mafic gneiss and anorthosite based on vp the observed vp values are attributed to mineralogy based on modal analyses of core samples from the same region poisson s ratio calculated from in situ vp and vs helps to discriminate between velocity variations caused by porosity effects from those due to lithology values of poisson s ratio obtained for the three lithologic units within the upper 100 m of the basement are anorthosite 0 35 0 39 mafic gneiss 0 34 0 38 and tonalite 0 32 0 34 these relatively high values of poisson s ratio indicate that the shallow basement is porous and water saturated poisson s ratio is a particularly effective discriminator in this instance because of its extremely low value in quartz 0 08 relative to other silicate minerals 0 21 0 32 and the greater quartz content of tonalite 27 36 relative to anorthosite and mafic gneiss 0 23 reflection coefficients calculated for the contrasting lithologies range up to 0 065 for tonalite mafic gneiss contacts and up to 0 044 for tonalite anorthosite contacts confirming that an interlayered sequence of tonalitic anorthositic and mafic rocks mapped at the surface may be the cause of the shallow reflections observed in the coincident seismic reflection data

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

tearing segmentation and backstepping of subduction in the aegean new insights from seismicity abstract this study revisits subduction processes at the hellenic subduction zone hsz including tearing segmentation and backstepping by refining the geometry of the nubian slab down to 150 180 km depth using well located hypocentres from global and local seismicity catalogues at the western termination of the hsz the kefalonia transform fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it respectively a discontinuity is suggested to exist between the two slabs at shallow depths the kefalonia transform fault is interpreted as an active subduction transform edge propagator fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it a model reconstructing the evolution of the subduction system in the area of peloponnese since 34 ma involving the backstepping of the subduction to the back side of adria provides seismological evidence that supports the single slab model for the hsz and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere in the area of rhodes earthquake hypocentres indicate the presence of a nw dipping subducting slab that rules out the presence of a ne sw striking subduction transform edge propagator fault in the pliny strabo trenches region earthquake hypocentres also allow refining the slab tear beneath southwestern anatolia down to 150 180 km depth furthermore the distribution of microseismicity shows a first order slab segmentation in the region between crete and karpathos with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east thermal models indicate the presence of a colder slab beneath the southeastern aegean that leads to deepening of the intermediate depth seismicity slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone

bridging onshore and offshore present day kinematics of central and eastern mediterranean implications for crustal dynamics and mantle flow we present a new kinematic and strain model of an area encompassing the calabrian and hellenic subduction zones western anatolia and the balkans using haines and holt s 1993 method we derive continuous velocity and strain rate fields by interpolating geodetic velocities including recent gps data in the balkans relative motion between stable eurasia and the western aegean sea is gradually accommodated by distributed n s extension from southern balkans to the eastern corinth gulf so that the westward propagation of the north anatolian fault naf throughout continental greece or peloponnesus is not required we thus propose that the naf terminates in north aegean and that n s extension localized in the corinth gulf and distributed in southern balkans is due to the retreat of the hellenic slab the motion of the hyblean plateau apulia peninsula south adriatic sea ionian basin and sirte plain can be minimized by a single rigid rotation around a pole located in the sirte plain compatible with the opening the pelagian rifts 2 2 5 mm yr and seismotectonics in libya we interpret the trenchward ultraslow motion of the calabrian arc 2 2 5 mm yr as pure collapse the calabrian subduction being now inactive in the absolute plate motion reference frame our modeled velocity field depicts two toroidal crustal patterns located at both ends of the hellenic subduction zone clockwise in nw greece and counter clockwise in western anatolia we suggest the nw greece toroidal pattern is the surface expression of a slab tear and consequent toroidal asthenospheric flow

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

chlorine isotopes in fluid inclusions determination of the origins of salinity in magmatic fluids abstract thermal ionisation mass spectrometry tims of the cs 2 cl ion has been used to determine the 37 cl value of paleofluids trapped in fluid inclusions modifications to the existing published method have lowered the precision by a factor of 2 0 09 1 thereby allowing real differences to be measured with greater certainty the method has been applied to determine 37 cl of two sets of magmatic fluids trapped in fluid inclusions samples were studied from the capitan pluton in new mexico and the sw england batholith both of which have high temperature fluids whose d and 18 o signatures are distinctly magmatic relative to standard mean ocean chloride smoc the 37 cl values of the capitan samples cluster around 0 whereas those from sw england cluster around 1 8 the br cl ratios of the capitan fluid inclusions are low indicative of halite dissolution and are comparable to local high salinity waters derived from leaching permian evaporites the br cl ratios of the sw england fluid inclusions are much higher and within the range of values normally considered to be magmatic we conclude that the 37 cl value of the capitan magmatic fluids confirms the evidence from br cl ratios that cl was derived from an evaporite source whereas the 37 cl value of the sw england fluids appears to be more representative of a deep magmatic source

rapid spatiotemporal variations in rift structure during development of the corinth rift central greece the corinth rift central greece enables analysis of early rift development as it is young 5 ma and highly active and its full history is recorded at high resolution by sedimentary systems a complete compilation of marine geophysical data complemented by onshore data is used to develop a high resolution chronostratigraphy and detailed fault history for the offshore corinth rift integrating interpretations and reconciling previous discrepancies rift migration and localization of deformation have been significant within the rift since inception over the last circa 2 myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift but this transition did not occur synchronously along strike isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620 340 ka characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at 30 m kyr since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin demonstrated by lateral growth of discrete depocenters at 40 m kyr a single central depocenter formed by circa 130 ka indicating full fault linkage these results indicate that rift localization is progressive not instantaneous and can be synchronous once a rift border fault system is established this study illustrates that development processes within young rifts occur at 100 kyr timescales including rapid changes in rift symmetry and growth and linkage of major rift faults

earliest laterites and possible evidence for terrestrial vegetation in the early proterozoic paleosols preserve information about the composition of the atmosphere and paleoclimatic conditions here we report the discovery of the first pisolitic laterites of precambrian age the laterites are immediately above the regional unconformity at the base of the early proterozoic gamagara formation in paleokarst depressions on the campbellrand dolomite transvaal supergroup south africa the paleosol profiles are very well preserved and show zoning relict soil textures and chemical composition virtually identical to those of modern laterites the pisolitic laterites provide not only evidence for a highly oxygenated atmosphere and possible hot and humid climatic conditions in early proterozoic time but also indicate the presence of terrestrial life on the kaapvaal craton 2 0 2 2 b y ago

high grade iron ore at windarling yilgarn craton a product of syn orogenic deformation hypogene hydrothermal alteration and supergene modification in an archean bif basalt lithostratigraphy banded iron formation bif hosted iron ore deposits in the windarling range are located in the lower greenstone succession of the marda diemals greenstone belt southern cross domain yilgarn craton and constitute a total hematite martite goethite ore resource of minimum 52 mt at 60 wt fe 0 07 p banded iron formation is interlayered with high mg basalts at windarling and precipitated during episodes of volcanic quiescence trace element content and the rare earth element ree ratios y ho 42 to 45 sm yb 1 5 together with positive la and gd anomalies in least altered hematite magnetite metachert bif indicate the precipitation from archean seawater that was fertilised by hydrothermal vent fluids with a basaltic hree y signature hypogene iron ore in sub greenschist facies metamorphosed bif formed during three distinct stages ore stage 1 was a syn to post metamorphic syn d1 fe ca mg ni co p ree metasomatism that produced local ni ree rich fe dolomite magnetite alteration in bif hydrothermal alteration was induced by hot fluid flow controlled by brittle ductile reactivation of bif basalt margins and crosscutting d1 faults the ni co rich content of dolomite and a shift in ree ratios in carbonate altered bif towards archean mafic rock signature y ho to 31 to 40 sm yb to 1 to 2 and gd gd to 1 2 to 1 4 suggest that high mg basalts in the windarling range were the primary source of introduced metals during ore stage 2 a syn deformational and likely acidic and oxidised fluid flow along bif basalt margins and within d1 faults leached carbonate and precipitated lepidoblastic and anhedral granoblastic hematite high grade magnetite hematite ore is formed during this stage ore stage 3 hydrothermal specular hematite spch fe dolomite quartz alteration was controlled by a late orogenic brittle compressional transpressional stage d4 the regional scale shear zone related d3 is not preserved in windarling this minor event remobilised iron oxides carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore ore stage 4 involved mesozoic to recent supergene oxidation and hydration in a weathering environment reaching down to depths of 100 to maximum 200 m below surface supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation important ground preparation for supergene modification and upgrade were mainly the formation of steep d1 to d4 structures steep bif basalt margins and particularly the syn d1 to syn d2 carbonate alteration of bif that is most susceptible to supergene dissolution the windarling deposits are structurally controlled supergene modified hydrothermal iron ore systems that share comparable physical chemical and ore forming characteristics to other iron ore deposits in the yilgarn craton e g koolyanobbing beebyn in the weld range mt gibson however the remarkable variety in pre syn and post deformational ore textures relative to d1 and d2 has not been described elsewhere in the yilgarn and are similar to the ore deposits in high strain zones such as of brazil quadrilatero ferrifero or iron quadrangle and nigeria the overall similarity of alteration stages i e the sequence of hydrothermal carbonate introduction and hypogene leaching with other greenstone belt hosted iron ore deposits supports the interpretation that syn orogenic bif alteration and upgrade was crucial in the formation of hypogene supergene iron ore deposits in the yilgarn craton and possibly in other archean paleoproterozoic greenstone belt settings worldwide

iron and manganese deposits of the urucum district mato grosso do sul brazil the urucum district is located in the brazilian state of mato grosso do sul near the city of corumba on the brazilian bolivian border the district comprises an area of 200 km 2 where eight up to 1 000 m high tabular mountains rise out of the swampy lowlands of the pantanal the tabular mountains are partly built by an up to 350 m thick rock sequence of the santa cruz formation from bottom to top the santa cruz formation is composed of manganese ore horizon mn 1 an 80 m thick unit of ferruginous sandstones and manganese ore horizon mn 2 which is overlain by an up to 270 m thick monotonous sequence of hematite jaspilites forty meters above mn 2 is manganese ore horizon mn 3 another 40 m above mn 3 is manganese ore horizon mn 4 both are intercalated with the hematite jaspilites the iron ore reserves of the urucum district amount to approximately 36 billion tons the calculation of the manganese ore reserves in the four horizons results in 608 million tons comprised of 23 4 million tons of measured 34 5 million tons of indicated 465 1 million tons of inferred and 86 million tons of hypothetical and speculative reserves in the urucum district predominantly layered detritus rich and concretionary cryptomelane ores are found the ores consist mainly of micro to cryptocrystalline cryptomelane associated with hematite and quartz braunite pyrolusite and lithiophorite are minor constituents layered braunite ores mined are composed of braunite cryptomelane pyrolusite hematite and quartz the average mn contents of these four different types of manganese ores range from 25 63 percent in the detritus rich cryptomelane ores to 49 51 percent in the concretionary ones the fresh hematite jaspilitic iron ores contain about 50 percent fe whereas in the weathered and quartz depleted hematite jaspilites the iron content increases to 67 percent most of the iron and manganese ores were precipitated as chemical sediments in a partly ice covered fjordlike basin field observations rb sr isotope studies and leaching experiments show that large scale solution transport and reprecipitation processes led to supergene concentrations of iron and manganese in the underlying sandstones at the bottom of the santa cruz formation and in manganese ore horizon mn 1

initial crustal thickness geometry controls on the extension in a back arc domain case of the gulf of corinth 1 since 60 myr peloponnesus and continental greece have been affected by the hellenidean compressional and the aegean extensional phases this complex evolution resulted in development of a strongly inhomogeneous crust in the gulf of corinth region to study this area we use a large strain thermomechanical numerical code paravoz previously used for a number of similar problems such as rift evolution yet instead of varying boundary and initial conditions applied to a plane layered model we use available geophysical constraints on the actual deep structure of the lithosphere to test its different possible initial structures by varying the position of the initial crustal heterogeneity versus the position of the lithospheric slab we are able to explain the origin of the internal structures and the kinematics of the gulf of corinth the results suggest that the development of shear zones in the lower crust is favored by the gravitational collapse of the thicker part of the crust whereas the geometry and the kinematics of these shear zones are controlled by the position of the edge of the slab asymmetry is seen in cases when a horizontal shift exists between the edge of the slab and the thicker part of the crust our model explains the differences between the northern shore and the southern shore as well as the east west variations observed in the gulf of corinth

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite hosted iron ore deposits in the quadrilatero ferrifero minas gerais brazil the quadrilatero ferrifero brazil is presently the largest accumulation of single itabirite hosted iron ore bodies worldwide detailed petrography of selected hypogene high grade iron ore bodies at e g the aguas claras conceicao pau branco and pico deposits revealed different iron oxide generations from oldest to youngest magnetite martite hematite pseudomorph after magnetite granoblastic recrystallised microplaty fine grained 100 m hematite laser fluorination oxygen isotope analyses of selected iron ore species showed that the 18o composition of ore hosted martite ranges between 4 4 and 0 9 and is up to 11 depleted in 18o relative to hematite of the host itabirite during the modification of iron ore and the formation of new iron oxide generations e g microplaty and specular hematite an increase of up to 8 in 18o values is recorded calculated 18o values of hydrothermal fluids in equilibrium with the iron oxide species indicate 1 the involvement of isotopically light fluids e g meteoric water or brines during the upgrade from itabirite hosted hematite to high grade iron ore hosted martite and 2 a minor positive shift in 18ofluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated 18o values and or the infiltration of small volumes of isotopically heavy metamorphic and or magmatic fluids into the iron ore system the circulation of large fluid volumes that cause the systematic decrease of 18o 16o ratios from itabirite to high grade iron ore requires the presence of e g extensive faults and or large scale folds

a pre 2 2 ga age for giant hematite ores of the hamersley province australia one current model for the origin of giant hematite ores of the hamersley province requires a two step process formation of martite and goethite by supergene enrichment followed by burial metamorphism to convert goethite to microplaty hematite our new structural stratigraphic and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower wyloo group a shrimp u pb zircon age from volcaniclastic breccia within the lower wyloo group suggests that hematite detritus in that group was derived from hematite ores older than 2209 or 15 ma the action of pre 2 2 ga oxidizing fluids is implied although their origin supergene versus hypogene remains controversial

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

rifting through a stack of inhomogeneous thrusts the dipping pie concept 1 orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages while the thermal state of the region is reequilibrating strong lateral variations of the depth to the brittle ductile transition develop as a consequence of these preexisting heterogeneities we use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension the competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments the results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation the presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge in this case the migration of the basin is slow continuous and limited by gravity driven processes that lead to the rise of hot weak material under the subsiding basin in case of a high competence contrast overall crustal strength is reduced by a dipping pie effect assuming overall high crustal strength the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults crustal scale planar faults rooting on the brittle ductile transition of the crust and thin skinned listric faults rooting on the nappe itself this bimodality results in a jump like migration of the basin downward along the dipping weak nappe applying this model to the case of the gulf of corinth greece allows us to explain in the case of assumed high competence contrast the observed microseismicity patterns the asymmetry of the gulf and the kinematics of fault migration within the basin

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

site amplification assessment in the east corinth gulf using 3d finite difference modeling and local geophysical data elastic 3d wave field simulations were performed in the seismically active region of eastern gulf of corinth in the area of loutraki city a new methodology was tested with the aim of performing multiple simulations for a large variety of realistic sources located around the study area by employing 3d finite difference modeling using matrix operations for the calculation of the spatial velocity and stress derivatives the new methodology was proven to be quite efficient in simulating the near surface 3d site effect of the study area by greatly minimizing the simulation time thus allowing the use of 3d finite difference modeling for a large number of simulations the complex geological features of the study area were obtained by performing multiple passive masw surveys within the busy urban area of loutraki by processing the acquired geophysical data a highly inhomogeneous near surface velocity structure in the study area was obtained and was implemented in the 3d wave field simulations the near surface amplification that is caused by the 3d subsurface structure was proven to be highly significant for the area of loutraki with high spectral amplification compared to the amplification that is caused by an equivalent 1d model in the area the dominant frequencies of the spectral amplification for the 3d model were also confirmed by processing hvsr measurements that were also taken in the area finally we also investigated how the propagation direction affects the near surface amplification

veins and hydrothermal fluid flow in the mt whaleback iron ore district eastern hamersley province western australia abstract with the advent of recent hydrothermal models for genesis of giant microplaty hematite ore deposits of the hamersley province we have undertaken regional scale analysis of veins and rocks surrounding the mt whaleback deposit with an aim to characterize the nature of fluid flow before during and after ore formation at least five groups of veins formed in the brockman iron formation in deformational events associated with the paleoproterozoic interaction between the yilgarn and pilbara cratons in a 600 km 2 area around the mt whaleback microplaty hematite orebody the five main veining events most likely correspond to the collisional d 2 and extensional collapse d 3 phases of the ca 2300 2200 ma ophthalmian orogeny with one or two later phases probably associated with the waning stages of the ca 1700 1650 ma capricorn orogeny d 4 and or later proterozoic events the abundance of veins associated with d 2 contraction and d 3 extensional fabrics and the composition up to 15 wt nacl equivalent and pressure corrected homogenisation temperatures of primary fluid inclusions 200 400 c indicate that the temperature salinity and likely volume of fluid reached a peak during the last stages of the ophthalmian orogeny and the first stages of post ophthalmian extension paragenesis of veins relative to the deformation history and localization of highest vein densities in the dales gorge member provides circumstantial evidence that the peak of hydrothermal activity in the district was synchronous with the main phase of microplaty hematite ore formation at mt whaleback we propose that the veins record part of the fluid flow system related to the genesis of this giant ore deposit

u th he geochronology of goethite and the origin and evolution of cangas abstract u th he geochronology of 147 grains of goethite cements extracted from ferruginous duricrusts cangas developed on banded iron formations from the quadrilatero ferrifero region minas gerais brazil records a history of protracted mineral dissolution reprecipitation that started at ca 48 1 4 8 ma and continues intermittently until the present a large majority of the samples more than 30 are younger than 2 ma revealing active mineral dissolution reprecipitation in the recent past within cangas goethite cements are younger near the surface and become progressively older towards the bottom of the weathering profile indicating that iron is more effectively cycled in the parts of the weathering profile more strongly affected by biogenic activity u th he geochronology of 14 goethite grains from saprolites in the same profiles yield results ranging from 55 3 5 5 to 25 7 2 6 ma for a single weathering profile goethite cements from cangas are invariably younger than goethite grains from the underlying saprolite indicating that the duricrust and the saprolite behave as independent and separate systems responding to different environmental controls thorium shows conservative behaviour during goethite dissolution reprecipitation and it is enriched towards the surface of the weathering profile uranium on the other hand is preferentially leached from the surface into the saprolite or out of the weathering profile recurrent goethite dissolution reprecipitation lends great textural complexities to cangas but it is also responsible for its capacity to reheal when physically disrupted this self healing property accounts for canga s role in armoring banded iron formation landscapes

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

thrust or detachment exhumation processes in the aegean insight from a field study on ios cyclades greece 1 whether back arc extension in the aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back arc extension new field observations structural analysis of finite and instantaneous strain and their relationship with metamorphism on ios island cyclades lead us to investigate the significance of the south cycladic shear zone scsz two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the scsz first penetrative top to the south shearing is observed in the upper cycladic blueschists and the lower cycladic basement close to their contact it is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals contemporaneous with overthrusting of the cycladic blueschists over the basement unit second large extensional top to the north shear zones lately crosscut the whole stack these constraints allow us to reinterpret the kinematics of the aegean domain during synorogenic and postorogenic periods when replaced in the reconstructed geometry of the hellenic eocene orogenic wedge the observed top to the south thrusting is coeval with extensional shear zones observed in syros that could represent the roof of the cycladic blueschists an extrusion structure is then proposed for the exhumation of the cycladic blueschists in the subduction zone top to the north asymmetric extension together with the andros tinos mykonos and paros naxos detachments can be correlated to oligo miocene extension in the back arc domain

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

genesis of itabirite hosted au pd pt bearing hematite quartz veins quadrilatero ferrifero minas gerais brazil constraints from fluid inclusion infrared microthermometry bulk crush leach analysis and u pb systematics fluid inclusions hosted in quartz and specular hematite from auriferous jacutinga and barren veins in the quadrilatero ferrifero qf have been studied using conventional and near infrared microscopy respectively the mineralization consists of veins that cross cut metamorphosed iron formation itabirite of the paleoproterozoic itabira group the sample suite comprises hematite from veins from the low strain domain in the w and sw of the study area as well as hematite samples from the eastern high strain domain in the central and ne parts of the qf halogen ratios of fluid inclusions in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite that was subsequently diluted fluid inclusions hosted in quartz and hematite are characterized by consistent na k ratios and considerable so4 contents and suggest similar formation conditions and perhaps fluid origin from a common source na k and na li fluid mineral geothermometers indicate water rock interaction at approximately 340 40 c hematites from the high strain domain contain fluid inclusion assemblages of high temperature aqueous carbonic and multiphase high salinity high temperature aqueous inclusions probably due to fluid immiscibility in the system h2o nacl co2 fluid inclusions hosted in hematite from barren veins in the low strain domain as well as in hematite from jacutinga type mineralization from the central part of the qf only host multiphase aqueous fluid inclusions all showing narrow ranges of salinity 7 2 11 7 wt nacl equiv and homogenization temperatures 148 to 229 c lower homogenization temperatures and the absence of co2 rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and or co2 escape due to cooling during fluid migration from the high to the low strain domain pb pb and u pb systematics of gold hematite and hematite hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for pd au and pb the formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the brazilian orogenic cycle 600 700 ma the pb isotopic characteristics of all samples are readily reconciled in a simple model that involves two different paleoproterozoic or archean source lithologies for lead and reflects contrasting depths of fluid percolation during the brasiliano orogeny

oxygen isotope compositions of iron oxides from high grade bif hosted iron ore deposits of the central hamersley province western australia constraints on the evolution of hydrothermal fluids the hamersley province of northwest western australia is one of the world s premier iron ore regions with high grade martite microplaty hematite iron ore deposits mostly hosted within banded iron formation bif sequences of the brockman iron formations of the hamersley group these high grade iron ores contain between 60 and 68 wt percent fe and formed by the multistage interaction of hydrothermal fluids with the host bif formation the oxygen isotope compositions of magnetite and hematite from bif hydrothermal alteration assemblages and high grade iron ore were analyzed from the mount tom price paraburdoo and channar iron ore deposits the 18o values of magnetite and hematite from hydrothermal alteration assemblages and high grade iron ore range from 9 0 to 2 9 per mil a depletion of 5 to 15 per mil relative to the host bif the 18o values are spatially controlled by faults within the deposits a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids the oxygen isotope composition of hydrothermal fluids 18ofluid indicates that the decrease in the 18o content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original bif late stage talc bearing ore at the mount tom price deposit formed in the presence of a pulse of 18o enriched basinal brine indicating that hydrothermal fluids may have repeatedly interacted with the bifs during the paleoproterozoic

prolonged history of episodic fluid flow in giant hematite ore bodies evidence from in situ u pb geochronology of hydrothermal xenotime abstract absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the earth s crust yet many ore deposits remain undated because suitable mineral chronometers have not been found the origin of giant hematite ore deposits which are hosted in precambrian banded iron formations bifs remains contentious several models have been formulated based on different sources and timing for the mineralizing fluids supergene metamorphic syn orogenic late orogenic extensional collapse and syn extensional precise geochronology of the ore offers a means of discriminating between these models in this study two u pb chronometers xenotime and monazite have been identified in high grade hematite ore bodies from the mount tom price mine in the hamersley province northwestern australia both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite hematite pseudomorph after magnetite and microplaty hematite indicating that the xenotime and monazite precipitated during mineralization in situ u pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events our results suggest that ore genesis may have commenced as early as 2 15 ga with subsequent hydrothermal remobilization and or mineralization at 2 05 ga 1 84 ga 1 67 ga 1 59 ga 1 54 ga 1 48 ga and 0 85 ga the location of the ore bodies along ancient fault systems and the coincidence of at least some of the u pb phosphate dates with episodes of tectonothermal activity in the adjacent proterozoic capricorn orogen implies that fluids were channelled through major structures in the southern pilbara craton during discrete phases of tectonic compression and extension our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow in contrast to the aforementioned models our results imply that iron ore formation was probably a long lived multi stage process spanning more than one billion years

fluid flow in extensional environments numerical modelling with an application to hamersley iron ores the mechanical feasibility of focusing both surface and basinal derived fluids towards sites of iron ore genesis during proterozoic deformation in the hamersley province is tested here by computer simulation finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain the addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra hydrostatic pore pressures downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes the introduction of sedimentary layering into the models allows lateral fluid flow such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis the involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the mt tom price deposit and possibly other large deposits with time

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km