the distribution of the mid to lower crustal low velocity zone beneath the northeastern tibetan plateau revealed from ambient noise tomography we collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the chinese provincial digital seismic networks a seismic collaborative experiment of northern tibet and the northeastern tibet seismic experiment cross correlations of vertical component records are computed to extract the rayleigh wave empirical green s functions group and phase velocities are then constructed from the empirical green s functions in 8 to 50 s period at periods 25 s more than 10 lower velocities are imaged beneath the qaidam basin and high velocities are observed beneath the nonbasin regions at periods 30 s up to 10 lower velocities are imaged in the qiangtang and songpan ganze terranes from these group and phase velocity maps a three dimensional 3 d vsv model of the crust is derived the model shows that the qiangtang and songpan ganze terranes have a very thick crust with a prominent low velocity zone lvz in the middle crust the lvz thins out in the vicinity of the eastern kunlun mountains providing a new constraint on the mode of deformation across the tibetan plateau the northwestern qilian orogen where receiver functions reveal a moho deeper than the surrounding areas also features a relatively weak midcrustal lvz which we interpret as an intracrustal response associated with the shortening between the north china craton and the tibetan plateau

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

taphonomy and paleoenvironmental conditions of deposition of fossil whales in the diatomaceous sediments of the miocene pliocene pisco formation southern peru a new fossil lagerstatte the pisco formation in peru contains abundant fossil cetaceans in middle miocene to lower pliocene sandstone siltstone and tuffaceous and diatomaceous mudstone fossil whales are especially abundant in the uppermost diatomaceous units most specimens are well articulated or partially disarticulated but associated degree of preservation is exceptionally high and uniform among articulated and disarticulated specimens and on the lower and upper surfaces of the bones some specimens have baleen preserved in anatomical position bones show no evidence of bioerosion by macro or micro invertebrates except for a very limited amount of microbial borings diatomaceous layers seem to lack bioturbation however bioturbation did occur in a few tuffaceous silty and sandy layers of the lower part of the formation shark teeth are found associated with many of the specimens however despite abundance of whale skeletons shark tooth marks are extremely rare several lines of evidence indicate that sediments and whales were deposited in a shallow to deep shelf environment an embayment and not in a beach environment the thick diatomaceous successions record conditions of strong ocean upwelling indicated by the abundant occurrence of the diatom species thalassionema nitzschioides the exceptional preservation of diatom frustules suggests that they reached the seafloor very rapidly and were not successively reworked the hundreds of whale specimens found their excellent preservation and their high degree of articulation make the pisco formation fossil whales the best representative assemblage of fossil mysticeti known so far the abundance of whale skeletons and other vertebrates in the diatomaceous beds their excellent preservation including evidence that soft tissue baleen was still present at burial and the degree of articulation point to frequent mortalities followed by rapid sedimentation that prevented decay of the skeletons and colonization by invertebrates sedimentation rates appear much higher than rates inferred from some other modern and ancient settings where whale skeletons have been found

early cretaceous synrift uplift and tectonic inversion in the loppa high area southwestern barents sea norwegian shelf tectonic inversion of rift basins is most commonly reported in the literature to occur after rifting has ceased in contrast we present evidence for synrift localized tectonic inversion from the loppa high area southwestern barents sea and present a model for the formation of inversion structures as a result of differential uplift the structures are of early barremian to mid albian age c 131 105 ma and are focused in or near pre existing extensional boundary faults along the margins of the loppa high inversion is interpreted to be the result of uplift of the high along its inclined boundary faults leading to space accommodation problems as uplift was not properly compensated by extension in the region the model constrains the initiation of uplift of the loppa high to the early barremian and shows that the asymmetric margin configuration of the high may have led to a bulk clockwise rotation of the high around a vertical axis during uplift the cause of uplift is not fully understood but is suggested to be linked to contemporaneous extreme lithospheric thinning in neighbouring basins to the west processes involved may include isostatic flexure thermal heating lithological phase changes and or far field stresses although these aspects need to be further tested

jurassic to early cretaceous basin configuration s in the fingerdjupet subbasin sw barents sea abstract the fingerdjupet subbasin in the southwestern barents sea sits in a key tectonic location between deep rifts in the west and more stable platform areas in the east its evolution is characterized by extensional reactivation of n s and nne ssw faults with an older history of late permian and likely carboniferous activity superimposed on caledonian fabrics reactivations in the listric nne ssw terningen fault complex accommodated a semi regional rollover structure where the fingerdjupet subbasin developed in the hangingwall in parallel the randi fault set developed from outer arc extension and collapse of the rollover anticline n s to nne ssw faults and the presence of other fault trends indicate changes in the stress regime relating to tectonic activity in the north atlantic and arctic regions a latest triassic to middle jurassic extensional faulting event with e w striking faults is linked to activity in the hammerfest basin cessation of extensional tectonics before the late jurassic in the fingerdjupet subbasin however suggests rifting became localized to the hammerfest basin the late jurassic was a period of tectonic quiescence in the fingerdjupet subbasin before latest jurassic to hauterivian extensional faulting which reactivated n s and nne ssw faults barremian se prograding clinoforms filled the relief generated during this event before reaching the bjarmeland platform high angle nw prograding clinoforms on the western bjarmeland platform are linked to early barremian uplift of the loppa high the terningen fault complex and randi fault set were again reactivated in the aptian along with other major fault complexes in the sw barents sea leading to subaerial exposure of local highs this activity ceased by early albian post upper albian strata were removed by late cenozoic uplift and erosion but later tectonic activity has both reactivated e w and n s nne ssw faults and also established a nw se trend

ambient noise tomography of north tibet limits geological terrane signature to upper middle crust 1 we use ambient noise tomography to map regional differences in crustal rayleigh wave group velocities with periods of 8 40 s across north tibet using the international deep profiling of tibet and the himalaya phase iv arrays 132 stations deployed for 10 24 months for periods of 8 24 s sensitive to midcrustal depths of 5 30 km we observe striking velocity changes across the bangong nujiang and jinsha suture zones as well as the kunlun qaidam boundary from south to north we see higher velocities beneath the lhasa terrane lower velocities beneath the qiangtang higher velocities in the songpan ganzi and kunlun terranes and the lowest velocities beneath the qaidam basin maps at periods of 34 and 40 s sensitive to the middle and lower crust at depths of 30 60 km do not show evidence of changes across those boundaries any differences between the tibetan terrane lower crusts that were present at accretion have been erased or displaced by cenozoic processes and replaced almost ubiquitously by uniformly low velocities

stochastic velocity inversion of seismic reflection refraction traveltime data for rift structure of the southwest barents sea we present results from an active source onshore offshore seismic reflection refraction transect acquired as part of the petrobar project petroleum related studies of the barents sea region the 700 km long profile is oriented nw se coincident with previously published multichannel seismic reflection profiles we utilize layer based raytracing in a markov chain monte carlo mcmc inversion to determine a probabilistic velocity model constraining the sedimentary rocks crystalline crust and uppermost mantle in a complex tectonic regime the profile images a wide range of crustal types and ages fromproterozoic craton to paleozoic to early cenozoic rift basins and volcanics related to eocene continental breakupwith greenland our analyses indicate a complex architecture of the crystalline crust along the profile with crystalline crustal thicknesses ranging from43 kmbeneath the varanger peninsula to 12 kmbeneath the bjornoya basin assuming an original post caledonide crustal thickness of 35 km in the offshore area we calculate the cumulative thinning factors along the entire profile the average factor along the profile is 1 7 0 1 suggesting 211 243 km of extension consistent with the amount of overlap derived from published plate reconstructions local factors approach 3 where bjornoya basin reaches a depth of more than 13 km volcanics carbonates salt diagenesis and metamorphism make deep sedimentary basin fill difficult to distinguish from original pre rift crystalline crust and thus actual stretching may in places exceed our estimates

southwestern barents sea margin late mesozoic sedimentary basins and crustal extension abstract the deep sedimentary basins of the southwestern barents sea were formed in response to several late mesozoic early cenozoic tectonic events within the north atlantic rift zone which culminated with continental breakup and formation of a mainly sheared margin in early tertiary times due to deteriorating data quality the development of the margin bordering sorvestsnaget basin is not well known to improve on this we use both seismic interpretation and gravity modelling to estimate the depth to the middle jurassic mj sequence boundary which marks the onset of mesozoic rifting the horizon represents a first order contrast in seismic velocity and density and the gravity field correlates well with its depth of burial only one seismic line enables tracing of the mj from surrounding areas into the sorvestsnaget basin down to a depth of at least 17 km in the northeastern part of the basin the deep basin is reflected in a general thinning of the crust from 30 36 km within the barents sea to 20 24 km in the margin area the gravity modelling show that the middle jurassic lower cretaceous sequences are of comparable thicknesses in the tromso and sorvestsnaget basins however adjacent to the rifted margin segment the deep northern part of the sorvestsnaget basin is affected by additional large late cretaceous normal faulting and subsidence these faults also caused additional structuring of the intrabasinal veslemoy high post middle jurassic crustal thinning in the southwestern barents sea shows maximum cumulative values exceeding 4 within the tromso and the northern sorvestsnaget basins the corresponding crustal extension is estimated to 70 85 km in a west to northwest direction there is no uniform increase in crustal thinning toward the continent ocean boundary which we attribute to the transform origin of the margin however there is a gradual migration of tectonic activity toward the incipient margin as the time of continental breakup was approached

why intracontinental basins subside longer 3 d feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

new insights into the tectono stratigraphic evolution of the southern stappen high and its transition to bjornoya basin sw barents sea abstract within the context of the southwestern barents sea the southern stappen high and its transition to the bjornoya basin are still underexplored improved quality seismic reflection data are utilised to describe new insights into the paleozoic to early cenozoic tectono stratigraphic evolution of the area as well as to discuss the structural inheritance and the rift development well defined syn rift wedges and better resolution images for both the deep carboniferous and permian successions are revealed in particular both the mid carboniferous and late permian earliest triassic extensional phases are characterized by widespread ne sw oriented normal faults that are mostly westward dipping although triassic is mostly considered as a tectonically stable period in the barents sea in the southern stappen high there is clear identification of a localised depocentre named herein intra stappen basin where syn tectonic geometries characterize the upper paleozoic and triassic deposits regional correlation to middle and upper triassic outcrops in southwestern svalbard reveals possible progradation from a west northwest northeast greenland provenance as a western sediment source area during the triassic in addition to the well known eastern sediment source area thin but distinct jurassic sequences are expected to be present on stappen high associated with prominent regional nw se extension throughout late jurassic that culminated during the earliest cretaceous furthermore structural and stratigraphic relations are observed within the study area that clearly indicate a distinct early aptian rift phase with increasing evidence for its occurrence in the southwestern barents sea upper cretaceous sequences bounded by major low angle west dipping detachment faults are observed in southwest stappen high during early cenozoic the study area was located at the proximity of the paleo coastline and paleo shelf edge for both paleocene and eocene gravity mass waste deposits these are most probably related to a progressively evolving steep bathymetric gradient between the developing margin mainly towards the west and to the south and the uplifted stappen high

basin modelling of the sw barents sea abstract the sw barents sea is an epicontinental platform consisting of n to nne oriented basins separated by basement highs the basins were formed during four distinct rift phases in the carboniferous late permian late jurassic early cretaceous and paleocene eocene progressive rifting culminated in continental breakup and seafloor spreading along the north atlantic axis approximately at 55 ma we present tectonic and thermal models of basin evolution along two seismic profiles crossing the sw barents sea the thermal and isostatic history of basins is constrained through time forward basin modelling based on an automated inverse basin reconstruction approach we estimate the effects of continental breakup and near margin processes magmatic underplating and sill intrusions on the thermal history and kerogen maturity below the vestbakken volcanic province basin models are calibrated against well data the results imply that both breakup and underplating alter the thermal and isostatic history of sediments along the margin the hydrocarbon potential of source rocks modelled along the margin suggests breakup has a permanent thermal effect on the present day sediments promoted by lateral heat flow while heat conduction by underplating is more diffuse inhibited to some degree by deeply buried low conductivity shales and limestones

silicate melt inclusion evidence for extreme pre eruptive enrichment and post eruptive depletion of lithium in silicic volcanic rocks of the western united states implications for the origin of lithium rich brines to evaluate whether anatectic and or highly fractionated lithophile element enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption lithification and weathering to generate significant li brine resources pre eruptive melt compositions preserved in inclusions and the magnitude of post eruptive li depletions evident in host rhyolites were documented at six sites in the western united states each rhyolite is a member of the bimodal basalt rhyolite assemblage associated with extensional tectonics that produced the basin and range province and rio grande rift an evolving pattern of closed drainage basins and geothermal energy or mineral resources results from the 0 8 ma bishop tuff geothermal in california 1 3 to 1 6 ma cerro toledo and upper bandelier tephra geothermal and 27 9 ma taylor creek rhyolite sn in new mexico 21 7 ma spor mountain tuff be u f and 24 6 ma pine grove tuff mo in utah and 27 6 ma hideaway park tuff mo in colorado support the following conclusions melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of sn mo and be are extremely enriched in li 1 000s of ppm those from spor mountain have the highest li abundance yet recorded max 5 200 ppm median 3 750 ppm forty five to 98 of the li present in pre eruptive magma was lost to the environment from these rhyolite tuffs the amount of li lost from the small volumes 1 10 km3 of li enriched rhyolite deposited in closed basins is sufficient to produce world class li brine resources after each eruption meteoric water leaches li from tuff which drains into playas where it is concentrated by evaporation the localized occurrence of li enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of li considering that hydrothermal deposits of sn mo be u and f may indicate potential for li brines in nearby basins we surmise that the world s largest li brine resource in the salar de uyuni 10 mt received li from nearby rhyolite tuffs in the bolivian tin belt

modelled palaeo temperature on voring offshore mid norway the effect of the lower crustal body abstract the voring area offshore mid norway has a complex geological history has experienced several extensional phases and was significantly influenced by the break up of the north atlantic we have modelled a cross section over the voring basin aiming to 1 reconstruct the basin evolution in a realistic way and 2 to investigate the heat flow and temperature history in the basin for the modelling we used the following tectonic events the opening of the basin during the permo triassic an event during the middle to late cretaceous and an event in the late cretaceous paleocene the theoretical effects of the lithospheric stretching are depending on the palaeo water depths of the area we present a prediction of the palaeo water depth and a sensitivity analysis of the influence of the palaeo water depth on the estimated beta factors in the area a lower crustal high velocity body found in the area is often interpreted as magmatic underplating related to the break up of the norwegian sea we show the temperature history calculated by models that were run with and without assumption of underplating by a magmatic body emplaced during the early tertiary the observed vitrinite reflectance in the voring area is best explained by numerical calculations of the vitrinite reflectance without a magmatic underplating our conclusion is that the lower crustal body is not related to magmatic underplating or to significant sill intrusions the body may consist of older mafic rocks or a mixture of old continental crust and mafic intrusions

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

effect of thermal contrasts on gravity modeling at passive margins results from the western barents sea the western barents sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling this margin developed by shear motion between the eurasian and greenland plates during the early tertiary and it is a significant border zone between young hot oceanic lithosphere and cooler continental lithosphere we construct two dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling the approach has a substantial impact over traditional shallow gravity models here demonstrated on a previously published model on the basis of a 140 mgal free air anomaly the old model proposes an anomalous high density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development however the lithospheric models predict a homogeneous oceanic crust while preserving regional isostasy at base lithosphere from continent to ocean two further tests agree with this conclusion a map of bouguer corrected ers 1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin admittance analysis shows a strong oceanic lithosphere and the high coherence between bathymetry and free air gravity discounts a significant subsurface load the high gravity anomalies at the margin are thus an edge effect enhanced by sedimentation onto the strong oceanic lithosphere and shaped by the effect of the lithospheric thermal field other results of this work include a new continent ocean boundary map and two crustal transects across the margin

volcano stratigraphy and 40ar 39ar geochronology of the macusani ignimbrite field monitor of the miocene geodynamic evolution of the andes of southeast peru abstract a total of 1400 m of ignimbritic ash flow tuffs distributed in five cross sections and extending from the pre neogene base to the uppermost flows were studied in the miocene macusani ignimbrite basin southeast peru volcano stratigraphic interpolation and precise 40ar 39ar dating of sanidine biotite and rhyolitic glass from the ash flow tuffs give rise to a southwest northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units i e 1 10 0 0 5 2 7 8 8 0 0 1 3 7 5 0 1 4 7 3 0 1 5 6 8 7 0 o 1 and 6 6 7 o 1 ma these delimit two brief e 10 1 and 7 1 ma which are sensibly synchronous with the quechua 2 and quechua 3 compressional events characterizing the tectonic regime in the central andes this close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the miocene tectono magmatic evolution of the cordillera oriental the stratigraphic relationships of the uranium mineralization of the macusani field are defined for the first time the stratiform stratabound occurrences are restricted to three main cooling units dated at 7 8 7 5 and 6 9 6 8 ma which constitute a maximum age for uranium deposition

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

optimising the use of marine tephrochronology in the north atlantic a detailed investigation of the faroe marine ash zones ii iii and iv abstract tephrochronology is central to the intimate 1 goals for testing the degree of climatic synchroneity during abrupt climatic events that punctuated the last glacial period since their identification in north atlantic marine sequences the faroe marine ash zone ii fmaz ii fmaz iii and fmaz iv have received considerable attention due to their potential for high precision synchronisation with the greenland ice cores in order to optimise the use of these horizons as isochronous markers a detailed re investigation of their geochemical composition sedimentology and the processes that deposited each ash zone is presented shard concentration profiles geochemical homogeneity and micro sedimentological structures are investigated for each ash zone preserved within core jm11 19pc retrieved from the southeastern norwegian sea on the central north faroe slope this approach allows a thorough assessment of primary ash fall preservation and secondary depositional features and demonstrates its value for assessing depositional integrity in the marine environment results indicate that the fmaz ii and iv are well resolved primary deposits that can be used as isochrons for high precision correlation studies we outline key recommendations for future marine tephra studies and provide a protocol for optimising the application of tephrochronology to meet the intimate synchronisation goals

cenozoic erosion and sedimentation in the western barents sea abstract seismic reflection data and well logs are used to interpret cenozoic sedimentary environments in the south western barents sea using volumetric calculations and compaction trend analysis the cenozoic erosion and sedimentation rates have been quantified marine environments ranging from neritic to bathyal water depths dominated the southern barents sea during the late palaeocene at the palaeocene eocene boundary a southwards progradation commenced along the uplifted and eroded loppa and stappen highs during the early eocene to mid miocene sedimentation continued in the intracratonic basins as well as along the newly established continent ocean boundary in the embryonic northern norwegian greenland sea substantial erosion occurred on the westernmost margin particularly on the stappen high during this time interval in the mid miocene during a period of relative lowstand of sea level a system of lowstand fans was deposited west of the stappen high later a pronounced progradation of the shelf edge commenced during the mid miocene to late pliocene the shelf edge prograded between 20 and 40 km during the mid pliocene to early pleistocene the barents sea and adjacent land areas were uplifted a fluvial glaciofluvial drainage system developed on the continental shelf resulting in pronounced erosion there and very high sedimentation rates on the continental margin during the last 0 8 ma several large glaciations on the barents sea shelf proper have resulted in overdeepened troughs and high sedimentation rates at the continental shelfbreak and on the continental slope the overall cenozoic erosion varies spatially as well as temporally in the svalbard area close to 3 km of rocks have been eroded since the oligocene the same amount or more was eroded from the stappen high during the palaeocene and early neogene in the south western areas during the eocene mid miocene erosion occurred along the margin in addition the loppa high and adjacent areas served as sediment source areas during the late pliocene and pleistoncene the average erosion in the south western barents sea amounted to about 600 m erosion was greater in the south east than to the west and north and was more extensive where the former glacial ice streams had flowed

neogene and quaternary ignimbrites in the area of arequipa southern peru stratigraphical and petrological correlations abstract this study focuses on the correlation of four newly identified rhyolitic ignimbrites located around arequipa which were previously described as a single ignimbrite sheet 1 the rio chili ignimbrite ca 13 33 ma crops out in the rio chili canyon and consists of a cooling unit of non welded to partially welded massive crystal rich lapilli tuff this ignimbrite represents the base of the neogene ignimbrite succession in the arequipa area and may be stratigraphically equivalent to the 14 13 ma huaylillas ignimbrites in southern peru 2 the second voluminous 20 km3 and widespread la joya ignimbrite sheet ca 4 87 ma filled the arequipa depression its probable source is now buried underneath the chachani volcanic complex the la joya ignimbrite covers both the arequipa batholith toward the rio vitor piedmont and also part of the altiplano to the east and north of the chachani complex 3 the arequipa airport ignimbrite aai sheet ca 1 6 ma 18 km3 the third and latest infill of the arequipa basin consists of a lower white columnar jointed massive lapilli tuff indurated by vapor phase recrystallization and is overlain by an upper pink lithic rich non welded massive lapilli tuff the source of this ignimbrite is also buried underneath the chachani complex as indicated by anisotropy of magnetic susceptibility measurements and component lithology 4 the yura non welded pumice flow deposits ca 1 02 ma 1 5 km3 are restricted to the northern and western flanks of the chachani complex and may correlate with tuffs of the capillune formation on the altiplano all ignimbrites are high k calc alkaline to alkaline rhyolites which contain variable amounts of plagioclase biotite quartz sanidine and fe ti oxides the modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites specific geochemical fingerprints such as sr y rb ba cs ree and sr isotopes were effective discriminator elements magmatic differentiation crustal residence time and contamination and source melting processes are discussed in order to explain compositional variations the older la joya ignimbrite and rio chili ignimbrite reflect initial wetter crustal melts closer to granite composition whereas with time the younger arequipa airport ignimbrite and yura tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting the voluminous 40 km3 neogene ignimbrites of the arequipa area were produced by three major explosive volcanic pulses in the central volcanic zone of southern peru whose genetic relation with tectonic uplift and crustal thickening of the andean cordillera are still a matter of debate

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

effects of lithosphere buckling on subsidence and hydrocarbon maturation a case study from the ultra deep east barents sea basin abstract lithospheric extension is proposed as one of the most common mechanism to explain the formation of intra continental sedimentary basins however extension is an unlikely mechanism for the formation and preservation of certain intra continental basins as for example the east barents sea basin ebb whose origin remains unexplained an alternative basin formation mechanism is buckling accompanying lithosphere shortening here we evaluate the conditions for lithospheric shortening buckling as a mechanism to form deep and isostatically compensated sedimentary basins we use a forward thermo mechanical finite element technique to evaluate this mechanism for the ebb where a very large and compensated subsidence is observed the lower crust is modeled with petrologic consistent densities that depend on pressure and temperature taking into account de hydration at high p t conditions various models are tested characterized by different compositions for the lower crust results show that lithospheric shortening buckling may result in compensated ultra deep basins like the ebb provided the presence of crustal heterogeneities such as a mafic underplate the implications of buckling for the maturation of source rocks are evaluated results show that maturation of kerogen in a contractional basin takes much longer time compared to a basin formed by extension owing to the significantly lower temperatures in the initial stages of basin formation in the case of the ebb buckling predicts better preservation of hydrocarbons as potential triassic source rocks may enter the oil and gas window after deposition of middle jurassic reservoir rocks and formation of late jurassic early cretaceous traps

a bayesian approach for thermal history reconstruction in basin modeling we present a novel method for the joint inversion of thermal indicator data vitrinite reflectance and apatite fission track and additional data bottom hole temperature and porosity for thermal history reconstruction in basin modeling a transdimensional and hierarchical bayesian formulation is implemented with a reversible jump markov chain monte carlo algorithm with a 1 d transient thermal model the objective of the inverse problem is to infer the heat flow history below a borehole given the data and a set of geological constraints e g burial histories and physical properties of the sediments the algorithm incorporates an adaptive data driven parametrization of the heat flow history allows for automatic estimation of the relative importance of each data type and quantification of parameter uncertainties and trade offs our approach deals with uncertainties on the imposed geological constraints in two ways first the amount of erosion and timing of an erosional event are treated as independent parameters second uncertainties on compaction parameters and surface temperature history are directly propagated into the final solution synthetic tests show that porosity data can be used to reduce uncertainties on the amount of erosion this work illustrates a truly probabilistic analysis of the trade off between the magnitude of erosion and variations in heat flow histories which is key in basin modeling the algorithm is then applied to real data from a well in the barents sea our algorithm can reconcile estimates of erosion from the thermal indicator and porosity data which is a difficult and subjective task in basin modeling

from pull apart basins to ultraslow spreading results from the western barents sea margin this paper describes results from a geophysical study in the area between the ultraslow knipovich ridge and bear island western barents sea the objective was to map the crustal structure along a profile crossing a pull apart rifted continental margin and oceanic crust generated by ultraslow spreading the results are based on modeling of wide angle seismic and gravity data together with interpretation of multichannel reflection data our results show a two layered oceanic crust in the western part of the profile the thickness of the oceanic crust is variable in the western 130 km ranging from 3 5 to 5 5 km east of km 130 the crustal thickness is relatively constant with values close to the global average for oceanic crust the oceanic crust is buried by a thick package of cenozoic sedimentary rocks the continent ocean transition cot is placed in the interval 207 255 km between unequivocal oceanic crust and the foot of the westernmost fault in the hornsund fault zone it is not possible to conclude whether this interval is oceanic crust or thinned and intruded continental crust but we favor the latter interpretation at least for the eastern part of the cot stretched continental crust is observed between hornsund fault zone and the knolegga fault here the sedimentary rocks have high velocities and are interpreted to be mainly of mesozoic and late paleozoic age in this interval moho depths increase abruptly from 15 km in the west to 27 km in the east crystalline basement velocities are observed close to the seafloor east of the knolegga fault we suggest that continental breakup north of greenland senja fracture zone occurred around 33 ma after a period of pull apart tectonics the spreading rate of the earliest seafloor spreading may have been higher than the present day spreading creating thicker oceanic crust close to the cot

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

two mica rhyolitic tephra in the east pisco basin peru new age and dispersion constraints for the eruptions of the eastern cordillera of central andes wo mica biotite and muscovite volcanics are particularly rare in the geological record one of the several dozen volcanic ash layers from central andes volcanoes found in the upper miocene marine succession of the pisco formation ica desert peru contains juvenile biotite and muscovite sillimanite andalusite feldspars and rhyolitic glass 39ar 40ar dating on biotite and muscovite concordantly constrain an age of 7 96 ma for this two mica ash layer a second tephra in the pisco formation has a similar biotite composition and an age between 7 45 and 6 93 ma the peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and together with the 39ar 40ar ages suggest to consider a correlation of these ash layers to the eruptions of the miocene macusani peru or morococala bolivia volcanic complexes in the eastern cordillera of central andes the major and trace element composition of glass supports the correlation with macusani a provenance from morococala seems less likely given the large distances involved these results provide new data on the volcanic activity of the eastern cordillera revealing ash that dispersed to over 500 km to the west in the forearc marine basins this finding highlights that the exhumed forearc east pisco basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of central andes during the miocene silicic flare ups

crustal structure and transform margin development south of svalbard based on ocean bottom seismometer data abstract the barents sea is located in the northwestern corner of the eurasian continent where the crustal terrain was assembled in the caledonian orogeny during late ordovician and silurian times the western barents sea margin developed primarily as a transform margin during the early tertiary in the northwestern part south of svalbard multichannel reflection seismic lines have poor resolution below the permian sequence and the early post orogenic development is not well known here in 1998 an ocean bottom seismometer obs survey was collected southwest to southeast of the svalbard archipelago one profile was shot across the continental transform margin south of svalbard which is presented here p wave modeling of the obs profile indicates a caledonian suture in the continental basement south of svalbard also proposed previously based on a deep seismic reflection line coincident with the obs profile the suture zone is associated with a small crustal root and westward dipping mantle reflectivity and it marks a boundary between two different crystalline basement terrains the western terrain has low 6 2 6 45 km s 1 p wave velocities while the eastern has higher 6 3 6 9 km s 1 velocities gravity modeling agrees with this as an increased density is needed in the eastern block the s wave data predict a quartz rich lithology compatible with felsic gneiss to granite within and west of the suture zone and an intermediate lithological composition to the east a geological model assuming westward dipping caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust as well as the observed structuring due to the transform margin setting the tectonic thinning of the continental block during opening of the norwegian greenland sea is restricted to the outer 35 km of the continental block and the continent ocean boundary cob can be located to within 5 km in our data distinct from the outer high commonly observed on transform margins the upper part of the continental crust at the margin is dominated by two large rotated down faulted blocks with throws of 2 3 km on each fault apparently formed during the transform margin development analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile and that the downfaulting dominates the whole margin segment from spitsbergen to bjornoya south of bjornoya the faulting leaves the continental margin to terminate as a graben 75 km south of the island adjacent to the continental margin there is no clear oceanic layer 2 seismic signature however the top basement velocity of 6 55 km s 1 is significantly lower than the high 7 km s 1 velocity reported earlier from expanding spread profiles esps and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7 8 km thick sedimentary overburden

estimation of crustal thinning by accounting for stretching and thinning of the sedimentary basin an example from the voring margin ne atlantic we propose a simple analytic way to estimate the stretching factors for basin formation by lithospheric extension the suggested approach accounts for the stretching and thinning of the sediments in a mass conservative manner it is shown that the stretching and thinning of the sediments may have an important impact on the magnitude of the factors especially for deep basins with thick pre rift formations this mass conservative way to estimate factors has been applied to a transect on the atlantic voring margin offshore mid norway where it undergoes four rift episodes from the end of the caledonian orogeny until today the results for the voring transect show that the first rift phases may have been more important than previously modeled these results also show that the magnitude of the late cretaceous paleocene rift phase becomes reduced by including extension and thinning of pre rift sediments the analytic results allow for a study of the sensitivity of the factors with respect to various parameters like for instance paleo water depth and rock densities

two dimensional inverse modeling of sedimentary basin subsidence recently developed mathematical models of subsidence in extensional basins provide a good basis for forward modeling however the large number of parameters to be specified by the user makes it difficult not only to do the modeling itself but also to judge the meaning of the results we present a new method for automatic searching of the best fitting parameter set of a 2d basin formation model weighted goal functions are used in the minimization process by the inverse problem solver algorithm the nominal parameter set in the present case includes profiles of the crustal and subcrustal thinning factors and the level of lithosphere necking the method was tested on synthetic data with parameters that were perturbed beforehand with some restrictions the algorithms are capable of resolving such perturbations the inversion technique has been applied to two profiles crossing the dnieper donets basin ukraine the crustal thinning factors obtained argue for a scissors like style of basin opening high subcrustal thinning values are necessary in order to explain the abnormally thick carboniferous section in the basin these values should be treated as cumulative ones due to the currently unresolvable influence of other rifting related processes particularly phase transitions

cenozoic exhumation on the southwestern barents shelf estimates and uncertainties constrained from compaction and thermal maturity analyses abstract the barents sea is believed to have been influenced in most parts by cenozoic uplift and erosion episodes the rocks in the area are not currently at their maximum burial depth the exhumation of the sedimentary rocks has had large effects on rock physical properties and hydrocarbon maturation and migration the current study seeks to estimate exhumation from shale compaction and thermal maturity techniques and discuss its implications for hydrocarbon exploration in the uplifted barents sea area this study uses well logs and thermal maturity data together with widely distributed shot gather data along long offset seismic reflection lines the use of shale compaction techniques to estimate exhumation was focused particularly on the regionally preserved aptian albian kolmule formation and paleogene torsk formation shales normal compaction reference curves were established for these units in areas currently at their maximum burial depth e g sorvestsnaget basin and vestbakken volcanic province the results suggest widespread cenozoic exhumation throughout the southwestern barents sea the exhumation magnitudes increase towards east and northeast the average exhumation estimates from the three data sources range from 800 to 1400 m within the hammerfest basin 1150 1950 m on the loppa high 1200 1400 m on the finmark platform and 1250 2400 m on the bjarmeland platform the marked differences in glacial erosion from mass balance and average erosion estimates from the current study suggest a significant pre glacial uplift and erosion in the southwestern barents sea area the observed stratigraphy and presence of significant volumes of late oligocene middle miocene sediments in basins at the outer margin and increased erosion rates at the same time in source areas suggest that maximum burial in the southwestern barents sea may have occurred sometime during the oligocene or even earlier in the eocene the results from this study are useful input for modelling of source rock maturation generation migration and trapping of hydrocarbons in the area these results are also an important input for the prediction of more precise reservoir and seal rock properties in frontier areas away from the exploration wells and provide valuable knowledge for the use of interval velocities in the uplifted areas

largest explosive eruption in historical times in the andes at huaynaputina volcano a d 1600 southern peru the largest explosive eruption volcanic explosivity index of 6 in historical times in the andes took place in a d 1600 at huaynaputina volcano in southern peru according to chronicles the eruption began on february 19 with a plinian phase and lasted until march 6 repeated tephra falls pyroclastic flows and surges devastated an area 70 40 km2 west of the vent and affected all of southern peru and earthquakes shook the city of arequipa 75 km away eight deposits totaling 10 2 13 1 km3 in bulk volume are attributed to this eruption 1 a widespread 8 1 km3 pumice fall deposit 2 channeled ignimbrites 1 6 2 km3 with 3 ground surge and ash cloud surge deposits 4 widespread co ignimbrite ash layers 5 base surge deposits 6 unconfined ash flow deposits 7 crystal rich deposits and 8 late ash fall and surge deposits disruption of a hydrothermal system and hydromagmatic interactions are thought to have fueled the large volume explosive eruption although the event triggered no caldera collapse ring fractures that cut the vent area point to the onset of a funnel type caldera collapse

the tectonized central peak of the mjolnir impact crater barents sea abstract the mjolnir structure sw barents sea is one of the best preserved marine impact craters on earth after impact on the paleo seafloor about 142 ma ago this crater experienced an atypical deformation of its central peak which is now elevated 435 m above the crater rims here we investigate the effect of far field tectonic stresses on the central peak uplift based on interpretation of new high resolution p cable and conventional seismic reflection data nearby wells provided stratigraphic control on the interpreted horizons the reconstruction of the crater sedimentary infill supports a subdued original central peak relief with a 5 km wide gentle mound 15 m below the rim our interpretation shows that subvertical outward dipping impact induced faults were reactivated by uplifted segments of the central peak up to 500 m above the platform level during one or several contractional episodes we postulate that post albian tectonic compressional events triggered the structural uplift of the mjolnir central peak differential compaction previously seen as the main deformation process may have increased the original central peak height by only 10 m the mobilization of impact shattered rocks by tectonic compression provides a new and robust explanation for the structural rise of mjolnir s central peak

geochronology and isotope transport systematics in a subsurface granite from the larderello travale geothermal system italy abstract a monzogranite here referred to as the montieri pluton was encountered along two depth profiles from ca 2 to 4 km depth in wells of the larderello travale geothermal system italy we obtained high resolution petrographic and microchemical data on seven fresh samples of the montieri pluton these data are required to correctly interpret the combined isotope data for the rb sr and k ar systems biotite and k feldspar from all samples were analyzed by rb sr and 39ar 40ar rb sr analyses gave extremely variable two point rb sr apparent ages and sr initials indicating that biotite did not equilibrate with feldspar bulk dissolution of k feldspars does not define a rb sr alignment that could have proved a common origin of all k feldspar samples from a single magma batch the leachable fractions of k feldspar separates have much more homogeneous 87sr 86sr ratios around 0 712 which may reflect the isotopic composition of a late stage circulating fluid because of the anatectic origin of the granites from the larderello travale geothermal system it is likely that every k feldspar separate contains at least three isotopically distinct feldspar generations relics of the hercynian gneiss basement pliocene magmatic minerals and hydrothermal retrogression products such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses the 39ar 40ar age spectra of the biotite samples show some internal discordance because of deviations from stoichiometry biotite discordance can be attributed both to chlorite intergrowths as predicted from the active fluid circulation and to the presence of multiple biotite generations such as have already been documented from other micas from the larderello travale geothermal system and as indeed confirmed by high resolution petrography total k ar biotite ages on cluster around 3 ma and broadly agree with a rb sr age obtained by regressing only the biotite analyses this suggests that the montieri pluton whose known volume is at least 25 km3 was emplaced at that time the k feldspar spectra are strongly discordant no step age approaches the zero age which would be predicted by ar diffusivity modelling the extraneous ar in the feldspars is not excess ar but inherited ar instead ar inheritance in the feldspar separates correlates with inheritance of sr arrhenius trajectories of the apparent ar diffusivities for all seven feldspars are astonishingly identical despite the heterochemism amongst samples however the apparent diffusivity calculated from the step heating experiments led to irreproducible models for the thermal history along the depth profile as well as time scales shorter than the historical record this indicates that ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions

is the asian lithosphere underthrusting beneath northeastern tibetan plateau insights from seismic receiver functions abstract whether or not the asian lithosphere has underthrusted beneath the tibetan plateau is important for understanding the mechanisms of the plateau s growth using data from the permanent seismic stations in northeastern tibetan plateau we studied seismic structures of the lithosphere and upper mantle across the plateau s northeastern margin using p and s receiver functions the migrated p and s receiver function images reveal a thick crust and a diffuse lithosphere asthenosphere boundary lab beneath the tibetan plateau contrasting sharply with the relatively thin crust and clear sharp lab under the bounding asian blocks the well defined lab under the asian blocks tilts toward but does not extend significantly under the tibetan plateau this is inconsistent with the model of asian mantle lithosphere underthrusting beneath the tibet plateau instead our results indicate limited passive deformation of the bounding asian lithosphere as it encounters the growing tibetan plateau

rare element enriched s type ash flow tuffs containing phenocrysts of muscovite andalusite and sillimanite southeastern peru ash flow tuffs of neogene age exposed over 2 500 km 2 in the macusani region of southeastern peru are the volcanic equivalent of s type granites the strongly peraluminous tuffs contain phenocrysts of andalusite sillimanite and muscovite and have high 87 sr 86 sr i 0 7258 and 0 7226 and 18 o 11 elevated concentrations of li cs be sn b and other minor elements compare with those in tin granites mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a hf a h 2 o the chemical isotopic and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock

crustal shear wave velocity structure of northeastern tibet revealed by ambient seismic noise and receiver functions abstract the tibetan plateau is formed by the persistent convergence between the indian and eurasian plates the northeastern tibetan plateau is undergoing young deformation that has been noticed for a long time we conduct a passive source seismic profile with 22 stations in ne tibet in order to investigate the crustal shear wave velocity structure and its relationship with tectonic processes in this paper we obtain the rayleigh wave phase velocity dispersion data among all station pairs within the period bandwidth of 5 20 s from the method of ambient noise cross correlations phase velocity variations correlate well with surface geological boundaries and tectonic features for instance low phase velocity beneath the songpan ganzi block and the guide basin we also compute p wave receiver functions based on the selected teleseismic events with similar ray parameters and perform the joint inversion of surface wave dispersion data and receiver functions to obtain the 2 d crustal shear wave velocity structure along the profile the inversion results show that low shear wave velocities beneath the songpan ganzi block are widespread in the middle to lower crust in together with high crustal vp vs ratios and high temperature suggested by the p wave velocities obtained from the active source seismic study we suggest that the low velocity zone beneath the songpan ganzi block is probably attributed to partial melting across the north kunlun fault there is no crustal lvz found beneath the kunlun block this structural difference may have already existed before the collision of the two blocks or due to limit of the northward extension for the crustal lvz across the north kunlun fault

numerical simulation of rifting in the northern viking graben the mutual effect of modelling parameters numerous basin modelling studies have been performed on the viking graben in the northern north sea during the past decades in order to understand the driving mechanisms for basin evolution and palaeo temperature estimations in such modelling it is important to include lithospheric flexure the values derived for the lithospheric strength by these studies vary considerably i e up to a factor of 30 in this study which is based on new interpretation of a regional transect we show that both the estimated value of the effective elastic thickness and the derived b profile are dependent on the assumed value of the depth of necking the use of models that implicitly set the level of necking at a depth of 0 km generally leads to an underestimation of the lithospheric strength and an overestimation of the thinning factors in the northern viking graben a necking depth at intermediate crustal levels gives results comparable to the observations extension by faulting is modelled to be a significant factor in conclusion rifting in the northern viking graben can be explained with various models of effective elastic thicknesses eet varying from 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking it is also shown that the development of the basin during the post rift phase cannot be explained by pure shear simple shear extension two mechanisms are proposed here to explain the post rift subsidence pattern namely intra plate stress and phase boundary migration the two extreme models for eet mentioned above 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking give very different responses to compressional stress the latter gives basically no response for realistic intra plate stress d 2004 elsevier b v all rights reserved

glass rich cordierite biotite rhyodacite valle ninahuisa puno se peru petrological evidence for hybridization of lachlan s type and potassic mafic magmas

biotite composition as a tool for the identification of quaternary tephra beds abstract stratigraphically important quaternary rhyolitic tephra deposits that erupted from the okataina and taupo volcanic centers in new zealand can be geochemically identified using the feo and mgo contents of their biotite phenocrysts the feo mgo ratio in biotite does not correlate with feo mgo in the coexisting glass phase so that tephra beds with similar glass compositions can be discriminated by their different biotite compositions some individual tephra deposits display sequential changes in biotite composition that allow separate phases of the eruption to be identified greatly increasing the potential precision for correlation in addition devitrified lavas that are unsuitable for glass analysis can be correlated to coeval tephra deposits by their biotite compositions biotite is common in high k2o 4 wt tephra beds and is widely dispersed in ash plumes because of its platy form thus making it important in correlation studies

a two dimensional inverse model for extensional sedimentary basins 1 theory 1 a simple two dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is described our starting point is a forward model which allows basin stratigraphy to be calculated from any given strain rate distribution this forward model includes potentially important effects such as flexural rigidity and the two dimensional conduction advection of heat conversely inverse modeling determines strain rate variation by minimizing the misfit between predicted and observed patterns of basin subsidence no a priori assumptions about the number duration or intensity of rifting episodes are necessary instead strain rate is allowed to vary smoothly throughout time and space an important restriction is that vertical lines remain vertical but this restriction can be relaxed if deemed necessary we have successfully inverted different synthetic sedimentary basin geometries which were generated by forward modeling sensitivity analysis and noise tests demonstrate that strain rate patterns can be recovered with confidence the relationship between stratigraphic misfit and elastic thickness shows that usually an upper limit but not a lower limit for elastic thickness can be retrieved our general approach is applicable to any extensional sedimentary basin including highly extended passive margins and it should be regarded as the stepping stone to more sophisticated two dimensional and three dimensional inverse modeling in the companion paper we apply our algorithm to a set of phanerozoic extensional sedimentary basins located worldwide

cenozoic erosion and the preglacial uplift of the svalbard barents sea region abstract the creation of the huge fans observed in the western barents sea margin can only be explained by assuming extremely high glacial erosion rates in the barents sea area glacial processes capable of producing such high erosion rates have been proposed but require the largest part of the preglacial barents sea to be subaerial to investigate the validity of these proposals we have attempted to reconstruct the western preglacial barents sea our approach was to combine erosion maps based on prepublished data into a single mean valued erosion map covering the whole western barents sea and consequently use it together with a simple airy isostatic model to obtain a first rough estimate of the preglacial topography and bathymetry of the western barents sea margin the mean valued erosion map presented herein is in good volumetric agreement with the sediments deposited in the western barents sea margin areas and as a direct consequence of the averaging procedures employed in its construction we can safely assume that it is the most reliable erosion map based on the available information by comparing the preglacial sequences with the glacial sequences in the fans we have concluded that 1 2 to 2 3 of the total cenozoic erosion was glacial in origin and therefore a rough reconstruction of the preglacial relief of the western barents sea could be obtained the results show a subaerial preglacial barents sea thus during interglacials and interstadials the area may have been partly glaciated and intensively eroded up to 1 mm y while during relatively brief periods of peak glaciation with grounded ice extending to the shelf edge sediments have been evacuated and deposited at the margins at high rates the interplay between erosion and uplift represents a typical chicken and egg problem initial uplift is followed by intensive glacial erosion compensated by isostatic uplift which in turn leads to the maintenance of an elevated and glaciated terrain the information we have on the initial tectonic uplift suggests that the most likely mechanism to cause an uplift of the dimensions and magnitude of the one observed in the barents sea is a thermal mechanism

automated thermotectonostratigraphic basin reconstruction viking graben case study we present a generic algorithm for automating sedimentary basin reconstruction automation is achieved through the coupling of a two dimensional thermotectonostratigraphic forward model to an inverse scheme that updates the model parameters until the input stratigraphy is fitted to a desired accuracy the forward model solves for lithospheric thinning flexural isostasy sediment deposition and transient heat flow the inverse model updates the crustal and mantle thinning factors and paleowater depth both models combined allow for automated forward modeling of the structural and thermal evolution of extensional sedimentary basins the potential and robustness of this method is demonstrated through a reconstruction case study of the northern viking graben in the north sea this reconstruction fits present stratigraphy borehole temperatures vitrinite reflectance data and paleowater depth the predictive power of the model is illustrated through the successful identification of possible targets along the transect where the principal source rocks are in the oil and gas windows these locations coincide well with known oil and gas occurrences the key benefits of the presented algorithm are as follows 1 only standard input data are required 2 crustal and mantle thinning factors and paleowater depth are automatically computed and 3 sedimentary basin reconstruction is greatly facilitated and can thus be more easily integrated into basin analysis and exploration risk assessment

severity and timing of cenozoic exhumation in the southwestern barents sea the juxtaposition of the north atlantic rift system with the borderlands of the eurasian arctic shelf has resulted in an unusual basin evolution in the southwestern barents sea exploration has concentrated on the mesozoic sandstone plays of the hammerfest basin these have yielded abundant gas and very little oil such findings are typical of peripheral north atlantic margin basins that have undergone cenozoic exhumation this is generally thought to have occurred during the palaeocene oligocene miocene or pleistocene hammerfest basin erosion is widely estimated to be in the range of 500 1500 m however the relative severity of individual episodes and impact on fluid dynamics within the basin are poorly constrained we apply basin modelling tools to establish the sensitivity of the petroleum system to three cited scenarios for cenozoic exhumation vitrinite reflectance and temperature data constrain burial and thermal histories calibration is not possible using heat flow variations alone erosion timing and magnitude are required to match available data the calibrated model indicates a significant thermal disequilibrium in the hammerfest basin at the present day resulting from late cenozoic exhumation forward modelling of pleistocene ice sheets allows for an estimation of pressure and temperature fluctuations in response to glacial interglacial cycles

the role of collisional tectonics in the metallogeny of the central andean tin belt abstract the inner arc of the central andes broadly corresponding to the eastern cordillera is the location of a rich tertiary and triassic sn w ag base metal metallogenic province commonly referred to as the bolivian tin belt we propose that the tertiary metallogeny which generated most of the tin ores was a direct consequence of discrete collisions between the south american plate and the nazca slab and sub slab mantle during the ongoing andean orogeny evidence supporting this proposal include 1 the coincidence of the tin province and the inner arc in a marked hump in the andean orogen which may represent tectonic indentation 2 the symmetry of the tin province with respect to the bolivian orocline the axis of which corresponds to the direction of highest compression 3 the relative symmetry of the magmatism and tin mineralization with respect to this axis 4 the concurrent timing of mineralization and compressional pulses 5 the similar host rock geochemistry and ore lead isotope data testifying to a common crustal reservoir and 6 the striking similarity of the igneous suites associated with the ore deposits to those from typical collisional orogens a number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region we propose instead that the latter is better explained by periodic compressional interaction between the farallon nazca oceanic plate and the south american continent this led to the generation of peraluminous magmas which during fractional crystallization exsolved the fluids responsible for the voluminous sn w mineralization

chemical and petrographic fingerprinting of volcanic ashes as a tool for high resolution stratigraphy of the upper miocene pisco formation peru tephra layers are a unique tool for stratigraphy their geologically instantaneous deposition across wide areas makes them a powerful instrument for dating and correlating distant localities in this paper we apply tephra fingerprinting for high resolution stratigraphic purposes in the upper miocene portion of the pisco formation which crops out along the southern peruvian coast the pisco formation diatomaceous strata host an important marine vertebrate fossil lagerstatte whose palaeontological relevance has entailed the necessity of reconstructing a detailed chronostratigraphic framework using 39 ar 40 ar dating and tephra correlations distal ashes from the central andes volcanoes are frequent in the pisco formation but their similar glass composition and mineral assemblage together with the shallow marine environment limiting tephra preservation could be unpromising for tephra based correlations in this study tephra layers from measured stratigraphic sections were fingerprinted with a combined petrographic and microanalytical approach including glass shard morphology and granulometric analyses based on the obtained results we correlated sections several kilometres apart greatly increasing the chronostratigraphic detail major element composition of biotite proved a valuable tool to discriminate the central andes metaluminous to peraluminous tephra deposited in the peruvian forearc basins this study highlights the applicability of tephra fingerprinting in unfavourable shallow marine environments supplementary material a compilation of tephra data field observations granulometric parameters petrographic data and epma analyses on glass epma single analyses on glass and biotite 39 ar 40 ar step heating results measured stratigraphic logs and biotite chemical diagrams are available at https doi org 10 6084 m9 figshare c 4201484

commingling and mixing of s type peraluminous ultrapotassic and basaltic magmas in the cayconi volcanic field cordillera de carabaya se peru abstract the cayconi district of the cordillera de carabaya se peru exposes a remnant of an upper oligocene lower miocene 22 2 24 4 ma volcanic field comprising a diverse assemblage of s type silicic and calc alkaline basaltic to andesitic flows members of the picotani group of the central andean inner arc basaltic flows containing olivine plagioclase clinopyroxene ilmenite and glass and glassy rhyolitic agglutinates with phenocrystic quartz cordierite plagioclase sanidine ilmenite and apatite respectively exhibit mineralogical and geochemical features characteristic of medium k mafic and lachlan s type silicic lavas cordierite bearing dacitic agglomerates and lavas however are characterized by dispersed melanocratic micro enclaves and phenocrysts set in a fine grained quartzo feldspathic matrix they contain a bimodal mica population comprising phlogopite and biotite as well as complexly zoned sieve textured plagioclase grains sector zoned cordierite sanidine quartz irregular patches of replaced olivine clinopyroxene and orthopyroxene and accessory phases including zircon monazite ilmenite and chromite the coexistence of minerals not in mutual equilibrium and the growth dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing trachytic textured andesite flows interlayered with olivine plagioclase glomerophyric calc alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite ca rich sanidine ilmenite and aluminous spinel the mineralogical and mineral chemical relationships in both the dacites and the trachytic textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts not represented by exposed rock types and both the s type silicic and calc alkaline mafic magmas such mixing relationships are commonly observed in the oligo miocene rocks of the cordillera de carabaya suggesting that the s type rocks in this area and by extension elsewhere derive their unusually high k 2 o ba sr cr and ni concentrations from commingling and mixing with diverse mantle derived potassic mafic magmas

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

cenozoic marine sedimentation in the sechura and pisco basins peru abstract the central and northern peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins we have examined the cenozoic stratigraphy of the onshore portions of the sechura basin 5 7 s and pisco basin 13 16 s two shelf basins which have accumulated marine sediment discontinuously since the mid to late eocene cenozoic sediments in the pisco basin were deposited during at least three major transgressive cycles each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones and capped by biogenic deposits including diatomites diatomaceous mudstones dolomitic horizons and phosphate deposits stratigraphic nomenclature for the pisco basin has recently evolved a stratigraphy presented here includes the eocene caballas fm upper eocene los choros fm upper eocene to lowermost oligocene yumaque fm uppermost oligocene to middle miocene chilcatay fm and upper miocene to pliocene pisco fm major hiatuses in the pisco basin span the late cretaceous to middle eocene early to late oligocene middle miocene and late pliocene pleistocene to recent cenozoic sediments of the sechura basin were deposited within at least 4 major transgressive cycles with hiatuses during the paleocene to middle eocene oligocene early to middle miocene and late miocene based on recent biostratigraphic studies sediments enriched in biogenic components accumulated between about 40 36 ma 24 16 ma and 11 3 ma in the pisco basin and between 40 37 ma and 8 5 4 5 ma in the sechura basin in both basins the most diatomaceous sediments are restricted to the late eocene and late miocene through pliocene the temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the late eocene the occurrence of diatomites and phosphorites is diachronous between the pisco and sechura basins and between the peruvian forearc and other circum pacific monterey formation analogs a reflection of the strong influence of local tectonism on sedimentation patterns the volume of neogene sediments along the peruvian forearc is nearly twice that of the monterey fm despite basin to basin facies diachroneity these deposits very likely contributed to fluctuations of the late miocene carbon co 2 system by acting as large carbon sinks

crustal scale subsidence and uplift caused by metamorphic phase changes in the lower crust a model for the evolution of the loppa high area sw barents sea from late paleozoic to present the loppa high area has been subject to several events of uplift and subsidence from the late paleozoic to present the driving mechanisms behind the vertical movements however are not fully understood we propose that uplift and subsidence were influenced by the combination of density changes caused by metamorphic phase changes in a 90 140 km wide mafic lower crustal body below the loppa high and local rift related and far field stress through a numerical modelling approach we analyse the tectonically induced variations in pressure and temperature in the lower crust their influence on phase changes in the mafic body and the affiliated vertical movements results show that 1 densification of the mafic body caused by far field compression associated with the late triassic westward translation of novaya zemlya could cause surface subsidence 2 heat and fluid influx provided by early cretaceous rifting could trigger density reduction and surface uplift and 3 the present day geometry of the loppa high as observed in seismic data can be reproduced by combining the modelled effect of rift flank uplift and phase changes in the mafic body phase change driven vertical movements may also have affected other structural highs in the western barents sea including the stappen high