sequence of thrusting within a thick skinned tectonic wedge evidence from40ar 39ar and rb sr ages from the austroalpine nappe complex of the eastern alps the sequence of thrusting within a regional scale alpine thick skinned tectonic wedge has been constrained by rb sr and 40ar 39ar dating of whole rock constituent white mica and biotite from structural units of the austroalpine nappe complex eastern alps austria preservation of variscan and pre variscan isotopic signatures in pre alpine basement units indicates a non penetrative metamorphic alpine overprint alpine nappe assembly and accompanying ductile deformation occurred during maintenance of lower greenschist facies metamorphic conditions c 300 450 c analyses from greenschist facies metamorphic mylonites and penetratively ductile deformed cover sequences display decreasing 40ar 39ar plateau ages structurally downward thrusting and accompanied ductile deformation commenced at c 100 90 ma in the uppermost structural levels footwall fault propagation effected internal deformation and out of sequence thrusting within intermediate structural levels at c 95 80 ma this was contemporaneous wit

hydrogen isotope composition of early proterozoic seawater the hydrogen isotope composition of early proterozoic seawater has been indirectly determined on the basis of d values measured in mafic ultramafic samples from a hydrothermally altered layered flow from the chukotat group of the lower proterozoic 2 0 1 9 ga cape smith fold belt northern quebec this flow and other mafic ultramafic units of the fold belt represent some of the oldest relict oceanic crust identified on earth measured d values of whole rock samples range from 53 to 40 matching closely values found in modern metavolcanic rocks and phanerozoic ophiolites a model of water rock interaction for conditions of metamorphism deduced from mineral parageneses and sr and o isotope data t 200 250 c water rock ratio close to 1 0 indicates that the d values of chukotat flow samples reflect interaction with seawater that had a d 0 20 this result implies that the hydrogen isotope composition of early proterozoic seawater was certainly similar to the modern value and that subduction and ridge processes that control the global water cycle have not changed significantly since the early proterozoic

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

sedimentary tectonics and subsidence modelling of the type upper cretaceous gosau basin northern calcareous alps austria the upper cretaceous gosau group of the austrian northern calcareous alps records sedimentation and subsidence after early cretaceous thrusting the gosau basin in the gosau russbach area is approximately 8 km wide and 10 km long it is filled by more than 1000 m alluvial shallow marine deposits of the lower gosau subgroup upper turonian lower campanian facies and thickness changes within short horizontal distances suggest synsedimentary basin margin faulting extensional structures within the triassic basement of the basin include fibrous gashes which indicate w nw e se directed extension of 6 18 w nw directed normal faults at the southeastern basin margin a prominent se directed normal fault with a minimum fault throw of approximately 500 m at the nw basin margin and dextral strike slip at the southwestern margin suggest a pull apart origin of the basin between nw se trending right overstepping dextral strike slip faults backstripped tectonic subsidence curves yield values of total subsidence of 700 m in approximately 6 ma forward computer modelling based on extensional models results in stretching factors between 1 09 1 15 local extension along strike slip faults is suggested as the main mechanism for basin formation

δ2h of water from fluid inclusions in proterozoic halite evidence for a deuterium depleted hydrosphere abstract 2h were measured in water present in tiny primary fluid inclusions trapped in mesoproterozoic 1 4 ga and neoproterozoic 0 8 ga halite crystals while the hydrogen concentrations range from 300 to 1500 ppm for the whole sample collection 2h values range from 74 to 54 vsmow for the mesoproterozoic halite sampled from the sibley group ontario canada and from 89 to 38 for the neoproterozoic halite from the browne formation officer basin australia the amount of evaporation required to precipitate halite is accompanied by a deuterium enrichment of 30 to 90 it means that the original aqueous solutions had 2h values comprised between 140 and 90 considering the estimated tropical paleolatitudes for both depositional environments surface waters were most likely significantly deuterium depleted relative to modern ones if such interpretation is partly at variance with the geological record it could be reconciliated with some modelling outputs of the long term water cycle indeed several mechanisms produce a progressive deuterium enrichment of the oceans those identified so far are the addition of deuterium rich extraterrestrial water cometary dust the loss of h during water photodissociation by ultraviolet radiation in the upper atmosphere and the isotopic fractionation that takes place during the hydroxylation of nominally anhydrous mantle rock forming minerals we also propose that the major process able to shift through time the 2 of the hydrosphere is the reduction of h2o into h2 during the serpentinization of mantle rocks at mid ocean ridges

proterozoic diabase sills of northern ontario magnetic properties and history 1 magnetic fabrics show that a series of massive diabase sills logan sills possess flow fabrics away from the midcontinental proterozoic rift system near its triple junction on the north shore of lake superior their intense remanences are commonly antiparallel to the geomagnetic field requiring some knowledge of natural remanent magnetization in order to interpret of ground magnetic surveys thermal demagnetization low temperature demagnetization and the combined techniques yield differently oriented characteristic remanence vectors in some of these sills although the commonly reported keweenawan reversed paleofield direction is verified by most a components the orientation of vectors within sites and through the thickness of sills is large and may be attributed to reversals excursions and secular variation b components are mostly down but their orientation distribution is poorly concentrated low temperature demagnetization prior to thermal demagnetization tends to improve the consistency of vectors defined in subsequent thermal demagnetization moreover in some cases three to five cycles of low temperature demagnetization alone produces the same result as the more time consuming thermal demagnetization some within site scatter of the a components is large and some anomalous specimen directions and alternate polarities may be associated with platinum group mineralization whereas the grand mean direction for all sites lies on the available apparent polar wander path between 1100 and 1300 ma the mean paleopole locations differ at the 95 level for different demagnetization treatments of specimens from the same sites

atmospheric oxygenation caused by a change in volcanic degassing pressure around two and a half billion years ago following the end of the archaean eon the atmosphere turned from anoxic to weakly oxic in what is known as the great oxidation event using a model of volcanic degassing gaillard et al demonstrate that a preceding period of continental crust formation may have been the trigger they propose that as continents emerged and volcanoes became increasingly subaerial rather than submarine magmatic volatiles were degassed at lower pressures leading to a progressive oxidation of the gases released this shift to a release of sulphur as sulphur dioxide rather than as hydrogen sulphide could then have fed marine sulphate reduction and the eventual oxygenation of earth s atmosphere the precambrian history of our planet is marked by two major events a pulse of continental crust formation at the end of the archaean eon and a weak oxygenation of the atmosphere the great oxidation event that followed at 2 45 billion years ago this oxygenation has been linked to the emergence of oxygenic cyanobacteria1 2 and to changes in the compositions of volcanic gases3 4 but not to the composition of erupting lavas geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3 5 billion years ago5 6 here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation using thermodynamic calculations simulating gas melt equilibria in erupting magmas we suggest that mostly submarine archaean volcanoes produced gases with so2 h2s 1 and low sulphur content emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late archaean then led to widespread subaerial volcanism which in turn yielded gases much richer in sulphur and dominated by so2 dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

mid cretaceous primitive alkaline magmatism in the northern calcareous alps significance for austroalpine geodynamics basanitic dykes local name ehrwaldite occur over a distance of at least fifty kilometres in a narrow east west trending zone within the lechtal nappe of the northern calcareous alps the dykes crosscut mesozoic sedimentary strata up to the lower cretaceous the sedimentary rocks show contact metamorphism in the immediate vicinity of the dykes the ehrwaldites vary in mineral content from olivine fo 91 87 titanian augite basanites to basanites containing also kaersutite titanian biotite and zeolites minor phases are titanian magnetite picotite llmenite carbonate and sulfides mantle derived xenoliths and xenocrysts form up to five percent of the rock bulk chemical analyses show the typical characteristics of nepheline basanites ne 11 17 wt mg number 74 78 and demonstrate that the ehrwaldites are primitive mantle derived melts trace element contents of the dykes are in excellent agreement with those for basanites and olivine nephelinites from tertiary and quaternary european provinces the same is true for the isotope characteristics of the dykes with average initial esr and end values of 8 2 and 4 7 respectively these data demonstrate similarities in the mantle source rocks and in the melting characteristics with european alkaline rocks and thus similarities in the european subcontinental mantle at different localities for the last 100 ma these data are in clear contrast with the value for the penninic suboceanic mantle geochemical constraints and piston cylinder experiments indicate a depth of origin of 80 kilometres at temperatures of at least 1250 c for the ehrwaldites potassium argon bulk rock data of petrographically different samples of the dykes from different localities yield consistent ages around 100 ma upper albian after emplacement the ehrwaldites underwent conditions of the zone of diagenesis with temperatures around 120 c which the late magmatic zeolites survived

kinematics of the inntal shear zone sub tauern ramp fault system and the interpretation of the transalp seismic section eastern alps austria abstract a new interpretation of the inntal tauern sector of the transalp seismic section is presented one of the most prominent contrasts in reflectivity in the transalp seismic section is the contact between the bajuvaric unit in the footwall and the overlying tirolic unit and its basement across a moderately south dipping interface we trace this contact from the surface at the southern margin of the inn valley to a depth of 5 km there the contact is deformed or cut by the tauern window northern margin we define the contact between bajuvaric and tirolic units as brixlegg thrust which is older than miocene tauern window exhumation and has a paleogene age the sub tauern ramp connects with the inntal fault system at the surface and roots below the tauern window oblique thrust movements across this fault system in the miocene caused exhumation of the hanging wall where the fault has a ramp geometry which is in the area of the transalp cross section and west of it east of the transalp cross section the fault system merges with alpine basal thrust which is a flat no miocene exhumation occurred above the flat

orogenic structure of the eastern alps europe from transalp deep seismic reflection profiling abstract the transalp group comprising of partner institutions from italy austria and germany acquired data on a 340 km long deep seismic reflection line crossing the eastern alps between munich and venice although the field work was split into four campaigns between fall 1998 and summer 2001 the project gathered for the first time a continuous profile across the alps using consistent field acquisition and data processing parameters these sections span the orogen itself at its broadest width as well as the editor fred davey and the two adjacent basins vibroseis and explosion data complementary in their depth penetration and resolution characteristics were obtained along with wide angle and teleseismic data the profile shows a bi vergent asymmetric structure of the crust beneath the alpine axis which reaches a maximum thickness of 55 km and 80 100 km long transcrustal ramps the southward dipping sub tauern ramp and the northward dipping sub dolomites ramp strongly reflective patterns of these ramps can be traced towards the north to the inn valley and towards the south to the valsugana thrust belt both of which show enhanced seismicity in the brittle upper crust the seismic sections do not reveal any direct evidence for the presence of the periadriatic fault system the presumed equivalent to the insubric line in the western alps according to our new evolutionary model the sub tauern ramp is linked at depth with remnants of the subducted penninic ocean the crocodile type model describes an upper lower crustal decoupling and wedging of both the european and the adriatic african continents

cretaceous to neogene cooling and exhumation history of the oetztal stubai basement complex eastern alps a structural and fission track study the central austroalpine oetztal stubai basement complex osk of the eastern alps together with its overlying sedimentary brenner mesozoic bmz and tectonic units upper austroalpine nappes underwent extension and exhumation during the cretaceous before its incorporation into the hanging wall of the neogene brenner normal fault during this late cretaceous to early tertiary deformation the brenner mesozoic and the immediately underlying uppermost part of the osk developed a strong stretching lineation within a near horizontal and often mylonitic foliation related to consistent top to ese directed transport quartz and calcite behaved ductilely throughout the area whereas dolomite shows a transition from brittle fracture in the north to plastic flow in the south consistent with the known cretaceous metamorphic temperature gradient the transition to crystal plastic flow of dolomite occurs at 480 c quartz textures indicate an increase in the contribution of prism glide together with a decrease in basal glide toward the south also consistent with the temperature gradient in the osk and bmz fission track analysis of both zircon and apatite along a n s profile in combination with existing data from the literature allows a continuous cooling path to be constructed for the osk cooling of the eastern osk started at 90 ma the thermal peak of metamorphism and ended some 30 myr later at near surface temperatures at 60 ma the northern region had already cooled below the upper limit of the apatite partial annealing zone whereas in the south cooling related to this exhumation phase stopped at temperatures between 100 and 200 c the ese directed shearing is interpreted as late cretaceous to early tertiary normal faulting that exhumed the osk and brought the upper austroalpine blaser and steinacher nappes into contact with the underlying brenner mesozoic the pattern of alpine metamorphic isograds established in the osk directly reflects the geometry and kinematics of this late cretaceous early tertiary exhumation due to low angle normal faulting

critical taper analysis reveals lithological control of variations in detachment strength an analysis of the alpine basal detachment swiss alps although evidence for weak detachments underlying foreland thrust belts exists very little is known about the lateral variations in effective strength as well as the geological nature of such variations using critical taper analysis we show that a detailed and systematic measurement of surface slope of the central european alps reveals variations in strength parameter f along the detachment based on the argument that the alps are close to the critical state we show that the basal detachment is very weak near the deformation front but strengthens toward the hinterland very low f effective coefficient of friction plus normalized cohesion values of 0 54 but may also require additional mechanisms of dynamic weakening

new evidence for block and thrust sheet rotations in the central northern calcareous alps deduced from two pervasive remagnetization events 1 we present 81 paleomagnetic sites early triassic to early cretaceous from the central sector of the northern calcareous alps nca eastern alps austria and germany stepwise thermal demagnetization defines three magnetic directions mostly carried by low unblocking temperature and low coercivity minerals j3 350 j2 500 j1 575 and 680 j3 and j2 show positive inclinations whereas j1 very seldom is of dual polarity the fold tests show that a j3 can be interpreted as a postfolding and posttilting remagnetization and j2 as a postfolding and pre syn tilting j1 can be considered as primary because of the occurrence of two polarities and evidence presented by other authors in the area all three components show a systematic and significant clockwise rotation after comparing with the expected european references j2 or j1 are marked by higher rotation values than j3 j2 shows different inclinations depending on the structural position north or southward dips considering the structural evolution and the observed inclinations the first postfolding and pretilting remagnetization event j2 could have taken place between late cretaceous and eocene times but certainly before the thrusting of the nca over the rhenodanubian flysch and northward tilting caused by the stacking of the lower austroalpine nappes the second postfolding and posttilting remagnetization j3 would have been acquired after the final thrusting of the nca over penninic units from then the nca behaved as a set of rigid blocks recording the main stage of vertical axis clockwise rotation 65 in average associated with the continental collision the variable degree of rotation in the different positions from 40 to 134 can be explained by individual vertical axis rotation in a block system trying to adjust to space problems the constant declination differences between j2 and j3 25 in average would reflect the rotation related with the lateral differences of shortening during the oblique thrust of the austroalpine units over the penninic units

triple oxygen and hydrogen isotopic study of hydrothermally altered rocks from the 2 43 2 41 ga vetreny belt russia an insight into the early paleoproterozoic seawater abstract the early paleoproterozoic represents a period of rapid changes in earth systems that could have affected the stable isotopic composition of seawater the well preserved pillow structures hyaloclastites and komatiitic basalts of the 2 43 2 41 ga vetreny belt baltic shield provide a record of high temperature water rock interaction induced by contemporaneous seawater here we present results of mineralogical fluid inclusion hydrogen and triple oxygen isotopic analysis of hydrothermal alteration products emphasis is given to vein filling quartz and epidote as they likely formed at high water rock ratios ten minerals pairs quartz epidote and quartz calcite returned temperatures of isotopic equilibrium between 286 and 387 c which compares well to the homogenization temperatures measured for saline fluid inclusions hosted in vein quartz the computed d and 18o values of equilibrium fluids range between 31 and 12 and 1 36 and 3 20 respectively which overlap with the isotopic composition of ice free world seawater and fluids generated at submarine hydrothermal systems this is the earliest piece of evidence suggesting that early paleoproterozoic seawater had a d value close to 0 we also present triple oxygen isotopic composition of quartz and epidote that formed in similar facies of hydrothermal alteration from the relatively young 6 7 ma oceanic crust as sampled by the odp hole 504b in the eastern pacific ocean these data show similarity to the triple oxygen isotope analyses of the vetreny belt rocks indicating that the 2 43 2 41 ga seawater had the 17o value close to that of modern day seawater due to small fractionation at 300 390 c epidote water 1 epidotes present a strong evidence that equilibrium fluids had 17o values close to 0 based on the previously published quartz water calibration the computed 17o values of equilibrium fluids range between 0 11 and 0 03 significantly lower than that of seawater or inferred seawater derived fluids at low water rock ratios this can be explained by multiple factors including phase separation of fluids or and presence of low temperature quartz overgrowths both are reflected in the fluid inclusion data and in situ 18o measurements by ion microprobe sims presented here overall our study suggests that the 18o 17o and d values of the 2 43 2 41 ga seawater were 1 7 1 1 0 001 0 011 and 0 20 respectively similar to the modern values which reflects the dominant role of submarine hydrothermal alteration in the stable isotopic budget of seawater throughout earth s history

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

geochemistry and origin of mafic rocks from the pelona orocopia and rand schists southern california abstract metabasites from the pelona orocopia rand por schist a presumed subduction complex in southern california have been analyzed for major minor and trace element concentrations to investigate the nature origin and tectonic implications of these rock bodies element abundances indicate that these rocks are tholeiitic and alkalic basalts generated from several sources most samples group 1 resemble normal to transitional mid ocean ridge basalts n type to t type morb variation of morb like samples can be accounted for by low pressure crystal fractionation a second group group 2 which includes five samples from the rand mountains and one from the san gabriel mountains has elemental abundances similar to those of tholeiitic ocean island basalts or e type morb although phosphorus is unusually low several samples of group 2 have high mgo ni and cr characteristics which are commonly associated with crystal accumulation a final group group 3 consists of two samples from the orocopia mountains that resemble alkalic ocean island basalts the analyzed samples were collected from six different bodies of por schist there seem to be no major differences in basalt type which would imply a unique and separate source for any one area various sites of accumulation have been suggested for the protoliths of the por schist open ocean gulf of california type basin trapped back arc basin and rifted back arc basin the first three settings are entirely consistent with the basalt geochemistry the last option is less likely but not impossible

growing folds and sedimentation of the gosau group muttekopf northern calcareous alps austria analysis of the three dimensional geometry of upper cretaceous clastics in the muttekopf area northern calcareous alps austria indicate fold and fault structures active during deposition coniacian continental to neritic sedimentation lower gosau subgroup was contemporaneous with displacements on nw trending faults and minor folding along ne trending axes from the santonian onwards sedimentation of the deep marine upper gosau subgroup the nw trending faults were sealed and large folds with wsw trending axes developed the direction of contraction changed to n s after the end of gosau deposition in the danian paleocene synorogenic sedimentation patterns indicate continuous contraction from the coniacian to the late maastrichtian danian therefore large scale extension as observed in the central part of the eastern alps cannot be documented in the western parts of the northern calcareous alps a combination of subduction tectonic erosion for the frontal parts and gravitational adjustment of an unstable orogen after nappe stacking for the internal parts possibly accounts for the different development of gosau basins in the frontal and trailing regions of the austroalpine wedge

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

isotope composition and volume of earth s early oceans oxygen and hydrogen isotope compositions of earth s seawater are controlled by volatile fluxes among mantle lithospheric oceanic and continental crust and atmospheric reservoirs throughout geologic time the oxygen mass budget was likely conserved within these earth system reservoirs but hydrogen s was not as it can escape to space isotopic properties of serpentine from the approximately 3 8 ga isua supracrustal belt in west greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater archaean oceans were depleted in deuterium expressed as d relative to vienna standard mean ocean water vsmow by at most 25 5 but oxygen isotope ratios were comparable to modern oceans mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in earth s oceans our calculations predict that the oceans of early earth were up to 26 more voluminous and atmospheric ch4 and co2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on archaean earth

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

a 400 km long piggyback basin upper aptian lower cenomanian in the eastern alps piggyback basins ori and friend 1984 comprise sedimentary basins that formed on active thrust sheets based on basin geometry and depositional systems two end member types of piggyback or thrust top basins have been distinguished type 1 basins are deep water basins that occur mainly as elongated synsedimentary deforming troughs in the footwall of thrusts in the early stages of orogenic wedge development and under lled foreland basins e g ori and friend 1984 lash 1990 type 2 piggyback basins are terrestrial to shallow marine intramontane basins that are formed predominantly during late orogenic stages e g ori and friend 1984 nijman 1998 this paper describes the geometry and evolution of an early deepmarine piggyback trough that formed on the cretaceous orogenic wedge of the eastern alps and the western carpathians the narrow basin extended over a considerable horizontal distance as suggested by similarities in stratigraphy and the timing of deformation the basin characterized a signi cant stage in the cretaceous evolution of the austroalpine northern boundary

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

low temperature demagnetization isolates stable magnetic vector components in magnetite bearing diabase summary it may be difficult to isolate stable palaeomagnetic vectors of different ages if they lie in grain assemblages with overlapping ranges of coercivity or of unblocking temperature this is because some moments associated with either vector may demagnetize at the same stage of experimental demagnetization the sharp transition between vector components may be obscured and stable components may appear less linear on the demagnetization plot thermal and alternating field af demagnetization techniques remove vector components on a quantitative basis according to a discrete limiting unblocking temperature or coercivity low temperature demagnetization ltd differs in that it removes vector components discretely wherever there are mobile domain walls experiments tested the ability of ltd to improve the effectiveness of af demagnetization on isothermal and anhysteretic remanent magnetizations irm arm in diabase multicomponent nrms were simulated irms or arms in different non overlapping coercivity ranges along three orthogonal axes or along two non orthogonal directions the known directions of the experimentally applied vector components were always more successfully verified by af demagnetization if ltd was first applied for the same specimens ltd reduced the same artificial remanences by 50 per cent for the coercivity range 0 15 mt by 25 per cent for the range 15 30 mt and negligibly for higher coercivity fractions ltd demagnetizes polydomain magnetite as domain walls rearrange on passing through a low temperature transition near 120 k

crustal contamination identified in keweenawan osler group tholeiites ontario a trace element perspective in the lake superior area voluminous middle proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the midcontinent rift and overlie archean and older proterozoic basement the 3000 m thick section of gently dipping 10 sse basalts on the black bay peninsula is divided into three suites based on the relationship between stratigraphy petrography and geochemical variations e g mg number a1 2 o 3 la sm and gd yb the lower suite consists of augite olivine porphyritic lavas with high mg number sio 2 tio 2 and gd yb but low la sm the overlying central suite flows are either aphyric or plagioclase porphyritic and have lower mg number sio 2 la sm and gd yb decline upward through the central suite the upper suite lavas are either aphyric or plagioclase porphyritic and element ratios plot on slightly different trends compared to the central suite the lower suite has a narrow compositional range the average of the four most primitive lavas is used

proterozoic and archean geology of the nipigon embayment implications for emplacement of the mesoproterozoic nipigon diabase sills and mafic to ultramafic intrusions the nipigon embayment is underlain by archean rocks of the english river wabigoon and quetico subprovinces and intruded along the west side by late to post tectonic mafic to ultramafic intrusions the early mesoproterozoic ultramafic to felsic badwater intrusion and felsic english bay complex are located in the northwest corner of the nipigon embayment three mafic to ultramafic intrusions the disraeli seagull and hele intrusions are located south of lake nipigon and the kitto intrusion is located east of the lake a number of mafic to ultramafic bodies jackfish island shillabeer kama hill nipigon bay have only limited outcrops the gabbroic nipigon diabase sills intrude all other rocks in the nipigon embayment and generally have a consistent mineralogy and geochemistry except for the inspiration sill s and the mcintyre sill geological and geophysical data suggest emplacement of the ultramafic intrusions by mechanisms similar to those controlling emplacement of the saucer shaped diabase sills these mechanisms are partially dependent on a series of pre existing north northwest and northeast trending faults formed prior to keweenawan magmatism the presence of sills rather than dykes indicates that the nipigon embayment was not extensional during the keweenawan midcontinent rift suggesting that the nipigon embayment is not a classic failed arm

oxygen and hydrogen isotope evidence for a temperate climate 3 42 billion years ago the widely held view that the archaean climate around 3 5 billion years ago was remarkably warm with ocean temperatures perhaps as high as 80 c has been questioned on the grounds that the established method of estimating ancient ocean temperature from the oxygen isotope ratios of sedimentary deposits is subject to significant uncertainty michael hren et al have adopted a different approach to estimating ocean temperature based on an analysis of both oxygen and hydrogen isotopes of 3 4 billion year old buck reef chert sediments from south africa the isotopes sampled are consistent with formation in waters no warmer than about 40 c suggesting that earth s early oceans may have been far cooler than previously thought the study of stable oxygen isotope ratios 18o of precambrian cherts suggests that ocean temperatures during the archaean era about 3 5 billion years ago were between 55 c and 85 c but uncertainty about the 18o of the primitive ocean has led to considerable debate regarding this conclusion here a combined analysis of oxygen and hydrogen istopes sampled from 3 42 billion year old buck reef chert rocks in south africa indicates that the ancient ocean was much cooler than previously thought stable oxygen isotope ratios 18o of precambrian cherts have been used to establish much of our understanding of the early climate history of earth1 2 3 and suggest that ocean temperatures during the archaean era 3 5 billion years ago were between 55 c and 85 c ref 2 but because of uncertainty in the 18o of the primitive ocean there is considerable debate regarding this conclusion examination of modern and ancient cherts indicates that another approach using a combined analysis of 18o and hydrogen isotopes d rather than 18o alone can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4 5 here we show that 18o and d sampled from 3 42 billion year old buck reef chert rocks in south africa are consistent with formation from waters at varied low temperatures the most 18o enriched buck reef chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below 40 c geochemical and sedimentary evidence suggests that the buck reef chert was formed in shallow to deep marine conditions so our results indicate that the palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler 40 c than previously thought2

kinematics of the alpine plate margin structural styles strain and motion along the penninic austroalpine boundary in the swiss austrian alps a structural and kinematic analysis of parts of the alpine suture zone i e the arosa zone and the edges of the overlying austroalpine and underlying penninic units is presented the arosa zone is interpreted as part of an accretionary wedge formed by the overriding of the austroalpine units the northern part of the adriatic plate over the penninic units structures strain and stretching trajectories indicate that two main deformation events followed each other discontinuously the earlier event was a wsw to nw directed thrusting under noncoaxial deformation and led to melange type structures the second event less penetrative in the arosa zone and with a downward increase in strain and noncoaxial flow formed n to ne verging folds and imbrication zones west directed displacement commenced in the early cretaceous and the motion changed to a prevailing northerly direction during the eocene as revealed by sediments involved in thrusting and by radiometric dating of the mylonites the kinematic history deduced from structural analysis agrees with the history obtained by recent work on plate margin sediments we correlate the anticlockwise rotation of the adriatic plate relative to europe between about 130 and 60 ma with the west directed motion in the arosa zone a change to relative n s motion during the lower tertiary led to the n to ne directed displacements a direct relationship thus exists between the motion of the adriatic plate and the resulting deformation along its northern margin during the early history of the eastern alps partitioning in brittle and ductile deformation and structural slicing under noncoaxial deformation is responsible for the melange type structures

deep structure and kinematics of the northern calcareous alps transalp profile the northern calcareous alps nca of southern bavaria and northern tyrol constitute a carbonate dominated polyphase fold thrust wedge together with its grauwacken zone basement it is the northernmost part of the far travelled upper austroalpine thrust complex of the eastern alps the present geometry developed in several kinematic stages jurassic extensional faults that affected large parts of the nca and their basement originated when the main part of the nca was still located southeast of the central alpine otztal silvretta complex these faults and related facies transitions influenced the later style of detachment of the nca thrust sheets mid to late cretaceous detachment thrust sheet stacking and motion over the central alpine complex are registered in clastic deposits of syntectonic basins the latest cretaceous to cenozoic nne to n directed motion of the nca towards europe in front of the central alpine complex created another set of significant contraction structures which at depth overprinted all previous structures during cretaceous to cenozoic deformation the nca experienced about 80 km of shortening i e about 73 along the transalp profile the european basement and autochthonous mesozoic cover beneath the allochthonous nca thrust sheets and flysch complexes seem to have remained relatively undeformed

transpressional collision structures in the upper crust the fold thrust belt of the northern calcareous alps abstract two major structural events characterize the tectonic evolution of the northern calcareous alps nca 1 late early cretaceous to late eocene nw directed dextral transpressional stacking of nappes as an expression of the formation of the austroalpine orogenic wedge and 2 miocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central eastern alps three first order detachment horizons are defined by differences of competence within areas of facies transition preexisting normal faults control the thrust architecture transpressional contraction in the nca is indicated by a nw directed thrusting oblique to both the long axis of the nca and the edge of the orogenic foreland b the occurrence of en echelon arrays of thrusts and folds laterally displaced by dextral strike slip faults the thrusts and strike slip faults are kinematically connected to each other and dissect the nca into rhomboidal blocks c ne directed extension parallel to fold and ramp axes and the internal strike of the nca and d clockwise rotation 30 of the entire nca around a vertical axis kinematic and dynamic analysis of mesoscale fault striae data related to transpressional stacking indicates a nw trend of contraction directions and in general a ne trend of the extension directions parallel to fold axes and branch lines of thrusts before rotation the average contraction direction was probably parallel to the generally w directed shear direction recorded in the crystal plastically deformed central part of the eastern alps three balanced cross sections across the nca yield a minimum of 54 65 total shortening

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

present day lithospheric strength of the eastern alps and its relationship to neotectonics 1 we calculate the present day lithospheric strength of the eastern alps along the new reflection seismic profile transalp to examine vertical and lateral strength variations and their implications on neotectonic activity of the eastern alps the large scale geometry of the eastern alps and the spatial distribution of upper and lower crustal layers and the lithospheric mantle is constrained by the deep seismic line two rheological models coupled to a kinematic thermal model that accounts for the thermal evolution of the eastern alps for the last 30 myr are investigated for the present day lithospheric configuration in the eastern alps models with strong model a and weak model b crustal rheologies predict the european and the adriatic plates to be stronger than the central zone of the orogen comprising the region between the inntal fault and the periadriatic fault model a is characterized by a brittle ductile boundary between 14 and 9 km depth and strong coupling of the mechanically strong lower crust to the upper mantle whereas model b suggests the presence of a thick decoupling zone between the upper crust and the upper mantle and a shallower brittle ductile boundary 7 10 km of these end member scenarios model a is in better agreement with neotectonic data including seismicity down to the upper lower crust boundary within the adriatic plate uplift of the central zone of the eastern alps and the southern alps and eastward escape of fault bound blocks such deformation pattern is best explained by lateral extrusion upon north south compression supporting a strong weak strong configuration of tectonic units along the transalp line

p t t history of the lower austroalpine nappe complex in the tarntaler berge nw of the tauern window implications for the geotectonic evolution of the central eastern alps new petrologic and 40ar 39ar geochronologic data constrain conditions of alpine metamorphism along the northwestern border of the tauern window the p t estimations based on phengite barometry were determined for samples from units of the lower austroalpine nappe complex exposed above the southpenninic interior of the tauern window and from upper parts of the southpenninic bundner schiefer sequence results suggest that both mesozoic metasedimentary nappe units reckner and hippold nappes and an ophiolitic nappe reckner complex of the lower austroalpine nappe complex have been metamorphosed at pressures between 8 and 10 5 kbar and temperatures around 350 c the structurally highest lower austroalpine unit quartzphyllite nappe was not affected by high pressure metamorphism and records maximum p t conditions of approximately 4 kbar and 400 c highest parts of the structurally underlying southpenninic bundner schiefer sequence were metamorphosed at intermediate pressures 6 7 kbar temperatures increased in all structural units during decompression whole rock 40ar 39ar plateau ages of silicic phyllites and cherts with abundant high si phengites record ages around 50 ma in the reckner nappe and 44 37 ma in the hippold nappe and southpenninic bundner schiefer sequence these ages are interpreted to date closely the high pressure metamorphism

proterozoic paleomagnetism in the nipigon embayment of northern ontario pillar lake lava waweig troctolite and gunflint formation tuffs abstract by using unusual combinations of demagnetization techniques proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post tectonic proterozoic units near armstrong northern ontario and also for well dated gunflint formation which by previous techniques yielded problematical paleomagnetic data the first paleomagnetic data are provided also for the seagull pluton and inspiration sills characteristic remanent magnetizations chrm for the pillar lake lavas indicate a keweenawan age more specifically 1000 1040 ma by comparison with the well established apwp for the late proterozoic superior craton four combinations of demagnetization techniques yield declinations in the range 108 133 and inclinations in the range 65 to 70 n 100 which define paleopoles near 200 w 48 n corresponding to a location on the keweenawan apwp near 1040 ma in the underlying basement a recently discovered proterozoic igneous complex the waweig troctolitic complex yields new paleomagnetic data with declination and inclination 42 54 n 14 defining a paleopole at 238 w 09 n its ages may be 1400 1600 or 2000 ma by comparison with the presently available ambiguous and sparsely populated apwp the first paleomagnetic results for the seagull pluton u pb age 1113 ma yield a mean declination of 87 4 75 7 n 32 corresponding to a keweenawan paleopole near 233 42 n consistent with other paleopoles near 1200 ma tuffs of the oft studied but problematical gunflint formation u pb age1878 ma yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens their mean primary declination and inclination 303 48 8 n 17 yields a paleopole now located near 178 w 42 n comparable with the published locations of paleopoles of 2000 ma of broader interest we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors combining the conventional techniques thermal and af demagnetization also improved the resolution of characteristic vectors not achieved by other means low grade metamorphism affected the non tectonized proterozoic cover to the canadian shield due to burial or hydrothermal effects obfuscating or erasing primary vectors in some lithologies and especially at certain sites

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

structural synthesis of the northern calcareous alps transalp segment the northern calcareous alps nca are the site of very large top to north convergent movements during cretaceous tertiary alpine mountain building to determine the amount of shortening the depth of detachment and the style of deformation we retro deformed an approximately 40 40 km area comprising the lechtal and allgau nappes on the basis of all available geological data and processed sections of the transalp reflection seismic experiment coherent 3d models were constructed by splining lines from n s cross sections integration of 3d kinematic modeling and field data shows the following the structure of the lechtal nappe is controlled by the triassic hauptdolomit four main thrusts link to a detachment at 2 6 km depth below sea level shortening estimates vary from 25 east to 42 west additional contraction is accommodated by folding in the east the subjacent allgau nappe can be traced about 10 km down plunge and is shortened by about one third in the western part the downplunge width is at least 15 20 km with restorable shortening of one third the triple inntal lechtal allgau nappes nca nappe system was moved uniformly n s to produce laterally heterogeneus shortening of 40 90 km or 50 67 we suggest that the nca are underlain by substantial amounts of buried molasse sediments and or overthrust units of helvetic and rheno danubian flysch indicating post eocene n s shortening of at least 55 km restored to an initial configuration the basin topography of the nca reveals strong e w thickness variations of the triassic wettersteinkalk and hauptdolomit platform carbonates such variations may pertain to n s trending growth faults which were important precursors to later jurassic extension of the austroalpine passive margin such structures are unlikely to be seen in the conventional n s cross sections but form an essential geometrical and mechanical element in later convergent mountain building

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

paleomagnetism and paleogeography of the northern calcareous alps austria prefolding magnetization components have been resolved at 38 sites of liassic adnet limestones in the central northern calcareous alps nca and at one site in the drauzug the site mean directions are northeasterly and down and are closer to coeval european directions than to southern alpine african directions the western end of both the nca and of the drauzug yield different site mean directions and negative fold tests indicating remagnetization the sampling localities have been rotated relative to one another during thrusting however comparison of paleomagnetic poles with the european apparent polar wander path implies an average clockwise rotation of about 30 for the nca relative to europe larger scale clockwise rotations in excess of 80 relative to the southern alps are implied by comparison with the southern alpine african polar wander path facies analysis of jurassic and lower cretaceous basinal sediments in the central part of the nca suggests that liassic subsidence was related to presently e w sinistral strike slip faults whereas malm berriasian subsidence was controlled by presently e w south dipping normal faults both paleomagnetic and facies data tend to indicate that the nca did not undergo the african rotation seen in southern alpine paleomagnetic data this implies the existence of a short lived tract of triassic jurassic ocean floor between the nca and the southern alps which linked the pindos ocean to the ligurian trough and allowed adria to rotate independently from the nca during the late jurassic eo alpine deformation and metamorphism are associated with the early cretaceous closure of this hallstatt trough

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

a natural and controlled source seismic profile through the eastern alps transalp abstract the combined passive and active seismic transalp experiment produced an unprecedented high resolution crustal image of the eastern alps between munich and venice the european and adriatic mohos em and am respectively are clearly imaged with different seismic techniques near vertical incidence reflections and receiver functions rfs the european moho dips gently southward from 35 km beneath the northern foreland to a maximum depth of 55 km beneath the central part of the eastern alps whereas the adriatic moho is imaged primarily by receiver functions at a relatively constant depth of about 40 km in both data sets we have also detected first order alpine shear zones such as the helvetic detachment inntal fault and sub tauern ramp in the north apart from the valsugana thrust receiver functions in the southern part of the eastern alps have also observed a north dipping interface which may penetrate the entire adriatic crust adriatic crust interface aci deep crustal seismicity may be related to the aci we interpret the aci as the currently active retroshear zone in the doubly vergent alpine collisional belt

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

earth s oldest reported glaciation physical and chemical evidence from the archean mozaan group 2 9 ga of south africa the pongola supergroup is a thick succession of archean 2 9 ga supracrustal rocks in the eastern part of the kaapvaal craton of southern africa its upper part the mozaan group 5000 m thick includes diamictites with a highly varied suite of clasts some of which are striated and faceted clasts in associated stratified siltstones are interpreted as ice rafted debris a glacial origin is also supported by geochemical investigation of diamictite matrix materials and associated mudstones plots of sio2 al2o3 and tio2 al2o3 suggest that these rocks underwent little chemical weathering an interpretation confirmed by calculation of a chemical index of alteration with low values typical of other glacial deposits the analyzed rocks are rich in fe average fe2o3 t is 24 wt like rocks of many other glaciomarine successions the diamictites and associated mudstones of the mozaan group have low th sc ratios suggesting preferential incorporation of mafic materials into the diamictite matrix material

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

provenance of cretaceous synorogenic sandstones in the eastern alps constraints from framework petrography heavy mineral analysis and mineral chemistry abstract the detrital components of cretaceous sedimentary rocks in the northern calcareous alps reflect the early alpine geodynamic evolution of the austroalpine microplate two contrasting source areas are distinguished on the base of light and heavy mineral analysis the first source area is located at the southeastern margin of the austroalpine and is composed of palaeozoic sediments and metamorphic rocks mesozoic carbonate rocks and ultrabasic rocks derived from the suture zone of the vardar meliata ocean the second source area is located in the northwest in a lower austroalpine position near the transpressive plate margin that juxtaposes austroalpine and penninic units this source area comprises palaeozoic low grade metamorphic rocks including high pressure hp rocks late palaeozoic meta sediments mesozoic carbonate rocks and ultrabasic rocks from obducted slices of penninic oceanic crust chemical analyses of detrital white mica amphibole and garnet support the discrimination between the two source areas tourmaline chemistry calls for a significant amount of metasedimentary rocks in the source area granitoid rocks and high grade metamorphic rocks are rare blue amphibole phengite and chloritoid composition suggest the erosion of lower blueschist facies rocks in the northwestern source area we suggest a modified model of austroalpine valanginian to coniacian tectono sedimentary evolution which is based on 1 an onset of subduction of the penninic ocean no earlier than late cretaceous 2 deposition of the analyzed sedimentary rocks in piggyback basins and 3 a reconstruction of provenance as proposed in this study

subduction and atmospheric escape of earth s seawater constrained by hydrogen isotopes abstract the hydrogen isotopic d h ratio reflects the global cycling and evolution of water on earth as it fractionates through planetary processes we model the water cycle taking seafloor hydrothermal alteration chemical alteration of continental crust slab subduction hydrogen escape from the early earth and degassing at mid ocean ridges hot spots and arcs into account the differences in d h ratios between present day oceans oceanic and continental crust and mantle are thought to reflect isotopic fractionation through seafloor alteration chemical alteration and slab dehydration however if the speed of plate tectonics has been nearly constant throughout earth s history the degassing and regassing rates are too small to reach the present day d h ratios we show that a hydrogen escape from reduced early atmosphere b secular net regassing or c faster plate tectonics on early earth is needed to reproduce the present day d h ratios of the water reservoirs the low d h ratio of archean seawater at 3 8 ga has previously been interpreted as a signature of a hydrogen escape but we find it can also be explained either by b secular net degassing or by c faster plate tectonics on early earth the rates of hydrogen escape from early earth and secular regassing on present day earth are constrained to be lower than 2 1 10 11 kg yr and 3 9 10 11 kg yr consequently the volume of water in the present day mantle could result entirely from the regassing through earth s history in that case the volume of initial oceans could be 2 to 3 times larger than that of current earth we suggest that in addition to the d h ratio of archean seawater identifying the d h ratios of both seawater and mantle throughout earth s history would allow to distinguish these evolutionary scenarios

petrology of middle proterozoic diabases and picrites from lake nipigon canada mafic rocks at lake nipigon provide a record of rift related continental basaltic magmatism during the keweenawan event at 1109 ma the mafic rocks consist of an early volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes sheets and sills the diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units compositional variation in the sills indicates that they crystallized from several magma pulses

radiogenic isotope characteristics of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario the ca 1110 1120 ma diabase sills of the nipigon embayment represent the oldest phase of the midcontinent rift recognized to date the sills can be subdivided into five geochemically distinct suites nipigon mcintyre inspiration shillabeer and jackfish sm nd rb sr and pb pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills the endt values for all the sill suites are consis tently negative 0 5 to 6 6 but show coherent variations both within and between suites the negative endt values can be interpreted as the result of contamination of a plume related mantle source by older crustal material the initial strontium sri values 0 7032 0 7068 for the sills indicate that at least two distinct contaminants are required 1 a source with strongly negative endt and lower sri likely archean metasediment or granite and 2 one with elevated sri likely sedimentary rocks of the proterozoic sibley group the pb data for the nipigon and inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the midcontinent rift the radiogenic isotopes preserve a complex contamination history best interpreted as the result of 5 contamination by a variety of archean and proterozoic sources the ultramafic jackfish sill shows the least ev idence of contamination and is interpreted to have been emplaced along a crustal scale fault with little interaction with crustal material in contrast the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

pre oceanic rifting in the alps abstract a review of mesozoic mainly earliest jurassic to mid jurassic rifting in the western and central alps normal faults with throws up to a few kilometers are moderately inclined and involve rotation of crustal blocks the amount of stretching and thinning of the crust by jurassic rifting is discussed the rifting 200 165 m y pre dates the opening of the alpine oceans by spreading and transform movements 165 110 m y

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

relating orogen width to shortening erosion and exhumation during alpine collision we investigate along strike width changes of the thickened accreted lower plate talp in the central and in the eastern alps we set the width of the talp in relation to the inferred amount of collisional shortening and exhumation along six orogen scale cross sections taking the present day along strike gradients in the amount of collisional shortening to represent the temporal evolution of the collisional wedge it may be concluded that the cross sectional area of the talp diminishes during ongoing shortening indicating that the erosional flux outpaced the accretionary flux higher amounts of collisional shortening systematically coincide with smaller widths of the talp and dramatic increases of the reconstructed eroded rock column higher amounts of shortening also coincide with larger amplitudes of orogen scale upright folds with higher exhumation and with higher exhumation rates hence erosion did play a major role in reducing by 30 km the vertical crustal thickness in order to accommodate and allow shortening by folding long term climate differences cannot explain alternating changes of width by a factor of almost 2 along straight segments of the orogen on length scales less than 200 km as observed from the western central alps to the easternmost eastern alps sedimentary or paleontological evidences supporting such paleo climatic differences are lacking suggesting that erosional processes did not directly control the width of the orogen

geochemical constraints on the growth of the continental crust several lines of evidence indicate that the archean upper crust was considerably more mafic than the present day upper crust there has been no significant change in ree and th abundances in post archean clastic sedimentary rocks suggesting that there has been no change in the composition of the upper crust during the post archean this indicates that if there have been any additions to the post archean upper crust they must have had similar composition to the upper crust itself geochemical modelling of ree and th abundances in sedimentary rocks suggests that the minimum ratio of post archean to archean upper crustal composition required to eliminate the archean upper crustal trace element signature within analytical uncertainty is about 4 1 such a model also is supported by isotopic data using plausible assumptions regarding the volume of archean crust isostatic relations and extreme models of the earth s degassing history it is proposed that approximately 65 75 of the continental crust formed